Widely Distributed Massive MIMO Communication Systems for High Mobility Users

Massive multiple input multiple output (MIMO) systems utilize tens to hundreds of base station (BS) antennas to exploit diversity in space, and serve several users simultaneously in the same time-frequency resource, thus achieving unprecedented levels of spectral efficiency, energy efficiency, and reliability. For this reason, they became the method of choice in the current roll-out of 5G deployments. However, a stringent requirement for massive MIMO systems is accurate and timely channel state information (CSI) from all users to all BS antennas. Currently, the only viable and scalable solution is the application of a time division duplex (TDD) scheme and relying on channel reciprocity. In high mobility scenarios, this approach inherently causes service degradation, as outdated CSI introduces inter-user interference and a reduced signal to noise ratio (SNR).
In this thesis, we investigate, both analytically and numerically, the adverse effects of users with high mobility on the reliability in a massive MIMO system, propose mitigation strategies, and evaluate their effectiveness. We consider (i) prediction to increase the quality of outdated CSI estimates; (ii) orthogonal precoding (OP) to further exploit spatial diversity; and (iii) distributed BS antenna geometries.
More specifically, we derive the instantaneous and asymptotic signal to interfer- ence and noise ratio (SINR) of a TDD massive MIMO system for a time-correlated fading process utilizing multi-step minimum mean square error (MMSE) CSI predic- tion and quantify the improvement over the utilization of aged CSI. We additionally prove and show by numerical simulation that the capability of massive MIMO sys- tems to reduce small-scale fading, i.e., the variance of the SINR, is independent of the CSI age. Further, we show the beneficial impact of OP in combination with CSI prediction on the reliability of a massive MIMO system by numerical link-level bit error rate (BER) simulation.
To substantiate the analytical and numerical findings, we develop a software- defined radio (SDR) based channel sounding framework. It has the capability to capture time-variant wireless channel impulse responses from two users to 32 BS antennas in highly dynamic scenarios in a fully parallel and time-synchronized manner. Moreover, the sounding framework is designed to support flexible BS antenna placements with an aperture of up to 90 m to characterize widely distributed (i.e., cell-free) setups. We conduct two vehicular measurement campaigns with highly mobile users in urban scenarios with BS antenna apertures ranging from 1 m to 50 m. The empiric evidence suggests that a widely distributed massive MIMO system, with BS antennas distributed over a wide aperture, mitigates the effects of aged CSI and reduces random fluctuations in signal strength, thus increasing reliability and energy efficiency.