Open, Closed, and Shared Access Femtocells in the Downlink

TL;DR

This paper analyzes access control strategies in femtocells, deriving SINR distributions and proposing a shared access scheme that balances the needs of home and cellular users based on their proximity to the macrocell base station.

Contribution

It provides analytical expressions for SINR and throughput in femtocells under different access policies and introduces a shared access method with an optimal time-slot allocation.

Findings

Shared access improves overall throughput and fairness.

Closed access favors home users; open access favors cellular users.

Shared access balances user requirements effectively.

Abstract

A fundamental choice in femtocell deployments is the set of users which are allowed to access each femtocell. Closed access restricts the set to specifically registered users, while open access allows any mobile subscriber to use any femtocell. Which one is preferable depends strongly on the distance between the macrocell base station (MBS) and femtocell. The main results of the paper are lemmas which provide expressions for the SINR distribution for various zones within a cell as a function of this MBS-femto distance. The average sum throughput (or any other SINR-based metric) of home users and cellular users under open and closed access can be readily determined from these expressions. We show that unlike in the uplink, the interests of home and cellular users are in conflict, with home users preferring closed access and cellular users preferring open access. The conflict is most pronounced for femtocells near the cell edge, when there are many cellular users and fewer femtocells. To mitigate this conflict, we propose a middle way which we term shared access in which femtocells allocate an adjustable number of time-slots between home and cellular users such that a specified minimum rate for each can be achieved. The optimal such sharing fraction is derived. Analysis shows that shared access achieves at least the overall throughput of open access while also satisfying rate requirements, while closed access fails for cellular users and open access fails for the home user.