Delay spread properties in a measured massive MIMO system at 2.6 GHz

Massive multiple-input multiple-output (MIMO) systems, where the base station (BS) is equipped with a large number of antennas and the mobile devices have a single antenna, can significantly enhance the system performance. In many wireless systems inter symbol interference (ISI) due to delay dispersion of the channel can dramatically affect the demodulation process of the received signals. Precoding in massive MIMO can reduce or eliminate ISI, while still exploiting the spatial diversity. In this paper, we investigate how much standard linear precoders help the ISI mitigation by shortening the rms delay spread. In order to evaluate the system performance, we measured a massive MIMO channel with 128 antennas at the base station and 36 users, including 26 line-of-sight and 10 non line-of-sight, each equipped with a single antenna. We also compare the results from the measurements with an independent identically distributed Gaussian channel with an exponentially decaying average power delay profile.