Analysis of Spectrum Efficiency and Energy Efficiency of Heterogeneous Wireless Networks With Intra-/Inter-RAT Offloading

Offloading the users from capacity-strained macrocells in a cellular network to small cells is an effective strategy to support the increasing mobile data traffic, but a key challenge is how to simultaneously achieve high spectrum efficiency (SE) and energy efficiency (EE) in the heterogeneous radio access technology (RAT) environment. This paper develops an analytical framework for studying the performance of a two-RAT heterogeneous network (HetNet) comprising cellular and wireless local area network (WLAN) RATs. Using the developed framework, the feasibility of enhancing the SE and EE via the implementation of biased intra- and inter-RAT offloading techniques is investigated. Findings from the analysis reveal that the performance gain for SE and EE is strongly dependent on the load level and the base station (BS) power consumption attributes. A multiobjective optimization problem that maximizes the SE and EE subject to quality-of-service (QoS) constraints is formulated and solved to give the Pareto-optimal operational regime specified in terms of the small-cell BS densities and biasing factors. The novelty of this paper is the quantification of the SE-EE tradeoff as an opportunity cost measure, which is defined by the constrained Pareto-optimal regime. The insight gained from analyzing the opportunity cost is used to formulate a strategy that exploits the varying load conditions to achieve a good balance in the SE-EE tradeoff. Numerical results show that, although the potential to reduce the performance gap between SE and EE is marginal under low and high load conditions, it is feasible to significantly improve the network performance by balancing the SE-EE tradeoff during the medium load condition, as well as satisfy the users´ QoS requirements by optimally adapting the small-cell BS density and offloading biasing factors.