"Phase Measurements for Coherent Transmission in MIMO System with USRP X310"

"In MIMO transceiver systems, coherence between RF chains is essential to allow for experimental evaluation of advanced signal processing algorithms. Most software-defined radio (SDR) systems do not provide phase coherence and have no self calibrating feature to allow for coherent transmission\/reception of signals at RF level. To allow for coherent transmission with an digital array, the phase responses of all RF chains feeding the array have to be calibrated in real time. The measurement features the phase responses of 6 RF chains on 3 USRP X310 variants over a duration of 1000 seconds. It can be seen that the starting phase of each chain is random, and that substantial phase drift can occur during the initial warm-up period (~5 minutes). This necessitates compensation of the measured phase offsets in real-time to allow for coherence between the RF chains."