Coded QAM in Multicode CDMA Systems

In this paper, the performance of the downlink multicode CDMA systems with bit interleaved coded multi-level quadrature amplitude modulation (BIC-MQAM) is studied with the consideration of interference cancellation. Generally, multicode transmission and adaptive multi-level modulation are used for flexible data rate transmissions in CDMA systems. In order to improve the system performance, multistage parallel interference cancellation and soft decision Viterbi decoding techniques are employed at the receiver. An analytical closed form of bit error rate (BER) is derived in presence of pilot channel estimation error due to multipath interference. Numerical results show that by using interference cancellation, the multipath interference can be effectively reduced, which leads to significant performance improvement even in presence of both channel estimation error and multipath interference. Also, system throughput can be increased by using high-order modulation without extra bandwidth expansion.