Linear multiuser receivers: Effective interference, effective bandwidth and user capacity

Multiuser receivers improve the performance of spread-spectrum and antenna- array systems by exploiting the structure of the multiaccess interference w hen demodulating the signal of a user. Much of the previous work on the per formance analysis of multiuser receivers has focused on their ability to re ject worst case interference. Their performance in a power-controlled netwo rk and the resulting user capacity are less well-understood. In this paper, me show that in a large system with each user using random spreading seque nces, the limiting interference effects under several linear multiuser rece ivers can be decoupled, such that each interferer can be ascribed a level o f effective interference that it provides to the user to be demodulated, Ap plying these results to the uplink of a single power-controlled cell, we de rive an effective bandwidth characterization of the user capacity: the sign al-to-interference requirements of all the users can be met if and only if the sum of the effective bandwidths of the users is less than the total num ber of degrees of freedom in the system. The effective bandwidth of a user depends only on its own SIR requirement, and simple expressions are derived for three linear receivers: the conventional matched filter, the decorrela tor, and the MMSE receiver. The effective bandwidths under the three receiv ers serve as a basis for performance comparison.