D-STAR: New Modes for VHF/UHF Amateur Radio, Part 1

D-STAR: New Modes for VHF/UHF Amateur Radio, Part 1
D-STAR: New Modes
for VHF/UHF
Amateur Radio, Part 1
Our friends in JARL have created a new
digital communication standard. Let’s look at
their new system, and what’s in it for hams.
By John Gibbs, KC7YXD
T
his article is the first in a threepart series describing and analyzing a new communication
standard developed by the Japan
Amateur Radio League (JARL). The
first part focuses on the advantages
of upgrading our VHF/UHF equipment to a new, more capable system.
The second article in the series addresses the technical design considerations of a digital voice and high-speed
data system in the VHF/UHF spectrum. The third, and final, article discusses the D-STAR standard and how
it addresses the needs and technical
issues raised in the two previous parts.
JARL has developed a new open
standard for VHF/UHF digital radio
called D-STAR. The system supports
both digitally modulated voice transmission and data transmission, including Internet connections, at DSL rates.
At a time when the third-generation
(3G) cell phone proposals for high-speed
data have been severely delayed,
18225 69th Pl W
Lynnwood, WA 98037
kc7yxd@arrl.net
D-STAR presents Amateur Radio operators with the opportunity to bring
the Internet Age to mobile and portable
operation.
I have been fortunate to be one of
the first hams to see and use the prototype transceivers of this new Amateur
communication system. Therefore, I
would like to take this opportunity to
share some of the knowledge and experience I have gained. In this article, I
will present the objectives of the new
JARL D-STAR system and provide a
glimpse of the capabilities of this new
system and the engineering tradeoffs
that went into the system design. I hope
you will find it interesting both in developing an understanding of this system and as an insight into the design
process of a digital radio system.
One of the major goals of D-STAR
is that it be an open system. This
series of articles contains enough system details for a skilled ham to develop a homebrew D-STAR digital
voice transceiver.1
If you are interested in this system,
you may soon have a chance to try it
yourself. Because the FCC encourages
1Notes
appear on page 34.
the Amateur Radio community to develop new digital modes, the US has
the regulatory structure in place for
hams to use an all-digital voice and
high-speed data radio system without
special licensing or permits. US hams
will therefore be the first in the world
with the opportunity to use the new
D-STAR system illustrated in Fig 1.
Regular readers of QEX know that
hams have been experimenting with
digital voice.2 In the US, the FCC encourages hams to continue such experimentation to demonstrate our
stewardship of our spectrum. In addition to individual efforts, Amateur
Radio organizations have also been
promoting digital radio. For years,
TAPR has focused exclusively on advancing the state of the art in Amateur Radio digital communications.
The ARRL has increased its efforts in
this area by sponsoring the establishment of Technology Task Force Working Groups on digital voice, high-speed
multimedia and software radio.
In addition to the efforts of individual hams and their organizations,
one manufacturer has already introduced a digital voice option to handheld
VHF/UHF radios. Unfortunately, these
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radios are of limited usefulness because
the necessary repeater infrastructure
for VHF/UHF digital voice operations
with these radios does not exist. These
radios will not work through existing
analog repeaters and the necessary
digital repeaters have not been developed. Early on in the D-STAR planning,
the JARL recognized that developing
VHF/UHF digital capabilities also requires developing new standards for
digital voice repeaters and the links between repeaters.
D-STAR History
The D-STAR standard not only addresses the needs of VHF/UHF voice
and data communications with mobile
and handheld radios, but also provides
the standards for repeater-to-repeater
linking and Internet access. It was clear
that developing and testing such a complex system would take many manyears of engineering and testing. The
efforts that would be required to achieve
this in a timely manner would far exceed any reasonable expectations of
volunteer ham labor, no matter how
dedicated. So, the JARL contracted with
the Amateur Radio manufacturer
ICOM to develop and evaluate D-STAR
prototype hardware. D-STAR has been
under development since 1998 and the
system operation has been proven in lab
and field tests. The result of all this
development effort is about to bear
fruit. The JARL expects to finalize the
D-STAR standard this summer.
A D-STAR mobile transceiver
called the ICOM ID-1 was used for
field trials in the Tokyo area (see the
cover photo on this issue) and shown
at three US Amateur Radio shows:
Dayton Hamvention 2002 and 2003
and the Digital Communication Conference (DCC) in Denver last fall.
Since then, repeaters and microwave
links have been developed and are cur-
rently available on a limited basis to
application developers in the US. All
these D-STAR compatible components
will soon be shipping in quantity, and
we expect that other manufacturers
will be shipping D-STAR-compatible
radios in the future.
Existing VHF/UHF System
Properties
To replace any existing system with
a new standard, there must be compelling reasons for incurring the expense
of new equipment. So it is good to start
the discussion of D-STAR with a look
at the capabilities and limitations of our
existing VHF/UHF Amateur communication systems in Tables 1 and 2. To do
this, let us look at the capabilities of a
representative voice repeater system
that covers the Pacific Northwest and
beyond: the Evergreen Intertie.
The Evergreen Intertie connects
more than 23 repeaters by full-duplex
UHF radio links that are transparent
to the user. From my location, there
are two main links in the system,
a North-South link that connects
Western Washington and Oregon and
an East-West link that crosses the
Cascade Mountains and connects to
cities in Eastern Washington.
Users can control switches using
DTMF tones to connect repeaters to
the link. The way this particular system is configured, a minimum of three
switches must be set by a user or control operator to connect two repeaters.
In a more-extreme case, seven
switches must be set to talk from Seattle repeater K7NWS to Portland repeater KJ7IY. Of course, each switch
that connects to the next link may be
already in use, so it can be difficult
for a user to establish such links if the
system is heavily used.
On a repeater link, only one contact can be held on a link at a time.
That is, unlike the telephone system,
there is no multiplexing on links. If a
link is in use or out of service, there is
no way to link the repeaters unless an
alternate path is available.
Another difference from the telephone trunk system lies in how a link
is established. In the telephone system, the system automatically picks
the link based on the call destination
and the trunk lines currently available. In the Evergreen Intertie, the
user must determine the logical path
through the repeater links and to
know the DTMF codes for each of the
switches.
Amateur Radio packet systems offer an analogous set of features
through dedicated packet nodes (simplex repeaters.) Packet radio is used
to transfer data (for example, computer files) and for keyboard contacts.
It might even be possible to have a
digital voice contact on a 9600-bps
system. However, the system is packetoriented, which means that real-time
communications are not guaranteed.
Unless the system was very lightly
loaded, some of the voice packets
would be unacceptably delayed.
Both the amateur voice repeaters
and packet nodes are FM systems.
Within the limitations of the existing
analog FM technology, some very creative communication solutions have
been developed. For instance, sub-audible tone codes are used to protect repeaters against accidental activation
by interference. DTMF is used to control some repeater functions, activate
a phone patch or for selective calling
of amateurs.
Enterprising hams continually add
new capabilities to the systems. A few
Table 1—Existing VHF/UHF Amateur
Radio System Features
•
•
•
•
•
•
•
•
Voice is FM, half duplex
Data is FSK, simplex
CTCSS protection
DTMF control
Linked repeaters
1200/9600 packet
APRS
Voice over Internet
Table 2—Existing VHF/UHF Amateur
Radio System Limitations
Fig 1—JARL’s proposed D-STAR system offers digital voice and data communication on
1.2-GHz with repeaters linked on 10 GHz and Internet gateways.
•
•
•
•
•
Spectrally inefficient
Low speed data
One QSO per link
Difficult to establish links
Cannot mix data and voice
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examples that come to mind include
satellite gateways, GPS-based location
systems (APRS) and worldwide communications with VHF/UHF transceivers when voice-over-Internet-protocol (VOIP) is added to a repeater. Yet
we are rapidly approaching the limit
of what we can do with the existing
infrastructure, as we can see by investigating the limitations of existing
VHF/UHF Amateur Radio systems
(see Table 2).
Spectral Efficiency
The first major limitation is spectral efficiency. The amateur community’s VHF/UHF spectrum usage has
not changed despite dramatic improvements in communication technology that have occurred in the last
few years. The FCC views the radio
spectrum as a finite resource that
must be efficiently shared among
many users. There are many new potential users appearing for the VHF/
UHF spectrum, and they are often
looking at the spectrum that has been
allocated for Amateur Radio. A growing part of ARRL resources are being
devoted to spectrum-defense.
