Analytical Ber Computation for Interference Cancellation

Multiple Access Interference (MAI) is a factor, which limits the capacity and performance of CDMA system. The conventional detector does not take into account the existence of MAI. Hence, a multi-user detection strategy is used for mitigating the effects of MAI. Novel receiver structures that take advantage of knowledge of MAI signal parameters are termed as Multi-user receivers and are more complex than conventional ones. This is because of their capability of using MAI signal information to help recover the desired user. Interference cancellation receivers have received a great deal of attention due to its advantages when compared with the other multi-user detectors. The interference cancellation techniques can be broadly broken into successive and parallel schemes for canceling multiple access interference. In order to utilize the advantage of both these two schemes, a Hybrid Interference Cancellation (HIC) scheme for MC CDMA system is proposed. HIC combines both SIC and PIC to the correct proportion so that the receiver performance is enhanced to reach the near optimal level. The objective of this work is to analytically compute BER for MC CDMA hybrid interference cancellation receiver and to justify its optimality. INTRODUCTION Cellular mobile communication is the mode of communication today, but will be the mode of living tomorrow. Future wireless services are intended to offer multimedia services to millions of subscribers, which will be obviously leading to bandwidth shortage. Considering the state of fact, International Tele-communication Union has identified CDMA as the wireless access scheme for 3G services. CDMA is a multiplexing technique where many users at the same time asynchronously access the same frequency band by spreading their information with pre assigned unique code sequence [1] and hence provides higher capacity over TDMA and FDMA scheme. However, OFDM is also a promising choice in the field of radio communication as it can transmit high data rate in a mobile environment. In 1993 three types of new multiple access schemes such as multi carrier CDMA (MC-CDMA), multi carrier DS-CDMA (MC-DS CDMA) and multi tone CDMA (MT CDMA) were proposed based on combination of CDMA and OFDM. MC-CDMA will be the probable access scheme for beyond 3G services. Since FFT is incorporated, it is possible to realize transmitter and receiver without much complexities for the above three schemes. These schemes have high spectral efficiency due to minimally densely sub carrier spacing. The main objective of this paper is to analytically compute the BER of the MCCDMA system with successive interference cancellation receiver, parallel interference cancellation receiver and hybrid interference cancellation receiver. INTERFERENCE CANCELLATION SCHEME Single user receivers treat multiple access interference (MAI), which is inherent in CDMA, as if it were additive random noise. Multi-user receivers use knowledge of the spreading sequences to exploit the structure of MAI. The major difference between single-user detector and Multi-User Detector (MUD) is that in MUD, the users are jointly detected for their mutual benefit. Multi user detection is a promising approach to overcome the limitations of single user CDMA receiver and improve system capacity. Since computational complexity of multi-user detectors like optimal receivers, De correlator, Minimum Mean Square Error receivers are computationally complex linear MUD like interference cancellation receivers are considered. In an interference cancellation scheme, the signal is first passed through a bank of correlators and then each user’s signal is re-constructed and cancelled from the received signal. Successive Interference Cancellation Scheme In each iteration of SIC [2], the entire users signal is estimated and the signal with the largest power is regenerated and subtracted from the buffered received signal. The remaining signals are now re-estimated and a new largest user is selected and the process continues until all the users’ signals have been recovered or the maximum allowable number of cancellations is reached. Successive interference cancellation receiver is shown in Fig.1. The SIC receiver almost has an optimal performance and is quite reliable but number of iteration to cancel out all the MAI is directly proportional to the number of users. Hence the computation time is quite large. Parallel Interference Cancellation Scheme In contrast to the SIC receiver, the PIC [3] receiver shown in Fig. 2 estimates and subtracts out all of the MAI for each user in parallel. The initial estimates of the transmitted data of the nth user are achieved by dispreading the received signal r(K) with the respective modified sequences Zn(K) leading to (1) ) ( ) ( ) ( ) 2 , 1 ( 1 ) 2 , 1 ( 0 , n n t k t j n L Z l L i r i X p