performance of multi tone code division multiple access (mt-cdma) in awgn channel and in presence of...

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

Khalifa Nasser K Jleta, Mahamod Ismail, Alina Marie HasbiDepartment ofElectrical, Electronics and System Engineering,

Faculty of EngineeringUniversiti Kebangsaan Malaysia436000 Bangi- Selangor, Malaysia

Tel: +6 03 8296322 Fax: +6 03 8296146Kjletagyahoo.com, (mbi, alina}@eng.ukm.my

Abstract- Multiton transmission was proposed becausethe associated larger symbol duration favorable to combatthe effects of multipath propagation 1i]. In fact the mainlimits of the conventional single carrier modulationtechniques are the restrictions imposed by the Multipathchannel, and the receiver complexity. On the hand themulticarrier techniques such as Multi-Carrier (MC) andMulti-Tone (MT) can provide high data rates at reasonablereceiver complexities. Jamming on the other hand is ofinterest in some communication application, and in militaryanti-jam systems. In this work, we study the performance ofMulti-Tone Code Division Multiple Access (MT-CDMA) inpresence of narrowband jamming. We investigate Bit ErrorRate (BER) performance as a function of some systemparameters such as number of sub-carriers, processing gain,and as a functions of channel conditions such as type ofchannel, channel K-factor, jamming center frequency,jamming power, and jamming to signal power ratio. Thesimulation has been carried out by using MATLAB packageand the results show, that the performance can be enhancedby increasing the processing gain, K-factor, or by decreasingthe number of sub-carriers, or the jamming to power ratio.Furthermore, that the effect ofjamming center frequency onthe performance is negligible.

Keywords-MT-CDMA, Narrowband Jamming, Fadingchannels

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 highperformance in presence ofjamming interference. Anothervery important advantage ofMT-CDMA gained from very

important advantage ofMT-CDMA gained from OFDM isits less sensitivity to the inter symbol interference (ISI)OFDM 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 ofthe sub-carriers, the sub-carriers are made orthogonal to each other. The possibleorthogonality in MT-CDMA is over the symbol time T,[2], [3].If the orthogonality is not altered by the channel,the modulating signal of each sub-carrier can be recoveredexactly by the receiver [4]. A comparison between MTand single carrier schemes was done in [41 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 [5]. Perfect symbol synchronizationand phase coherence are assumed. And the fading isassumed flat (no inter symbol interference).

In the next section we describe MT-CDMA system. Insection III we study the channel model. Section IVprovides the simulation results, discussion and section Vincludes a summary and the conclusion.

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II. SYSTEM DESCRIPTION

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

CI(t) Cos(2*pi*fIT)

C2(t) Cos(2*pi*f2T)

I II I

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

Fig. 1. MT-CDMA Transmitter

£ t)S

W ~~~Transmitted bandwidth 0

modulated and spread waveforms are scaled and thensummed. The final carrier waveform is denoted as S(t) [5]

M'i

V It Ij( N2)C(n)cos(2irfJT(3)

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

Lx] is the integer part of x.

cos 2*fjT (t) is the RF carrier for the corresponding.MT-CDMA receiver is shown in Fig. 3. The received

signal is entered to M parallel paths, on each path wedemodulate the received signal by multiplying it bycos 27f (t) and c(t), then integrating over the symbol

period (Ta). The integrator removes (shown in Fig. 4) thedouble or high frequency tenn 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*fIT)

I I I ifl f2 f3 f3 * *fM f

Fig. 2. MT-CDMA Spectra

The incoming bit stream denoted by d(t) is first serial-to-parallel converted into M data streams (Mis number ofsub-carriers). Then the data on each path is spread byusing orthogonal codes denoted by C (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 + T where f; is the RF and T is the

symbol time, and p = 0,1,....M-l. The modulated andspread waveforms are scale and then summed. The J«sub-carrier data waveform (transmitter input) is[51:

d] (t) 2jB E dj,,PT(t-T (1)n

where the nth data symbol is di, 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)= Ij(n)pT (t- nT) (2)

The Processing Gain (P) is defined as P = T,/Tc, where Tc= l/Rc the chip time, and Rc is the chip rate. The

Fig. 3. MT-CDMA Receiver

Fig. 4. De-spread block ofMT-CDMA receiver

III. CHANNEL MODEL

Two types of fading effects characterize the mobileradio channel: large-scale fading and small scale fading.Small-scale fading is also called Rayleigh or Rician fadingbecause if a large number of reflective paths is

