D ATE
M EETING
23 R D J ANUARY 2001
FLCC P OLICY M EE TING
D OCUMENT R EFERENCE
W ORKING P APER N O :
PR RSWG
0101 002 0
RSWG(01-01)/002
RADIOCOMMUNICATIONS AGENCY
M O D UL ATION S C HE M E
E FFI C IE N C Y S TU DY
U SING C OMPUTER S IMULATION T O
D ETERMINE T HE S PECTRAL E FFICIENCY
O F M ODULATION S CHEMES
Christopher Pratt RA2 / FTSLU
38 G H Z M OD U L AT I ON
SCHEME EFF ICIENCY STUDY
C H RI S T OP H E R P RA T T , R A 2 F T S L U
ABSTRACT
A study was conducted to find the relative spectral efficiency of four modulation schemes in
the 38GHz band. The study was conducted by adapting a computer program to randomly place
links in a sample area subject to the frequency assignment criteria laid out in RA350. The factors
used to determine spectral efficiency were the maximum link density in the sample area, and the
bit rate and bandwidth for each link. The report indicates that higher modulation schemes
provide better spectral efficiency in both the single node and multiple node cases that were
investigated.
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C H RI S T OP H E R P RA T T , R A 2 F T S L U
TA B L E O F C ON T E N T S
1. INTRODUCTION ....................................................................................................................... 4
2. AIMS AND OBJECTIVES ........................................................................................................... 5
3. SATURATION SIMULATION ................................................................................................... 5
CHANNEL DATA ........................................................................................................................................................... 5
LINK PLACEMENT........................................................................................................................................................ 6
PROGRAM OPERATION ............................................................................................................................................... 7
4. SIMULATION TESTS ................................................................................................................. 9
SINGLE NODE SIMULATION ....................................................................................................................................... 9
MULTIPLE NODE SIMULATION .................................................................................................................................. 9
PLACEMENT FAILURES VS LINKS PLACED ............................................................................................................... 9
5. RESULTS AND ANALYSIS ....................................................................................................... 10
SINGLE NODE SIMULATION ..................................................................................................................................... 10
MULTIPLE NODE SIMULATION ................................................................................................................................ 12
PLACEMENT FAILURES VS LINKS PLACED ............................................................................................................. 13
6. CONCLUSION .......................................................................................................................... 15
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C H RI S T OP H E R P RA T T , R A 2 F T S L U
1 . I N T ROD U C T I ON
As the radio spectrum in the UK becomes more and more congested, there is an increasing
need for more spectrally efficient links. Firstly to enable links to be assigned in places where it
hitherto would not have been possible, and secondly to minimise the possible interference to
future links in the surrounding area.
New technological advances are allowing higher order modulation equipment to be
produced – allowing higher data rates to be transmitted in bandwidth currently used for medium
rate traffic. This is clearly advantageous in the case where the signal is confined in a medium
where it cannot interfere with or be corrupted by other signals e.g. in the case of an optical fibre
link. But the benefits are not clear when the medium in question is free space, in the case of a
radio link. In this case, in order to achieve a satisfactory BER for a higher modulation order, the
additional power necessary increases the likelihood of interference, thereby reducing the benefits
gained from the improved bandwidth utilisation efficiency.
There is therefore a need to compare – in a spectral efficiency sense – different orders of
modulation when applied to fixed point to point links in a realistic simulation. This information
is useful to the RA to help promote better spectral efficiency in the UK.
Four modulation schemes were chosen for the study, 155Mbit/s over 28MHz and 56MHz
channels and 34Mbit/s over 28MHz and 14MHz channels. Comparisons were then made for
two scenarios; the first was to find the relative spectral efficiency of the different schemes around
a single nodal point with no additional interference. The second was for a realistic situation in
which links were placed on 500 nodes in a sample area of 50x50km.
The simulations were carried out using a realistic computer simulation of link assignment in
the UK. The computer program placed links in the sample area until the area became saturated.
