MIMO Broadcast Channels with Spatial Heterogeneity

We develop a realistic model for multiple-input multiple-output (MIMO) broadcast channels, where each randomly located user's average SNR depends on its distance from the transmitter. With perfect channel state information at the transmitter (CSIT), the average sum capacity is proven to scale for many users like αM/2 log K instead of M log log K, where α, M, and K denote the path loss exponent, the number of transmit antennas, and the number of users in a cell. With only partial CSIT, the sum capacity at high SNR eventually saturates due to interference, and the saturation value scales for large B like MB/M-1, where B denotes the quantization resolution for channel feedback.