Asymptotic Multiuser Efficiency for 2-Stage Detectors in Awgn Channels

Abstract

In the AWGN multiple-access channel with binary phase-shift keying modulation, the k/sup th/ user error probability for a given demodulator vanishes exponentially with the noise level as -n/sub k/ SNR/sub k/2, where n/sub k/ is the asymptotic multiuser efficiency (AME), and SNR/sub k/ is the received signal-to-background-noise ratio. Thus, the asymptotic multiuser efficiency is an attenuation of the error rate exponent for isolated transmission and maximum a posteriori demodulation, and provides a simple yet precise means of comparing multiuser receivers for sufficiently low noise levels. To date, this parameter is only known for the following receivers in the 2-user, asynchronous AWGN channel: the maximum likelihood sequence detector, the decorrelating detector, the linear MMSE detector, and the conventional detector. In this talk the asymptotic multiuser efficiencies for a class of detectors for the 2-user, asynchronous AWGN channel will be presented. This class may be loosely described as receivers which estimate and subtract multiple-access interference (MAI) by using tentative data decisions, and includes the two stage detectors with both conventional or decorrelated tentative decisions. The asymptotic multiuser efficiencies for this class of detectors clearly indicate regions for which a given user should avoid updating tentative decisions and suggest combinations of the above receivers to improve single-user performance. This technique applies to the AME of soft tentative decision strategies as well, and we demonstrate that the near-far resistance of two-stage detectors may be markedly improved using soft decision nonlinearities. Below we present an outline of the approach for conventional tentative decisions.