A Low-Complexity Precoder for Large Multiuser MISO Systems

In this paper, we consider the problem of preceding in large multiuser MISO systems, where by ´large´ we mean (i) large number of transmit antennas (N_t) at the base station of the order of tens to hundreds of transmit antennas, and (ii) large number of downlink users (N_u) of the order of tens to hundreds of users where each user has one receive antenna. Such large MISO systems will be of immense interest because of the high capacities (sum-rates) of the order of hundreds of bits/channel use possible in such systems. We propose a vector perturbation based low-complexity precoder, termed as norm descent search (NDS) precoder, which has a complexity of just O(N_uN_t) per information symbol. This low complexity attribute of the precoder is achieved by searching for the perturbation vector over a reduced search space. Interestingly, in terms of BER performance, the proposed precoder achieves increasingly better BER for increasing N_t, N_u, such that for large N_t, N_u it achieves near-exponential diversity with some SNR loss, thus making it suited for large MISO systems both in terms of complexity as well as performance. The results of uncoded/turbo-coded simulations without and with channel estimation errors are presented.