A novel multiple access scheme for mobile communications systems

Transcription

1 Indian Journal of Radio & Space Physics Vol. 36, October 7, pp A novel multiple access scheme for mobile communications systems Poonam Singh, R V Raja umar & S Lamba Department of Electronics & Electrical Communication Engineering, Indian Institute of echnology, haragpur 71 3 (WB), India {psingh, rkumar, Received 19 June 7; accepted 3 August 7 his paper presents a novel multiple access scheme for future mobile communications using ime Division Multiple Access (DMA), Frequency Division Multiple Access (FDMA) and Code Division Multiple Access (CDMA). he proposed system can support different classes of users each with different data rates and can provide very high spectral and system efficiency for the uplink and downlink. It is able to meet the demands on flexibility in data rate and provides scalability with respect to bandwidth using variable time slots, spreading factors and number of subcarriers. It exploits the advantages given by the combination of the spread spectrum technique with multicarrier modulation as well as DMA. eywords: Code division multiple access (CDMA), Orthogonal frequency division multiplexing (OFDM), Multi carrier CDMA, Multi-carrier direct sequence code division multiple access (MC-DS-CDMA) PACS No: 84.4 Ua 1 Introduction he primary goal of next generation wireless system will be the convergence of multimedia services such as speech, audio, video, image and data. his implies that a future wireless terminal will be able to connect to different networks in order to support various services by guaranteeing high speed data. he rapid increase in the number of wireless mobile terminal subscribers and users of wireless local area networks (WLAN) and wireless local loops (WLL) highlights the importance of wireless communication. he most important objective of the global 4th generation (4G) wireless systems is to offer cellular users broadband multimedia services everywhere. he 4G cellular systems will support much higher data rates (1 Mbps-1 Gbps) than 3G cellular. When selecting a multiple access scheme, perhaps the most important question is the number of admissible users per cell for a given available total bandwidth, for given radio propagation conditions and for a required transmission quality. All the existing multiple access schemes, orthogonal frequency division multiple access (OFDMA), time division multiple access (DMA) and code division multiple access (CDMA), taken together provide plenty of resources for providing subscribers with a wide variety of services and applications and accommodating new applications yet to be imagined. Frequency Division Multiple Access (FDMA) is the multiple access technique employed in the first generation of cellular communication systems, e.g. the Analog Mobile Phone Service (AMPS) system, where the system bandwidth is divided into several channels and each user is assigned a distinct channel. he commonly used multiple access schemes for second and third generation wireless mobile communication systems are based on either DMA, CDMA or the combined access schemes with FDMA. In a DMA system all users can use the entire channel bandwidth and are distinguished by allocating short and distinct time slots to each user. In CDMA, all users are allowed to use the entire system bandwidth all the time. he signals of users are distinguished by assigning different spreading codes. he FDMA and DMA techniques can accommodate N users on a channel whose bandwidth is N times the bandwidth of individual user signals without any mutual interference, but not a single additional user can be supported beyond this limiting number. he CDMA does not have a hard limit on the number of users that can be accommodated, but is subject to multi-access interference (MAI), which increases linearly with the number of users 1. Orthogonal Frequency Division Multiplexing (OFDM) is a special case of multi-carrier modulation, where a single data stream is transmitted over

