C802 Sample Report-Manufacturing Warehouse

WAREHOUSE AND MANUFACTURING FACILITY
RF Spectrum
S
t
A
Analysis,
l i
Calibration and System Design
111 Deerwood Road, Suite 200
San Ramon, California 94583
(925) 552-0802
www.Connect802.com
sales@Connect802.com
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This is a “sterilized” version of a report that was delivered to a client.
This client needed a wireless network to support hand-held
inventory scanners, wireless VoIP phones, and notebook
computers used on the production floor.
floor
The site had laser etching machines, many electric saws and
lathes, and other RF noise sources. An on-site RF spectrum
analysis sweep and signal propagation measurements were
0
used to ensure the accuracy of the predictive RF CAD model.
TABLE OF CONTENTS
Topic
Page
Overview and Background
Important Installation Guidelines
Executive Overview
Overall Conclusions
Project Assumptions
Design Specifications
Equipment Specifications
Interpreting the RF Models
Original Drawing Files
3
4
6
7
8
9
17
20
RF CAD Modeling and Simulation
Isometric Models
Fl
Floor
1 RF D
Design
i
Floor 2 RF Design
Cisco 7920 Phone Coverage
21
22
24
29
RF Spectrum Analysis
Noise and Interference Assessment
Interference Power and Existing Access Points
Recommendation
Coverage Testing in the Main Production Area
Coverage in the Drying Rooms
Perspective on the RF Spectrum Analysis Measurements
32
33
34
35
36
37
Installer’s Working
g Plans
Access Point Deployment
Floor 1 – Overview and Detailed Plans
Floor 2 – Overview and Detailed Plans
39
40
46
Technical Appendix
Applying the Design Specifications
Signal Strength Analysis
Spectrum Analyzer Traces
Standard Spectral Footprints
Antenna Polarization Issues
Indoor Polarization Testing
General Interference Issues
U b l
Unbalanced
dP
Power Eff
Effectt
How a Model is Created
50
54
56
59
65
74
75
77
78
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1
EXECUTIVE OVERVIEW
RF System Design Utilizing CAD Modeling and Simulation
This report provides predictive RF coverage models, installation plans, and the details
necessary to support the installation of an In-Building 802.11b/g system to provide
Wi-Fi connectivity throughout specified areas of the Southern Division production,
warehouse, and office facility.
Building characteristics that would impact RF transmission and reception were identified
and antennas were placed for proper radio performance, ease of installation, and
aesthetics.
On-Site RF Analysis
After completion of the predictive CAD models an on-site visit was performed. In addition
to general calibration and verification of the predictive models, special testing was
performed to compare a directional Yagi antenna to an omnidirectional antenna in the
warehouse storage rack area
area. It was found that an omnidirectional antenna
antenna, mounted
above the racks (on the roof trusses and away from the fire curtains) provided better
overall, wider-area coverage than a directional Yagi mounted against an exterior wall
and pointing down an aisle. This was predicted during the modeling and confirmed on
site. A discussion is included in this report.
Overall, the predictive CAD models were found to be accurate to within +/- 3 dB including
the metal
metal-walled
walled paint drying rooms.
rooms Coverage in the warehouse area was better than
predicted. The result was that one less access point was required in the warehouse
area and one antenna was moved roughly 15-feet in the paint drying room area.
Required Equipment
The present design requires the following radio equipment:
Floor 1 – 20 X Cisco 1242 each with 1 X 1781 omnidirectional antenna
5 X Cisco 1130 access point with integrated antenna
Floor 2 – 4 X Cisco 1130 access point with integrated antenna in office area
2 X Cisco 1242 each with 1 X 1781 omnidirectional antenna in main area
Total:
T
t l 31 Access
A
P
Points
i t (9 Ci
Cisco 1130
1130, 22 Ci
Cisco 1242 and
d 22 1781 antennas)
t
)
Note: Each access point must provide a minimum 30 mW transmit power, 50 mW
recommended. The use of diversity antennas is optional and does not change the
current design.
This equipment list does not consider the necessary cable, connectors, Ethernet infrastructure or power supply components, or other
materials and supplies that may be necessary to complete the actual on-site implementation of the system design in the present report.
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2
Significant Factors in the Design Process
The inventory rack areas were less obstructive to RF signal penetrating than initially
considered in the RF CAD modeling. This was because many shelves contained
relatively small boxes of inventory with a significant amount of open space behind
and
db
between
t
ititems stacked
t k d on th
the racks.
k
As a result of the on-site findings, the predictive model that suggested a requirement for
four access points across the tops of the inventory racks was adjusted so that three
access points are in the final design.
IMPORTANT NOTE: At the time of the on-site measurements the inventory levels in the
racks was considered typical. If the racks are, at a later time, filled to full capacity
with highly obstructive material it may result in reduced performance in the rack
area. It’s not anticipated that this type of significantly increased inventory loading
will actually present a problem however, in the event that a problem does occur,
then adding one additional access point over the rack area would be the corrective
action.
No other aspects of the design, or the on-site engagement, were unexpected or unusual.
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3
OVERALL CONCLUSIONS
General Conclusions
Th predictive
The
di ti CAD models
d l accurately
t l provided
id d the
th design
d i for
f the
th network
t
k and
d this
thi d
design
i
was verified through on-site measurements. The warehouse inventory racks introduced
less RF attenuation than predicted by the model due to the fact that inventory was stacked
on the shelves with many gaps between boxes and many areas where small boxes did not
occupy the full surface area of the shelves on which they were stacked. The better-thanexpected characteristics in the warehouse allowed the removal of one access point in the
rack area
area.
On-site, it was initially thought that two access points could be removed in the rack area but
post-visit evaluation led to the current design recommendation that only one of the original
access points be removed. This is to assure proper signal coverage in all areas within the
rack space.
Significant
Si
ifi
Fi
Findings
di
Several unexpected, existing Wi-Fi transmitters were identified, including one that was
broadcasting the SSID “UCLAWLAN”. This access point’s location was undetermined but
assumed to be outside the building (but within 1500 feet of the site.) The most significant
RF activity was the result of the existing access points currently in use at the site. In
addition, a number of 2.4 GHz cordless phones and Bluetooth devices (wireless headsets,
k b d and
keyboards,
d mice?)
i ?) were seen.
The most significant utilization was seen on 802.11 Channel 11, however “significant” is
relative in this case. Although Channel 11 was the most utilized, and although the other
devices seen did present transmissions on other channels, the overall impact is felt to be
normal and not a factor at this time.
In general, however, if future performance problems or VoIP call quality problems should ever
be suspected in an area of the facility it’s recommended that a check be done to see if the
use of Channel 11 is a factor in the troubleshooting. If problems are encountered with
devices operating on Channel 11 then it’s recommended that the areas where these
problems are seen be reconfigured to move to Channel 1 or Channel 6. From a
troubleshooting perspective, the potential for channel overlap will not be a factor during
system testing (because only a limited number of devices will be active at any one time.
Overall, the environment is suitable for the deployment of the proposed network system.
Suitability of the Present Design
This report shows the correct installation locations for radio equipment to meet the specified
requirements but does not suggest or guarantee that the RF characteristics of the space
will remain constant or that the information p
provided for the creation of the design
g is
complete. The design should be validated by making simple RF signal strength
measurements as equipment is installed, confirming that the actual RF propagation is
consistent with the prediction.
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4
PROJECT ASSUMPTIONS
The site is a single warehouse building with a large ground floor (152,000 sq.ft.) and
some smaller office and assembly areas as a partial second floor (26
(26,104
104 sq
sq.ft.).
ft ) Ceilings
in warehouse and industrial-type areas are 40 feet. Ceilings in office areas are 9 feet.
100% coverage in the facility is desired. The design is for an 802.11 b/g network that will
support wireless VOIP phones, wireless scanners that they use to conduct inventory,
and wireless scanning devices used to goods being manufacture red through each
department (i.e. sanding, painting, polishing, finish). Some areas require more coverage
than others. Some existing Cisco 1200 series access points are being used in some of
the indoor office areas; these will be deactivated when the final network system is
installed and operational and these were not considered as interferers or factors in the
present design.
The following equipment has been selected for the current project:
AIR-LAP1242AG-A-K9
AIR-ANT1728
AIR-LAP1131AG-A-K9
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802.11ag LWAPP AP Dual 2.4,5GHz RP-TNC FCC Cnfg
2.4 GHz, 5.2 dBi Ceiling Omni Ant. w/RP-TNC Connector
802.11ag LWAPP AP Integrated Antennas FCC Cnfg
5
DESIGN SPECIFICATIONS
ABOVE
TARGET
Overall Considerations
The models created in this design
g will target
g -57 dBm to meet the specifications
p
listed below. When examining a Grid Coverage Model in this report you’ll see
the target signal level represented as a color hue as shown on the sample
legend to the right.
BELOW
TARGET
The Design Specifications are used to gauge the correctness of each predictive
RF CAD model and simulation in this report. Developing a correct design
often involves a trade-off between alternatives available to the RF engineer
engineer.
The present design is based on an aggressive coverage strategy in which
engineering compromises were resolved in favor of improved signal
coverage.
A detailed discussion of the significance of each of the Design Specifications
may be found in the Technical Appendix at the end of this report.
Target Mobile Client Devices
The present design supports mobile client devices with the following specifications:
Transmit Power Output (TPO)
30 mW (14.7 dBm)
Receiver Effective Antenna Gain
2 dB
Effective Isotropic Radiated Power (EIRP)
16.7 dBm
Minimum Modulation Rate
54 Mbps (802.11g)
Minimum Required Receive Signal Strength (RSSI)
-67 dBm
Fade Margin
10 dB
Maximum Modulation Rae (0 dB Fading)
54 Mbps (802.11g)
Access Point Radio Specifications
The following Access Point specifications were applied to the present design:
Air Standard
802 11g
802.11g
Transmit Power Output (TPO)
30 mW (15 dBm)
Transmitter Effective Antenna Gain
5.2 dBi
Effective Isotropic Radiated Power (EIRP)
20.2 dBm
Minimum Required Receive Signal Strength (RSSI)
-67
67 dBm
Minimum Modulation Rate
54 Mbps (802.11b)
Maximum Modulation Rate (0 dB Fading)
54 Mbps @ -70 dBm
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EQUIPMENT SPECIFICATIONS
Aruba 800 Mobility
y
Controller
The Aruba 800 is a full-featured wireless LAN mobility controller that aggregates up to 16
controlled Access Points (APs) and delivers centralized control and security for wireless
deployments. The Aruba 800 is designed for small/branch offices, the Aruba 800 mobility
controller delivers integrated mobility, security and convergence services for both wired
and wireless users and can be easily deployed as an overlay without any disruption to
the existing branch office network and be centrally managed from the corporate
headquarters or data center using the Aruba Mobility Management System.
System In addition
addition,
the Aruba 800 can be deployed as an identity-based security gateway to authenticate
wired and wireless users, enforce role-based access control policies and quarantine
unsafe endpoints from accessing the corporate network. Guest users can be easily and
safely supported with the built-in captive portal server and advanced network services,
such as EAP offload and DHCP server, allowing branch office network operations to
continue uninterrupted even when the WAN link fails.
fails Features that allow the Aruba 800
to create a secure branch office environment without requiring additional VPN/firewall
devices include integrated site-to-site VPN, split-tunneling, ICSA-compliant stateful
firewall and NAT capabilities. Site-to-site VPN can be integrated with all leading VPN
concentrators to provide seamless integration into existing corporate VPNs. In addition,
advanced convergence features such as Call Admission Control (CAC), voice-aware RF
g
and strict over-the-air QoS allow the Aruba 800 to deliver mobile VoIP
management
capabilities.
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Aruba AP60 and
AP61 Access Point
Ideal for dense AP deployments, the Aruba AP 60 and
61 access points are dual-function, single-radio 802.11a
or b/g access points designed for use only with Aruba
mobility controllers. Each AP provides a dedicated or
shared air monitoring, giving administrators a full view of
and control over the 2.5 and 5 GHz RF spectrums and
eliminating the need for a discrete network of RF
sensors. The
Th A
Aruba
b AP60 supports
t dual-band
d lb d
detachable antennas while the AP61 provides built-in
90-degree rotational dual omni-directional high-gain
antennas.
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Aruba AP65 Access Point
The Aruba 65 is Aruba’s smallest, dual-radio thin architecture
access point the provides concurrent operation of 802
802.11a
11a
and 802.11b/g services. The self-contained design includes
T-bar hanging brackets for mounting directly to the dropped
ceiling or wall mounting on two screws. The unit can act as
both a standard 802.11 access point and an RF monitor
concurrently, across the 2.4 GHz and 5 GHz spectrums.
Manageability
Network-wide management through the Aruba Mobility System via CLI, Web GUI, and
SNMP
Management
g
by
yg
geographical/physical
g p
p y
location, BSSID, Radio Type
yp
Antennas
Dual, integral, tri-band, omni-directional diversity antennas with 180-degrees rotational
movement. Non-detachable.
VSWR 1.5:1
G i 3.30
Gain:
3 30 dBi
Interfaces
1 X 10/100 Base-TX (RJ-45) Ethernet (Auto-sensing MDI/MDX)
PoE 48 VDC / 220 mA 802.3af compliant
Rear mounted
Supports Serial over Ethernet
1 X 5V DC External Power Interface
Transmit Power
Configurable in steps between 1 mW and 65 mW
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Wi-Jack Access Point with Ethernet Port
The Wi-Jack access point provides a stylish, dual-band (802.11b/g and
802.11a configuration) access point and a standard RJ-45 Ethernet jack in a single, easily
installed unit.
All Wi-Jack units are centrally controlled and managed by the Aruba Networks Mobility System
Controller and require no maintenance or user/administrator interaction. The Wi-Jack is
installed in the same enclosure, and in exactly the same manner as a standard RJ-45 jack and
faceplate.
Because the Wi-Jack radio is a
full-power access point
transmitter it provides wireless
coverage that’s consistent with
what would be expected from a
stand-alone, full-sized Wi-Fi
access point.
Wi-Jack Wall
Outlet
The RJ-45 Ethernet port provides
wired connectivity while
maintaining all security and
access control configurations
associated with wireless users.
A Wi-Jack unit is installed exactly like a standard RJ-45
network jack and provides both an Ethernet connection
and an unobtrusive 802.11 Wi-Fi
Wi Fi access point radio.
