Soft iterative multisensor multiuser detection in coded dispersive CDMA wireless channels

A soft iterative multisensor array receiver for coded multiuser wideband code-division multiple-access wireless uplink channels is proposed, such channels are typically both frequency- and time-selective. A new equivalent discrete-time synchronous representation is used to model asynchronous multiuser dispersive channels that employ, in general, random spreading. The proposed scheme suppresses multiuser interference over a wide range of user signal powers, by iteratively exchanging soft information between a minimum mean-square-error (MMSE) multiuser demodulator and a bank of single-user decoders, feeding back the outputs of the latter to aid in soft multiuser multisensor MMSE-RAKE demodulation and subtractive interference cancellation, in the second and subsequent iterations. It displays near-far resistance since it behaves like a successive interference subtracter across iterations. The array responses are obtained via a multipass estimation scheme that uses both (temporal) pilot symbols and soft estimates fed back from the decoders to effectively provide multisymbol pilot signals and thence successively refined estimates with increasing iterations, while seeking neither to rely on the array geometry nor to estimate the directions from which users' signals arrive at the receiver. Simulation studies indicate that this scheme performs close to the single-user case with a two-sensor receiver array, and perfect channel state information, after four iterations; alternatively, it allows significantly increased user capacity compared with conventional receivers, and suffers only a modest loss with estimated array responses.