Calibration Techniques for Fully Parallel 24 × 24 MIMO Sounder

Multiple-input multiple-output (MIMO) systems have attracted lots of attentions, for their potential for high gain in channel capacity. For evaluating the performance of MIMO systems in more practical situations, more accurate channel models are necessary; and channel measurement is necessary to estimate the parameters of the channel models [1] [2]. In [2], we developed a fully parallel wideband MIMO channel sounding system at 11GHz with 400MHz bandwidth, which utilized software defined radio (SDR) architecture and is considered more suitable for real-time evaluation of link-level performance and channel properties, comparing with switching-based time division multiplexing (TDM) sounder, that has been commonly used [3]. However, one of the drawbacks of the fully parallel MIMO channel sounder system is that it needs complex procedures of baseband circuit adjustment, radio frequency (RF) circuit compensation, and back-to-back (B2B) calibration measurement, comparing with switching-based system; and the complexity increases with the numbers of transmit (Tx) and receive (Rx) channels. Since currently, the sounder system developed in [2] has been extended into 24 × 24 MIMO sounder, it becomes a very serious problem that it will take very long time to carry out the B2B calibration measurement for all possible MIMO connections. In this paper, automatic system calibration schemes are introduced and a simplified B2B calibration method is also proposed to shorten the measurement time.