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10
Ruckus MetroFlex Repeater
The Ruckus MetroFlex Repeater
The Ruckus MetroFlex repeater connects automatically to the
outdoor Wi-Fi system and repeats the signal into the building. With
pre-configuration by Connect802 to link your Ruckus MediaFlex
repeaters to the outdoor system the building occupant simply plugs
the unit it without any configuration or interaction.
The MetroFlex unit is a small (5.5” X 4.8” X 3”) device that can be
inconspicuously placed on a bookshelf, end table, or countertop near
a window or patio door facing the outdoor wireless system.
Key Features
• The Ruckus MetroFlex is two products in one. It acts as a wireless access gateway, providing
reliable connectivity to outdoor Wi
Wi-Fi
Fi networks
networks, as well as an extended range indoor 802
802.11b/g
11b/g
access point.
• The Ruckus MetroFlex is the world’s first customer premise equipment specifically designed and
optimized for broadband operators building large-scale metro Wi-Fi networks.
• Patent-pending hardware and software let subscribers connect faster and more reliably to
outdoor metro Wi-Fi networks – automatically optimizing around changing environmental
conditions. Intelligent connection algorithms determine the fastest Wi-Fi mesh node with which to
associate
i t based
b
d on th
the hi
highest
h td
data
t rates
t and
db
bestt signal
i
l strength.
t
th
• The Ruckus MetroFlex is the only system that provides unprecedented diversity through the use
of a unique, dual polarized antenna array. This provides the largest coverage area possible, the
strongest resilience to interference and the highest receive sensitivity.
• The Ruckus MetroFlex has 5 10/100 autosensing Ethernet ports for the connection of wired
computer or multimedia devices.
The MetroFlex Repeater can be easily placed on an end
table, bookshelf, or countertop where it has a “view” of
the outdoor wireless network through a window or
patio door.
p
Coverage is extended throughout up to a 4000 square
typical foot indoor area.
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11
Cisco 1242 Series Access Point
(802.11a/b/g)
Cisco® Aironet® 1240AG Series Access Points
deliver the versatility, high capacity, security, and
enterprise-class features demanded by WLAN
customers. These IEEE 802.11a/b/g access points
are designed specifically for challenging RF
environments such as factories
factories, warehouses
warehouses, and
large retail establishments that require the antenna
versatility associated with connectorized antennas,
a rugged metal enclosure, and a broad operating
temperature range. The Cisco Aironet 1240AG
Series provides local as well as inline power,
including support for IEEE 802.3af
802 3af Power over
Ethernet (PoE).
Receive Sensitivity (Typical)
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Transmit Power Options
12
Cisco 1130 AG Series Access Point
Cisco Aironet 1130AG Series IEEE 802.11a/b/g access points
provide high
high-capacity,
capacity high
high-security,
security enterprise-class
enterprise class
features in an unobtrusive, office-class design, delivering
WLAN access with the lowest total cost of ownership. With
high-performing dual IEEE 802.11a and 802.11g radios, the
Cisco Aironet 1130AG Series provides a combined capacity
of up to 108 Mbps to meet the needs of growing WLANs.
Transmit Power (Maximum)
802.11a
17 dBm (50 mW)
802.11b/g
20 dBm (100 mW)
Receive Sensitivity
802.11a:
6 Mbps:
9 Mbps:
12 Mbps:
p
18 Mbps:
24 Mbps:
36 Mbps:
48 Mbps:
54 Mbps:
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-87 dBm
-86 dBm
-85 dBm
-84 dBm
-80 dBm
-78 dBm
-73 dBm
-71 dBm
802.11g:
1 Mbps:
2 Mbps:
5.5 Mbps:
p
6 Mbps:
9 Mbps:
11 Mbps:
12 Mbps:
18 Mbps:
24 Mb
Mbps:
36 Mbps:
48 Mbps:
54 Mbps:
-93 dBm
-91 dBm
-88 dBm
-86 dBm
-85 dBm
-85 dBm
-84 dBm
-83 dBm
-79
79 dB
dBm
-77 dBm
-72 dBm
-70 dBm
13
Cisco 7920 and 7921 Wireless VoIP Phone
The Cisco 7920 Wireless IP Phone demands special attention both in the
design of the wireless network and during the on-site pre-installation
survey The phone will operate at RSSI values as low as -80
survey.
80 dBm
(or below) however a -67 dBm level is recommended for highest call quality.
802.11b Transmit Power: 100 mW EIRP with scaling
at 50, 20, 5, and 1 mW. The same transmit power should be used on all phones
and should be equal to the highest transmit power used for any access point.
Authentication: 802.1x, WEP, WPA, EAP-FAST, TKIP
Target Design Values (as depicted below): RSSI = -67 dBm, CIR = 19 dB
The Conceptual Channel Plan and RSSI Targets
Sh
Shown above
b
iis a conceptual
t l representation
t ti off h
how adjacent
dj
t coverage areas should,
h ld id
ideally,
ll iinterleave.
t l
In practice, signal strength boundaries are never circular (as implied by the diagram) but, rather, follow
patterns affected by the attenuation and reflection of the walls and obstructions.
The most significant aspect of the signal coverage representation is the stipulation that a Carrier-toInterference Ratio (CIR) minimum of 19 dB be maintained at a minimum signal level of -67 dBm. These
specifications have been applied to the present designs.
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Vocera Communications Badge
RF System Requirements
R
Recommended
d d Mi
Minimum
i
Si
Signall L
Level:l -65
65 dBm
dB 2.4
2 4 GHz
GH 802
802.11b
11b
RF Output Power: 75 mW (18 dBm)
Target SNR: 25 dB
From the Vocera Website:
The Vocera Communications Badge is a wearable device that weighs less than two
ounces and can easily be clipped to a shirt pocket or worn on a lanyard. It
enables instant two-way voice conversation without the need to remember a
phone number or manipulate a handset.
The Vocera Communications Badge is controlled using natural spoken
commands. To initiate a conversation with Jim and Mary, for example, the user
would simply say, "Conference Jim Anderson and Mary Guscia." In addition,
when a live conversation is not necessary, text messages and alerts can be sent
to the LCD screen on the back of the Vocera Communications Badge.
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15
Dolphin 9500 Data Collection Terminal
The Dolphin 9500 Data Collection Terminal is an mobile
computer device with a built-in 802.11b Wi-Fi radio.
The unit can authenticate through LEAP
LEAP, MD5
MD5, TLS
TLS, TTLS
TTLS,
PEAP, and WEP. Through the SIM card interface, this unit
also supports GSM/GRPR in the 900, 1800, and 1900 MHz
frequency bands.
A built-in Bluetooth radio is standard. It should be noted that the built-in Bluetooth does not
interfere with the built-in Wi-Fi even though both operate in the 2.4 GHz ISM frequency
band However,
band.
However it is possible that a 9500 unit using Bluetooth could interfere with another
9500 unit (or other Wi-Fi device) if within 10 feet of the other device. Although interference
of this sort is not anticipated, it is possible. This fact should be kept in mind if a
connectivity or performance troubleshooting problem arises during normal activities.
802.11b Transmit Power Output:
Antenna Gain: 2.15 dBi with a polarization drop to 0 dBi
Specified EIRP: 30 mW
Antenna Polarization Considerations
The Dolphin 9500, like most handheld scanners of similar construction, incorporates an internal omni-directional
antenna that is p
polarized along
g the long
g axis of the device.
An omni-directional antenna has its maximum gain perpendicular to its axis of polarization. Gain decreases
towards the axis of polarization with essentially no signal transmission or reception ability at the center of the
donut-shaped propagation volume, as depicted in the diagram below.
This implies that it is possible for a worker to hold a handheld terminal at an orientation where the weakest
signal is received from the access point intended to provide coverage to the work area. Signal reflection
normally makes it unlikely that the lack of reception ability along the longitudinal axis of the device will cause
loss of connectivity it can become a problem if the signal in the environment is already weak due to
attenuation.
Imagine that a worker is back in a corner, behind stacked inventory and perhaps behind a wall, and they
complain that they can’t scan merchandise located over their head. Addressing the complaint includes
considering whether or not the problem is related to someone “pointing the handheld directly at an access
point” in an area of already diminished signal coverage.
Maximum
Gain
Signal from this antenna
would benefit from the
2.15 dBi gain in the
handheld.
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Minimum
Gain
Signal from this antenna would
strike the handheld in the zone
with the weakest antenna gain.
16
Symbol MC9090-G Handheld
Mobile Computer/Scanner
The MC9090-G is a p
portable data collection
terminal and scanner for
applications including shelf price audits and
inventory management.
Wireless 802.11a/b/g support is provided
through the internal Symbol
Photon Wireless Module, detailed specifications
for which are
presented on a separate page to follow.
Of significance is the fact that the MC9090-G
specifications list the device as having a 100
mW output power. Cross-referencing the
Photon module specification reveals that the
actual
t lT
Transmitit Power
P
Output
O t t is
i less
l
than
th 100
mW implying that the 100 mW specification is
an Effective Isotropic Radiated Power (EIRP)
value that includes the MC9090-G’s antenna
gain.
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17
Symbol VC5090 Vehicle/Fixed Mount
Mobile Computer
The VC5090 Mobile Computer
p
is a ruggedized
gg
‘thin client’ device
which can be used as a portable computer on the warehouse or
production floor, or which can be mounted to a forklift or other
vehicle. In addition to its internal diversity antenna pair the
VC5090 supports an external antenna connector. Wireless 802.11a/b/g
support is provided through the internal Symbol Photon Wireless Module,
detailed specifications for which are presented on a separate page to follow.
Of significance is the fact that the VC5090 specifications list the device as having a 100
mW output power. Cross-referencing the Photon module specification reveals that the
actual Transmit Power Output is less than 100 mW implying that the 100 mW
specification is an Effective Isotropic Radiated Power (EIRP) value that includes the
VC5090’s antenna gain.
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Zebra 105SL Wireless Print Server
The Zebra 105SL printer is a ruggedized, all-metal printer
for label and paper widths of 4
4.09
09”. The 105SL supports
the Zebra Wireless Printer Server option which allows a
Cisco or Symbol PCMCIA card to be inserted into the
printer for 802.11b connectivity.
Assumed Transmit Power: 30 mW
Assumed Antenna Gain: 2.15 dBi
Assumed Minimum RSSI: -90 dBm
Examples of PCMCIA 802.11b Adapters Used With the Zebra Printer
Lucent (Proxim) ORiNOCO (example: 848441556)
Note that this card can be p
purchased
rchased with
ith an optional
external 7 dBi antenna. Using an external antenna
can help improve data rates and connectivity
compared to an installation location where a
standard PCMCIA adapter is “buried” behind
the printer, particularly when “buried” below
tabletop height
height.
Symbol Spectrum 24 (example: LA-4121-1000-1C)
Cisco Aironet PCMCIA (example: AIR-CB21AG-A-K9)
Transmit Power and Receive Sensitivity
Many PCMCIA adapter manufacturers do not make technical specifications regarding transmit power and
receive sensitivity readily available. Most 802.11b-only adapters provide specifications in terms of
“range” (in feet). These specifications are essentially useless since they have no way of being
consistently
i t tl compared
db
between
t
products
d t or applied
li d tto a d
design
i with
ith any accuracy.
Many 802.11b/g manufacturers do make their specifications available for this second generation of Wi-Fi
standards. You’ll find some specific transmit power and receive sensitivity figures elsewhere in this
report but, in general, assume the following:
Although Cisco Aironet adapters have a 100 mW transmit power output, 30 mW is more typical for many
802.11b-only adapters. Using 30 mW as an assumed transmit power output errors on the side of
caution in a design.
Although some PCMCIA adapters provide 1 Mbps 802.11b connectivity to RSSI values as low as -96 dBm,
assume a reasonable lowest-limit of -90 dBm for minimum, consistent connectivity.
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19
INTERPRETING THE RF MODELS
The Application of RF CAD Models
Th RF CAD models
The
d l and
d plans
l
presented
t d in
i this
thi reportt representt radio
di signal
i
l
propagation, signal strength predictions, and the installation locations of radio
transmitter/receiver antennas. The models allow an understanding of the design
process and allow the proposed design to be treated with credibility.
Despite the complicated data and analysis that underlies an RF CAD simulation the
resulting model communicates its specific message with a minimum of effort. The RF
d i
designer
sets
t outt to
t ascertain
t i and
d ill
illustrate
t t a particular
ti l aspectt off th
the system
t
design.
d i
The selection of details discussed in any particular model may vary but the goal is to
communicate a specific element or objective. Each model is accompanied by
explanatory text that details the idea being presented.
Several basic types of models are common to many designs although other model types
may be included as required to accurately determine and depict specific
characteristics These basic model ttypes
characteristics.
pes are e
explained
plained belo
below.
Isometric Models
Synopsis:
A 3-dimensional representation of the installation
site where different materials (having different RF
attenuation and reflection characteristics) are
represented by different colors.
Detailed Explanation:
In this form of projection an illusion of perspective is obtained while the
dimensions are retained to scale. The RF engineer creating the design
will use Isometric models as part of the 3-dimensional predictive
modeling
d li process. Th
The engineer
i
iis able
bl tto explore
l
th
the RF characteristics
h
t i ti
of the installation environment “behind” and “above” obstructions, partitions, and buildings. Visualization
of RF signal coverage in 3-dimensional space reveals patterns and relations which would not be clear
through the analysis of on-site measurements.
Isometric models also help both the design engineer and the reader of delivered report visualize the space
being modeled. Different colors on the rendering represent materials with different RF characteristics. A
color legend is provided in the report.
It’ important
It’s
i
t t to
t note
t that
th t the
th Partition
P titi Categories
C t
i used
d in
i a particular
ti l design
d i reportt may differ
diff ffrom th
those
shown here. Also, a partition type may be used to represent another material with the same RF
characteristics. A common example of this is the use of “Cubicle Wall” to represent all furniture in an
office building.
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Contour Coverage Models
Synopsis:
Colored regions represent signal strength zones
surrounding a radio’s transmit/receive antenna.
D t il d E
Detailed
Explanation:
l
ti
A Contour Coverage prediction model presents the contour of a colored
boundary region in which a certain minimum level of received signal
strength will be present. Multiple contours may be presented in a single
model and contours from multiple transmitter/receiver devices may
overlap. A transmitter/receiver is in a centralized location in each
bounded region. Occasionally Contour Coverage models may be used
to represent signal
to interference or other metrics.
metrics
signal-to-interference,
In general there are two color schemes that may be employed when presenting Contour Coverage models.
