  We present a reciprocity calibration method for dual-antenna repeaters in
wireless networks. The method uses bi-directional measurements between two
network nodes, A and B, where for each bi-directional measurement, the
repeaters are configured in different states. The nodes A and B could be two
access points in a distributed MIMO system, or they could be a base station and
a mobile user terminal, for example. From the calibration measurements, the
differences between the repeaters' forward and reverse gains are estimated. The
repeaters are then (re-)configured to compensate for these differences such
that the repeaters appear, transparently to the network, as reciprocal
components of the propagation environment, enabling reciprocity-based
beamforming in the network.
