Coverage in Multi-Antenna Two-Tier Networks

TL;DR

This paper analyzes how multi-antenna techniques can improve coverage in two-tier cellular networks with hotspots, addressing interference issues and proposing decentralized power control to enhance reliability and capacity.

Contribution

It derives the maximum number of femtocells that can operate simultaneously under outage constraints and compares single-user and multiuser antenna strategies for coverage improvement.

Findings

Single-user MIMO outperforms multiuser MIMO in coverage and spatial reuse.

Decentralized power control effectively manages interference at cell edges.

Approximately 60 femtocells per cellsite can be supported with reliable coverage.

Abstract

In two-tier networks -- comprising a conventional cellular network overlaid with shorter range hotspots (e.g. femtocells, distributed antennas, or wired relays) -- with universal frequency reuse, the near-far effect from cross-tier interference creates dead spots where reliable coverage cannot be guaranteed to users in either tier. Equipping the macrocell and femtocells with multiple antennas enhances robustness against the near-far problem. This work derives the maximum number of simultaneously transmitting multiple antenna femtocells meeting a per-tier outage probability constraint. Coverage dead zones are presented wherein cross-tier interference bottlenecks cellular and hotspot coverage. Two operating regimes are shown namely 1) a cellular-limited regime in which femtocell users experience unacceptable cross-tier interference and 2) a hotspot-limited regime wherein both femtocell users and cellular users are limited by hotspot interference. Our analysis accounts for the per-tier transmit powers, the number of transmit antennas (single antenna transmission being a special case) and terrestrial propagation such as the Rayleigh fading and the path loss exponents. Single-user (SU) multiple antenna transmission at each tier is shown to provide significantly superior coverage and spatial reuse relative to multiuser (MU) transmission. We propose a decentralized carrier-sensing approach to regulate femtocell transmission powers based on their location. Considering a worst-case cell-edge location, simulations using typical path loss scenarios show that our interference management strategy provides reliable cellular coverage with about 60 femtocells per cellsite.