However, defending our usage of
these valuable frequencies will become more difficult because the current amateur FM system is not spectrally efficient. Today, the FCC only
grants new licenses in Land Mobile
services to users that meet reduced
spectrum-occupancy requirements.
The FCC calls this “refarming.” The
FCC has extended this principle to
other radio services. For example, the
existing GMRS spectrum was
refarmed with FRS channels placed
between the old GMRS channels.
There is no reason why we should feel
that the Amateur Radio Service would
be exempt from the requirement for
spectral efficiency.
As a matter of fact, the quest for
spectral efficiency is increasing. Current Land Mobile services refarming
is from 25-kHz channel spacing to
12.5 kHz. However, in the next few
years, the FCC plans to repeat this
process and force all new Land Mobile Service licenses to use equipment
compatible with 6.25-kHz channel
spacing. It is not clear that FM radios
can be developed that will meet the
stability and bandwidth requirements
of such a system and be sold at an affordable price, so radios using other
technologies may need to be developed.
But what is certain from modulation
theory is that as the deviation is reduced, the signal quality advantage of
FM over AM systems (including SSB)
quickly disappears.
Data Rate
Compatible with Regulations
A second limitation is the 9600baud rate limitations of existing commercially available radios. In this
data-intensive Internet age, this speed
is woefully inadequate. Any new system should have the capability of supporting data transfers at speeds rivaling DSL.
If a new system required changes in
the FCC regulations, it might take
years before it could be adopted. Fortunately, this is not necessary for digital
voice and data transmissions in the US.
The FCC encourages the Amateur
Radio community to develop digital
voice and new digital data communication systems. One example is the FCC’s
attitude toward the new HF digital
modes such as PSK31. There was some
concern in the amateur community that
the encoding used in PSK31, called
Varicode, would be considered a form
of encryption and hence barred by the
Part 97 regulations. However, the FCC
has clearly and repeatedly stated that
encoding is not encryption and that as
long as the encoding method is public
there is no regulatory problem.
Where should digital voice transmissions occur in the current band plan?
Again the FCC in its encouragement of
digital radio has already decided that
digital voice operations belong in the
phone bands.3
What regulatory issues are there
for new linked systems? William
Cross, W3TN, of the FCC Policy and
Rules Branch, made it clear at his presentation at Dayton last year that
there are no rules specifically written
for linked systems; the FCC regulates
stations, not systems.
Finally, emergency operation is
clearly one of the requirements for any
Amateur Radio system to meet one of
the Part 97 justifications for the Amateur Radio Service. Any new system
must not only be available to support
communication needs during an areawide emergency, but an amateur must
be able to break in and use the system during an accident or other local
emergency.
Limited Linking
As mentioned above in the Evergreen Intertie example, a severe
limitation of the current FM-analog repeater system is the number of contacts
that can be handled by a link. An ideal
repeater-to-repeater link would have
much wider bandwidth than the existing links. This bandwidth would then
be dynamically allocated between voice
contacts and high-speed data users.
In addition, as we have seen, it is
difficult for the user to establish links
between repeaters. With today’s lowcost computing power, a more automated method of calling a distant ham
could certainly be developed.
Data and Voice
The final and very significant limitation of the existing systems is that
repeaters can only handle voice or data,
not both simultaneously. As we shall see
later, there are many applications that
could be opened to Amateur Radio operators if this feature could be incorporated in a new VHF/UHF system.
Desired Properties
Having considered the features and
limitations of current analog FM systems, let us next consider what properties any new analog or digital systems should have. Ideally, any new
system should solve the limitations of
the existing systems without losing
any of the features. In addition, any
new system should have the properties described below and in Table 3.
Table 3—Desired Properties of New
VHF/UHF Amateur System
•
•
•
•
•
•
•
•
•
•
•
•
•
Compatible with regulations
Worldwide standard
Enables new applications
Enhancement friendly
Scalable
Open standard
Repeater operation
Linking repeaters
Simplex
High speed data
ANI
Expandable
Affordable
A Worldwide Standard
Why is it desirable to be compatible with an international standard?
The primary reason for all countries
to share the same system architecture
is to make the radios affordable. If the
radios and the repeater infrastructure
are not within the financial reach of
the majority of the ham community,
no amount of extra features will make
a new system successful.
Radio costs are dramatically dependent on sales volume both because research and development costs can be
spread over a larger number of radios
and because manufacturing unit costs
decrease as volume increases. Today
North America accounts for about a
third of the world’s Amateur Radio
licenses. Japan, with a far smaller population, has about another third. So if
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Japan and the US agree on a single
standard, the manufacturing volume
could double, which would dramatically
reduce radio costs for all amateurs. If
the rest of the world also joined in the
standard, further cost reductions would
follow. As an example of what can happen when there is not a world standard,
consider the 222-MHz band.
Because the band is not available
worldwide, manufacturers offer a limited number of transceiver models. And
we find that equivalent rigs (if even
available) tend to be more expensive
than the high-volume 2-meter rigs.
A second reason for a worldwide
standard is that tying repeaters together via the Internet is becoming a
popular feature of today’s repeater
systems. Any new system must support this trend for both voice and highspeed data. This could be done by
specifying a protocol that two otherwise incompatible systems would use
to exchange data; but due to the economic issues discussed above, this is
a less-than-ideal solution.
New Applications and
Enhancements
To take full advantage of the digital revolution in Amateur Radio, minimum standards will need to be established. Unlike a telecom system, which
needs rigid standardization, an Amateur Radio system must have just
enough standardization to allow communication, without inhibiting innovation.4 This is a difficult balance and
requires a great deal of work during
the system design to properly blend
these conflicting requirements.
Ideally, any new communication
system would be a perfect “wireless
cable.” Of course, one of the things of
interest to QEX readers is that no
communication system is perfect. The
study of the impairments and experimenting with ways to improve communications over an impaired channel are interesting areas of our hobby.
Every system involves a great deal of
compromise; that is a part of daily life
for the communication-system design
engineer. An ideal system for Amateur
Radio would allow a great deal of experimentation that could be layered
on top of a well-functioning, but not
overly constraining, radio system.
A Scalable System
A cell-phone system will not work
until the complete infrastructure is
deployed in an area. Clearly this is not
practical for Amateur Radio. Any new
Amateur Radio VHF/UHF system
must be able to work with only one
repeater and even—within the limits
of line-of-sight propagation—without
repeaters at all. Multiple repeaters
and the linking of repeaters can come
later as the user base develops or as
funds become available.
In addition, it is important to be
able to communicate with other hams
who have not upgraded to the new
system. This can be done in two ways:
The radios themselves could have analog FM capability, or the repeater can
be capable of interfacing with the existing analog radio repeaters.
The system should also be scalable
to facilitate emergency operation. Natural or man-made disasters can destroy
both the commercial and amateur communication infrastructure. A new communication system should be able to
work immediately without repeaters
and be flexible enough so that spare
transceivers can be connected to quickly
form an emergency, temporary repeater.
An Open Standard
As this is an Amateur Radio system,
the system should be available from
more than one manufacturer. It is desirable to have competition between the
radio manufacturers to keep prices low
and encourage innovation within the
framework of a new standard.
Yet it is at least as important to those
of us with QEX leanings that the system technology is such that a ham with
a sufficient technical background can
make any part of the system, including
radios, repeaters and repeater links.
Because so much of leading-edge communication technology is the intellectual property (IP) of communication
companies, the requirement that hams
be able to develop and publicize equipment without violating patent rights
becomes a system-design challenge.
Further Requirements
High-Speed Data
Fixed site-to-site data links at
greater than 9600 bits/s are rare, but
not unknown in Amateur Radio. Any
new VHF/UHF system should support
high-speed data, not only for these fixed
links but also for mobile and portable
operation. This means the system must
tolerate channel impairments like
multipath and Doppler shift.
To be able to interface into the vast
array of low-cost hardware and software available today, the new VHF/
UHF system should appear as a “wireless Ethernet cable” to a PC. You
should be able to use any software that
can interface with the IEEE 802.3
(10Base-T) Ethernet, connect a cable
from the PC to the transceiver and use
the computer just as if it was a wired
connection. For example, if the other
half of the RF link is connected to an
ISP, then an Internet browser will
work seamlessly and the Amateur
world will have high-speed wireless
Internet connections.
