Transmit Diversity Vs Beamforming for Multi-User OFDM Systems

In an Orthogonal Frequency Division Multiplexing (OFDM) system, downlink beamforming can be implemented either before IDFT (frequency domain) or after IDFT (time domain) module in a base station transmitter. We denote the former scheme as PreIDFT downlink beamforming, and the latter as Post-IDFT downlink beamforming. In this work, we have compared Pre-IDFT downlink beamforming with Space-Time Block coded and Space-Frequency block coded transmit diversity schemes for 4× 1 downlink multi-user OFDM (OFDM-TDMA and OFDMA) systems. The study is performed for indoor micro and pico cells and urban macro cells. Regardless of the multiple access scheme, it is found that beamforming always performs better in outdoor environment, where angular spread is lower, thus spatial correlation is higher. Similarly, indoor environment (high angular spread and low spatial correlation) suggests that transmit diversity schemes performs better than beamforming strategies. Keywords— Multi-user OFDM, Beamforming, Transmit Diversity