The choice of modulation format for a mobile satellite service is complex. The subjective performance is summarized of candidate schemes and voice coder technologies. It is shown that good performance can be achieved with both analogue and digital voice systems, although the analogue system gives superior performance in fading. The results highlight the need for flexibility in the choice of signaling format. Linear transceiver technology capable of using many forms of narrowband modulation is described

Bateman, A.

Mcgeehan, J. P.

Whitmarsh, W. J.

English

NASA Technical Reports Server

N92-24171
Analogue and Digital Linear Modulation
Techniques for Mobile Satellite
W.J. Whitmarsh, A. Bateman and J.P. McGeehan
University of Bristol
Centre for Communications Research
Queens Building, University Walk
Bristol BS8 1TR, United Kingdom
Tel: +44 272 303726, Fax: +44 272 255265
Abstract
Tile choice of modulation format for a mobile satellite
service is complex. This paper summarises the subjec-
tive performance of a variety of candidate schemes and
voice coder technologies. It is shown that good perfor-
mance can be achieved with both analogue and digital
voice syst_ems, although the analogue system gives su-
perior performance in fading. The results highlight
the need for flexibility in the choice of signaling for-
mat. Linear transceiver technology capable of ut,lising
many forms of narrowband modulation are descr,bed.
Introduction
Tile mobile satellite environment places severe con-
straints on the selection of modulation systems. It
is important to balance the power efficiency of any
scheme that is chosen with its bandwidth utilisation.
The selecnon should seek to minimise the bandwidth
required for a single voice channel (or corresponding
data channel) while maximising the quality of perfor-
mance at any given carrier to noise level. This paper
discusses the subjective performance of various inodu-
lation schemes and speech processing techniques with
application to the mobile satellite service. The discus-
sion is based on the mean opinion score (MOS) assess-
ment of voice quality with measurements drawn from
a number of recent publications and laboratory mea-
surements. A comparison is made between the schemes
on the basis of both bandwidth and speech quality at
given carner to noise levels.
It is interesting to note how the performance of an
analogue form of linear modulation (ACSSB) compares
with that of a digitised voice source used in conjunc-
tion with an appropriate data modulation scheme. For
a given power/bandwidth budget, the linear analogue
system performs at least as well as digital equivalents
using the present generation of voice coder technology.
This is perhaps surprising given the current trend to-
wards digital implementations. It should be noted that
the analogue scheme is most favourable in a fading en-
vironment (Ricean) due to the absence of any sharp
performance threshold as received signal strength is
lowered.
Future advances in vocoder technology will un-
doubtedly make digital voice more attractive and it
is prudent to select a modulation system, and conse-
quently a transceiver design, that can efficiently convey
both digital and analogue traffic with the same hard-
ware. A pilot aided form of linear modulation based
on the Transparent Tone in Band (TTIB) technique [1]
has been shown to be directly compatible with both
analogue and digital traffic and would allow a smooth
and cost effective transition between analogue and dig-
ital voice operation. The design of mobile terminal
equipment intended for pilot aided linear modulation
is briefly reviewed here before discussing the compari-
son of modulation systems.
The Linear Modulation Mobile
Terminal
Recent developments in digital signal processing tech-
nology has made the exciting prospect of a reconfig-
urable transceiver, capable of handling a variety of
analogue and digital signaling formats a realistic goal.
Prototype equipment based on direct conversion tech-
tuques has been developed at the Centre for Comnm-
nications Research at the University of Bristol for use
in a variety of frequency bands [2]. Such a system is
normally described as hnear so long as its operation is
612
International Mobile Satellite Conference, Ottawa, 1990
https://ntrs.nasa.gov/search.jsp?R=19920014928 2020-03-17T12:27:47+00:00Z
Weaver Method
Recelver DSP
Front
. . Audio
°'_°du"t°t }O-.te
/waveform
decoder ] Output
Audio
Modulalor { /Data
ANaveform
encoder 1 input
Linearised PA
Figure 1: A Pilot Aided Linear Transceiver
independent of the amplitude and phase characteristics
of the signaling format.
Both the transmitter and receiver systems are
based on the Weaver frequency translation technique.
This means that each frequency translation requires
a quadrature local oscillator to drive the mixers. All
