  Benefitting from multi-user gain brought by multi-antenna techniques, space
division multiple access (SDMA) is capable of significantly enhancing spatial
throughput (ST) in wireless networks. Nevertheless, we show in this letter
that, even when SDMA is applied, ST would diminish to be zero in ultra-dense
networks (UDN), where small cell base stations (BSs) are fully densified. More
importantly, we compare the performance of SDMA, single-user beamforming
(SU-BF) (one user is served in each cell) and full SDMA (the number of served
users equals the number of equipped antennas). Surprisingly, it is shown that
SU-BF achieves the highest ST and critical density, beyond which ST starts to
degrade, in UDN. The results in this work could shed light on the fundamental
limitation of SDMA in UDN.
