A combinational scheme of pre-FFT adaptive beamforming and frequency-domain adaptive loading for OFDM

We propose a transmission scheme combined with orthogonal frequency division multiplexing (OFDM), adaptive beamforming and adaptive loading. Two adaptive beamforming schemes based on the least mean square (LMS) algorithm and pre-FFT combining are used to obtain the space diversity gain. Their difference lies in either using the frequency-domain or time-domain pilot vector in the beamforming. The beamforming scheme based on the time-domain pilot vector has comparatively low complexity. In order to counter the severe frequency-selective fading which is inevitable for broadband wireless communications with or without an adaptive antenna array, we use frequency-domain adaptive loading to load the bits and transmit power to the individual subcarriers. The performance investigations show the combinational, scheme based on the adaptive beamforming, and adaptive loading is very effective in decreasing the bit error rate (BER) for OFDM.