Two different methods of generating spread spectrum signals for an overlay service are discussed, and the data rate and efficiency which can be achieved while maintaining low interference with existing traffic are examined

Ha, Tri T.

Pratt, Timothy

English

NASA Technical Reports Server

THE BRADLEY DEPARTMENT 
OF ELECTRICAL ENGINEERING 0 
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E]IIIECUTIVESEMMARY 
STUDY OF SPREAD SPECTRUM MULTIPLE ACCESS 
SYSTEMS FOR SATELLITE COMMUNICATIONS 
WfTH WERLAY ON CURRENT SERVICES 
by T r i  T. Ha and Timothy Pratt 
VIRGINIA POLYTECHNIC INSTITUTE AND STATE UNIVERSITY 
BLACKSBURG, VA 24061 
prepared fo r  
NATIONAL 'AERONAUTICS AND SPACE ADMINISTRATION 
NASA Lewis Research Center 0 
Contract NAS 3-24887 
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VIRGINIA POLYTECHNIC INSTITUTE AND STATE UNIVERSITY 
Blacksburg, Virginia 24061 (703) 961 - 6646 
https://ntrs.nasa.gov/search.jsp?R=19890014385 2020-03-20T03:12:47+00:00Z
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EXECUTIVE SUMMARY 
Contract NAS2 - 24887 
"A Study of Spread Spectrum Multiple Access Systems for 
Satellite Communications with Overlay on Current Service.I1 
by Tri T. Ha and Timothy Pratt 
Virginia Polytechnic Institute and 
State University, Blacksburg, VA 24061 
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1. Introduction 
There are many applications for low cost satellite com- 
munication links in which a large number of small terminals send 
data at a low rate to a central terminal. This report describes 
two approaches to the design of such systems using spread spec- 
trum techniques. Spread spectrum provides a means for multiple 
access of a single communication channel with low interference to 
other users: in this application, it is used to overlay a low 
data rate signal on top of an occupied channel. 
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The report discusses two different methods of generating 
spread spectrum signals for an overlay service, and examines the 
data rate and efficiency which can be achieved while maintaining 
low interference with existing traffic. 
In the first part of the report, m-ary FSK is used as 
the spread spectrum technique. 
switched between a large number of frequencies and the average 
power density at any frequency is very small. 
of the report, chirp spread spectrum is described. 
system, each data bit is sent as a series of frequency sweeps 
over a wide frequency range. 
In this method, the signal is 
In the second part 
In the chirp 
This ensures that spectral density 
is low at any frequency. 
Spread spectrum is an important technique in any satel- 
lite communication system using very small antennas (VSAT’s). A 
small antenna has a broad beam, which results in interference at 
adjacent satellites in the geostationary orbit. 
tional multiple access techniques are used, such as FDMA and 
TDMA, the adjacent satellite interference may be excessive. 
If conven- 
In the work reported here, the capacity and performance 
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of VSAT links overlaid on an occupied channel were investigated, 
for the m-ary FSK and chirp systems. Results are presented 
showing how many channels can share a transponder, and the 
resulting interference levels. 
is a good technique for multiple access by a number of small 
terminals. 
spectrum systems, so more channels can be accommodated. Chirp 
spread spectrum is shown to be a simple technique for overlaying 
an occupied channel with a low data rate signal. 
It is concluded that m-ary FSK 
Its efficiency is higher than direct sequence spread 
2 .  Multiple Access Using m-ary FSK 
We investigated two forms of MFSK which incorporate 
spread spectrum techniques to reduce interference levels by 
spreading the spectral density of the signal over a wide range 
of frequencies. 
their performance in a VSAT network. 
Part A of the report describes these systems and 
The two techniques investigated were MFSK/DS and 
MFSK/DS-FH. The first, MFSK/DS, converts data bits into a set  of 
multiple frequencies, and the resulting signal is then multiplied 
by a pseudo-random (PN) sequence with a high bit rate to spread 
the signal energy over a wide bandwidth. In MFSK/DS-FH, the same 
frequency modulation is used initially, but the FSK signal is 
then frequency hopped at a high rate over a wide bandwidth to 
provide the required spectrum spreading. 
In both cases, we investigated systems using non- 
coherent receivers because these are much simpler and cheaper 
than coherent receivers. 
Figures 9A - 12A of the report show how many users can 
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share a 36 MHz transponder at C band and Ku band. Each user 
sends a 1200 bps data stream, and a practical system needs an 
Eb/No of about 14 dB to achieve a satisfactory error rate at the 
receiver when non-coherent detection is used. It can be seen 
from these figures that the MFSK/DS/FDM systems achieve more 
channels/transponder than the MFSK/DS/FH systems, and that larger 
values of M lead to improved efficiency. This is because 
interference effects are reduced with high-order m-ary FSK. 
We designed some typical links at C band and Ku band 
I using these systems. 
1.2 - 1.8 m diameter and transmitted 2W at C band and 0.5W at Ku 
band. The receiver used non-coherent detection, without coding. 
