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TERM PAPER

PERFORMANCE OF DS SSMA SYTEMIN A RICIAN CHANNEL

COURSE : WIRELESS COMMUNICATION(EEL860)

CORDINATOR: Prof. V K Jain

Submitted byDeepak Jose 2010EEE2451Thomas Joseph 2010JOP2229


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1. AIM

Implementing a DS SSMA system considering a Rician Channel using Matlab Study the behaviour of the channel under various multipath situations. To get a understanding of the data formats for implementing the different blocks in SimVerify the performance of the system.

2. INTRODUCTION

We analyze the performance of a DS-SSMA system in a Rician faded channel. The basic modulation scheme of BPSK and used the Rake Receiver to accommodatmultipath effects . Our work is restricted to a single user system and consider oneffect of AWGN in the signal along with the multipath effects. We are comparinresults with the standard results available from research works.

2.a Basic procedure followed:

Modulation scheme BPSK.Processing gain of 60 ( f c / f b )Considering a 3-ray multipath signal.Mobile communication carries frequency is900MHz we use this value in Doppler frequency measurements.

The data rate is the standard taken from 2.5G where the data rate varies from 30kto 90kbps.We choose a data rate of 30kbps.

3 . FADING IN A WIRELESS ENVIRONMET

Radio waves propagate from a transmitting antenna, and travel through free spacundergoing absorption, reflection, refraction, diffraction, and scattering. They ar

greatly affected by the ground terrain, the atmosphere, and the objects in their palike buildings, hills, vegetation, etc. These different physical phenomena determmost of the characteristic features of the received signal.

In most of the cellular systems, the height of the mobile antenna is lesser than thsurrounding structures. Thus, the existence of adirect or line-of-sight path betweenthe transmitter and the receiver is not possible. In such a case, propagation is madue to reflection, diffraction and scattering from the buildings. So, in practice, th


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transmitted signal arrives at the receiver via several paths with different time delcreating amu ltipath situation at the receiver, these multipath waves with randomlydistributed amplitudes and phases combine to give a resultant signal that fluctuattime and space. Therefore, a receiver at one location may have a signal that is mudifferent from the signal at another location, only a short distance away, becausethe change in the phase relationship among the incoming radio waves. This caussignificant fluctuations in the signal amplitude. This phenomenon of randomfluctuations in the received signal level is termed as fading [4]

The short-term fluctuation in the signal amplitude caused by the local multipath called sm all-scale fading, [4] and is observed over distances of about half awavelength.

On the other hand, long-term variation in the mean signal level is calledlarge-scale fading [4]. The latter effect is a result of movement over distances large enough tocause gross variations in the overall path between the transmitter and the receive

Large-scale fading is also known as shadowing , because these variations in the meansignal level are caused by the mobile unit moving into the shadow of surroundinobjects like buildings and hills.

Small-scale fading can be further classified as flat or frequency selective and slofast. A received signal is said to undergo flat fading , if the mobile radio channel has aconstant gain and a linear phase response over a bandwidth larger than the bandwof the transmitted signal. Under these conditions, the received signal has amplitufluctuations due to the variations in the channel gain over time caused by multipHowever, the spectral characteristics of the transmitted signal remain intact at threceiver. On the other hand, if the mobile radio channel has a constant gain and l phase response over a bandwidth smaller than that of the transmitted signal, thetransmitted signal is said to undergo freq u ency selective fading [4]. In this case, thereceived signal is distorted and dispersed, because it consists of multiple versionthe transmitted signal, attenuated and delayed in time. This leads to time dispersithe transmitted symbols within the channel arising from these different time delaresulting inintersy mb ol interference (ISI).

When there is relative motion between the transmitter and the receiver, Doppler spread is introduced in the received signal spectrum, causing


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frequency dispersion. If the Doppler spread is significant relative to the bandwidthe transmitted signal, the received signal is said to undergo fast fading . This form of fading typically occurs for very low data rates. On the other hand, if the Dopplerspread of the channel is much less than the bandwidth of the baseband signal, thesignal is said to undergo slow fading.

