A investigation on the correlation-coefficient and power metrics for MIMO antennas in a reverberation chamber

Abstract

Many MIMO antenna specifying aspects have been investigated. These aspects include capacity, diversity gain, BER between multiple antennas, K-factor and their mutual relations in both a reverberation chamber and an anechoic chamber. In this presentation, the correlation-coefficient (ρCC) of the transmission coefficients will be compared to the complex transmission-coefficient squared (TCS) by investigating the behavior of the coefficient of variance (Cv) for the two measures. The Cv is calculated by normalizing the standard deviation by the mean for the different positions in the reverberation chamber. The transmission coefficients S13 and S23 are obtained from a three-port vector network analyzer measurement done between two monopole antennas (connected on ports 1 and 2, on a single ground-plane) and a brass-horn antenna (on port 3). Three sets of monopoles are at fixed separation distances on a ground plane (called fix separation parallel-monopole antenna (FSPA)), and are representative of good, bad, and average multiple-input multiple-output (MIMO) antenna's. Another antenna with an adjustable separation distance between the monopoles (called variable separation parallel-monopole antenna (VSPA)), allows variation of the MIMO antenna over a range from good to bad. For both types of MIMO antennas, the ρCC as function of the monopole separation follows the trend of the analytic model (Bessel-function). The ρCC is better (i.e., a smaller value) at larger separations as can be seen in Figure 1a and 1b. With increasing separation the Cv increases to values of approximately 0.4 to 0.5, which implies that the ρCC is becoming less accurate. However, if we look at the case were there are 6 typical RF absorbers-sections added for loading, the Cv of the TCS almost doubles while the Cv of the ρCC stays more-or-less the same. In conclusion, for a small separation between the monopoles, especially in a heavily loaded environment, the ρCC would likely exhibit less uncertainty as a performance metric for a MIMO system, than the TCS measurement.