Date: 5th March 2001
Meeting: Frequency Management Ad-hoc Group.
Document Reference: prFMAC 0201 006 0
Working Paper No: FMAC (02-1)/6
Radiocommunications Agency
Subject:
University of Bristol Study – Alternative Schemes for Microwave Fixed Links (1994)
Contact:
Brian Harrison – Tel: 0207 211 0292. E-mail: brian.harrison@ra.gsi.gov.uk
3.
Introduction. At the FMAC meeting held on the 6th February 2001 the RA agreed to summarise
the results of the University of Bristol modulation efficiency study which was completed during
March 1994. Two approaches were taken to estimate spectral efficiency. First minimum theoretical
bandwidth to transfer 34 Mbit/s at four modulation orders (QPSK, 16 QAM, 64 QAM and 128
TCM) was the basis for a series of simulations. This was followed by a set where a theoretical
maximum traffic rate for each type of modulation was transferred via a bandwidth of 28 MHz.
Simulations covering random and nodal site distributions were completed. Bristol’s executive
summary, final concluding remarks plus four tables showing efficiency comparisons are shown
below:
2.
Executive Summary.
‘The aim of this project was to determine whether higher level modulation schemes can
improve the spectral efficiency, and consequently the capacity of, a practical fixed-link
point-to-point network. In order to analyse the problem in detail two pieces of software
have been used. TPATH is a proprietary package with particular strengths in the area of
propagation modelling. A second piece of software (Microwave Link Simulator) was
written in the Centre for Communications Research in order to provide a means of
extending the capabilities of TPATH thus enabling the evaluation of the comparative
performance of different combinations of modulation and coding schemes in the fixedlink environment.
The results show that although high-level modulation schemes are more susceptible to
noise and interference than the lower level schemes, these effects can almost always be
mitigated by employing simple techniques such as higher performance antennas,
diversity combining, or tighter channel filtering in the transmitter and receiver. Other
techniques such as linearised transmitter power amplifiers can be used to reduce the
generation of interference in adjacent and offset channels.
The performance of modulation schemes in a temporally dispersive environment was
found to be dependent mainly on the equaliser architecture. For high-level modulation
schemes, where the symbol rate is relatively low, equaliser design can in fact become
easier than for a system with and equivalent data rate using a low-level modulation
scheme, although other aspects of the system design do become more complex.
Of all the modulation and coding schemes examined, Trellis Coded Modulation which is
a synergistic combination of modulation and coding appeared to offer the potential of the
greatest improvement in capacity. These schemes are currently little-known, and
complex to implement. However they are rapidly gaining favour in bandwidth limited
channels as they effectively give a significant performance improvement when
compared to their un-coded counterparts, with no penalty in data throughput or
bandwidth expansion.
In conclusion, it was shown that by examining hypothetical and actual fixed-link
scenarios, the theoretical improvement in bandwidth efficiency, expressed as Mbit/s per
MHz per km2, of approximately 200% is possible when trellis-code 128 QAM is used in
place of conventional QPSK.’
Page 1 of 3
3.
Efficiency Tables.
These tables were compiled from the results of constant traffic rate, variable bandwidth
simulations. However, the report stated that the similar results would apply if bandwidth were
kept constant as opposed to traffic rate. Results are those for 7.5 GHz.
Results for a Hub using Standard Performance Antennas
Links Supporteda
Min. Angular Separation
Bandwidth Efficiency
(Mbit/s/MHz/km2) x 10-3
QPSK
5(.75)
630
12.5
Modulation
16-QAM
64-QAM
4(.25)
3(.5)
850
1030
18.5
22.8
128 TCM
4(.0)
900
26.1
a
The number of links supported can obviously only take integer values, however the fractional part
shown in brackets have been used in the calculation of angular separation and bandwidth efficiency.
Results for a Hub using High Performance Antennas
Links Supported
Min. Angular Separation
Bandwidth Efficiency
(Mbit/s/MHz/km2) x 10-3
QPSK
9(.5)
380
20.7
Modulation
16-QAM
64-QAM
5(.5)
4(.0)
650
900
23.9
26.1
128 TCM
5(.0)
720
32.6
Results for a Random Layout using Standard Performance Antennas (150 x 150 Km)
Estimated Links Supported
Bandwidth Efficiency
(Mbit/s/MHz/km2) x 10-3
QPSK
63
4.3
Modulation
16-QAM
64-QAM
50
37
6.8
7.6
128 TCM
48
9.8
Results for a Random Layout using High Performance Antennas (150 x 150 Km)
Estimated Links Supported
Bandwidth Efficiency
(Mbit/s/MHz/km2) x 10-3
4.
QPSK
70
4.8
Modulation
16-QAM
64-QAM
58
46
7.9
9.4
128 TCM
56
11.5
Concluding Remarks.
‘The results of this study clearly indicate that a considerable increase in the capacity of a fixed-link
network can be brought about by the use of higher level modulation. Such systems are however more
complex and costly to engineer than the simpler low-level systems, and as market forces may limit the
extent to which the ultimate theoretical capacity could be realised, with both manufacturers and
operators reluctant to invest in such equipment.’
RA2/FTSLU
Page 2 of 3
Page 3 of 3