Performance of Two-Hop DS-CDMA Systems Using Amplify-and-Forward Protocol over Different Fading Channels

Transcription

1 Performane of Two-Hop DS-CDMA Systems Using Amplify-and-Forward Protool over Different Fading Channels Nuri Kapuu 1 Mehmet Bilim 1 Ibrahim Develi 1 1 Department of Eletrial and Eletronis Engineering Eriyes University Kayseri Turkey nurikapuu@eriyes.edu.tr 1 Abstrat This study analyses the performane of diretsequene ode division multiple aess (DS-CDMA) based on two-hop amplify-and-forward protool over Weibull symmetri fading hannels as well as Rayleigh/Riian Riian/Rayleigh asymmetri fading phenomenas. We investigate the bit-error rate (BER) of the onsidered system using multiple relays by onsidering the effet of Weibull fading parameter and Riian K fator on the system performane. Our simulation results demonstrate the positive impats of the value of fading parameter Riian K fator and inreasing number of relay nodes on BER performane. It is also onfirmed that the Riian K fator is more effetive on the system performane over Riian/Rayleigh fading hannels in omparison with Rayleigh/Riian fading environment. Index Terms Two-hop relaying DS-CDMA fading hannels amplify-and-forward. I. INTRODUCTION Diret-sequene ode division multiple aess (DS- CDMA) whih is one of the wireless and mobile ommuniation tehniques has an important role and is ommonly used in our daily ommuniation tehnology. The main objetive of DS-CDMA tehnique is inreasing bandwidth and providing a seure ommuniation in a frequeny limited system [1] [6]. In some ases there may not be possible to ommuniate between transmitter and reeiver due to path loss shadowing and fading effets. In this ases two-hop relaying in whih a relay node is used for the transmission of soure information an be a useful alternative to maintain the ommuniation in DS-CDMA systems due to its onnetivity. Two hop relaying is an effetive method to ahieve the high transmission rates and extend the overage range. In two-hop transmissions two different tehniques an be employed suh as amplify-andforward (AF) and deode-and-forward (DF) relaying. In DF relaying method relay sends an estimation of soure information to destination. On the other hand the relay amplifies the soure information and sends the amplified soure information to the destination in AF method [7]. Manusript reeived 4 Deember 014; aepted 9 May 015. This work was supported by the Sientifi and Tehnologial Researh Counil of Turkey (TUBITAK) under projet grant 114E4. The authors thank to the Sientifi Researh Projets Coordinating Offie of Eriyes University (EU-BAP Projet No: FDK ). Several papers have analysed performane of DS-CDMA systems based on ooperative diversity in whih diret transmission is onsidered in addition to two-hop relaying over different fading hannels suh as Rayleigh Nakagamim and frequeny seletive fading hannels [8] [17]. In [8] the authors investigated the bit-error rate (BER) performane of DS-CDMA systems by applying ooperative DF relaying tehnique over Rayleigh faded hannels. A joint suessive interferene anellation (SIC) detetion method with a multi-relay seletion algorithm was proposed for the uplink of ooperative DS-CDMA systems in [9]. Li et al. evaluated the apaity of ooperative DS-CDMA systems based on both AF and DF methods in Rayleigh fading hannels [10]. The authors in [11] onsidered power alloation problem for DS-CDMA system based on ooperative ommuniations when the transmission hannels are subjet to Rayleigh distribution. In another work the performane of AF/DF based DS-CDMA systems are investigated with use of non-orthogonal spreading odes in Rayleigh fading environments [1]. The study presented in [13] gives the evaluation of downlink transmission in a ooperative DS-CDMA in Nakagami-m distributed hannels by assuming preproessing at the reeiver. Mehemed et al. [14] evaluated the outage probability of AF ooperative diversity for DS-CDMA systems under Nakagami-m faded transmission environments. Convolutional oded ooperative DS-CDMA systems are studied over frequeny seletive fading environments in [15]. In