Repeater Operation
Because the FCC requires that the
control operator is able to shut down
the repeater and repeaters are often
in remote locations, remote control of
the repeater must be designed into any
new system. With today’s systems,
shutting down the system is the only
option available if a user abuses the
repeater. A new system should have
the ability to block offenders from repeater access while still allowing others to access the system.
Control over landlines is certainly
required, but radio control operation is
necessary for those sites without phoneline access. Of course, it is highly desirable that the control operator can use
the data capabilities of the system to
monitor the status of the system and
control many other features.
Linking Repeaters
Any new Amateur Radio system
must have a wide-bandwidth links capable of supporting multiplexed contacts. Multiple contacts are necessary
because the system should support
multiple repeaters at a single site as
well as different pairs of sites using the
link at the same time. The link must
also support both voice and data so that
we do not have to invest in two links.
The best way to meet this need is for
the link itself to be digital and the voice
digitized for transmission over the link.
Because more than one pair of repeaters is using the link at a time (multiplexed), the link must be full duplex.
Each site in the system repeats the
high-speed link signal and extracts
and adds the contacts that are appropriate for its site. Normally only one
repeater in the system would broadcast the contact. The other repeaters
ignore contacts that are not directed
to them. Otherwise one contact would
tie up the entire repeater system.
Most often, these repeater links
would be microwave links. However,
because of the distances involved, it
may be more attractive to use Internet
linking with some repeater systems.
Any new VHF/UHF Amateur system
should support both types of links.
Simplex
Any new system, no matter what
benefits are available from repeater
operation, must be able to work simplex without a repeater. And unlike
typical fixed digital radio networks, it
is critical that anyone tuning the bands
can immediately listen in on a contact.
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This requires two properties that are
not available on many digital radio systems. First, the digital voice system
must work without handshakes. That
way it is possible to have one talker and
many listeners. Second, it must not be
necessary to wait for the start of a new
transmission to acquire the carrier,
frame and bit synchronization necessary to demodulate the contact.
Automatic Number Identification (ANI)
Any new Amateur system should
support a higher level of automation
in establishing a contact. An equivalent feature to what the commercial
Land Mobile market calls “Automatic
Number Identification” (ANI) needs to
be developed. With this feature enabled, your radio opens squelch only
when your call sign is received. Some
hams do this today with a DTMF code
rather than their call sign, but DTMF
codes are not unique and DTMF signaling is very slow. If desired, the
radio can beep when you are called or
in mobile applications, the horn can
sound.
This call-sign squelching principle
should also be extended to repeaters.
Repeaters today use CTCSS tones to
keep from being opened accidentally
by interfering signals. On any new
system, you should use the repeater
call sign to unambiguously and easily
open the repeater. This would be followed by the call signs of the party you
are calling and the repeater they use
so that the system can route your call.
Affordabilty
The system must be designed to be
tolerant of the performance limitations of reasonably priced components.
Particularly with high-speed data at
UHF, the frequency and time accuracy
requirements of many modern digital
radio systems are so great as to be
prohibitively expensive for amateur
usage.
Another reason for designing the
system to be reasonably tolerant of
component and system variations is
to allow enterprising Amateurs the
opportunity to homebrew their own
D-STAR hardware.
Also to save user cost, the system
should not be designed for full duplex
operation. Full-duplex operation requires expensive isolation between the
transmitter and the receiver. Halfduplex and simplex operations allow
the sharing of many expensive components between the receiver and
transmitter. Finally, radio amateurs
almost always operate in these lowcost modes so there is no problem with
conversion to new modes.
Advantages of Digital Modulation
In the next part of this series, we
delve into the engineering design
considerations that were made in developing D-STAR and the technical details of its implementation. Yet, let us
conclude by investigating the advantages of a new system based on digital instead of analog modulation.
The first advantage of digital modulation is the ability to reduce occupied
spectrum. To meet the regulatory pressures discussed above and to reduce
the congestion on our bands, any new
system must be spectrally efficient.
One solution would be to stay with an
analog FM system and reduce the deviation, as the FCC has required of the
Land Mobile Service. However, doing
so reduces the audio quality that is the
major benefit of FM.
It is a better solution to change
the modulation completely and
transmit voice using digital modulation. However, without careful system design, switching to digital
voice could actually increase the
bandwidth required for voice communication because of the high bit
rates required by uncom-pressed
voice. For instance, pulse-code
modulation (PCM), as used by the
US telephone standard, requires a
digital stream of 64,000 bits/s. Even
with very elaborate modulation
schemes, that high bit rate would require a much wider bandwidth than
current FM voice radios.
The enabling technology for digital
voice is digital signal processing
(DSP). It has long been realized that
the information in a voice signal is
highly redundant and that it should
be possible to establish good transmissions without sending the redundant
information. Modern high-speed, lowcost signal processors and very clever
algorithms can dramatically reduce
the bit rate required to accurately reproduce a human voice in real time.
We shall see that it is possible to get
similar voice quality at only 2400 bits/
s and therefore occupy far less spectrum than today’s FM systems.
A second advantage of digital
modulation is improved quality. With
wide-band systems like HDTV and
high-speed wireless Internet service,
the most important advantage of digitizing transmissions is the ability to
use DSP to correct for transmission
errors. This results in improved performance over the vast majority of the
operating area. In analog radio communication systems we have little
choice but to live with the errors
caused by propagation, noise and interference (both natural and man-
made). We can sometimes increase the
received signal-to-noise ratio by increasing transmitter power and/or
using gain antennas. However the fading caused by multipath propagation
is not improved by increasing the
transmitted signal power. Particularly
in mobile wide-band systems, multipath can be a serious problem. You
have probably heard multipath impairment if you listen to FM broadcasts in your car. It is perhaps most
noticeable if you are at a stop light and
hear distortion but move a few feet
and the distortion disappears.
But perhaps the greatest advantages
of digital voice transmission are the
added features that are possible when
a digital data payload is added to a voice
contact. The availability of simultaneous low-speed data transmission
with voice transmissions opens up a
whole world of new possibilities for
Amateur Radio. Imagine sending still
pictures, maps, small data files and
GPS position while rag chewing. What
would it be like to have “instant messaging” on your radio? What a great way
to politely break into a contact!
Notes
1Remember, however, the D-STAR specifications discussed in this article have not
yet been finalized.
2See for example “Practical HF Digital
Voice,” by Charles Brain, G4GUO, and
Andy Talbot, G4JNT, QEX May/June
2000, pp 3-8.
3See, for instance, the editors preface and
Paul Rinaldo’s, W4RI, comments in a
sidebar in “Practical HF Digital Voice,”
pp 3 and 4.
4See “Technical Standards in Amateur Radio,” Doug Smith, KF6DX, QEX Mar/Apr
2003, p 2.
An Extra class license holder, John
usually is found on the HF bands, primarily operating PSK31. At age 3,
John exhibited early talents in electronics by “helping” his dad fix a TV.
He plugged the speaker into a wall
socket! Despite this traumatic start, he
spent his youth building Heathkit and
Eico equipment, repairing vacuumtube radios and TVs and designing
and building numerous homebrew
projects including a Morse decoder
high-school project built with resistortransistor logic in the mid 1960s.
With BSE and MSEE degrees in
control and communication theory, he
has worked for Hewlett-Packard in the
fields of spectrum and network analysis and frequency synthesis. He is currently the research department engineering manager at ICOM America,
where his primary interests are digital communications and DSP. John
has eight patents and is currently applying for four more.
††
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D-STAR, Part 2:
Design Considerations
Come learn what JARL put into their
proposed new VHF communication standard.
By John Gibbs, KC7YXD
I
n the first segment of this series,
we considered the attributes we
would like to see in any new VHF/
UHF Amateur Radio system. In this
second segment, we discuss the technical issues involved with selecting the
parameters of an Amateur Radio digital system to meet those attributes.
Perhaps the easiest way to organize
the design considerations for a digital
radio system is to use the OSI Model.
The OSI Model is officially known as
The Basic Reference Model for Open
Systems Interconnection. We start the
description with the bottom level of the
OSI model, the physical layer. Then we
will work our way up to the top levels,
which are open to amateur experimentation and application development.
