A method for increasing downlink capacity by coded multiuser transmission with a base station diversity array

In this paper, a simple structure is proposed for supporting multiple intracell users in the same time/frequency slot in the downlink using a transmit antenna array at the base station and joint detection at the single-antenna mobile receivers. To support multiple users, the system requires the simultaneous transmission of several successive code symbols from different diversity antennas. As a consequence, a single multi-antenna channel usage spans several trellis transitions - an unconventional situation which is handled with a novel merged trellis. Using the merged trellis, the optimal decoder is identified, and an analytical expression for the average bit-error rate is developed, based on soft-decision joint decoding at the mobiles. Unusual behaviour is demonstrated in terms of diversity order: as the number of transmit antennas increases due to an increasing number of users, the diversity order actually decreases due to the simultaneous transmission of successive code symbols. Even with the loss in diversity order, the method provides a major increase in downlink capacity while maintaining good performance for all users at low signal-to-noise ratios with moderate computational load