A comparison of bandwidth-efficient multiple access to other signal designs for correlated waveform multipleaccess communications

There have been several papers in the literature that deal with the design of signature waveforms for use by the transmitters in uplink, single-cell, multiple-access communications. In particular, we consider the approach introduced by Guess and Varanasi (1996, 1997), where the signature waveforms are specifically designed for the centralized multiuser receiver at the base so that each transmitter can be guaranteed a preassigned quality-of-service (QoS) requirement in terms of the received signal-tointerference ratio (SIR). The resulting strategy is called bandwidth-efficient multiple access (BEMA). When all users employ pulse amplitude modulation (PAM) and a common signaling rate, the key question in BEMA is how the waveforms must be designed to occupy as little bandwidth as possible and still meet the QoS objectives. For a strict measure of bandwidth, and for a given set of received powers, this question was addressed by the authors for the maximum SIR decision-feedback (MSIR-DF) receiver of Varanasi and Guess (1998). A similar question was addressed by Viswanath, Anantharam and Tse (see ibid., vol.45, p.1968-1983, Sept. 1999) where, for a sum constraint on the received powers, optimal signature signals and transmit powers were obtained for the linear MSIR receiver (without decision feedback). A somewhat different but related non-QoS approach proposes the design of signature signals that maximize the total capacity of the multiple-access channel under a spreading-gain constraint. This article undertakes a comparison of the minimum bandwidth required (to achieve the QoS requirements) for the signals designed for the MSIR-DF receiver, for the linear MSIR receiver, and for sum-capacity maximization as shown by Viswanath and Anantharam (see ibid., vol. 45, p.1984-1991, Sept. 1999). We show that the bandwidth required for multiuser receivers with decision feedback can be significantly less than that required for linear receivers or for sum-capacity maximization when the MSIR-DF receiver is used.