Optimum modulation and multicode formats in CDMA systems with multiuser receivers

Abstract

We study the problem of maximizing the total system throughput under a bit error rate constraint for all users in the uplink of a single-cell synchronous CDMA system. Users realize variable bit rates by using a combination of multicode transmission and adaptive QAM modulation. We assume random signature sequences for all users, and perform an asymptotic analysis. We parametrize each user's resource allocation scheme by two parameters, viz., the number of signatures the user transmits with and the number of signal points in the user's QAM constellation, and optimize the total throughput over this parameter space. We examine four different settings: single-user matched filter (SUMF) and minimum-mean-square error (MMSE) receiver at the base station, with and without maximum power constraints. For a single user system, we describe the jointly optimum number of multicodes and constellation sizes for these four different system models. When multiple users are present, we show that the total throughput is maximized when only one user transmits: with no maximum power constraints this user can be chosen arbitrarily, otherwise it should be the one with the largest SNR. This solution, although optimal in the sense of maximizing the total throughput, is unfair to all but one user: thus, we examine a scheduling mechanism that assigns equal time frames to all users, thus yielding maximum fairness, and discuss the resulting total throughput loss.