PERFORMANCE COMPARISON BETWEEN MC-CDMA AND DS-CDMA SYSTEMS FOR AWGN AND RAYLEIGH FADING CHANNEL

Transcription

1 ISSN: X Volume, Issue 11, November 215 PERFORMANCE COMPARISON BETWEEN MC-CDMA AND DS-CDMA SYSTEMS FOR AWGN AND RAYLEIGH FADING CHANNEL R.Ramalakshmi, S.Karthikeya Abstract- The latest techology i cellular mobile commuicatio systems has become more demadig for better quality service. It requires broad badwidth for huge data trasfer. I this paper, we compare the performace Direct Sequece Code Divisio Multiple Access (DS-CDMA) ad Multi- Carrier Code Divisio Multiple Access (MC-CDMA) which are the major coteders for the et geeratio wireless sigalig techiques. Code Divisio Multiple Access (CDMA) is a multiple access techique i which several trasmitters ca sed iformatio simultaeously over a sigle commuicatio chael. Walsh-Hadamard (W-H) Code is chose for spreadig, which reduces Multiple Access Iterferece (MAI) due to its orthogoal property. Miimum mea square equalizatio (MMSE) ad tched filter detectio is assumed for both DS-CDMA ad MC-CDMA. The comparative study Bit Error Rate (BER) performace over differet modulatio techiques is doe ad their performaces are compared for BER ad Eb/N. data stream. Orthogoal sub-carriers are used to carry data o parallel chaels. OFDM eeds accurate frequecy sychroizatio ad its sub-carriers are ot orthogoal due to frequecy deviatio. Frequecy deviatio betwee trasmitter ad receiver oscillator cause Carrier Frequecy Offset (CFO). CFO causes Iter-carrier Iterferece. It has the oises like amplitude variatio ad Peak to average power ratio. OFDM requires very accurate frequecy sychroizatio betwee the trasmitter ad the receiver; with frequecy deviatio the sub-carriers will o loger be orthogoal, causig Iter-Carrier Iterferece (ICI) (i.e, cross-talk betwee the subcarriers). Frequecy fsets are typically caused by mismatched trasmitter ad receiver oscillators, or by Doppler shift due to movemet. Ide Terms- BER, CDMA, DS-CDMA, MC-CDMA, OFDM, W-H Code. I. INTRODUCTION I recet years Orthogoal Frequecy Divisio Multipleig (OFDM) ad CDMA systems have gaied attetio due to their use i high speed wireless commuicatio. Both OFDM ad CDMA have differet features, for eample, the former is a completely immue to multipath fadig effects, ad the later has multi-user capability. OFDM-CDMA to combie these features, so that we ca achieve higher data rates for multiple users simultaeously. OFDM is a digital multi-carrier modulatio scheme that trasmits high data rate stream by several low rate R.Ramalakshmi, VLSI Desig, PSNA CET, Didigul, Tamil Nadu, Idia. S.Karthikeya, Departmet ECE, PSNA CET, Didigul, Tamil Nadu, Idia. Fig. 1 Block diagram CDMA system CDMA is oe several methods multipleig wireless users. I CDMA, users are multipleed by distict codes rather tha by orthogoal frequecy bads, as i Frequecy Divisio Multiple Access (FDMA), or by orthogoal time slots, as i Time Divisio Multiple Access (TDMA). II. LITERATURE REVIEW Multicarrier sigalig techique is applied i [1] to a DS-CDMA system, where a data sequece is multiplied by a spreadig sequece modulates multiple carriers, rather tha a sigle carrier. To prevet self-iterferece they use bad-limited spreadig waveforms, ad system performace over a All Rights Reserved 215 IJARECE 271

2 ISSN: X Volume, Issue 11, November 215 frequecy selective Rayleigh chael i the presece partial bad iterferece are evaluated; also compare system performace with that a siglecarrier RAKE system. The arrowbad iterferece suppressio capability this multicarrier system is useful to reject the iterferece from a arrowbad sigal to a CDMA sigal. I [2], the overview multiple access schemes such as MC-CDMA, DS-CDMA, ad multi-toe CDMA (MT-CDMA) ad discussio advatages ad disadvatages with a ormal DS-CDMA scheme. Simulatio results are show that the MC-CDMA scheme with MMSEC is a promisig protocol i a 2- path frequecy selective slow Rayleigh fadig chael. MC-DS-CDMA ca adapt the trasmissio rates by chagig both time ad frequecy spreadig factors. I [3], a aalytical method is proposed