Performance Of Multi Tone Code Division Multiple Access (mt-cdma) In Awgn Channel And In Presence Of Narrowband Jamming

Performance of Multi tone Code Division Multiple Access (MT-CDMA) in Fading Channels and in Presence of Narrowband Jamming Khalifa Nasser K Jleta, Mahamod Ismail, Alina Marie Hasbi Department of Electrical, Electronics and System Engineering, Faculty of Engineering Universiti Kebangsaan Malaysia 436000 Bangi- Selangor, Malaysia Tel: +6 03 8296322 Fax: +6 03 8296146 Kjletagyahoo.com, (mbi, alina}@eng.ukm.my

Abstract- Multiton transmission was proposed because the associated larger symbol duration favorable to combat the effects of multipath propagation 1i]. In fact the main limits of the conventional single carrier modulation techniques are the restrictions imposed by the Multipath channel, and the receiver complexity. On the hand the multicarrier techniques such as Multi-Carrier (MC) and Multi-Tone (MT) can provide high data rates at reasonable receiver complexities. Jamming on the other hand is of interest in some communication application, and in military anti-jam systems. In this work, we study the performance of Multi-Tone Code Division Multiple Access (MT-CDMA) in presence of narrowband jamming. We investigate Bit Error Rate (BER) performance as a function of some system parameters such as number of sub-carriers, processing gain, and as a functions of channel conditions such as type of channel, channel K-factor, jamming center frequency, jamming power, and jamming to signal power ratio. The simulation has been carried out by using MATLAB package and the results show, that the performance can be enhanced by increasing the processing gain, K-factor, or by decreasing the number of sub-carriers, or the jamming to power ratio. Furthermore, that the effect of jamming center frequency on the performance is negligible.

Keywords-MT-CDMA, Narrowband Jamming, Fading channels I. INTRODUCTION

Multimedia applications of mobile communications need modulation, and multiple access techniques that, can deliver very high data rates, which cannot be offered, by the traditional single carrier modulation techniques. MTCDMA is a combined technique between Orthogonal Frequency Division Multiplexing (OFDM) and Code division Multiple Access (CDMA) and it is an attempt to provide a communication system that inherits the advantages of both CDMA and OFDM, and provides the new probable technique for the fourth generation of mobile communication systems. MT-CDMA inherits the powerful features of CDMA, which allow number of users to access the channel simultaneously. This is achieved by, modulating and spreading the signals with pre-assigned codes sequences. Since CDMA provides better spectral efficiency and easier base station placement compared to second generation systems and beside its high performance in presence of jamming interference. Another very important advantage of MT-CDMA gained from very

1-4244-0000-7/05/$20.00 02005 IEEE.

important advantage of MT-CDMA gained from OFDM is its less sensitivity to the inter symbol interference (ISI) OFDM is its less sensitivity to the inter symbol interference (ISI) caused by radio channel impairments. This advantage is achieved by using more than one carrier to carry the symbol and then a lower data rate for the same user data comparing to single carrier modulation is achieved. By well-chosen system parameters, we can reduce the effect of unfriendly channels such as frequency-selective channels. Another very important advantage using MT-CDMA is its spectrum efficiency due to allowing the sub-carriers to largely overlap with each other. Actually High Spectral Efficiency is the main advantage of MT-CDMA over single and the multicarrier techniques. At the same time, to facilitate inter-carrier interference free demodulation of the sub-carriers, the subcarriers are made orthogonal to each other. The possible orthogonality 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 recovered exactly by the receiver [4]. A comparison between MT and single carrier schemes was done in [41 and both schemes show equivalent performance in presence of multi user interference. Jamming noise is of interest in some fields and applications such as anti-jam systems used in military applications and in the spectral overlay of narrowband pulsed signals over Direct Sequence Spread Spectrum (DS-SS) transmission [4]. In this work we assume single user to simplify the work (no Multi User Interference) and we use conventional receiver which is recommended in case of single user detection. Spreading technique is DS-SS and the modulation technique is assumed Binary Phase Shift Keying (BPSK), since it is the most common modulation type, used in direct sequence systems. The type of data mapping to sub-carrier is considered as Serial to Parallel (S:P) transformation. The codes on all sub-carriers are the same, in order to obtain a high correlation among the sub-carriers modulation symbols [5]. Perfect symbol synchronization and phase coherence are assumed. And the fading is assumed flat (no inter symbol interference). In the next section we describe MT-CDMA system. In section III we study the channel model. Section IV provides the simulation results, discussion and section V includes a summary and the conclusion.

