Performance evaluation of open loop multi-user MIMO systems

Significant throughput gains can be achieved in a multi-user MIMO (MU-MIMO) wireless system by exploiting the combination of multi-user scheduling and multi-user diversity. Open-loop MU-MIMO (OL-MU-MIMO) is a codebook based precoding technique where precoders are fixed a priori at the base station (BS) in a known fashion and the user needs to feedback which precoding vector is to be chosen referred to as precoding vector index (PVI) or stream indicator. This scheme feedbacks the channel quality indicator (CQI) which is used by the BS for allocation of modulation and coding scheme (MCS) to the scheduled users. As the precoders used at all the base stations are known a priori, estimation of co-channel interference (CCI) is accurate and there is negligible mismatch between the CQI fed back by the user in present frame and SINR experienced by the user during next frame for low Doppler, resulting in stable CQI modeling. In this paper, an extensive study is made on OL-MU-MIMO and open loop single-user MIMO (OL-SU-MIMO), with an emphasis on how OL-MU-MIMO exploits multi-user diversity to achieve high spectral efficiencies. We also derive the SINR and CQI expressions for such MU-MIMO systems, and provide simulation results which indicate that OL-MU-MIMO outperforms OL-SU-MIMO only when there are large number of users in the system.