Physical Layer
Transceiver Frequency
We hams have a large spectrum allocation at 1.2 GHz (60 MHz wide in
18225 69th Pl W
Lynnwood, WA 98037
kc7yxd@arrl.net
the US) that is little used today. In fact,
we should use this spectrum or risk losing it to commercial interests. Also, if
we want to develop a system to send
high-speed data, we will need a widebandwidth signal and there is little
available spectrum at 70 cm. If it is desired to use the D-STAR protocol at
lower frequencies, the high-speed data
mode could be dropped. In fact, a prototype portable 2-m HT using only the
digital voice mode has been developed
and was shown this spring at IWCE.
Like the previous FM system, the
system design logically calls for halfduplex operation for digital voice and
simplex for high-speed data.
Repeater Link Frequency
Since it is desired to have multiple
contacts and high-speed data packets
on the repeater link, a wide bandwidth
is required. Therefore the repeater
backbone must be at microwave frequencies and the amateur band at
10 GHz is a logical choice. Today’s
equipment generates usable 10 GHz
power, is affordable and the 10 MHz
of bandwidth is practical. The bi-
directional, asynchronous nature of
multiple contacts demands a fullduplex repeater link.
Given the high-speed data requirements of the system and the desire to
use digital voice to reduce transceiver
spectrum requirements, the repeater
link should be digital. With the asynchronous nature of the system, packet
mode is a natural choice. However,
because a packet system does not
guarantee real-time communication,
voice should be given priority over
data to minimize the possibility of
voice disruptions.
Modulation
An ideal modulation system should
generate a signal that has a narrow
spectrum, with low side lobes so that
it does not interfere with other nearby
users. On our crowded ham bands, this
is becoming more of an issue daily.
There are several commonly used
modulations for digital data. The digital modulation scheme chosen will significantly affect the performance of a
communication system. Generally we
want to maximize the data rate within
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the constraints of acceptable level of
latency, available bandwidth, acceptable error rate, product costs and operating environment (that is mobile,
portable, fixed-link). In particular,
mobile and portable operation causes
variable multipath fading and fast
phase shifts that can wreak havoc
with digital radios.
Less spectrally efficient modulations
generally have better operation characteristics in poorer SNR conditions. Also,
they are more forgiving of frequencyoffset errors between the transmitter
and receiver and frequency and phase
response error on the channel—an
important consideration if costs are to
be kept down in a UHF system.
4 FSK— FSK, MSK and GMSK are
very attractive because they are constant amplitude modulation. This
means that the power amplifier can
be class C, which offers low cost and
excellent power efficiency.
FSK has of course been used in
amateur systems for years, dating
back at least to the introduction of
RTTY. Newer variations on FSK use
more frequencies than just mark and
space. For instance, the new weak signal mode, JT44 uses very slow FSK
(about 5 Hz data rate) with 44 different frequencies each corresponding to
a character.1 But at the higher data
rates needed for a VHF/UHF digital
voice system, four FSK frequencies
offers an attractive option for
improved FSK performance.
GMSK— Among FSK, MSK and
GMSK, GMSK offers the best spectral
efficiency with only a slight degradation in the BER compared to FSK and
MSK. These advantages have made
GMSK one of the most popular digital modulations worldwide. Other
more complex modulations like QPSK
require a more expensive linear power
amplifier that also typically requires
more current, which is critical in portable operation.
GMSK low-pass-filters the data
stream with a filter that approximates
a Gaussian time and frequency
response. A Gaussian filter is used
because of its desirable properties in
both the time and frequency domains.
This filtering reduces the high-frequency content of the modulation and
therefore narrows the frequency spectrum of the modulated signal while
widening the data response minimally.
However, as you continue to narrow
the filter, the spectrum continues to
narrow and the time response of the
filter lengthens. This causes the peak
amplitude to decrease and the adjacent data tails of the time response to
1Notes
interfere with the decoding of the desired symbol, a phenomenon called
inter-symbol interference (ISI).
A typical compromise between ISI
and bandwidth used by many systems
is for the bandwidth/data rate ratio to
be equal to 0.5. This yields almost no
degradation due to ISI compared to
MSK and yet dramatically reduces the
spectral occupancy of GMSK compared to MSK.
QPSK— In theory, quadrature
phase-shift modulation could have a
constant amplitude format. However,
the rapid switching of the input data
causes a QPSK signal to have large
sidebands that destroy its spectral efficiency. Therefore in practice, raised cosine filters are used on input data to
reduce these sidebands. To preserve the
wave shape induced by these filters requires the use of a more expensive and
less power-efficient linear amplifier. If
a class-C amplifier were used with
QPSK, the sidebands that were
removed by the cosine filter would be
regenerated.
In the presence of additive white
Gaussian noise (AWGN), QPSK requires about 3 dB less signal-to-noise
than does FSK. However, in real channels, with multipath and poorly
synchronized receivers, the 3-dB
advantage quickly disappears.
Data Link Layer
Time Division Multiple Access (TDMA)
TDMA is one of the two commonly
used multiplexing standards for cellular phones. The cell tower site acts as
the master clock and assigns a time slot
to each of several cell phones that are
assigned the same frequency. For proper
operation, it is critical that each phone
transmit and receive exactly in its assigned time slot. This is not attractive
for amateur simplex operations because
operations are as two or more equals,
and there is no master to determine the
clock and assign time slots.
Any Amateur Radio system has to
work without a centralized frequency
reference and master clock. In addition, the radios must be able to acquire
signals that are somewhat off frequency and acquire timing without the
need for a separately transmitted
clock signal. These requirements may
make an amateur system less spectrally efficient than a centrally-controlled system like the cell phone, but
they are more in keeping with the
spirit of Amateur Radio, particularly
the capability to operate when the
infrastructure is destroyed.
Code Division Multiple Access
(CDMA)
CDMA (also known as spread spec-
appear on page 28.
trum) is also used for multiplexing cellular phones. In CDMA, several cell
phones share the same frequency and
transmit simultaneously. Each phone
on a frequency is modulated with a code
sequence that spreads the spectrum in
a unique way. If the receiver is synchronized and has the same code sequence,
then the signal is restored. Otherwise,
the signals from other phones become
part of the background noise.
An important limitation on the system is that undesired signals are not
completely rejected. Depending on the
length of the codes used and the attendant difficulty in synchronizing, perhaps 20-30 dB of so-called processing
gain can be attained. Therefore, a strong
nearby CDMA signal can overpower a
more distant signal. This classic problem with spread-spectrum communications is called “the near-far problem.”
In a cell-phone system, this problem
is addressed by power control. Since all
the nearby phones are communicating
with the same nearby cell site, the cell
site remotely controls the power level
of each phone to minimize the possibility of interference. However, in Amateur
Radio, particularly with multiple simplex contacts, this is not a solution.
Frequency Domain Multiplexing
(FDM)
TDMA, CDMA and other modern
multiplexing schemes require coordination between the units that is incompatible with the basic goals of the
Amateur Radio Service. One of the
major justifications for our service in
the US is emergency service. TDMA
and CDMA require an infrastructure
to provide the coordination. This infrastructure would quite possibly be
destroyed in an emergency. So, the best
solution for Amateur Radio is what we
have traditionally used, FDM.
Network Layer
In the network layer, the binary
data stream is divided into discrete
packets of finite length. In addition,
error checking is performed by cyclic
redundancy check (CRC) at this level.
If an error is detected, it is corrected
by the retransmission of packets.
Transport Layer
In the transport layer, we multiplex
and split all the data streams we need
to send and receive. In an Amateur
Radio system, we would typically need
to include repeater control data;
source, destination and routing information (that is, call signs of both operators and repeaters used); and what
is called the payload, which is the voice
or data to be sent.
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Presentation Layer
Codec
As mentioned in the first segment
of this series, simple PCM encoding of
voice results in a 64,000 bits/s data
stream. Codecs have been developed
to compress voice with good quality
down to 2400 bits/s and lower. These
codecs develop their extreme data
compression by modeling short segments of the human voice and only
transmitting the reduced information
needed to describe the voice model.
One of the major difficulties in
designing a digital voice radio is in testing the voice quality. High-compression
codecs are designed to work with a human voice; traditional tests like frequency response and harmonic distortion with sinusoidal tones do not generate meaningful results. Consequently,
a subjective method of testing called
mean opinion score (MOS) has been
developed. MOS is estimated by a test
with a group of normal listeners who
are asked their opinion on a five-point
scale (1 = bad, 5 = excellent) and the
results are averaged together.2
A very important factor in conducting MOS tests is the acoustical environment. Since the codec is designed
to highly compress the information in
a human voice, it is easy to imagine
that the presence of other signals and
noise can severely affect the performance of the system. An excellent test
for Amateur Radio is performance in
an automotive environment including
engine noise and wind noise from an
open window.