In one case reds are considered “hot” and dark-blue/violet is considered “cold” so that the highest signal
strength is represented by the red (hot) areas. Another scheme is to use green for the “maximum bit rate
coverage” zone, yellow for “minimum bit rate coverage” zone and red or blue for an “intermittent
coverage zone”. Be sure to check the legend and accompanying text when interpreting a Contour
Coverage model.
Grid Coverage Models
Synopsis:
Colored grid areas represent the best signal
strength predicted in each location.
NOTE: Grid squares
bounded with orange
p
borders represent
-60 dBm
Detailed Explanation:
The typical Grid Coverage model is technically referred to as a “Non-Composite
RSSI” coverage prediction. Unless otherwise stated, this is the type of Grid
Coverage model used in design reports. RF signal strength in each grid
square is considered relative to all specified transmitter/receiver antennas.
The strongest possible signal strength available in the grid square is
represented. The Grid Coverage model thus presents a representation of the
signal strength associated with a transmission and reception for a radio device
in the grid square and the best possible radio in the environment.
The color legend for Grid Coverage models is generally consistent with the
example shown below. Details regarding suitable signal levels will be
discussed in the report. As a general guideline for 802.11a/b/g/n and 802.16
unlicensed WiMax designs, light-blue (-80 to -90 dBm) is close to the minimum
usable signal strength
strength, greenish and yellow hues represent normal coverage
zones (-60 to -75 dBm), and reddish areas are usually only found very close to
an antenna (> -50 dBm)
When included, the percentage figures indicate how much of the predicted area
contains a particular signal strength level.
Ray Tracing Grid Coverage
A grid coverage model may be created using propagation algorithms or ray
tracing
depending
the requirements
the project.
Ray ttracing
t i techniques,
t h i
d
di on th
i
t off th
j t R
i
works by tracking the path taken by multiple rays of RF signal through the
space, obeying the laws of reflection and refraction. A Ray Tracing Grid
Coverage model can often show how reflections in a building can provide
otherwise undisclosed coverage behind obstructions.
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STRONG
NORMAL
MARGINAL
UNUSABLE
21
Installation Plans
Synopsis:
Floorplans and/or outdoor area maps depicting
exact installation mounting locations for use by
the equipment installer. Installation plans appear
together in a section immediately preceding the
Technical Appendix in this report.
Detailed Explanation:
These plans are intended for use by the field installer. They shows the
exact location for each antenna including any specific installation
instructions. These plans also serve as the basis for creating a bill of
materials for the completion of the project.
Channel Plan and AP Numbering
Access points are identified with a unit number
followed by the channel on which the access point
should be configured
configured, as shown to the right
right.
Unit Number “03”
Channel Number “11”
AP03_11
3-D Contour Coverage Model
Synopsis:
The imaginary boundary in space
formed by a particular signal
strength is shown as a
3-dimensional
3
dimensional solid figure
in a model.
Detailed Explanation:
This model gives a visual depiction of where, in
3-dimensional space, a particular level of coverage
extends. It is commonlyy used to show how a signal
g
propagates out of a building or from one floor of a
building to the next.
Compass Directions Used in Model Discussions
All compass directions given in this document are relative to the
model space and not to the real world. Hence, the top of the
page is considered “north”, the left side is “west”, and so forth.
These references may or may not correspond to the actual
orientation of the building or outdoor area relative to a map.
A small compass symbol with the words “Model Space” wrapped around
the top (shown to the left) is used throughout the report as a reminder that
discussions referencing directions refer to model space and not to
magnetic north.
Note: AutoCAD filenames may appear at the bottom of some pages as an internal
reference. If present, they’re shown in courier font for identification.
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ORIGINAL DRAWING FILES
The original AutoCAD drawing files, shown below, were formatted to create the 3dimensional models used for the preliminary
p
y design
g and then validated during
g the
on-site survey engagement.
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23
ISOMETRIC MODELS
A 3-dimensional model of the facility (Floor 1 shown below) was created and formatted to
accurately represent the RF characteristics of the space
space. On
On-site
site verification
confirmed that this model was accurate and results were found to be within 3 dB of
those predicted (with the warehouse area being better than predicted.)
Metal-walled
paint drying rooms
Special attention was given to the
metal-walled paint drying room area
Production Areas
Inventory Racks
Office Areas
On-site testing included raising a test
transmitter to the ceiling with a lift
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FLOOR 1 RF DESIGN
Coverage from a Single Access Point
The Contour Coverage model shown below shows how a single access point is expected
to perform. It can be seen that the coverage zone, as it extends outwards from the
access point, is shaped by the attenuation and reflection in the building.
Single Access Point Grid Coverage
The Grid Coverage Model below shows the same access point as above.
It can be seen (by comparing the colored hues in the model to the
legend to the right) how the signal power level varies with distance
from the access point. In addition, the Grid Coverage Model allows
an assessment of how signal coverage extends throughout the building
floor, as opposed to the presentation of a single set of coverage
contours as was shown in the Contour Coverage model above.
NOTE: Grid squares
bounded with orange
p
borders represent
-60 dBm
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25
Full Coverage on Floor 1
When all access points are active, coverage meets and exceeds the
target
g design
g g
goal of -60 dBm (p
(providing
g a 10 dB fade margin
g above
the -70 dBm minimum required RSSI) as shown below.
Reduction from 4 Access Points to 3
In the final design presented in the Installer’s Working Plans, the access
point count in this area has been reduced from four to three. The on-site
RF analysis determined that the inventory racks were slightly less
obstructive than was originally considered during the predictive modeling.
This installation
location was moved
slightly as a result
of on-site calibration.
In the unlikely event that future inventory stocking creates a situation where VoIP call quality or data
throughput
th
h t performance
f
is
i reduced
d
d in
i this
thi area th
the addition
dditi off one more access point
i t (f
(for a ttotal
t l off ffour as
shown above) will correct the problem. Increased loading of the racks was taken into consideration when the
decision was made to reduce the count to three in this area and no problems are anticipated.
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26
FLOOR 2 RF DESIGN
Coverage from a Single Access Point
The Contour Coverage model shown below shows how a single access point is expected
to perform. It can be seen that the coverage zone, as it extends outwards from the
access point, is shaped by the attenuation and reflection in the building.
Single Access Point Grid Coverage
The Grid Coverage Model below shows the same access point as above.
It can be seen (by comparing the colored hues in the model to the
legend to the right) how the signal power level varies with distance
from the access point. In addition, the Grid Coverage Model allows
an assessment of how signal coverage extends throughout the building
floor, as opposed to the presentation of a single set of coverage
contours as was shown in the Contour Coverage model above.
NOTE: Grid squares
bounded with orange
p
borders represent
-60 dBm
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27
Full Coverage on Floor 2
When all access points are active, coverage meets and exceeds the
target
g design
g g
goal of -60 dBm (p
(providing
g a 10 dB fade margin
g above
the -70 dBm minimum required RSSI) as shown below.
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28
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29
FIRST
T FLOOR FULL G
GRID PREDICTIO
ON (SIZED FULL PAGE FOR DETA
AILS)
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30
SECON
ND FLOOR FULL
L GRID PREDICTION (SIZED FULL PAGE FOR DE
ETAILS)
ACTIVE
TRANSMITTER
ACTIVE
TRANSMITTER
Comparison of 8 dBi panel vs. 5 dBi omni. In neither case can the AP
cover more than 1 aisle
aisle, but overall coverage is better with the omni
omni.
Specifically, notice that the directional antenna has less coverage at
the far end of the aisle, while the center-mounted omni has better
coverage at the ends of the aisles.
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31
CISCO 7920 PHONE COVERAGE
Grid Coverage for Currently Recommended Design
The Grid Coverage model shown below depicts the signal coverage based on the design
presented in the Installer’s Working Plans. Notice that there are three access points
horizontally across the inventory storage racks as previously discussed.
The present design will provide the required -67 dBm signal strength for Cisco VoIP
phones.
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32
Considering Cisco’s Specification for 19 dB Signal-to-Interference Ratio (SIR)
As discussed in the Equipment Specifications at the beginning of this report, Cisco
specifies that a 19 dB SIR is recommended for best call quality (in addition to the -67
dBm RSSI demonstrated on the previous page.)
The Grid Coverage model shown below depicts predicted SIR on Floor 1 of the facility.
Any area that is green, orange, yellow, or red has an SIR above 20 dB. Blue-green
areas begin to approach 15 dB.
There are some areas that a clearly blue or purple. These are predicted
to have an SIR below 15 dB. This will only be a factor if multiple
wireless VoIP users are present in the area, along with multiple
data transfer users. In this case the VoIP users may experience
some degradation in call quality. Data users will be unaffected.
Considerable effort was made to create the design in such a way as to provide the best
SIR over as much floor area as possible. The wide open warehouse areas were a
challenge because access points could easily “see” each other (reducing the SIR) and
the paint drying rooms were also a challenge since the metal walls demanded a higher
density of access points (also reducing SIR.)
We do not anticipate any significantly noticeable problems, even with the areas of most
reduced SIR, because the environment will not typically be experiencing high data
volume.
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33
Comparing the Present Design to the Cisco Conceptual “Circles” of Coverage
The Equipment Specifications for the Cisco 7920 phone
includes a discussion of the conceptual ideal coverage
model provided by Cisco. The conceptual model
explains that a -67 dBm RSSI with a 19 dB SIR is
recommended. The model further recommends
coverage zones spaced at -89 dBm.
Shown below is a model depicting predicted coverage for
Cisco Conceptual Ideal
a single access point on Floor 1. The red area is
Coverage Diagram
(Discussed in the Equipment
bounded by -67 dBm on the warehouse floor and the
Specifications for the 7920 phone)
yellow
ll
area is
i bounded
b
d d by
b -89
89 dBm.
dB
It can be seen that the -89 dBm coverage area is not circular, as represented in Cisco’s
conceptual model. In some directions it extends all the way across the floor of the
building.
Because of the large open areas involved in the design, and the metal walls of the paint
drying rooms, it is impossible to create a design that provides an absolute minimum 19
dB SIR in all areas. The present design, as previously discussed, does provide optimal
SIR in all areas based on the -67 dBm RSSI that is also provided in all areas.
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34
RF SPECTRUM ANALYSIS
Noise and Interference Assessment
A large
g number of individual and area-wide spectrum
p
analysis
y sweeps
p were p
performed
throughout the entire facility. No significant interference or noise in the 802.11b/g 2.4
GHz spectrum was identified either from the environment in general or from any piece
of electrical machinery in use at the site.
The top display (below) shows signal energy across the 2.4 GHz frequency band. The
upper light-blue trace is the peak signal level observed during the measurement. On
the left is a background interference source and to the right is a Channel 11 802.11b/g
transmitter. Although the peak RF signal level for these sources is high (close to and
above -70 dBm) the amount of time these signals is present is very short.
The bottom display (below) shows the Duty Cycle during the same measurement period.
Duty Cycle shows the amount of time signals are present in the spectrum. Notice that
the left-hand scale on the Duty Cycle graph goes from 0% to 10%. In almost all cases
the presence of any noise or interference was accompanied by Duty Cycle
measurements similar to these, with events occupying significantly less than 4% of the
cycle time. As a result, these background transmissions may be considered to be no
factor in the present design.
-70 dBm
5% Duty Cycle
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35
Interference Power and Existing Access Points
The graph shown below depicts overall signal power and interference seen on each
802.11b/g and 802.11a channel during the spectrum sweep.
The small plus signs (+) indicate that an 802.11 access point was visible on the indicated
channel.
The access points seen that manifested stronger signal levels were always those
currently in use at the site. External interferers were always at a power level that was
considered insignificant to the proper operation of the in-building network.
The table below shows a listing that was typical of most locations at the site. There are
some Bluetooth and other interferers, but it can be seen that they manifest power
levels below -80 dBm. The current design targets -70 dBm for the newly proposed
wireless network making these interferers of little consequence.
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36
Recommendation: Migrate Away From the Use of 2.4 GHz Cordless Phones
One of the most consistent, and strongest interferers identified were the 2.4 GHz
cordless phones in use at the site. These do not present a current obstacle to
i l
implementing
ti th
the Wi
Wi-Fi
Fi network
t
k however
h
it’s
it’ recommended
d d that
th t a generall plan
l b
be putt
into place to migrate away from the 2.4 GHz spectrum for non-Wi-Fi use as much as
possible.
Going forward, these phones could be replaced with similar phones operating in the 5.8
GHz or 900 MHz spectrum. This would help reduce background interference for the
Wi-Fi network.
The impact of these interferers will be to occasionally reduce the data transfer rate of a
Wi-Fi connection or appear as a slight reduction in call quality if using wireless Voiceover-IP.
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38
Coverage Testing
T
in the Ma
ain Production A
Area
A test access point was placced in the central llocation denoted by the red circle. The access pointt was lifted to a he
eight of
roughly 35 feet to simulate in
nstallation at the cceiling. The signa
al levels from this ttest access point are shown on the
e floorplan
ments close to the
e test access poin
nt are above -70 d
dBm (the target fo
or the design) and
d almost all
above. Notiice that measurem
measureme
ents are above -8
80 dBm (a level att which 18 Mbps 8
802.11g is assumed.) The test acce
ess point was tran
nsmitting
with 6 dB more
m
power than the minimum assu
umed for the design so it can be asssumed that post--installation meassurements
(of a 30 mW
W transmitter) wou
uld be 6 dB less tthan shown.
Coverage in the Drying Rooms
It was predicted in the RF CAD model that an access point in the hallway, outside the
drying rooms, would provide coverage through the metal walls and into the rooms
themselves. This was confirmed during the on-site engagement.
As seen to the right, the predictive model
suggests that a signal level of -60 dBm
would be obtained in the drying room
with an access point placed in the
hall, as shown.
Seen below is the AirMagnet signal strength
graph with the area in the dotted oval
being the point at which the -60 dBm
prediction was assumed. It can be seen
that the prediction exactly matches
the actual on-site measurement.
-60
60 dBm
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39
Perspective on the RF Spectrum Analysis Measurements
The RF spectrum at the site was generally consistent in all areas. There was a constant
level of background noise and interference but, when the existing access points are
deactivated the background signal levels should not introduce a problem
deactivated,
problem.
