Spectral Efficiency of Massive MIMO Systems With Multiple Sub-Arrays Antenna

In this paper, we study the achievable spectral efficiency (SE) for multi-user massive multiple-input multiple-output (MIMO) systems in a single cell, where the base station (BS) has the perfect channel state information and utilizes the maximum ratio transmission precoding. The channel model is built based on a finite dimensional mmWave channel by taking the azimuth angle into account and all users are assumed to have a random distribution in a circular-shaped cell. When the BS is deployed with a full uniform linear array (ULA) configuration, an analytical expression is derived by taking into account the expectation of the squared inner product between every two channel vectors that is viewed as inter-user interference. To effective suppress inter-user interference, we focus on studying the BS installing a multiple sub-arrays antenna configuration, in which all antennas are divided into three sub-ULA configurations to cover three regions of the cell, respectively. A closed-form expression on the inter-user interference is derived. Furthermore, we evaluate the performance of full ULA and three sub-ULA configurations and derive approximated expressions on the achievable SE. Numerical results reveal that the achievable SE of system increases with the number of antennas, while it approaches saturation in the high-SNR regime. Interestingly, it is also found that three sub-ULA configurations are able to suppressive user interference and their achievable SE is superior to full ULA configuration, which has a great potential of mmWave massive MIMO systems.