A final issue in codec selection is
the MOS performance in the presence
of the channel impairments we commonly find in VHF/UHF commun-
ication paths. As Digital Voice
Systems point out on their Web site,
“Vocoders…designed for extremely
low bit-error rates, such as those encountered in land-line communications, often experience serious
degradation when applied to the much
higher bit-error rates found in wireless communications. Consequently,
it is important to consider robustness
to channel degradations during the
vocoder-algorithm design process.”
Scrambling
Bit synchronization and accurate
level slicing in the receiver require frequent transitions in the data (no long
Table 1—D-STAR Transmission Characteristics
Mode
Backbone
Data
Digital Voice
ITU
AMBE
Transmission Speed
10 Mbps or less
128 kbps or less
8 kbps
2.4 kbps
Fig 1—Header additions with TCP/IP protocol.
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Bandwidth
10.5 MHz
130 kHz
9 kHz
5 kHz
strings of ones or zeros). To ensure that
condition, most digital radio systems
use a device called a scrambler to randomize the input data stream.
“Scrambler” is an unfortunate term
for people who are familiar with analog radios; it is common to interpret this
as encryption. The FCC of course
forbids encryption for amateurs. Scrambling is not an attempt to hide the message content, however; it is a fixed and
published method known by all potential receivers for converting the input
data stream into a data stream with
short strings of ones or zeros.
Scrambling is typically done with
a shift register and exclusive OR gates.
CCITT recommendation V.26 recommends this procedure.
Application Layer
This is the layer where hams can
begin to customize the system and add
their own applications. In addition,
this is the level where the system design allows for user control entry and
data entry, both data from the IEEE
10BaseT Ethernet and analog audio
from the microphone.
D-STAR Proposed Standards
As stated in the first part of this
article, D-STAR is not a finalized standard at the time of this writing. However, the field trials are finished and
standard publication begins here.
Table 1 shows the system as it stands
at this writing.
To describe the proposed D-STAR
standard, we will start at the input
side of the transceiver and work our
way out to the antenna, working our
way down the OSI model. We will
then see how the standard defines
the repeater operation and the links
between repeaters. First we will consider the high-speed data mode and
then the digital voice.
High-Speed Data
The standard interface for highspeed data into and from the D-Star
system is IEEE802.3 (10BaseT Ethernet.) In Fig 1 you can see how the
D-STAR transmitter adds a radio
header extension to the Ethernet message just as the Ethernet protocol added
a header to the Internet Protocol, TCP/
IP. Since this radio header is stripped
off in the receiver, the radio link appears
to be a “wireless Ethernet cable.” Therefore, it is possible using existing
software (such as browsers) to communicate the same images, text and voice
as is handled by Ethernet, including
links to the Internet, without modification.
Data Multiplexing
Fig 2 shows the details of a communication packet from the radio part
in Fig 1. Each packet consists of a
radio header and the Ethernet packet
described above followed by an errorchecking frame. The radio header is
worthwhile to study in some depth as
it shows many of the D-STAR system
capabilities.
Each frame of the radio header is
identical in both the high-speed data
mode and in the digital voice mode.
If the standard is approved as proposed, it will contain the following
information:
The first two fields are common to
most digital radios, the bit sync and
the frame sync. These preambles are
designed to allow the receive modem
to establish timing and level lock as
quickly as possible.
The flag field describes the content
of the data field.
Bit 7
Data or voice communication
flag.
Bit 6
Repeater or simplex flag.
Bit 5
Communication-interruption
flag.
Bit 4
Control signal, data or voice
signal flag.
Bit 3
Emergency/normal signal
flag.
Bits 2-0 T r a n s m i s s i o n - c o n t r o l
bits (see below).
The ID field can hold four call signs:
1. The local repeater you are accessing (optional).
2. The linked distant repeater the
called party is using (optional).
3. The station you are calling (can be
CQ).
4. Your own call sign.
The PFCS field is a check word for
the header. Some of these bits require
a little more explanation if we are to
understand the operation of the system. Notice that when bit 3 of the flag
field is set, you are asking for an emergency break-in. (On many FM repeaters you would say “break” today.) For
Fig 2—Proposed bit pattern in high-speed digital mode.
Table 2—Call-Sign Combinations and the System Function
(Uses Evergreen Intertie Call sign examples3)
Called
Departure
Destination
Station
Repeater
Repeater
CQCQCQ
KB7WUK
K7NWS
N7ABC
KB7WUK
K7NWS
N7ABC
K7NWS
K7NWS
N7ABC
DIRECT
DIRECT
Own
Station
KC7YXD
KC7YXD
KC7YXD
KC7YXD
Function
CQ Portland
Call N7ABC in Portland
Call N7ABC on local repeater
Simplex
Sept/Oct 2003 25
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instance if you want to report an accident, pushing the emergency key on
the radio will set bit 3 and all D-STAR
radios within range will open squelch
and their volume to be set high.
The flag field bits 0, 1 and 2 are
used for transmission control. They
implement functions like ACK, ARQ
and repeater control.
One of these repeater control functions is repeater lockout. Repeater
lockout is used mainly to block illegal
stations. A D-STAR repeater can hold
a black list of call signs that have consistently violated repeater and/or FCC
rules. If a blacklisted station calls the
repeater, the repeater does not repeat
the message but instead calls the
offending station back with the lockout
bit set. The offending station’s radio will
then display a message indicating that
it is blocked from the repeater. So now,
it is not necessary to shut down the
repeater for everyone when one individual is misusing the repeater.
Another important field for understanding the capabilities of the system
is the ID field. Understanding the ID
field is important because it shows the
great flexibility available in the system calling capabilities. The first thing
to notice is that the D-STAR protocol
automatically IDs at every transmission. This easily meets the FCC ID
requirements for ID at start, end and
every 10 minutes of transmission.
Next, to understand how the four ID
fields work, Table 2 illustrates the contents of each field if KC7YXD were to
transmit on a fictional D-STAR Evergreen Inter-tie system. It is not necessary to always fill in all the ID fields. If
you respond to a CQ or a call directed
at you, your D-STAR transceiver will
automatically fill in the fields for you.
Digital Voice
Codec— Those of you who had a
chance to see the D-STAR presentation
at last year’s Dayton Hamvention or at
the DCC in Denver last fall may recall
that the Digital Voice mode occupied
8 kHz of bandwidth using the ITU
G.723.1 Codec standard. At that time,
the two codecs were undergoing field
trials; today the JARL has selected
AMBE as the standard. The two standards under consideration were the ITU
standard G723.1 and a Digital Voice
Systems proprietary codec that uses the
AMBE algorithm.
ITU G.723.1 uses an ACELP (algebraic code-excited linear prediction)
algorithm that generates a 5.3-kbps
data stream. With an algorithm delay
of 37 ms, the total wireless-communication-throughput delay is a little over
100 ms, quite reasonable for halfduplex communications.
AMBE stands for advanced multiband excitation. AMBE can use different levels of compression to trade off
voice quality and bit rate. Tests show
that at the 2.4-kbps data rate, the voice
quality was at least as good as the
higher-data-rate ITU G.723.1 over real
radio links. The algorithmic delay is
only slightly longer than G723.1
(44 ms), so the factor-of-two improvement in data rate (and spectral efficiency) comes with no noticeable latency
increase.
The data-rate reduction from the
AMBE codec is particularly significant
because of worldwide pressure from
regulatory agencies to reduce the occupied bandwidth of voice communications sufficiently to allow 6.25-kHz
signal spacing. When using a modulation scheme sufficiently robust to give
reliable communication in mobile and
portable applications, only the AMBE
data rate meets this signal-spacing
requirement.
The decision between codecs is complicated by the fact that G.723.1 is an
Table 3—AMBE Vocoder-Based
Systems
Inmarsat
Thuraya
Iridium
APCO Project 25 (IMBE)
G4GUO & G4JNT HF Digital Voice
System
open public standard codec whereas
AMBE is the patented intellectual
property of Digital Voice Systems.
Unlike many companies, however, the
present owner of this technology supports the Amateur Radio community
and is willing to sell these parts in
small quantities.