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r -@| -l| II. SYSTEM DESCRIPTION

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

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

CI(t)

M

Cos(2*pi*fIT)

V

I I

I I CM(t)

Cos(2*pi*fMT)

Fig. 1. MT-CDMA Transmitter

-14

~~~Transmitted bandwidth

W

It

0

h.-

I

.1

I I I

i

N2)C(n) cos(2irfJT (3)

£ t)S

S: P

It Ij(

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 we demodulate the received signal by multiplying it by cos 27f (t) and c(t), then integrating over the symbol period (Ta). The integrator removes (shown in Fig. 4) the double or high frequency tenn resulting from the multiplication by the sinusoidal carrier signal. The decision circuits are used then to collect the symbol samples to make a decision and get the data symbol. The decision static is denoted by 4. This is done on each subcarrier, hence Mof Fig. 4 is required [5].

Cos(2*pi*f2T)

C2(t)

'i

Cos(2*pi*fIT)

11

S2i25

fl f2 f3 f3 * * fM

f

Fig. 2. MT-CDMA Spectra

The incoming bit stream denoted by d(t) is first serialto-parallel converted into M data streams (Mis number of sub-carriers). Then the data on each path is spread by using orthogonal codes denoted by C (t).Same or different codes can be used on each path. After spreading the spread data modulates the orthogonal sub-carriers. In MTCDMA, the sub-carriers are orthogonal over the symbol duration, and hence the sub-carriers frequencies are given

by f

=

f +

T

where f; is the RF and T is the Fig. 3. MT-CDMA Receiver

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

d] (t)

2jB E n

dj,,PT (t-T

(1)

where the nth data symbol is di, E D = {+ 1}, and T is the bit time. The modulation symbol Ij(n) are analogous to the data symbol and the modulation waveform for sub-

Fig. 4. De-spread block of MT-CDMA receiver

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

III.

CHANNEL MODEL

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

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

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

random process each with variance 02. The ratio of specular to defuse energy defines the so-called Rician Kfactor, which is given by [6] 2

(5)

2o

the best-and worst-case Rician fading channels associated with K-factor of K = oo and K=O are the Gaussian and Rayleigh channels with strong LOS and no LOS path, respectively. So, the Rayleigh fading channel can be considered as a special case of a Rician channel with K=0. The Rician PDF is given by [6].

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

(=

Lf

(6)

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

[6]

[I

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

r

.

0

(7)

the Rician cumulative distribution function(CDF) takes the shape of [61

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

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

[24

*,__-*

10

a

;

,,,,,,;, , .... ,, ,,,; .;-..-;-; .........

;

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

--------------_ S

DL

--

_ _.__ _ - ____

' ''---

_

_

0

,

2

--

3

4

-~~

--------

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

irF IU

6

5

SNR(L1B)

-------

a

7

9

10

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

, -------------

.--s *---e------e---.5ayel gh

10

-c

(K + 22) jU m

4

-

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9,Rayleigh

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..................... _ .........

-

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-s---.--- 3 Rayleigh

------t-------------'-- ----'

-

.

.......

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---------;------.--------

10 r

@

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in'L 1u 0

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--r---- --+---- --.--- -------..

------'------'-----............ ~ ---

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------t--------------:-------F------t--------------~-

It is known that, the throughput of the MT-CDMA is increased as the number of the sub-carriers increased. But some limits taken place by the environment on the number of sub-carriers, which we can use. Fig. 5 shows two environments, the first one when it is difficult to get LOS path between the transmitter and the receiver, and the

0 :::r:::r:s::::--Rt-----s::::

-.------*

------

--

