| The capacity of Direct Sequence Code Division Multiple Access (DSCDMA) systems is limited by multiple access interference (MAI) instead of thermal noise. Commercial CDMA systems regard MAI as additive noise which limits the number of users that can be supported in the system to less than 10% of the spreading gain. Moreover, the bit error rates for individual users degrade rapidly with an increase in the number of users.; A new sub-symbol based multiuser detection technique addresses the two major concerns in DS-CDMA systems: high bit error rates and low system capacity. The two filters in traditional multiuser detectors—a code-matched and a multiuser linear filter—are designed independently, resulting in an increase in system complexity due to asynchronous operation. Multiuser detection based on sub-symbols is a joint optimization of the code-matched filter and the linear multiuser detection filter. This design approach drastically reduces the implementation complexity while asymptotically approaching the ideal performance.; In the sub-symbol scheme, data symbols of users are partitioned into sub-symbols such that the resulting sub-symbols from different users are time-aligned at the receiver. Simplified optimal linear filters operate in each sub-symbol interval, and the filtered outputs are processed using various combining algorithms. Simulation results indicate that a condition number based combining algorithm approaches the ideal bit error rate performance. Other combining schemes, such as simple accumulation, select-best, and maximal ratio, demonstrate immunity to the near-far effect but do not perform as well.; Although the sub-symbol scheme in conjunction with the diversity combining algorithms yields lower bit error rates, it does not guarantee a significant increase in the number of simultaneous users. Assuming that the system can be made to operate in a quasi-synchronous mode, the system capacity can be increased to the number of chips in the longest sub-symbol by adjusting the parameters used in sub-symbol detection.; While multiuser detectors perform well in the reduction of intra-cell MAI, they fail to suppress the inter-cell MAI. The use of multiple antennas in conjunction with linear multiuser detectors conveys both immunity to inter-cell MAI and near-far resistance. Simulations of the resulting system indicates an improved overall performance. |
