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DIRECT SEQUENCE SPREADSPECTRUM COMMUNICATIONS SYSTEM IN A MULTIPATH FADING CHANNEL USED IN WCDMASYSTEM. RAKE RECEIVER PERFORMANCE ANALYSISIN WCDMA SYSTEMCHIDERA MAC-ROWLAND EJIOFOR20159045DATA COMMUNICATIONSDEPARTMENT OF ELECTRICAL ANDELECTRONICS ENGINEERING NEAR EAST UNIVERSITY, TRNC.  Abstract- We are go?ng to look at the modulat?on anddemodulation of d?rect sequence spread spectrum ?n th?s paper and also ?tsperfomance ?n a mult?path fad?ng channel in the first part, we cons?deredd?fferent parameters used ?n the system and for the commun?cat?on channels ?nthe network. Also mainly we use a rake rec?ever to show how well it cancels outthe effect of multipath fading in WCDMA system or network. MATLAB SIMULINK  ?s the software used for simulation by makingdifferent models as we would see in the paper, results from the s?mulat?on showthe use of different parameters and how it affects the perfomance of a WCDMAsystem.This was acheived by checking the BER (bit error rate)perfomance of theparameters by changing variables of the parameters in the WCDMA system.

Wechecked the effect it has on the systems perfomance when two differentmodulation techniques are applied, when noise is added to signal, variables ofthe rake receiver gets changed and how the system performs when there is rakereciever and when it is witouth one. Hence showing how valuable it is to use arake receiver to increase the perfomance and capacity of system.Keywords-modulat?on, spread?ng, mult?path channel, rake rec?ever, transm?tter, demodulat?on,pseudo no?se sequence, despreading,pulse shift filter, wireless , performance,WCDMA(wideband code division multipath access), CDMA(code division multipathaccess).

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                           I. INTRODUCTIONWe shouldfirst know that code division multipath access is different from wideband codedivision multipath access but the difference isn’t really much because it’s thebandwidth that differentiates them. CDMA is basically an algorithm that is usedin telecommunications to ensure use of more useable channels within a bandwidthcreating more access for users. While WCDMA still uses the same process todivide the channels for more users.

The major difference in the world between WCDMAand CDMA is the way it is technologically grouped round the world. CDMA is a 2Gtechnology directly linked in competition with GSM. While WCDMA is a 3Gtechnology and can perform both functions of 3G and 2G within the same area ofcoverage.  We can say that WCDMA producesa better function than CDMA because it offers a much quicker speed and takesfull advantage of more recent technological services that a 2G network doesn’thave.WCDMA is an ITU standard derived from Code-Division Multiple Access (CDMA),we should know that it is popularly known as IMT-2000 direct spread.

WCDMA is a third-generation wirelesstechnology that gives a higher data speeds or transmission to mobile andportable wireless devices than previous 2G networks. Also we can say WCDMA hasthe following advantages better system capacity, immune to fading effectsbecause of DSSS, stronger signal at receiver, better data services, safer datatransmitted and wider bandwidth. Here we would lookat the direct sequence spread spectrum of CDMA since WCDMA is derived from CDMA.Direct sequence spread spectrum is also called direct sequence code divisionmultiple access (DS-CDMA), is one of two approaches to spreadspectrum modulationfor digital signal transmission over the airwaves. In direct sequence spreadspectrum, the data to be transmitted is divided into small sectors, each ofwhich is allocated across to a frequency channel across the spectrum. A datasignal at the point of transmission is combined with a higher data-rate bitsequence (also known as a chipping code) that divides the data accordingto a spreading ratio.

This chipping code helps the signal try to preventinterference and also enables the initial data transmitted to be recovered ifdata bits are affected or spoilt maybe because of noise. We should know that itis incorporated into CDMA or WCDMA to create the advantages we stated abovethat is accommodate a large number of users in one channel depending on thevoice activity level of the radio direct Spread Spectrum has many uniqueproperties that we cannot find  in manyother enhancement techniques  like theability to eliminate or reduce the multi-path interference which we would focusmuch in this paper, also privacy due to the encoding, low power spectraldensity or congestion in the network since the signal spreading is done over alarger frequency band and again as we said support multiple users at same time.Rake receiver which was first proposed by Robert priceand Paul green is a radio receiver made with the aim of fighting andeliminating the effects of multipath fading of signals. This can be done byusing sub receivers or what we also call fingers having various correlatorsthat are assigned to each multipath component. _that it has been assigned toand a the later stage in the system each fingers transmission is combined togetherto collectively make the most use of different characteristics gotten from  the fingers or transmission paths. Note thatthis whole process at times could result in a higher signal to noise ratio inthe transmission path than in the receiving environment. The essence of thewhole process is since each multipath or sub path components has the originaldata transmitted then at the receiver’s end it could all be combined for abetter data or signal and to provide a reliable data or transmittedinformation. The signals gotten from the different multipath can be separated anddata from each used in improving signal to noise ratio.

