PERFORMANCE EVALUATION OF IEEE 802.16e STANDARD

PERFORMANCE EVALUATION OF IEEE 802.16e STANDARD
İSTANBUL TECHNICAL UNIVERSITY  INSTITUTE OF SCIENCE AND TECHNOLOGY
PERFORMANCE EVALUATION OF IEEE 802.16e STANDARD
MSc. Thesis by
Pedro Francisco ROBLES RICO
(990071803)
Department: Electronics and Communication Engineering
Programme: Telecommunication Engineering
Supervisor: Associate Prof. Dr. Selçuk Paker
DECEMBER 2007
ACKNOWLEDGEMENT
I would like to express my deep appreciation for my advisor’s help, Selçuk Paker.
He has made possible that I can finally finish my degree. Thank you for those
meetings on Tuesday. At the same time, I still have to say thank for my old partner
in Sweden and Lucia who were always worried about my problems and progress.
An important remind for Fatih Murat who was helping me with the measurements.
Thank you for bringing me to the Asian side of the city.
The last remind for my Lab’s partner Mustafa Turkmen who was there always
translating, even the day I was handing this Thesis.
December 2007
Pedro Francisco Robles Rico
ii
TABLE OF CONTENTS
ABREVIATIONS AND ACRONYMS
LIST OF TABLES
LIST OF FIGURES
ÖZET
SUMMARY
1.
v
vii
viii
ix
x
INTRODUCTION
1.1
Frequency Bands
1.2
Topology and Architecture
1.2.1
PMP
1.2.2
Mesh
1.3
Reference Model
1.3.1
MAC layer overview
1.3.2
PHY layer overview
1
2
3
4
4
5
6
6
2. IEEE 802.16e TECHNICAL OVERVIEW and AMENDMENTS to IEEE
802.16-2004
9
2.1
Introduction
9
2.2
Physical Layer Description
10
2.2.1
OFDM Technique
11
2.2.2
OFDMA Symbol Structure and Sub-channelization
12
2.2.3
TDD Frame Structure
14
2.2.4
Advanced PHY layer features
15
2.3
MAC Layer Description
16
2.3.1
Quality of Service (QoS) Support
17
2.3.2
Mobility Management
18
2.4
Comparison between 802.16-2004 and 802.16e profiles
20
2.4.1
OFDM and OFDMA
21
2.4.2
Handoffs
23
3.
ADDITIONAL INFORMATION
3.1
Standard Mobile WiMAX parameters
3.1.1
Receiver sensitivity in Mobile WiMAX technology
3.1.2
Performance Analysis
3.2
Current equipment parameters
3.2.1
Available Products
3.2.2
Comparison among products
3.3
ETSI Frequency usage plan
3.4
Propagation models available
3.4.1
Free Space Model
3.4.2
SUI Model
3.4.3
COST-231 Hata
3.4.4
COST-231 Walfisch-Ikegami model
iii
24
25
25
27
30
31
36
40
43
43
43
48
49
3.4.5
ECC-33 Path Loss Model
52
4.
CALCULATION
4.1
Coverage Prediction Evaluation
4.1.1
Scenario A
4.1.2
Scenario B
4.2
Theoretical Maximum throughput
54
55
57
60
62
5.
MEASUREMENTS TAKEN
5.1
Environment
5.2
Used devices
5.3
Measurements
66
66
68
69
6.
CONCLUSIONS and FUTURE WORK
74
REFERENCES
76
BIOGRAPHY
79
iv
ABREVIATIONS AND ACRONYMS
AAS
AMC
AMS
ARQ
BE
BER
BPSK
BS
BW
BWA
CC
CDMA
CPS
CPE
CQI
CRC
ECC
ErtPS
ETSI
FBSS
FDD
FEC
FFT
FUSC
HARQ
HHO
IEEE
ISI
LAN
LLC
LOS
MAC
MAP
MDHO
MIMO
MS
NF
nrtPS
OFDM
OFDMA
PDU
: Adaptive Antenna System
: Adaptive Modulation and Coding
: Adaptive MIMO switching
: Automatic Repeat Request
: Best Effort
: Bit Error Ratio
: Binary Phase Shift Keying
: Base Station
: Bandwidth
: Broadband Wireless Access
: Convolution Code
: Code Division Multiple Access
: Common Part Layer
: Costumer Premise Equipment
: Channel Quality Indicator
: Cyclic Redundancy Code
: Error Correction Code
: Extended real-time Polling Service
: European Telecommunication Standards Institute
: Fast Base Station Switching
: Frequency Division Duplex
: Forward Error Correction
: Fast Fourier Transform
: Full Usage of Sub-Channels
: Hybrid Automatic Repeat Request
: Hard Handoff
: Institute of Electrical and Electronics Engineers
: Inter-Symbol Interference
: Local Access Network
: Logical Link Control
: Line Of Sight
: Medium Access Control
: Media Access Protocol
: Macro Diversity Hand Over
: Multiple Input Multiple Output
: Mobile Station
: Noise Figure
: non-real-time Polling Service
: Orthogonal Frequency Division Multiplex
: Orthogonal Frequency Division Multiple Access
: Packet Data Unit
v
PHY
PL
PMP
PUSC
PTP
QAM
QPSK
RTG
rtPS
SC
SDU
SNR
SS
SUI
TCP
TDD
TTG
TTI
UGS
VoIP
WiMAX
WLAN
: Physical Layer Protocol
: Path Loss
: Point to MultiPoint
: Partially Used Sub-Carriers
: Point to Point
: Quadrature Amplitude Modulation
: Quadrature Phase Sift Keying
: Receive/transmit Transition Gap
: real-time Polling Service
: Single Carrier
: Service Data Unit
: Signal Noise Ratio
: Subscriber Station
: Standford University Interim
: Transmission Control Protocol
: Time Division Duplex
: Transmit/receive Transition Gap
: Transmission Time Interval
: Unsolicited Grant Service
: Voice over IP
: Worldwide Interoperability for Microwave Access
: Wireless Local Area Network
vi
LIST OF TABLES
Page No
Table
Table
Table
Table
Table
Table
Table
Table
Table
Table
Table
Table
Table
Table
Table
Table
Table
Table
Table
Table
2.1 Mobile WiMAX Applications and Quality of Service [19] .................. 18
3.1 Receiver SNR assumptions [1].............................................................. 26
3.2 Receiver Sensitivity (dB) according 802.16e Standard......................... 27
3.3 OFDMA scalability parameters ............................................................ 29
3.4 Base Stations information ..................................................................... 38
3.5 Subscriber Station information.............................................................. 39
3.6 Frequency usage plan ............................................................................ 42
3.7 Numerical values for the SUI model parameters [9]............................. 44
3.8 COST-231 Hata limitations................................................................... 49
3.9 COST-231 Walfisch-ikegami model limitations................................... 52
4.1 Outdoor Scenarios ................................................................................. 54
4.2 Estimated coverage for 5 MHz into Scenario A.................................... 58
4.3 Estimated coverage for 10 MHz into Scenario A.................................. 58
4.4 Estimated coverage for 5 MHz into Scenario B.................................... 60
4.5 Estimated coverage for 10 MHz into Scenario B.................................. 61
4.6 OFDMA parameters according with 802.16e-2005 Standard............... 63
4.7 Estimated Maximum Data rate in Mbps................................................ 64
5.1 Measurements results for Area 1........................................................... 70
5.2 Measurements results for Area 2........................................................... 71
5.3 Measurements results for Area 3........................................................... 72
vii
LIST OF FIGURES
Page No
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
1.1: PMP Network topology........................................................................... 4
1.2: Mesh mode Topology.............................................................................. 4
1.3: IEEE 802.16 Protocol layering................................................................ 6
2.1: Orthogonal Sub-Carriers ....................................................................... 11
2.2: Symbol Structure with Cyclic Prefix (CP) insertion ............................. 11
2.3: OFDMA Sub-Carrier Structure ............................................................. 12
2.4: DL PUSC sub-channel .......................................................................... 13
2.5: Tile Structure for UL PUSC .................................................................. 14
2.6: WiMAX OFDMA Frame Structure [19]............................................... 15
2.7: OFDM and OFDMA [20]...................................................................... 22
2.8: Uplink in OFDM and OFDMA [20]x ................................................... 23
3.1: Different MAC PDU formats ................................................................ 30
3.2: Uplink and Downlink ............................................................................ 40
3.3: Constructive and destructive interference ............................................. 48
3.4: COST-231 W-I environment................................................................. 50
3.5: Angle for Base Station........................................................................... 51
4.1: Adaptive Modulation in WiMAX technology....................................... 55
4.2: Coverage Radius for scenario A............................................................ 59
4.3: Coverage Radius for scenario B ............................................................ 62
4.4: Different modulation techniques areas.................................................. 65
5.1: Electrical Faculty in Maslak Campus.................................................... 66
5.2: Elevation Map of Istanbul and Bosphorus ............................................ 67
5.3: Coverage areas for testing ..................................................................... 68
5.4: WiMAX Network.................................................................................. 68
viii
ÖZET
Bu çalışma Mobil WiMAX ‘te kapsama alanını ve veri hızı hakkındaki çalışmaları
içerir. Tez WiMAX hakkında genel bilgi, gelişimi ve tanımları vererek başlıyor.
Okuyucu WiMAX hakkında ve faydaları hakkında tam bir fikre vardığında, Fiziksel
Katmanı da, kapsama alanına ve veri hızına etki eden özel nitelikle daha detaylı
açıklanacak. Bu nokta Mobil ve Sabit WiMAX Fiziksel Katları arasında
karşılaştırma ile bitecektir.
Daha detaylı hesaplamalar için diğer görüşler belirtilip çalışılmıştır. Bu grupta
802.16e-2005 Standardının özel parametreleri, mevcut donanım parametreleri,
frekans kullanım planı ve muhtemel yayılım modelleri bulunmaktadır.
Gerekli tüm bilginin açıklanmasından sonra hesaplamalar çok kolay bir şekilde
anlaşılacaktır. Bunlar teorik kapsama alanı yarıçapı ve her bir kapsama alanı için
elde edilen maksimum veri hızıdır.
Sonuçtan önce WiMAX ‘in ne olduğu hakkında daha iyi bir genel düşünce
sağlaması açısından İstanbul çevresinden bazı ölçümler elde edilmiştir. Bir Sabit
WiMAX Baz İstasyonu Elektrik Elektronik Fakültesine yerleştirilmiştir, Mobil
WiMAX ‘e göre birçok farklı özellikler ile çalışır ve hala ilgi çekicidir.
ix
SUMMARY
This paper consists in a research of coverage and data rate for Mobile WiMAX. The
thesis commences with a general introduction about WiMAX, evolution and general
overview. Once the lector has obtained a complete idea of WiMAX and its benefits,
the Physical Layer will be further explained with special attention on attributes
which affect the coverage and the maximum data rate. This point is finished with a
short comparison between Mobile and Fixed WiMAX Physical Layers.
In order to achieve rigorous calculations, other aspects have been studied and
described. In this group it can be found: specific parameters of 802.16e-2005
Standard, current equipment parameters, frequency usage plan and possible
propagation models.
After the explanation of all required information, the calculations can be easily
realized. Thus, theoretical coverage radius and maximum data rate for each coverage
area are obtained.
Before conclusions in order to provide a better concept about what WiMAX is, some
measurements have been taken in Istanbul. A Fixed WiMAX Base Station is located
in Electrical Faculty, which works with many different features comparing with
Mobile WiMAX but it is still interesting.
x
1.
INTRODUCTION
In the last ten years Wireless Network usage and development has been established
in the society firmly. Due to this sharp progress and investment, it has been
necessary to found a single Standard for each Wireless Network. These Standards
are developed by the IEEE organization (Institute of Electrical and Electronics
Engineers) placed in United States.
The widest Wireless Standard is IEEE 802.11, well known as WiFi. The first release
of this Standard was finished in 1999 (ANSI/IEEE Std 802.11 1999 Edition) but
afterwards there has been successive Specifications as 802.11a, 802.11b and 802.11g
where alterations are located mainly in the physical layer, remaining basically the
same MAC layer since the original Specification.
During the development of the IEEE 802.11 Standard, a new Wireless Network idea
emerged, instead of being developed for a Local Area Network it is patterned for
Metropolitan Area Network. It means, for a whole city. This Standard was defined as
IEEE 802.16 and it was developed in order to fill the gap between 802.11 LAN IPbased network and GSM. The new Standard will achieve high bandwidth efficiency
(higher than 802.11 or GSM) and in the last amendment it will support mobility,
although more restricted than GSM.
The first Specification was approved in 2001 however it has still short integration in
the society. This new Standard is commonly named WiMAX since this is the name
for the organization responsible of certifying products related with IEEE 802.16.
WiMAX is based on the IEEE 802.16 standard and on ETSI HiperMAN. The next
version of IEEE 802.16, 802.16-2004 (previously known as Revision D, or 802.16d),
was ratified in July 2004. 802.16-2004 includes previous versions (802.16-2001,
802.16c in 2002, and 802.16a in 2003) and covers both LOS and NLOS applications
in the 2-66 GHz frequencies. As is habitual with IEEE standards, it specifies only the
Physical (PHY) and Media Access Control (MAC) layers.
1
The changes introduced in 802.16-2004, respect previous Standards, were focused
on fixed and nomadic applications in the 2-11 GHz frequencies. Two multi-carrier
modulation techniques are supported in 802.16-2004: OFDM with 256 carriers and
OFDMA with 2048 carriers. The first WiMAX Forum certification profiles are
based on OFDM as defined in this version of the standard.
In December 2002, Task Group was created to improve support for combined fixed
and mobile operation in frequencies below 6 GHz. Work on the 802.16e amendment
is already completed. The new version of the standard introduces support for
scalable OFDMA (a variation on OFDMA) which allows scalability for a variable
number of carriers (at least 512, 1024 and 2048), in addition to the previouslydefined OFDM and OFDMA modes. The carrier allocation in OFDMA modes is
designed to minimize the effect of the interference on user devices with
omnidirectional antenna. Furthermore, IEEE 802.16e offers improved support for
Multiple Input Multiple Output (MIMO) and Adaptive Antenna Systems (AAS), as
well as hard and soft handoffs. It also has improved power-saving capabilities for
mobile devices and more extensive security features. As with 802.16-2004, 802.16e
incorporates previous versions of the standard and adds support for fixed and mobile
access. The Amendments of Mobile WiMAX respect IEEE 802.16-2004 Standard
will be further explained and described in this document.
In the following points of the Introduction, basic concepts of WiMAX are
explicated, since otherwise Standard would hardly be understood.
1.1
Frequency Bands
Three different frequency bands are described in WiMAX Standard (both 802.162004 [4] and 802.16e-2005 [1]): two Licensed Bands, 10 – 60 GHz for line-of-sight
(LOS) and Frequencies below 11 GHz for both near-LOS and non-line-of-sight
(NLOS) and on the other hand, the last frequency band is License-exempt and below
11 GHz.
However, there are only certified products for frequencies below 11 GHz, therefore
the frequency band 10 – 60 GHz is only described for the theory but not for practice.
