Performance of space-time coding for 3GPP HSDPA service under flat and frequency selective fading conditions

There is little doubt that an important feature of third generation (3G) mobile systems is going to be the ability to send and receive large volumes of data at high speed. The Third Generation Partnership Project (3GPP) is currently considering suitable systems for high-speed downlink packet access (HSDPA) services to provide the required data throughput. One of the systems currently under consideration is a space-time coding (STC) architecture using the vertical Bell Labs layered space-time (VBLAST) system on the downlink-shared channel (DSCH) to allow simultaneous transmissions over multiple channels destined for a single user. Although link level results for flat fading channels indicate significant performance advantages over conventional single antenna systems, it is only under frequency selective fading conditions that we are able to make a realistic analysis. We simulate the system and obtain its performance in frequency selective fading environments. Our results show severe system performance degradation in comparison to flat fading channels. This loss in performance is due to the loss of orthogonality between spreading codes induced by such channels. This leads us to the conclusion that changes in the structure of the system are necessary in order to overcome this issue.