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In this paper, we provide joint subcarrier assignment and power allocationschemes for quality-of-service (QoS)-constrained energy-efficiency (EE)optimization in the downlink of an orthogonal frequency division multipleaccess (OFDMA)-based two-tier heterogeneous cellular network (HCN). Consideringunderlay transmission, where spectrum-efficiency (SE) is fully exploited, theEE solution involves tackling a complex mixed-combinatorial and non-convexoptimization problem. With appropriate decomposition of the original problemand leveraging on the quasi-concavity of the EE function, we propose adual-layer resource allocation approach and provide a complete solution usingdifference-of-two-concave-functions approximation, successive convexapproximation, and gradient-search methods. On the other hand, the inherentinter-tier interference from spectrum underlay access may degrade EEparticularly under dense small-cell deployment and large bandwidth utilization.We therefore develop a novel resource allocation approach based on the conceptsof spectrum overlay access and resource efficiency (RE) (normalized EE-SEtrade-off). Specifically, the optimization procedure is separated in this casesuch that the macro-cell optimal RE and corresponding bandwidth is firstdetermined, then the EE of small-cells utilizing the remaining spectrum ismaximized. Simulation results confirm the theoretical findings and demonstratethat the proposed resource allocation schemes can approach the optimal EE witheach strategy being superior under certain system settings.

Energy-Efficient Heterogeneous Cellular Networks with Spectrum Underlay and Overlay Access