Then, within this context:
2
Frequencies below 11 GHz
In the frequencies below 11GHz due to the longer wavelength line-of-sight is no
required and multipath may be negligible. For supporting near-LOS and non-line-ofsight (NLOS) scenarios it is required additional PHY functionality. Optionally some
MAC features can be introduced, such as Mesh topology and Automatic Repeat
Request (ARQ).
This range of frequencies may be licensed or license-exempt bands. They are
similar; however license-exempt bands introduce additional interference. The PHY
and MAC introduce Dynamic Frequency Selection (DFS) mechanism in order to
detect and avoid interference.
License-exempt frequencies below 11 GHz (primarily 5-6 GHz)
The physical environment for the license-exempt bands below 11 GHz is similar to
that of the licensed bands in the same frequency range. However, the license-exempt
nature introduces additional interference and co-existence issues, whereas regulatory
restrictions limit the allowed radiated power. In addition to the characteristics
described, the PHY and MAC introduce mechanisms such as dynamic frequency
selection (DFS) to facilitate the detection and avoidance of interference and the
prevention of harmful interference into other users including specific spectrum users
identified by regulation.
In the context of the IEEE 802.16 Standard, the use of the term “license-exempt
frequencies” or “license-exempt bands” should be taken to mean the situation where
licensing authorities do not coordinate individual assignments to operators,
regardless of whether the spectrum in question has a particular regulatory status as
license-exempt or licensed.
1.2
Topology and Architecture
When there is a common air medium which must be shared, an efficient sharing
mechanism has to be used to utilize it in an efficient way. In the IEEE Standard
802.16 there are two different sharing wireless media; Point to Multipoint (PMP)
and Mesh topology wireless networks.
3
1.2.1
PMP
This topology operates with a central Base Station (BS) and its sectorized antenna
which has the capability of handling multiple independent sectors simultaneously.
Within a given frequency and antenna sector, when the BS transmits all the
Subscriber Stations (SSs) receive the same transmission. The BS owns the control of
the downlink. Respect the Uplink, all the transmissions are directed to the BS. The
BS manages the network by coordinating the transmission of the SSs. It does not
require coordinating its transmission with other stations.
Figure 1.1: PMP Network topology
1.2.2
Mesh
The main difference between the PMP and Mesh mode is related with the link
among the stations. In PMP all the transmission occurs between the BS and SSs,
whereas in Mesh mode the traffic can be placed directly between two SS and the SSs
do not must transmit directly to the BS. The traffic can be enrouted through other
SSs.
Figure 1.2: Mesh mode Topology.
4
In Mesh mode the concept BS refers the station which has a direct connection to the
backhaul services outside the Mesh Network. All the others stations are termed SSs.
Within Mesh Networks there are not Downlink or Uplink concepts.
Nevertheless a Mesh Network can perform similar as PMP, with the difference that
not all the SSs must be directly connected with the BS. The resources are granted by
the Mesh Bs. This option is termed Centralized Scheduling.
Concurrently there is another manner to schedule the transmissions, Distributed
Scheduling. In this case all the stations even the Mesh BS must coordinate their
transmissions with the others. And all the stations shall broadcast their schedules.
Both distributed or centralized scheduling algorithms have been considered in the
standard for mesh mode operations. In general, in mesh mode all the nodes have to
coordinate their transmissions in their two-hop neighbourhood and shall broadcast
their schedules (available resources, requests and grants) to all their neighbours. In
particular, nodes have to ensure that the resulting transmissions do not cause
collisions with the data and control traffic scheduled by any other node in the twohop neighbourhood.
1.3
Reference Model
The MAC layer in the IEEE Standard 802.16 is composed by three sublayers.
— Service-Specific
Convergence
Sublayer
(CS):
It
provides
any
transformation or mapping of external network data.
— MAC Common Part Sublayer: This is the largest sublayer. It provides the
core MAC functionality of system access, bandwidth allocation,
connection establishment, and connection maintenance.
— Security Sublayer: It provides authentication, secure key exchange, and
encryption.
It is shown in the following figure.
5
Figure 1.3: IEEE 802.16 Protocol layering
The first three sublayers constitute MAC layer and the other layer, PHY layer, shall
be studied deeper.
1.3.1
MAC layer overview
As it is mentioned below, MAC layer embraces two main sublayers. ServiceSpecific Convergence Sublayer is used to map the transport-layer-specific traffic to a
MAC that can efficiently transport any traffic type. The Common Part Sublayer is
responsible for fragmentation and segmentation of MAC service data units (SDU)
into MAC protocol data units (PDU), QoS control, and scheduling and
retransmission of MAC PDUs. The bandwidth request and grant mechanism has
been proposed to be scalable, efficient, and self-correcting. The 802.16 access
system allows multiple connections per terminal, multiple QoS levels per terminal,
and a large number of statistically multiplexed users. It provides a wide variety of
request mechanisms.
1.3.2
PHY layer overview
The primary purpose of the PHY layer is to process correctly the raw bit information
in order to minimize the errors at the receiver and maximize the throughput. For
achieving the high performance levels required to support wireless broadband
6
services, advanced modulation, equalization, multiplexing, diversity schemes and
error control schemes are specified. The multiple versions of PHY layer are listed
below [11]:
•
WirelessMAN-SC: corresponds to the single-carrier version, designed
to support LOS operation in the 10 to 66 GHz frequency range. The goal
is to provide flexibility in LOS operation scenarios, in terms of planning,
cost, services and capacity;
•
WirelessMAN-SCa: this is the single-carrier solution for NLOS
operation in frequencies below 11 GHz. The frame structure is designed
to be robust against multipath fading. Furthermore, it supports Mobile
receiver stations, channel estimation and equalization, space time coding,
adaptive modulation, Automatic Repeat Request (ARQ), multiple error
correcting
coding
schemes,
Adaptive
Antennas
System
(ASS),
transmission diversity and power control.
•
WirelessMAN-OFDM: designed to support NLOS operation in
frequencies below 11 GHz, based on Orthogonal Frequency Division
Multiplexing (OFDM), which consists of a multicarrier modulation
scheme.
This
version
extends
the
functionalities
of
version
WirelessMAN-SCa, to support Mesh topology and subchannelization on
the uplink, thus providing advanced resources for coverage optimization;
•
WirelessMAN-OFDMA: this version supports NLOS operation in
frequencies below 11 GHz, based on an Orthogonal Frequency Division
Multiple Access (OFDMA), which consists of an extension of OFDM
technique to allow multiple users access a shared channel. Number of
carrier are scalable (some document; SOFDMA).It consists of many of
WirelessMAN-SCa
functionalities,
including
support
to
subchannelization on uplink and downlink;
•
WirelessHUMAN: due to the support to functionalities for operation in
license-exempt frequencies, this version is named “High-speed
Unlicensed Metropolitan Area Network” (HUMAN). It can operate at
frequencies between 5 and 6 GHz, which includes 10 and 20 MHz
channels. However, the channelization scheme to be adopted in particular
7
deployment will depend on regulatory aspects. It is important noting that
this version implements SCa, OFDM and OFDMA versions of PHY
layer.
8
2.
IEEE 802.16e TECHNICAL OVERVIEW and AMENDMENTS to
IEEE 802.16-2004
2.1
Introduction
Although 802.16e is generally recognized as the mobile version of the standard,
actually it serves the dual purpose of adding extensions for mobility and including
new enhancements to the Orthogonal Frequency Division Multiplexing Access
(OFDMA) physical layer. This new enhanced 802.16e physical layer is now being
referred to as Scalable OFDMA (SOFDMA), a multi-carrier modulation technique
that uses subchannelization to support scalable channel bandwidths from 1.25 MHz
to 20 MHz. It includes a number of important features for fixed, nomadic, and
mobile networks such as handover between WIMAX cells and roaming among
WIMAX and other networks.
It is important indicating that this thesis is mainly focused in OFDMA PHY layer
since it permits multiple accesses in the same band. Some aspects from Mobile
WiMAX for Base Stations profiles are specified as optional in order to provide
additional flexibility for deployment based on specific environments which may
require different configurations that are either capacity-optimized or coverageoptimized.
High Data Rates: The inclusion of MIMO antenna techniques in addition with
flexible sub-channelization schemes, Advanced Coding and Modulation enable the
Mobile WiMAX technology to support High Data Rates in downlink and uplink.
Quality of Service (QoS): The fundamental premise of the IEEE 802.16 MAC layer
is QoS (it is very important in 802.16-2004 Standard as well). It defines Service
Flows that enable end-to-end IP based QoS. Additionally, sub-channelization and
MAP-based signaling schemes provide a flexible mechanism for optimal scheduling
of space, frequency and time resources over the air interface on a frame-by-frame
basis.
9
Scalability: Since spectrum resources for wireless broadband worldwide are still
quite disparate in its allocations, increasingly globalized economy, Mobile WiMAX
technology therefore, is designed to be able to scale different channelizations from
1.25 to 20 MHz to consent with varied worldwide requirements as efforts proceed to
achieve spectrum harmonization in the longer term. WirelessMAN-OFDMA PHY
layer in the 802.16e Standard describes the OFDMA PHY. This mode is based on at
least one of the FFT sizes 2048 (compatible to IEEE Std. 802.16-2004), 1024, 512
and 128 shall be supported. This shall facilitate support of the various channel
bandwidths for example 1.25 MHz (128 size FFT), 5 MHz (512), 10 MHz (1024)
and 20 MHz (2048).
The Mobile Station may implement a scanning and search mechanism to detect the
DL signal when executing initial network entry and this could include dynamic
detection of the FFT size and the channel bandwidth employed by the BS.
This also allows diverse economies to realize the multi-faceted benefits of the
Mobile WiMAX technology for their specific geographic needs such as providing
affordable internet access in rural settings versus enhancing the capacity of mobile
broadband access in metro and suburban areas.
Security: The new features provided for Mobile WiMAX security aspects are best in
class with EAP-based authentication, AES-CCM-based authenticated encryption,
and CMAC and HMAC based control message protection schemes.
Mobility: Mobile WiMAX supports optimized handover schemes with latencies less
than 50 milliseconds to ensure real-time applications such as VoIP perform without
service degradation. Flexible key management schemes ensure that security is
maintained during handover.
2.2
Physical Layer Description
Next paragraphs describe Physical Layer features. 802.16e Standard provides a
further explanation of this layer, however for this thesis the description have been
focused in main aspects.
10
2.2.1
OFDM Technique
Orthogonal Frequency Division Multiplexing (OFDM) is a crucial technique for
supporting NLOS operation. It is used as well in WiFi technology due to the higher
multipath robustness against path loss. OFDM is a multiplexing technique that
subdivides the bandwidth into multiple frequencies sub-carriers:
NFFT Subcarriers
Spanning Δf = BW/NFFT
f
Figure 2.1: Orthogonal Sub-Carriers
In this case, a data stream transmitted at a rate of R bps is divided into several
parallel sub-streams of reduced data rate, achieving a transmission rate of R/N bps,
where N is the number of sub-carriers. By reducing data rate, symbol duration is
increased. The increased symbol duration improves the robustness of OFDM to
delay spread. Moreover, the introduction of Cyclic Prefix (CP) increases robustness
against multipath fading and it can completely eliminate Inter-Symbol Interference
(ISI). The CP is typically the last samples of data portion, of the useful symbol,
appended to the beginning of the data payload as shown in next figure:
Figure 2.2: Symbol Structure with Cyclic Prefix (CP) insertion
11
2.2.2
OFDMA Symbol Structure and Sub-channelization
The OFDMA symbol structure consists of different types of sub-carriers as shown in
Figure 2.3. The pilot sub-carrier does not carry data or signalling information, they
are used for estimation and synchronization purposes (equalization, power control
mechanism, etc). The DC sub-carriers allow the inclusion of guard band between
groups of sub-carriers. This sub-carrier is defined in the Standard as “the sub-carrier
whose frequency would be equal to the RF centre frequency of the station”. DC subcarriers together with Guard sub-carriers (used for guard bands) are commonly
denominated Null sub-carriers.
Figure 2.3: OFDMA Sub-Carrier Structure
Active sub-carriers (data and pilot) are group into subsets of sub-carriers called subchannels. Sub-channelization in both DL and UL are supported by WiMAX
OFDMA PHY. There are two types of grouping sub-carriers for sub-channelization:
diversity and contiguous.
First one, diversity, draws sub-carriers pseudo-randomly to form a sub-channel.
Pseudo-random intercalation provides frequency diversity and inter-cell interference
averaging. The Diversity permutation include DL FUSC (Fully Used Sub-Carrier),
DL PUSC (Partially Used Sub-Carrier) and UL PUSC and additional optional
permutations. This permutation technique will be further described below.
On the other hand, contiguous permutation draws contiguous sub-carriers to form a
sub-channel. These permutations include DL AMC (Adaptive Modulation and
Coding) and UL AMC, and have the same structure.
In general, diversity permutations perform well in mobile applications while
contiguous permutations are well appropriated for fixed, portable or low mobility
12
environments. For this reason, diversity permutations will be further explained for
UL and DL.
In the case of DownLink, DL PUSC is mandatory and DL FUSC is an optional
feature. Therefore DL PUSC sub-carrier group will be described. For this case,
usable sub-carriers (pilot and data) are grouped in clusters. Each cluster contains 14
contiguous sub-carriers per symbol. Each cluster will be integrated by 12 data subcarriers and 2 pilot sub-carriers with different distribution depending of the symbols
number; even or odd as it is shown in next figure:
Figure 2.4: DL PUSC sub-channel
Only pilot positions in the cluster are shown, data sub-carriers in the cluster are
distributed to multiple sub-channels. In the previous figure it can be noticed, as well,
that a sub-channel contains 2 clusters and is formed up of 48 data sub-carriers and 8
pilot sub-carriers. It can be found out better with an example:
For 5 MHz Bandwidth channel there are 360 data sub-carriers and 60 pilot subcarriers. Then, one cluster is formed by 12 data sub-carriers and 2 pilot sub-carriers,
therefore there will be (360+60)/(12+2) = 30 clusters. Since each sub-channel is
composed by 2 clusters, there will be 30/2 = 15 sub-channels in DL PUSC.
For UpLink the main group of sub-carriers change the name, in this case this group
is called tile instead of cluster. This is defined for the UL PUSC where a sub-channel
is constructed from six uplink tiles, each tile has four successive active sub-carriers
and its format is as shown below:
13
Symbol 0
Symbol 1
Symbol 2
Data Sub-carrier
Data Sub-carrier
Figure 2.5: Tile Structure for UL PUSC
According to 802.16e Standard, a slot in the uplink is composed of three OFDMA
symbols and one sub-channel, within each slot, there are 48 data sub-carriers and 24
fixed-location pilots. Hence, a sub-channel is formed by 24 usable sub-carriers. It
can be realized better with an example:
In the case of 5 MHz Bandwidth, it can be found in 802.16e Standard that the
number of all sub-carriers used in a symbol is 408 (409 minus DC sub-carrier).
Then, because the number of sub-carriers for tile is 4, number of tiles is 408/4 = 102.
