I. Introduction
Massive multiple-input multiple-output (MIMO) is widely acknowledged as a key enabling technology for the next generation mobile communication networks. Massive MIMO offers an increased spectral and energy efficiency as compared to regular MIMO [1], [2]. In massive MIMO, the base stations (BSs) are equipped with a very large number of antennas and serve many users simultaneously. The BSs' antenna arrays can be deployed in either a co-located or a distributed manner. In co-located deployment, the BS consists of a single antenna array, where all the antenna elements are in close proximity to each other. Differently, in distributed deployment, the antenna arrays belonging to a BS are not necessarily deployed in close proximity to each other [3], [4]. As such, the antenna arrays can be placed at arbitrary locations within a cell. We refer to this as distributed antenna array (DAA) massive MIMO.