An Analytical Framework for Evaluating Spectrum/Energy Efficiency of Heterogeneous Cellular Networks

Achieving high spectrum efficiency (SE) and energy efficiency (EE) is of primary importance for the sustainability of future cellular networks, but a key challenge is on balancing the tradeoff that arises when maximizing both of these performance metrics simultaneously. This paper develops a framework for analyzing the SE and EE of a two-tier heterogeneous cellular network consisting of macrocell and femtocell base stations (BSs) operating under a shared-spectrum scenario. It is shown that both the SE and the EE can be significantly enhanced with the overlaid deployment of the femto tier. However, the performance gain achievable is found to be strongly dependent on the load level and the BS power consumption attributes. A multiobjective optimization problem that maximizes the SE and the EE subject to quality-of-service (QoS) constraints is formulated and solved to give the Pareto-optimal operational regime. The novelty of this work is the quantification of the SE-EE tradeoff as a Lebesgue measure, which is defined by the Pareto-optimal regime. The developed framework is useful for studying the impact of the load on the SE-EE tradeoff, based on a strategy that exploits the varying load conditions to achieve good balance in the SE-EE tradeoff is formulated. Numerical results show that, while the improvement achieved in minimizing the SE-EE performance gap is marginal under high-load conditions, it is feasible to significantly increase the SE and the EE during low-load conditions and satisfy the users' QoS requirements by optimally adapting the density of the femto-tier BSs accessing the shared spectrum.