The antenna correlation coefficient in wireless sensor networks

Diversity antennas are for improving communications and sensing systems. The concept has been around for well over half a century and overlaps with sensor array design. Diversity in communications has come to the fore because most of our radio links are now mobile, seldom having a line-of-sight but being rich in multipath where the signals from single port antennas are degraded. It was originally developed for the receiving case, but recent advances, driven by consumer demand for better performance, has seen diversity deployed at both ends, allowing multiple input, multiple output (MIMO) communications. With wireless sensor networks (WSNs) gaining broader interest - i.e., systems other than cellular and wifi networks, antenna diversity techniques take on a broader interest for improving WSN sensor node and network performance. The most important parameter in diversity design is the correlation coefficient, but this is not widely understood. We present an update which aims to clarify some common misconceptions. The various definitions of correlation coefficients for similar elements, are shown analytically to be the same and this is demonstrated for practical antennas for WSN nodes. Finally, it is clarified why the ensemble signal correlation coefficient, or its estimate, is often different from that commonly calculated in commercial antenna simulators.