Multicoding and adaptive modulation for uncertain CDMA channels

In this paper, an adaptive modulation technique for time-varying, frequency-selective CDMA channels with a high level of uncertainty is proposed and studied. The ability of the adaptive modulation to perform well in the presence of channel estimation errors results from the use of associated linear-quadratic (LQ) receivers that are adapted in parallel with the modulation. We propose a simple, intuitively appealing cost function that can be maximized to find signal constellations and associated LQ receivers that are optimal in a certain sense. We discuss the properties of the proposed LQ cost function and derive a related adaptive algorithm for the simultaneous design of signals and receivers based on a simple multicoding technique. Finally, we present the results of a simulation study that clearly demonstrates the potential performance advantage of the proposed approach vis-a-vis MMSE linear receivers.