966

S: Pr - - @ | -- l | -S2i25-14

It h.-

I .111


encountered the received signal envelope is described by aRayleigh or a Rician probability density function (PDF)[6]. The fading amplitudes can be modeled by a Rician ora Rayleigh distribution, depending on the presence orabsence of specular signal component. Fading is Rayleighif the multiple reflective paths are large in number andthere is no dominant line-of-sight (LOS) propagation path.If there is also a dominant LOS path, then the fading isRician- distributed. The fading amplitude ri at the ith timeinstant can be represented as [6]

ri = V(x, +g6)2 +y" (4)

where O is the amplitude of the specular component and

xi, y1 are samples of zero-mean stationary Gaussian

random process each with variance 02. The ratio ofspecular to defuse energy defines the so-called Rician K-factor, which is given by [6]

K = fl 2 (5)2othe best-and worst-case Rician fading channels associatedwith K-factor of K = oo and K=O are the Gaussian andRayleigh channels with strong LOS and no LOS path,respectively. So, the Rayleigh fading channel can beconsidered as a special case of a Rician channel with K=0.The Rician PDF is given by [6].

(= rjexp[- (r + ,82)/20r2]I fL (6)

where r . 0where I0 [J is the zero-order modified Bessel function ofthe first kind. If there is no dominate propagation path,K=O and I0 [I =1 yielding the worst-cases Rayleigh PDF[6]

frayliegb(f)= 2Lj.exp[r/2. r. 0 (7)

0othe Rician cumulative distribution function(CDF) takesthe shape of [61

second one when it is possible to get some LOS paths. Weinvestigate how many of the sub-carriers we can use toachieve the same or comparative performance in both ofthem.

peofoyance of MT-CDMA in presence of single janring over Rayleigh and Rician channels~-: I:::: I:::::a:::::.:::::::;::: --t- Rayleigh channel

- Jo-J.aRictO nRiciachsnrtel::::- -::::,:: K-factor=2f Rician channel

......... ....10 *,__-* ; ; ,,,,,,;, , ,, ,,,; .;-..-;-;

------------- . .....................---

--------------_S , -~~ -------- . _ ._,

--- ______.__ _ _ _ __ __ -.------* 0 :::r:::r:s::::--Rt-----s::::

' ''--- -'''----- -r--- -- -- ------ -------

0 2 3 4 5 6 7 a 9 10SNR(L1B)

Fig. 5. Effect of number ofLOS paths on number sub-carriers

it can be noticed from Fig. 5, that having more LOS pathsin the channel allows using more sub-carriers and hencemore system throughput. For example using five sub-carriers in Rician channel with ten LOS paths gives thesame performance as using three sub-carriers in Rayleighchannel. Duplicating the number of sub-carriers and thenumber ofLOS paths gives comparative performance withrespect to three sub-carriers in Rayleigh channel. It can beconcluded that choosing the number of the sub-carriersstrongly relates to the environment that the MT-CDMAsystem will be installed at. The effect of single jammingcan be neglected due to effect of the correlator andintegrator used in the receiver, the jamming is spread firstby the corelator over the whole system bandwidth and theuseful signal is despread then integrated over T, so, onlysmall portion ofjamming power will be collected by theintegrator, and a big portion will be removed or passfiltered and the similar explanation can be carried out forthe multiple jamming cases. The number of sub-carrier islimited also by the interfering among the sub-carrier,which is actually increases as the number of the sub-carrier increases and the performance degraded as shownin Fig. 6.

fRce(r) = 1- exp(-y)l[r ] [24

where y = (K + 22)

(8)

10

jU 10m

VI. SIMULATION RESULTSIt is known that, the throughput of the MT-CDMA is

increased as the number of the sub-carriers increased. Butsome limits taken place by the environment on the numberof sub-carriers, which we can use. Fig. 5 shows twoenvironments, the first one when it is difficult to get LOSpath between the transmitter and the receiver, and the

1D3

pedorr,nce ofMT-CDMA in pressnce of singke jamming aome Rayligh

0 1 2 3 l 5 S6 7 8 9 10SNR(dB)

Fig. 6. Interfering among sub-carriers effect in Rayleigh channel and inpresence of single jamming