advantage of using the Weaver method is that the im-
age products fall within the users own channel, thus
minimising the accuracy needed ia tile gain and phase
match on the two quadrature signal paths.
Figure 1 gives a block diagram of a proposed
transceiver architecture for a pilot aided linear
transceiver. In such a system, distortions due to fad-
ing are removed in the receiver baseband processing
by ascertaining the fading characteristics from tile na-
ture of the received pilot tone. The transceiver is di-
vided into the fixed hardware sections of the linearised
transmitter and the zero 1F/single IF receiver, and the
software reconfigurable baseband processing sections.
In a pilot aided system, the baseband processing in-
eludes the generation of TTIB and fading correction
using feedforward signal regeneration [3]. Processing
is also required for whatever signaling format is to be
transmitted through the system. It is intended that
the particular signaling format chosen is not limited
by the transceiver design.
Linear Transmitters
The transmit section of the transceiver must meet
stringent requirements for linearity and power effi-
ciency. Considerable success has been achieved at Bris-
tol using feedback amplifier linearisation techniques,
although feedforward techniques also have potential.
A block diagram of feedback amplifier linearisation is
shown in figure 2. The baseband control function may
take a number of forms, the simplest and most well
understood being the cartesian feedback system [4] in
Baseband
Signa,}
Processing
ooup,.r
FunctionB_::?:::] _ ,_
"--'°r
Figure 2: Feedback Linearisation of an RF Amplifier
which the control function consists of a pair of low-
pass differential amplifiers. A well known alternative
is adaptive predistortion [5] in which the feedback path
is sampled (not necessarily continuously) and the gain
and phase response of the amplifier assessed across
its operating range. The transmitted signal is predis-
torted to correct for any non linearities in the amphfiers
characteristic.
With careful design, feedback linearisation can give
excellent performance. Linearised class C amplifiers
yield third order intermods better that 60dB down on
PEP for a two tone test. Performance is limited by
the loop gain that can be achieved within the limits of
stability•
The gain and phase match of the I/Q downcon-
verter limits the image rejection at the transmitter out-
put. 40 dB image rejection requires a phase match of 1
degree and an amplitude match of 0.1 dB. This can be
achieved over a reasonable bandwidth using RF tech-
niques alone. To extend the bandwidth further, an
automatic calibration procedure can be carried out in
production to calculate correction factors for any gain
and phase mismatch. In service, the baseband pro-
eessing can use these to predistort the drive to the
amplifier. In a transmitter fed with I and Q b_seband
signals, DC offsets will coincide with the pilot tone and
can easily be made small enough not to dominate (off-
sets 45 dB down on PEP can be achieved, while the
pilot is often around 10 dB down).
Linear Receivers
Weaver method zero IF receiver systems (direct con-
version) have been suggested for linear transceivers [6].
They offer the advantage of minimising the component
count, but require two linear mixers operating at. the
incoming frequency and an accurate 90 degree phase
International Mobile Satellite Conference, Ottawa, 1990
613
Delta-Sigma
Converter
_ Front Roofing _i_'_'_ _
E_ ___71 f_r I _ Channel Bit [ t_ SIgna.t I
Low Noise
Op-Amp
Figure 3: A Single IF Weaver Method Receiver.
shift network which operates across the entire range
of the systems frequency synthesiser. Carrier leakage
can also be problematical at high frequencies, causing
large DC offsets in the demodulated I/Q signals. These
problems mean that direct conversion will probably be
unsuitable for use at mobile satellite frequencies for the
immediate future.
Figure 3 shows a block diagram for a single IF lin-
ear receiver. This alleviates many of the problems by
performing the I/Q generation at a low, fixed IF. If
a suitable frequency is chosen, then mixers with good
linearity, low carrier leakage and low cost can be ob-
tained. The quadrature phase shift network need only
operate at a single frequency, making accuracy and sta-
bility easy to achieve. Unfortunately, a roofing filter
will now be required to remove the image frequencies
which, after the first mixing stage, do not lie within tile
wanted channel. Such filters can be obtained cheaply
if a prudent IF is chosen.
Carrier leakage around the final mixers results in a
DC offset in the resulting I/Q baseband signals. Such
offsets can be removed by filtering, but they can alter
slightly with incoming signal strength. When operat-
ing in weak signal areas, this variation means that DC
rejection must be achieved up to 5 to 10 Hz. Such fil-
