Wideband distributed transmit beamforming using channel reciprocity and relative calibration

This paper describes a technique for transmit beam- forming from the nodes in a distributed radio network to a distant target node across a frequency-selective channel. The approach exploits signals transmitted by the target node and channel reciprocity to avoid requiring explicit channel state feedback from the target. Channel estimates between network radios are used for relative calibration to address non-reciprocal effects due to independent clocks and electronic component variability. Variants of the technique allow wideband coherent beamforming to the target when the target signal is known or when it is unknown.