Performance of Multi tone Code Division Multiple Access (MT-CDMA) in anAWGN Channel and in Presence of Narrowband Jamming

Khalifa Nasser K Jileta, Mahamod Ismail, Alina Marie HasbiDepartment of Electrical, Electronics and System Engineering,

Faculty of EngineeringUniversiti Kebangsaan Malaysia436000 Bangi- Selangor, Malaysia

Tel: +6 03 8296322 Fax: +6 03 8296146kjleta(yahoo.com, {mbi, alina)}eng.ukm.my

Abstract- The main limits of the conventional singlecarrier modulation techniques are the restrictions imposedby the multipath channel, and the receiver complexity. Onother hand the multicarrier techniques such as Multi-Carrier (MC) and Multi-Tone (MT) can provide high datarates at reasonable receiver complexities. Jamming on theother hand is of interest in some communicationapplications, and in military anti-jam systems. In this work,we study the performance of Multi-Tone Code DivisionMultiple Access (MT-CDMA) in presence of narrowbandjamming. We investigate Bit Error Rate (BER) performanceas a function of some system parameters such as number ofsub-carriers, processing gain, and as a function of channelconditions such as jamming type, number of jammers,jamming center frequency, jamming power, and jamming tosignal power ratio. We compare the performance with theanalytical performance in absence of any jamming. Thesimulation results show, that the performance can beenhanced by increasing the processing gain, number ofjammers or by decreasing the number of sub-carriers, thejamming to power ratio or jamming power. Furthermore theeffect of jamming center frequency on the performance isnegligible, and good agreement with the analyticalperformance in absence ofjamming is achieved.

Keyword&-OFDM, CDMA, MT-CDMA, Narrowbandjamming

I. INTRODUCTION

Multimedia applications of mobile communicationsneed modulation, and multiple access techniques that, candeliver very high data rates, which cannot be offered. bythe traditional single carrier modulation techniques. MT-CDMA is a combined technique between OrthogonalFrequency Division Multiplexing (OFDM) and Codedivision Multiple Access (CDMA) and it is an attempt toprovide a communication system that inherits theadvantages of both CDMA and OFDM, and provides thenew probable technique for the fourth generation ofmobile communication systems. MT-CDMA inherits thepowerful features ofCDMA, which allow number of usersto access the channel simultaneously. This is achieved by,modulating and spreading the signals with pre-assignedcodes sequences. Since CDMA provides better spectralefficiency and easier base station placement compared tosecond generation systems and beside its highperfonnance in presence ofjamming interference. Another

very important advantage of MT-CDMA gained fromOFDM is its less sensitivity to the inter symbolinterference (ISI) caused by radio channel impairments.This advantage is achieved by using more than one carrierto carry the symbol and then a lower data rate for the sameuser data comparing to single carrier modulation isachieved. By well-chosen system parameters, we canreduce the effect of unfriendly channels such asfrequency-selective channels. Another very importantadvantage using MT-CDMA is its spectrum efficiency dueto allowing the sub-carriers to largely overlap with eachother. Actually High Spectral Efficiency is the mainadvantage ofMT-CDMA over single and the multicarriertechniques. At the same time, to facilitate inter-carrierinterference free demodulation of the sub-carriers, the sub-carriers are made orthogonal to each other. The possibleorthogonality in MT-CDMA is over the symbol time T,[1], [2]. If the orthogonality is not altered by the channel,the modulating signal of each sub-carrier can be recoveredexactly by the receiver [3]. A comparison between MTand single carrier schemes was done in [4] and bothschemes show equivalent performance in presence ofmulti user interference. Jamming noise is of interest insome fields and applications such as anti-jam systemsused in military applications and in the spectral overlay ofnarrowband pulsed signals over Direct Sequence SpreadSpectrum (DS-SS) transmission [4]. In this work weassume single user to simplify the work (no Multi UserInterference) and we use conventional receiver which isrecommended in case of single user detection. Spreadingtechnique is DS-SS and the modulation technique isassumed Binary Phase Shift Keying (BPSK), since it isthe most common modulation type, used in directsequence systems. The type of data mapping to sub-carrieris considered as Serial to Parallel (S:P) transformation.The codes on all sub-carriers are the same, in order toobtain a high correlation among the sub-carriersmodulation symbols. Perfect symbol synchronization andphase coherence are assumed. And the channel is assumedas Additive White Gaussian Noise (AWGN).

In the next we describe MT-CDMA system. In sectionIII we describe the simulation methodology. Section IVprovides the simulation results, discussion and section Vincludes a summary and the conclusion.

