MISO in Ultra-Dense Networks: Balancing the Tradeoff between User and System Performance

TL;DR

This paper investigates the limitations of multi-antenna beamforming in ultra-dense small cell networks, revealing a fundamental tradeoff between user coverage and network throughput, and proposing a critical density concept to optimize system performance.

Contribution

It introduces the concept of critical density to balance user coverage and throughput, and quantifies how system parameters affect this tradeoff in ultra-dense networks.

Findings

Both coverage probability and spatial throughput diminish with increasing base station density.

Single-user beamforming increases the critical density, allowing denser networks without degrading performance.

Critical density is inversely proportional to the square of the antenna height difference.

Abstract

With over-deployed network infrastructures, network densification is shown to hinder the improvement of user experience and system performance. In this paper, we adopt multi-antenna techniques to overcome the bottleneck and investigate the performance of single-user beamforming, an effective method to enhance desired signal power, in small cell networks from the perspective of user coverage probability (CP) and network spatial throughput (ST). Pessimistically, it is proved that, even when multi-antenna techniques are applied, both CP and ST would be degraded and even asymptotically diminish to zero with the increasing base station (BS) density. Moreover, the results also reveal that the increase of ST is at the expense of the degradation of CP. Therefore, to balance the tradeoff between user and system performance, we further study the critical density, under which ST could be maximized under the CP constraint. Accordingly, the impact of key system parameters on critical density is quantified via the derived closed-form expression. Especially, the critical density is shown to be inversely proportional to the square of antenna height difference between BSs and users. Meanwhile, single-user beamforming, albeit incapable of improving CP and ST scaling laws, is shown to significantly increase the critical density, compared to the single-antenna regime.