Downlink beamforming under EIRP constraint in WLAN OFDM systems

Range extension by means of downlink beamforming in a wireless local area network (WLAN) orthogonal frequency-division multiplexing (OFDM) system is addressed. A maximum ratio (MR) combining beamformer scaled according to the equivalent isotropic radiated power (EIRP) constraint is compared to the conventional total power (TP) restricted solution in typical propagation conditions. It is shown that the EIRP-scaled MR beamformer demonstrates significant performance degradation compared to the TP case. A joint optimization problem for the beamforming weights over all the sub-carriers subject to the EIRP constraint is formulated based on maximization of the minimum signal-to-noise ratio (SNR) at the receiver. A two-stage sub-optimal solution is proposed that exploits optimization over groups of non-adjacent sub-carriers and normalization to maintain the overall EIRP constraint. Its efficiency is illustrated in the IEEE 802.11 environment.