1-4244-0000-7/05/$20.00 C2005 IEEE. 979

11. SYSTEM DESCRIPTION ANT MODEL

Fig. 1, Fig. 2, show the MT-CDMA transmitter andMT-CDMA spectra respectively.

C (t) Cos(2*pi*fxT)

C (t) Cos(2*pi*f2T)

I II I

CM(t) Cos(2*pi*fMT)

Fig. 1. MT-CDMA Transmitter

S(t)*00

'- I I I I !f1 f2 f3 f3...fM

Fig. 2. MT-CDMA spectra

The incoming bit stream denoted by d(t) is first serial-to-parallel converted into Mdata streams (Mis number ofsub-carriers). Then the data on each path is spread byusing orthogonal codes denoted by Cj(t).Same or differentcodes can be used on each path. After spreading thespread data modulates the orthogonal sub-carriers. In MT-CDMA, the sub-carriers are orthogonal over the symbolduration, and hence the sub-carriers frequencies are given

by f = f +P where fo is the RF and T is the

symbol time, and p = 01,.....M-l. The modulated and

spread waveforms are scale and then summed. The Jlhsub-carrier data waveform (transmitter input) is[5]:

dj(t) = 2 djn p t-n (l)n

where the nib data symbol is djn E D = {1}and T is

the bit time. The modulation symbol Ij(n) are analogous tothe data symbol and the modulation waveform for sub-carrierj can be defined as[5].

Ij(t) =EIj(n)pTs (t- nTs) (2)

The Processing Gain (P) is defined as P = T/TC, where T,= 1/R1 the chip time, and R& is the chip rate. Themodulated and spread waveforms are scaled and thensummed. The final carrier waveform is denoted as S(t) [5]

Sj , x{ Ij j QC(n)cos(2Tj,t)4(3)

where:S(t) is the transmitted signal.M is the number of sub-carriers.

Lxj is the integer part of x.

cos 2z'rfjT(t): is the RF carrier for the correspondingsub-carrier at the transmitter.At the receiver, the transmitted signal is added to n(t),

the AWGN noise signal with zero mean and variance ofNJ2 W/Hz. The received signal can be

r(t) = s(t) + n(t) (4)MT-CDMA receiver is shown in Fig. 3. The receivedsignal is entered to M parallel paths, on each path wedemodulate the received signal by multiplying it bycos 2fjR(t) and c(t), then integrating over the symbolperiod (Ts). The integrator removes (shown in Fig. 4) thedouble or high frequency term resulting from themultiplication by the sinusoidal carrier signal. Thedecision circuits are used then to collect the symbolsamples to make a decision and get the data symbol. Thedecision static is denoted by 4. This is done on each sub-carrier, hence Mof Fig. 4 is required [5].

Cos(2*pi*f,T)

Fig. 3. MT-CDMA Receiver

1.(t) Ts _(t)

C (t)Fig. 4 De-spread block ofMT-CDMA receiver

980

d(t)

MT-CDMA analytical model is given in [5]. [5]Assumes AWGN and doesn't consider any jamminginterference. The AWGN channel receiver decision staticsZj is given as [5]

TsZ= r(t)cos(27dtRt)c(t)dt (5)

0

The probability of error was derived in [5] for BPSKand it is given by

PB = QEaAjJ (6)

Equation (6) is used to compute the BER analyticallyfor MT-CDMA in absence ofjamming interference and itis used in this work to compare the performance inpresence and absence ofjamming interference.

1Il. SIMULATION DESCRIPTION

We have employed Monte Carlo simulation techniqueto estimate the performance of the system in terms ofprobability of error. Monte Carlo simulation methodutilizes sequence of random numbers to perform thesimulation. To study the MT-CDMA system performance,a software model of the system is developed (transmitter,communication channel, and receiver). The simulationprogram started by setting the simulation parameters andthen a random data is generated. The simulation modelcan be summarized as follows:

A Serial to parallel (S: P)Converting the serial data to parallel is a simple

process. Dividing the input data with rate Rb bps to Mparallel streams each ofwhich is Rb/M bps where M is thenumber of sub-carriers does the whole process.

B. Spreading code generatorThe code that, used to spread the data is also randomly

generated The spreading process is done based on DS-SStechnique, where each data bit is repeated first P times,then randomized by multiplying it by a random number(pseudorandom code) generated above. By repeating eachbit P times, we achieve the first condition for the DS-CDMA signal; we get chips with chip time equal to Tb/Por R, equal to P*Rb (spread the data over widerbandwidth), and then by multiplying the chips withrandom number we achieve the second of DS-CDMAsignal which seems random to the jammers.