the studies above [8] [15] ooperative diversity based DS-CDMA systems are investigated under symmetri fading onditions where all hannels between ooperating nodes have the same fading distributions. However symmetri fading environment is not suitable for the real wireless hannel harateristis. On the other hand the hannels between ooperating nodes an experiene different fading onditions (asymmetri fading onditions) in realisti wireless relay networks. In the literature there are several studies that are presented the performane of ooperative diversity and two-hop relaying systems without integrating DS-CDMA tehnique under asymmetri hannel onditions [16] [1]. In [16] [17] the authors studied the outage probability and bit-error probability of two-hop AF relaying (without DS-CDMA system) protools when the hannels 97

2 are subjet to Rayleigh/Riian and Riian/Rayleigh fading ases. Xu et al. [18] examined performane of two-hop AF transmission without DS-CDMA priniple in Nakagamim/Riian fading hannels. In [19] outage probabiliy of twohop DF relaying sheme was investigated for Rayleigh/Gamma fading onditions. The authors in [19] proposed an analytial expression for the outage probability of the onsidered ooperation model in their work. Spalevi et al. [0] examined a dual-hop AF relaying transmission model under mixed Rayleigh/Hoyt fading. In [1] the performane of two-hop DF transmission is studied in Rayleigh/Weibull asymmetri environments. Despite the many reent studies about ooperative DS- CDMA systems to the best of authors knowledge none of them has presented the performane of DS-CDMA based on two-hop AF relaying over symmetri and asymmetri fading hannels. Motivated by these observations for the first time we study DS-CDMA systems based on two-hop AF relaying over symmetri Weibull fading hannels that the use of Weibull hannel model is reommended by IEEE Vehiular Tehnology Soiety Committee on Radio Propagation. In addition to these we analyse the performane of DS-CDMA by using two-hop AF transmission model over asymmetri fading hannels where the hannels of two hops experiene Rayleigh/Riian and Riian/Rayleigh onditions respetively. Moreover we illustrated BER performane of two-hop AF relayed DS-CDMA system for different senarios. Furthermore we present the effets of Weibull fading parameter and Riian K fator on the system performane by providing omputer simulations. II. DS-CDMA SYSTEMS Spread spetrum based CDMA has an important role in our daily life ommuniations. Multiple aess systems allow more than one user to share frequeny (bandwidth) and time simultaneously. One of the important CDMA tehniques is DS-CDMA. In a traditional DS-CDMA system eah user is separately treated as an information while the other users are onsidered as interferene [1]. In Fig. 1 transmitter of K users DS-CDMA system is shown. After demodulation proess deision is made to estimate the kth user information d k. Fig.. Blok diagram of K simultaneous users DS-CDMA reeiver. A. System Model III. SYSTEM AND CHANNEL MODELS Two-hop transmission is a signifiant way to maintain the ommuniation when diret transmission is impratial. In this work AF protool is onsidered with DS-CDMA system whih was explained in the previous setion. Two-hop transmission model with one relay is given in Fig. 3. Fig. 3. Illustration of two-hop relaying sheme with one relay. A soure (S) broadasts the information signal to the destination (D) with the help of a relay (R) in a two-hop system model. hsr and hrd indiate the hannel oeffiients of S-R and R-D links respetively. For the onsidered two-hop AF relaying DS-CDMA system S node has a DS-CDMA transmitter while D node has a DS-CDMA reeiver. In addition to these R node has DS-CDMA transeiver. In this work L 1 and 3 relay nodes are used. The R nodes are assumed to be operating in the half duplex mode. The transmission period onsists of two intervals. In the first interval S transmits user information to the R nodes. The reeived information at lth R node is given as f 1 K rl ( t) dk i Ck t k itd hsr i nl t i0 k1 kl (1) Fig. 1. Blok diagram of K simultaneous