tering removes some of the information content of tile
received pilot. It may be prudent to frequency lock
the receiver so that DC falls in the gap between the
signaling modulation and the pilot. DC rejection can
then be achieved with no information loss and removal
of the frequency offset is straightforward (figure 4).
The availability of modern delta-sigma analogue
to digital converters with 18 bit resolution means that
conversion into the digital domain can be achieved with
dynamic range well in excess of that required for mo-
bile satellite receivers. The signals can then be filtered,
amplified and mixed as required with no fear of insta-
bility. (High resolution ADCs are becoming cost effec-
tive due to the economies of scale in the digital audio
market). The limitation in receiver performance; ad-
jacent channel performance, sensitivity, selectivity and
DC
REJECTION
FILTER
_ODULATION
SIDEBANO 1
NEGATIVE
FREQUENCY DC
PILOT
IODULATION
SIDEBAND 1
POSITIVE
FREQUENCY
Figure 4: Placing DC in the demodulated I/Q signals.
overload point, are now largely dependant on the lin-
earity of the RF pre-amplifier and subsequent mixer
stages. Field testing of a 900MHz single IF linear re-
ceiver is currently in progress at Bristol and prototype
hardware for 1.7 GHz operation is under development.
Subjective Comparison of Mod-
ulation Systems
Before comparing some modulation formats which may
be used in a mobile satellite system (whether linear of
not) it must be stated that voice assessment based on
MOS is only truly valid if all modulation candidates are
tested by the same panel of listeners under the same
test conditions. (Clearly, the source tape must be com-
mon for all systems under trial and the playback to
assessment panels, customers etc, must be randomised
to prevent learning of the information content) As the
results for this assessment are drawn from a number of
sources [7,8,9,10], this condition is not fully satisfied,
however, the subjective performance of analogue lin-
ear modulation with respect to a high quality 9.6kbps
codec has been validated at the laboratories at Bristol
University and thus provide a good reference point for
scaling the other results. It is on this basis that the re-
sults are presented and discussed, giving, it is felt, and
accurate picture of modulation and coder performance
with current technology. Further work is in progress
at Bristol to complete the analysis for a full range of
coder types and modulation techniques.
Modulation Systems
All modulation systems will be compared on the basis
of average signal power with respect to noise power
density (dB/Hz) required to maintain a given voice
quality (MOS). Three digital modulation schemes have
been studied, namely:
614
international Mobile Satellite Conference, Ottawa, 1990
• 25% raised cosine filtered 16-QAM
(coherent detection).
Bandwidth efficiency = 3.2 bps/Hz
BER of 10 -3 at an _ of 13.5 dB
BER of 10 -2 at an _o of 11 5 dB
• 25% raised cosine filtered QPSK
(coherent detection).
Bandwidth efficiency = 1.6 bps/llz
BER of 10 -3 at an Ne--_oof 9 dB
BER of 10 -2 at an _ of 7 dB
• 100 % raised cosine filtered trellis coded 8-PSK
Bandwidth efficiency = 1 bps/Hz
BER of 10 -3 at an NoE--_of 5.8 dB
BER of 10 -2 at an _ of 3.8 dB
Jvo
All three systems assume a 2 dB implementation
loss.
Three vocoder rates are investigated of 9.6kbps,
4.8kbps, and 2.4kbps. Channel bandwidth for the dig-
ital systems is calculated as the vocoder rate divided
by bandwidth efficiency plus 300 Hz guard band to
allow for Doppler spread.
The carrier to noise ratio, _oo is calculated as
E__ x 10 log (vocoder rate)
No
The fourth system is an analogue form of linear
modulation based on TTIB. The _ values for this sys-
No
tem are measured as the average received signal power
when the speaker is active (including pilot tone) di-
vided by the received noise power density. It should
be noted that this figure is somewhat pessimistic as
it does not take account of periods of voice inactivity.
Clearly, the average received power would be lower if
voice inactivity were accounted for resulting in lower
power consumption in both the mobile and the satel-
lite. For the equivalent power saving in a digital voice
system, a robust voice activity detection is required.
The channel bandwidth for the analogue system
is 3.7 kHz, consisting of a 300 to 3000 Hz voice band-
width, a 600 Hz TTIB notch and a 400 Hz guard band.
Figure 5 shows the results for a static AWGN. As
would be expected, the analogue system out-performs
the digital systems at high signal strengths, the lat-
ter exhibiting an upper threshold due to vocoder al-
gorithm limitations which degrade as tile coder rate is
reduced. As the signal strength falls, certain digital
systems out-perform the analogue scheme. For exam-
ple, TC-8PSK with a 9.6 kbps coder is much better
