Performance analysis of transmit antenna diversity in 3G WCDMA system

Transmit diversity schemes employ several antennas at the cellular base station to improve the forward link transmission performance (throughput, range, capacity and quality) without increasing the complexity of the mobile station. This paper attempts to evaluate the forward link performance improvement using different transmit diversity schemes by means of computer simulation. The schemes considered are space-time transmit diversity (STTD) and feedback mode transmit diversity (FBTD) proposed in a third generation (3G) WCDMA system. The effect of noisy channel estimates, due to imperfect channel estimation, and of imperfect closed loop power control, due to imperfect signal to interference ratio (SIR) estimation, on the performance is analyzed under different channel conditions and mobile velocities. The channel environments considered in the simulations are single ray Rayleigh fading, vehicular channel and pedestrian channel. It is observed that, with power control, the difference in the performance of ND, STTD and FBTD is not significant (around 1 dB to 2 dB) at high user velocities (>120 kmph). It is hence concluded that that with SIR based closed loop power control and imperfect channel estimation, diversity gains are not significant in the downlink of WCDMA system.