This improvement can bedone only if the time spread of the channel or used for transmission is greaterthan the time resolution of the whole system.                 II.SYSTEM WORKING PRINCIPLE OF DSSS  Fig.1 DSSS Block Model The figure above is based on my assumption showing the system model. Weshould note that after the data is generated at the input, the information ordata is transmitted in bits(data) either +1 or -1 as we generally know andrandomly distributed also. For proper spreading of data a complex Pseudo Noisecode arragnement is used.  an = a1n + j*.

a2n            eqn(1) where an , a1n ?{-1, +1} meaning that an , a1ncan be +1 or -1 bits.While j2 = -1.The data transmission rate is (DR) is N times smaller than thechip rate (CR) N is the spreading gain and as we can see from the model it is applied tothe signal.We should know N(spreading gain)= chip rate (CR) ÷ Data rate (DR) An oversampling occurs of the block before spreading and the data orinformation transmitted is oversampled by N (spreading gain) times beforespreading takes place in the next block. After this occurs we then spread thesignal as we already talked of in equations (1) then next the signal spreadedtakes a pulse shaped form because a filter is appleid to it a (Square RootRaised Cosine) filter is used fopr filtering having a roll off factor variableof ? ? 0,1, it is also in Continous time we should rememberthat.

An equation below shows the non casaul impulse response of the filter.                 eqn(2) We should know that TC is thechip period = 1/CRFor an execution of the pulse shape filterwe must delay, truncate and properly observe Pn i.e;  Pn would be     for 0? n ? ?.?                                   0,              when otherwise  ?(over sampling variable) and ? is the(forced impulse response filter) FIR filter channel length and it is a naturalodd number too we should remember that.  The digital to analog conversion blocklabeled as DAC and it helps create a smooth transition of the signal or sampleddata. The next block does a modulation using the RF Quadrature modulationtechnique (Radio Frequency) with the block being used a Rayleigh Fading channelwith L representing (the number of multipath where signal is spread into) L =int (bandwidth of signal (BW) × time spread of channel (TD) )  Finally we would look at the receivingend, after signal passes the channel it goes through demodulation at thedemodulator block and here a local carrier (with PN sequence) that has beensynchronized initially with the transmitters carrier is used to check forerrors and also recover affected or lost signals or data bits that weretransmitted.

We can only recover the signal, message or data of the identicalPN sequence to that of the modulator carrier is used. Then it is converted backfrom analog to digital so we can have the initial mode of transition that thereceiver can fully understand. Below we see a digital rake receiver systemmodel and focus on its performance fully in the next chapter. We should knowthat the correlation arms which depends also on the number of arms we chooseworks using a copy or reference PN sequence that was used at the transmittingend.

Fig.2 Digital Rake Receiver System Model. We should know that each arm has to usesame PN sequence as to achieve proper assembling of spreaded data after it wastaken from the spreaded paths, each correlation arm is delayed by one chip andthe data in it is extracted from the input signal so it uses the same PNsequence. Meaning that for each arm there is a delay of on chip in the PNsequence.

A scaling is also used at the correlation arms and it is to ensure abound on lower probability of error when assembling or desporeading data in thearm or multipath. This scaling is done on the outputs of correlation arms andMaximum Ratio Combination (MRC) requirements must be fulfilled meaning, eachsignal or data from correlation outputs get added together, after being rotatedand weighed or compared according to the phase and strength of each correlationarm. MCR is done so that the data or signals added from each output after beingadded yields the maximum ratio between data and noise terms. Below we see a systemmodel of a correlator arm for better understanding and in the next chapterfocus on the rake receiver as whole and certain factors or parameter componentsin it that affects its performance and also how it can be improved.           III.SIMULATION USING MATLAB AND RESULTS For our simulation and the various results. ? used MATLAB SIMULINKprogramme R2017a to run them and some models built from the inbuilt modelexample. I built a complete WCDMA model from the transmitting end to thereceiving end.