Presented here, as a perspective, are other measurements made throughout the site.
The previous sections of the RF Spectrum Analysis section have detailed specific
factors and these following spectrum analyzer traces are included as additional
information.
2nd Floor Office Area
A transmitter is seen on Channel 1 and Channel 11 with some background noise in-between channels.
In the Machine Area in the Warehouse
Some intermittent spikes are visible in the lower frequencies but the Duty Cycle time for this type
of interference is less than 3% making it a non-issue relative to the design. On the right is the
“hump” associated with an access point operating on Channel 11. Again, the Duty Cycle time is very low.
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40
Outdoors, in the Paint Shop Storage Area
Evidently there is an 802.11 transmitter operating on Channel 11 very close to this location.
The “hump” on the right (Channel 11) is the signal from this transmitter.
O the
On
th left,
l ft the
th “flat-top”
“fl t t ” signal
i
l is
i consistent
i t t with
ith an 802.11g
802 11 transmission,
t
i i
perhaps
h
on Channel 2. Neither of these spectral observations is detrimental to the proposed network.
In the Floor 1 Office Areas
Again, notice the strong Channel 11 transmitter on the right. The red trace (at the bottom) is the
average signal level across the spectrum. Notice that it remains very low, consistent with the fact
that the Duty Cycle time for the other, stronger transmissions is very short.
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41
INSTALLER’S WORKING PLANS
Access Point Deployment
T
Two
different
diff
t Ci
Cisco access point
i t models
d l will
ill b
be used
d iin th
the currentt
design.
The Cisco 1130 access points will be installed in the
interior office areas. They have an integrated antenna so no external
antenna will be required.
The Cisco 1242 with an external 1781 omnidirectional antenna will be
used for all other (non-office) areas in the facility. The 1781
antenna should be mounted pointing straight down from the
ceiling. If it’s necessary to mount the antenna higher than the bottom
of a fire curtain then it should be no closer than 8 feet to the
fire curtain.
The Installer’s Working Plans on the following pages denote either
1130 or 1242 adjacent to the installation location.
CISCO 1130
CISCO 1242 with
1781 Antenna
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42
Floor 1 - Overview
Installation plans for Floor 1 are divided into five separate areas to allow the plans to be
presented with appropriate detail
detail. The areas are shown below
below.
Note that any installation location that’s on the 45-foot ceiling, over the top of everything
else, can be moved as required to installation. Ideally moving an antenna up to 8 feet
will have no impact on these areas and an insignificant impact if movement up to 15
feet is required. If a suitable mounting location can not be found within 15 feet of a
specified point a discussion should take place regarding options and impact.
I t ll ti llocations
Installation
ti
iin areas where
h
th
the antenna
t
iis mounted
t d tto an enclosed
l
d ceiling
ili
(offices, paint-drying rooms) should not be relocated more than 5 feet from the
indicated locations.
AREA 2
AR
REA 3
AREA 1
AREA 5
AREA 4
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43
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18’
~18 feet West of w
wall edge.
Mount 2’ so
outh of the support column that’s almo
ost
exactly at th
he location shown fo
or the AP. Don’t mo
ount
the
e AP flush to the sup
pport column.
1242
Acce
ess point antenn
nas are mounted
to the ceiling,
c
no close
er than 8’ from an
ny
fire curtain. Any mounting position may
y be
ed by as much as 8’ as required for
relocate
convenie
ent mounting. In general, these three
access poin
nts are in the cen
nter of their resp
pective
areas. If they
y must be moved
d, they should be
e moved
to as closely as possible maiintain a central location.
10’
1242
2
28’
of the outside corne
er edge which
28’ west o
places the AP in the center of the wider area.
12
242
6’
6’ south and 10’ east
o
of outside corner.
Note h
how the AP is not in line
with the su
upport column to th
he north.
Floor 1 – Area 1
44
Floor 2 – Area 2
{AP05 and AP06 are part of the installation plan for Area 5}
AP04 – Centered at the junction of the two walkway areas
AP22 – In line with the adjacent wall, centered in the open area
AP23 – Centered in the walkway, halfway down the length of the drying room
AP26 – Centered in walkway area, half-way down the length
AP27 – Essentially centered in the square area shown
1242
1242
1242
1242
Installation location for AP06
is detailed in Area 5
1242
Installation location for AP05
is detailed in Area 5
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Floor 1 – Area 3
AP18
Install in the corner of the room
as shown, 1-foot from both
j
adjacent
walls.
1130
AP17
Mount in the corner of the room,
not in the hallway. Mount 1-foot
from the diagonal wall section.
1130
AP16
Mount 1-foot east of the wall, in
the open area (not in the
enclosed offices). The mounting
location should be in-line with the
east-west wall.
1130
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46
Floor 1 – Area 4
AP8, AP11, and AP12 are mounted in a line, half-way down the longer aisles between
racks.
AP12 iis mounted
t d9
9-feet
f t westt off the
th west-most
t
t racks
k centerline.
t li
AP25 is mounted at/over the point where the north-south short wall meets the east-west
long wall. It’s mounted on the south side of the wall (or, if mounted above, 1-foot
south of the wall.
AP13 is mounted half-way across the east-west wall, four feet south of the wall.
AP09 is mounted in-line with the wall section shown to the west.
1242
Half-way down the length
of the aisle
22’
1242
In-line with this
wall section.
9’
45’ ffrom th
the
west-most wall
1242
1242
1242
1242
Centered on this wall section,
4’ south of the wall
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47
Floor 1 – Area 5
AP05 is mounted in the center of the rectangular area in which it’s located, as shown.
AP06 is mounted in line with the wall to the north,, as shown
AP07 is mounted in the center of the square area shown
AP14 is half-way between the two support columns as shown
AP19 and AP20 are installed either centered on the ceiling in the square areas shown, or
on the walls in the locations indicated.
AP24 is mounted in the center of the square area shown
1242
1242
This is essentially in the middle, between
these two support columns (even though it
is shown slightly to the right)
1242
1242
AP14 is half-way
between these two
support columns
1242
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AP19 and AP20 can be
installed flush to the
walls in the positions
indicated, or the
antenna can be ceiling
mounted in the center
of the hallway, in the
square areas
indicated.
1130
1130
48
Floor 2 - Overview
Floor 2 has been divided into two areas as shown below. Area 1 is comprised of the two
separate second floor areas above the production floor
floor. These areas are of a
sufficiently small square footage size as to make the location for installation
completely non-critical. These areas need a separate access point because they’re on
the second floor and it’s felt that separate coverage by floor is optimal.
Area 2, the office, cubicle, and training room area is presented with standard installation
detail. In this area the installation locations should not change by more than 5 feet.
AREA 1
AREA 2
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Floor 2 – Area 1
AP37 is shown in the center of the room in which it’s installed. The exact location is
completely non
non-critical
critical and AP37 may be installed anywhere over the room
room.
AP36 is shown mounted to the wall of the room with glass (light-blue) in the walls. As
with AP37, the exact location is non-critical and AP36 may be installed anywhere
over the room.
1242
1242
AP37
AP36
These two antenna locations can be relocated anywhere over their respective rooms,
as indicated by the dashed boxes.
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1130
1130
1130
1130
he wall, 1-foot belo
AP
P30, AP31, and AP
P35 can be installled with their ante
ennas either on th
ow the ceiling, or
w
on the ceiling anywh
here in the dashed
d boxes shown. T
The installation loccations must be within
the areas
unded by these dashed boxes.
bou
ecording area. This location
AP
P29 mounting loca
ation is critical to provide
p
coverage into the sound re
sho
ould not move by more than 2-feet.
10’6”
Install on the ceilin
ng half-way down th
his wall. The
a
antenna
should be no more than 1-foo
ot west of the
wall, on the wes
st side of the wall as
s shown.
Floor 2 – Area 2
51
This section provides an engineering perspective
and details on various analysis and design aspects
discussed and applied in the report.
report
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APPLYING THE DESIGN SPECIFICATIONS
The Project’s Design Specifications
A set of specifications
p
is developed
p and agreed
g
upon
p for each p
project.
j
They
yq
quantify
y the RF
characteristics for the mobile (or fixed) client devices as well as the access points. Client
devices may include wireless notebook computers, handheld scanners or data
acquisition terminals, VoIP handsets, cameras, PDAs, in-hospital computer-on-wheels
carts, etc. The specifications also quantify the capabilities of the access points. Between
the two, the less capable communicating device (client or access point) must be the
determining factor with regard to the minimum values used in design. Compared to the
Access Points, the mobile clients are typically less capable as wireless communicators.
Consequently, the designs are typically based on the limitations of the least capable
client device. This device becomes the Target Mobile Client Device.
Overall Considerations Specified for the Design
The target signal strength used to assess the correctness of the designs is presented. The
target is based on the minimum acceptable receive sensitivity and the specified fade
margin. Hence, if the minimum receive sensitivity were -91 dBm and a 10 dB fade
margin was specified the target signal strength for the design would be -81 dBm.
This section also stipulates the strategy used to evaluate RF coverage during modeling and
simulation and it sets forth anything that is significant relative to the way the design
specifications were determined.
All of the design strategies provide a commercial-grade wireless network system that will
provide RF connectivity for users in the designated coverage areas. The three potential
strategies are:
Aggressive
Engineering compromises are resolved in favor of improved signal coverage. Additional access points
are added to a design to compensate for all RF shadows that are below the target signal strength in
areas where user connectivity is required. This strategy helps assure full performance and capacity
under all circumstances but can raise the overall cost and complexity of a system. A beneficial
byproduct of this strategy is that, in most cases, the system that will tolerate the failure of any single
access point and still provide highly usable coverage from adjacent access points.
Moderate
Engineering compromises are resolved in favor of budgetary considerations.
considerations Some RF shadows are
tolerated down to a 0 dB fade margin before additional access points are added. This strategy attempts
to find a balance between performance, capacity, cost, and complexity while maintaining a high level of
service.
Conservative
Engineering compromises are resolved based on practical usage assessments. Additional access
points are only added when absolutely necessary to provide user connectivity
connectivity. Limited areas of slightly
reduced coverage for active users or RF shadowing may be tolerated down 0 dB fade margin and some
very limited RF shadow areas may be tolerated to below usable levels . This strategy assumes that the
operating environment will remain consistently optimal and therefore may have limited areas in which
user connectivity is degraded during sub-optimal environmental conditions. This strategy can help
reduce the cost and complexity of a system while still providing general wireless network connectivity.
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53
Specifications for Target Mobile Client Devices
A table, like the one shown to the right,
lists the specifications for mobile
client devices that were used in the
creation of the report.
The way each specification is applied
is described below.
Transmit Power Output (TPO)
CLIENT DEVICE SPECIFICATIONS
RF power output in milliwatts and dB-milliwatts provided by the client device’s radio. The value
specified
p
is typically
yp
y the maximum value available but may
y be the maximum value specified
p
for the
project. This value is normally taken from manufacturer’s equipment specifications for typical
devices that will be used. When a manufacturer’s specification is unavailable a value of 30 mW is
used by default. This is typical of a conventional wireless notebook computer with an internal Wi-Fi
adapter.
Receiver Effective Antenna Gain
This is basic antenna gain, reduced by any cable or connector loss between the client device and
the antenna.
antenna Thus,
Thus the effective antenna gain may be slightly less than the rated dBi gain for the
antenna element itself. When a manufacturer's specification is not available the default value is
2 dBi which is consistent with a built-in antenna in a conventional wireless notebook computer.
Effective Isotropic Radiated Power (EIRP)
Final output power from the client device. The sum of TPO and Effective Gain. This value is
presented in the table as a reference to compare the current design with any other design.
Minimum Modulation Rate
The Client Device must be able to operate at this 802.11 modulation rate as a minimum.
Modulation rates are often mistakenly referred to as “data rates” for 802.11 Wi-Fi systems. The
rates of 1, 2, 5.5, and 11 Mbps for 802.11b and 6, 12, 18, up to 54 Mbps for 802.11g and 802.11a
are actually references to the way user’s data bits are impressed on the transmitted RF signal.
Modulation rates, however, map directly to vendor’s specifications concerning the required signal
power (and noise levels) necessary for operation. Hence, a vendor’s specification may state “11
Mbps @ -96 dBm” or some similar values. It should be noted that with regard to 802.11b/g and
802 11a the actual user throughput for TCP/IP data is roughly half the modulation rate
802.11a
rate. Hence
Hence, an
11 Mbps modulation rate yields roughly a 5 Mbps to 6 Mbps aggregate data rate for all users
simultaneously active on a single channel. In like manner, a 1 Mbps modulation rate provides
approximately 512 Kbps for TCP/IP throughput on a channel.
Minimum Required Receive Signal Strength (RSSI)
This is the minimum signal level required by the client to operate at the specified Minimum
Modulation Rate. The value is obtained from manufacturer’s equipment specifications. In the
absence
b
off vendor
d specifications
ifi ti
or prior
i experience
i
with
ith th
the equipment,
i
t ad
default
f lt value
l off
-91 dBm is used for 1 Mbps 802.11b connectivity. The specified Fade Margin value is added to the
minimum RSSI to arrive at the target signal strength for the design.
Fade Margin
This is an additional level of signal power that is added to the requirements for a project to account
for normal environmental variation. From a purely technical standpoint, fade margin specifically
compensates for multipath fading when receivers (or transmitters) are in motion. From a practical
standpoint, this engineering margin (as used in the report) includes compensation not only for
multipath fading but for environmental electromagnetic fluctuation, noise, out-of-band and cochannel interference, and tolerance range used by equipment manufacturers.
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54
Maximum Modulation Rate (0 dB Fading)
This is the modulation rate that is predicted in a case when there is absolutely no multipath fading
from signal reflections, no noise or interference of any kind, and when all equipment is operating at
the best-case factory specification; hence, 0 dB fading. The Maximum Modulation Rate indicates
th
the modulation
d l ti rate
t specified
ifi d b
by th
the equipment
i
t manufacturer
f t
when
h actually
t ll operating
ti with
ith 0 dB off
real-world degradation. In practice, this situation is unlikely but the Maximum Modulation Rate
provides an indication of what the theoretical best-case could be for the design.