2 SINGH et al.: MULIPLE ACCESS SCHEME FOR MOBILE COMMUNICAIONS SYSEMS 431 a number of lower rate subcarriers. By using a large number of subcarriers, a high immunity against multipath dispersion can be provided since the useful symbol duration s on each sub-stream will be much larger than the channel time dispersion. Hence the effects of inter-symbol interference (ISI) will be minimized. o eliminate ISI almost completely, a guard time is introduced for each symbol. he guard time is chosen larger than the expected delay spread. he OFDM can be easily realized by using discrete Fourier transform (DF) or more computationally efficient Fast Fourier ransform (FF) 3. oday, progress in digital technology has enabled the realization of a FF also for large number of subcarriers, through which OFDM has gained much importance 4,5. In OFDMA (Orthogonal Frequency Division Multiple Access), the channel bandwidth is divided into a number of subchannels. he subchannel is a subset of carriers out of the total set of available carriers. In order to mitigate the frequency selective fading, the carriers of one subchannel are spread along the channel spectrum. A CDMA scheme is a potential candidate for a third generation system. However, it has to cope with the presence of MAI. he present third generation (3G) systems which use CDMA can provide a maximum data rate of Mbps for indoor environment, which is quite less than that needed for recent multimedia applications that require very high bandwidth with mobility. he most important objectives of 4G wireless systems are to take care of severe ISI, which results from the high data rates and to provide spectral efficiency in the available limited bandwidth. Multi-carrier modulation with spread spectrum technique known as Multi Carrier Code Division Multiple Access (MC-CDMA) is a promising technique for future wireless multimedia communications. A lot of research has been devoted to hybrid schemes such as MC-CDMA, MC-DS-CDMA (Multi-Carrier Direct Sequence Code Division Multiple Access), where multi-carrier and spread spectrum system have been combined 8,9. he MC- CDMA system avoids MAI due to an FDMA scheme at subcarrier level and also exploits the diversity gain offered by spread spectrum technique. It allows one to benefit from several advantages of both multi-carrier modulation and spread spectrum system by offering high flexibility, high spectral efficiency, simple and robust detection techniques and narrow band interference rejection capability 1,11. he performance analysis of MC-CDMA and MC-DS-CDMA in multipath fading channels are given in 1-14 and their performances have been compared in 15,16. In this paper, a new hybrid multiple access scheme is proposed to get higher capacity and more flexibility. his scheme uses DMA, CDMA and OFDMA to accommodate data rates from 8 kbps to 1 Mbps in a bandwidth of MHz. he data rates can be increased further by using higher modulation schemes, e.g. 16-QAM or 64-QAM under good channel conditions. he number of time slots per frame, spreading factor and number of subcarriers are variable and provide flexibility to support variable data rates. his paper is organized as follows. Section discusses briefly the existing multiple access schemes, Section 3 discusses the proposed multiple access scheme, Section 4 gives the system model for implementation of this scheme and the performance analysis for the proposed MC-CDMA based system is given in Section 5. Simulation results are discussed in Section 6 to show the effectiveness of the proposed method. Some concluding remarks are made in Section 7. Proposed scheme he proposed multiple access scheme uses a combination of DMA, CDMA and OFDMA and thus increases the number of users that can be accommodated in a channel. Each user is assigned a time slot or a number of time slots depending on its data rate. he spreading factor is variable for different service classes and hence the number of subcarriers is also different. he spreading factor is chosen according to data rates for each application. hen the data are spread using the user specific spreading code of length L and the spread data are transmitted on different subcarriers using OFDM. he chips of spread data symbol are transmitted in frequency direction over several parallel subchannels (MC- CDMA) or in time direction over several multicarrier symbols (MC-DS-CDMA). he MC-CDMA based system has been proposed for the downlink wireless access of the 4G mobile radio system, because the orthogonality among the channels is maintained by using orthogonal codes when synchronous transmission is used from a base station. he channel estimation accuracy is also maintained by using a common pilot channel with