to evaluate the effects the power cotrol elemet ad the complete Multiple Access Iterferece (MAI) o the multi-rate MC-DS-CDMA system. A larger power cotrol ca make the frequecy-domai spreadig gai is usually better tha that with a larger time-domai spreadig gai. I [], the effects log spreadig sequeces ad Carrier Frequecy Offset (CFO) o the performace asychroous MS-DS-CDMA systems with correlated fadig amog sub chaels were ivestigated. A epressio for approimate BEP was obtaied based o Gaussia ad local-mea approimatio. The derived aalytic results for MC- DS-CDMA by appropriate modificatios with by meas umerical simulatios; they demostrated that ICI ca be mitigated by usig optimum combiig ad by assigig a commo radom spreadig sequece over all subcarriers a give user. [5], ivestigates the trade-f betwee diversity ad chael estimatio ad chael estimatio errors for asychroous MC-CDMA ad MC-DS-CDMA. Both systems perform pilot-symbol aided chael estimatio maimal ratio combiig at the receiver, where the weightig coefficiets are determied by the chael estimated at each sub-carrier. The bit error rates the two systems for differet iformatio rates, umber users ad umber pilot symbols used i each chael estimate is compared. I [6], gives the most suitable techique for implemetig G commuicatio systems. The modems OFDM, MC-CDMA ad OFCDM have bee redesiged ad performace is aalyzed i terms probability error. The results reveal that OFCDM provides the lowest BER for give SNR ad the selectio a higher spreadig factor for frequecy-domai spreadig led to a icrease i the BER, whe the chael chose to be highly frequecy-selective ad a higher spreadig factor for time-domai spreadig led to a icrease i the requiremet for the umber subcarriers. The pricipal goal [7] is desigig ad testig the performace source ad chael codig ad decodig circuits are implemeted for CDMA Trasceiver usig etremely simple circuit cocept as trasmitter ad receiver hardware cellular commuicatio system is implemeted for CDMA scheme. It also describes the modulatio ad demodulatio circuits for CDMA. Trasceiver is realized by VHDL codes ad testig the same for a wide rage iput coditios. I [8], a improved MC-CDMA system usig Frequet Domai Differetial Detectio (MC- FDDD-CDMA) for low SNR liks is proposed. They are aalyzed the system performace BER uder the fast fadig HF chael, with sychroizatio errors, icludig frequet fset ad timig fset. The results show that the sychroizatio errors reduce the desired sigal amplitude, ad itroduce Iter-Carrier Iterferece, which limits the system BER performace. They improved a system, which ca effectively reduce the CPE ad cosiderably promote the BER performace uder the low SNR HF chaels. I [9], they desiged the CDMA trasmitter ad receiver ad they are tested usig ad arbitrary ad receiver ad they are tested usig a arbitrary chose data stream. A compariso has bee doe betwee the trasmitted ad received data ad satisfactory results have bee achieved. Icreasig the umber bits is possible to reach the stadard rates specified for CDMA. [1], aalyzes the performace BER uder Rayleigh fadig chael coditios MC-CDMA i presece AWGN (Additive White Gaussia Noise) usig BPSK modulatio for differet umber subcarrier, differet umber users usig MATLAB program, ad compariso betwee simulated results, which shows the reductio i BER performace. [11], implemets the MC-CDMA system ad derive the compariso BER Vs Eb/N for MC- CDMA commuicatio system usig variable umber bits with QPSK ad BPSK modulatio o Rayleigh chael ad Additive White Gaussia Noise. Simulatio result BER Vs Eb/N with All Rights Reserved 215 IJARECE 2715