The bit rates were 1200 bps at C band and 56 kbps at Ku band. 
The resulting number of users in each system was about 1000 at 
C band, giving a transponder throughput of 1.2 Mbps, and around 
50 at Ku band, for a throughput of 2.8 Mbps. 
inefficient use of a 36 MHZ transponder: an efficient FDMA system 
could achieve at least 6.7 Mbps with 1000 users in a SCPC mode. 
Our typical VSAT systems used an antenna of 
This represents 
Interference levels at adjacent satellites with 2 O  
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spacing were calculated at -100 dBW/m2 
users and well below -112 dBW/m2 at Ku band. 
figure is acceptable, but the C band figure is too high for such 
a system to be employed when satellite spacing is 2O. 
broader beam of the C band antenna is the prime cause of the high 
interference level. This suggests that VSAT networks may be 
confined to Ku band operation when a large number of stations 
transmit simultaneously. 
for C band with 968 
The Ku band 
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3 .  MFSK Overlay service 
Overlay services place a low data rate signal over 
traffic in a channel which is already filled to its nominal 
capacity. Spread spectrum techniques allow the spectral energy 
of the overlay signal to be spread across the bandwidth so that 
the ratio of traffic energy to overlay signal energy becomes very 
high. Under these conditions, the overlay signal appears like 
low level noise to the receiver and normal operation of the 
channel is undisturbed. Obviously, very low data rates must be 
used in the overlay signal to avoid interference. Applications 
for such services are: a low data rate return signal overlaid on 
an outgoing television signal, engineer order wires, signaling 
channels to accompany TV or telephony, and other data communi- 
cation links to accompany a broadband signal. 
We have investigated the performance and capacity of 
MFSK-DS spread-spectrum overlay signals when the traffic is FDM 
or SCPC telephony, or FM-TV. The data rate which can be achieved 
for the overlay signal is in the 2.5 - 20 kbps range. Highest 
data rates are achieved by using convolutional encoding of the 
overlay data stream, and lowest interference is achieved with an 
FM-TV signal in the occupied channel. Interference ratios for 
the traffic channel were held to very low levels to guarantee 
zero impairment of the traffic. 
4. Conclusions on Part A. 
MFSK spread spectrum systems offer good multiple access 
capability and reasonable transponder utilization in a VSAT net- 
work. Efficiency is well below that achievable with SCPC or 
FDM/FDMA multiple access, but is much higher than for direct e 
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sequence spread spectrum multiple access. Interference levels at 
adjacent satellites are unacceptably high for a large network of 
C band terminals, but are tolerable at Ku band. 
MFSK seems to be a good choice for a Ku band network of 
Using 1.8 m VSAT's operating at data rates up to 56 kbps. 
antennas and 0.5W transmitters, good performance can be achieved 
with a network of 60 to 80 terminals. 
The advantages of MFSK spread spectrum are lower cost 
terminals and higher tolerance of interference. 
5. C h i r p  Spread Spectrum 
Chirp is a technique in which binary signals are 
represented by a rapid up or down sweep in frequency. 
nique has been widely used in radars, but rarely for communica- 
tions. 
tiple chirps to represent data l's and 0's in an overlay service. 
The tech- 
In Part B of this report, we propose a system using mul- 
The major advantages of chirp as a spread spectrum 
technique lie in the ease with which the chirp signal can be 
detected (de-spread). In all other spread spectrum systems, a 
high-speed random (PN) sequence is used to spread the spectrum of 
the signal. At the receiver, a synchronization system must lock 
to the weak spread spectrum signal so that the incoming signal 
can be multiplied by a stored PN sequence, correctly synchron- 
ized, to achieve de-spreading. In low data rate systems, the 
time to lock a de-spreading sequence may be very long, and 
intermittent operation by terminals becomes impossible. 
In a chirp system, de-spreading is achieved with passive 
dispersive filters, typically surface acoustic wave (SAW) 
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devices, resulting in very much simpler receivers with no lock 
delay time. 
Multiple access with chirp is poor when only the slope 
of the chirp signal is varied. 
bility, we proposed a system which we call llMulti-ChirplB, in 
which a sequence of fixed slope chirps is transmitted. The 
chirps can be up-or down-chirps, and various combinations of up 
and down slopes in a 16 or 32 bit word provide a means of iden- 
tifying individual signals. 
To improve multiple access capa- 
Multiple access performance with multi-chirp is well 
below that of the MFSK systems reported in part A, and we do not 
recommend chirp for this purpose. 
6. Chirp Overlay Service 
We investigated chirp for an overlay service in which 
data could be sent at a low rate over an FM-TV channel. The 
multi-chirp signal was designed to spread a 2.4 kbps data signal 
over 5 MHz bandwidth using a 16-chirp word of up and down 
chirps. A major difficulty which we encountered was a lack of 
published data on the spectrum of FM-TV signals. We simulated 
