Multiuser MIMO channel equalization

In MIMO receivers, the channel state needs to be estimated for equalization, detection, and for feedback to the transmitter in case of adaptive modulation and coding. Most current iterative [7, 16, 27] and non-iterative [25] schemes in the single-user MIMO case are training-based and rely on the transmission of pilot symbols. Alternatives to pilot-based algorithms are semi-blind schemes which exploit e.g. the known structure of the space-time code to allow reliable channel estimation during ongoing data transmission [2, 3, 18, 20]. Channel estimation and equalization for multiuser MIMO systems involve both information-theoretic [5, 17] and signal processing aspects. Early contributions to the field of MIMO communications assumed that the receiver has perfect channel state information. Recently there has been increased interest in the case where neither the receiver nor the transmitter know the channel state [11–13, 26]. We consider this case in Section 1.4.2. In [2], a linear space-time modulation technique was proposed which allows the receiver to jointly estimate the channel and demodulate the data without prior channel knowledge at the transmitter or receiver. In Section 1.4.2, this modulation scheme is extended to the case of multiple users. Section 1.4.3 investigates iterative equalization, detection, and interference cancellation for CDMA with random spreading. An iterative space-time receiver is considered for the uplink of a coded CDMA system. This type of receiver was studied in [1, 19, 29] for perfect channel knowledge. The extension to multipath fading channels incorporating soft decisions for improved