Transmitter and receiver calibration for obtaining the channel reciprocity for time division duplex MIMO systems

A communication device having a receive path comprising:

• a number M antennas and respective radio modules, wherein the M antennas and respective radio modules support N×N multiple-input multiple-output (MIMO) transmissions and receptions, wherein M≧N;
• a processor operable in a baseband domain and configured to perform a self-calibration procedure by which a correction factor is computed for a channel reciprocity level, determined by said self-calibration procedure;
• a plurality of switches in each one of said radio modules configured to change the receive path for regular MIMO operation or during said self-calibration procedure;
• a digital attenuator configured to adjust calibration signals in the baseband domain;
• and
• a low noise amplifier (LNA), wherein there are multiple receive paths and at least one path loss included in the receive path between one of the M antennas used for receiving and the LNA,
• wherein the M antennas are configured to alternate transmitting and receiving periods on a single channel according to a time division duplex (TDD) communication protocol, wherein the M antennas are configured to define phases employed in MIMO transmission and selected using information measured at the communication device during receiving periods based on the channel reciprocity level,
• wherein the digital attenuator is further configured to digitally attenuate the calibration signal by an amount that is sufficient to prevent the LNA in the path undergoing calibration from saturating,
• wherein the communication device is further operable to perform the self-calibration procedure in which one antenna/radio (i) acts as a transmitter and the other antenna/radio (j) acts as the receiver during the self-calibration procedure, the self-calibration procedure comprising:
• performing the following for each of two antennas/radios (i and j):
• emitting a first calibration signal with a signal strength equivalent to a signal strength used during regular MIMO operation, the receiver using one receive path with some path loss to prevent the LNA from saturation and recording a received calibration signal Ra—ji;
• transmitting a second calibration signal that is digitally attenuated, the receiver continually employing the same one receive path used in said emitting step and recording a received calibration signal Rb—ji;
• transmitting a third calibration signal that is digitally attenuated, the receiver switch selecting the receive path used for regular MIMO operation and recording a received calibration signal Rc—ji;
• and
• reversing roles of transmitter and receiver for the two selected antennas/radios to repeat said self-calibration procedure and recording the measured calibrated signals:
• Ra—ij, Rb—ij, and Rc—ij.