3 ISSN: X Volume, Issue 11, November 215 QPSK modulatio shows that as BER performace decreases, Eb/N icreases. The received MIMO MC-CDMA sigals will ever be corrupted because copy same sigals are trasmitted over all subcarriers ad error or overlappig sigals will ever take place because orthogoal property. Wavelet packet waveform set is the set waveform geerated from a full biary wavelet packet tree which is used as the modulatio waveform i a MC-CDMA system. [12], presets a desig covetioal MC-CDMA ad wavelet packets based MC-CDMA ad their compariso o the basis performace matrices. BER Comparative aalysis MC-CDMA system with wavelet packet based MC-CDMA system usig differet modulatio techiques is preseted. Wavelet packet based MC- CDMA system usig BPSK modulatio is quite satisfactory as compared to other modulatio techiques i AWGN chael. I this paper, we compare the performace DS-CDMA ad MC-CDMA for differet modulatio techiques such as BPSK, QPSK ad their performaces are compared for BER ad Eb/N. III. SYSTEM MODEL A. MC-CDMA MC-CDMA is the combiatio frequecy domai spreadig data ad OFDM schemes, combies the advatages both the schemes. The arrow badwidth message is multiplied by a large badwidth sigal that is a pseudo radom oise code. Users i a CDMA system use the same frequecy bad ad trasmit simultaeously. The trasmitted sigal is recovered by correlatig the received sigal with the PN code used by the trasmitter. Spreadig codes are pseudo-orthogoal codes. MC-CDMA systems are more sesitive to the carrier frequecy fset tha sigle-carrier systems. Carrier frequecy fset removes the orthogoality amog subcarriers i MC-CDMA systems ad degrades the system performace seriously. (i) MC-CDMA Trasmitter MC-CDMA scheme trasmits the same symbol i parallel through several subcarriers. The OFDM trasmits differet code the user i the frequecy domai. Data stream from each user is divided i to several low rate parallel data stream. The iput data is multiplied by the spreadig code. The MC-CDMA trasmitter spreads the origial data stream over differet subcarriers usig a give spreadig code i frequecy domai. We ca use the W-H codes as a optimum orthogoal set, because we do ot have to pay attetio to the auto-correlatio characteristics the spreadig codes. Fig. 2 MC-CDMA Trasmitter The OFDM system with the CDMA system coverts the symbols to time domai samples by Iverse Fast Fourier Trasform (IFFT) ad assigs a sub-carrier for each symbol. The the sub-carriers are multipleed to form as a serial stream. Before the trasmissio the serial data stream, it is coverted to blocks ad each block is separated by a guard frame. The total umber sub-carriers are modulated i basebad by a IFFT ad coverted back i to serial data. A cyclic prefi is iserted betwee the symbols which are a repeat the ed the symbols at begiig, to reduce ISI ad ICI caused by multipath fadig. The cyclic prefi legth is chose such that it is greater tha the delay spread the chael. I MC-CDMA trasmissio, it is essetial to have frequecy o selective fadig over each sub-carrier. (ii) MC-CDMA Receiver Fig. 3 MC-CDMA Receiver The, the cyclic prefi is removed ad the samples are serial to parallel coverted to obtai the m-subcarriers compoets. The m-subcarriers are demodulated by a Fast Fourier Trasform (FFT) ad multiplied by the gai to combie the received sigal eergy scattered i the frequecy domai. All Rights Reserved 215 IJARECE 2716

4 ISSN: X Volume, Issue 11, November 215 B. DS-CDMA DS-CDMA is the combiatio time domai spreadig data ad OFDM schemes, combies the advatages both the schemes. Sequece to be trasmitted is spread i the time domai ad trasmitted by multiple sub-carriers. Pseudo-oise sequece is used as the spreadig code. (i) DS-CDMA Trasmitter DS-CDMA scheme trasmits the time domai data sigal through sub-carriers. The DS-CDMA trasmitter spreads the origial data stream usig a give spreadig code i time domai. Istead PN sequece we may use the W-H code as a optimum orthogoal set. The spreadig sigal is modulated by either BPSK or QPSK modulatio ad trasmitted through chael. Fig. DS-CDMA Trasmitter (ii) DS-CDMA Receiver Received sigal is the modulated form spreadig sigal. The received sigal is demodulated ad de-spread usig the reverse form trasmitter. Fig. 5 DS-CDMA Receiver I Direct Sequece Spread Spectrum trasmissio, the user data sigal is multiplied by a code sequece, as biary sequeces are used. The duratio a elemet i the code is the chip time. The ratio betwee the user symbol time ad the chip time is called the spread