C. Sinusoidal Carriers GeneratorsSinusoidal sub-carriers generators are used to generate

sinusoidal vectors for modulation. The sub-carriers ofMT-CDMA as mentioned before are separated by l/(T,) infrequency domain, and since we have one sample per chipand each spread bit is sampled P times, then in simulationthe sinusoidal vectors are separated by I/P. The data ismultiplied by the spreading code and sinusoidal signal for

each sub-carrier and all the sub-carriers samples are thensummed to perfonn the transmitted signal given by (3)Fbut for one user (1=1).

D. AWGNchannelRand Matlab function is used to generate AWGN. The

variance ofAWGN is changed with the desired Eb/NO. Thetransmitted signal is then added to the AWGN to get thetransmitted signal with AGNW noise. That signal is thenadded to the jamming noise to perform the input signal tothe receiver.

E. Jamming genera torJamming noise can be considered as sinusoidal signals.

Both of single tone and multiple tone jamming noises canbe generated by the same method used to generate thesinusoidal carriers.

F. Detection and error estima tionThe receiver is simulated based on the system shown in

Fig. 3. We use conventional receiver for the detectionprocess, which consists of correlator, integrator andcomparator. The received signal enters to the receiver isfirst serial to parallel converted into M parallel paths. Ateach path it is demodulated and despread by the correlatorby multiplying it with the sinusoidal signal and spreadingcode. After that it is integrated or accumulated over thesymbol time, where the double frequency term resultingfrom the multiplication by the carrier signal is removed.Then, decisions are made on the bits by the decisioncircuit shown in Fig. 4 after collecting the symbol samplesand translating them to data symbol based on thresholdvalue (greater than or equal to 0.5 is considered 1, and lessthan 0.5 is 0).To calculate the errors exist in the received data, we

compare these received bit estimates with the transmittedbits. In other words, the bits of each transmitted sub-carrier are compared with the corresponding received sub-carrier bit. The error counter is also increased if thetransmitted and received bits are not the same. The biterror of the all system (aggregate bit error rate) is then theaverage of these bits error of all sub-carrier.The output of this simulation program is a plot of the

obtained bit error ofthe system versus SNR (ENo).

IV. SIMULATION RESULTS

A Performance in Presence ofSingle JanmingThe most important parameter that effect design of MT-

CDMA is the number of carriers, since in MT-CDMA thebandwidth is divided among large number of sub-carriers,and the sub-carriers are largely overlapped. Fig. 5 presentsthe effect of number of carriers on the systemperformance.

981

0peformnance of MT-CDMA in presence of single jamming over AWGN channel1n0 . .::.::-:_-.:-= :.t-.::-. :=:::: -_:I . . _

lo-'I

o-210U

s. 10,~

lo'

110 1 2 3 4 5 6 7 8 9 10

SNRtdB)

Fig. 5. Effect of number of sub-carriers

Fig. 5 presents clearly that by increasing number ofcarriers, the BER will increase, due to the increase in thesub-carriers interfering, and hence the system performancewill be degraded. The result is based on three simulationsperformed with different number of carriers (3 carriers, 5carriers and 15 carriers) for various SNR (O dB to 10 dB).Indeed some factors may limit the number of carriers inthe system, such as sub-carriers interfering, systemcomplexity and processing time. With small number ofsub-carrier (M13), the performance obtained was veryclose to the theoretical BER in absence of any jamming.We can also notice that the effect of single jamming noiseis negligible due to the effect of corrleator and theintegrator where the jamming is first spread over the entiresystem bandwidth by the correlator and then lowpassfiltered by the integrator so, that only small portion of itspower will be collected. The effective power of thejammer is reduced by a factor equals to the processinggain [7]. The effect of processing gain is showrn in Fig. 6.

performance of MT-CDMA in presence of single jamming over AWGN channel.i.~~ ~ ~~~--------4 P-32

:::.1~~~~~~~~-- -P=64---- ---- ----- ------ ------1 P=128

10 . ........... AWGN

Li 10

10m

0 1 2 3 4 5 6 7 8 9 10SNR(dB)

Fig. 6. The effect of processing gain in presence of single tone jamnmingnoise and over AWGN channel

achieved by using higher processing gain but the cost ofincreasing the processing gain is less throughput, lower bitrate, and higher system complexity.

The effect of jamming interference comes from itscenter frequency and its power with compared to thesignal power. Maximum jamming effect is achieved if thetone is placed on one of the sub-carrier center frequencies[6]. Fig. 7 shows the effect of single tone jamming centerfrequency and jamming to signal power ratio. First thejammer was replaced at the center of the first sub-carrier,and then at the center frequency of the transmitted signal.As shown in Fig. 7, it is clear that, the jamming has noeffect on the transmission even if it has centeredfrequency as one of the sub-carrier center frequencies(worst case). However, by increasing the jamming tosignal power ratio from 3 dB to 9 dB, the jamming noisehas clear effect on the performance of the system.