Specifications for Access Point Radios
A table, like the one shown to the right,
li t the
lists
th specifications
ifi ti
ffor th
the access
points that were used in the creation
of the report.
The way each specification is applied
is described below.
Air Standard
ACCESS POINT SPECIFICATIONS
Specifies whether the design was based on the transmission characteristics of 2.4 GHz 802.11b,
2.4 GHz 802.11g, 5.8 GHz 802.11a or other standards for wireless transmission. It’s important to
note that a design based on a higher frequency or higher bit-rate will almost always require a
higher level of signal coverage and a better signal-to-noise ratio for proper operation. Hence, if a
design is based on 802.11b communicators it may have some weak areas relative to 802.11g or
802.11a communicators (for multi-mode radios or mixed mode clients.) On the other hand, a
design based on 802.11g communicators will always provide sufficient RF coverage for an 802.11b
communicator using the same multi
multi-band
band access point transmitters for communication
communication. In many
cases, however, the practical aspects of creating a design will allow an 802.11b design to serve
the needs of an 802.11g user community.
Transmit Power Output (TPO)
RF power output in milliwatts and dB-milliwatts provided by the access point. The value specified is
typically the maximum value available but may be the maximum value specified for the project.
This value is normally taken from manufacturer’s equipment specifications for the specific access
points that will be used
used. When a manufacturer’s specification is unavailable a value of 65 mW is
used by default. This is typical of many mid-range commercial access points. Note that the power
output of the access point is normally not a factor since the limiting factors in a design are the
weakest transmitter and least sensitive receiver. Normally the client device is a weaker transmitter
than the access point and the client’s transmit power is used to develop a design.
Transmitter Effective Antenna Gain
This is basic antenna gain, reduced by any cable or connector loss between the access point and
th antenna.
the
t
Thus,
Th
the
th effective
ff ti antenna
t
gain
i may b
be slightly
li htl lless th
than th
the rated
t d dBi gain
i ffor th
the
antenna element itself. When a manufacturer's specification is not available the default value is
3 dBi which is an arbitrary compromise between the minimum 2.15 dBi “rubber duck” style antenna
found on many access points and 5 dBi which is a common value for widely used integrated and
external antenna access point units.
Effective Isotropic Radiated Power (EIRP)
Final output power from the access point. The sum of TPO and Effective Gain. This value is
presented in the table as a reference to compare the current design with any other design.
Because the access point is typically the more powerful transmitter (relative to client devices) this
value also must comply with FCC maximum EIRP regulations (which vary depending on the type
of implementation.)
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55
Minimum Required Receive Signal Strength (RSSI)
This is the minimum signal level required by the access point to operate at the specified Minimum
Modulation Rate. The value is obtained from manufacturer’s equipment specifications. In the
absence
b
off vendor
d specifications
ifi ti
or prior
i experience
i
with
ith th
the equipment,
i
t ad
default
f lt value
l off -91
91
dBm is used for 1 Mbps 802.11b connectivity. Normally the sensitivity of the access point exceeds
that of the client devices which makes the client device’s sensitivity the limiting factor in a design.
Minimum Modulation Rate
Refer to the description for this specification as it relates to the client device. The issues are
identical but often not a factor since the specifications that constrain modulation rate are more
typically related to the client device (because it’s often the less capable transmitter and receiver.)
Maximum Modulation Rate (0 dB Fading)
(Same comment as Minimum Modulation Rate above)
Additional Discussion Relative to Applying the Design Specifications
Manufacturer’s Signal Strength Requirements are Approximations
Equipment manufacturers specify the level of signal strength required to obtain various levels of
throughput. Because of the underlying engineering technicalities in stipulating these signal strength
requirements the values given by manufacturers are only generalized approximations.
Nonetheless, they serve as a basis for creating system design requirements.
Modulation Rate and Actual Data Throughput are Different
In every case, the “data rates” specified by manufacturers at a given signal strength are not
representative of the actual user data throughput that will be obtained in practice. The technical
t
term
for
f the
th “data
“d t rate”
t ” specified
ifi d by
b a manufacturer
f t
in
i this
thi context
t t iis ““modulation
d l ti rate”.
t ” A
As a
reasonable estimate it may be expected that the combination of protocol overhead, packet loss,
and required acknowledgement traffic will result in a user throughput data rate that is close to half
the specified modulation rate for a particular signal level. For example, when 54 Mbps is specified
by a manufacturer then, in a real-world, practical scenario, one might expect to measure roughly
23 Mbps across a standard (non-proprietary) wireless link. An 11 Mbps 802.11b connection may
be expected to provide something close to 5 Mbps of actual user data throughput.
A Fade Margin Value is Used to Compensate for Fluctuations
A correct system design begins with a stipulation of the minimum acceptable user data rate
required for proper application performance and capacity. To achieve the required minimum signal
strength for the corresponding manufacturer’s specification the environmental variance must be
taken into consideration. Simple and normal electromagnetic fluctuation in the RF spectrum results
in a signal whose strength measurement will fluctuate by +/- 3 dB without any visible change in
environment or relative position of transmitter and receiver. Multipath reflections, noise, and
interference may degrade an indoor signal by another 7 dB or more
more. If a manufacturer specifies
that -80 dBm is required to obtain a desired data rate then the system must be designed to provide
-70 dBm. The 10 dB difference is referred to as a fade margin and, for ISM and U-NII unlicensed
systems (802.11a/b/g and unlicensed WiMax) the use of a 10 dBm fade margin is considered
standard practice. In a warehouse or manufacturing environment, and whenever the physical
configuration of an area changes dynamically during use, an additional 5 dB of margin is added as
compensation. Hence, although the term used is fade margin, the margin also compensates for
noise, interference, and environmental fluctuations.
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56
SIGNAL STRENGTH ANALYSIS
The Access Point fades to -100 dBm
but the Repeater and the Interferers
do not show any correlation
802.11 Repeater ((-61 dBm))
Excellent Connectivity Above this Level
802.11 Access Point (-84 dBm)
Co-channel
Co
channel Interference (<-90
(< 90 dBm)
Shown above
Sh
b
iis a Si
Signall S
Strength
h graph
h ffrom measurements made
d iin the
h engineering
i
i
lab. This measurement is presented as a point of perspective for the reader and other
graphs in the present report can be compared and contrasted with this one as a point
of reference.
The two “pintado476” 802.11 radios are in the engineering building and the other three
access points are undesired transmitters located in buildings adjacent to and greater
th 300 ffeett ffrom th
than
the b
building.
ildi
– Both the Access Point (green) and Repeater (blue) are transmitting at 18 dBm
with a 2.15 dBi dipole antenna (“rubber duck” style.)
– The Access Point is located 50 feet from the point of measurement, through
two ½” drywall interior partition walls.
– The Repeater
p
is located 10 feet from the p
point of measurement,, through
g a
single ½” drywall interior partition wall.
Connectivity from a notebook computer directly to the Access Point is intermittent and at
the limit of usefulness. In general, a received signal that drops below -90 dBm is
always assumed to provide marginal performance. Notice that the other access points
in the environment (the Co-channel Interference) is almost the same peak signal
g as the Access Point. An 802.11 connection with less than 6 dB between the
strength
signal and the noise also provides marginal performance.
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57
The bottom three transmitters (3C:91/red, 30:2A/yellow, and 4E:FF/white) can be
considered to be noise and interference. The Access Point and Repeater are the
desired communicators. These other three transmitters simply introduce unwanted RF
energy into the environment. Hence, while the Access Point is less than 6 dB above
th 3C
the
3C:91/red
91/ d ttransmitter
itt ((an unacceptably
t bl smallll Si
Signal-to-Noise
l t N i ratio),
ti ) th
the R
Repeater
t
itself is 20 dB greater than everything below it.
In the example, the important transmitter is the Repeater. This is the radio to which the
client notebook computer will connect. As such, the fact that a Repeater is being used
becomes irrelevant. The Repeater plays the role of a standard Access Point as far as
evaluation of the results is concerned. Connectivity through the Repeater is excellent.
p
is visible with a signal
g
level above -60 dBm. In g
general, a
Notice that the repeater
received signal that is at, or above -70 dBm provides maximum bit-rate connectivity
(and -60 is 10 dB above -70.)
The Co-channel Interference (from the three other access points) is in an acceptable
power range relative to the Repeater signal. All interferers are close to and below -90
dBm and the Signal-to-Noise ratio approaches 30 dB (between the interferers and the
Repeater signal.) A SNR of greater than 16 dBm is desirable for consistent, maximum
bit-rate
bit
rate 802
802.11
11 operation
operation.
Notice that there is very little correlation between the signal strength of the various
transmitters. For example, the Access Point fades to -100 dBm two times, but the
Repeater and the 3C:9A (red) Linksys continue to present their own uncorrelated and
independent signal levels at the point of measurement.
Key Points
When assessing
g a Signal
g
Strength
g Graph
p yyou look first at the varying
y gg
graph
p line
representing the desired (or test) transmitter. This line should remain at least 10 dB
above the specified minimum signal strength for a project to allow for the worst-case
environmental variances in propagation. This is the 10 dB fade margin which is built
in, by standard practice, to a Wi-Fi design. As a general rule, an absolute minimum
measured signal strength of -90 dBm may be considered an arbitrary minimum level,
with a more reasonable target being -80 dBm. When attempting to minimize the
number of access p
points in a p
plan (p
(perhaps
p for budgetary
g
y reasons)) the 10 dB fade
margin can be relaxed and, if the measured signal strength remains above the
specified minimum, then the measurement can be considered appropriate.
The second thing you look for is the degree of noise and interference underlying the
desired (or test) transmitter. The desired transmitter must be at least 6 dB above the
interference and noise, with greater than a 16 dB Signal-to-Noise ratio being
considered excellent.
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58
SPECTRUM ANALYZER TRACES
Representing
R
ti D
Data
t with
ith RF Signals
Si
l
Many types of devices exchange data using Ethernet data frames. Typically these
Ethernet frames carry IP packets with TCP or UDP used to convey blocks of user
data. The frames and packets consist of data bits.
A wireless LAN transmitter first translates the raw data bits into data structures that
provide the best format for transmission through a process called coding. The
transmitter then introduces specifically
f
defined
f
fluctuations
f
into a radio frequency
f
(RF)
( )
signal such that a particular fluctuation pattern represents a particular bit pattern. This
is the process called modulation of the RF carrier.
The physics of electromagnetic waves explains that bit coding and modulation cause the
original RF carrier frequency to occupy more than one frequency. In its simplest form
the result is a series of additional frequencies called harmonics. More complex coding
and
d modulation
d l ti cause th
the original
i i l carrier
i ffrequency tto spread
d outt over a range off
frequencies. In the case of an 802.11 transmitter this range of frequencies occupies
roughly 22 MHz in the transmission band.
Application of Spectrum Analysis
A spectrum analyzer measures the amount of RF energy present at each frequency in a
transmission band. The spectrum analyzer display is a graphic representation of the
frequency band (across the horizontal X-axis) with the amount of signal power
represented for each frequency (on the vertical Y-axis).
The 802.11 standards define limitations on the amount of energy that may be transmitted
above and below the center carrier frequency on any channel. These limits are
presented in the form of a spectral mask plot in the IEEE standards documents.
To the right
g are the spectral
p
mask p
plots for both
802.11b/g transmission in the 2.4 GHz
band and 802.11a transmission in the
5.8 GHz band. The horizontal axis is
frequency, from low to high in a single
channel, and the vertical axis is the
amount of signal energy allowed at each
frequency.
In an engineering lab, with complete shielding
from all outside RF energy, a spectrum
analyzer would display a trace that was
almost identical to the spectral mask
plots for a properly operating radio. In the
reall world,
it often
the
to
ld however,
h
ft requires
i
th trained
t i d eye off an RF engineer
i
t see the
th
representative signature of an 802.11 communicator when background noise or
multiple, interfering transmitters are present.
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Spectrum Analyzer Traces
Shown to the right are three spectrum
analyzer traces typical of a real-world
environment with very little noise or
interference. They are not
“laboratory” examples and so it can
be seen that the pure signal pattern
defined by the spectral mask is only
generally
g
yp
present.
The bottom example, showing an
802.11a transmitter in the 5.8GHz
band, most closely resembles the
spectral mask waveform from the
IEEE standard. That is because the
5.8 GHz band is relativelyy
uncluttered and generally noise free.
Noise (random electromagnetic
influences) and interference (nonrandom transmissions from other
radios) distort and disrupt the RF
signal
in the air. The p
purpose
of a
g
p
spectrum analyzer is to display the
resulting plot of spectral energy so a
trained RF engineer can draw
conclusions about the otherwise
invisible electromagnetic waves
carrying information across the
wireless network.
The present report will present and
explain various spectrum analyzer
traces obtained during the on-site
engagement. In some cases the
representative 802.11 spectral
di t ib ti plots
distribution
l t will
ill b
be evident,
id t iin
other cases they will be lost to
background noise and interference.
In addition, the effects of RF
attenuation, reflection, refraction,
diffraction, and diffusion will be
manifest
if t in
i the
th spectrum
t
analyzer
l
plots. When significant, these
characteristics will also be
discussed.
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802.11b Channel 11 Transmitter
from XL2261 Spectrum Analyzer
802.11b Channel 1 Transmitter
from Cognio IMS Spectrum Analyzer
802.11a Channel 60 Transmitter
from Cognio IMS Spectrum Analyzer
60
The Swept Spectrogram Display
The spectrum analysis results in this report will sometimes be presented in the form of a
“swept spectrogram” or “waterfall” graph, pictured below.
Weak Energy: Blue
Strong Energy: Red
Spectral energy in
802.11b Channel 11
Time
Low
Frequency
High
The swept-spectrogram graph represents the strength of RF energy at multiple
frequencies over time. The X-axis (horizontal) indicates the frequency that is being
measured. In the screen shot above, the left-most end of the axis represents 2.400 GHz
and the right-most end of the axis represents 2.500 GHz.
Each horizontal sweep across the frequency space takes up one line on the graph, with
new lines appearing at the bottom and pushing older lines up
up. The Y-axis
Y axis (vertical)
represents time, with the “present” being at the bottom of the axis and the “past” being
at the top of the axis.
Finally, the color of the graph represents the strength of the RF energy at the given
frequency and time. In the graphs in this report, red indicates strongest energy, with
yellow, green, light blue, and dark blue indicating progressively less energy.