At the same time, it is written that number of tiles for sub-channel is 6, therefore in a
5 MHz Bandwidth UL PUSC there are 102/6 = 17 sub-channels.
2.2.3
TDD Frame Structure
In spite of 802.16e Physical Layer supports TDD and Full and half-duplex FFD
operations, TDD is defined for the initial mobile WiMAX profiles for its added
efficiency in support of asymmetric traffic and channel reciprocity for easy support
of advanced antenna systems, although TDD does require global synchronization..
When implementing a TDD system, the frame is built from BS and SS
transmissions. Next figure illustrates the OFDM frame structure for a Time Division
Duplex (TDD) implementation. Each frame is divided into DL and UL sub-frames
separated by Transmit/Receive and Receive/Transmit Transition Gaps (TTG and
RTG, respectively) to prevent DL and UL transmission collisions.
14
Figure 2.6: WiMAX OFDMA Frame Structure [19]
Each frame in the transmission starts with a preamble (incorporated in the Downlink
subframe) followed by a DL transmission period and UL transmission period.
Besides preamble, other control information is inserted in the frame structure. In the
Downlink subframe DL-MAP and UL-MAP are used to provide sub-channel
allocation and other control information for the DL and UL sub-frames respectively.
At the same time in this subframe FCH is added to provide frame configuration
information. On the other side, UL subframe incorporates Ranging and optional
CQICH and ACK-CH.
2.2.4
Advanced PHY layer features
Within Mobile WiMAX QPSK, 16-QAM and 64-QAM are mandatory in the DL. In
the UL 64-QAM is optional. It also supports Convolutional Code (CC) and
Convolutional Turbo Code (CTC) with variable code rate. These are the mandatory
modulation features; however Block Turbo Code and Low Density Parity Check
Code (LDPC) are optional. This set of different modulation techniques is commonly
denominated Adaptive Modulation and Coding (AMC) and it provides a fine
resolution of data rates depending of the Mobile Station.
A new feature of Mobile WiMAX is the Channel Quality Information (CQI)
Channel (CGICH) which is used to provide channel-state information from the
15
Mobile Terminal to the Base Station scheduler. This information shall include:
Physical CINR, effective CINR, MIMO mode selection and frequency selective subchannel selection. With this information Base Station scheduler shall determine the
appropriate data rate for each burst allocation. This feature is also mentioned with
802.16-2004 Standard but it completely introduced in Mobile WiMAX.
As in the case of CQICH, Hybrid Auto Repeat Request (HARQ) is briefly described
in 802.16-2004 Standard as an optional feature. HARQ is made capable using N
channel “Stop and Wait” protocol which provides fast response to packet errors and
improves coverage. A dedicated ACK channel is provided in the uplink for HARQ
ACK/NACK signalling which allows fully asynchronous operation. The
asynchronous operations are more flexible to the scheduler at the cost of additional
overhead for each retransmission allocation.
The combination of HARQ, CQICH and AMC increase coverage and capacity for
WiMAX in mobile applications. They provide a robust link adaptation in mobile
environments at vehicular speeds up to 120 km/h.
2.3
MAC Layer Description
Since the beginning 802.16 Standard has been developed for the delivery of
broadband services including voice, data and video. It can support bursting data
simultaneously with streaming video and latency-sensitive voice traffic over the
same channel. Since the resource allocation information is carried in the Map
messages within the beginning of each frame, explained in 3.2.3, the MAC scheduler
can effectively change the resource allocation on any frame. It can vary the resource
to one terminal from a single time slot to the complete frame; this is basic to adapt
the bursting nature of the traffic.
Quality of Service (QoS) Support and Mobility Management are the most important
features for this thesis (they will be further explained) however other features can
not be despised. Mobile WiMAX adds a MAC scheduling service which is designed
to efficiently deliver broadband data services such us voice, data and video overtime
varying broadband wireless channel. This service has the following properties: Fast
Data Scheduler, Scheduling for DL and UL, Dynamic Resource Allocation, QoS
Oriented and Frequency Selective Scheduling.
16
On the other hand, as it is mentioned in the Introduction of this point (IEEE 802.16e
Technical Overview and Amendments to IEEE 802.16-2004), Mobile WiMAX
supports best in class security features by adopting the best technologies available
today. Mobile WiMAX can support device/user authentication, flexible key
management protocol, strong traffic encryption, control and management plane
message protection and security protocol optimization for fast handovers.
2.3.1
Quality of Service (QoS) Support
In the Mobile WiMAX MAC Layer, QoS is provided via service flows. This service
is a unidirectional flow of packets which are provided with a particular set of QoS
parameters. The Base Station and the Subscriber Station establish a unidirectional
logical link between the peer MACs called connection. After connection
establishment, the QoS parameters, depending of the application, define the
transmission ordering and scheduling on the air interface. Therefore QoS
(connection-oriented) can provide exact control over the air interface. This service
flow is supported for both directions, DL and UL. The management of service flow
parameters is realized by exchange of MAC messages.
17
Table 2.1 Mobile WiMAX Applications and Quality of Service [19]
QoS Category
Applications
QoS Specifications
UGS
Unsolicited Grant Service
VoIP
Maximum Sustained Rate
Maximum Latency Tolerance
Jitter Tolerance
rtPS
Real-time Polling Service
Streaming Audio or Video
Minimum Reserved Rate
Maximum Sustained Rate
Maximum Latency Tolerance
Traffic Priority
ErtPS
Extender Real-Time Polling
Service
Voice with activity
nrtPS
Non-Real-Time Polling Service
File Transfer Protocol
BE
Best-Effort Service
Data transfer, Web
Detection (VoIP)
(FTP)
browsing, etc
Minimum Reserved Rate
Maximum Sustained Rate
Maximum Latency Tolerance
Jitter Tolerance
Traffic Priority
Minimum Reserved Rate
Maximum Sustained Rate
Traffic Priority
Maximum Sustained Rate
Traffic Priority
Mobile WiMAX supports a wide range of data services and applications with diverse
QoS requirements. These different applications are summarized in the table shown
above.
2.3.2
Mobility Management
The IEEE 802.16e Standard defines a framework for supporting mobility
management. Battery life and handoff are two critical issues for mobile applications.
Power Management
In order to support battery-operated portable devices, mobile WiMAX has powersaving features that permit portable Subscriber Stations to operate for longer periods
of time without having to recharge. Power saving is achieved by turning off parts of
the Mobile Station in a controlled manner when it is not actively transmitting or
receiving data. Mobile WiMAX defines signaling methods that allow the Mobile
Station to retreat into two different modes: sleep mode or idle mode when inactive.
Sleep mode is a state in which the MS effectively turns itself off and becomes
unavailable for predetermined periods, to DL or UL traffic. The periods of absence
are negotiated with the serving Base Station. Sleep mode is intended to minimize
18
Mobile Station power usage and therefore air interface resources. To facilitate
handoff while sleep mode, the MS is allowed to scan other Base Stations to collect
information related with handoff.
Idle mode permits even greater power savings. Idle mode allows the MS to
completely turn off and to become periodically available for DL broadcast traffic
messaging without registration at a specific Base Station as the MS cross an air link
environment populated by many Base Stations. Idle Mode profits the Mobile Station
by eliminating the requirement for handoff and other normal operations and benefits
the network and Base Station by removing air interface and network handoff traffic
from mainly inactive Mobile Stations while still providing a simple and opportune
method (Paging) for alerting the Mobile Station about pending DL traffic.
Handoff
The IEEE 802.16e Standard defines signaling mechanisms for tracking Subscriber
Stations as they move from the coverage range of one base station to another when
active mode or as they move from one paging group to another when idle mode. The
Standard supports seamless handoff to permit the MS to switch from one BS to
another at vehicular speeds without interrupting the connection.
Three handoff methods are supported in IEEE 802.16e Standard; one is mandatory
and other two are optional. The mandatory handoff method is called the hard handoff
(HHO) and is the only type required to be implemented by mobile WiMAX certified
products initially. HHO implies an abrupt exchange of connection from one BS to
another. The handoff decisions are made by the BS, MS, or another entity, based on
measurement results reported by the Mobile Station. The Mobile device periodically
does scan of frequency and measures the signal quality of neighbouring Base
Stations. During these intervals of time, the Mobile device is also allowed to
optionally execute initial ranging and to associate with one or more neighbouring
Base Stations. Once a handover decision is made, the MS begins synchronization
with the DL transmission of the objective BS, performs ranging if it has not been
realized while scanning, and then terminates the connection with the previous BS.
Any undelivered MAC PDUs at the BS are retained until the timer finish.
The two optional handoff methods supported by Mobile WiMAX are Fast Base
Station Switching (FBSS) and Macro Diversity Handover (MDHO). In both
19
methods, the MS maintains a valid connection at the same time with more than one
BS. In the FBSS case, the MS maintains a list of the Base Stations included, this list
is called the Active Set. The MS constantly monitors the Active Set, performs
ranging, and maintains a valid connection ID with each of them. However the MS
only communicates with the Anchor BS, from the Active Set, for uplink and
downlink messages including management and traffic connections. When a change
of anchor BS is required, the connection is switched from one base station to another
without invocation of explicitly handoff signaling messages. The MS simply reports
the selected anchor BS on the CQICH. An important requirement of FBSS is that
data is simultaneously transmitted to all members of an Active Set of Base Stations
that are able to serve the MS.
Macro diversity handover is similar to FBSS about Active Set and Anchor BS,
except that the MS communicates with all the Base Stations in the Active Set
simultaneously of downlink and the uplink unicast messages and traffic. A MDHO
commences when a MS decides to transmit or receive unicast messages and traffic
from various Base Stations in the same time interval. In the downlink, multiple
copies received at the MS are combined using diversity combining techniques. In the
uplink, the MS sends data to multiple Base Stations, where selection diversity of the
information received is performed to pick the best uplink.
Both FBSS and MDHO offer better performance to HHO, but they require that Base
Stations are synchronized, use the same carrier frequency and share network entry
information. Support for FBHH and MDHO in Mobile WiMAX networks is not
fully developed yet and is not part of WiMAX Forum Release 1 network
specifications.
2.4
Comparison between 802.16-2004 and 802.16e profiles
The amendments introduced in 802.16-2004, by incorporating features of previous
versions, were focused on fixed and nomadic applications in the 2-11 GHz. Two
multi carrier modulation techniques are supported in 802.16-2004: OFDM and
OFDMA with 256 and 2048 sub-carriers respectively. Although the Standard
supports both modulation techniques, first WiMAX Forum certification profiles are
based on OFDM.
20
At the same time, as 802.16-2004, 802.16e-2005 incorporates features of previous
versions and adds support for mobile access. This mobility support is based on
Scalable OFDMA. This aspect will be studied since different modulation techniques
mean different performances. Since they are not the same modulation technique,
OFDM and OFDMA are not compatible.
There are several optional features that are supported in 802.16-2004 profile and
they are implemented in 802.16e in order to obtain a better performance for mobile
services. Among these functionalities, improved support for MIMO and AAS will
contribute for a considerable increase in throughput and NLOS capabilities. In the
same set of functionalities, HARQ and CQICH are mentioned in 802.16-2004 as
optional features, further explained in paragraph 3.2.4.
2.4.1
OFDM and OFDMA
A key difference between Fixed and Mobile WiMAX is the multiplexing technique:
they use OFDM and OFDMA respectively. OFDM multiplexing technique is less
complex than Scalable OFDMA, thus 802.16-2004 WiMAX Forum Certified
products are supposed to be lower cost than future Mobile WiMAX products.
Therefore Fixed WiMAX network may be deployed faster by using directional
antennas.
On the other hand, OFDMA gives 802.16e profiles more flexibility for managing
different devices with variety of antenna types and form factors. This means a
reduction in interference with omnidirectional antennas and improved NLOS
capabilities. Within this multiplexing technique, subchannelization is defined as a
group of different sub-channels which can be allocated to different subscribers
depending of the channel condition and their data requirements. These features
introduce wide flexibility in managing the bandwidth and transmit power.
21
OFDM
Frequency (sub-carriers)
OFDM sub-carriers
OFDMA
Group M-1
Group 2
Group M
{
Group 1
N sub-carriers
Pilot
Sub-channel 1
Sub-channel 2
Sub-channel 3
}
Frequency (sub-carriers)
OFDMA Sub-carriers
Figure 2.7: OFDM and OFDMA [20]
In the figure above it can be noticed that in OFDM all sub-carriers are transmitted in
parallel with the same amplitude. Contrary, OFDMA divides the sub-carriers space
into M sub-channels with N sub-carriers each sub-channel. For instance, OFDMA of
1024 sub-carriers is divided in 30 sub-channels of 28 sub-carriers in the Downlink
and 35 sub-channels of 24 sub-carriers in the Uplink, including in these groups Data
and Pilot sub-carriers. Coding, modulation and amplitude are set separately for each
sub-channel based on channel conditions to optimize the use of network resources.
In OFDM subscriber devices are assigned time for Uplink transmissions. The slot
can be only used by one user device, it can be observed in the next figure where the
first user will use every 256 sub-carriers for the transmission. Contrary, in OFDMA,
subchannelization in the Uplink enables various Subscriber Stations to transmit at
the same time over the sub-channel(s) allocated to them.
22
Sub-channels
Carriers
Figure 2.8: Uplink in OFDM and OFDMA [20]x
OFDMA within 802.16e Standard has additional advantage since it can scale the
number of FFT according to the channel Bandwidth (128, 512, 1024 or 2048). It
changes the number of FFT in order to keep the sub-carrier spacing constant across
different channel Bandwidths. By keeping this value the bandwidth is better utilized.
2.4.2
Handoffs
Support for handoffs is another key addition in the 802.16e amendment for mobile
access. The capacity of maintaining a connection, while moving across coverage
borders of Base Stations, is a prerequisite for mobility and it is included as a
requirement in the 802.16e Standard. While the 802.16-2004 Standard offers
optional handoff capabilities, support for handoffs is not required by the Fixed
WiMAX profiles.
Mobile WiMAX will support different types of handoff, from hard to soft and it will
be up to the operator to choose among them, although only Hard Handoff (HHO)
will be mandatory. Hard handoffs use a period of time before make the approach, the
user device is connected to only one Base Station at any given time, which is less
complex than soft handoffs but has a higher latency. Soft handoffs are comparable to
those used in some cellular networks and allow the user device to retain the
connection to a Base Station until it is associated with a new one, thus reducing
latency. While applications like mobile Voice over Internet Protocol (VoIP) or
gaming greatly benefit from low-latency soft handoffs, hard handoffs typically
suffice for data services.
23
3.
ADDITIONAL INFORMATION
As it is mentioned before in the Introduction, the investigation of coverage and
theoretical maximum throughput in the new 802.16e Standard, Mobile WiMAX is
the aim of the Thesis. For this target, it is essential explaining the process from the
beginning.
Performance analysis requesting information can be almost completely obtained
from 802.16-2004 and 802.16e-2005 Standards, with the complement of WiMAX
Forum certified profiles. Here it can be found out the necessary information for the
calculations of theoretical maximum throughput. This value will be decreased
mainly because three factors: PDU header, guard time and pilot sub-carriers. This
information is not only related with the Physical Layer, also MAC Layer influences
in the maximum throughput.