factor. IV. MODULATION TECHNIQUES I telecommuicatios, modulatio is the process coveyig a message sigal, for eample a digital bit stream or a aalog audio sigal, iside a sigal that ca be physically trasmitted. Modulatio a sie waveform is used to trasform a basebad message sigal i to a pass bad sigal. A device that performs modulatio is kow as a modulator ad a device that performs the iverse operatio modulatio is kow as a demodulator. The aim digital modulatio is to trasfer a digital bit stream over a aalog bad pass chael, for eample over the public switched telephoe etwork or over a limited radio frequecy bad. (a) Biary Phase Shift Keyig (BPSK) BPSK also called as 2PSK, is the simplest form Phase Shift Keyig (PSK). It uses two phases which are separated by 18 ad so ca also be termed 2-PSK. It does ot particularly matter eactly where the costellatio poits are positioed. This modulatio is the most robust all the PSKs sice it takes the highest level oise or distortio to make the demodulator reach a icorrect decisio. I the presece a arbitrary phase shift itroduced by the commuicatios chael, the demodulator is uable to tell which costellatio poit. As a result, the data is te differetially ecoded prior to modulatio. BPSK is fuctioally equivalet to 2-QAM modulatio. (b) Quadrature Phase Shift Keyig (QPSK) Sometimes this is kow as quaterary PSK, Quadriphase PSK, -PSK or QAM. QPSK uses four poits o the costellatio diagram, equi-spaced aroud a circle. With four phased, QPSK ca ecode two bits per symbol. The mathematical aalysis shows that QPSK ca be used either to double the data rate compared with a BPSK system while maitaiig the same badwidth the sigal, or to maitai the data-rate BPSK but halvig the badwidth eeded. I the latter case, the BER QPSK is eactly the same as the BER BPSK. Give that radio commuicatio chaels are allocated by agecies such as the Federal Commuicatio Commissio givig a prescribed badwidth, the advatage QPSK over BPSK becomes evidet: QPSK trasmits twice the data rate i give badwidth compared to BPSK, at the same BER. All Rights Reserved 215 IJARECE 2717

5 Bit Error Rate Bit Error Rate ISSN: X Volume, Issue 11, November 215 V. SPREADING CODE VII. SIMULATION RESULT Spreadig will covert arrowbad sigal to widebad sigal to suppress the self iterferece by multipath effect. Various types spreadig codes, like W-H code, Pseudo Noise (PN) spreadig code, Gold code ad Golay code are used to spread the user data. These codes ca be distiguished by their correlatio properties, orthogoal ad peak average to power ratio (PAPR). Sice W-H code is orthogoal code its performace is good amog the above spreadig code. To miimize the MAI, spreadig code should be orthogoal. I uplik PN code is used because due to distortio spreadig codes orthogoal gets lost. For a stream digits as ad 1, have the followig two properties, such as balace property ad idepedece. The Walsh-Hadamard codes are geerated with set N=2 codes. Orthogoality property shows crosscorrelatio betwee ay two W-H codes the same set is zero. W-H code spreadig, is used for all users the same chael which are sychroized i time. VI. (i) PERFORMANCE ANALYSIS BIT ERROR RATE The bit error rate or bit error ratio (BER) is defied as the umber bit errors divided by the total umber trasferred bits durig a studied time iterval. BER is a uit less performace measure. The bit error probability is the epectatio value the BER. The BER ca be cosidered as a approimate estimate the bit error probability. BER measure requires a trasmitter, a receiver ad a chael. Geerate log sequece radom bits, which is the iput to the trasmitter. BER as fuctio the Eb/N is give by: The user bits are radomly geerated ad the compariso is performed betwee MC-CDMA ad DS-CDMA. Simulatio BER Vs SNR with BPSK modulatio MC-CDMA shows that as BER performace decreases, the bit eergy to oise ratio, Eb/N icreases. We have oticed that BER for MC- CDMA BPSK shows low value compared to all other techiques. BER for MC-CDMA with BPSK for 2 users Fig. 6 USER1 BER FOR MC-CDMA BPSK Theoratical BER for BPSK o AWGN Theoratical BER for BPSK o Rayleigh Chael Theoratical BER for BPSK o AWGN Theoratical BER for BPSK o Rayleigh Chael (ii) BER = 1 2 erfc SNR E b N (1) Fig. 7 USER2 BER FOR MC-CDMA BPSK SNR is the Sigal to Noise Ratio, is defied as the ratio sigal power to the oise power. It measures the relatioship betwee the stregth a iformatio sigal ad the radom fluctuatios o amplitude, phase ad frequecy o that sigal as oise. For digital sigals, SNR is defied as the amout eergy i the sigal per bit iformatio carried by the sigal. SNR=Sigal Power/Noise Power (2) All Rights Reserved 215 IJARECE 2718

