Soft-decoder feedback aided space-time processing in multiuser interference suppression

This work presents a soft iterative minimum mean square error (MMSE) multiuser interference suppressor for coded direct sequence code division multiple access (CDMA) uplink channels that employ space-time processing at both the mobile transmitters and the base station receiver. With a new equivalent discrete-time synchronous model for randomly spread asynchronous multiuser CDMA channels, we develop an iterative scheme that exchanges soft information between an MMSE multiuser demodulator (that also concurrently performs subtractive interference cancellation) and a bank of single-user decoders. It performs well both at low and high received signal powers, and displays the successive cancellation property across iterations, exhibiting good near-far resistance. Introducing multisensor arrays at the receiver relaxes the limitations imposed by the inherent suboptimality of MMSE multiuser demodulation, in so far as it enhances interuser separation, now in the spatial sense, and thereby allows significant overload capability. Further, this work explores new territory by studying multisensor transmission in multiuser channels. A new multisensor block modulation scheme called space-time spreading, for mobile transmitters is proposed; it is observed to provide significant benefits especially when the spatial separation afforded by the use of multisensor receiver arrays, is otherwise limited