3 43 INDIAN J RADIO & SPACE PHYS, OCOBER 7 high transmission power. However, if MC-CDMA based approach is used in the uplink, the orthogonality among signals of different users is destroyed increasing the MAI, because the transmitted signal of each user is affected by different channel variations he MC-DS-CDMA system has been proposed for uplink as it transmits the same spread data symbol in parallel over a number of subcarriers, so the signals of different users can be distinguished in the receiver. But spectral efficiency of the system decreases and more complex receivers are needed to handle large number of users. he MC-CDMA is proposed for the downlink and MC-DS-CDMA for uplink in order to optimize both the spectral efficiency and mobile power consumption. In both cases, variable time slots, spreading factors and variable number of subcarriers are used. he maximum achievable data rate depends on the available channel bandwidth, number of time slots and the spreading factor. 3 System model 3.1 ransmitter and receiver Figures 1 and show respectively, the structures of proposed transmitter and receiver. he proposed system supports M different service classes, each with different data rates R m. he users are assigned one or more time slots per frame depending on their data Fig. 1 Proposed transmitter Fig. Proposed receiver rates. he spreading factor and the number of subcarriers are also variable and are chosen according to data rates for each application. he modulation scheme employed is Quadrature Phase Shift eying (QPS), 16-QAM (Quadrature Amplitude Modulation) or 64-QAM depending on the data rate of users and channel conditions. he complex valued data symbol b k is multiplied with the user specific spreading code of length L. he complex valued data sequence obtained after spreading is then modulated onto a number of subcarriers using OFDM. In MC-CDMA based systems, the number of subcarriers are equal to number of chips after spreading. In MC-DS-CDMA based systems, the number of subcarriers are chosen according to the bandwidth requirement, which is decided by the data rate and modulation scheme used. hus each data symbol is spread over many subcarriers. A guard time of g is usually inserted. he OFDM symbol duration including a guard interval is s = + g, where is the actual symbol duration. Here, one data symbol per user is transmitted in one OFDM symbol. In the synchronous downlink channel, the modulated signals of active users are added before transmission. In receiver, after removing the guard interval and taking inverse OFDM the received sequence is equalized to get the transmitted data. After channel estimation, the output of each subcarrier is equalized and then coherently combined over the parallel subcarrier components, i.e. despreading of signals is performed. Finally, the regenerated symbol sequences are parallel to serial converted to recover the transmitted binary data. he proposed scheme can transmit data at rates varying from 8 kbps to 1 Mbps. he system bandwidth is assumed to be MHz and the final chip rate is 4 Mcps. he number of subcarriers varies from 8 to 496 with a subcarrier spacing varying from 4.88 khz to.5 MHz. Duration of one frame is 1 ms, consisting of 16 time slots, each slot duration being 6.5 µs. It consists of 56 chips. Each user is assigned 1 to 16 time slots per frame depending on its data rate. he spreading factor varies from 4 to 3. he data symbols are time-multiplexed with the pilot symbols and the resultant symbol sequence is converted from serial to N parallel sequences. Each data-modulated symbol sequence is duplicated into M parallel copies and each duplicated symbol is multiplied by a chip from the spreading code. An orthogonal multi-carrier signal is generated using the IFF

4 SINGH et al.: MULIPLE ACCESS SCHEME FOR MOBILE COMMUNICAIONS SYSEMS 433 and a guard interval is inserted every generated symbol. able 1 summarizes the simulation parameters assumed in this paper. 3. Channel model A mobile radio propagation channel is characterized by frequency selective multipath channel consisting of many propagation paths with different time delays. he simulation is first carried out under ideal channel conditions, where the noise is considered only due to AWGN (Additive White Gaussian Noise). hen the IU-R defined Vehicular A propagation channel model having six Rayleigh faded discrete paths is considered. As per this model, a channel is modeled as an FIR filter, whose impulse response can be expressed as P 1 h ( n) = a ( n) δ( n τ ) (1) i i i i= with a i (n) and τ i being the complex path gain and time delay of the ith propagation path in a P path model. he channel parameters are summarized in able. 3.3 Channel estimation When coherent detection is used in