On transmit diversity for TD-SCDMA

In TD-SCDMA (UMTS TDD 1.28 Mcps mode) advanced techniques such as closed loop transmit diversity (CLTD) are foreseen. Deployment of CLTD influences the choice of receiver architecture and depends on propagation conditions, mobile speed, feedback delay, antenna weight computation and quantization. The impact of these factors is quantified and analyzed. In addition, due to recent interest in increasing the number of diversity antennas, results are presented using four antennas for CLTD. All simulations were performed using a setup very similar to TD-SCDMA conformance tests. The results obtained advise the use of a MMSE joint detection receiver architecture. For two transmit antennas, large gains up to 10 dB are achieved for low diversity propagation conditions, however the gains are smaller (∼2 dB) otherwise. The use of four antennas approximately doubles the gain achieved by two antennas. Also, it was found that higher mobile speeds do not decrease the gain of using CLTD, whereas larger feedback delays negatively impact performance.