Open vs Closed Access Femtocells in the Uplink

TL;DR

This paper analyzes open versus closed access femtocells in uplink cellular networks, revealing that the optimal access mode depends on the multiple access scheme and user density, with significant implications for network performance.

Contribution

It provides a mathematical and simulation-based comparison of open and closed access femtocells across different multiple access schemes, highlighting conditions favoring each mode.

Findings

Closed access preferable at high user densities in TDMA/OFDMA networks.

Open access can improve femtocell performance by over 200% in CDMA systems.

Access mode should be adapted based on multiple access scheme and user density.

Abstract

Femtocells are assuming an increasingly important role in the coverage and capacity of cellular networks. In contrast to existing cellular systems, femtocells are end-user deployed and controlled, randomly located, and rely on third party backhaul (e.g. DSL or cable modem). Femtocells can be configured to be either open access or closed access. Open access allows an arbitrary nearby cellular user to use the femtocell, whereas closed access restricts the use of the femtocell to users explicitly approved by the owner. Seemingly, the network operator would prefer an open access deployment since this provides an inexpensive way to expand their network capabilities, whereas the femtocell owner would prefer closed access, in order to keep the femtocell's capacity and backhaul to himself. We show mathematically and through simulations that the reality is more complicated for both parties, and that the best approach depends heavily on whether the multiple access scheme is orthogonal (TDMA or OFDMA, per subband) or non-orthogonal (CDMA). In a TDMA/OFDMA network, closed-access is typically preferable at high user densities, whereas in CDMA, open access can provide gains of more than 200% for the home user by reducing the near-far problem experienced by the femtocell. The results of this paper suggest that the interests of the femtocell owner and the network operator are more compatible than typically believed, and that CDMA femtocells should be configured for open access whereas OFDMA or TDMA femtocells should adapt to the cellular user density.