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ASYNCHRONOUS DS-CDMA SYSTEM FOR VSAT

S A T E L L I T E C O M M U N I C A T I O N N E TW OR KS

Young K . K i m , S r . M e m b e r o f I E E E

G TE S p a c e n e t C o r p o r a t i o n

1700 Old Meadow RoadM cL e a n , V i r g i n i a 22102

ABSTRACT

Recently spread spectrum has found Its way intocommercial satellite communications mainly in

low data rate applications. In the paper. forstandard 56 Kbps point-to-point communications.DS-CDMA system was assessed from the technicalfeasibility standpoint for cse with Wide C-bandtransponders.

discussed and the performances of DS-CDMA system

(asynchronus and synchronus) were explored.

revealed that DS-CDMA could produce a goodthroughput result.system was provided, and briefly the tradeoffbetween standard SCPC and DS-CDMA was addressed.

communications, DS-CDMA system could be a

promising alternative to TDMA or FDMAincorporating with small economic earth stations.

I. INTRODUCTION

In recent years there has been considerableinterest in satellite comunications with spreadspectrum and multiple access capabilities.

spread spectrum effect is clearly of use forsecure conmunication pruposes. For satelliteapplications, spread spectrum modulation yieldsa conceptually simple solution to the

requirement of lowering radio frequencyinterferences of one user by another.other hand, multiple access capability is of use

for many users to share the bandwidth andtransmission capability of a satellite

comnunication system with a single repeater.

In code division multiple access, the channelseparation is primarily due to coding embedded

within the carrier waveform.

station uses the entire satellite bandwidth andtransmit through the satellite whenever desired,with all active stations superimposing their

waveforms on the downlink. Thus, unlike time

division multiple access (TDMA) and frequency

division multiple access (FDMA). there is norequirement for precise timing and frequency

coordination between the various transmitting

stations. However. system performance depends

quite heavily on the ability to recognizeaddress codes [l].

Spread spectrum techniques use a muchlarger-than-required bandwidth to transmit

information.

have been used as the predominant multipleaccess techniques in satellite communications.

The basic concept of the DS-CDMA was

Extensive link analysis and system tradeoffs

A recomendation for optimum

Because of a vast market in interactive data

The

On the

Each uplink

Because of this, FDMA and TDMA

However, both techniques employ expensive earth

stations with large antenna diameters.

Interference coordination is also the mainproblem with the orbital spacing of 2 " . In

satellite communication networks, the recurringcost per node becomes a significant factor. An

obvious cost reduction tradeoff is to reduceremote terminal cost, which always implies usinga small antenna.

access appears to be the right candidate tooffer this inexpensive alternative for low data

rate application [ 2 1.The two most connon forms of spread spectrum

techniques employed in CDMA arefrequency-hopping and direct sequencemodulation. The direct sequence (phase-coded)

CDMA method is very attractive for comunicationsystems which also require protection against

malicious interferences and unauthorizedlistening.

Spread spectrum multiple

11. DIRECT SEQUENCE (DS) DMA SYSTEM

To study DS-CDMA, the concept of DS spreadspectrum system using coherent PSK as carriermodulation is discussed. Unlike the standardunspread modem, here the resulting PSK carrierat the modulator output is spread by multiplying

it by another carrier that has been modulated by

a pseudo-noise (PN) sequence represented by thebipolar waveform with a chip rate much larger

than the information rate. At the receiver, theinformation is removed by multiplying the

channel wave form by a synchronized replica ofth PN sequence. This operation therefore

spreads the interference signal over the spread

bandwidth determined by the PN sequence, andhence the interference is reduced.BY far the most widely studied binary PNsequence is the maximal length linear feedback

shift register Sequence (m-sequence) that can be

generated with m-stage shift registers [ 3 ] .