Results regarding the number of links assigned for different modulation schemes were used to
accurately determine the spectral efficiency of the scheme.
A third simulation was carried out to investigate the theoretical maximum link density for the
given sample area. In addition, an attempt has been made to determine the maximum density
possible for the chosen real world area on which the simulation was based.
The rest of the report follows the following structure. The next chapter outlines the aims and
objectives of the study. Chapter 3 explains the saturation simulation program, outlining how to
use the program and its operation. Chapter 4 details the tests conducted using the simulation
program and chapter 5 presents the results and analysis of the simulations. Finally, chapter 6
contains the conclusions deduced from the study.
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C H RI S T OP H E R P RA T T , R A 2 F T S L U
2 . A I M S A N D OB J E C T I V E S
The primary aim of the study was to quantify the affect that different modulation schemes
have on spectral efficiency. It is currently assumed that higher order modulation implies better
spectral efficiency. This study attempted to find - through thorough and realistic simulation –
whether this is the case, and if so, the spectral efficiencies for each scheme.
The secondary aim related to the spectral efficiency of links around a single node. Many
assume that lower modulation orders provide better spectrum usage around a nodal point. This
study attempted to find – through thorough and realistic simulation – the relative spectral
efficiency of different modulation schemes in a single node situation.
The third aim was to formulate a method of finding the theoretical maximum link density for
a given area. The data collected could then be used to quantify how full the 38GHz band is in
different areas of the UK.
3 . S A T U R A T I ON S I M U L A T I O N
A C program was written in 1997 which randomly assigned links in a 100 x 100km test area
in 60 channels of various widths (from 28MHz down to 3.5MHz) across 112MHz of bandwidth
in the 38GHz band. Links were placed until a specified number of successive placement failures
occurred per channel. A link failed if there was excessive interference between it and any other
link in the test area. The program then displayed on the screen the number of links placed in each
of the 60 channels and the total number of links placed. The program could be run n times and
the average number of total links placed and the average per channel displayed.
The program was adapted to place links in 4 adjacent channels; all links having the same
modulation scheme in any one run. This would then allow for comparison between the different
schemes. The modulation schemes chosen for the study allowed the following traffic rates and
bandwidths: 155/28, 155/56, 34/14 and 34/28; using modulation orders of 128, 16, 16 and 4
respectively. In order to provide a thorough and fair comparison, the following alterations were
made to the program:
CHANNEL DATA
Channel data for the chosen schemes were obtained from RA document RA350 – Frequency
Assignment Criteria for the 38GHz band. The following data changes were made:





The specific centre frequencies for the four adjacent channels for each of the
four modulation schemes was entered into the initialisation part of the program.
The specific RSLs (receiver sensitivity levels) for the different schemes were
input into the initialisation part of the program.
The specific wanted to unwanted ratios for the different schemes were input
into the initialisation part of the program.
The EIRP calculation formula was adjusted for each scheme.
The maximum and minimum path lengths for each scheme were calculated and
changed.
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SCHEME EFF ICIENCY STUDY
C H RI S T OP H E R P RA T T , R A 2 F T S L U
LINK PLACEMENT
The link placement function was changed from the original program as follows:
In order to emulate more correctly the real world situation, the program was changed so that
a number of nodes are randomly set up at the beginning of each run. The program then
randomly selected one of these nodes as the start point for a link. It then randomly chose a
channel and an end point anywhere in the plane subject to the maximum and minimum path
length restriction. The links were always two way.
The number of nodes was based on the RA’s database for 38GHz sites around London. The
area was reduced to 50 x 50km and the number of nodes chosen was 500. The reduction in area
was necessary to reduce computational time. It did not affect the relative results gained from the
program.
The program was modified so that links were placed until a pre-defined number of
successive placement failures occurred per channel per node. It is considered that this method
provides and accurate and realistic comparison between modulation schemes.