Although this graph is somewhat complex at first look
look, it is one of the most effective ways
of visualizing RF energy over time. Digital radio transmissions are bursty in nature, so a
simple “snapshot” of the energy at one point in time usually fails to capture the full
nature of the RF energy in the area. Because the waterfall graph shows RF energy over
time, a more complete perspective on the transmissions is possible.
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61
STANDARD SPECTRAL FOOTPRINTS
To better appreciate the results of the on-site RF spectrum analysis survey it is instructive to
see what typical signal traces look like apart from any dramatic outside influences.
Sh
d di
dh
l off ttypical
i l spectral
t l ffootprints,
t i t th
Shown and
discussed
here are examples
the
representation of RF signals as seen with a spectrum analyzer.
CENTER LOBE
The 802.11b Spectral Footprint
SIDE LOBE
The power limits are defined by the
IEEE 802.11 standards as shown in the
diagram to the right, referred to as the
spectral mask.
Since the goal of this discussion is to
CENTER FREQUENCY
provide a perspective on how to make
THE 802.11b SPECTRAL MASK
sense out of a spectrum analyzer trace
the in-depth engineering details of the spectral mask can be ignored. The overall shape
and structure are significant, however. The shape and structure are representative of
signals transmitted using a modulation scheme called Phase Shift Keying (PSK).
Because 802.11b uses spread spectrum transmission, the signal energy is spread out, left
and right, around the center frequency (which is the middle frequency in an 802.11
channel.) A center lobe carries most of the energy (and hence, most of the bit
information in a transmitted packet.) Upper and lower side lobes result from the
transmission of the signal and they, too, carry some of the useful RF energy
contributing to the bit information being transmitted. In a theoretical world the actual
signal would have a shape that looked very much like the spectral mask. In the realworld, however, the spectral footprint seen with a spectrum analyzer always shows the
deformities associated with actual signal transmission. There is a point at which the
signal is deformed to such a degree that the bits can no longer be recovered from the
ttransmission.
i i
To the right (below) is a spectrum analyzer trace of an 802.11b transmitter acquired in the
engineering lab. The three trace lines (blue, yellow, purple) represent the average
signal level, the current maximum level, and the maximum level ever measured (the
Max Hold line.) This is a representative example of a
good signal with a reasonable amount of
CENTER FREQUENCY NOTCH
normal distortion
distortion. Notice that the center lobe
NOISE
has a ‘notch’ at the center frequency. This is SIDE LOBE
DEGRADATION
an artifact of normal signal transmission
(called the center frequency null or DC null)
and it’s always an easy way to spot the
center frequency of a spread spectrum
802.11b
channel. The center lobe has lost
802 11b channel
NOTCH BETWEEN
MAXIMUM (HOLD)
CENTER LOBE AND SIDE LOBE
its theoretical curve shape but the notch
NOW (MAX)
between the center lobe and the side lobes
AVERAGE
can be seen.
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802.11b SEEN IN THE ENGINEERING LAB
62
In fact, this 802.11b engineering lab trace (shown again to
the right for reference) is providing connectivity with an
11 Mbps modulation rate, the best possible for 802.11b.
Notice that the actual average amount of signal energy in
the channel (bottom, blue trace) is very low. In fact, the
average is close to -100 dBm which is not much different
than the background noise in the lab. It’s common to find
that when a single access point, or only a small number
of lightly used access point are present in an environment
that the average signal power is low
low. In this situation it
can be expected that the number of users of the system
could increase dramatically (perhaps adding 20 to 40 additional users in the area
represented by the analysis trace) without having “too many” users in the environment.
The signal power across the channel being measured in the current sweep of the spectrum
analyzer (middle, yellow trace) shows the signal “now” and is the maximum signal power
for each point
point, left-to-right,
left to right across the display
display. It can be seen that at this instant (selected
for the example being discussed) the signal doesn’t look very much like the ideal
spectrum mask. It’s typical to find that many moments in time present a signal that is only
generally reminiscent of the ideal waveform. It’s often in these instantaneous moments
when undesired transmissions, that would otherwise remain hidden, become visible
“under” the upper, overall maximum trace (the purple line.)
It should be noted that the colors used for average
average, max
max, and max hold traces may vary
from one presentation of data to another. It’s important to remember that the relationship
between traces (bottom, middle, and top) remains consistent but the colors may change,
and some screen presentations do not depict all three metrics.
Signal Degradation and Packet Retransmission
In general, the more well-formed the maximum value trace (upper, purple line) appears, the
greater the probability that a receiver will be able to extract uncorrupted bits from the
transmission. It must be realized, however, that the waveform can be significantly
distorted and some data will still get through. Because an 802.11 transmitter must receive
an ACK packet in response to transmitted data packets it knows when a transmission was
lost, and the lost data packet is retransmitted. The retry mechanism is complicated but it’s
sufficient to consider that a retry will occur no more than 7 times
times. For each failed
sequence of 7 retries the overlying TCP/IP protocol may retry 15 times. Of these potential
105 retransmitted packets, it’s not unrealistic to think that as many as 30% of them would,
themselves, fail. Hence, a degraded environment may see an average of 73
retransmissions of every TCP packet.
Since a 1 Mbps 802.11 modulation rate offers roughly 512 Kbps of best-case TCP/IP
throughput (after normal overhead and interframe spacing delay) this implies that a
heavily corrupted network could have performance as low as 7 Kbps. This is in the range
of performance that might be expected from a telephone dial-up link and would be
unsuitable for commercial applications like wireless Voice-over-IP or high quality video.
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63
The 802.11a and 802.11g Spectral Footprint
Unlike 802.11b which uses PSK modulation in a spread spectrum transmission, 802.11a
and 802.11g identically use a transmission mechanism called Orthogonal Frequency
Division Multiplexing (OFDM). The OFDM
spectral mask (shown to the right) applies
to both 802.11a and 802.11g.
From the waveform perspective, 802.11a
and 802.11g differ only in that 802.11a
operates in the 5.8 GHz band and 802.11g
operates in the 2.4 GHz band. The
underlying OFDM transmission mechanism
is identical and the waveforms are identical. 802.11a and 802.11g (OFDM) SPECTRAL MASK
Shown below is a spectrum analyzer trace showing an 802.11a transmitter. The trace
comes from the engineering lab is does not the degree of environmental degradation
that is typical for real-world system.
EDGE CUTOFF
CENTER FREQUENCY NOTCH
802.11a SEEN IN THE ENGINEERING LAB
The Center Frequency Notch (as was seen for 802.11b PSK modulation, but for a slightly
different reason) is visible in the middle of the channel. Above and below the center
frequency there is a relatively consistent signal strength to the edges of the channel
and then a tapering down of signal strength, ending in an abrupt cutoff at the edges of
the next channel.
The Spectral Implications of 802.11g Backwards Compatibility with 802.11b
802.11b uses PSK modulation while 802.11a and 802.11g use OFDM modulation, as
discussed previously. The difference in modulation methods gives rise to the two
uniquely different spectral footprints. When 802.11g and 802.11b clients operate
together in the same network (using the same access points) the performance of the
802.11g clients degrades because a portion of each transmission must be done using
PSK modulation (which is done at the 1 Mbps rate.)
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64
Estimates of the degree of degradation are as dramatic as a 50% reduction in throughput
for 802.11g, OFDM-capable clients when 10% of overall clients are using 802.11b
PSK. Of course, this implies that an 802.11g client that would have been able to
connect with only an 24 Mbps modulation rate is now degraded to 12 Mbps, which is
still better than the best that an 802.11b-only device can offer.
The Reason for Mixed-Mode Performance Degradation
Access points may be transmitting some packets to clients who are capable of
demodulating an OFDM transmission (802.11g clients) and other who can only
demodulate PSK (802.11b
(802.11b-only
only clients.)
When any device (client or access point) wants to transmit, all other stations must defer
throughout the duration of the transmission and for a sufficiently additional time (10’s
of microseconds) to allow the intended recipient to send an 802.11 ACK packet.
Every data packet begins with a preamble sequence of bits followed by a packet header.
The packet preamble alerts other stations to the fact that a new transmission is
beginning In the packet header is a variable
beginning.
variable, called the duration field
field, which conveys
the length of the payload (measured in microseconds). This information is then used
by all other nodes to set an internal timer (called the network allocation vector, NAV)
which ensures that they won’t try to transmit until the current packet has been
transmitted and the ACK will be completed. The channel is assumed to be occupied
until the NAV timer expires.
This process of media access control requires that all stations within range of an access
point are capable of demodulating the packet preamble and header. Herein lies the
challenge. 802.11b-only devices can only demodulate PSK transmissions. Hence, in
a mixed-mode environment, access points must transmit all preambles and packet
headers using PSK modulation. Moreover, 802.11b specifies that this part of any
packet be transmitted using 1 Mbps modulation rates to minimize the chances of bit
p
((In fact,, even a simple
p 802.11b-onlyy transmission at 11 Mbps
p begins
g
with
corruption.
a 1 Mbps preamble and header, then switches to 11 Mbps on the basis of a variable
transmitted as part of the preamble.)
When an access point is configured to support mixed-mode operation (and this is the
default for all 802.11b/g access points) it must transmit all packets with a PSK
preamble and header at a 1 Mbps modulation rate. When 802.11g clients detect the
y too, switch to mixed-mode operation and use PSK
PSK preamble and header they,
for their own preambles and headers.
The Significance of Mixed-Mode Relative to RF Spectrum Analysis
In a mixed-mode environment the spectral footprint for transmissions will be a
summation of the PSK preambles and headers on all packets, the PSK payloads in
802 11b-only packets,
802.11b-only
packets and the OFDM payloads in 802
802.11g
11g packets
packets. The result will
look almost exactly like a typical PSK spectral footprint if there is very little actual user
data being transmitted, or if most users are 802.11b transmitters. Mixed-mode
devices must detect suitable signal and noise levels before they switch to OFDM for
their payloads and so these, too, may actually not be switching up to OFDM.
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65
As a result, it’s common to see a spectrum analyzer trace acquired in an environment
where it’s known that 802.11g devices are in use, but find that the trace most
closely resembles the spectral footprint for an 802.11b transmission.
Mi d M d Spectral
Mixed-Mode
S
t l Footprint
F t i t with
ith 802
802.11g
11 Predominance
P d i
Shown below is the spectrum analyzer trace from the engineering lab showing a mixedmode, 802.11g and 802.11b network where the predominant usage is, in fact,
802.11g OFDM.
Notice that the frequencies below and above the Channel 11 frequency band show the
gradual tapering-off of signal energy that is consistent with the OFDM footprint. On
the other hand, the signal energy immediately below and above the center
frequency notch bows upwards, forming ‘humps’ that are consistent with the PSK
spectral footprint (and unlike the ‘flat’ top of the OFDM spectral footprint.)
GRADUAL TAPERING OFF
LIKE OFDM
PSK-LIKE ‘HUMPS’
BELOW AND ABOVE
CENTER FREQUENCY NOTCH
MIXED-MODE WITH PREDOMINANT 802.11g
5.8 GHz Multi-Beam Space-Time Coding (MB-STC) Spectral Footprint
There is another spectral footprint that is present when transmitters are using a
modulation technique called Multi-Beam Space-Time Coding (MB-STC). MB-STC
uses aspects of the engineering that’s found in the emerging 802.11n 100 Mbps
standards (Multiple Input / Multiple Output, MIMO) and in the 802.16 WiMAX
standards (Space-Time Coding, STC). Specifically, Motorola Canopy utilizes this
technology in their 5.8 GHz unlicensed outdoor radio equipment.
With MB-STC, data is modulated onto multiple RF signals, transmitted simultaneously
with different angles of polarization (multi-beam). The signals don’t ‘see’ each other
and are influenced independently by the environment. This is analogous to the way
some traffic lights are equipped with polarized glass covers so that the traffic light
can only be seen by drivers who are close to and directly in line with the traffic
signal. The technique is part of a family of engineering methods called Multiple
I
Input,
t Multiple
M lti l O
Output
t t (MIMO).
(MIMO)
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66
A space-time code (STC) is a method employed to improve the reliability of data
transmission. STCs rely on transmitting multiple, redundant copies of a data stream
to the receiver so even if some of the streams are corrupted in transit there will still
b a hi
be
high
h probability
b bilit that
th t th
the original
i i ld
data
t can still
till b
be recovered.
d
The data stream transmitted on the multiple beams is deliberately de-correlated so that a
single corruption event (i.e. noise or interference) will have very little likelihood of
simultaneously making any sequential stream of bits unrecoverable. This is
accomplished by shifting the time synchronization between beams (space-time
coding) As such, Multi-Beam Space Time Coding is resistant to multi-path fading,
atmospheric
t
h i di
disturbances,
t b
noise
i and
d iinterference,
t f
and
d other
th environmental
i
t l RF
events that could degrade performance.
Because MB-STSC is a derivative of OFDM, the general shape of the spectral footprint
has an overall shape that’s similar to the 802.11a/g footprint. The prominent spikes
at the lower and upper channel boundaries identify the transmission as MB-STC.
Shown below is an example of an MB-STC spectral footprint. In a very general way the
lower and upper portions of the footprint are reminiscent of the gradual tapering off
that is seen with OFDM (since MB-STC incorporates OFDM sub-carriers). The most
notable characteristic, however, is the dramatic high signal energy spike found at
the boundaries of the channel.
Notice that the average signal power (bottom purple trace) is very low (close to
-100 dBm) with the exception of the spike points and the center frequency which,
with MB-STC, does not have a null (i.e. does not have the ‘notch’ seen with PSK
and OFDM.) Readers with a strong RF engineering perspective may recognize the
similarity between the average trace line and a trace of an analog transmission with
an upper and lower sideband.
PROMINENT SPIKES AT
CHANNEL BOUNDARIES
CENTER
FREQUENCY
(AVERAGE)
MB-STC SPECTRAL FOOTPRINT
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67
ANTENNA POLARIZATION ISSUES
Comparison of Antenna Alternatives
This discussion of antennas is specifically focused on PCMCIA and miniPCI antennas
used on notebook computers but is expanded to describe characteristics that relate
to all dipole antennas. This discussion provides a perspective on the two basic types
of adapter designs using two types of antennas. The effects noted are characteristic
of the microwave bands and the principles are true for all transmission bands. The
differences between a typical PCMCIA Wi-Fi adapter (with a completely internal
antenna structure) and an adapter utilizing a fold-out
fold out antenna are presented. This
discussion explains the degradation in reception that can occur when the transmitting
and receiving antennas are depolarized to a small or great degree. Polarization
refers to the angle at which the antennas are positioned and depolarization implies
that they’re positioned at different angles relative to each other.