For the case of coverage prediction, principally, it must be shown the basic formula
of a link budget:
Pr = Pt + Gt − PL + Gr
(3.1)
where Pr is the minimum received power in the receiver, is usually related with the
Sensitivity, Pt is the transmitted power, Gt is the gain of the transmitter, PL is the
Path Loss, it depends of the environment and Gr is the gain of the receiver.
Therefore, all of these parameters must be explicated before being used. Thus, in the
next point, all the parameters are further explained. First of all, it must be shown the
specifications of 802.162 Standard, although this information will not be used for the
receiver sensitivity since the thesis will be more focused in the actual certificated
products. For this reason some of the certificated products will be studied. At the
same time, different Propagation Model will be examined in order to get correct
information for each environment.
This link budget would not have purport without information about the work
frequency, since it is known for all that Frequency conditions completely the
24
calculations. In this point, at the same time, number of sub-carriers is further
described depending of the frequency band and direction; downlink or uplink.
3.1
Standard Mobile WiMAX parameters
First of all, Standard Mobile WiMAX parameters must be described. All of this
information can be found in 802.16-2004 and 802.16e-2005 Standards. These data
are divided into two groups according to the target of investigation. The first point is
related with the maximum radius coverage and the following one is relevant with
maximum throughput.
3.1.1
Receiver sensitivity in Mobile WiMAX technology
Although the minimum sensitivity for the receiver is not going to be used according
to the Standard, this value has extremely importance since is the principle for
certificated products. The receiver minimum sensitivity level, RSS, is derived
according to next Equation [1]:
⎛F N
Pr ,min = −114 + SNR Rx − 10 log R + 10 log⎜⎜ S used
⎝ N FFT
⎞
⎟⎟ + Im pLoss + NF
⎠
(3.2)
where,
SNRRx is the receiver SNR as in the Table 3.1.
R is the repetition factor. It can take the next values: 2, 4 or 6. The difference
between the best case (R = 6) and the worst case (R = 2) is 4.77dB.
FS is the sampling frequency in MHz (4.1.2.1).
ImpLoss is the implementation loss, which includes non-ideal receiver effects such
as channel estimation errors, tracking errors, quantization errors, and phase noise.
The assumed value is 5 dB.
NF is the receiver noise figure, referenced to the antenna port. The assumed value is
8 dB.
25
Table 3.1 Receiver SNR assumptions [1]
Modulation
QPSK
16-QAM
64-QAM
Coding Rate
Receiver
SNR (dB)
1/2
5
3/4
8
1/2
10.5
3/4
14
1/2
16
2/3
18
3/4
20
This table has been modified in 802.16e-2005 Amendment, their values have been
changed. And they can be modified further, depending of the new WiMAX profiles.
It is important to notice that Nused in this case is the set of pilot and data sub-carriers
because it is related with the required power for those usable sub-carriers. Remind
that pilot sub-carriers are used for management information.
According to this formula, some values have been calculated for QPSK, 16-QAM
and 64-QAM Modulation and 5 MHz and 10 MHz Channel Bandwidth. These
parameters have been utilized since they are the most interesting for this thesis. It
will notice below, in others paragraphs, that there already are these two channels
certified by WiMAX Forum.
These calculated values will be compared with the values provided by the device
producers in order to find out the differences and similitudes.
26
Table 3.2 Receiver Sensitivity (dB) according 802.16e Standard
QPSK
16-QAM
64-QAM
5 MHz
10 MHz
Nfft
512
1024
Nused
420
840
1/2
5
-92,30
-89,29
3/4
8
-89,30
-86,29
1/2
10,5
-86,80
-83,79
3/4
14
-83,30
-80,29
1/2
16
-81,30
-78,29
2/3
18
-79,30
-76,29
3/4
20
-77,30
-74,29
Above minimum required receiver sensitivity according to 802.16e-2005 Standard is
shown. This is the minimum value required by the Standard however companies
may make even better products with a lower Noise Figure which is the main point of
improvement.
3.1.2
Performance Analysis
Different literatures use various different statics in measuring the performance of
wireless network. Throughput is defined as the amount of data (bits) transferred
successfully from one node to another in a specified amount of time. This definition
is the same in Mobile WiMAX where it can be found the formula to calculate it in
the Standard. At the same time, it can be forgot that many aspects in the MAC layer
decrease the total throughput such as: header, subheader, CRC, preamble, etc.
Therefore, it is also important to explain all of these overloads.
Physical layer throughput
As it is mentioned before the physical layer throughput is defined in the Standard
although it only defines it as all people know; amount of data (bits) transferred
successfully from one node to another in a specified amount of time. Thus, in
802.16-2004 Standard [4]rata bit rate is defined for OFDM physical layer as:
R=
N used bm c r
TS
(3.3)
27
where bm is the number of bits per modulation symbol and cr is the coding rate. The
symbol duration TS, according to Figure 2.2, is given by
TS = Tg + Tb
(3.4)
= [G + 1]Tb
where G is the ratio Tg/Tb, this value can be: 1/4, 1/8, 1/16 or 1/32. And Tb = 1/Δf,
with the sub-carrier spacing Δf is given as
Δf =
FS
N FFT
(3.5)
and,
⎞8000
FS = floor ⎛⎜ n BW
⎟
8000
⎝
⎠
(3.6)
where FS is the sampling frequency, n is the sampling factor, BW is the nominal
channel bandwidth and NFFT is the number of points for FFT.
The Sampling factor in conjunction with BW and Nused determines the sub-carrier
spacing, and the useful symbol time. This value has changed from OFDMA 802.162004 Standard is set to 8/7 as follows: for channel bandwidths that are a multiple of
1.75 MHz then n = 8/7 else for channel bandwidths that are a multiple of any of
1.25, 1.5, 2 or 2.75 MHz then n = 28/25 else for channel bandwidths not otherwise
specified then n = 8/7.
This is the theoretical and direct calculation however it should take into account that
for example that TS period corresponds to Figure 2.2, therefore R must be reduced in
a factor of 4/5, 8/9, 16/17 or 32/33 according to the configuration. Only those rates
of TS period are used for data payload.
At the same time, it can be noticed in Figure 2.3 that not every OFDMA symbol is
used for data. According to [5] Mobile WiMAX uses only 44 data OFDMA symbol
from a total of 48. This reduction of performance will take into account in the
calculations.
28
Next figure shows information about two different possible bandwidths 5MHz and
10MHz which will be used for the research. Number of sub-channels are calculated
according to paragraph 2.2.2 depending if it is downlink or uplink.
Table 3.3 OFDMA scalability parameters
Parameter
Downlink
Uplink
Downlink
Uplink
System Bandwidth
5 MHz
10 MHz
FFT size
512
1024
Null sub-carriers
92
104
184
184
Pilot sub-carriers
60
136
120
280
Data sub-carriers
360
272
720
560
Sub-channels
15
17
30
35
Frame duration
5 milliseconds
OFDMA symbol/frame
48
Data OFDMA symbols
44
Within the table it can be observed that number of Data OFDMA symbols is only 44
from 48. This is because 1.6 symbols are used for TTG and RTG gaps and the rest
from 4 OFDMA symbols are used for locating other useful information such as ULMAP, DL-MAP or FCH.
MAC layer overload
Connections are also affected but the MAC Layer. All MAC PDUs are required to
load some overload for different purposes. Minimum overload would be 10 bytes (6
bytes of header and 4 bytes of CRC) of a total amount of 2047 bytes. This load is not
considerable by if MAC PDUs must to be overload with different subheaders such
us Mesh, Fragmentation or Packing subheaders, this will influence in the efficiency
of the connections by decreasing the maximum throughput.
29
Below some different MAC PDU formats are shown:
Generic MAC
Mesh Subheader
header (6bytes)
(2bytes)
Payload (optional)
Generic
Other
Fragmentation
Payload (One SDU or fragment
MAC Header
subheaders
subheader
of an SDU)
Generic
MAC
header
Grant Management subheader
(UL only)
Packing
subheader
Payload (One SDU or
SDU fragment or a set
of ARQ Feedback IEs)
Packing
subheader
Payload (One SDU
or SDU fragment)
CRC
(4bytes)
CRC-32
CRC-32
Figure 3.1: Different MAC PDU formats
Although it is clear that these different configurations would have different
performances, this influence is not going to be studied since the complexity of
developing a formula for knowing how many PDU would be fragmented or packed
for instance.
3.2
Current equipment parameters
There are various devices which can perform Mobile WiMAX. There exist many
different certified products from few companies since, although most of wireless
companies are interested on developing WiMAX devices, they did not get yet a
certified product. In the next paragraphs some of these certified products will be
presented, focusing in the most important features for the research, such as:
Frequency Band, Channel Size (FFT), Minimum sensitivity, transmitter and receiver
gain and transmitted power.
There are two important aspects for noticing. First one, although devices are able to
transmit a high power, this transmitted power is limited by CPE (Costumer Promise
Equipment); therefore for calculations these supervised values will be used. The
second important aspect is that, since Mobile WiMAX supports VoIP (Voice over
IP), connection between Base Station and Mobile Station can be limited by uplink
30
given that the transmitted power of the Mobile Station is lower than Bases Station’s.
This is because Mobile Station will be usually closer to people that Base Stations.
Anyway coverage prediction will use Base Station as transmitter and Mobile Station
as receiver in order to ease calculations.
3.2.1
Available Products
Since Mobile WiMAX profiles have not been developed yet, it is especially difficult
to describe the properties of the future devices. In the cases there are not data sheets
of Base and Subscriber Station, the aim of the thesis will be finding the new
Sensitivity from Fixed WiMAX devices.
Airspan has already developed future device for Mobile WiMAX therefore, the
values of gain, transmitted power and Sensitivity will be directly accepted. In other
cases calculations will have to be realized. This causes that table made below with
all companies contains information contributed by the company’s website (Airspan)
and calculated information from company’s website. Then, table will be made just to
provide an idea of different devices, but it will not be very useful for a acceptable
comparison.
In order to determine new sensitivity value for Mobile WiMAX next formulas will
be used. Thus, Noise is defined as:
N = −174 + 10 log BW + NF
(3.7)
and sensitivity:
S min = N + SNR
(3.8)
then,
S min = −174 + 10 log BW + NF + SNR
(3.9)
Within these formulas, in those cases where Companies have not already developed
equipments for Mobile WiMAX, the values of Bandwidth, Noise Figure and SNR
will changed in accordance with 802.16-2004 and 802.16e-2005 Standards.
Most of equipment manufacturer define the sensitivity at the lowest bandwidth and
modulation, usually 3.5 MHz at BPSK 1/2. Moreover, since their products are
31
designed for Fixed WiMAX they are developed for OFDM PHY layer. Then, they
use SNR requirements from point 8.3.11.1 in [5] and for this thesis the values must
be in accordance with SNR requirements from point 8.4.13.1.1 in [1].
At the same time Bandwidth (in this thesis 5 and 10 MHz) and Noise Figure must be
in accordance with the new requirements from 802.16e-2005 Standard. Therefore
Noise Figure will have a value of 8 dB instead of the 7 dB used in 802.16-2004
Standard. And the sensitivity will be modified in accordance with the new
Bandwidth.
Thus, for instance, when a Subscriber Station designed for Fixed WiMAX
(developed for OFDM PHY) had a sensitivity of -98 dBm at BPSK 1/2 (SNR = 6.4
dB) and bandwidth of 3.5 MHz, new sensitivity for QPSK 1/2 (SNR = 5 dB) and 5
MHz bandwidth would be:
⎞ + (−7 + 8) ≈ −97 dBm (3.10)
S NEW = −98 + (−6.4 + 5) SNR + 10 log⎛⎜ 5 MHz
NF
3.5 MHz ⎟⎠
⎝
In accordance with this calculation, sensitivity for every Fixed WiMAX device will
be calculated. Thus, comparison with Table 3.2 can be realized in order to find a
value for sensitivity which represents the logic average of all of those values.
Some WiMAX companies are described below. Only main companies have been
selected from WiMAX Forum Certified products. All of these products are already
certified for Fixed WiMAX, but few presented devices for Mobile WiMAX did not
get the certification yet.
Airspan
The first product of Airspan was based on CDMA radio technology. It was adapted
for fixed wireless access and was a market success. The company currently provides
a wide range of WiMAX Base Stations and customer premise devices. The company
currently has over 100 engineers developing Mobile WiMAX solutions.
This company has already made an effort to develop new and old device for the new
standard IEEE 802.16e. Some of the devices will be able to work with Mobile
WiMAX in the future however they are not capable to operate in 2.3 GHz frequency
band, therefore these devices will not be interesting for this thesis. They are EasyST-
32
2 and ProST-2. On the other hand in Airspan’s catalogue it can be found two
possible Base Stations and one Mobile Station:
•
Base Station: MicroMAXe and MacroMAXe. Both initially support
5MHz and 10MHz channel sizes. However, the product is capable of
supporting 20MHz channels (Mobile WiMAX profile Rel. 1.5) as well.
Both have been designed to support either 2x10MHz and 2x5MHz (using
dual PHY/MAC) or 1x20MHz channel. They support 512, 1024 and 2048
FFT Scalable OFDMA. MacroMAXe is optimized for 2.3 GHz and 2.5
GHz frequency bands whereas MicroMAXe comprises wider range of
frequency bands.
•
Mobile Station: It consists in a WiMAX USB and it supports Mobile
WiMAX. The WiMAX USB packs a big RF performance despite it is
diminutive size delivering up to +22dBm into the antenna. The product is
capable of supporting 10MHz, 8.75MHz, 7MHz and 5MHz (1024 and
512 FFT Scalable OFDMA). It can operate in wide range of frequency
bands as MicroMAXe.
Alvarion
Alvarion was one of the first companies to produce 802.11 WLAN equipments.
Within WiMAX system, currently, BreezeMAX is the most important product for
Alvarion. It keeps the companies into the selected company’s BWA industry
leadership. At the same time Alvarion will be one of the first companies offering
mobile WiMAX although only new family’s member BreezeMAX 2300/2500/3500
were ready when this thesis was written.
Then, BreezeMAX is the family of WiMAX products for Alvarion. It comprises
Base Station as well as Subscriber Station (indoor and outdoor). Within this family
BreezeMAX 2300/2500/3500 Subscriber Stations will be used for getting required
information and , on the other hand, general features for Base Stations will be
obtained from [16] (alv_BreezeMAX_pbp.pdf).
•
Base Station: BreezeMAX Base Station solution features advanced
OFDM technology to support NLOS operation, adaptive modulation up
to QAM64, and the highest spectral efficiency available. Operating in the
33
3.3, 3.5 and 3.6 GHz licensed frequency bands. Different bandwidth can
be software selected (3.5 and 1.75MHz).
•
Mobile Station: This Mobile Station is operating in 2.3, 2.5, and 3.5
GHz and related licensed frequency bands. The product is capable of
supporting 10MHz, 7MHz, 5MHz and 3.5MHz (software selectable).
Although it is designed for 802.16e-2005 Standard, it does not support
yet Scalable OFDMA. The sensitivity is also described for OFDM FFT
256, therefore it must be also extrapolated. Antenna has different gains
depending on the frequency band. It is TDD-based platform.
