Channel-Access-Aware User Association With Interference Coordination in Two-Tier Downlink Cellular Networks

The diverse transmit power levels of base stations (BSs) in a multitier cellular network, on the one hand, lead to an uneven distribution of traffic loads among different BSs when received signal power (RSP)-based user association is used. This causes underutilization of resources at low-power BSs. On the other hand, strong interference from high-power BSs affects downlink transmissions to the users associated with low-power BSs. In this context, this paper proposes a channel-access-aware (CAA) user association scheme that can simultaneously enhance the spectral efficiency (SE) of downlink transmission and achieve traffic load balancing among different BSs. The CAA scheme is a network-assisted user association scheme that requires traffic load information from different BSs, in addition to channel quality indicators. We develop a tractable mathematical model to derive the SE of the network and the SE of downlink transmission to a user who associates with a BS using the proposed CAA scheme considering almost blank subframe (ABS)-based interference coordination at a macro BS. The framework can model and analyze the individual traffic load distributions of different BSs in a two-tier network. The derived expressions provide approximate solutions of reasonable accuracy, compared with the results obtained from Monte Carlo simulations. Numerical results comparatively analyze the gains of the CAA scheme over conventional-RSP-based association and biased-RSP-based association. Based on this, important insights are extracted, which are related to the selection of the proportion of an ABS in various traffic load scenarios.