Massive MIMO takes advantage of the reciprocity of the over-the-air (OTA) channel in TDD to reduce the overhead in channel estimation. It estimates the downlink channel based on the uplink channel obtained from SRS in uplink. For this to work, the Tx and Rx chains in the base station RU must be calibrated so that they are reciprocal.
This module is similar in function to our 8×8 MIMO SDR except this module has built-in circuit to perform the reciprocal calibration of the 8Tx and 8Rx chains. More importantly, the built-in calibration circuits can be chained for reciprocal calibration of a larger RU, e.g., a RU with 64Tx and 64Rx chains.