than ACSSB at 45 dB/Hz. The improvement in per-
formance, however, is achieved at the price of band-
. width expansion. The TC-SPSK system with a 9.6
MOS
35 40 45 50 55
C/No (dB/Hz)
ACSSB I
Figure 5: Subjective Performance of all Systems.
MOS
5
35 40 45 50 55
C/No (dB/Hz)
analogue 16-QAM QPSK TC- SK
Figure 6: Subjective Performance of Systems with
Channel Bandwidth below 4 kHz.
kbps coder requires a channel bandwidth of approxi-
mately 9.9 kHz, compared with the 3.7 kHz channel
bandwidth for ACSSB.
Taking into account the narrow band requirement
for mobile satellite operation, figure 6 highlights the
performance of those systems which require a channel
bandwidth of less than 4 kHz. It is now clear that AC-
SSB performs as well as any of the digital competitors,
except for a 2.4 kbps coder with TC-8PSK at very low
signal strengths. It is well recognised that 2.4 kbps
coders do not, at present, provide sufficient quality of
service for a public telephony mobile satellite service.
If services with a MOS below 2.5 are neglected, then
the choice of modulation system becomes that of fig-
ure 7. It is now clear that ACSSB performs at least as
well &s present generation modulation and voice cod-
ing technologies. At low signal strengths the amplitude
companding maintains a usable voice channel without
recourse to bandwidth expansion.
International Mobile Satellite Conference, Ottawa, 1990
615
MOS
5
i ....... 2:" :i
40 45 50
C/No (dB/Hz)
ACSSB
analogue 16-QAM QpSK
--------o-------- .... e .........
55
Figure 7: Subjective Performance of Systems with
Channel Bandwidth below 4 kHz and MOS above 2.5.
Performance in Fading
Taking an operating point of 51 dB/ttz as a typical
link budget for the M-SAT system, the 9.6 kbps 16 -
QAM system will not operate satisfactorily, 4.8 kbps
16 - QAM will allow a fade margin of 2 dB, 4.8 kbps
QPSK will give a fade margin of 6 dB, and ACSSB a
fade margin of 7 - 8 dB.
Conclusions
With the application of feedback amplifier linearisa-
tion techniques, along with recent advances in device
technology (particularly ADC, DAC and DSP technol-
ogy) a umversal linear transceiver can be realised. This
could convey voice traffic via an analogue or a digital
modulation format. Thus a smooth and cost effective
transition between signaling formats could be achieved
as and when superior speech vocoders become available
for tile mobile satellite service.
Based on some recent subjective performance
measurements of candidate modulation schemes and
speech coders for mobile satellite, it is concluded that
digital vocoder technology, with a suitable data mod-
ulation format can now approach the performance
af an analogue linear modulation system in a given
power/bandwidth budget. Speech quality, channel oc-
cupancy, fade margin and power consumption, how-
ever, are all superior in the analogue implementahon.
be pubhshed, IEEE Transactzons on Vehicular
Technology,
[2] A. Bateman, D. Haines and R.J. Wilkinson, "Di-
rect Conversion Linear Transceiver," 5th Interna-
tional Conference on Mobzle Radio and Personal
Communications, December 11-14 1989, United
Kingdom.
[3] J.P. McGeehan and A. Bateman "Theoretical and
Experimental Investigation of Feedforward Signal
Regeneration as a means of Combating Multipath
Propagation Effects in Pilot-Based Mobile Radio
Systems," IEEE Transact,ons on Vehzcular Tech-
nology, VT-32, 1983, pp.106-120
[4] A. Bateman, R.J. Wilkinson and J.D. Marvill
"The Application of Digital Signal Processing to
' Transmitter Linearisation," 8th European Confer-
ence on Electrotechniques, June 13-17, 1988, Swe-
den.
[5] Y. Nagata "Linear Amplification Technique for
Digital Mobile Communications," 39th IEEE Ve-
hicular Technology Conference, May 1-3, 1989,
USA.
[6] A. Bateman and D.M. Haines "Direct Conver-
sion Transceiver Design for Compact Low-Cost
Portable Mobile Radio Terminals." 39th IEEE Ve-
hzcular Technology Conference, May 1-3, 1989,
USA.
[7] "AEEC Subjective Test Report - Selection of
Voice Codec for the Aeronautical Satellite Ser-
vice," Brutish Telecom R_JT Memorandum No.
RT53/001/88.
[8] "On the Subjective Voice Quality Test of ACSSB
Modulators and Digital Vocoders," INMARSAT
Internal Document, 1989.
[9] "Relative Performance of Linear Modulation
(analogue) vs a 9.6kbps digital vocoder," Centre
for Communzcations Research, Bristol Umverszty,
Internal Report, May 1989.
[10] "Trials Report on the Subjective Performance of
Modulation Techniques for Mobile Satellite," Aus-
traha Telecom Internal Report, 1989.
References
[1] A. Bateman, "Feedforward Transparent Tone In
Band: Its Implementation and Applications," to
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International Mobile Satellite Conference, Ottawa, t990


Analogue and digital linear modulation techniques for mobile satellite

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