Uplink Capacity and Interference Avoidance for Two-Tier Femtocell Networks

TL;DR

This paper analyzes uplink capacity and proposes interference avoidance strategies for two-tier femtocell networks, demonstrating significant capacity improvements through time-hopping CDMA and sectorized antennas.

Contribution

It introduces an uplink capacity analysis framework and interference mitigation techniques for two-tier femtocell networks with universal frequency reuse.

Findings

Interference avoidance via time-hopped CDMA increases femtocell density by 7x.

Sectorized antennas improve uplink capacity and reduce interference.

Femtocell exclusion regions and tier selection policies offer modest capacity gains.

Abstract

Two-tier femtocell networks-- comprising a conventional macrocellular network plus embedded femtocell hotspots-- offer an economically viable solution to achieving high cellular user capacity and improved coverage. With universal frequency reuse and DS-CDMA transmission however, the ensuing cross-tier cochannel interference (CCI) causes unacceptable outage probability. This paper develops an uplink capacity analysis and interference avoidance strategy in such a two-tier CDMA network. We evaluate a network-wide area spectral efficiency metric called the \emph{operating contour (OC)} defined as the feasible combinations of the average number of active macrocell users and femtocell base stations (BS) per cell-site that satisfy a target outage constraint. The capacity analysis provides an accurate characterization of the uplink outage probability, accounting for power control, path-loss and shadowing effects. Considering worst case CCI at a corner femtocell, results reveal that interference avoidance through a time-hopped CDMA physical layer and sectorized antennas allows about a 7x higher femtocell density, relative to a split spectrum two-tier network with omnidirectional femtocell antennas. A femtocell exclusion region and a tier selection based handoff policy offers modest improvements in the OCs. These results provide guidelines for the design of robust shared spectrum two-tier networks.