A novel technique to increase the capacity of code division multiple access system using scrambled spreading sequences

Abstract

A multistage Linear Parallel Interference Cancellation (LPIC) approach is presented to mitigate the effect of multiuser interference in uplink Direct Sequence Code Division Multiple Access (DS CDMA) system. The system is overloaded to accommodate more number of users than the processing gain of the system. The performance of this overloading scheme is evaluated by assigning one set of orthogonal Gold codes to the first N users and another set of orthogonal Gold codes to extra users. Both the sets are overlaid by a common pseudo noise scrambling sequence. This paper proposes a novel technique of using Weighted Linear Parallel Interference Cancellation (WLPIC) for overloaded CDMA system resulting in bandwidth saving, better performance and increase in the speed of interference cancellation. Also this technique considerably reduces the multipleaccess
interference, since set-1 users suffer from interference of the set-2 users only, while the set-2 users suffer from
interference of the set-1 users. This provides a way for accommodating additional users in the same available bandwidth. Considerable conservation of time is achieved by avoiding the usage of conventional successive Interference Cancellation (IC) for scrambled CDMA system. The empirical results show that this scheme provides 50% overloading with three-stage weighted
linear parallel interference cancellation (WLPIC) technique for N = 64 at a Bit Error Rate (BER) of 10-3 and 75% overloading at a BER of 10-2. The three-stage WLPIC scheme clearly outperforms matched filter detector, Conventional LPIC and the twostage WLPIC on Additive White Gaussian Noise (AWGN) channel.