And also mainly the perfomance of WCDMA with and without a rakereceiver was observed. The Bit Error Rate (BER) was used to ananlyze the rakereceivers perfomance using various design parameters which i would give below. Setup for simulation As i said earlier i used WCDMA models which was inbuilt in MATLAB toperform simulation after modification to fit my required system models. I trieddoing this using an assumed situation close to real tiome to get adequateresults. Below we see the steps and various models used in the simulation.First i tried to calculate and show the BER when Eb/No has range 0-12dB usingBPSK (Binary Phase Shift Keying ) and QPSK (Qaurter nary Phase Shift Keying )modulation techniques and compare the outcomes, then secondly calculate and showagain the BER when Eb/No has range still 0-12 dB with channels AWGN andMultipath Rayleigh Fading channels, then calculating and showing the BER whenEb/No has some range as above but comparing the effect with the presence of arake receiver and aslo without it. Then also calculating the BER with Eb/Nohaving same range but different Spreading Factors and fingers (under AWGN andMultipath Fading Rayleigh Fading Channel but only QPSK modulation technique) toshow the maximum or optimum perfomance of the system and at which parameters itis achieved. For a BPSK modulation the model in SIMULINK is shown below and the signalis modulated using this method.

The input is a column vector because it is aframe based input, where the input frame is equivalent to product of number ofsymbols and sample per symbol value.   Fig. 3 BPSK Modulation Technique Model  For QPSK modulation, the input is in integers andbinary mapped into symbols. Its input is a column vector like in BPSK and alsobecause it is framed based integer input. Below we have the QPSK modulationtechnique model.

  Fig. 4 QPSK Modulation Technique Model Below we have the WCDMA physical layer block built forthe simulation. We can say that the WCDMA channel model subsystem simulates awireless link channel which has (Additive White Gaussian Noise) AWGN andMultipath Fading channels too. The AWGN channel block adds white Gaussian noiseto a real or complex signal coming from the input as its name implies. If inputis real a real Gaussian noise is added and a real output is produced.

If inputis complex it adds a complex Gaussian noise and output signal is also complex.The sample time used is gotten from the input signal.Fig. 5 WCDMA Model With AWGN and Multipath FadingChannel  We should note that the Eb/No (signal to noise ratio)is calculated as;    {S/R}/ {(1+N}/W}Where S being the received signal power, R given astransmission rate, I is the interference level, N is the noise and W is thebandwidth. We use different propagation conditions environments such asMultipath and AWGN channel, with fingers set from 1-4, Signal to noise ratioEb/No in (dB) and speed of terminal in Km/h.  Now we would look at the rake receiver’s performanceanalysis.

This is the effect it has on the signals BER when it is present andwithout it also. Fig. 6 WCDMA Model With A Rake Receiver  Above figure shows the simulation model block of WCDMAsystem with the rake receiver in the WCDMA receiving end. The rake receiverconsists of correlators or fingers which received the signals from differentmultipath channels and combine them with appropriate delays to recover thetransmitted signal. In the rake the first binary data is EX-ORed with the chipcode and the spread sequence is modulated and transmitted to the channel, dueto multipath effects, the various signal copies of the same signal transmittedis demodulated, the chip stream from the demodulation is fed to thecorrelators, each providing different amount of delays. Finally the signalshave to be combined back adequately based on estimated weight factors orspreading factors. A rake receiver consists of down sampler, decorrelators fordata and pilot, channel estimation (compares receiving driving signal withreference signal and phase correction, where data is phase corrected.In the appendix we see the rake receiver block modelused for simulation.

Note we adjusted the number of fingers and spreadingfactor in the correlator. To compare an ideal rake receiver we change certainparameters variables such as spreading factors, number of fingers and type ofchannels. The parameters focused on are given as follows to actually check theeffect on quality of signal and systems capacity/performance.

Here we see them: Eb/No (dB)- 0 to 12 for the range                               Spreading Factor –  4, 8, 32, 64, 128, 256.                              Samples per chip-1                              Channels – AWGNand Multipath Fading                              Modulation type-QPSK, BPSK                             Rake Fingers- 1,2,3,4fingers respectively                              Number of Frame –1 We change the following parameters to find the bestcombined parameter variables and we would see that in the conclusion whichcombination produces optimum performance of the rake receiver in the WCDMAsystem. Results  For the QPSK and BPSK after our simulation I plottedthe BER performance for the rake receiver under QPSK and BPSK modulationtechniques on the same graph, so we can compare the results. Here we used theproposed parameters for the simulation basically changing the modulationtechniques and simulink model block is shown in Fig.6 and 7. Below is the BERperformance vs Signal to noise ratio in decibels graph for the analysis. Fig.