This binary m-sequence has been successfully

employed in communication, navigation, and

related systems over the past several years.For the early applications, m-sequences were

used primarily because of their excellent

periodic autoco rrelat ion properties. For many

of the recent applications, however, the

cross-correlation properties of such sequences

are at least as important a s the autocorrelationproperties [ 5 ] .

in which the utilization of sequences with lowcross-correlation is important in maximizing t k

total number of simultaneous users.

One application is ou r DS-CDMA

0896-582X/87/0000/0140$01 OO 0 988 IEEE

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Non-maximal length sequences which arebasically sums of pair of m-sequences (i.e..

Gold sequences and large and small sets ofKasami sequences). all with period 2”-1,exhibit much smaller peak cross-correlation values that are appropriate for

DS-CDMA.cross-correlation for these deterministicsequences [2 .

In DS-CDMA, each carrier in the grouprepresents a low interference signal to theothers. The level of interference is directlydetermined by the ratio of peakcross-correlation to peak autocorrelation of thePN-related sequence. Thecarrier-to-interference ratio is largelydetermined by the number of simultaneous activeusers and the peak cross-correlation values.

The DS-CDMA system model that we consider isshown in Figure 1 for K users. The k’-th user’sdata signal bk(t) is a sequence of unit ampli-tude, positive and negative, rectangular pulsesof duration T. This Signal represents the k-thuser‘s binary information sequence.user is assigned a code waveform ak(t) which

consists of a periodic sequence of unitamplitude, positive and negative, rectangularpulses of duration Tc.sequence of elements of (, t1, -1) then we canrepresent ak(t) as

Table 1 lists the peak values of the

The k-th

If (aik’) is a

a , ( t ) = ;-ay’ pc ( t - j T c )

where PTC(t) is the rectangular pulse ofchtf:,duration Tc.(a1 ) has period N=T/Tc so that there isone code period a:k’, aik),---,

ai!; per data symbol.bk(t) is modulated into the phase-codedcarrier Ck(t), which is given by

c,, c t ) =G , c t ) c o r (w,t + e,)

We as sme that each sequence

The data signal

Thus, the transmitted signal for the k-th usersis

In the above expressions 8 k represents thephase of the k-th carrier, wo represents thecommon center frequency, and P represents thecommon signal power.

synchronized, then the time delay ? k shown inthe model of Figure 1 can be ignored (i.e.,

rk = 0 for k = 1,2,---,K). This would requirea common timing reference for the K transmittersand it would necessitate compensation for delaysin the various transmission paths.majority of DS-CDMA systems such compensation isnot feasible and hence the transmitters are nottime-synchronous.

For asynchronous systems the received signal

r(t) in Figure 1 is given by

If the CDMA system is completely

For the

r ( t ) =f f i a , ( t - r n ) b , r t - T , , ) Co l ( ~ g t ’O r ) + n ( t J

the ChaMel noise process which we assm e to be

a white Gaussian with two-sided spectral densityN0/2.

..I

Where #k - 6 k - wc 7 k and n(t) is

Since we are concerned with relative phaseshifts modulo 2 and relative time delaysmodulo T. there is no loss of generality inassuming &=O and 7 I = 0 and consideringonly 05 7 < T L 3 .c_& m o r kfi.

If the received signal r(t) is the input tocorrelation receiver matched to Si(t). theoutput is

Z, = i ’ r ( t ) ai ( t ) COS act I t

The data signal br(t) can be expressed as

bm ( t ) -k - hap , (t- ir)

where (bk,l ) S { + 1, -11, The Output Of thecorrelation receiver at t=T is given by

where Rki and 6 k l are the continuous-timepartial cross-correlation functions defined by

111. PERFORMANCE ANALYSIS OF DS-CDMA SYSTEM

Up to this point we have not explicitlyindicated which parameters of thecross-correlation functions should beoptimized.

a code for which the error probabilities Pr (2 ;> Ol bi,o=-1) and Pr (Zi < 04 bi,o=+l) and smllfor all the values of the parameters 7 k ,

The ideal situation would be to find

8 k . bk,-l. and bk.0.

