Fundamental Limits of Ultra-dense Networks

Abstract

Mobile traffic has significantly increased over the last decade, mainly due to the stunning expansion of smart wireless devices and bandwidth-demanding applications. This trend is forecast to be maintained, especially with the deployment of fifth generation (5G) and beyond networks and machine-type communications. A major part of the mobile throughput growth during the past few years has been enabled by the so-called network densification, i.e. adding more base stations (BSs) and access points and exploiting spatial reuse of the spectrum. Emerging 5G cellular network deployments are envisaged to be heterogeneous and dense, primarily through the provisioning of small cells such as picocells and femtocells. Ultra-dense networks (UDNs) will remain among the most promising solutions to boost capacity and to enhance coverage with low-cost and power-efficient infrastructure in 5G networks. The underlying foundation of this expectation is the presumed linear capacity scaling with the number of small cells deployed in the network. In other words, doubling the number of BSs doubles the capacity the network supports in a given area and this can be done indefinitely. Nevertheless, in this context, several important questions arise: how close are we to fundamental limits of network densification? Can UDNs indefinitely bring higher overall data throughput gains in the network by just adding more infrastructure? If the capacity growth arrives to a plateau, what will cause this saturation and how the network should be optimized to push this saturation point further? These are the questions explored in this chapter. The performance of wireless networks relies critically on their spatial configuration upon which inter-node distances, fading characteristics, received signal power, and interference are dependent. Cellular networks have been traditionally modeled by placing the base stations on a regular grid (usually on a hexagonal lattice), with mobile users either randomly scattered or placed deterministically. Tractable analysis can sometimes be achieved for a fixed user location with a small number of interfering BSs and Monte Carlo simulations are usually performed for accurate performance evaluation. As cellular networks have become denser, they have also become increasingly irregular. This is particularly true for small cells, which are deployed opportunistically and in hotspots and dense heterogeneous networks (HetNets). As a result, the widely used deterministic grid model has started showing its limitations and cannot be used for general and