Energy-efficient downlink transmission in two-tier network MIMO OFDMA networks

We propose an energy-efficient resource allocation scheme for downlink transmission in two-tier Network MIMO OFDMA-based macrocell-femtocell networks where the femto-cells form clusters of equal size. The proposed scheme uses a joint zero-forcing beamforming with semi-orthogonal user selection (ZFBF-SUS) transmission at each network tier to perform allocation of subcarrier and precoding coefficients. Then, power allocation is optimized in order to maximize the total system energy efficiency (i.e., average number of successfully transmitted bits per energy unit [bit/Joule], or equivalently, the average data rate per unit power [bps/Watt]). The macro base stations (MBSs) and the femto base stations (FBSs) in a cluster maximize their energy efficiency in a distributed manner while considering the cross-tier interference and the capacity limitations of backhaul links. The problem of maximizing energy efficiency is formulated as a fractional program and solved by using the Dinkelbach iterative algorithm. Numerical results show that the proposed scheme outperforms the scheme that maximizes the system average capacity, in terms of energy efficiency, and also improves the total system performance in terms of energy efficiency and average system capacity when compared to a single-tier system.