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C H RI S T OP H E R P RA T T , R A 2 F T S L U
SCHEME EFF ICIENCY STUDY
PROGRAM OPERATION
The program requires the user to enter one or more options in the command line. These
options specify the modulation scheme, the number of saturation runs to complete, the number
of consecutive placement failures per channel per node permissible and the output file name. The
program then starts placing links in the test area for the first run. The program operation is best
described by the following flowchart.
START –User runs program,
specifying options
Initialisation
Randomly assign start
nodes
Randomly select start
node and channel
Channel on
node full?
Yes
No
Randomly assign a finish
point subject to max and
min path length
Calculate EIRP
Do hi-hi and lo-lo checks
with an existing link
No
Interference
acceptable?
Yes
Checked with
all existing links?
Yes
No
Increment fail counter for
channel on current node
No
Increment success counter
for current node
All nodes
full?
Yes
No
Completed
all runs?
Yes
END – output
results to screen
Fig. 3.1 – Flowchart of saturation simulation program
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C H RI S T OP H E R P RA T T , R A 2 F T S L U
The output from the program is depicted in Fig. 2 below.
Fig. 3.2 - Screen dump of output from saturation simulation program
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C H RI S T OP H E R P RA T T , R A 2 F T S L U
4 . S I M U L A T I ON T E S T S
The following tests were conducted using the saturation simulation program:



Simulation of the mean number of links assigned around a single node with no
other source of interference.
Simulation of the mean number of links assigned over a 50 x 50km square,
placing links around 500 randomly positioned nodes.
Simulation to determine the link between placement failures and number of
links assigned in a test area. The aim of this work is to estimate the theoretical
maximum link density for the test area.
The first two test scenarios were conducted using the four chosen modulation schemes and
corresponding channel widths. The third was conducted on the 155/28 system only.
SINGLE NODE SIMULATION
The program was altered to place a single node in a 50km square. It was decided that the link
length be kept constant at 3km. This provided the fairest test and comparison between systems.
The simulation outputs the mean and maximum number of links assigned per run.
For each modulation scheme, 1000 runs were completed, with 1000 placement failures
allowed per channel. 1000 placement failures per channel were chosen so that the maximum
number of links possible was placed on each run. 1000 runs were conducted and the results
averaged to provide the minimum of variance in each set of results. This was again to provide the
fairest comparison between systems.
MULTIPLE NODE SIMULATION
The simulation placed links around 500 nodes in a 50 x 50km area over four adjacent
channels until the area was saturated. For each modulation scheme, 100 runs were completed
with 20 consecutive placement failures allowed per channel per node. These numbers were
chosen as a trade-off between minimising variance in the results and computation time. A batch
file was set up and executed to run the simulations for the four modulation schemes sequentially.
The four simulations took 3 ½ days to complete.
The simulation outputs the mean number of links assigned per run and per node per run.
PLACEMENT FAILURES VS LINKS PLACED
The program was altered so that the area and number of nodes were reduced to 10x10km
and 20 respectively. The reason for this was to speed up the computation to allow a high number
of placement failures to be tested. Keeping the nodal density the same gave the chance to find
trends in the results. 100 runs were completed for each placement failure value. A number of
placement failures were tested for the 155/28 system and the corresponding number of mean
links successfully placed per run recorded.
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SCHEME EFF ICIENCY STUDY
5 . R E S U LT S A N D A NA LY S I S
SINGLE NODE SIMULATION
For 1000 runs, 1000 placement failures per channel and link length equal to 3km, the mean
and maximum number of links assigned per run are presented.
SYSTEM
MEAN
MAXIMUM
155/28
155/56
34/14
34/28
16.040
20.287
20.056
36.928
20
24
24
44
LINKS PER CHANNEL
(FOR MAXIMUM RUN)
5
6
6
11
Table 5.1 – Single node simulation results
The ‘links assigned per channel’ column shows the number of links per channel for the run
(or runs) in which the maximum number of links were assigned. In each case, the same number
of links was assigned in each channel. This is a good indication of the theoretical maximum
number of links that could be assigned i.e. each channel on the node has reached its capacity.