Axxcelera
Axxcelera Broadband Wireless is a data networking solutions company, developing
technology to deploy networks for broadband wireless communications over
Internet. ExcelMAX and AB-MAX fixed wireless broadband platforms are used to
bridge the last mile of broadband wireless communications, with a point-tomultipoint solution. Axxcelera has put together their ExcelMAX and Excel Air
products for Licensed WiMAX, and their AB MAX and AB Access for Unlicensed
WiMAX applications.
•
Base Station: Axxcelera’s ExcelMAX Base Station is a Point to
Multipoint (PMP) BS product designed to operate in the 3.3-3.8 GHz
spectrum and supports Full FDD (Frequency Division Duplex)
architecture. The 802.16-2004 Standard compliant NLOS platform
supports a strong suite of Quality of Service (QoS) features and multiple
services. The product is capable of supporting 14 MHz (optional), 7
MHz, 3.5 MHz and 1.75 MHz bandwidths. Antenna has different gains
depending on the degrees; 60 or 90.
•
Subscriber Station: Axxcelera’s ExcelMAX Indoor CPE is a selfinstallable Point to Multipoint (PMP) CPE designed to operate in the 3.33.8 GHz spectrum and supports a Half Duplex FDD (Frequency Division
Duplex) or TDD (Time Division Duplex) architecture. NLOS operations
are supported. It works only in 7 and 3.5MHz channel size. And the
antenna has a gain of 10dBi with 90 degrees. The ExcelMAX CPE 3400
supports also a strong suite of Quality of Service (QoS) features,
34
including Committed Information Rate (CIR), Peak Information Rate
(PIR), BE, nrtPS, rtPS, and UGS.
Redline
Redline Communications is a technology company specialized in the design and
manufacture of standards based on broadband and wireless access solutions. Redline
launched its family of WiMAX products called RedMAX. The RedMAX family
includes products for each of the 802.16 variants (a, 2004) and to guarantee also that
future of the next amendment of IEEE 802.16 Standard (802.16e) to support
mobility, although they were available when this thesis was written.
•
Base Station: RedMAX AN-100U is a High performance PMP Base
Station platform. It operates in the 3.3 to 3.5; 3.4 to 3.6; and 3.6 to 3.8
GHz RF bands. It supports 2nd generation 802.16 MAC layer and 3rd
generation OFDM PHY layer, 7 and 3.5 MHz bandwidth are supported.
Maximum transmitted Power is 23 dBm across all modulation/coding
levels. It has a extremely low latency and superior reliability. Dynamic
QoS.
•
Subscriber Station: RedMAX Subscriber Unit is 802.16-2004
Standard compliant. It operated in the 3.3 to 3.5; 3.4 to 3.6; and3.6 to 3.8
GHz RF bands. This Subscriber Station has self-installation. 7 and 3.5
MHz bandwidth are supported as well as extremely low latency. 24 dBm
is the maximum transmitted Power. Dynamic Quality of Service (QoS)
settings are also supported.
Telsima
Telsima Corporation is a leading developer and provider of WiMAX based
Broadband Wireless Access and mobility solutions for media rich applications. The
Company develops and markets Base Station and Subscriber Station systems and
network management software for the WiMAX telecommunications market.
They have developed already two Subscriber Stations for 802.16e Standard
(StarMAX 3100 and 3200), however, in the website too few information is provided
about these future products. Therefore, in the same line than previous companies, old
devices will be used in order to provide guiding information.
35
This family is called StarMAX and it contains Base Station (6400) and Subscriber
Station (2100). Actually, they are both Fixed WiMAX-designed, however, they can
be configured to support of IEEE 802.16e-2005 Mobile WiMAX. Thus they would
support Scalable OFDMA. Anyway, future Mobile WiMAX features are not deeply
explained therefore information from Fixed WiMAX data sheet will be used.
•
Base Station: Telsima’s StarMAX 6400 Base Station is a Point to
Multipoint (PMP) BS product designed to operate in the 3.30-3.40 GHz,
3.40-3.60GHz and 2.50-2.69 GHz spectrum and supports TDD as duplex
method. The 802.16-2004 Standard compliant NLOS platform supports a
strong suite of Quality of Service (QoS) features and multiple services.
The product is capable of supporting 7 MHz, 6 MHz, 3.5 MHz and 3
MHz bandwidths, software configurable. Moreover, bandwidth can be
configured form 1.5 to 14 MHz in 250 KHz steps on request. The
maximum transmitted power is 30 dBm.
•
Subscriber Station: StarMAX 2100 Subscriber Unit is 802.16-2004
Standard compliant. It operates in the 2.50-2.69 GHz, 3.30-3.40 GHz,
3.41-3.60 GHz RF bands. The product is capable of supporting 7 MHz, 6
MHz, 3.5 MHz and 3 MHz bandwidths, software configurable. Moreover,
bandwidth can also be configured form 1.5 to 14 MHz in 250 KHz steps
on request. 20 dBm is the maximum transmitted Power. There are three
different Subscriber Station; 2130, 2140 and 2160, with different gains.
3.2.2
Comparison among products
A comparison in terms of the main performance is reported into two different tables.
First one is filled for Base Stations and second one for Subscriber Stations. More
features than required are described within the tables. It is important to distinguish
between actual Mobile WiMAX devices and Fixed or coming Mobile WiMAX
devices.
In those cases where Sensitivity is described in the data sheet for Fixed WiMAX, the
value has been extrapolated as it is explained before. Thus, these values would
theoretically be in accordance with the new 802.16e Standard.
36
Base Stations
In the case of Base Stations information is shown as: Firstly Company and Model
are shown, afterwards all interesting and desired information for this thesis. Used
WiMAX Standard will affect all features of the Base Stations.
About Frequency Bands; only TDD is certified by WiMAX Forum for the first
profile (it will be further explained in the next point 4.3). It is the same about
Bandwidth since it is depends of the new Mobile WiMAX profiles, and the
Bandwidth would determinate the FFT size while within Fixed WiMAX only 256
OFDM is used. Gain and maximum transmitted power have a similar influence of
the Fixed WiMAX. They do not use MIMO systems and other techniques to improve
the connections. Thus, new devices will enhance the performance.
37
Table 3.4 Base Stations information
Producer
Airspan
Airspan
Alvarion
Axxcelera
Redline
Telsima
Model
MicroMAXe
MacroMAXe
BreezeMAX
ExcelMAX
BS
RedMAX
BS AN100U
StarMAX
6400
802.16e2005
802.16e2005
802.16-2004
802.16-2004,
upgrade to
802.16e
planned
802.16-2004
802.16-2004,
future 802.16e
2.3GHz,
2.5GHz,
3.3GHz,
3.5GHz,
3.7GHz
2.3GHz,
2.5GHz
3.3GHz,
3.5GHz,
3.6GHz FDD
3.4GHz,
3.6GHz
3.3-3.5; 3.43.6; 3.63.8GHz
2.50-2.69,
3.40-3.60,
3.30-3.40GHz
20MHz,
2x10MHz,
10MHz,
5MHz
20MHz,
2x10MHz,
10MHz,
5MHz
3.5MHz,
1.75MHz
14MHz (opt),
7MHz,
3.5MHz,
1.75MHz
7MHz,
3.5MHz
7MHz, 6MHz,
3.5MHz,
3MHz (SW
configurable)
N.D.
N.D.
-99dBm for
OFDMA
5MHz @
QPSK 1/2
-99dBm for
OFDMA
5MHz @
QPSK 1/2
-97dBm for
OFDMA
5MHz @
QPSK 1/2
-99dBm for
OFDMA
5MHz @
QPSK 1/2
N.D.
N.D.
17 dBi typical
16dBi (60
degree), 14dBi
(90 degree)
N.D.
N.D.
28dBm
+28.0 dBm
(BPSK)
+23 dBm
across all
modulation/
coding levels
30 dBm
WiMAX
Standard
Frequency
Bands
Channel
Size
Sensitivity
Antenna
Gain
Tx. Power
Up to 2 x
+36dBm
2x +40dBm
This table will not determine the used Frequency bands for the final calculations
since it is required to follow ETSI Frequency usage plan, explained in the next point.
It will condition the bandwidth as well as the frequency bands, although this thesis is
focused in 5 and 10 MHz bandwidth.
At the same time maximum transmitted power for the Base Station is limited by the
CPE to 20 dBm. Thus, Tx. Power information is only useful in order to know what
the limit of the transmitter would be. On the other hand, at least, gain of Base Station
for calculations can be accepted as approximately 16 dBi.
38
Mobile Stations
This second table has a very similar performance. It has exactly the same structure.
And it also has parallel disadvantage since it is filled as well as before with Fixed
WiMAX certified products, except of Airspan’s and Alvarion’s Subscriber Stations.
Thus, for instance, Axxcelera and Redline Subscriber Station do not work in 2.5
GHz Frequency Band and 5 and 10 MHz Bandwidth. This can be noticed in the next
table:
Table 3.5 Subscriber Station information
Producer
Airspan
Alvarion
Axxcelera
Redline
Telsima
Model
MiMAX USB
BreezeMAX
(2300/2500/3500)
ExcelMAX
Indoor CPE
RedMAX
Subscriber
Unit (SU-I)
StarMAX
2100
802.16e-2005
802.16e-2005
802.16-2004
802.16-2004
802.16-2004,
future 802.16e
3.3-3.8 GHz
3.3-3.5; 3.43.6; and 3.63.8GHz
2.502.69GHz,
3.403.60GHz,
3.30-3.40GHz
WiMAX
Standard
Frequency
Bands
Channel
Size
Sensitivity
2.3-2.4GHz,
2.4962.69GHz,3.33.8GHz, 4.95.8GHz
10MHz,
8.75MHz,
7MHz, 5MHz
10MHz, 7MHz,
5MHz, 3.5MHz
7 MHz,
3.5MHz
3.5 MHz,
7MHz
7MHz,
6MHz,
3.5MHz,
3MHz (SW
configurable)
-100dBm for
QPSK 1/2
OFDMA @
5MHz
-98dBm for
QPSK 1/2
OFDMA @
5MHz
-92dBm for
QPSK 1/2
OFDMA @
5MHz
-97dBm for
OFDMA
5MHz @
QPSK 1/2
-97dBm for
OFDMA
5MHz @
QPSK 1/2
Antenna
Gain
Tx. Power
2.3GHz, 2.5GHz,
3.5GHz
N.D.
up to 17dBm
(4.9-5.8GHz)
Up to 22dBm
(others)
2130:
10.5/8dBi
Outdoor 14 dBi
(2.5GHz)
10dBi (90º)
Indoor 7 dBi
(2.5GHz)
N.D.
2140: 7/5dBi
2160:
16/14dBi
ND
24dBm
39
Up to +24
dBm
20dBm
In the previous table all sensitivities are described for Mobile WiMAX OFDMA. In
the cases they were presented in the data sheets for Fixed WiMAX they have been
extrapolated as it is explained before.
Although there are various characteristics shown for Base and Subscriber Stations
only Tx. power and gain will be used for Base Stations and sensitivity and gain for
Mobile Stations. This is due to evaluation is focused in downlink. In this situation
Base Station is the transmitter and Mobile Station the receiver.
Figure 3.2: Uplink and Downlink
Since a lonely Subscriber Station possess sensitivity for Mobile WiMAX, it is
difficult to decide the value of the sensitivity in accordance with 802.16e Standard.
Thus, a value between estimated devices sensitivities and Standard requirements
could be -94 dBm. And the gain would take a value of 8 dBi approximately since
there is not so much information about gain in Mobile WiMAX devices and 8 dBi at
the same time is more restrictive.
3.3
ETSI Frequency usage plan
WiMAX is designed to operate in the frequency range 2 to 66 GHz, however, most
of interest is focusing on the 2 to 6 GHz range where LOS and NLOS scenarios are
supported and in the new Standard 802.16e mobile operation is possible.
There are three main bands being considered for Mobile WiMAX implementation in
Europe: 2.3 GHz and 3.5 GHz for licensed bands and 5.8 GHz for unlicensed bands.
In addition, there are further bands which are being considered by the WiMAX
forum: 2.5 GHz, 2.8 GHz and 3.3 GHz. The position with respect to the availability
of this spectrum in Europe varies from country to country, since each country
40
supervises the frequency spectrum. It makes it difficult to see how any European
decision could be taken on an allocation.
As one can see, the IEEE 802.16 standard is quite flexible in terms of operation
frequency, supporting both license and license-exempt bands. However, only
licensed bands have been approved by the WiMAX Forum for the initial system
profiles.
The WiMAX Forum has thus far defined five fixed certification profiles and
fourteen mobility certification profiles (see Table). To date, there are two fixed
WiMAX profiles against which equipment have been certified:
41
Table 3.6 Frequency usage plan
Frequency Band
Channel Width OFDM FFT size
Duplexing
(Band index)
Fixed WiMAX 802.16-2004
3.5 GHz
(1)
5.8 GHz
(2)
3.5 MHz
256
FDD
3.5 MHz
256
TDD
7 MHz
256
FDD
7 MHz
256
TDD
10 MHz
256
TDD
Mobile WiMAX 802.16e-2005
2.3 - 2.4 GHz
(1)
2.302 - 2.32 GHz
2.345 - 2.36 GHz
(2)
2.496 - 2.69 GHz
(3)
3.3 -3.4 GHz
(4)
3.4 -3.8 GHz
(5)
5 MHz
512
TDD
8.75 MHz
1024
TDD
10 MHz
1024
TDD
5 MHz
512
TDD
10 MHz
1024
TDD
5 MHz
512
TDD
10 MHz
1024
TDD
5 MHz
512
TDD
7 MHz
1024
TDD
10 MHz
1024
TDD
5 MHz
512
TDD
7 MHz
1024
TDD
10 MHz
1024
TDD
These are the frequency bands and channel bandwidths selected by the WiMAX
Forum for the initial system profiles, which cover many of the worldwide spectrum
allocations suitable for mobile WiMAX. Other frequency bands, channel bandwidths
and FDD will be considered for future profiles based on specific market
42
opportunities; 1.25 MHz (FFT size 128) and 20 MHz (FFT size 2048), thus as
different combinations for frequency band and bandwidth.
3.4
Propagation models available
Propagation modeling is fundamental to the planning of Mobile WiMAX System
and several propagation models have already been proposed for a wide range of
scenarios of wireless communications. Since WiMAX system can operate in
different frequencies this thesis is focused on propagation models that provides
acceptable accuracy for LOS and NLOS scenarios and operation frequency up to
6GHz. At the same time it is significant the environment WiMAX is operating in. It
will be distinguished two different environments: Urban and Suburban.
Thus, with these conditions, best fit propagation models are described. Last
propagation model described, ECC-33 Path Loss Model, is not used for final
calculations but it is explained since it is mentioned in many documents about
wireless communications.
This propagation models are not specially described for WiMAX. Most of them are
described for frequencies below 2.3 and 3.5 GHz.
3.4.1
Free Space Model
Assuming free-space propagation (without obstructions between the transmitter and
receiver) path loss in dB, PL, is evaluated as.