users DS-CDMA transmitter. The information of eah user d tk 1... K k modulated and spreaded by the unique spreading ode of the kth user k(t) that is different for eah user. After that sum of the K users information y(t) is transmitted to the reeiver. K simultaneous users DS-CDMA reeiver blok diagram is demonstrated in Fig.. At the reeiver y(t) is multiplied by eah user s unique spreading ode to separate the user information and then demodulation is applied. is where l 1 3 K is the number of simultaneous users f is the frame length dk i is the ith symbol of kth user and Ck is the spreading ode of the kth user. T d T and k represent bit period hip period and random transmission delay of kth user respetively. hsr is the hannel fading oeffiient between S-R and nl ( t) is the additive white Gaussian noise (AWGN) whih has zero mean and n N 0 variane. In the seond transmission interval eah relay node 98

3 amplifies the reeived information aording to an amplifiation fator whih an be expressed as [7] Eb Eb hsr N0 where Eb is energy of transmitted signal. R nodes use the amplifiation fator and send the amplified soure information to the D node. At D node reeived signal from lth relay node is written as f 1 K L rd ( t) r l l t Dk r iltb hrd i n l D t l i0 k1l 1 () (3) where r is the transmission delay of lth relay Dk indiates the time delay of the seond transmission phase hrd l is the hannel oeffiient between lth R-D and nd l ( t) is AWGN of the lth R-D hannel. At the D node reeived signal from eah relays are ombined with the use of maximum ratio ombining (MRC) tehnique for two-hop system as [] ( ) L rd t (t) MRC wl rd (4) l l1 where L 1 3 wl is the MRC oeffiient of the reeived * * signal from the lth R node and wl Eb hsrd / SRD. h SRD is the onjugate of the equivalent hannel oeffiient and SRD is the noise variane of the S-R-D link. B. Channel Models This subsetion explains the fading models used in the onsidered two-hop AF relaying with DS-CDMA network. In this study three different fading senarios are onsidered for the system hannels. In senario 1 S-R and R-D hannels are subjet to Weibull distribution whih is the symmetri fading ondition. Probability density funtion (PDF) of Weibull distribution an be given as [3] respetively whih is the asymmetri fading onditions. The PDF of S-R link an be exponentially written aording to Rayleigh PDF as [3] where f exp (6) is random Rayleigh variable for S-R link. The PDF of R-D link an be expressed aording to Riian distribution as [3] f K K 1 K 1e e K K 1 I0 where 0 0 the zeroth order modified Bessel funtion of the first kind. In the senario 3 the hannels between S-R and R-D links are distributed LoS Riian fading and non LoS Rayleigh fading respetively. In this senario 1 is the Riian random variable of S-R link is while is the Rayleigh random variable of R-D hannel. In senarios and 3 fading oeffiients of the two-hops are generated aording to (6) and (7). K indiates the Riian K fator and IV. SIMULATION RESULTS (7) I is We present the BER simulations of two-hop DS-CDMA systems with AF relaying tehnique for three different ases in this setion. All simulation urves are plotted versus average SNR per hop. In the onsidered system the number of simultaneous users is assumed K 4 and Hadamard spreading odes are used for spreading with length of N 3. In addition to these assumptions all performane results are shown by varying fading parameters and number of relays using binary phase shift keying (BPSK). f 1 1 exp 1 (5) where 0 is the gamma funtion is fading parameter is the saling parameter and is the random Weibull variable. In this senario the oeffiients of S-R and R-D hannels are generated aording to the PDF in (5). In senario S-R and R-D links experiene non line-ofsight (LoS) Rayleigh fading and LoS Riian fading Fig. 4. BER performane of two-hop DS-CDMA with the use of AF relaying over Weibull symmetri hannels using BPSK signalling. In Fig. 4 the BER performane of DS-CDMA system 99

