Channel Hardening in Massive MIMO - A Measurement Based Analysis

Wireless-controlled robots, cars and other critical applications are in need of technologies that offer high reliability and low latency. Massive MIMO, Multiple-Input MultipleOutput, is a key technology for the upcoming 5G systems and is one part of the solution to increase the reliability. More specifically, when increasing the number of base station antennas in a massive MIMO systems the channel variations decrease and the so-called channel hardening effect appears, which makes the channel seem more deterministic. In this paper, channel hardening in massive MIMO systems is assessed based on an analysis of measurement data. For an indoor scenario, the channels are measured with a 128-port cylindrical array for nine single-antenna users. The analysis shows that in a real scenario a channel hardening of 3.2–4.6 dB, measured as a reduction of the standard deviation of the channel gain, can be expected depending on the amount of user interaction. Also, some practical implications and insights are presented.