4. TRANSMITTER In the transmitter part a random integer generator which generates a

binary random number is used. The random integer generator generates onesazeros with equal probability. This data is given to the BPSK generator whichgenerates the BPSK modulated signal. This BPSK signal is given to the spreaderspreading by multiplying it with a PN sequence, which is a unipolar signal, whicconverted to a bipolar signal by using a converter. The spreader causes the signaspread to a higher bandwidth by multiplying data symbols, bit rate equal to T b with ahigh bit rate code sequence, chip rate equal to Tc.

Here we use the following data sets:-1. Data rate of 30Kbps2. Chip rate of 1.8Mbps.3. So the processing gain is 60.

F ig u re 1. DS SS MA Trans m itter


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4.a Spreading Sequence Generator

The PN Sequence Generator block generates a sequence of pseudorandom binarynumbers.The PN Sequence Generator block uses a shift register to generate

sequences. Table below shows the parameter that has been used in the simulationTa b le 1. PN Seq u ence Generator b lock para m eters Parameters value

Generator Polynomial [1 0 0 0 011]Initial States [1 0 0 0 0 1]Shift(or mask) 0Sample time,T c Tchips(1/1.8Mbps)Attribute of output signal Sample based output

5. CHANNEL

5. a Rician Fading The Rician distribution is observed when, in addition to the multipath componenthere exists a direct path between the transmitter and the receiver. This model is in a sm all-scale fading scenario. Thus in the presence of multipath, the transmittedsignal can be written as [4]

wherekd is the strength of the direct component.[ d is the Doppler shift along the LOS path, and[ diare the Doppler shifts along the indirect paths .

i is the random phase due to mutipath


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The envelope in this case has a Rician density function given by [3]

Where I00 is the 0th order modified Bessel functions of the first kind. The cumulatdistribution of the Rician random variable is given as [3]

whereQ( , ) is the Marcums Q function The Rician distribution is often describedterms of the Rician factor K , defined as the ratio between the deterministic signal power (from the direct path) and the diffuse signal power (from the indirect pathis usually expressed in decibels as

K= ranges from 1 to for a Rician Channel [3]

The above can be written in dB as below:-

In equation if k d goes to zero (or if k d 2/2 2 r 2 /2 2 ), the direct path is eliminated andthe envelope distribution becomes Rayleigh, with K (dB) = - .

The different fading models available are:-

N akagami Distribution

[5]


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Nakagami distribution can be used to incorporate the effect of both Rayleigh anRician Faded channel. As for m=1 we have a Rayleigh channel and m > 1 gia Rician Channel.

L ognormal Fading

[5]This is used in long term fading situations.

Suzuki Distribution

[5]

4.b Outage probability

In a fading radio channel, it is likely that a transmitted signal willsuffer deep fades that can lead a complete loss of the signal or outage of the signThe outage probability is a measure of the quality of the transmission in a mobileradio channel. Outage is said to occur when the received signal power goes belowcertain threshold level .It can be calculated as the integral of thereceived signal power p(t) as [4]

Where P th is the threshold power.


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F ig u re 2. M odel of the Channel

The channel we considered here a Rician faded channel. In Rician channel, ther

exist a direct path along with multiple numbers of indirect paths in betweentransmitter and receiver. The multiple path phenomenons are due to diffraction,reflection and scattering by buildings, hills and trees etc. The multipath fading camodelled as a linear finite-impulse response (FIR) filter. It is assumed that delay power profile and Doppler spectrum of the channel are separable. Let { si }denotes theinputs to the channel .Then {yi}are the samples at the output of the channel. Theinput-output relation is given by

Yi= si-n *gn [5]

Where{ gn} is the set of tap weights given by

gn= ak sinc [(k/Ts)-n] -N1 n N2 [5]

where Ts is the input sample period to the channel

{ k} where 1 k K is the set of path delays andK is the total number of paths in the multipath fading channel.

{ak} where 1 k K, is the set of complex path gains of the multipath fading chaThese path gains are uncorrelated to each other.

N1and N2 are chosen so that gn is small when n is less than N1 or greater than N2


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Ta b le 2. Rician Channel para m eters

Here1 s corresponds to an extra 300 meter 1st multipath8 s corresponds to an extra 2400 meter 2ndpath

F ig u re 3. I m pu lse response of channel in resolva b le for m ( M atla b simu lation)

parameter valuesK factor 2Maximum Doppler shift 50HzPath delay vector [0,1 s,8 s]


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F ig u re 4. Channel response when two of two paths are in a non resolva b le for m ( M atla b simu lation)

5. RECEIVER

The receiver we are using here is a rake receiver. The rake receiver consists of multiple correlators , in which the receive signal is multiplied by timeshifted versions of a locally generated code sequence. The time shifted versions code sequence is used to separate signals such that each figure of the rake receivsees signals coming from a single path. The spreading code is chosen to have a vsmall autocorrelation value for any nonzero time offset. This avoids crosstalk .


