Spatial separation of closely-spaced users in measured massive multi-user MIMO channels

Fully-synchronous measurements of a massive multi-user multiple-input multiple-output (MU-MIMO) radio propagation channel are presented. We evaluate the ability of a massive MIMO system to spatially separate users located close to each other in line-of-sight (LOS) propagation conditions. The system consists of a base-station (BS) antenna array equipped with 64 dual-polarized antenna elements (128 ports) arranged in a cylindrical configuration, and eight single-antenna users. The users are confined to a five-meter diameter circle and move randomly at pedestrian speeds. The BS antenna array is located on top of a 20 m tall building and has LOS to the users. We examine user separability by studying singular value spread of the MU-MIMO channel matrix for several subsets of BS antenna array ports, along with sum-rate capacity and achievable sum-rates with both zero-forcing and matched-filtering linear precoders. We also analyze the performance of the user with the lowest rate. Finally, a comparison between the performance offered by the massive MIMO system and that of a conventional MU-MIMO system is provided. To the best of our knowledge, this is the first report of fully-synchronous dynamic measurements of a massive MIMO system. Our investigation shows that even users located close to each other in LOS propagation conditions can be spatially separated in a massive MIMO system.

I. Introduction

Massive MIMO is an emerging communication technology promising order-of-magnitude improvements in data throughput, link reliability, range, and transmit-energy efficiency [1]–[4]. These benefits arise from leveraging additional degrees of freedom provided by an excess of antenna elements at the BS side. A typical massive MIMO system can consist of one or more BSs equipped with many, say, , antenna elements serving single-antenna users in the same time-frequency resource. is in the order of 10 to 20 users, possibly more. Due to its potential to greatly increase spectral efficiency compared to today's systems, massive MIMO is considered as one of the main directions towards future 5G communication systems [5]–[7].