The bit error probability of the carrier Sk(t)is

U

pb ( s M ) = , i - 1 t,C1 C ( f l ) ~ , i (r,,)+(*l)T;c;

C O S ( $ , , ~ ~ L O ~ INow assume that the random variables ? k ,

k=1.2,---, K are uniformly distributed over

( O P T ) *

For the asynchronous DS-CDMA systems, the exact

calculation of error probability seems aformidable task. In this work, we present oneapproach based on the Chemoff bound to obtainupper bound on the error probability expression

in eq ( 3 ) [21.

For a real number 0, the Chernoff bound

states that

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By using the Chernoff bound on the varjable

z I -1.8 ( 1 1 ) , ~ c ~ , ) + ( ~ L ) R ~ ; c ~ , ) I

cos #&t ?L

U. 1..K I L ( 5 )

The bit error probability in ( 3 ) is bounded by

Then the bit error probability of the entire

population of users is bounded by

[ e x p ( x \ pmo , ~ o ~ + e * p ( - ~ l

pb 5 pc =Fro ) K ex p ( - X ) C P p (No_4Eb

a x

emoxcos 0)( 7 )

A

+ e x P ( - 1 emo* cos e)+ x p < - x

I 1 COP e) d e l K

The evaluation of the gdd cross-correlationcan be found in [ 7 ] , but f

for deterministic sequences (Gold or Kasami)

as fwbJ( I was. Therefore, the performancebound in eq ( 7 ) for asynchronous DS-CDMA system

is not that easy to be calculated.

Recently E . A . Geraniotis [ 8 1 compared theperformances of DS-CDMA systems for both

asynchronous and synchronous cases, and hisresult showed that for all modulation types(PSK, DPSK, and,FSK), synchronous resultgenerally performed better than synchronous

(which means, less requred Eb/No to guarantee an

error probability objectivek.out that these results could be applied to bothdeterministic PN sequences and randomsequences. From thi,s result the synchronousDS-CDMA can be viewed as an upper bound for theasynchronous DS-CDMA.

From eq ( 7 ) . the bit error probability ?f

synchronous DS-CDMA system can be bounded

using 7 k = O , k=1,2.---,K, by

is not available

He also pointed

pb 5 pc = F T ~ cxp (- >) exp x 2 )4Eb

enp ( - x l f " , , I =os e11dwhere now the cross-correlation of a selected PNrelated sequence 1s bounded by I Wax I, -3ich

can be available from Table 1.

If the Gaussian assumption is invoked for

inter-user interference, then Pb is Chernoff

bounded by

( 9 )

is the

' - - X 2 / l d x

where O(xl = G l. .

standard Gaussian integral.

In the following, considering thecomputational difficulties in error probability

for asynchronous DS-CDMA systems, we Will usethe bit error probability bound for synchronous

DS-CDMA system to assess the feasibility OfDS-CDMA system for use with Wide C-band SPACENETtransponders.

IV. DS-CDMA SYSTEM DESIGN AND LINK ANALYSIS

The DS-CDMA spread spectrum system design

tradeoffs to optimally utilize the satellitechannel include the following:

Information data rate

Forward error correction (FEC) coding

Spreading rate

Access capacity (CDMA)

Earth station antenna size

Satellite channel power and bandwidth-nterference with Small Antennarate and gain

The low gain associated with small antennas can

be overcome by increased transponder power.This increase is bounded however, by a FCCspecification on the maximum permissible carrierpower density. This restriction protectsco-frequency users from interfering with eachother. The increased beamwidth caused by

smaller aperture size has perhaps the mostsignificant impact on system design. Antennawith 4 foot (1.2m) diameters have a half-power(3dB) beamwidth of 5." at 4 GHz. This implies

that when satellites are spaced 2" apart, the

antenna will receive mainbeam signals from threedifferent satellites (one desired and twoundesired). The magnitude of this interferenceis a function of the traffic in co-frequency

transponders on the adjacent satellites. For 6foot (1.8m) and 8 foot (2.4m) aperture antennashalf-power beamwidth can be 3.5" and 2.4O,respectively. And in these cases, the

interferences from the adjacent 2" spacedsatellites could be much reduced.

