A low-complexity precoder for large multiuser MISO systems

In this paper, we consider the problem of precoding 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-u N-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.