The JARL is not alone in deciding
on AMBE for its high voice quality and
very low bit rate. Table 3 shows several
digital systems that have standardized
on this codec technology. For instance,
the Telecommunications Industry Association (TIA) selected DSVI’s codec technology over CELP and other codecs for
the APCO Project 25 North American
land-mobile radio-communication system. This is particularly significant because at least two Amateur Radio
groups are evaluating Project 25 radios
as an alternative digital radio standard
for amateur usage.
Fig 3 illustrates the bit pattern used
in the digital voice mode when the
AMBE codec is used. As mentioned
before, the radio header is identical to
the high-speed digital mode radio
header and so will not be discussed
here.
The most interesting part of Fig 3
is that the digital-voice data frames
are interleaved with data frames.
These frames are currently reserved
by the D-STAR standard with no dedicated usage by the system overhead.
This means that the system is capable
of supporting a 2400-bits/s data
stream from a user application while
the user is talking on the system!
Notice that the D-STAR system itself
provides no error detection for this
data, so it would be up to the user’s
application to provide error detection
and error correction. This and other
overhead would decrease the end-toend data rate slightly; but if radios are
built to exploit this capability, hams
could potentially add many interesting features to the D-STAR system.
What is not shown in Fig 3 is that
the frame and sync fields are repeated
often so that the errors between the
transmitter and receiver clocks can be
Fig 3—Bit pattern in digital-voice mode.
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corrected without requiring a master
clock signal. It also means that another
amateur can tune into the middle of a
contact and listen to the conversation
without waiting for the sync frames of
the radio header at the next over.
Modulation
Several modulation methods were
investigated during the development of
the D-STAR standard. Modulations
tested included GMSK, FSK, 4-FSK,
MSK and QPSK. GMSK has been
selected for the backbone line between
repeaters. The standard for the portable
and mobile transceivers may include
more than one modulation format.
Gaussian minimum-shift keying
(GMSK) and quadrature phase-shift
keying (QPSK) are the two finalists. A
third, 4-FSK, has been recently proposed as an alternate standard and is
now under investigation. The reason for
the delay is that selecting the best
modulation for D-STAR real world
applications is not a trivial exercise. In
real mobile communications systems,
the link between a moving node and a
base station will be subject to
multipath, which results in Rayleigh
fading. This will have a significant effect on the resultant BER performance,
possibly increasing the required C/N for
a specific BER by as much as 10 dB.
QPSK is commonly used in fixedlink communication systems. Under
ideal conditions, QPSK would give
better performance than GMSK or
4-FSK and its higher spectral efficiency is obviously attractive. However, QPSK’s higher spectral efficiency
also leads to higher susceptibility to
transmission impairments such as
multipath and phase hits. Yet, the biggest disadvantage of QPSK is the need
for extremely linear power amplifiers
to avoid spectral growth—what we
Amateurs call splatter.
The front-runner at the time this
article is written is GMSK. In its favor,
GMSK is a well-proven technology, and
probably the most commonly used digital modulation in the world for portable
applications (see Table 4). GMSK has
two basic advantages. First, it is more
robust than QPSK to common transmission impairments. Second, GMSK, as a
form of FSK, has constant amplitude
and can therefore use very efficient
class-C power amplifiers. Third, GMSK
is not as sensitive to frequency errors
between the transmitter and receiver.
Because no master frequency reference
is available in the D-STAR system, tuning errors on a 1.2-GHz signal can be
substantial, particularly with the extremes of temperature found in portable
operation. The alternatives are to suffer the expense of a precision frequency
reference in all the radios or adopt a
modulation method like GMSK that is
more tolerant of frequency errors.
However, GMSK is not so spectrally
efficient as QPSK. For instance, at
128 kbps, GMSK with a BT product of
1
/2 occupies a bandwidth of 135 kHz.
For the same data rate, QPSK requires
only 83 kHz.
The best solution is probably to use
a codec with the AMBE algorithm described earlier that reduces the data
rate as far as possible and then use
GMSK for more robust communications, but the tests still continue.
Fig 4 is a somewhat busy graph
that dramatically shows the difference
in occupied bandwidth. The existing
FM system bandwidth can be determined by Carson’s Rule to be about
16 kHz. While not all combinations of
modulation and codec algorithms are
shown, you can clearly see that you
can fit many more digital voice contacts into the same spectrum.
Table 4—GMSK is used in Systems
Worldwide
Repeater
GSM cell phone
DECT
Cellular Digital Packet Data (CDPD)
Mobiltex
The digital voice mode is halfduplex with a 20-MHz offset between
transmit and receive frequencies.
High-speed data is simplex.
As shown in Fig 5, much of the repeater site function is to provide a
Fig 4—Occupied bandwidth of digital radio.
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Fig 5—Integrated site with analog and digital radio repeaters, high-speed backbone and Internet connection.
gateway to other repeaters, both at
other sites and to repeaters on different modes. The repeater also provides
an interface to the repeater backbone
link to the Internet if desired. The system is designed to support remote control of the repeater over radio and/or
landline links.
Repeaters could be linked via the
Internet instead of the backbone, but
because of bandwidth limitations,
much of the high-speed multiple contact capability would be lost.
Repeater Call Sign Protection
To protect repeaters from co-channel interference, CTCSS tones are
used to prevent interfering signals
from triggering the repeater. In the DSTAR system, the digital header contains the call sign of the repeater to
be accessed. If the repeater does not
see its call sign in the message, the
repeater is not opened.
The Backbone Repeater Link
One of the major advantages of the
D-STAR system is the full-duplex
10-MHz-bandwidth backbone link between repeaters. This wide bandwidth
allows multiple voice and data contacts to occur simultaneously on the
link. An analysis of the frequency of
use of data and voice communications
demonstrated that a 10-Mbit/s fullduplex link would support the needs
of up to 12 linked repeaters.
The high-speed data and digitalvoice data streams from multiple repeaters are multiplexed into a single
data stream according to the asynchronous transfer mode (ATM) standard.
This 10-Mbit/s data stream is GMSK
modulated onto a 10-GHz carrier, resulting in an approximately 10-MHz
wide signal.
The ATM cell is made up of a short
53-byte packet that consists of a 5-byte
header and a 48-byte payload. The ATM
cell is sent to the required destination
according to the preset list that is set
by the ATM switch set at each repeater
site. Because the priority level can be
designated in the header, voice signals
arrive in real time. This avoids the delays that happen with VoIP on the
existing Internet Protocol.
Backbone field tests have been carried out with a 36-dB-gain parabolic
antenna and a 1-W transmitter. Heavy
rains in southern Japan of more than
12 inches per hour limit the practical
distance that the repeaters can be
separated. It was found that taking into
consideration these extreme weather
conditions, the maximum range for
uninterrupted communications is about
12 miles. Obviously, the fog and rainfall at the location and the acceptable
probability of communication interruption dramatically affect this number.
Notes
1“JT44: New Digital Mode for Weak Signals,”
(World Above 50 MHz) QST, June 2002,
pp 81-82.
2D. Smith, KF6DX, “Digital Voice: The Next
New Mode?” QST, January 2002. For a
discussion of MOS, see the sidebar, “How
Do I Sound?” on pp 29.
3K7NWS is a Seattle, Washington, repeater and
KB7WUK is a Portland, Oregon, repeater on
the Evergreen Intertie. These examples assume an identical system to the Evergreen
Intertie but based on D-STAR.
††
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D-STAR, Part 3:•
Implementation•
We’ve seen the “whys” and “hows” of D-STAR. Let’s
look at the hardware and possible uses for the system.
By John Gibbs, KC7YXD
T
his article, the final part of the
series, investigates the block
diagram and performance of the
prototype equipment to better understand the design issues of a D-STAR
digital radio.
The hardware used in testing the
D-STAR standard is shown in Fig 1
and the performance of the mobile unit
is summarized in Table 1. Some of this
hardware is available today and we
expect several manufacturers will offer hardware soon.
Recall that the D-STAR standard
has only recently finalized the selection of the modulation and codec. Prototype testing demonstrated that
GMSK modulation and the AMBE
2020 codec gives the best combination
of spectral efficiency and robust communications.