receivers, information about the channel state is required and has to be estimated by the receiver. he basic Parameters able 1 Simulation parameters Values Bandwidth MHz Chip rate 4 Mcps Data rate 8 kbps-1 Mbps Spreading Codes Walsh Codes Spreading factor 4-3 Modulation QPS, 16-QAM, 64-QAM Number of sub-carriers able IU-R defined Vehicular A propagation channel model ap Relative delay, ns Average gain, db principle of pilot symbol aided channel estimation is to multiplex reference symbols, known as pilot symbols, into the data stream. he receiver estimates the channel state information based on the received, known pilot symbols. he pilot symbols can be scattered in time and/or frequency direction in OFDM frames. Here frequency domain equalizer has been used, so pilot symbols are multiplexed with data symbols in frequency direction. At the receiver, the guard intervals of the received signals are removed and the resultant symbol sequence is converted to a modulated signal of each subcarrier using FF. For channel estimation, the channel impulse response at each subcarrier is found by averaging the impulse response measured for each of the dedicated pilot symbols. Using the obtained channel impulse response, the output of each subcarrier is equalized and then coherently combined over the N parallel subcarrier components, i.e. de-spreading the signals. Finally, the regenerated symbol sequences are parallel to serial converted to recover the transmitted binary data. 4 Performance analysis It is found that MC-CDMA systems suffer from multi-access interference when the channel is frequency selective fading. In fact, MAI is a major factor that limits the performance of CDMA based systems. he MAI can be reduced by using orthogonal codes. But the orthogonality of these codes could be destroyed in a multi-path environment. For high data rates, the delay spread may be longer than the duration of several chips and the induced MAI will limit the system performance. he transmitted signal of kth user is given by sk ( = bk ( ck ( () where b k ( is the data and c k ( is code for kth user. Due to multi-path fading, the received signal will be given by r( = s ( t τ ) h( + n( (3) k= 1 k where represents convolution, τ the propagation delay, h( the channel impulse response and n( the AWGN, with a double-sided power spectral density of N o /. Also r(= bk ( t τ) c( t τ) h( + n( (4) k = 1

5 434 INDIAN J RADIO & SPACE PHYS, OCOBER 7 In the receiver for first user, the received signal is multiplied by the corresponding code in the receiver and the detected signal after equalization is given by d ( n) r( c ( dt = 1 1 = [ bk ( t τ) ck ( t τ ) + n( ] c 1 ( dt (5) k = 1 Equation (5) can be written as d 1( n) = bk ( t τ ) c1( t τ ) c1( dt + b k ( t τ ) c k ( t τ ) c1 ( dt + n( c1( dt k = (6) he first term in the above equation represents the desired signal at the output of first user's receiver, second term represents the interference from other ( 1) users, known as MAI and the third term corresponds to the output due to AWGN. In the first term, the delayed version of the received signal is being multiplied by the code of first user, so some amount of self-interference is introduced. In the second term, the delayed signal is multiplied by codes of other users, so even if the codes are chosen to be perfectly orthogonal, the cross-correlation will not be zero and a significant amount of MAI will be present. In general, the MAI for kth user is given by MAI = b l ( t τ) c l ( t τ) c k ( dt l = 1l k = bl ( t τ)ρkl (7) l = 1l k where ρkl = cl ( t τ) ck ( dt is the crosscorrelation between different users spreading sequences. he MAI will be more if this crosscorrelation is large, which increases with the delay spread σ τ. For high data rates, the delay spread may be longer than the duration of several chips and the induced MAI will limit the system performance. In MC-DS-CDMA systems all the chips are transmitted on the same subcarrier, which experiences correlated fading, so MAI is less as compared to MC-CDMA system. It is difficult to analyze MAI exactly, because it is the sum of many interferences, which may not be independent in general. Since the QAM data symbols b k s are independent and identically distributed (i.i.d.) with zero mean and variance E s (the symbol energy) and the spreading codes c k 's are also sequence of i.i.d. random variables with equal probabilities. One can conclude that the interference is at least uncorrelated with zero mean. So applying central limit theorem, MAI can be approximated by a Gaussian process with zero mean and variance N c N σ c = n N( 1) 1 σ I (8) 8π n= 1 l = 1, l n ( n l) where σ n is the noise variance, the number of users, N c the number of subcarriers and N the spreading factor. he probability of error for each user is given by P ( k) = Q SINR = Q Eb (9) e σ + σ n MAI In a multi-user system, the average bit error rate for users is given by 1 1 BER = Pe ( k) k = where is the number of users. 