it can be noticed from Fig. 5, that having more LOS paths in the channel allows using more sub-carriers and hence more system throughput. For example using five subcarriers in Rician channel with ten LOS paths gives the same performance as using three sub-carriers in Rayleigh channel. Duplicating the number of sub-carriers and the number of LOS paths gives comparative performance with respect to three sub-carriers in Rayleigh channel. It can be concluded that choosing the number of the sub-carriers strongly relates to the environment that the MT-CDMA system will be installed at. The effect of single jamming can be neglected due to effect of the correlator and integrator used in the receiver, the jamming is spread first by the corelator over the whole system bandwidth and the useful signal is despread then integrated over T, so, only small portion of jamming power will be collected by the integrator, and a big portion will be removed or pass filtered and the similar explanation can be carried out for the multiple jamming cases. The number of sub-carrier is limited also by the interfering among the sub-carrier, which is actually increases as the number of the subcarrier increases and the performance degraded as shown in Fig. 6.

(8)

VI. SIMULATION RESULTS

_ ._,

.

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

------

where y =

-::::,:: K-factor=2f Rician channel

::::-

(4)

where O is the amplitude of the specular component and xi, y1 are samples of zero-mean stationary Gaussian

K = fl

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

--:------------.----------

I. l 5

SNR(dB)

6S

7

-

------

---

8

9

10

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

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The other important factor of the single jamming is its center frequency, the worst case when it is located at the same frequency as one of the sub-carriers [7]. Fig. 7 shows the effect of the center frequency of the jammer and the jamming to signal power ratio. At first we have replaced the jammer at the same frequency as the first sub-carrier to get the worst case and then at the MTCDMA spectra and the jamming to signal power was the same for both cases, and then we increase the jamming to signal ratio power. From Fig. 7 we can easily note that, the performance is almost for the first two cases and that means replacing the jamming on one of the sub-carrier center frequencies will not affect the performance. But by increasing the jamming to signal power ratio from 1dB to 3dB the performance has been degraded clearly.

V. CONCULSION

In this work, the performance of MT-CDMA in fading channels, various system conditions and in presence of narrowband jamming noise has been investigated. Two fading channels have been considered, Rayleigh and Rician fading channels. The performance in Rician channels is enhanced due to the existing of LOS paths between the receiver and transmitter (high K-factor). The number of the sub-carriers that can be used is limited by the chanel characteristics. The number of sub-carrier increases as the number of LOS paths increases in the channel. 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 almost the same as replacing it at the center of the spectrum of the transmitted signal however, the performance is degraded as jamming to signal power ratio increases.

performance of MT-CDMA in presence of single jamming over Rayleigh J-&-- Ff4requency of first subcarter, Rayleigh

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Fig. 7 ..Ef-f .. erto frequency of t ransmdted peRat signal, weh a-ti n=3ffrequency offcrst subafrier,Rayleigh

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IV. REFERENCES [1 L.Vandendorpe, Multitone Spread Spectrum Multiple Access Communications System in a Multipath Rician Fading Channel. IEEE Transactions on Vehicular AR technology. Vol. 44. NO. 2. 1995. [21 D. W. Matolak, V. Deepak, F. A. Alder, Perfonnance of Multitone and Multicarrier DS-SS in the Presence of Imperfect Phase Synchronization. School of Electrical Engineering & Computer Science, Athens, OH 45701.2002. [3] S. Hara, R. Prasad, Overview of Multicarrier CDMA, IEEE Communication Magazine, vol. 35, no. 12, pp. 126-133, December 1997. [4] D. W. Matolak, F. A. Alder, V. Deepak, Performance of Multitone and Multicarrier DS-SS in The Presence of PartialBand Pulse Jamming/Interference. School of Electrical Engineering & Computer Science, Athens, OH 45701.2002. [5] 1. Sen, Bandwidth Efficient Reduced-complexity MT-DS-SS via Reduced sub-carrier Frequency Spacing. M.Eng. Thesis. Carleton University. Ohio University. 2004. [61 Nikolay.K. 2003. Mobile Radio Channels Modeling in MATLAB. RADIOENGINEERING, Vol. 12, no. 4, December 2003. [71 R. L. Peterson, R. E. Ziemer, D. E. Borht, Introduction to Spread Spectrum Communication, Prentice-Hall, Upper Saddle River, New Jersey, 1995.

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

the results in Fig. 8 show that, the performance of MTCDMA system can be enhanced and fading effect can be overcomed by increasing the processing gain. But using high processing gain reduces the bit rate (P = B/R) so it depends on the desired bit rate (the application). High processing gain will not be the best choice to overcome the channel impairments if the application requires high data rate such as multimedia applications.

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