The design of multiuser detectors for guaranteed bit error rate in CDMA

We consider the design of code-division multiple-access (CDMA) systems in which the users are each guaranteed some quality-of-service (QoS) threshold that is given in terms of a bit error rate (BER). In long-code CDMA systems for which the load (defined as the ratio of the number of users in the system to the processing gain) is less than or equal to unity, we show that a multiuser linear receiver with appropriately designed powers does well at meeting the BER requirements with reasonable power consumption. In fact, the design significantly surpasses a recent approach from the literature both in terms of increased user capacity and reduced power requirements. For short-code CDMA systems, we investigate the effects of erroneous feedback in multiuser decision-feedback detectors in overloaded systems (i.e., more users than processing gain). An exact analysis of systems designed for a load ratio of two indicates that feedback errors are deleterious at lower signal-to-noise ratios (SNR). In this regime significant increases in SNR have almost no effect on the error rate. However, above a certain SNR threshold the error rate is found to decrease as it would in a single-user channel, but with a SNR penalty of approximately 4.6 dB.