Another indicator of this fact is that if 10000 runs are conducted, or 10000 placement failures are
allowed, the maximum in the sample remains the same.
In order to make a useful and fair comparison between the systems their characteristics and
capacities needed to be taken into account. This is done by taking account of the bit rate and
channel width of each of the systems. The bit rate must be taken into account, because this is the
measure of the amount of data being transmitted by the links in the area in question. The channel
width must be taken into account because this affects the number of channels that can be
assigned in one band. So, multiplying the number of links assigned in the area by the bit rate (in
Mbit/s) and dividing by the bandwidth gives a measure of how much data can be communicated
in a specified bandwidth within a given time within the sample area. This result will be called the
normalised spectral efficiency of the systems (bits/Hz) and allows a direct comparison
between different modulation schemes.
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C H RI S T OP H E R P RA T T , R A 2 F T S L U
SCHEME EFF ICIENCY STUDY
SYSTEM
155/28
155/56
34/14
34/28
NORMALISED MEAN
SPECTRAL EFFICIENCY
(BITS/HZ)
88.79
56.15
48.71
44.84
NORMALISED
MAXIMUM SPECTRUM
(BITS/HZ)
110.7
66.43
58.28
53.43
Table 5.2 – Normalised single node simulation results
It can be seen from the above table and graph that in a single node situation, it is clear that
the 155Mbit/s over 28MHz channel system is the most efficient. This means that although fewer
155/28 links may be placed around a single node, the links placed allow more data to be
communicated per second in a referenced bandwidth. The results show that as the modulation
order decreases, so does the spectral efficiency.
Normalised Spectral Efficiency (bits/hz)
120
100
80
Normalised mean
Normalised maximum
60
40
20
0
155/28
155/56
34/14
34/28
System
Fig. 5.1 – Graph of normalised Spectral Efficiency (bits/Hz) for a single node
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C H RI S T OP H E R P RA T T , R A 2 F T S L U
SCHEME EFF ICIENCY STUDY
MULTIPLE NODE SIMULATION
For 100 runs, 20 placement failures per channel and 500 nodes in a 50x50km sample area,
the mean number of links assigned per run and per node per run are presented.
SYSTEM
MEAN PER RUN
155/28
155/56
34/14
34/28
3474.80
4268.51
4288.19
8379.92
MEAN PER NODE
PER RUN
6.95
8.54
8.58
16.76
Table 5.3 – Multiple node simulation results
As with the single node situation, account has to be taken for the different characteristics and
capacities of the four different systems on test. To this end, the normalised mean values (number
of links placed x bit rate / channel bandwidth) are presented below.
SYSTEM
155/28
155/56
34/14
34/28
NORMALISED MEAN PER
RUN (BITS/HZ)
19235.5
11814.6
10414.2
10175.6
NORMALISED MEAN PER
NODE PER RUN (BITS/HZ)
38.5
23.6
20.8
20.4
Table 5.4 – Normalised multiple node simulation results
Normalised spectral efficiciency per run for multiple node
scenario (bits/hz)
25000
20000
15000
10000
5000
0
155/28
155/56
34/14
34/28
System
Fig. 5.2 – Graph of normalised spectral efficiency per run
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C H RI S T OP H E R P RA T T , R A 2 F T S L U
SCHEME EFF ICIENCY STUDY
The results clearly show that from a simulation realistically representing the real world situation,
the 155Mbit/s over 28MHz channel system comes out as the most spectrally efficient. Although
fewer links were actually placed in the sample area for the 155/28 system, when account is taken
for the higher bit rate and/or the smaller channel width, it is shown that this system uses the
spectrum the most efficiently.