Many engineers believe that spread spectrumalleviates the interference problems which are

driven by the use of small aperture antennas.But this satellite interference issue inconjunction with spread spectrum appears to be a

complicated matter to handle. Generally

agreeable statement can be made as follows:

against narrowband interference. by spreadingthe interfering carrier, the spread spectrum can

reduce this interference significantly. Butagainst widebiand interference (typical TDMA orEM/TV carrier). it is difficult to expect

interference reduction since the interfering

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carrier will not be spread as it is already

wideband.The DS-CDMA system is capable of rejecting

narrowband interference from terrestrial radio

system. But against wideband terrestrial

interference, the use of spread spectrum doesn'tlo& pr ai si ng in reducing the interference-

Intennodulation

The intermodulation in CDMA is due to the

nonlinear amplification of overlapping carrierspectra. ,Th is is in contrast to the case in

F'DMA where th e carrier spectra are contiguous.Nevertheless, intermodulation spectra for the

two cases have been observed to be quitesimilar, and intermodulation effects for the

FDMA case are often used in CDMA analysis as

well [I]. FOC CDMA the power backoff for linear

performance of nonlinear amplifiers is required.

It was reported [9 ] that in the QpsK DS-CDMAsystem the interference of the intermodulation

products (IMPS) caused by the interaction

between a desired biphase spread spectrum Signal

and each of the undesired biphase spreadspectrum signal in a nonlinear transponder can

be much reduced, though these intermodulationinterferences cannot be reduced by the

inteference rejection capability of a BPSK

DS-CDMA system.Error-Correction Coding

The use of error-correction coding in DS-CDMAcan greatly improve the number of simultaneous

users because coding in effect reduces the noise

floor of the satellite channel as far as signaldetection is concerned.

code rates and values of specified

achieving acceptable performance (BER

with a Practical convolutional code and a softdecision decoder [4].Link Analysis Cases

In the following, the link analysis cases andassumptions are summarized.

SPACENET 1 (12OOW)

56 W p s information rate

1.8m and 2.4m VSATs

FM: rate of 71 8 or 1/2

Modulation: BPSK or QPSK

Low noise amplifier temperature: 90°K

Filtering: Square root 60% cosine

Guaranteed BEI? of 10"

The spread spectrum channel capacities (max.

Table 2 shows several

for

Wide C-band transponder (72 MHz)

COllQf f

no. of 56 Kbps channel permissible) per SPACENETWide C-band transponder for different PN related

codes, FM: rates and modulation types are

sunmnarized in Table 3.

For both BPSK and QPSK modulations, codeperiod 63(2'-1) Kasami sequence (small set)

turned out to be the best selection in terms of

the bandwidth throughput (bit/Hz). Gold codehas larger set si=, but it has bigger peak

cross-correlation value than Kasami small set,which limits the no. of simultaneous users

significantly under a given BER objective. Inthe calculation. the Gaussian assumption was

invoked for DS-CDMA user interference and the

energy-per-bitluser-interference-density ratio

was determined from the no. of simultaneous

users and the normalized maximum

cross-correlations.

io-', system performance was calculated using

eq (9). With Kasami sequence (small set) of

code period 63, we have maximum eight

simultaneous users per a spread spectrumchannel.

In the follow-on coded DS-CDMA linkperformance analysis, it was generally conceivec

that the spread spectrum channel capacity waslimited mostly by satellite bandwidth than

satellite power.

