Optimum Infinite-Length MMSE Multi-User Decision-Feedback Space-TimeProcessing in Broadband Cellular Radio

Abstract

We present an investigation of space-time receiver architectures in multi-user wireless systems where optimal idealized (infinite-length) space-time filtering is applied to minimize the mean-square error (MSE). Feedforward, decision-feedback and cross-decision feedback filters are employed; hence, the system eliminates the post-cursor ISI (intersymbol interference) and CCI (co-channel interference). While this is a form of MIMO-DFE (Multi-Input Multi-Output Decision-Feedback Equalization), no joint transmitter-receiver optimization is performed since the transmit sources are assumed to be individual users. Our analysis leads to an entirely novel closed-form expression for the minimum achievable MSE for any single user in both fully- and partially-connected cross-decision feedback systems (XDF). This differs from previous results on MIMO-DFE obtained in a CDMA context which rely on the Gaussian assumption. Numerical results compare the performance of XDF systems with standard space-time DF and linear processing as well as the MIMO matched-filter bound in both a generic Rayleigh-fading system context and a broadband wireless context with fixed, highly directional subscriber antennas and correlated fading based on a 3D channel model parameterized according to measurements at 29.5 GHz. Results indicate that generally, an N-element array with fully-connected XDF processing provides the same performance as an N+1-element array with standard linear space-time processing.