The results also show that the benefit of transmitting 34Mbit/s over a 14 MHz channel is
nominal compared to transmission over a 28MHz channel (34/14 gives an improvement in
spectral efficiency of 2.34% over 34/28). This is due to the difference in the minimum carrier to
noise ratio (C/N) required at the receiver for the two systems. Since the 34/14 scheme is a 16QAM system, it requires a greater C/N than the QPSK system used to transmit 34/28. The
greater C/N requirement means that for the same link length, a higher EIRP is necessary. The
increased interference caused reduces the spectral efficiency benefit obtained from halving the
bandwidth.
PLACEMENT FAILURES VS LINKS PLACED
For 155/28 system, 100 runs per placement failure test, and 20 nodes in a 10x10km sample
area, the mean number of links assigned per run is compared against the number of allowable
placement failures per channel per node.
PLACEMENT FAILURES
MEAN NUMBER OF
PER CHANNEL
LINKS PLACED PER RUN
1
69.31
2
90.62
5
123.32
10
156.05
20
198.19
50
254.72
100
291.07
200
316.68
300
323.04
Table 5.5 – Links placed vs. placement failures per channel
38GHz nodal congestion test results
Mean number of links assigned per run
350
300
250
200
150
100
50
0
0
50
100
150
200
250
Consecutive placement failures per channel per node
Fig. 5.3 – Links assigned vs. Placement failures
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38 G H Z M OD U L AT I ON
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C H RI S T OP H E R P RA T T , R A 2 F T S L U
The above graph serves to show the limit towards which the number of links placed per run
is tending. It can be seen that as the consecutive number of placement failures per channel per
node is increased, the number of links placed increases at a decreasing rate. The figure towards
which the mean number of links placed is tending can be thought of as a theoretical maximum
i.e. towards the limit of link density for the particular system.
It can be seen by extrapolation of the curve that a theoretical maximum is close to being
reached. This theoretical maximum is for a nodal density approximately equal to the density in a
similar area centred on London, and therefore attempts to show the realistic maximum link
density in this area of the UK. It is accepted, however, that in reality this would never be
achieved due to the constraints placed on the positions of links in the real world.
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C H RI S T OP H E R P RA T T , R A 2 F T S L U
6 . C O N C L U S I ON
The aim of the study was to investigate the affect of modulation order on spectral efficiency
when applied to fixed point to point microwave links. This was done for the modulation
schemes, 155/28, 155/56, 34/14 and 34/28, which have modulation orders 128, 16, 16 and 4
respectively. The measure of spectral efficiency used was maximum link density x bit rate /
bandwidth. Three tests were conducted. The first to find the maximum number of links assignable
around a single nodal point. The second involved randomly placing links around 500 nodes in a
50x50km sample space to find the maximum achievable link density. As a side issue, the third test
attempted to propose the maximum achievable link density for 155/28 systems in the sample
area.
The results show that for each of the tests undertaken, the higher order modulation schemes
are the most efficient, with the 128-ary 155/28 scheme performing appreciably better than the
other schemes on test. They do show, however, that the modulation order is not the sole factor
determining spectral efficiency; carrier to noise ratio can be seen to have a bearing on it. This is
evident in two cases: 155/56 provides better spectral efficiency than 34/14 for the same
modulation order, and 34/14 provides only a nominal advantage over 34/28.
The third test shows that the number of links assignable using the 155/28 scheme tends
towards an upper limit as the successive number of placement failures per channel per node is increased.
This then gives the theoretical maximum link density for the 155/28 scheme. This would not be
achievable in reality due to the non-random nature of link placements in the real world. Even so
this upper bound could be used to determine some sort of measure of how full a band is in a
particular area and allow a comparison between different areas and between different bands.
As an extension to this study, a further study could be performed using the same computer
program to investigate the spectral efficiency of a wider range of modulation schemes. The affect
of carrier to noise ratio and any other possible effects would then be known, and an increased
knowledge of spectral efficiency would be realised.
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