4 with two-hop AF relaying is shown for the symmetri Weibull fading hannels (senario 1) in whih S-R and R-D links are Weibull distributed. Weibull distribution inludes Rayleigh fading (non LoS onditions) as a speial ase when. In this figure performane urves are obtained for different values of fading parameter for 1.5 and 3. From Fig. 4 it is obvious that the performane is improved as the value of parameter is inreased as expeted. When the hannel fading parameter value is 1.5 the performane is slightly improved as the number of relays are inreased if we ompare the ase of 3. It an be learly observed that inreasing the number of relays has a great effet on the performane of proposed system when the fading parameter equals to 3. For instane when 1.5 the BER values at 15 db are equal to and for L 1 and L 3 respetively. In ontrast to these results if is inreased to 3 the BER values are redued to 6110 with L 1 and 110 with L 3 at 15 db. fading is investigated by varying the Riian K fator as K 5 db and K 10 db. Figure 5 onfirms that if the first hop experienes Rayleigh fading inreasing the number of relays and higher value of K fator do not improve the performane for onsiderable amount. For example in the ase of L 1 the performane is almost same after 10 db when K 5 db and K 10 db respetively. This observation is also valid for the ases of L and L 3. Figure 6 shows the BER of the onsidered system model over Riian/Rayleigh (senario 3) asymmetri fading hannels. In ontrast to the results in Fig. 5 the performane is improved when the Riian K fator is inreased as an be seen in Fig. 6. As an example the BER value is for the ase of two relays and K 5 db at 15 db. However if K is inreased to 10 db the BER value is redued to with two relays at 15 db. In senario 3 the amplifiation fator is alulated aording to Riian fading (LoS onditions) oeffiients so the performane is improved onsiderably as the number of relay nodes and Riian K fator are inreased. Fig. 5. BER performane of two-hop DS-CDMA using AF relaying over Rayleigh/Riian asymmetri faded hannels with BPSK modulation. V. CONCLUSIONS We investigated two-hop DS-CDMA system in whih AF method is deployed at the relay nodes over different wireless hannel onditions for S-R/R-D links suh as Weibull/Weibull Rayleigh/Riian and Riian/Rayleigh fading onditions. Fousing on the simulation results some useful issues were revealed. It was demonstrated for Weibull/Weibull hannels if hannel onditions are worse than non LoS Rayleigh fading ( 1.5 ) inreasing the number of relays in the system does not bring a onsiderable amount of performane improvement. Partiularly simulation results for Rayleigh/Riian asymmetri ase illustrate that the system performane is almost same from medium SNR to high SNR range even if the value of K fator is inreased. Moreover inreasing the value of Riian K fator exhibits onsiderable performane improvement when S-R link is distributed aording to LoS Riian fading onditions as ompared to the ase of S-R link is non LoS Rayleigh distributed whih was onfirmed by the simulation results. This is due to the fat that the system performane depends on the quality of S-R hannel in AF relaying method. Fig. 6. BER performane of two-hop DS-CDMA using AF relaying over Riian/Rayleigh asymmetri fading hannels with BPSK modulation. In Fig. 5 the BER of the onsidered system model where S-R/R-D links experiene Rayleigh/Riian (senario ) REFERENCES [1] S. Moshavi Multi-user detetion for DS-CDMA ommuniations IEEE Communiations Magazine vol. 34 no. 10 pp [Online]. Available: [] C. Ciftlikli I. Develi A simple and useful approah for the determination proess of the weighted despreading sequenes in a DS-CDMA system European Trans. on Teleommuniations vol. 14 no. 4 pp [Online]. Available: /ett [3] A. Litvinenko E. Bekeris Probability distribution of multiple-aess interferene in haoti spreading odes based on DS-CDMA ommuniation system Elektronika ir Elektrotehnika vol. 13 no. 7 pp [Online]. Available: /j01.eee [4] I. Develi Detetion of optimal spreading odes for DS-CDMA wireless systems with despreading sequenes weighted by adjustable hip waveforms Journal of the Franklin Institute vol. 34 pp

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