(available Eb/No) and the calculated

energy-per-bit/ user-interference-density wereincorporated with eq (9) to meet with quaranteed

BER of lo-' (see Table 2).

Tradeoffs between satllite bandwidth andsatellite power provide the summary Table 4 for

channel capacity (max. no. of 56 kbps channels)

and VSAT HPA sizes. Here we considered two

point-to-point services (1.8 m-to-1.8 m and 2.4m-to-2.4m).

result between synchronous and asynchronousDSICDMA system [ a ] showed that for a case withPSK modulation, code length 63 and ten

simultaneous users which was almost identical toour case, there was about 2.4dB Eb/No difference

(with guaranteed BER of

observed that for higher BER objective (lo-'or lo-' BER) the FMNo difference gap became

wider, which means we may expect at least this

Eb/No improvement with asynchronous DS-CDMAsystem over synchronous system. But to be more

realistic, this Eb/No improvement was allocated

as the overall implementation margin of

asynchronous DS-CDMA system.

Through the study, we found that QPSK with

7/8 rate ??EC and 2.4 m VSAT could be the best

selection in balancing the satellite bandwidthand power tradeoffs (providing 130 channel

capacity) with 3.0 dB nominal transponder output

backoff and in meeting small HPA sizeStandard SCPC Services

link was considered to study the tradeoffsbetween DS-CDMA and SCPC. With larger C-band

antennas (4.5m-to-4.5m), the SCPC link showed

better channel capacity (180 Channels per Wide C

transponder) than spread spectrum link, but itrequires expensive large antenna ground systems

and bigger HPA size (75 Watts). Interferencecoordination in this regular C-band SCPC link is

also the major problem with the orbital spacing

With the guaranteed BE R of

The link budget result

For your more information, the comparison

Also it was

A non-spread spectrum, standard 56Kbps SCPC

of 2O.

V. CONCLUSION

For standard 56 kbps point-to-point

communications service, DS-CDMA system wasconsidered from the feasibility standpoint f or

the use of SPACENTCT Wide C-band transponders.

Through the link designs and performance

tradeoff study, the optimum CDMA system

throughput result (bit/H z) appeared to bepromising in utilizing the Wide C rich bandwjdth

(72 MHz) transponder. A recommendation was made

for the optimum asynchronous DS-CDMA system and

the tradeoff between standard SCPC and DS-CDMAwas addressed.

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Despreading process in DS-CDMA can reduce the

interferences (especially narrowbandinterfernce) from the adjacent satellites and

terrestrial radios considerably. DS-CDMA systemlink analysis results concince us to use smallaperture C-band antennas ( 2 . h ) and small

solid-state power amplifiers which can gear tovery low cost network design.

density which leads to a low probability ofdetection (provision of message privacy), could

be a merit feature.Also, the very high availability which is an

inherent advantage in C-band satellite operationcannot be deemphasized.

In a CDMA approach, a low carrier power

REFERENCES

1.

2.

3 .

4 .

5.

6.7 .

8

M. Gagliardi, Satellite Communications.Chapter 1984 Wadsworth Inc.T. T. Ha, "Spread Spectrum for Low CostSatellite Services", International Journal

of Satellite Communications, Vol 3 , June 1985.R. Dixon, Spread Spectrum Systems, Chapter 3.

A.J.

Viterbi, "When Not to Spread Spectrum -A Sequel". IEEE Communication Magazine,April 1985.

D. V. Sarwate and M. B. Pursley,"Cross-correlation Properties of Pseudorandomand Related Sequences", Proceedings of theIEEE, Vol 68, No. 5, May 1980.J. Campanella. Private Communication.M. B. Pursley. "Performance Evaluation forPhase-Coded Spread-Spectrum Multiple-Access

Communication - Part I: System Analysis",IEEE Trans. on Comm.. Vol COM-25, No. 8,August 1977.

