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

TL;DR

This paper proposes joint resource allocation schemes for energy-efficient downlink transmission in heterogeneous cellular networks, balancing spectrum underlay and overlay access to optimize energy efficiency under QoS constraints.

Contribution

It introduces a dual-layer optimization approach and a novel resource allocation method combining spectrum overlay access and resource efficiency concepts.

Findings

Proposed schemes approach optimal energy efficiency.

Underlay access can degrade EE in dense small-cell deployments.

Overlay access with resource efficiency improves EE performance.

Abstract

In this paper, we provide joint subcarrier assignment and power allocation schemes for quality-of-service (QoS)-constrained energy-efficiency (EE) optimization in the downlink of an orthogonal frequency division multiple access (OFDMA)-based two-tier heterogeneous cellular network (HCN). Considering underlay transmission, where spectrum-efficiency (SE) is fully exploited, the EE solution involves tackling a complex mixed-combinatorial and non-convex optimization problem. With appropriate decomposition of the original problem and leveraging on the quasi-concavity of the EE function, we propose a dual-layer resource allocation approach and provide a complete solution using difference-of-two-concave-functions approximation, successive convex approximation, and gradient-search methods. On the other hand, the inherent inter-tier interference from spectrum underlay access may degrade EE particularly under dense small-cell deployment and large bandwidth utilization. We therefore develop a novel resource allocation approach based on the concepts of spectrum overlay access and resource efficiency (RE) (normalized EE-SE trade-off). Specifically, the optimization procedure is separated in this case such that the macro-cell optimal RE and corresponding bandwidth is first determined, then the EE of small-cells utilizing the remaining spectrum is maximized. Simulation results confirm the theoretical findings and demonstrate that the proposed resource allocation schemes can approach the optimal EE with each strategy being superior under certain system settings.