Channel access-aware user association in two-tier cellular networks

The diverse transmit powers of the base-stations (BSs) in a multi-tier cellular network lead to uneven distribution of the traffic loads among different BSs and thus cause underutilization of the available resources at low power BSs. In this context, this paper proposes a channel access-aware (CAA) user association scheme that can simultaneously enhance the system spectral efficiency and balance the traffic loads among different BSs. The CAA scheme is a network-assisted user association scheme that requires the traffic load informations from different BSs in addition to the channel quality indicators. Also, in this paper, we develop a tractable mathematical framework to characterize the spectral efficiency of downlink transmission to a user who associates to a BS using CAA scheme. Numerical results demonstrate the performance gains of CAA scheme over conventional received signal power-based association and biased-received signal power-based association. The derived expressions provide approximate solutions of reasonable accuracy when compared to the results obtained by Monte-Carlo simulations. Moreover, the impact of state-of-the-art almost blank sub-frames (ABS)-based interference coordination scheme on the proposed CAA scheme is also investigated using Monte-Carlo simulations.