E. A. Geraniotis, "Performance of Noncoherent

Direct-Sequence Spread-SpectrumMultiple-Access Communications", IEEE Jour.on Selectd Areas in Comm., Vol SAC-3, No. 5,September 1985.

Delov

W 1 ) TS~O,JI)COS~I& I

F I G U R E I . ASYHCHROXO'U'S DIInECT SEQIJLNCE

CDMA SYSTEM HODEL

T a b le I . P r o p e r t i e s of S e q u e n c e S e t s

Famlly m S e t Peak

S i z e C r o s s - c o r r e l ati nGo1d odd 2" + 1 1 + 2 (m r" 1 2

1 + 2 ( " + 2 > / 2Gold Z( m od 4 ) 2" + 1

Kasaml e v e n 2"" 1 + 2 m

(small)

Kasami e v e n 2"'2(2"+1) 1 + 2 ( m r 2 ' / 2

(large)

T a b l e 2

R e q u i r e d E b I N o for IO-' E R

C o d e R a t e ( E i s l C o d e S y m b o l ) E b / N o( d B )

I ( u n c o d e d )

31 4I / 2

i t a

10 .57 . 1

6 . 2

5 3

T a b l e 3

( I ) BPSK w l t h 7 1 8 Rate FEC

PN C o d e Simul. p e r Wide C Channe lP e r i o d C o d e Famllv U s e r s T r a n s p o n d e r s CaDaCI Y

No . o f No. of C K R S

8 1 1 883 Kasaml ( s m a l l )G o l d & K a s a m l (large) 3 1 1 33

1 2 7 G o l d ( o d d ) IO 5 50

255 Kasaml (small) 1 5 2 30

C o l d & K a s a m l ( l a r g e ) 1 5 2 20511 C o l d ( o d d ) 38 1 38

( 1 1 ) BPSK r l t h 11 2 Rate FEC

NO. of NO . o f C a r r le r r

simultaneous per nlde C ChannelPN Code user s rranrpon derr C a p a c i t y

ertod Coda F a n l l v

8 6 48

4 6 2 4

14 3 42

16 I

63 Kasanl ( s m a l l )c o l d h K a s a nl ( la r ge )

G O I ~ K a s a m l ( I t r a t ) 15

12 7 Gold ( O d d ) I 16625 5 Kasani ( S M l l )

(111)

PM code users Transwnderr Capacitye r iod Code F d 1 Y

QPSK r l t h 718 Rate FEC

no. o no. o C dr r le r s

s i w i t a n k u s pe: niae c Channel

8 21 I6 83 21 6 39 I 1 99

5 805 so2 76

63 K u m l ( S M 1 1 )

t o l d h Karaml ( large)

I 6Gold h K a r a m I ( l a r g e ) 10

38

12 7 mid (odd)

5 1 1 G O I ~ odd)

25 5 Kasaml ( S M l l )

(1'4) QPSK r l t h I i 2 Rate FE C

PU codePeriod Code F a d I Y

no. of no. of C a r r l e r s

SImuitanMus Per nide c Channelusers TransWnders CaPaCltY

8 12 96

4 12 4814 6 84

Gold 6 K 1 s U I ( l a r g e )

3 b8

3 45

16

Go l d b K a r u l ( l a r g e ) 10

63 KaSaml ( S M l l )

I2 7 Gold (odd)25 5 K a s u I (small)

T a b l e 4C h a n ne l C a p a c i t y (No. o f 56 k b p s C h a n n e l )

a n d V SA T H PA S i z e

(I.am-tol.am12.4m-to-2.4m)F EC

M o d u l a t i o n 718 R a t e 112 R a t eaa I aa 4a I 6ah a n n e l B P S K

C a p a c i t yP e r S P A C E N E T

W i d e C - b a n dT r a n s p o n d e r Q p SK 1 1 4 I 130 96 / 96

VSAT BPSK 50 I 30 25 I 10

HP A

50 I 30 25 1 10i z e

( W a t t s ) Q P S K

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