PL(d ) = 32.44 + 20 log 10 ( f ) + 20 log 10 (d )
(3.11)
Where f is the working frequency in MHz and d represents the distance between the
transmitter and receiver in Km. In this case LP is commonly considered L0.
3.4.2
SUI Model
Stanford University developed channel models for the frequency below 11GHz
which, in this case, are the aim of the researching. This model has been proposed for
Broadband Wireless Access (BWA) systems operating under NLOS and LOS
conditions. The SUI model is divided into three different types of terrain, known A,
B and C. It considers in Type A the path loss associated with hilly terrain with
43
moderate to heavy foliage environment that will result the maximum pass loss. In
Type B model it considers either mostly flat terrains with moderate to heavy tree
densities or hilly terrain with light tree densities. In Type C model it results the
minimum path loss that associated with the flat terrain with the light tree densities.
The path loss equation with correction factors is as follow [9]:
⎛ d ⎞
PL = A + 10γ log 10 ⎜⎜ ⎟⎟ + X f + X h + s
⎝ d0 ⎠
for d>d0
(3.12)
Where d is the distance between transmitter and receiver, d0 is the reference distance
and s is the lognormally distributed factor which is used to account for the shadow
fading owing to trees and other mess and the value is between 8.2dB and 10.6dB.
The others parameters are defined as:
A = 20 log 10 (
4πd 0
λ
with λ = c
)
in meters
f
(3.13)
γ = a − bhBS + c h
BS
(3.14)
γ is the path loss exponent, the parameter hBS is the subscriber station antenna height
above ground and should be between 10 m and 80 m. The constants a, b and c
depend on the terrain type, as presented in the table below:
Table 3.7 Numerical values for the SUI model parameters [9]
Model Parameter
Terrain A
Terrain B
Terrain C
a
4.6
4.0
3.6
b ( m-1 )
0.0075
0.0065
0.005
C(m)
12.6
17.1
20
The SUI model is originally competed to frequencies close to 6GHz and receiver
antenna height between 10 m and 80 m. In order to surpass such limitations,
correction factors are commonly used for the operating frequency:
44
X f = 6.0 log 10 (
f
)
2000
(3.15)
and the IEEE 802.16d channel model specifies a correction for the terminal antenna
height,
X h = −10.8 log10 (
hr
)
2000
for Terrain types A and B
(3.16)
hr
)
2000
for Terrain type C
(3.17)
= −20.0 log 10 (
Where the frequency is in MHz and the height is in meters above the ground. The
first expression seems to come from the AT&T measurements. The second is
supposed to be the terminal height correction factor defined by Okumura. There
seems to be an error here as the Okumura correction factor is:
X h = −10 log 10 (
hr
)
3
= −20 log 10 (
hr
)
3
hr < 3m
(3.18)
10m > hr >3m
(3.19)
It can be shown that model needs corrections so that the reference distance increases
with receiver height and it decreases with frequency. Then, calculating a new
breakpoint distance:
⎧
⎛d ⎞
⎪ A + 10γ log⎜⎜ ⎟⎟ + X f + X h + s
⎪
⎝ d0 ⎠
PL = ⎨
⎪20 log⎛ 4πd ⎞ + s
⎜
⎟
⎪
⎝ λ ⎠
⎩
for
d > d ´0
(3.20)
for
d ≤ d ´0
Where;
⎛ 4πd´0 ⎞
A = 20 log10 ⎜
⎟
⎝ λ ⎠
(3.21)
d 0 = 100m
(3.22)
45
d ´0 = d 010
⎛ Xh+X f
−⎜⎜
⎝ 10γ
⎞
⎟⎟
⎠
(3.23)
γ = a − bhBS + c h
BS
(3.24)
d = distance between base and terminal
hBS = height of base station
s = shadowing
and correction factors Xf and Xh are as described above.
Fading
A defining characteristic of the mobile wireless channel is the variations of the
channel strength over time and frequency. The variations are denominated fading.
Different classifications can be found depending on the book we are reading. In the
case of [23] they can be divided into two types:
•
Large-scale fading, due to the signal path loss as a function of distance
and shadowing by big objects such as buildings and hills. This occurs
when the mobile device moves through a distance of the order of the cell
size, and is typically frequency independent.
•
Small-scale fading, due to the constructive and destructive interference
if there is multiple signal paths between the transmitter and receiver. This
occurs at the spatial scale of the order of the carrier wavelength, and is
frequency dependent.
According to [22] within Small-scale fading another division can be noticed. For one
side the “multipath delay time spread leads to time dispersion and frequency
selective fading”. And the other effect would be produced by the Doppler since its
“spread leads to frequency dispersion and time selective fading”.
The most recognized classification for fading in the one which divides it into two
types; Slow and fast fading. This division can be found in [24]. In this case it is
considered that the major difficulties for a transmission within a city are caused by
the fact that the most communication are affected by others via scattering of
electromagnetic waves from surfaces or diffraction over and around buildings. These
46
multiple propagation paths, generally defined multipath, have both slow and fast
aspects:
•
Slow fading appear due to the fact that most of the large reflectors and
diffracting objects along the transmission path are remote from the
terminal. The movement of the terminal relative to these remote objects is
small; as consequence, the corresponding propagation changes are slow.
The statistical variation of these mean losses due to the variation of
intervening terrain, vegetation, etc., is modeled as a lognormal
distribution for terrestrial applications. The slow-fading process is also
referred to as shadowing or lognormal fading.
•
Fast fading is the fast variation of signal levels when the user station
moves short distances. Fast fading is due to the reflections of local
objects and the motion of the mobile device relative to those objects.
Thus, the received signal is the sum of a certain number of signals
reflected from local surfaces, these signals can be summed in a
constructive or destructive manner, depending on their relative phase
relationships. The resulting phase relationships are dependent on the
relative path lengths to the close objects, and they can change with
significance over short distances. In the case of the figure 3.3 S1 and S2
are summed in a constructive manner at T1 however in a destructive
manner at T2. Particularly, the phase relationships depend on the speed of
the motion and the frequency of transmission.
47
Figure 3.3: Constructive and destructive interference
There are many fading models for representing the distribution of the attenuation:
Nakagami, Weibull, Rayleigh, Rician, Dispersive fading model and lognormal
shadowing fading.
In the case of SUI propagation model the fading effect is assumed as lognormal
shadowing fading and its expression is given by Okumura as:
σ = 0.65[log f ]2 − 1.3 log f + A
(3.25)
with f in MHz,
A = 5.2dB (urban) or 6.6dB (suburban)
3.4.3
COST-231 Hata
The Hata model is an empirical approach of the graphical path loss data provided by
Okumura and the operating frequency is between 150MHz and 1500MHz. The
predictions of Hata model is closely related to the Okumura model, as long as
distance exceeds 1 km. Therefore, this model is well suited for the large cell mobile
systems (cell radius above 1 km, but it provides good results only for d > 5 km).
COST-231 Hata model was devised as an extension to the Hata-Okumura model.
This model is purported to be used in the frequency band from 500MHz to
2000MHz. It is restricted to large and small macro-cells. Four parameters are used
for estimation of the propagation loss by Hata´s well-known model: frequency f
48
(MHz), distance d (Km), base station height hb (m) and the height of the mobile
antenna hm (m). COST-231 Hata model can be written as follow:
PL= 46.3 + 33.9 log10 ( f ) −13.82log10 (hb ) − ahm + (44.9 − 6.55log10 (hb ))log10 d + Cm
(3.26)
Where the parameter Cm is defined as 0dB for medium sized city and suburban
centres with medium tree density and 3dB for metropolitan centres. The parameter
ahm is defined for urban environments as follow:
ahm = 3.20(log10 (11.75hm )) 2 − 4.97
for f > 400MHz
(3.27)
and for suburban or rural (flat) environments
ahm = (1.1 log 10 f − 0.7)hm − (1.56 log 10 f − 0.8)
(3.28)
This factor is a correction factor for effective mobile antenna height, which depends
on the size on the coverage area. The validity domain of COST-231 Hata model is:
Table 3.8 COST-231 Hata limitations
3.4.4
Frequency (f)
1500-2000 MHz
Base Station Height (hBS)
30-200 m
Mobile Height (hMT)
1-10 m
Distance (d)
1-20 km
COST-231 Walfisch-Ikegami model
COST-231 Walfisch-Ikegami model allows an improvement of the estimation of the
path loss in suburban and urban environments where buildings are quasi-uniform, by
considering additional environment data: height of building hroof, width of roads w,
buildings separation, b, and road orientation with respect to the direct radio path, γ.
Furthermore, this model is valid when base station antenna is situated below rooftop
level. It has been obtained from [21]:
49
Figure 3.4: COST-231 W-I environment
In addition, it discerns between LOS and NLOS conditions. For the LOS case, the
propagation loss is expressed as follow:
LP = 42.6 + 26 log d [km] + 20 log f [ MHz ]
d≥20m
(3.29)
Under NLOS, the path loss is evaluated from:
⎧ L0 + Lrts + Lmsd
LP = ⎨
⎩ L0
for Lrts + Lmsd ≥ 0
for Lrts + Lmsd ≤ 0
(3.30)
where L0 is the free space loss, Lmsd is the multiple screen diffraction loss. The Lrst
describes the coupling of the wave propagating along the multiple-screen path into
the street where the mobile station is located:
Lrts = −16.9 − 10 log(w[ m ] ) + 20 log f [ MHz ] + 20 log ΔhMT [ m ] + LOri
(3.31)
where LOri is an empirical correction factor, obtained from a few experimental
measurements:
LOri
⎧− 10 + 0.354ϕ
⎪
= ⎨2.5 − 0.075 (ϕ − 35)
⎪
⎩4.0 − 0.114 (ϕ − 55)
for 0 o ≤ ϕ < 35 o
for 35 o ≤ ϕ < 55 o
(3.32)
for 55 o ≤ ϕ < 90 o
where j is the angle between incidences coming from base station and road , in
degrees shown in following figure:
50
Figure 3.5: Angle for Base Station
and,
ΔhMT = hRoof − hMT
(3.33)
The multiple screen diffraction loss, Lmsd, is defined as:
Lmsd = Lbsb + K a + k d log d [ km ] + k f log f [ MHz ] − 9 log b[ m ]
(3.34)
where:
⎧⎪− 18 log(1 + ΔhBS [ m ] )
Lbsb = ⎨
⎪⎩0
⎧
⎪54
⎪⎪
K a = ⎨54 − 0.8ΔhBS
⎪
⎪54 − 0.8Δh d [ km]
BS
⎪⎩
0.5
⎧18
⎪
kd = ⎨
ΔhBS
⎪18 − 15 h
Roof
⎩
⎧ ⎛ f[ MHz] ⎞
− 1⎟⎟
⎪0.7⎜⎜
⎠
⎪ ⎝ 925
⎪
k f = −4 + ⎨
⎪
f
⎪1.5⎛⎜ [ MHz] − 1⎞⎟
⎜
⎟
⎪⎩ ⎝ 925
⎠
for hBS > hRoof
for hBS ≤ hRoof
(3.35)
for hBS > hRoof
for d ≥ 0.5 km and hBS ≤ hRoof
(3.36)
for d < 0.5 km and hBS ≤ hRoof
for hBS > hRoof
for hBS ≤ hRoof
(3.37)
for medium sized city and suburban centres
with medium tree density
for metropolitan centres
51
(3.38)
with:
ΔhBS = hBS − hRoof
(3.39)
The term ka denotes the increase of the path loss for base station antennas below the
rooftops of adjacent buildings. The terms kd and kf control the dependence of the
multi screen diffraction loss versus distance and radio frequency.
Restrictions of the model are given as follow:
Table 3.9 COST-231 Walfisch-ikegami model limitations
Frequency (f)
800-2000 MHz
Base Station Height (hBS)
4-50 m
Mobile Height (hMT)
1-3 m
Distance (d)
0.02-5 km
In case of that data on the structure of buildings and roads are not available,
following values could be taken as default.
hRoof= 3m{number of floors}+{roof-height}
roof-height =3 m for pitched or 0 m for flat
b=20...........50 m, w=b/2, φ=90º
3.4.5
ECC-33 Path Loss Model
The ECC-33 path loss model, which is developed by Electronic Communication
Committee, is extrapolated from the original measurements by Okumura, which
were gathered in the suburban areas of Tokyo. The authors divided the urban areas
into two categories; ‘large city’ and ‘medium city’ and it classifies the suburban
areas into ‘open’ and ‘quasi-open’ areas. A typical European city is quite different
from the Tokyo´s environment. It can be therefore categorized as a ‘medium city’.
The basic path loss is as follow:
PL = A fs + Abm − Gb − G r
(3.40)
52
Where Afs, Abm, Gb and Gr are the Free space propagation loss, the basic median loss,
the bass station height gain factor and the mobile station height gain factor
respectively, and they have the values:
A fs = 92.4 + 20 log 10 d + 20 log 10 f
(3.41)
Abm = 20.41 + 9.83 log 10 d + 7.894 log 10 f + 9.56(log10 f ) 2
(3.42)
Gb = log 10 (
hb
{
) 13.958 + 5.8[log 10 d ]
200
2
}
(3.43)
G r = [42.57 + 13.7 log 10 f ][log 10 hm − 0.585]
(3.44)
Where, f is the frequency in GHz, d is the distance between the base station and the
mobile station, hb is the Base station antenna height in meters and hm is the Mobile
station antenna height in meters.
53
4.
CALCULATION
Preamble point is essential for comprehending the following calculations. As it has
been mentioned before, this thesis is focused in 5 MHz and 10 MHz Bandwidth,
since they are both the most important bands for Mobile WiMAX. At the same time
WiMAX forum has only accept the licensed band of 2.3 GHz yet and the other
important central frequency will be 3.5 GHz. Therefore, in accordance with current
regulatory rules, the resulting scenarios are described in the next table, where 2.3
GHz and 3.5 GHz are working into Urban and Suburban environments:
Table 4.1 Outdoor Scenarios
Scenario
Description
Parameters
BW: 5 and 10 MHz
A
Licensed Bands
FC: 2.3 GHz
Suburban and Urban
Modulations: QPSK and 64-QAM
Environment
Coding rates: 1/2, 2/3 and 3/4
Sensitivity: -94 dBm
Tx Power: 20 dBm
Antennas gain: 16 dBi BS
and 10 dBi MS
BW: 5 and 10 MHz
B
Licensed Bands
FC: 3.5 GHz
Suburban and Urban
Modulations: QPSK and 64-QAM
Environment
Coding rates: 1/2, 2/3 and 3/4
Sensitivity: -94 dBm
Tx Power: 20 dBm
Antennas gain: 16 dBi BS
and 6 dBi MS
It must be reminded that the sensitivity written above is for a 5 MHz bandwidth and
QPSK 1/2 modulation, which mean that it is the best possible sensitivity, that means
best coverage radius but lowest throughput.
Sensitivity, transmitted power and antennas gains have been chosen in order to point
3.2 where features from Base and Subscriber Stations where explained. Within this
54
context, Mobile Station gain is different for 2.3 GHz and 3.5 GHz environments in
order to perform in a better manner the real products and surroundings effects, both
are around the value determined from Products, 8 dBi. Anyway, it is difficult to
simulate conditions when the Mobile WiMAX products have not been certified yet,
thus, this thesis contributes an orientation about Mobile WiMAX performance.
