On the Coverage and Capacity of Ultra-Dense Networks With Directional Transmissions

We investigate the performance of a downlink ultra-dense network (UDN) with directional transmissions via stochastic geometry. Considering the dual-slope path loss model and sectored beamforming pattern, we derive the expressions and asymptotic characteristics of the coverage probability and constrained area spectrum efficiency (ASE). Several special scenarios, namely the physically feasible path loss model and adjustable beam pattern, are also analyzed. Although signal-to-interference-plus-noise ratio collapsing still exists when the path loss exponent in the near-field is no larger than 2, using strategies like beam pattern adaption, can avoid the decrease of the coverage probability and constrained ASE even when the base station density approaches infinity.

I. Introduction

Network densification, millimeter-wave (mmWave) communications and massive multiple-input multiple-output are regarded as the ‘big three’ promising key technologies in 5G mobile communication systems [1]. Performing directional transmissions in the mmWave band can help compensate for the strong path loss and reduce the inter-cell interference when moving towards ultra-dense networks (UDNs). Dual-slope path loss model shows good precision in mmWave UDNs [2], but it also brings difficulties in network performance analyses.