Krylov Subspace Method Based Low-Complexity MIMO Multi-User Receiver for Time-Variant Channels

We consider the uplink of a time-variant multiple-input-multiple-output (MIMO) multi-user (MU) multi-carrier (MC) code division multiple access (CDMA) system. The linear minimum mean square error (LMMSE) filters for channel estimation and multi-user detection at the receiver side require too high complexity to be implementable in a system operating in time-variant channels. We develop a low-complexity implementation of an LMMSE filter based on the Krylov subspace method. We are able to reduce the computational complexity by one order of magnitude. Furthermore, in a system with K users having N _T transmit antennas, parallelization of the computations of the multi-user detector into KN_T branches is achieved as well as considerable storage reduction. We discuss more specifically a fully loaded system (the number of subcarriers N is equal to the number of users K) with N_T = 2 transmit antennas per user, N_R = 4 receive antennas and K = N = 64