It is primordial to notice that the environment will affect the coverage and as
consequence the maximum data rate since it is related with the modulation
technique. Depending of the coverage area different modulation techniques can be
used. This aspect can be observed in the next figure:
Figure 4.1: Adaptive Modulation in WiMAX technology
The shortest coverage area is for 64-QAM however, this modulation technique
achieves the highest data rate. On the other hand QPSK has the widest radius of
coverage with the lowest data rate.
4.1
Coverage Prediction Evaluation
For calculating the coverage radius excel has been used. Within a excel sheet all
formulas from all propagation models have been written in order to find the correct
value of distance (d).
Pr = Pt + Gt − PL + Gr
(4.1)
From the precious formula, as minimum required received power, both gains and
transmitted power are already know, possible value for Path Loss can be found, as
follow.
55
PL = Pt − Pr + Gt + Gr
(4.2)
Thus, for instance, the maximum value for Path Loss would be:
PL = 20dBm + 94dBm + 10dBi + 16dBi = 140dB
(4.3)
It must be noticed that the result is in dB because Pr is with a symbol minus (-). This
means that transmitted and received power in a logarithmic form would be dividing
each other, as consequence the both dBm would transform to dB. So, this total Path
Loss is equalized with every Propagation Model, hence a formula can be achieved.
Solving properly this formula, the maximum coverage radius can be obtained for
each different Propagation Model.
The impact of Bandwidth on coverage is in accordance to OFDMA characteristics.
Actually, by increasing BW and corresponding number of used sub-carriers, the
antenna’s sensitivity worsens in this manner (complete formula in 4.1.1):
⎛ F xN
10 log⎜⎜ S used
⎝ N FFT
⎞
⎟⎟
⎠
with
(
FS = floor nBW
8000
)8000
(4.4)
Since the thesis is focused in 5 and 10 MHz Bandwidth, it can be easily noticed that
sampling frequency is double in 10 MHz with respect of 5 MHz (n=28/25 in both
cases), therefore the difference between these two bandwidths is 3dB.
On the other hand the effect of using different modulation techniques on coverage
prediction is greater than the bandwidth’s one. The influence of modulation is
completely related with the requirements of SNR, these values are written in Table
3.1. Thus, the different between the worst (64-QAM at 3/4) and the best case (QPSK
at 1/2) is 15 dB. Taken QPSK 1/2 as 0 dB, QPSK 3/4 would be 3 dB and on the
other side 64-QAM 2/3 and 64-QAM 3/4 would be 13 and 15 dB respectively.
For a more rigorous study of the radius coverage, as mentioned before, the results
are shown in the next order; Since SUI propagation model has three types of terrain,
firstly type C, B and last one type A because it is been used for urban environment
therefore it will obtain the worst results. On the other hand, both COST-231 models
consider urban and suburban environments therefore formulas will be distinguished
in the results.
56
At the same time Free Space propagation model is used in both Scenarios in order to
compare easily values with the theoretical maximum coverage. Thus, the influence
of varied determining factors can be assessed.
There are various important values for calculations. They are related with each
propagation model, different references for each one. Therefore, within this context,
the following values have been taken:
•
Base Station height is 30 meters since it is more or less the height of 5
levels building plus the antenna height, 20 meters for the building and 10
for Antenna. Mobile Station is declared as 2 meters.
•
Reference distance, as it is taken in most of propagation models, is 100
meters.
•
Building separation and width of roads have been chosen in accordance
with COST-231 Walfisch-Ikegami model recommendations (at the final
of point 4.4.4). Thus, building separation has been set to 25 meters and as
consequence width if roads are 12.5 meters. As it is mentioned in the
recommendations as well the road orientation is taken as 90 degrees.
Anyway, values can be easily changed since they are written in excel. Thus, it could
be adapted to various different environments.
4.1.1
Scenario A
With all the explanations about steps followed to calculate the coverage radius, it is
clear how results will be obtained. Then, with the mentioned values for the
sensitivity, gains and transmitted power, the coverage for Scenario A results as
follow:
57
Table 4.2 Estimated coverage for 5 MHz into Scenario A
5 MHz
Propagation
QPSK
Model
64-QAM
1/2
3/4
2/3
3/4
Free Space
103,82
73,50
23.24
18,46
SUI C
1,59
1,35
0,77
0,69
SUI B
1,35
1,15
0,68
0,61
SUI A
1,07
0,93
0,57
0,52
COST-231 Hata Sub
1,12
0,92
0,48
0,42
COST-231 Hata Urb
0,89
0,73
0,38
0,33
COST-231 W-I Sub
0,87
0,73
0,40
0,35
COST-231 W-I Urb
0,58
0,49
0,26
0,23
This table summarizes the coverage radius for 5 MHz Bandwidth. It can be noticed
the difference among different path loss models, where Free Space model is shown
in order to collate with the others more realistic models.
There is an enormous difference between Free Space model and others models.
Logically, Free Space model achieves higher coverage radius because no parameters
are reckoned. Only frequency affects the result. This phenomenon is repeated in
every scenario and bandwidths.
Table 4.3 Estimated coverage for 10 MHz into Scenario A
10 MHz
Propagation
QPSK
Model
64-QAM
1/2
3/4
2/3
3/4
Free Space
73,50
52,03
16,45
13,07
SUI C
1,35
1,14
0,65
0,58
SUI B
1,15
0,98
0,58
0,52
SUI A
0,93
0,80
0,50
0,45
COST-231 Hata Sub
0,92
0,76
0,39
0,34
COST-231 Hata Urb
0,73
0,60
0,31
0,27
COST-231 W-I Sub
0,73
0,61
0,33
0,29
COST-231 W-I Urb
0,49
0,40
0,22
0,20
58
In this second table, the difference with previous values is solely the effect of 3 dB,
previously mentioned. The difference is approximately around 20% (mostly a bit
less than 20%) wider the coverage for 5 MHz than for 10 MHz.
Finally a graphic with different coverage radius for various path loss values is shown
in order to provide information for any device, with its respectively sensitivity, gain
or transmitted power, at the same time it can be used to compare with better devices
(higher gain or transmitted power as well as better sensitivity). Free Space
propagation model is not shown since its values are much greater than in other
models, and it can be considered solely as an ideal case.
3000,0
Coverage Radius [m]
2500,0
2000,0
1500,0
1000,0
500,0
0,0
120 122 124 126 128 130 132 134 136 138 140 142 144 146 148 150
Path Loss [dB]
SUI C
SUI B
SUI A
COST-231 W-I sub
COST-231 Hata urb
COST-231 W-I urb
COST-231 Hata sub
Figure 4.2: Coverage Radius for scenario A
Observing Figure 4.2, it can be noticed that SUI A and COST-231 Hata Suburban
are crossed somewhere around 137 dB. For upper Path Loss values COST-231 Hata
Suburban will obtain better coverage radius and vice versa. COST-231 W-I
Suburban and COST-231 Hata Urban have similar performance, however, in this
case, both lines are very together within low path loss and after 140 dB it looks they
are going to get separated more and more, at a low rhythm.
59
4.1.2
Scenario B
Same tables and graphic are shown in this Scenario B. Similar results can be
observed, although as higher is the central as worse are the coverage radius,
therefore this environment, because of the central frequency’s influence, is more
adverse for coverage radius the resulting values decrease comparing with Scenario
A. At the same time, within Scenario B, the Mobile Station has a gain of 6 dB
instead of those 10 dB used in the Mobile Station for Scenario A.
Then the total different values are shown in the next table for 5 MHz Bandwidth.
Table 4.4 Estimated coverage for 5 MHz into Scenario B
5 MHz
Propagation
QPSK
Model
64-QAM
1/2
3/4
2/3
3/4
Free Space
65,50
46,37
14,66
11,65
SUI C
1,27
1,08
0,62
0,55
SUI B
1,09
0,93
0,55
0,50
SUI A
0,88
0,76
0,47
0,43
COST-231 Hata Sub
0,86
0,71
0,37
0,32
COST-231 Hata Urb
0,68
0,56
0,29
0,26
COST-231 W-I Sub
0,68
0,57
0,31
0,28
COST-231 W-I Urb
0,46
0,38
0,21
0,18
Table 4.4 contains the results for 10 MHz bandwidth with 3.5 GHz central
frequency. In fact, this table contains worst results for coverage radius.
60
Table 4.5 Estimated coverage for 10 MHz into Scenario B
10 MHz
Propagation
QPSK
Model
64-QAM
1/2
3/4
2/3
3/4
Free Space
46,37
32,83
10,38
8,25
SUI C
1,08
0,91
0,52
0,47
SUI B
0,93
0,80
0,47
0,42
SUI A
0,76
0,76
0,41
0,37
COST-231 Hata Sub
0,71
0,58
0,30
0,27
COST-231 Hata Urb
0,56
0,46
0,24
0,21
COST-231 W-I Sub
0,57
0,48
0,26
0,23
COST-231 W-I Urb
0,38
0,32
0,17
0,15
In this second table, the difference with previous values is solely the effect of 3 dB,
as well as before. And it can be also noticed that the difference is also approximately
around 20% (in this cases a bit more than 20% in some cases) wider the coverage for
5 MHz than for 10 MHz.
Finally a graphic with every propagation model is shown as before in order to
provide information for any device, with its respectively sensitivity, gain or
transmitted power, at the same time it can be used to compare with better devices
(higher gain or transmitted power as well as better sensitivity). Free Space
propagation model is again not shown since its values are also much greater than in
other models, and it can be considered solely as an ideal case.
61
3000,0
Coverage Radius [m]
2500,0
2000,0
1500,0
1000,0
500,0
0,0
120 122 124 126 128 130 132 134 136 138 140 142 144 146 148 150
Path Loss [dB]
SUI C
SUI B
SUI A
COST-231 W-I sub
COST-231 Hata urb
COST-231 W-I urb
COST-231 Hata sub
Figure 4.3: Coverage Radius for scenario B
In this table it can be noticed that higher central frequency affects especially both
COST-231 propagation model. Coverage radius is lower for every propagation
model, for SUI models (all types) this difference is very small however for COST231 propagation models this difference can reach 100%.
This is the case of COST-231 W-I for urban terrain. For a Path Loss of 140 dB,
within 3.5 GHz environment, 279 meters is resulted from the calculations as the
coverage radius. If it is compared with 582 meters resulted within 2.3 GHz
environment, the increase is 108%.
4.2
Theoretical Maximum throughput
Once, in accordance with sensitivity, antenna gains and path loss, that the Mobile
Station has established a connection with the Base Station with one of the
modulation techniques, the achieved maximum data rate can be calculated.
For this goal, the values of the most relevant system parameters are defined, in
accordance to the IEEE 802.16e Standard.
62
Table 4.6 OFDMA parameters according with 802.16e-2005 Standard
5 MHZ
Parameter
Nused
10 MHZ
Downlink
Uplink
Downlink
Uplink
420=360d+60p
408=272d+136p
840=720d+120p
840=560d+280p
NFFT
512
1024
bm
2 (QPSK), 4 (16-QAM) and 6 (64-QAM)
cr
1/2, 2/3 and 3/4
G
1/4, 1/8, 1/16 and 1/32
8/7 for BW a multiple of 1.75 MHz
n
28/25 for BW a multiple of 1.25, 1.5, 2 or 2.75 MHz
8/7 for rest of cases
In the table above, the number of used sub-carriers (Nused) is divided into two groups;
data sub-carriers and pilot sub-carriers, denoted with d and p respectively. It is very
important distinguishing this number since the total (data plus pilot sub-carriers) is
used for solving the value of bandwidth influence in sensitivity (described above,
5.1) and calculation for the theoretical maximum data rate use only the number of
data sub-carriers in order to be stricter with acquired results.
On the same path of getting a strict value in calculations, data rate has been
decreased by a factor of 9/8 and 44/42, this means that final result of calculations are
multiplied by 8/9 and 42/44. These factors are related with Cyclic Prefix and frame
structure respectively.
Therefore, the maximum theoretical throughput has been calculated as follow: first
of all sub-carrier spacing (Δf) is calculated from sampling frequency (FS) by using
formulas from additional information (3.1.2):
Δf = 10937.5 Hz
(4.5)
then, symbol duration can be obtained as the sum of useful symbol interval
(Tb=1/Δf) and guard interval (Tg). In this thesis, guard interval has a value of 1/8.
Hence,
TS = Tb + Tg = (G + 1)Tb = 102.9 μs
(4.6)
63
this value is used for both bandwidths. In accordance of preamble, the inverse of this
number must by multiplied by the Nused (in this case only data sub-carriers), number
of bits per modulation symbol (bm, depending on the modulation technique) and the
coding rate, cr.
In this context and reminding that number of data sub-carriers is: 360 for downlink
and 272 for uplink in a bandwidth of 5 MHz and 720 for downlink and 560 for
uplink in 10 MHZ. The obtained result of these calculations is:
Table 4.7 Estimated Maximum Data rate in Mbps
5 MHz
Transfer Rate
QPSK
10 MHz
64-QAM
QPSK
64-QAM
1/2
3/4
2/3
3/4
1/2
3/4
2/3
3/4
Downlink (MHz)
2,91
4,36
11,64
13,09
5,82
8,73
23,27
26,18
Uplink (MHz)
2,20
3,30
8,79
9,89
4,52
6,79
18,10
20,36
Thus the maximum downlink and uplink date rate are 26 and 20 Mbps respectively,
which both are achieved for 10 MHz channel bandwidth, 64-QAM modulation with
3/4 coding rate. In this case, coverage radius is between 580 and 270 meters
(depending on the propagation model) for Suburban environments and between 410
and 150 meters for urban environments.
Comparing maximum throughput and coverage radius, it can be perceived that
operation in 5 MHz bandwidth provides better coverage but with lower throughput
than in 10 MHz bandwidth. Thus, for maximum coverage radius (with 5 MHz
Bandwidth) 2.9 and 2.2 Mbps will be achieved at least for Downlink and Uplink
respectively.
Then, C Area shall contain the maximum data rate, with every Mobile Station using
64-QAM modulation technique. The maximum data rate for set of Mobile Station
which uses 64-QAM is call denominated C (Mbps). And the name of the data rate
for Mobile Station placed within A Area is called consequently A (Mbps).
64
Figure 4.4: Different modulation techniques areas
In this manner, Mobile Station placed into B Area will reach a data rate which
comply this condition:
A( Mbps) < B( Mbps) < C ( Mbps)
(4.7)
65
5.
MEASUREMENTS TAKEN
5.1
Environment
In order to compare theoretical calculations with real values some measurements
have been taken around the Base Station which is placed in the roof of Electrical
Faculty within Maslak Campus (Istanbul Technical University).
Figure 5.1: Electrical Faculty in Maslak Campus
Places around Campus have been chosen with the help of the program “Radio
Mobile” and “Google Earth”. Program “Radio Mobile” is a tool used to predict the
performance of a radio system. It uses digital terrain elevation data for automatic
extraction of path profile between a transmitter and a receiver.