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F ig u re 5. Receiver m odel

The received signal can be represented by a complex model given by

r(t)=c(t)s(t)+n(t) [2]

where ,c(t) is fading co-efficient(multiplicative),s(t) is the transmitted signal, and

n(t) is the additive white Gaussian noise.Let the transmitted signal s(t)= b(k)g(t-KT) [2]

where b(k) is the data sequence andg(t) is the pulse shape function andT is the symbol delay.

5.a Rake receiver

If, in a mobile radio channel reflected waves arrive with small relative time delayself interference occurs. Direct Sequence (DS) Spread Spectrum is often claimedhave particular properties that makes it less vulnerable to multipath reception. In particular, the rake receiver architecture allows an optimal combining of energyreceived over paths with different. It avoids wave cancellation (fades) if delayedarrive with phase differences and appropriately weighs signals coming in withdifferent signal-to-noise ratios. [4]


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F i u 6

i bl

[5]

The rake rece i er cons ists of mu lti le corre lators, in wh ich the rece i e si na l ismulti lied by time-sh if ted vers ions of a loca lly genera ted code sequence. Theintention is to separa te signa ls such that each f inger on ly sees s igna ls com ing in over asingle (reso lvab le) pa th. The spread ing code is chosen to have a very sma ll autocorre lation va lue for any nonzero time offse t. Th is avo ids cross talk be tweenf ingers.The equa tion re lated to each of the corre lators in the R ake is

Where T var ies accord ing to the de lay cons idered in the R ake

In our s imulation we var ied the de lay vec tor is as:-

[0 2 6] and [0 3 5] are the va lues of n in the d ifferen t corre lators

The rake rece iver is des igned to op timally de tect a DS-SS M signa l transm itted over a dispers ive mu lti path channe l. It is an ex tens ion of the concep t of the ma tched f ilter.


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6. PERFORMANCE ANALYSIS

F ig u re 7. Scatter plot of BPS K signal considering AW GN and mu tlipath

F ig u re 8. Perfor m ance of DS SS MA in a Rician channel when m ob ile m oves at a speed of 5 m /s(corresponds to a Doppler shift of 15 Hz ).


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Ta b le 3. SNR v/s BER for DS SS MA in Rician Channel

Signal-to-noise ratio(E b/N0)

Bit Error rateBER

0 0.12 0.077354 0.04636 0.020338 0.00621910 0.000990912 6.656e-00514 8.6e-007

.

F ig u re 9 . Perfor m ance Co m parison of W CD MA in 2-Rays Between AW GN and M u ltipath Rayleigh F ading Channels for 1 u ser [3]


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Ta b le 4. Si mu lation res u lt for eval u ation on BER vs. SNR for 2.ray M u ltipath Rayleigh channel for 1 u ser when n umb er of data is 2 00 , 000 [3]

7. CONCLUSION

BER of a DS SSMA system has been modelled and evaluated on a Rician chanAnd it is having good BER , but it is at the cost of data rate.

Behaviour of the channel was found under various multipath situationsWe made a comparison with a known research paper in WCDMA.


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8. REFERENCES

[1] Lei WEI, Performance of DS-SSMA systems with Differential phaseDetection on Rician channels, sixth IEEE international symposium on27-29sep 1995,173-177 vol.1.

[2] Tugay Eyceoz and Alexandra Duel-Hallen, Deterministic Channelmodeling and Long Range Prediction of Fast Fading Mobile RadioChannels, communication Letters, IEEE volume 2,issue 9.

[3] Performance study on high data rates modulation techniques of w-cdmain multipath fading channel , Project Report Muhammad Najib BinIsmail

[4] J.G. Proakis, Digital Communications, McGraw-Hill, New York,1995

[5] P.S. Rappaport, Wireless Communications, Prentice Hall, NJ, 1996.

[6] www.mathworks.com

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