Massive MIMO is considered a key technology for 5G. Various studies analyze the impact of the number of antennas, relying on channel properties only and assuming uniform antenna gains in very large arrays. In this paper, we investigate the impact of mutual coupling and edge effects on the gain pattern variation in the array. Our analysis focuses on the comparison of patch antennas versus dipoles, representative for the antennas typically used in massive MIMO experiments today. Through simulations and measurements, we show that the finite patch array has a lower gain pattern variation compared with a dipole array. The impact of a large gain pattern variation on the massive MIMO system is that not all antennas contribute equally for all users, and the effective number of antennas seen for a single user is reduced. We show that the effect of this at system level is a decreased rate for all users for the zero-forcing MIMO detector, up to 20% for the patch array and 35% for the dipole array. The maximum ratio combining on the other hand, introduces user unfairness.
Massive MIMO (MaMIMO) is a technology of primary interest for sub-6 GHz operation in the next generation cellular systems. While MaMIMO is most often linked to macrocell scenarios, where a single cell serves many users distributed over a large area, network densification will also result in scenarios where many users are served by a MaMIMO base station (BS) that is nearby. A key question is how to scale up MaMIMO: should we add more antennas to a given cell, or create multiple smaller and distributed cells that can cooperate? This paper documents the measured performance of a very dense MaMIMO system for an indoor-to-outdoor propagation environment. The impact of the number of antennas, and the distribution of the antenna elements is experimentally verified for a simplified linear deployment of the BSs. Concretely, we serve 12 closely located users with 16, 32 or 64 antennas. We compare a centrally positioned collocated array and two distributed arrays with their uplink throughput in a licensed 2.6 GHz band. The experimental results show that 12 users can be served with only 32 antennas for the distributed topology, which is effectively only 16 antennas per MaMIMO BS. For the specific case analyzed in our measurement campaign, with the centralized deployment, 64 antennas are needed to obtain good performance, while distributing the antenna elements in two sub-arrays improves total performance and fairness between the users.
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