66
Figure 5.2: Elevation Map of Istanbul and Bosphorus
Above Elevation map of Istanbul and surroundings is shown. With the help of this
map and by introducing all the information about the transmitter (Base Station) and
receiver (Subscriber Station) places for measurements have been chosen. This
information includes antennas height, gains, transmitted power, etc. Thus, next map
is showing the final possible areas of coverage.
67
Figure 5.3: Coverage areas for testing
In the previous map, points within each area have been already shown in order to
avoid adding many different maps with little information. These points have been
selected because of the situation; high part of the terrain, no buildings in front or
other reasons. Always, in order to get the best link quality. However in the Area 4,
even with a good position, it was impossible to access to the network, neither in
Area 5.
5.2
Used devices
BreezeMAX family from Alvarion has been used for these measurements. Thus,
omnidirectional antenna, Access Unit ODU (OutDoor Unit) and Micro Base Station
have been used for the Base Station. On the other side, PRO CPE ODU and
Broadband Data CPE have been utilized for the Subscriber Station. All of these
products and the final WiMAX Network can be seen in the next figure.
Figure 5.4: WiMAX Network
In the previous figure, it can be noticed that more devices have been utilized such as:
Router Linksys, Toshiba laptop (client) and Asus laptop (server).
The WiMAX Network is operating in the frequency of 3.7 GHz and a bandwidth of
3.5 MHz with TDMA FDD as Radio Access Method. In the case of FDD, the
bandwidth for Uplink and Downlink are half of the total bandwidth and as
68
consequence the data rate for UL and DL should be the same. In this case, only
Uplink has been tested.
5.3
Measurements
Once the WiMAX Network has been completely described, the followed procedure
for testing the connection can be explained. In this case Iperf has been used in order
to measure the data rate between the Base Station (Server) and Subscriber Station
(client). Iperf was developed by NLANR/DAST as a modern alternative for
measuring maximum TCP and UDP bandwidth performance. Iperf allows the tuning
of various parameters and UDP characteristics. However, in this case, only TCP
connections have been tested.
The commands for Server and Client are a bit different, although in these
measurements very basic commands have been utilized. Then, hence, following
commands are:
iperf –s –i1
for server
iperf –c 1.1.1.5 –i1 –t10
for client
-s and –c are respectively the notation for server and client. –i1 means seconds
interval between periodic bandwidth reports, in this case one second. 1.1.1.5 is the IP
address of the server and –t10 is the time the test is running, in this case ten seconds.
Thus, the realized test was running only during 10 seconds however more than one
test for each point has been done; two tests for Area 1 and 3 and three tests for Area
2. At the same time Iperf was measuring the Bandwidth, the Base Station Program
installed in the Laptop acting as server was collecting all the information about the
Link. This includes SNR UL and DL, RSSI UL and DL and used Modulation
technique for UL and DL as well.
The weather conditions were not the most suitable for the objective but they had to
be taken that day. Wind was 10 Km/h, decreasing during the day. Temperature was
between 8 and 5 Celsius degrees and the humidity was around 76%.
It should be reminded that the connection is FDD; thus, the bandwidth is divided
symmetrically for Uplink and Downlink. In this case only Uplink is tested since is
69
the client which is sending packets to the server. These packets have a size of 8
kbits.
It is also important to remind that the multiplexing technique is OFDM instead of
OFDMA used in Mobile WiMAX. In this case the number of FFT is fixed and equal
to 256, where the number of used sub-carriers is 192.
The last line of every following table contains the results of measurements which
were taken in May of 2007 with much better weather conditions and clear sky. These
conditions were:
temperature: 17º - 24º
humidity: 20 % - 32 %
wind speed: 10 – 40 km/h
These measurements were taken by Istanbul Technical University students who were
working for an Undergraduate Thesis. These theses can be found in the main Library
and the results were provided by the Advisor. In this case, both links were tested;
Uplink and Downlink by performing a full duplex TCP test. Also Iperf was used in
these tests but with different commands, thus, it was possible to test the link with
various configurations. Data Rates are an approximation of the average that it would
be obtained from all the acquired values.
Point Area 1
Area 1 is located the closest of all of them to the Campus and as consequence to the
Base Station. The exact distance for the Base Station to the Subscriber Station when
the measures were taken was 2.6Km. These measurements were taken in the middle
of the street with many cars passing around and likely without line of sight. This
aspect was especially due to it was a raining day with fog.
Table 5.1 Measurements results for Area 1
SNR UL
SNR DL
RSSI UL
RSSI DL
Modulation
Modulation
Data Rate
(dB)
(dB)
(dBm)
(dBm)
UL
DL
(Kbps)
18,6
20
-86,8
-83
BPSK 3/4
BPSK 1/2
1920
21,1
20
-83,9
-83
BPSK 3/4
BPSK 1/2
1830
70
17
20,4
-87,1
-82,3
QAM-16 3/4
QAM-64 3/4
- QAM-16 1/2
- QAM-16 3/4
N.D.
In the previous table it was be noticed that ratios for signal and noise are quite high
and as consequence the Uplink is working with BPSK 3/4 Modulation technique,
which in these measurements is the highest Modulation technique achieved.
The measurement from May is higher in the Modulation Technique but almost equal
in SNR and RSSI levels, this is mainly because of the humidity which was quite high
(73%) that day of May. However there is not information about the Data Rate for
that day.
Point Area 2
In this case, this area is located in the Asian side of the city, with the Bosphorus in
the middle. A very suitable place was got in this case, with possible line of sight with
the Base Station. It was only “possible” because as it is mentioned before the day
was not very clear and there was fog.
The distance between the Base Station and Subscriber Station was around 4.17Km.
And there were not high buildings around, cars neither.
Table 5.2 Measurements results for Area 2
SNR UL
SNR DL
RSSI UL
RSSI DL
Modulation
Modulation
Data Rate
(dB)
(dB)
(dBm)
(dBm)
UL
DL
(Kbps)
5,4
9
98,8
93
BPSK 1/2
QPSK 1/2
870
8,1
11
96,5
91
BPSK 3/4
BPSK 1/2
1660
6
3
98,7
97
BPSK 1/2
BPSK 1/2
869
7,8
13,9
-95,9
88,8
QPSK 3/4
QAM-16 3/4
- QAM-16 1/2
1800
Three measurements were taken. Fortunately in one of them BPSK 3/4 Modulation
technique were achieved and it can be noticed that the Data rate is almost the double.
In the two others the Modulation technique was the lowest. At the same time it can
be observed that QPSK 1/2 was achieved for Downlink in one of the measurements.
71
The last measurement taken in December is the worst of them. The Data Rate is the
same than the first one but it can be seen that all the values about SNR and RSSI are
worse in this last case.
Comparing the results from December and those from May it can be easily noticed
that weather conditions affect the Network throughput as well as in the previous
Area. By observing all the values, it can be seen that the second measurement in
December is almost equal than in May according to SNR and RSSI values. However
achieved Modulation techniques are quite worse and as consequence the Data rate is
affected.
Point Area 3
This is the last Area where it was possible to get in the WiMAX Network. Even
when it is further than Area 4 it was possible to get signal because there is a hill used
for multiple antennas for different targets. This hill can be viewed from the roof of
electrical faculty, where the Base Station is placed. Thus, there was direct line of
sight between Base Station and Subscriber Station. The distance between both
antennas was around 9.1 Km.
Table 5.3 Measurements results for Area 3
SNR UL
SNR DL
RSSI UL
RSSI DL
Modulation
Modulation
Data Rate
(dB)
(dB)
(dBm)
(dBm)
UL
DL
(Kbps)
5,6
12
-98,1
-91
BPSK 1/2
QPSK 1/2
870
5,6
12
-98,3
-91
BPSK 1/2
BPSK 3/4
870
10,7
16,75
-92,6
85,75
QAM-16 1/2
- BPSK 1/2
QAM-64 2/3
- QPSK 3/4
3500
In this last case both measures are almost the same. With a lonely difference, they
achieved different Modulation technique for Downlink; QPSK 1/2 and BPSK 3/4.
However this difference does not influence in the Data Rate because only the Uplink
is used.
This is the case where there is more difference between December and May
measurements. This is likely due to the long distance between Base and Subscriber
Station, around 9km. All the values are higher and as consequence a better
performance of the Network is achieved.
72
73
6.
CONCLUSIONS and FUTURE WORK
Conclusions are difficult to obtain since there are not equipments for Mobile
WiMAX yet. Anyway, some conclusions are already evident without performing
Mobile WiMAX equipment.
The main difference in the Physical Layer between Mobile and Fixed WiMAX is
related with the multiplexing technique; OFDM comparing with OFDMA. By
operating with OFDMA multiplexing technique the Network can perform in a more
flexible way for different users. At the same time the Frequency resource is utilized
in a more efficient manner.
The new features for Mobility will provide to WiMAX a wider spread in the used
Wireless Technologies. This new characteristic, Mobility, is very important because
in the last years Fixed WiMAX has already been produced and installed somewhere
but less than what it was expected. WiMAX should find the place between WiFi and
GSM.
Another conclusion that it can be clearly acquired in that weather influences
extremely in the WiMAX Links. It is logical to notice that as long is the link as
higher is the weather’s influence. It should be possible to ensure a minimum
throughput for an internet connection. The maximum coverage is something
interesting for studying but the Network provider, the one which is installing the
WiMAX Base and Subscriber Stations, should guarantee a minimum bandwidth for
each user. This would be impossible if several Antennas are not placed.
At the same time, as weather’s influence, building’s influence should be inquired.
Although it is very troublesome to get a clear idea about this influence since there
are a lot of aspects which would affect in the results and most of the cases, especially
in Istanbul, each building is different with the building behind. This last comment is
supported by the taken measurements since the three Areas were not same far away
from the Base Station, but the point whose Data Rate was lowest was not the furthest
one.
74
For the last point it must be noticed that the difference between this two distinctive
central frequencies 3.5GHz and 2.3 GHz, deduce that the second one would be
properly chosen.
The possible future work is quite clear; some measurements should be taken with
Mobile WiMAX equipments. These measurements should be compared with the
others taken with Fixed WiMAX equipments and at the same time it would be also
interesting to calculate theoretically the maximum throughput of 802.16-2004
Standard, that, by the way, it has been already realized by other Universities. Then,
all of these results shall provide a better idea about the most suitable Standard to use
in each situation.
75
REFERENCES
[1] Standard IEEE 802.16e-2005. Part 16: Air Interface for Fixed and Mobile
Broadband Wireless Access Systems Amendment 2: Physical and
Medium Access Control Layers for Combined Fixed and Mobile
Operation in Licensed Bands
[2]
Worldwide
Interoperability
Microwave
forum.
Access
http://www.wimax.org
[3]
Tutorial:
802.16
MAC
Layer
Mesh
Extensions
Overview.
www.ieee802.org/16/tga/contrib/S80216a-02_30.pdf
[4] Standard IEEE 802.16.
http://standards.ieee.org/getieee802/download/802.16-2004.pdf
[5] Mobile WiMAX – Part I: A Technical Overview and Performance
Evaluation.
http://www.wimaxtrends.com/docs/technical_specs/WiMAX_Overvie
w_v2.pdf
[6] Hassan Yagoobi. 2004. “Scalable OFDMA Physical Layer in IEEE 802.16
WirelessMAN”, Intel Technology Journal, Vol 08.
[7] Fixed, nomadic, portable and mobile applications for 802.16-2004 and
802.16e WiMAX networks.
http://www.wimaxforum.org/technology/downloads/Applications_for_
802.16-2004_and_802.16e_WiMAX_networks_final.pdf
[8] IEEE Standard 802.16: A technical Overview of the WirelessMAN Air
Interface for Broadband Wireless Access. IEEE Communication
Magazine, June 2002.
76
[9] Multihop Path Loss Model (Base-to-Relay and Base-to-mobile).
http://www.ieee802.org/16/relay/contrib/C80216j-06_011.pdf
[10] Below Rooftop Path Loss Model.
http://wirelessman.org/relay/contrib/C80216j-06_010.pdf
[11] Coverage prediction and performance evaluation of wireless
metropolitan area networks based on IEEE 802.16.
http://iecom.dee.ufcg.edu.br/~jcis/dezembro2006/volume20/JCIS_2005
_20_006_On.pdf
[12]Comparison of Empirical Propagation Path Loss Models for Fixed
Wireless Access Systems.
http://www.cl.cam.ac.uk/research/dtg/publications/public/vsa23/VTC05
_Empirical.pdf
[13] Pedro Francisco Robles Rico. 2006. Investigation of IEEE standard 802.16
Medium Access Control (MAC) layer in Distributed Mesh Networks
and comparison with IEEE 802.11 ad-hoc networks.
www.diva-portal.org/diva/getDocument?urn_nbn_se_liu_diva-70201__fulltext.pdf
[14] WiMAX Forum™ Mobile System Profile Release 1.0 Approved
Specification.
http://www.wimaxforum.org/technology/documents/WiMAX_Forum_
Mobile_System_Profile_v1_2_2.pdf
[15] “PREPARATORY STUDY ON INTEROPERABLE WIMAX AND
BROADBAND MOBILE SATELLITE NETWORKS”. WISAT.
WiSat_WP1_D1_v4.pdf.
[16]
Alvarion.
alv_BreezeMAX_pbp.pdf.
3500_datasheet.pdf. www.alvarion.com
77
BreezeMAX_2300-2500-
[17] WiMAX Network Performance Analysis. BSc Thesis. Nilcan Özcan.
Istanbul Technical University.
[18] WiMAX Coverage Analysis. BSc Thesis. Miray Özel. Istanbul Technical
University.
[19]
Mobile_WiMAX_Part1_Overview_and_Performance.pdf.
WiMAX
Forum.
http://www.wimaxforum.org/news/downloads/Mobile_WiMAX_Part1_
Overview_and_Performance.pdf
[20] Applications _ for _ 802.16-2004_ and_ 802.16e _WiMAX_ networks_
final.pdf. WiMAX Forum.
http://www.wimaxforum.org/news/downloads/Applications_for_802.16
-2004_and_802.16e_WiMAX_networks_final.pdf
[21] Multi-hop Relay System Evaluation Methodology (Channel Model and
Performance Metric). www.ieee802.org/16/relay/contrib/C80216j-
06_052r1.pdf
[22] Theodore S. Rappaport. 2002. Wireless Communications, principles and
practice. Prentice Hall PTR.
[23] David Tse, Pramod Viswanath. 2005. Fundamentals of Wireless
Communications. Cambridge University Press.
[24] Simon Haykin, Michael Moher. 2005. Modern Wireless Communications.
Prentice Hall.
78
BIOGRAPHY
Pedro Francisco Robles Rico was born in Madrid, Spain, in September 1983. He
graduated from Arturo Soria School in June 2001 and attended to the
Telecommunications Engineering Faculty of Alcalá de Henares University. He spent
one year of University from august 2005 until May 2006 in Sweden as Erasmus
Student realizing a Thesis about Distributed Networks within WiMAX technology.
In October 2006 he was working in Telefonica I+D, Madrid, as intern until last of
July 2007.
His research interests are in networks, specially wireless.
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