Analog combining of multiple receive antennas with OFDM

Optimal signal processing of M receiver antennas requires digitization of each of the M RF signals. For example, MRC (maximal ratio combining) of M receiver antennas requires M ADCs (analog to digital converters). Since the ADC is a highly power-consumptive component in current receiver chain architectures, we investigate suboptimal signal processing schemes that combine M analog signals before digitization, thus enabling a much cheaper implementation with a single ADC. We focus on IEEE 802.11a like OFDM LAN systems with one transmit and multiple receiver antennas (SIMO - single input multiple output), with perfect channel knowledge at the receiver. Our scheme uses scalar weights to combine the receiver antennas in the time-domain. These weights are a function of the spatio-temporal channel matrix. The results are very promising: 4-antenna linear combining (LC) outperforms 2-antenna selection diversity by 5-7 dB at an uncoded BER of 0.01, and this margin is even wider at lower BERs.