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 when demodulating the signals of a user. Much of the previous work on the performance analysis of multiuser receivers has focused on their ability to reject worst-case interference. Their performance in a power controlled network and the resulting user capacity are less well-understood. In this paper, we show that in a large system with each user using random spreading sequences, the limiting interference eeects under several linear multiuser receivers can be decoupled, such that each interferer can be ascribed a level of eeective interference that it provides to the user to be demodulated. Applying these results to the uplink of a single power-controlled cell, we derive an eeective bandwidth characterization of the user capacity: the signal-to-interference requirements of all the users can be met if and only if the sum of the eeective bandwidths of the users is less than the total number of degrees of freedom in the system. The eeective bandwidth of a user depends only on its own SIR requirement , and simple expressions are derived for three linear receivers: the conventional matched lter, the decorrelator and the MMSE receiver. The eeective bandwidths under the three receivers serve as a basis for performance comparison.