Antenna Polarization
The present discussion relates directly to a typical dipole
antenna and is extended to consider applicability to
directional antennas as well. At issue is the fact that the
electromagnetic wave propagating outwards from the “stick”
of a dipole antenna oscillates in a plane that is consistent with
the orientation of the antenna. This is referred to as the
p
p g
g wave. When a dipole
p
polarization of the p
propagating
antenna Vertical
e t ca
o o ta
Horizontal
Polarization
is pointing straight up it is said to be vertically polarized. On Polarization
its side it’s referred to as being horizontally polarized. There
are some subtleties relating to the electrical versus the magnetic portion of the wave
but the relevant concepts can be discussed without delving too deeply into the
underlying physics.
g and Receiving
g Antennas Should Be Oriented the Same Way
y
Transmitting
A receiving antenna can acquire the strongest signal when it’s oriented so that it
matches the polarization of the received signal. When a receiving antenna’s
polarization does not match that of the transmitter it’s said to be depolarized.
Depolarization is measured in degrees with 180-degrees being the maximum amount
of depolarization. In this situation the two antennas are at right-angles relative to
each other. This situation is referred to as cross
cross-polarization
polarization and is the worst-case
worst case
scenario.
In open, unobstructed space a cross-polarized receiving antenna will experience the
maximum degradation in signal strength. Indoors, however, the scattering of the
signal from obstructions in a building may cause the transmitted signal to be
reflected and refracted from many different angles so it arrives at the receiver in
many different angels of polarization. In this case the effect of depolarization and
cross-polarization may be negligible relative to the multi-path reflection degradation
that might be taking place.
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Prior Research Has Been Conducted to Study Antenna Polarization Issues
Studies conducted at Lucent Technologies, Bell Laboratories and by the U.S. Navy have
focused on the effects of polarization in indoor locations. A vertically polarized dipole
antenna was used at the transmitter which is consistent with a typical 802.11 access
point’s
point
s antenna orientation
orientation. It was recognized that the signal arriving at the receiver would
undergo multiple scattering from obstacles. The following conclusions are consistent with
this prior research.
Large Building Floors With Low Ceilings Retain Transmission Polarization
Most building floors have horizontal dimensions that are significantly larger than the height
of the ceiling. In this environment, most rays of a propagating wave tend to arrive with
angles
l close
l
tto th
the h
horizontal
i
t l plane.
l
Th
The b
building
ildi flfloor acts
t iin a manner similar
i il tto a wave
guide. In this environment the original polarization of a transmitted wave tends to remain
relatively consistent throughout the signal coverage area.
In this environment it was found that 90% of depolarization errors resulted in a receive
signal loss of less than 1 dB.
Horizontal Polarization is Often More Effective in Large Building Floors
With Low Ceilings
It was also found that when limited clutter was present on this type of building floor (thereby
reducing the signal scatter and reflection) horizontal polarization of both transmitter and
receiver could result in as much as 10 dB greater overall receive signal strength as
compared to vertical polarization for transmitter and receiver.
Very
y High
g Ceilings
g Reduce the Benefits of Polarization
In a building that has a very high ceiling and where the ratio of horizontal to vertical
dimension more closely approaches 1:1, the oblique reflections from walls and scattering
in the building may result in a sufficiently diverse combination of polarization effects that
orientation differences between transmitting and receiving antennas cease to introduce
significant changes in the signals acquired at the receiver.
As the ratio of vertical to horizontal dimension approaches 1:1 a receive signal loss of up to
5 dB was observed.
Directional Antennas are Effected More by Depolarization
The more the azimuth propagation pattern (top-down view) of an antenna differs from the
elevation pattern (side view), the greater the effect of depolarization. In this regard, a
dipole antenna’s torridial propagation pattern (doughnut shape) is circular when viewed
from the top and relatively circular (in an generalized overall perspective) when viewed
from the side. A sector antenna, on the other hand, may have a 60-degree horizontal
beamwidth but only a 20-degree vertical beamwidth. Depolarizing a pair of sector antenna
causes more dramatic changes than depolarizing a pair of dipoles with extreme examples
approaching as much as 20 dB of loss at 180-degrees of depolarization.
Low Gain Dipole Antennas are Effected the Least by Depolarization
A dipole antenna designed to produce no gain (beyond the 2
2.15
15 dB relative to isotropic) will
have an elevation propagation pattern that is more similar to the spherical azimuth pattern
than a dipole designed to introduce higher gain. Hence, as dipole gain is increased the
capacity to experience greater depolarization losses is also increased, up to the
suggested 5 dB loss figure in a practical worst-case scenario.
69
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Dipole Antenna Propagation Patterns
DIPOLE ANTENNA
Consider a vertically polarized dipole antenna, as depicted to
the right. The electromagnetic field propagates outwards in
a manner that can be conceptualized as a torus (a
flattened doughnut shape
shape.))
Viewed from the top the torus would look like a circle. This is
the 360-degree, omnidirectional azimuth propagation
pattern of the antenna. Viewed from the side, a crosssection of the torus (elevation pattern) would generally
resemble a figure-eight (actually, it would resemble the
cross section of a doughnut
cross-section
doughnut.))
Axial Field Reduction Zone
The propagation of an electromagnetic signal from a dipole
antenna is consistent with the way magnetic
lines of flux surround a bar magnet. As
depicted to the right, notice that directly along
Dipole Propagation
th extended
the
t d d axis
i off th
the antenna
t
(the
(th black
bl k
Pattern
double-headed arrow) is an area into which
the signal does not propagate. A dipole
antenna manifests a zone of weak
Magnetic Flux
transmission and reception that extends
Surrounding a Bar
outwards along the axis of the antenna. This
Magnet
is referred to as the Axial Field Reduction
Zone.
Thus far, depolarization has been considered when two
Azimuth (Top) View
antennas are in parallel vertical planes but their angle of
orientation within the planes differ. If, however, you were
to point the “stick” of a receiving dipole directly at a
transmitting antenna (as if you were using the antenna as
a whiteboard pointer) the two antennas would no longer be
in parallel vertical planes. Weak or zero signal would be
present in the transmitting antennas’ Axial Field Reduction
Zone and otherwise strong signals arriving in a receive
Elevation (Side) View
antennas’ Axial Field Reduction Zone would be highly
attenuated or undetectable.
The Axial Field Reduction zone is normally not significant until the receiver is more than
roughly 30 feet (9 meters) from the transmitter. The central core of the zone has not yet
“spread out” enough to manifest itself to any great degree. Since the floor and ceiling of a
building floor are generally much lower than this height there is no design constraint or
consideration to be addressed.
Axial Field Reduction is almost never an issue with a verticallyy polarized
p
dipole.
p
Receivers
are not usually “above” or “below” the radio, they’re typically to the sides of the radio in
some part of a building. If an antenna in a large indoor area with a low ceiling is
horizontally polarized there may be areas beyond 30 feet (9 meters) from the axial line of
the antenna that suffer degraded performance.
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Worst-Case Axial Field Reduction Scenario for Dipole Antennas
If a dipole is “pointed” (like a whiteboard pointer) at another antenna then the Axial Field
Reduction is maximized, as depicted below. As the distance between antennas begins to
exceed roughly 30 feet (9 meters) the effects quickly become dramatic with no signal
transmission or reception inside the Axial Field Reduction Zone expected beyond roughly
50 feet (15 meters).
Conic Axial Field
Reduction Zone
Worst-Case Scenario for Antenna Orientation
{Distance Greater Than Roughly 30 Feet (9 Meters)}
Dipole Antennas Have a Conic Axial Field Reduction Zone
The Axial Field Reduction Zone can be thought of as a pair of cone-shaped regions
extending along the longitudinal axis of the dipole antenna, out both ends of the
dipole. This is in comparison to the circular shape of the Field Reduction Zone to be
described for a patch antenna.
Don’tt “Point”
Don
Point a Dipole at Its Intended Target
The worst-case situations shown above should never be intentionally designed into an
antenna system. With mobile or portable radio equipment a situation may occur when
the orientation of a user’s device causes their dipole antenna to “point” directly at the
intended target radio. This can cause dropped connections or speed reduction caused
by packet retransmission.
Correct Dipole Orientation
A dipole antenna should be oriented in a manner that is most consistent with the antenna
to which intended communication is intended. Outdoor areas typically benefit more
from vertical polarization and many indoor areas (with relatively low ceilings) benefit
from horizontal dipole polarization.
A simple test can often be performed when in doubt as to the most effective polarization.
By simply changing from a vertical to a horizontal polarization by reorienting one
antenna it’s often possible to measure relative signal strength directly on the receiving
device. During the conduct of this test be sure to measure results at multiple locations
throughout the intended coverage area.
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The PCMCIA Patch Antenna
The most common type of antenna found in PCMCIA adapter cards is a small rectangle
of metal on the circuit board referred to as a patch antenna (also a biscuit antenna.) A
large version of this type of flat plate antenna is called a panel antenna
antenna. It’s
It s common to
hear the term “patch” used to refer to large, directional antennas when it would be more
accurate to refer to the larger version as a “panel”. Shown below is a typical PCMCIA
adapter and a detail view of a pair of patch antennas. The epoxy coating on the “nub” of
the card has been cut off to disclose the metal elements on the circuit board.
Antenna Elements
on Circuit Board
Typical PCMCIA Patch Antenna Design
Antenna Diversity Using Two Elements
Often a pair of patch antennas is used to provide antenna diversity. Diversity is a method
whereby reflected signals are received by both elements of the diversity pair and the
higher quality signal is used. While antenna diversity in a PCMCIA form factor may
have advantages over non-diversity designs the advantages are minimal compared to
the potential for depolarization loss.
Because the worst-case reflection problem results in signals arriving 180-degrees out of
phase and canceling each other at the receiver the best design for a diversity pair of
antennas is to separate them by an equal multiple of the wavelength. In this way a
signal may be cancelled by its reflection at one antenna, but unaffected at the other.
Diversity Improvement is Negligible in a PCMCIA Form Factor
Because PCMCIA diversity patch antennas are so close together the ability of one
antenna to receive a significantly better signal than the other is negligible in almost all
situations when indoor scatter and reflection attenuate and distort most wave paths to a
generally equal degree.
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Polarization of the PCMCIA Patch Antenna
Comparing the propagation pattern of a dipole antenna
(torus) to that of a patch (shown to the right) reveals that
although they are different in some details they’re
they re both
the strongest in a direction that’s perpendicular to the
axis of the radiating element. For the dipole that’s
perpendicular to the long axis of the “stick”, and for the
patch it’s perpendicular to the flat plate of the element.
The “ends” of the dipole (where little or no signal
p
emanates)) define its p
polarization. A vertical dipole
doesn’t transmit strongly at 0- and 180-degrees and
that’s the axis that’s used to define polarization. By the
same logic, examination of the patch antenna pattern
shows that it doesn’t transmit strongly along the plane of
the antenna element. The orientation of the plane of the
antenna element defines the polarization of a patch
antenna.
The horizontal nature of the polarization in the depiction to
the right is best seen in the Elevation View at the bottom.
It could be imagined that if the cross-section of the flat
plane of the antenna patch were rotated into a vertical
position it would closely resemble the vertical pattern of a
dipole that was in an upright position.
In Practical Circumstances, PCMCIA Patch Antennas
are Typically Horizontally Polarized
Typically a PCMCIA card is inserted into a notebook or
tablet computer in such a way that its orientation is the
same as that of the computer. When the notebook
computer is placed flat on a horizontal surface, the patch
antenna is horizontally polarized. If the computer or
tablet is raised up and held out vertically in front of a user
then the patch antenna is vertically polarized.
PATCH ANTENNA
Patch Propagation
Pattern
(Horizontally Polarized)
Azimuth (Top) View
Elevation (Side) View
Axial Field Reduction of Patch Antennas
It
It’s
s uncommon for two communicating radios to both have patch antennas unless two users
are connected in a peer-to-peer network with their notebook computers. The most
common situation is an access point with a dipole antenna communicating with a
notebook computer or handheld device that’s using a PCMCIA patch antenna. In this
situation the depolarization effects are somewhat more complicated than when two
identical antennas are used, but the effects can be considered.
To begin,
g , consider two identical p
patch antennas,, one transmitting
g and the other receiving.
g If
both patch elements are horizontal then the weakest transmission from one is being
directed to the weakest receiving area of the other. This is the worst-case Axial Field
Reduction scenario.
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Patch Antennas Have a Circular Axial Field Reduction Zone
The patch antenna, as a flat piece of metal, does not have a
single, narrow longitudinal axis as does a dipole. The “edge”
of the patch acts like the longitudinal axis of a dipole and,
i
th
d
th
ld R
d ti Z
since
there are ffour edges,
there iis a Fi
Field
Reduction
Zone
extending out at an angle extending slightly above and
slightly below this horizontal plane. It encompasses the patch
Field Reduction Zone
Surrounding a Patch Antenna
in all directions and, if viewed from above (perpendicular to
the flat patch) it would be circular.
Worst-Case Axial Field Reduction Scenario for Patch Antennas
Consider the typical way that a number of notebook computers might be used in an
ordinary situation. It’s reasonable to assume that they all might be placed horizontally on
desktops or on the laps of users. As depicted to the below, all of the PCMCIA adapters
would be in the same horizontal orientation.
The important thing to realize is that no matter how the individual devices are turned or
positioned, the PCMCIA adapters will remain horizontally polarized and their Axial Field
R
d ti Z
ill di
tl
Reduction
Zones will
directly
overlap, resulting in the
worst possible connectivity
when distances begin to
exceed roughly 30 feet
These Machines Are in Each Other’s
Axial Field Reduction Zones
(9 meters).
P t h Antennas
Patch
A t
U
Used
d With a Standard
St d d Access
A
Point
P i t
Shown below is what may be the most common Wi-Fi scenario. A horizontally polarized
patch antenna (notebook computers) communicating with a vertically polarized dipole on
an access point. In most cases the notebook computers or handheld devices are used at
a height that’s sufficiently below the mounting location for
the access point that the angle from the access point to
the
the patch
Axial
Field
th notebook
t b k iis above
b
th
t h antenna’s
t
’ A
i l Fi
ld
Reduction Zone. This situation is represented here.