some typical and worst-case TV spectra so that we could assess 
the mutual interference problems for a chirp overlay system. 
FM-TV presents a problem for a chirp overlay system. 
Chirp is a frequency modulation technique and an FM-TV receiver 
uses an FM demodulator to recover the TV signal. We simulated 
the response of a TV demodulator to a multi-chirp signal to 
assess the extent to which the overlay signal would cause 
interference, and were able to show that a 36 dB power ratio for 
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TV to overlay signal would result in unnoticeable impairment of 
the TV video. 
The TV signals we used for overlay testing were a 9 step 
staircase and a random video waveform. Figures 10B and 14B show 
the time waveform and frequency spectrum of the staircase signal. 
Figures 18B and 22B show the waveform and spectra of the random 
signal. It can be seen that the random signal produces a much 
broader spectrum, representing a more difficult case for recovery 
of the overlay signal. 
with FM to obtain a S / N  improvement in the video signal. 
calculated new spectra for random signals after TV pre-emphasis 
had been applied; the result is shown in Figure 33B. The spec- 
trum is now very broad and almost flat. 
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Satellite TV systems use pre-emphasis 
We 
We used the random-waveform case to study mutual 
interference effects when a multi-chirp signal was overlaid on 
the satellite TV channel. We set the overlay signal power in the 
transponder to a level 36 dB below that of the TV signal, to 
ensure no impairment of the recovered video. 
position of the 
band and calculated bit error rates for the data link. Figure 
43B shows the results of this analysis. Placing the chirp signal 
in the center of the transponder leads to very poor performance 
due to the concentration of TV signal energy there. When the 
chirp signal is moved towards the edge of the transponder, 
acceptable error rates are obtained. Data rates up to 25 kbps 
are achievable when the chirp signal is located at the edge of 
the transponder band. With smaller frequency offsets, data rates 
of 2.4 and 4.8 kbps would be possible. 
We then varied-the 
5 MHz wide chirp signal within the transponder 
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7. Conclusions 
Low data rate systems using multiple chirp spread 
spectrum can be used to overlay an occupied channel. 
FM-TV spectra shows that the chirp signal should be set 36 dB 
below the TV signal in the transponder to avoid TV impairment. 
Analysis of 
Data rates of a few kilobits are then possible. At C band, a 
350 mW transmitter and 2.4 m dish could send such a signal if 
the chirp RF frequency is offset at least 8 MHz from the video 
carrier. 
Chirp systems have the advantage of a passive despread- 
ing circuit. There is no delay in synchronizing to the spread 
spectrum signal, which makes the technique attractive for short 
bursts of data. Applications in which terminals need to join a 
network briefly, send a few hundred bits of data and then leave 
the network, are well suited to chirp operation. 
We conclude that chirp is a feasible technique for 
overlay service, although capacity of the overlay circuit is 
small. Performance of C band is good, with a low power trans- 
mitter and minimal adjacent satellite interference. 
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Figura 9A. Performance of MFSWDSEDM Systems (C band) 
111. MFSK Spread Spectrum Multiple Access 
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Flgun 1OA Performance of MFSKfDSlFH System8 (C brlid) 
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111. MFSK Spread Spectrum Multiple Access 
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Figure 11A Performance of MFSKlOSIFDM Systems (Ku band) 
Ill. MFSK Spread Spectrum Multiple Access 
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Figure 12A Perfonmnca of MFSKIDSffH Systems (Ku band) 
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111. MFSK Spread Spectrum Multiple Access 
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Figure 10B Simulated 9 step staircase function (time plot) 
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5. Interference Analysis I (General Approach) 
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Figure 14BSpectrum of 9 step staircase function 
5. Interference Analysis I (General Approach) 
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Figure 18BTime plot of simulated 256 step random amplitude model for video signal 
6. lnterlerence Analysis II (Practical Modeling and Simulation) 
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FREQ, MHz 
Figure 228 Frequency spectrum of figure 21 (linear) 
6. Interference Analysis iI (Practical Mcdeling and Simulation) 
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Figure 338 Transmitted FM-TV spectrum ( m( 1) = random, preemphasis version ) 
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6. Interference Analysis II (Practical Modeling and Simulation) 
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8. Performance Analysts of Overlay Service it- Precmpharis Case. 


Study of spread spectrum multiple access systems for satellite communications with overlay on current services: Executive summary

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