Analytical study of QAM with interference cancellation for high-speed multicode CDMA

In this paper, we consider the multilevel quadrature amplitude modulation (M-QAM) for forward link of multicode code-division multiple-access (CDMA) systems with interference cancellation to support high data rate service, and provide an analytical bit error rate (BER) performance of the system. In the current third-generation (3G) wide-band CDMA systems, in addition to multicode transmission, M-QAM is employed for high-speed downlink packet access (HSPDA) due to its high spectral efficiency. In frequency-selective fading channels, multipath interference seriously degrades the system performance, therefore interference cancellation technique is employed at the receiver. In this paper, an analytical closed-form BER of multicode CDMA systems with QAM is presented, where two important factors (channel estimation error and additive interference) on system performance are considered. Numerical results show that the system is very sensitive to additive multipath interference. With the help of interference cancellation technique, M-QAM can be adaptively employed in high signal-to-noise ratio cases to increase system throughput, within 5 MHz bandwidth. Meanwhile, simulation results are shown to illustrate the accuracy of the analytical method. Moreover, it is found that extra pilot power should be invested for more accurate channel estimation and consequently better BER performance when using the interference cancellation technique.