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

TLDR

Multiuser receivers enhance spread‑spectrum and antenna‑array performance by exploiting multiaccess interference structure, yet their behavior in power‑controlled networks and resulting user capacity remain poorly understood. The study demonstrates that in large systems with random spreading, limiting interference effects of linear multiuser receivers can be decoupled, assigning each interferer an effective interference level toward the demodulated user. The authors derive simple expressions for the effective bandwidth of a user—dependent only on its SIR requirement—for three linear receivers: matched filter, decorrelator, and MMSE. They show that user capacity in a power‑controlled uplink is achieved when the sum of users’ effective bandwidths, computed from their SIR requirements for matched filter, decorrelator, or MMSE receivers, is below the system’s degrees of freedom, providing a basis for comparing receiver performance.

Abstract

Multiuser receivers improve the performance of spread-spectrum and antenna-array systems by exploiting the structure of the multiaccess interference when demodulating the signal 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. We show that in a large system with each user using random spreading sequences, the limiting interference effects under several linear multiuser receivers can be decoupled, such that each interferer can be ascribed a level of effective 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 effective 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 effective bandwidths of the users is less than the total number 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 decorrelator, and the MMSE receiver. The effective bandwidths under the three receivers serve as a basis for performance comparison.