Downlink Beamforming in Multi-Antenna Two-Tier Networks with User Selection

We consider a two-tier cellular network with universal frequency reuse, consisting of a macrocell overlaid with shorter range, low power multi-antenna base stations. Each transmitter selects a set of users among all active users in its cell and serves them using multiuser linear precoding. The effect of user selection in throughput and spatial reuse is investigated and analytical expressions and bounds for the success probability, area spectral efficiency, and average user rate are derived. Our results indicate that downlink multiuser transmission shows throughput gains at both tiers as compared to single-user communications, if combined with scheduling, while the optimal number of served users per cell depends on different network parameters such as pathloss exponent, interferer density, and outage constraints. A multi-mode switching strategy, which instantaneously adapts the number of served users, is then proposed. Furthermore, we provide insights on efficient user selection metrics. Both analytical and numerical results show that zero-forcing precoding may increase coverage and spatial reuse by reducing both intra-tier and cross-tier interference through user scheduling.