The Access Point is Above the Angle of the
Axial Field Reduction Zones
Correct Antenna Orientation
When combining a horizontally polarized patch antenna with a dipole it may be beneficial
to place the dipole in a horizontally polarized orientation. Careful consideration must be
given because when the dipole is horizontally polarized the Axial Field Reduction Zone is
lowered into a horizontal orientation as well
well.
If the narrow dimension of an interior building floor allows no more than 30 feet (9 meters)
beyond the “top” and “bottom” of the horizontal orientation of a dipole then Axial Field
Reduction is not a serious concern.
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74
Adapters Utilizing a “Flip-Out” Antenna
Some PCMCIA or miniPCI adapters incorporate a “flip open” dipole
antenna. A mock-up of the MiniPCI adapter card is shown to the
right Below it is a photograph of an actual antenna
right.
antenna, then a
Mock-Up of
photograph of an actual cut-away view showing the construction
MiniPCI
Adapter
of the antenna after the plastic arm was removed. A traced outline
has been overlaid on the cut-away to represent the plastic
housing that was removed.
Notice that this is a single (non-diversity) dipole antenna. As such, it
obeys all of the principles and behaviors discussed previously for
dipole antennas. An Axial Field Reduction Zone is present off the
“end” of the antenna.
Antenna Polarization is Dependent on WebPad Orientation
As will be discussed in the next section of this report, the antenna
moves with the host notebook computer or PDA and may be in
any orientation
how th
the user iis h
holding
i t ti based
b
d on h
ldi or positioning
iti i
their device.
Photo of
Antenna Polarization is NOT Dependent on
Flip-Up Antenna
the Open or Closed Position
Assume that the user’s device is placed in a fixed position, perhaps
flat on a table or at some fixed angle in a user’s lap. With the
user’s device in a fixed position, consider the effect of opening
the antenna. Flipping the antenna “open” doesn’t really
“extend” it or somehow lengthen it. It simply
re-orients the antenna.
If the user’s device is flat on a table and the antenna is
opened the polarization remains horizontal and the angle of the
A i l Fi
Axial
Field
ld R
Reduction
d ti Z
Zone moves.
If the user’s device is being held in a horizontal position and the
antenna is opened then its polarization changes from horizontal
Disassembled Antenna
(closed) to vertical (open) with the angel of the Axial Field
showing Dipole Element
Reduction Zone rotating from horizontal to vertical along with the
dipole.
Antenna Diversity for Single-Dipole Radios
The lack of antenna diversity is not considered a dramatic deficit in the design of adapters
because of its intended use in a relatively small indoor environment where scatter and
reflection will minimize any benefits that diversity antennas might offer in an outdoor or
large-scale indoor area environment.
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Antenna Polarization Simulations
The top model shows the prediction with the access
point antenna oriented vertically. A shadow area
appears in the bedroom as a result of the corner of
the one of the interior rooms.
It can be seen that this shadow area (which is the
most potentially problematic area in the floorplan)
completely loses coverage with horizontal antenna
polarization.
The middle model shows horizontal polarization with
the antenna oriented left-to-right relative to the
floorplan (and relative to this page.) As pointed out
in the previous discussion of antenna polarization,
horizontal polarization can provide improved signal
coverage in some cases while other environments
show no improvement with horizontal polarization
polarization.
In the present design horizontal polarization
actually causes a reduction in coverage.
At the bottom is a model showing the antenna,
horizontally polarized, but oriented the “long way”
relative to the floorplan. In this case the previously
reasonable coverage in the lower room has now
shrunk back away from the wall.
Shadow
Area
Vertical
Polarization
Oriented
Straight-Up
Horizontal
Polarization
Oriented
Horizontally
Left-to-Right
Horizontal
Polarization
P
Coverage Has Shrunk Back
Away From the Wall
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Oriented
Horizontally
Front-to-Back
76
INDOOR POLARIZATION TESTING
Testing Performed by Connect802 Corporation
Metal
A real-world test was performed by engineers at Connect802
Obstruction
to determine the degree to which signal strength varied
E
D
when access point antennas were oriented for vertical and
horizontal polarization. With the antennas pointing straight
B
C
up (vertical polarization) signal strength was measured
Access
while walking the path from (A), through the building to
Point
A
(C), and then back along a slightly different path past (D),
75 feet (23 m)
ending in location (E).
(E) A metal obstruction (large water
The Test Area:
heater) was in a service closet directly between the
Start (A) to hallway (B) across open area
access point and location (E). The letter locations relate
and into far room (C), back through
the floorplan to the signal strength graph below.
alternate room (D), ending at (E) behind
large metal obstruction.
Test Results
In many areas the horizontal polarization (#1) had better signal strength than the vertical
polarization
the
B to
l i ti (#2).
(#2) Moving
M i away from
f
th access point
i t (#3,
(#3 from
f
t C) the
th horizontal
h i
t l
polarization maintained signal strength close to 10 dB better than vertical polarization (as
indicated on the graph below at #3.) Behind the metal obstruction (#4) however, vertical
polarization provided a 10 dB improvement over horizontal polarization.
Conclusions
In most areas, horizontal polarization provided better signal strength than vertical polarization
with the exception of the area in the shadow of the large metal obstruction. For large building
floors with relatively low ceilings (8 to 12 feet / 2.5 to 3.5 m) horizontal antenna polarization
appears be the
better choice for
Vertical Polarization (Green)
antenna orientation.
#1
These results will
Horizontal Polarization (Purple)
vary depending
d
di on
#2
the way a specific
A
environment scatters
B
#4
and reflects signals
#3
B
so use of horizontal
10dB
polarization should
E
not be considered
D
an absolute rule
for system
implementation.
C
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77
GENERAL INTERFERENCE ISSUES
Environmental Noise Relative to 802.11 Receivers
An 802.11 receiver may experience interference from undesired 802.11 transmitters or
other similar radio equipment. In addition, environmental noise, which is predominantly
random in nature, may be present in the environment. Two common sources of
random energy are cordless phones and microwave ovens.
Interference from 2.4 GHz Cordless Phones
802.11b devices transmit in the 2.4 GHz ISM band using Spread Spectrum techniques.
802.11g
g devices transmit in this spectrum
p
using
g Orthogonal
g
Frequency
q
y Division
Multiplexing (OFDM). Both of these techniques confine the predominant energy of
transmission to a 22 MHz wide “channel”. (A transmitted signal actually spreads into a
band wider than 22 MHz but the energy beyond the 22 MHz width is only significant
when two devices are separated by less than roughly 3 ft (1 m) so the concept of
“channel” is typically applied to the 25 MHz spread.)
For interference to occur,, an undesired transmission must take place
p
such that a
sufficient amount of its energy overlaps into the 25 MHz channel of the victim. If two
Wi-Fi transmitters on the same, or sufficiently overlapping channels, were to transmit
at the same time their signals could interfere.
A 2.4 GHz cordless phone uses a transmission technique called Frequency Hopping.
Unlike Spread Spectrum and OFDM transmissions a Frequency Hopping cordless
phone
p
utilizes the entire allowable 2.4 GHz ISM band but sends onlyy veryy short
duration, very narrow bandwidth pulses in a pseudo-random pattern across the entire
band.
Frequency Hopping is compared to
Spread Spectrum in the specialized
spectrum analyzer display shown to
the right called a Swept Spectrum
display. The spectrum analyzer
sweeps from low to high frequency
every 200 ms, and represents the
signal strength at each frequency
with a blue to red hue. A single
sweep (the dashed line from A to C)
d i t th
depicts
the RF environment
i
t att a
given instant in time. The spectrum
analyzer continues to sweep and the RF environment over time is depicted moving
upward in the Swept Spectrum display.
Notice the green “smear” from B to D. This is an 802.11 Spread Spectrum transmitter in
a 22 MHz channel. The green indicates that higher signal energy existed in that part of
the
th spectrum
t
on every sweep off the
th spectrum
t
analyzer.
l
In
I the
th dashed
d h d circle
i l att E you
see some of the many little red and yellow “dots” that a scattered across the display.
These are the hopping pulses from a 2.4 GHz cordless phone. Their scattered nature
is clearly visible in the display.
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78
The consequence of Frequency Hopping is that it’s extremely unlikely that it will
introduce sufficient energy into a single 22 MHz portion of the band to disrupt an
802.11 transmission in the same 22 MHz portion of the spectrum. The reverse is not
always true. It is possible for an 802.11 transmitter to introduce sufficient energy into
the environment that it creates noise or disruption in a cordless phone conversation. In
general, interference from Wi-Fi devices is not observed beyond a few feet (1 m) from
a cordless phone. Using a cordless handset while using a Wi-Fi device can result in
noisy calls, but it’s unlikely that the Wi-Fi connection will suffer. Placing a Wi-Fi device
only a few inches (7 cm) from a cordless phone may cause data corruption on the WiFi connection but it’s much more likely that the phone will be impacted more than the
Wi Fi device
Wi-Fi
device.
Interference from Microwave Ovens
A microwave oven produces between 500 and 1000 watts of RF energy, typically at
2450 MHz (2.450 GHz). The oven is carefully shielded to contain the energy and
protect people nearby in accordance with regulatory requirements for health and
safety. Depending on the age of the oven and its physical condition the degree to
which
hi h RF energy iis contained
t i d varies
i ffrom a llow lleakage
k
llevell att th
the titime off
manufacture to a higher level for older, or damaged ovens.
The RF model to the right depicts three microwave
ovens in three kitchens in three adjacent dwelling
units. It can be seen how the RF noise generated
by the ovens spreads out directly through the open
entryway
t
to
t the
th kit
kitchen
h and
d indirectly
i di tl through
th
h the
th
interior walls of the dwelling. The legend indicates
only “weak” and “strong” relative noise levels since
the actual dBm measurement is contingent on the
degree of leakage from the oven. It may be
assumed that a typical oven will create sufficient
noise to be disruptive to a Wi-Fi receiver in areas
with orange and reddish hues. An oven
experiencing more severe leakage may cause
Wi-Fi disruption in the areas with yellow and green
hues as well.
802.11b/g channel 9 has a center frequency of
Weak
Strong
2452 MHz.
MHz This places a microwave oven
oven’s
s noise
frequency directly in the middle of the channel 9 bandwidth. It can be expected that
the noise will impact the adjacent two channels resulting in potential problems on
channels 7 through 11.
Microwave noise can cause lost connections when in close proximity to the oven but,
more typically, results in reduced throughput (resulting from retransmission of
corrupted packets
packets.))
The design solution for environments with microwave ovens is to configure Wi-Fi
equipment for operation on channels below 7. Configuring equipment to operate on
channel 1 is ideal and operation up to and including channel 6 will generally create
appropriate noise immunity for a Wi-Fi system.
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79
UNBALANCED POWER EFFECT
When the radio on one side of a communication link has greater Transmitter Power
Output (TPO) than the radio to which it’s
it s communicating (on the other side of the link)
then an Unbalanced Power Effect (UPE) Zone exists at some distance from the more
powerful radio. To define the UPE zone, assume that the radio with the stronger TPO
is in the center of an unobstructed area (as depicted by the model shown below).
The inner UPE radius is the point at which the radio with the higher TPO (in the middle)
can no longer hear the transmission from the radio with the lower TPO (moving away).
Th outer
The
t radius
di is
i the
th point
i t att which
hi h th
the radio
di with
ith th
the llower TPO (moving
(
i away)) can no
longer receiver the signal transmitted by the radio with the higher TPO (in the middle).
When the inner UPE radius is reached the connection between the two radios fails.
Connectivity must be bi-directional and at the inner UPE radius the weaker radio is no
longer able to push its signal to its partner. The lower power transmitter is the weak link
in this chain of connectivity. Notice, in the model shown below, that with a 1 dB
difference bet
between
een the stronger and weaker
eaker TPO the UPE zone
one has a width
idth of 640
feet.
-90 dBm
Inner Boundary
820 Feet
STRONGER
RADIO
(+1 dBm)
WEAKER
RADIO
-95
95 dBm
Outer Boundary
1460 Feet
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80
HOW A MODEL IS CREATED
In today’s sophisticated engineering disciplines, the use of computer aided design (CAD)
to simulate real
real-world
world systems is commonplace. Aircraft designers, civil engineers, and
even chemical engineers use computer models and simulation systems to test and
confirm their designs before putting them into practice. The same engineering
sophistication is applied to the design of wireless network communication systems.
The RF modeling and simulation software used to create the design plans presented in
this report is based on a number of patented and patent pending technologies and
methods. This section describes the model g
generation p
process and is intended solely
y
to satisfy the intellectual curiosity of the technically inclined reader. A detailed
discussion of RF modeling, including the math and physics underlying the modeling
software system, may be found in the book, “Wireless Communications Principles and
Practice”, by Theodore S. Rappaport (Prentice Hall 2002 ISBN 0-13-042232-0)
The software system is based on an OEM version of the Autodesk AutoCAD design
engine.
g
Overlaid on this engine
g
are a number of p
plug-in
g calculation modules that
perform path loss measurements, taking into consideration the construction
characteristics in a building, the noise factors in the environment, and the effects of cochannel interference, signal reflection, refraction, and diffraction in the space being
simulated. When accurate plans are properly formatted the predicted results are
typically within ±2 dB of empirical measurements. This accuracy has been confirmed
by direct field measurements at various customer sites.
Simulations are calculated in 3-dimensions. A
data set is created for each transmitter (and/or
receiver) by applying electromagnetic wave
propagation formulae to a “ray” extending
outward from each transmitter (or inward to
each receiver.) The ray is mathematically
rotated through 360-degrees in the horizontal
plane. The plane is then rotated through 360degrees with the ray being recalculated in its
own 360-degree rotation for each step of the
plane. The result is a set of data points
describing a spherical volume surrounding
each
h ttransmitter
itt (or
( receiver.)
i
)
From the resulting set of data points it becomes
possible to calculate the relationships between
transmitters both in real-time (during the
simulation and antenna placement phase of
design work) and for the purposes of report
output.
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y
x
z
Data points are calculated along a
ray rotated through 360-degrees
y
x
z
A spherical volume of data points is
created by
y rotating
g the calculation plane
through 360-degrees, repeating the plane
calculations at each step.
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