Optimum Signal Design and Power Control for Multiuser Wireless Communications Under Location-Invariant Bandwidth Constraints

This paper considers the design of signature waveforms that optimize the total capacity of multiuser decoders for the QAM-synchronous Gaussian multiple-access channel under a Fractional Out-of-Band Power (FOBP) bandwidth constraint. In doing so, it imposes a hitherto unrecognized constraint that is relevant in wireless communication networks: the bandwidth must be independent of the location at which it is measured, and hence independent of the received powers. For a fixed signaling rate, and under the location invariant bandwidth constraint, signature waveforms and power allocations are obtained that maximize the total capacity of the Post-Decorrelative Decoder (PDD) and the Optimum Successive Decoder (OSD) [1] . A further maximization over the signaling rate reveals that the maximum total capacity of the OSD for the optimal (correlated) signals and signaling rate can be significantly higher than that achievable by the OSD for systems with either orthogonal (maximum spreading) or identical signals (no spreading). Moreover, if the decoder is restricted to be the PDD, orthogonal signaling is nearly optimal. Hence, the gain in total capacity that is possible with optimally correlated signals (relative to orthogonal signals) in conjunction with the OSD are not available for systems that use the PDD.