7 BER Performance Analysis Comparing BPSK andQPSK   Modulation Techniques In WCDMA.  The graph above shows when using BPSK the WCDMAtransmission can tolerate a Signal to noise ratio (Eb/No) of >6-8 decibelswhile that of QPSK tolerates a Signal to noise ratio (Eb/No) of > 10-12decibels. Know that the BER in QPSK gets worse as it drops lower than 6dB. Notethat using BPSK allows the BER performance to be enhanced in a noisy channel.So from the graph we say that the BER for QPSK is much higher than that under BPSKwhile the processing time for Qpsk IS much smaller. Next below we see the result gotten when the channel isAdditive White Gaussian Noise (AWGN) and Multipath Rayleigh Fading Channel.Here we compared using both AWGN and Multipath Fading channel together and whenan AWGN channel is used and having variable number of paths, multipath delayand power profile.

Note we control the fading rate of the Multipath RayleighFading Channel by using specific velocity of mobile terminal.                               Fig. 8 BER Performance Analysis Of Channels In WCDMASystem. From the graph of the simulation we can see that theBER of the two channels together is way more acceptable than just an AWGNchannel. Therefore we can say that using AWGN + Multipath Rayleigh FadingCHANNEL together at once creates a more efficient capacity of WCDMA system thanjust separately using the channels. In the graph above note that the normalplotted graph is assumed as the two signals combined together while the otheris just an AWGN channel WCDMA simulated system.  Then we look at the effect of WCDMA system with thepresence of a rake receiver and also without it, by simulating and plotting agraph to compare the BER performance of the two situations.   Fig.

9 BER Performance Comparing WCDMA Receivers The  figure 12which is the graph of the comparison of the system with and without a rakereceiver at the receivers end and note we maintained an AWGN channel, a QPSKmodulation technique and spreading factor of 256. From the graph we can seethat the system when no rake receiver is present at the receiving end haslimited interference, with BER approaching to > 10% even when Eb/No variesfor 0-15dB. We cannot accept such a performance for the system. But for thepresence of rake receiver in the WCDMA system the BER has an acceptable limit.  The next simulation we looked at was to see the BERperformance of the WCDMA system when the spreading factors are varied, withEb/No varying from 0-12db and number of fingers in the receiver being 4. Welook at BER performance when spreading factor is (4, 8. 32, 64, 128,256)respectively.

  Fig. 10 BER Performance Analysis Comparing SpreadingFactors Of WCDMA System From the above graph of BER performance of differentspreading factors using the parameters shifted above we can see that Eb/No isconstant just when the spreading factor is 4 and different for other spreadingfactors. The graph shows the BER performance decreased as the spreading factorsincreased (BER is inversely proportional to the spreading factor). The maximumperformance of the rake receiver is when spreading factor is at 256. Lastly we look at the BER performance for differentnumber of fingers at the rake receiver, with spreading factor= 256, under AWGNchannel, QPSK modulation technique, where Eb/No ranges from 0=12dB. Note thatwe did the simulation when the fingers varied from 1-4.

From the simulation wegot the graph of BER vs Signal to noise ratio shown below. We can say that theincrease in number of fingers in the rake receiver reduces the BER thereforethe more the fingers used in the rake receiver the better the systemperformance. Also know that the number of finger used depends on the number ofmultipaths the path searcher can find.  CONCLUSION In conclusion for the system we see that the followingparameters variable produces optimum WCDMA system performance        : Eb/No (dB)- 0 to 12 for the range                               Spreading Factor– 256                              Samples per chip-1                              Channels – AWGNand Multipath Fading                              Modulation type-QPSK                              Rake Fingers- 4fingers                 And the rake receiver is a very key technique used forWCDMA system performance enhancement and system capacity enhancement.  FUTURE WORK  I hope in the future to look at other receivers and doa research on how much they all improve the performance of the network as awhole with increasing technology as we have 4G and 5G technologies now.

It wasan interesting experience working on this paper and I do look forward to moreexperiments to see how much the communication network can be advanced and how Ican contribute to it.              Appendix A Model of RAKE receiver Four selectors Fig.11    Model Of A Rake Reciever With Four Selectors  REFERRENCES  1)       http://kilyos.

ee.bilkent.edu.tr/~signal/BCSP/szabo.pdf 2)       https://www.slideshare.net/bhagwatsinghmahecha/pulse-shaping-fir-filter-for-wcdma  3)   S.

Daumont, R. Basel, Y. Louet, “Root-Raised Cosine filterinfluences on PAPR distribution of single carrier signals”, ISCCSP 2008, Malta,12-14 March 2008. 4)   Proakis, J. (1995).

 Digital Communications (3rded.). McGraw-Hill Inc. ISBN 0-07-113814-5. 5)      http://www.

srmuniv.ac.in/sites/default/files/files/lab_experiment.pdf 6)      Simon Haykin, “DigitalCommunications”, John Wiley & Sons, 1988.

 ISBN 978-0-471-62947-4                                             

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