5 Simulation results Figure 3 shows the symbol error rate (SER) performance of the proposed multiple access system in multipath fading channel for different number of users for a data rate of 8 kbps. he modulation used is QPS and spreading factor is 3. he required bandwidth for one symbol is 18 khz and number of subcarriers is 3 with a subcarrier spacing of 4 khz. he RF bandwidth is MHz and so the total number of subcarriers can be 5, but the number of subcarriers is taken in powers of to reduce computational complexity, so one can take 496

6 SINGH et al.: MULIPLE ACCESS SCHEME FOR MOBILE COMMUNICAIONS SYSEMS 435 Fig. 3 he SER plot for the proposed system subcarriers with a subcarrier spacing of 4.88 khz. hus, using CDMA, DMA and OFDMA, the maximum number of users can be 5 at a data rate of 8 kbps, giving a maximum spectral efficiency of. It is observed that in multi-path fading channel the performance degrades as the number of active users increase due to MAI. Even when Walsh codes are used, which are perfectly orthogonal codes, the MAI is not zero, since each chip of the PN sequence experiences independent fading, which tends to destroy the orthogonality between spreading sequences. his increases the MAI and degrades the SER performance. 6 Conclusion A new multiple access scheme has been proposed, which uses FDMA, DMA and CDMA and can be used for transmission of different classes of data, e.g. audio, video, internet, ISDN, multimedia, etc. having different data rates and modulation schemes. he parameters, e.g. spreading factor, number of time slots, number of subcarriers, etc., are optimized to get suitable bandwidth scalability. hese parameters are chosen according to the required data rate, the available bandwidth, the number of subscribers, etc. he performance of proposed system has also been studied for mobile channels. References 1 Sari H, Vanhaverbeke F & Moeneclaey Marc, Extending the Capacity of Multiple Access Channels, IEEE Commun Mag (USA), () 74. Chang R W & Gibby R A, A theoretical study of performance of an orthogonal multiplexing data transmission scheme, IEEE rans Commun echnol (USA), 16 (1968) Weinstein S B & Ebert P M, Data transmission by frequency-division multiplexing using the discrete Fourier transform, IEEE rans Commun echnol (USA), 19 (1971) van Nee R & Prasad R, OFDM for Wireless Multimedia Communications (Artech House, Boston/ London),. 5 Z Wang & Giannakis G B, Wireless Multicarrier Communications, Where Fourier meets Shannon, IEEE Signal Process Mag (USA), May () 9. 6 Proakis J G, Digital Communications (McGraw Hill, New York), Rappaport S, Wireless Communications: Principles and Practice (Prentice-Hall, Amsterdam), Hara H & Prasad R, Overview of multicarrier CDMA, IEEE Commun Mag (USA), 35 (1997) Yee N, Linnartz J P & Fettweis G, Multi-carrier CDMA in indoor wireless radio networks in Proc IEEE International Symposium on Personal, Indoor and Mobile Radio Communications 1993 (PIMRC, 93), Yokohama, Japan, Sept. 1993, pp ,. 1 Chouly A, Brajal A & Jourdan S, Orthogonal multi-carrier techniques applied to direct sequence spread spectrum CDMA systems in Proc IEEE Global elecommunications Conference, 1993 (GLOBECOM,93), Nov., 1993, pp Fazel, Performance of CDMA/OFDM for mobile communication system in Proc IEEE International Conference on Universal Personal Communications 1993 (ICUPC,93), Oct. 1993, pp Hara S & Prasad R, Design and Performance of Multicarrier CDMA System in Frequency-Selective Rayleigh Fading Channels, IEEE rans Veh echnol (USA), 48 (5) Sept. (1999). 13 Sourour E A & Nakagawa M, Performance of orthogonal multicarrier CDMA in a multipath fading channel, IEEE rans Commun (USA), 44 (1996) ondo S & Milstein L B, Performance of multi-carrier DS- CDMA systems, IEEE rans Commun (USA), 44 (1996), Fazel & aiser S, Multi-Carrier and Spread Spectrum Systems (John Wiley, New York), Suwa S, Atarashi H & Sawahashi M, Performance Comparison Between MC/DS-CDMA and MC-CDMA for Reverse Link Broadband Packet Wireless Access, IEEE rans Commun (USA), 44 () 356.