The IF and RF parts of the block
diagram (see Fig 2) of the ID-1 shows
a straightforward dual-conversion
18225 69th Pl W
Lynnwood, WA 98037
kc7yxd@arrl.net
superheterodyne design that should
look familiar to those experienced with
analog rigs. However, several issues in
a digital-radio IF are not clear from
the block diagram.
IF Design Issues
The first issue with digital-radio
IFs is that the group delay of the IF
structure is critical. While analog radio designers can ignore phase linear-
ity, group-delay variations need to be
less than about 10% of the data period to avoid excessive BER due to
intersymbol interference.
The second issue with digital radio
IFs is that IF bandwidth must be
wider than that of an equivalent analog design. It must be wider so that
significant energy does not fall near
the band edges of the filter because
there the group delay is not constant.
Table 1
ID-1 Specifications Summary
Operating frequency
Operating Modes
(FDMA)
Data Rate
CODEC
Data Interface
RF Power
Receive Sensitivity
(typical)
Switching time
GMSK Modulation
1.2 GHz Amateur Radio Band
FM (analog voice)
0.5GMSK (digital voice / data)
4.8 kbps (voice) / 128 kbps (data)
AMBE
IEEE802.3 (10Base-T)
10 W/1 W
FM
–16 dBu
4.8 kbps GMSK Voice
–10 dBu
128 kbps GMSK Data
+ 2 dBu
10 mS (digital mode)
Quadrature Modulator / FPGA (baseband)
42 Nov/Dec 2003
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It often rises significantly and displays
what are called “ears” (from their
shape). This is particularly true in the
receiver IF where transmitter and receiver relative-frequency tuning errors
may cause the signal to be off center
in the IF. Unfortunately, this increases
the noise and interfering signals that
pass through to the detector.
The quality of these IFs is measured by the sensitivity numbers in
the specifications and in the “eye” diagrams in Fig 3. The well-open eye
means that the receiver can easily distinguish between the plus and minus
signal sent and therefore decode with
very little BER. Fig 4 shows how the
BER improves as S/N increases in the
digital voice mode.
The final issue with digital radio
IFs is the quality of the local oscillators. First, as implied above, the frequency reference must be accurate
and temperature-stable if communication is to be established at UHF with
a reasonably wide receiver IF. Second,
the close-in phase noise of the local
oscillators must be kept low, particularly if QPSK and other high-data-rate
modulations are used. Excess oscillator noise can increase the BER just as
effectively as actual channel impairments. One of the advantages of
using GMSK is its relatively low sensitivity to these receiver problems, as
shown in Fig 5.
Mobile and Portable Internet Access
The application that springs almost
instantly to everyone’s mind is highspeed wireless Internet access. Part of
the reason is that the Internet has become such an important communication and information tool in hams’ lives
today. Another reason is all the hype
built up around third-generation (3G)
cell phones and the DOCOMO system
in Japan. Yet, with today’s meltdown
in telecommunication commerce, it
could be years before a 3G phone system is deployed in the US. So, with the
deployment of D-STAR, hams could
once again have a leading technology
that the rest of the population would
envy and that might encourage more
people to get their tickets.
In support of this vision of D-STAR
as an Amateur Radio community
growth agent, it is interesting to watch
the reaction of inactive no-code hams.
For a variety of reasons, they got their
tickets, but never really got interested
in the hobby. Often when they see a
D-STAR demonstration, you can see
their eyes light up and almost hear the
gears turning in their head! Several
have said that a system like D-STAR
would get them active again.
Because this is Amateur Radio,
there will be some restrictions on this
vision of high-speed wireless Internet.
The FCC does not allow encryption,
so there is no guarantee of privacy.
Anyone can look over your shoulder
and read your e-mail.
Some hams bring up the issue of
advertising and pornography. Control
operators will be responsible for the
content passing through their repeaters exactly as they are today. However,
this does not seem to be a very difficult issue. Inexpensive software exists
today that can filter out this offending material. Control operators can
easily incorporate so-called “kiddy filter” software into the repeater’s
Internet interface. If the existing software does not quite fit our application,
then resourceful hams will develop
better software!
Then there is the issue of thirdparty traffic. Again, the control operator is responsible for ensuring that no
illegal third-party traffic passes
through his or her station.
Baseband Design Issues
The baseband hardware and modulators have far more obvious differences in this digital radio block
diagram in Fig 2. For instance, on the
transmitter side, the audio input is
immediately converted to digital form,
even if the radio is in the analog FM
mode. This digital information is then
signal-processed digitally and modulated onto the first IF. The modulation
is accomplished by an I/Q modulator
made with an FPGA. When teamed
with DSP, an I/Q modulator is a very
versatile component that can handle
any form of modulation needed in the
ID-1. It is even possible to produce
narrowband-FM with the digitized
voice. (The analog FM feature is desired for compatibility with existing
analog radios.)
D-STAR Applications
D-STAR is very much a “blank
slate,” waiting for amateurs to write
upon it. We can exploit its capabilities
for a variety of old and new uses. Here
are a few of the many suggestions we
have heard from the Amateur Radio
community as possible applications of
D-STAR.
Fig 1—Currently available hardware (counterclockwise, from upper right): RC-24 Control
Head, ID-1 1.2 GHz transceiver, ID-RP1D 1.2 GHz data repeater, ID-RP1VS 1.2 GHz voice
repeater, ID-RP1L 10 GHz backbone repeater and AH-1045/1080 parabolic antenna.
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Fig 2—1.2 GHz transceiver block diagram.
Fig 3—“Eye” patterns for digital voice (A) and high-speed data (B).
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Combination of 802.11 and D-STAR
It would be surprising if manufacturers did not quickly develop a
handheld D-STAR-compatible radio,
but the high-speed data mode will necessarily have reduced range compared
to a mobile rig with a good antenna
and more power. What could you do if
you wanted to connect a notebook
computer to the Internet, but you are
beyond the limited range of a
handheld?
When hams have a range problem
with handhelds on today’s analog FM
system, they sometimes cross-band
repeat using their car’s mobile radio.
A similar solution could be implemented for high-speed data using
D-STAR and a wireless LAN access
point. A D-STAR mobile in your car
could be cross-band (and cross-mode)
connected to an access point installed
in your car. Only an Ethernet cable is
needed for this connection (no PC). If
you already had a wireless LAN card
in your notebook computer, you would
be ready to go. Your notebook computer now has high-speed Internet
access with the range of the highpower mobile.
posting of hams’ experiences with the
system as well as freeware and
shareware that they have found useful in D-STAR operation.
Visitors to the area could download
information they need, even at 3 AM.
Are you looking for a good Mexican
restaurant, or do you need a quiet motel away from the highway? The
Intranet could have suggestions from
other hams on file, and you could
download maps, driving directions and
even pictures.
The possibilities multiply enormously if your notebook computer has
GPS. Now D-STAR can guide you exactly to your destination with accurate
maps and directions that better reflect
the local driving conditions than those
provided by major services on the
Internet. Local hams could help you
avoid traffic problems caused by temporary road closures and accidents.
Emergency Communications: Another D-STAR Intranet application is
emergency communications. Even if
the local D-STAR repeater were
knocked out, temporary repeaters
could quickly be assembled using two
transceivers back-to-back. Training
needs are minimized by using standard Internet browsers. When an operator comes onto the system, he can
easily access stored files and bring
himself up-to-date on the situation
without distracting others.
Possible Add-Ons and
Enhancements
We wrap up this discussion of the
new D-STAR system with a treatment
of the possible directions in which applications might evolve. D-STAR is not
meant to be a turnkey communication
system like the cell-phone system. Instead, it is an infrastructure that hams
can use to meet current and future
communication needs. Most importantly, it is a flexible, highly capable
system that allows amateurs, them-
Other High-Speed Data Applications
The Internet is so pervasive today
that we sometimes forget that there
are many other uses for high-speed
data transmission. Here are two highspeed data applications that have
arisen in D-STAR discussions.
Local Amateur Intranet: Rather
than connecting to the Internet, a
club-sponsored repeater could offer a
wireless, wide-area Intranet. What
might they put on the site? It certainly
is a good place to make available the
repeater system’s operation guide and
rules. To encourage D-STAR experimentation, it would be useful to have
Fig 4—Bit-error rate versus RF level.
Fig 5—Frequency error versus sensitivity.