967

.--s -c -s---.--- 3 Rayleigh*---e------e----.5ayelgh------ - - 4 .- . 9,Rayleigh. . _................... .........

~~~v --;.' --'--- -- _ ' ....... _ _ ' !

--------r------ ----q ---------;------.--------

r::

:: @ :: t - --_-e--- - ---------'

------t--------------:-------F------t--------------~- ------- ----+ .-----'~ -~~;~~~------ --r---- --+---- --.--- -------..

_ __ . . -------- ------'------'-----............ ~ - ------__------------ :------ - ---:-------------.----------- --- ---

I.

IU ------------- - ------t-------------'-- ----'irF ,

1uin'L

aDL


The other important factor of the single jamming is itscenter frequency, the worst case when it is located at thesame frequency as one of the sub-carriers [7]. Fig. 7shows the effect of the center frequency of the jammer andthe jamming to signal power ratio. At first we havereplaced the jammer at the same frequency as the firstsub-carrier to get the worst case and then at the MT-CDMA spectra and the jamming to signal power was thesame for both cases, and then we increase the jamming tosignal ratio power. From Fig. 7 we can easily note that,the performance is almost for the first two cases and thatmeans replacing the jamming on one of the sub-carriercenter frequencies will not affect the performance. But byincreasing the jamming to signal power ratio from 1dB to3dB the performance has been degraded clearly.

performance of MT-CDMA in presence of single jamming over Rayleigh- J-&-- Ff4requency of first subcarter, RayleighFig. 7 ....Ef-f erto frequency of t ransmdted peRat signal, weha-ti n=3ffrequency offcrst subafrier,Rayleigh

InFi.8, th efec of incrasin th prcssn gai is.

o------------- - -------.---------------

.. . .._..._ _

----------- ------- -----'-------___ -_ __ _ _ _ _

a: s__._. .......... s.k-- ---.---5------- ------r-------.----t,-____,___-______-

-------'--- ---.- - -... .... ...... ... .-_-.-_--

.- - .- .--- -------- .- -.- .-- .----

.,.,------,-- -- --;- --------;- --- --.------n----- --;----- '---- -- -- --,2 3 A .8 9 1

Fig.~ ~7.Efec offrqec of th .amn an .hoeosgaoe

Fig. Fi.Effethoffeffenctof increammingan the procesosinggnainpowe

shown.

V. CONCULSION

In this work, the performance ofMT-CDMA in fadingchannels, various system conditions and in presence ofnarrowband jamming noise has been investigated.Two fading channels have been considered, Rayleigh

and Rician fading channels. The performance in Ricianchannels is enhanced due to the existing of LOS pathsbetween the receiver and transmitter (high K-factor). Thenumber of the sub-carriers that can be used is limited bythe chanel characteristics. The number of sub-carrierincreases as the number of LOS paths increases in thechannel.The sub-carriers interfering also limit the number of

sub-carriers and degrade the system performance.Increasing the processing gain enhances the system

performance.It has been shown that, replacing the single jamming at

the centre frequency of one of the subcarriers frequencies(worst jamming case) has a negligible effect and almostthe same as replacing it at the center of the spectrum ofthe transmitted signal however, the performance isdegraded as jamming to signal power ratio increases.

IV. REFERENCES[1 L.Vandendorpe, Multitone Spread Spectrum Multiple Access

Communications System in a Multipath Rician FadingChannel. IEEE Transactions on Vehicular AR technology.Vol. 44. NO. 2. 1995.

[21 D. W. Matolak, V. Deepak, F. A. Alder, Perfonnance ofMultitone and Multicarrier DS-SS in the Presence ofImperfect Phase Synchronization. School of ElectricalEngineering & Computer Science, Athens, OH 45701.2002.

[3] S. Hara, R. Prasad, Overview of Multicarrier CDMA, IEEECommunication Magazine, vol. 35, no. 12, pp. 126-133,December 1997.

[4] D. W. Matolak, F. A. Alder, V. Deepak, Performance ofMultitone and Multicarrier DS-SS in The Presence of Partial-Band Pulse Jamming/Interference. School of ElectricalEngineering & Computer Science, Athens, OH 45701.2002.

[5] 1. Sen, Bandwidth Efficient Reduced-complexity MT-DS-SSvia Reduced sub-carrier Frequency Spacing. M.Eng. Thesis.Carleton University. Ohio University. 2004.

[61 Nikolay.K. 2003. Mobile Radio Channels Modeling inMATLAB. RADIOENGINEERING, Vol. 12, no. 4,December 2003.

[71 R. L. Peterson, R. E. Ziemer, D. E. Borht, Introduction toSpread Spectrum Communication, Prentice-Hall, UpperSaddle River, New Jersey, 1995.

Fig. 8 Processing gain effect in presence of multiple tone jamming inRayleigh fading channel

the results in Fig. 8 show that, the performance of MT-CDMA system can be enhanced and fading effect can beovercomed by increasing the processing gain. But usinghigh processing gain reduces the bit rate (P = B/R) so itdepends on the desired bit rate (the application). Highprocessing gain will not be the best choice to overcomethe channel impairments if the application requires highdata rate such as multimedia applications.

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performance of multi tone code division multiple access (mt-cdma) in awgn channel and in presence of...

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