Dperformance of MT-CDMA in presence of single jamming over AWGN channel

SNR(dB)~~~~~J S-d

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--,*- J-S--,,*,}9dB.

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..... .... ..... ....... .. . ..3... 3sE..

redce by the inert. Tis is tru if thiamn

- 1 2 3 4 5 6 7 8 9 10SNR(dq-

Fig. 7. The effect ofjamming to signal power ratio in presence of singletone jamming over AWGN channel

B. PERFORM4ANCE OVER AWGNCHAMFL AND) INPRESENCE OF MULTIPLE TONE JAMM7INGIn this part the effect of the number of jammers is

investigsed. Fig. 8 shows the simulation results. It is clearfrom Fig. 8 that, increasing the number of jammingsignals enhances the performance. This is because as weincrease the number of jamming noises as we go towardwhite wide band jamming case which is actually thedesired jamming case and most of designers do so effortsto push the jammer to be at that case because the totaljamming power will be divided among a number ofjammers and then spread over a wide band by thecorrelator at the receiver side so then the jamming effectwill be spread over all the sub-carriers and then will bereduced by the integrator. This is true if the jammingpower is not sufficient enough to interrupt thetransmission, as shown in Fig. 9.

It is clear from the results in Fig. 6 that increasing theprocessing gain can enhance the system performance.Higher capacity in terms of number of users can be also

982

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SNR(dB)Fig. 8. The effect ofnumber ofjammers over AWGN channel

Fig. 9 represents the effect of increasing the jammingpower on the system performance. The result is based onthree simulations perfonned with jamming power (equalto the transmitted power, double of the transmitted power,triple of the transmitted power). From Fig. 9, we caneasily notice that the BER of the system decreases as thejamming power increases even when the number ofjammers is higher than the number of the sub-carriers sowe can conclude that the effect of multiple jamming noiseis controlled by its power and the increasing of thenumber of jamming signals can enhance the performanceof the system unless the jamming power is not sufficientenough to interrupt the transmission.

-peformance of MT-CDMA in presence of Mutipte tone jamming over AWGN channel

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Replacing the single jamming at the center frequencyof one of the sub-carriers frequencies has a negligibleeffect and almost the same as replacing it at the center ofthe spectrum of the transmitted signal however, theperformance is degraded as jamming to signal power ratioincreases.

Increasing the number of jammers will enhance theperformance, but increasing jamming power degrades theperformance. Excellent agreement between performancein AWGN in presence ofjamming and the theoretical plotof the performance in AWGN without any jamming.

VI. REFERENCES

[11 D. W. Matolak, V. Deepak, F. A. Alder, Perfornance ofMultitone and Multicarrier DS-SS in the Presence of ImperfectPhase Synchronization. School of Electrical Engineering &Computer Science, Athens, OH 45701.2002.[21 S. Hara, R. Prasad, Overview of Multicarrier CDMA, IEEECommunication Magazine, vol. 35, no. 12, pp. 126-133, December1997.[3] L.Vandendorpe, Multitone Spread Spectrum Multiple AccessCommunications System in a Multipath Rician Fading Channel.IEEE Transactions on Vehicular AR technology. Vol. 44. NO. 2.1995.14] D. W. Matolak, F. A. Alder, V. Deepak, Performance ofMultitone and Multicarrier DS-SS in The Presence of Partial-BandPulse Jamming/Interference. School of Electrical Engineering &Computer Science, Athens, OH 45701.2002.[5] 1. Sen, Bandwidth Efficient Reduced-complexity MT-DS-SSvia Reduced subcarnier Frequency Spacing. M.Eng. Thesis.Carleton University. Ohio University. 2004.[6] R. L. Peterson, R. E. Ziemer, D. E. Borht, Introduction toSpread Spectrum Communication, Prentice-Hall, Upper SaddleRiver, New Jersey, 1995.[71 Tan. F. Wong. (2004, May 12). Spread Spectrum and CodeDivision Multiple Access. Wireless communication course.University of Florida. Available:http://www.wirelesss.ece ufl-edu/twong/eel6503/

10.10 1 2 3 4 5 6 7 a 9 10

SNR(dB)

Fig. 9. The effect of the power of multiple tones janming noise overAWGN channel

V. CONCLUSION

In this work, the performance of MT-CDMA undervarious channel, various system conditions and inpresence of narrowband jamming noise has beeninvestigated. Increasing the number of sub-carriers willdegrade the system performance. The sub-carriersinterfering limit the number of sub-carriers.

Increasing the processing gain enhances the systemperformance.

983

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