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selves, to expand their service. Rather
than depending on manufacturers to
provide new features and applications,
we expect the amateur community will
develop add-ons to the system that will
address the major goals of Amateur
Radio including emergency communications, experimentation and just
plain fun! Hams who have seen early
demonstrations of the D-STAR system
have generated the following ideas.
Power to the People!
We hams have our own opinions of
how products should be designed and
which features should be added. One
of the great things about the D-STAR
system is that for a large part, it is
possible for us to try out our ideas and
further the state of the art. Error correction is one area that is ripe for contribution by hams.
As data rates increase or as we
push the range, decoding errors begin
to be significant in any digital radio
system. This is less of a problem for a
properly designed digital voice system,
because it is not significantly disturbed by BER levels that would render digital file transfers impossible.
Yet any high-speed digital mode can
use any help it can get.
Because of the importance of transferring data quickly and accurately,
there has been a great deal of theoretical work done on coding and error correction. Tom McDermott, N5EG, gives
a good introduction to the many coding
techniques used in digital radio including Reed-Solomon, Golay and convolutional codes.1 Newer codes called “turbo”
codes have been developed that approach the theoretical limit on how fast
information can be transmitted over a
noisy band-limited channel.
However, these codes are only optimal if the interference is what we call
AWGN (additive white Gaussian noise).
This is true because the mathematics
of AWGN is well understood. The bad
news is that most of the impairments
we find in real radio communications
do not match this nice mathematical
model. The good news is this is an opportunity for Amateur Radio to again
advance the state of the art.
An interesting example of the possibilities of error correction is the ubiquitous CD player. A few years ago, I
saw a demonstration of the power of
the error correction used in CDs. The
professor had drilled large holes in a
CD and despite these obvious faults
in the data stream, the music played
perfectly without a click, pop or drop
out! Perhaps some enterprising ham
can discover the way to make just as
1Notes
appear on page 47.
dramatic improvement in radio.
Interleaving Spreads
Bursts of Errors
Wireless communication channels
with fades of the signal power are
prone to errors occurring in bursts.
Burst errors can cause problems by
breaking error-control codes when the
number of errors exceeds the maximum number of correctable errors for
the specific code used. For short bursts,
intraframe interleaving improves performance by spreading the burst of
errors over several different code
words. For example, if four code words,
each containing 23 bits that can correct up to 3-bit errors, are used in a
frame consecutively, then a burst error 4 bits long will break a single code.
However, if the four code words were
interleaved (that is, bit 1 codeword 1,
bit 1 codeword 2, bit 1 codeword 3, bit
1 codeword 4, bit 2 codeword 1, and so
forth), each code word would contain
only one error, which could easily be
corrected. Since intraframe interleaving only modifies the bit ordering
within the current frame, no additional delay is generally needed for
implementation. If additional delay
can be tolerated, interframe (more
than just the current frame) interleaving can be used to further increase the
performance with longer burst errors.
Mixed Voice and Data
As we saw in the section on the
D-STAR standard, the proposed digital
voice protocol has the ability to transmit low-speed user data simultaneously
with voice. The first-generation D-STAR
transceivers minimally support this
feature. However, as new radios are introduced, it is expected that hams will
develop applications that exploit this
capability. Notice that in the D-STAR
system, this is referred to as low-speed
data. Yet the data rate is actually about
2400 baud, faster than the old 1200
baud of amateur systems (and yes,
slower than the 9600 baud used in
higher-speed systems).
What could we do with this feature?
How about the equivalent of the
Internet’s “instant messaging”? With
instant messaging, messages could be
added by the sender or even from a
third party (where legal) and added
by the repeater. Imagine that you are
in the middle of a contact when
• A DX alert displays on your mobile
for a country you need, or
• A printer attached to your transceiver prints out route instructions
to your club’s Field Day site, or
• Your spouse sends you the grocery
list and reminds you that the lawn
needs mowing—well, maybe that
isn’t such a good feature!
How about doing instant messaging one better and send instant pictures. The miniature cameras used
recently in cellular phones are about
96×96 pixels; that is less than 10 kbits.
So, a picture could be sent in less than
30 seconds simultaneously with a
voice contact.
In a sense, this voice and data capability is like DSL: you can talk over
the same channel while data are
transmitted—although not at DSL
speeds in this mode. The data you can
send through this channel are limited
only by your imagination. For instance, what do you think about mixing voice and next-generation APRS?
VoIP and D-STAR
VoIP voice communication is of
course possible in the digital data
mode because it does not matter what
information is carried in the data.
However, VoIP is not a very attractive
method of communication via Amateur Radio today. It often suffers from
poor voice quality scores due to the
very long latency from the intensive
signal processing and because the
Internet does not give priority to voice
packets.2 These voice quality problems
would certainly not be helped by the
128 kbps data rate of D-STAR.
Finally, VoIP on D-STAR is spectrally inefficient, requiring 130 kHz of
bandwidth compared to less than
6 kHz for the highly compressed
D-STAR digital voice mode. Still, for
applications that require higher-speed
data simultaneous with voice, inventive amateurs may find solutions to
these problems.
Registration
The D-STAR proposal currently
keeps a list of amateurs (call signs)
who have accessed the system. So, if
you want to call me, KC7YXD, you
don’t need to know the linking repeater. The system simply finds the
repeater I last accessed and automatically routes your call to me. A logical
extension of this capability is that if
my radio is on the repeater frequency,
the system can poll it and automatically register me onto that repeater.
This feature could be extended to
keep a database at each repeater of
each registered amateur’s interests.
How would our hobby change if you
could call “CQ Collins radio collector”
and automatically link to someone on
the other side of the country or perhaps the other side of the world?
Roaming
Another feature that hams could
add to the system is roaming. What if,
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when driving through an area, the repeater could download into the radio
memory the frequencies and call signs
of nearby D-STAR repeaters? Then as
I drive away from the repeater, the
radio is all set to access the next repeater. Never again sit down with a
repeater book and program the radio
before the next trip!
Of course, if we were to add GPS
capability and the D-STAR repeater
database held the footprints, calls and
frequencies of adjacent repeaters, the
radio could automatically switch repeater sites as you drive through an
area!
Trunking
“Trunking” is a land-mobile-radio
term for a system that uses multiple
repeaters to support many contacts at
once. Most trunking systems use a
“home channel” for calling, then the
system assigns a clear repeater frequency to complete the contact. The
radios then automatically go to the
assigned frequency. The basic advantage of trunking is that the system can
support many more users simultaneously than with individual systems.
Effectively, it lets one listen to all repeaters in an area by only monitoring
the home channel for a call. Since the
D-STAR system sends call signs digitally, it is easy to envision a simple
computer program that would monitor the home channel and alert me
when I am being called.
Conclusion
Clearly, Amateur Radio is at a
crossroads today. Technical and regulatory forces are pushing us out of our
well-proven but inefficient ways. The
possibilities that digital radio brings
to our hobby are truly limited only by
our imagination.
I hope this article has stirred
your imagination and stimulated
your interest in the possibilities of
digital voice and high-speed data in
Amateur Radio today. Perhaps you
will be inspired to try the D-STAR system and maybe even develop applications or variations of the D-STAR
system.
Recommended Reading
Visit www.dvsinc.com to read more about
AMBE and to hear voice samples at various coding rates.
D.W. Griffin and J.S. Lim, “Multiband Excitation Vocoder,” IEEE Transactions on
Acoustics, Speech and Signal Processing,
Vol 36, No 8, August 1988, pp 1223-1235.
Notes
1Wireless Digital Communications: Design
and Theory, Tom McCermott, N5EG, Tucson Amateur Packet Radio Corporation,
1996.
2The D-STAR digital voice mode addresses
this problem by giving real-time data, such
as voice, priority over repeater links.
At age 3, John exhibited early talents
in electronics by “helping” his dad fix a
TV. He plugged the speaker into a wall
socket! Despite this traumatic start, he
spent his youth building Heathkit and
EICO equipment, repairing vacuumtube radios and TVs and designing and
building numerous homebrew projects
including a Morse decoder high-school
project built with resistor-transistor
logic in the mid-1960s.
With BSE and MSEE degrees in
control and communication theory, he
has worked for Hewlett-Packard in the
fields of spectrum and network analysis and frequency synthesis. He is
currently the research department engineering manager at ICOM America,
where his primary interests are digital communications and DSP. John
has eight patents and is currently applying for four more.
An Extra class license holder, John
usually is found on the HF bands, primarily operating PSK31.
††
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