  In this paper we present the first comprehensive study of the multi-user
capacity of millimeter-wave (mm-wave) urban cellular networks, using
site-specific ray-tracing propagation data and realistic antenna array
patterns. We compare the performance of TDMA and SDMA (time and spatial
division multiple access, respectively) for diverse network scenarios and
antenna configurations. We propose a greedy heuristic algorithm to solve the
network-wide directional link allocation problem, thereby estimating the
achievable capacity and coverage of multi-user mm-wave networks. Our results
show that inter-cell interference is negligible, so that TDMA performance is
strictly limited by air-time sharing. By contrast, the major limiting factor
for SDMA is intra-cell interference, emphasizing the impact of real antenna
array sidelobes. Nonetheless, SDMA significantly outperforms TDMA in terms of
average UE throughput, by up to 2 Gbps using 8x8 arrays. As an important design
insight, our results show that larger base station antenna arrays limit
intra-cell interference while compensating for small UE arrays, reducing costs
and beamforming requirements in practical SDMA networks. Our analysis also
shows that the limited number of antenna sub-arrays in a practical hybrid
beamforming architecture may force SDMA to drop UEs with good coverage,
highlighting a tradeoff between base station densification and antenna
resources.
