In this paper, we study how the capacity and coverage of voice over internet protocol (VoIP) in the WCDMA uplink can be improved by us- ing advanced base station receivers capable of multi-user detection (MUD). To avoid the complexity of joint-detection, a multi-stage successive interfer- ence cancellation (SIC) detector or parallel interference cancellation (PIC) detector is used as an implementation of MUD. To further reduce complex- ity, time-division multiplexing (TDM) is used to separate user groups in time so that MUD can focus on a smaller number of overlapping signals. SIC or PIC cancels a number of temporarily detected user signals from the receive signal before detecting another user signal. A temporarily detected user signal can be regenerated based on demodulated data or decoded data, and based on hard-decision values or soft-decision values. We find that a two-stage SIC or PIC process improves VoIP performance significantly and having more detection stages beyond two does not further improve perfor- mance by much. The improved performance allows a VoIP user to reduce the average transmit power, which translates to VoIP capacity and coverage increases. Furthermore, we find that the individual user performance is not sensitive to its detection order. We also quantify the relative performance between the various signal regeneration schemes considered. As expected, post-decoding interference cancellation (IC) offers a significant gain over pre-decoding IC. Furthermore, we find that post-decoding IC is robust to traffic load variations. Also, soft-decision based cancellation achieved the best performance. However, the performance improvement is small com- pared to hard-decision based cancellation. Finally, we see that MUD can work with G-Rake to suppress one dominant other-cell interfering signal, e.g., due to a high-data-rate user in another cell.
['Y.-P. Eric Wang', 'Stephen Grant']
Multi-User Detection for Improving VoIP Capacity and Coverage in WCDMA Uplink