6 Bit Error Rate Bit Error Rate Bit Error Rate ISSN: X Volume, Issue 11, November 215 BER for MC-CDMA with QPSK for 2 users variable umber bits with BPSK ad QPSK modulatios o Additive White Gaussia Noise, ad data is geerated radomly. I tabulatio, we cosider the umber users 2 with the umber sub-carriers, ad umber users with umber sub-carriers, which shows MC-CDMA with BPSK shows better performace compared to all other methods. Thus the performace MC-CDMA with BPSK is better tha the QPSK modulatio techique based o simulatio results Fig. 8 USER1 BER FOR MC-CDMA QPSK Theoratical BER for QPSK o AWGN Theoratical BER for QPSK o Rayleigh Chael SN R Ra g e (d B) Table 1 MC-CDMA with BPSK Modulatio BER AWGN for BER for Rayleigh Chael Mi Mi M i users=2, subcarriers= users=, subcarriers= M i Fig. 9 USER2 BER FOR MC-CDMA QPSK BER for DS-CDMA with BPSK for 2 users Theoratical BER for QPSK o AWGN Theoratical BER for QPSK o Rayleigh Chael BER for BPSK modulatio with Spread Spectrum thechiques User 2 2 users Oe user system e^ e^ Table 2 MC-CDMA with QPSK Modulatio SNR, db Fig. 1 BER FOR DS-CDMA BPSK VIII. CONCLUSION This paper compare the BER betwee MC- CDMA ad DS-CDMA for high speed data commuicatio ad tried to study the BER Vs Eb/N performace for both commuicatio system usig SN R Ra ge (db ) BER for AWGN Mi BER for Rayleigh Chael Mi users=2,n umber subcarriers= Mi users=, subcarriers= Mi All Rights Reserved 215 IJARECE 2719

7 ISSN: X Volume, Issue 11, November e^ e^ IOSR Joural Electroics ad Commuicatio Egieerig, SEPTEMBER, 213. [12] moha Sigh Chadoliya, Raji Jaiwal, Om Prakesh hela, Comparative Aalysis MC-CDMA system with Wavelet Packet Based MC-CDMA System Usig Differet Modulatio Techiques, IOSR Joural Electrical ad Electroics Egieerig, VOL 9, MAY, 21. REFERENCES [1] Shiro Kodo, Laurece B. Milstei, Performace multicarrier DS-CDMA systems, IEEE TRANSACTIONS ON COMMUNICATIONS, VOL, NO. 2, FEBRUARY, [2] Shisuke Hara, Ramjee Prasad, Overview multicarrier CDMA, IEEE Commuicatios. g, DEC, [3] Li-Chu Wag, Chih-We Chag, O the Performace Multicarrier DS-CDMA With Imperfect Power Cotrol ad Variable Spreadig Factors, IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS, VOL 2, NO.6, JUN, 26. [] Feg-Tsu Chie, Chie-Hwa Hwag, ad C.-C. Jay Kuo, Performace Aalysis Multicarrier Offsets ad Radom Spreadig Uder Optimum Combiig, IEEE TRANSCATIONS ON COMMUNICATIONS, VOL 5, NO., APRIL, 26. [5] Adreq S. Lig, Laurece B. Milstei, Asychroous MC- DS-CDMA ad MC-CDMA, IEEE TRANSACTIONS ON COMMUNICATIONS, VOL 56, NO., APRIL, 28. [6] Syed M. Za. S. Shah, A.W.Umrai, ad Aftab A. Memo, Performace Compariso OFDM, MC-CDMA ad OFCDM for G Wireless Broadbad Access ad Beyod Progress, Electro-magetic Research Symposium, MARCH, 211. [7] Amsavalli A, Kashwa K.R, FPGA Implemetatio Low Compleity VLSI Architecture for DS-CDMA Commuicatio System, Iteratioal Joural Computer Applicatios, VOL 2, NO. 2, MARCH, 212. [8] Habib. U, Desig ad Implemetatio Computatioally Efficiet MC-CDMA Trasceiver ad Performace Aalysis i Fadig Chael, Itelliget Systems Modelig ad Simulatio, 213. [9] Vaibhav K Kakade, Implemetatio DS-CDMA Trasmitter ad Receiver i VHDL for FPGA, Iteratioal Joural Latest Treds i Egieerig ad Techology, 213. [1] G. Bridha, Performace Aalysis MC-CDMA usig BPSK Modulatio, Iteratioal Joural Research i Egieerig ad Techology, VOL 1, JUNE, 213. [11] Karmjeet Sigh, Rajbir Kaur, Performace Compariso Multi-Carrier CDMA Usig QPSK ad BPSK Modulatio, All Rights Reserved 215 IJARECE 272