An Opportunistic Downlink Mimo-Ofdm Scheme

This paper presents an opportunistic multiple-input multiple-output (MIMO) scheme that achieves increased throughput of multi-user downlink transmission in frequency-selective channels. Developed for OFDM systems, the main idea of the proposed scheme is to find the optimal subset of users for each subchannel and optimal transmit beamforming vector which would maximize the sum-rate. Assuming perfectly known channel information, an optimal solution is derived and a suboptimal solution with much lower complexity that provides comparable results to the optimal solution is also presented. The proposed scheme outperforms the existing FDMA-based and SDMA-based schemes significantly in terms of sum-rate by efficiently exploiting the time-varying nature of radio channels and multi-user diversity