LTE-U WiFi HetNets: Enabling Spectrum Sharing for 5G/Beyond 5G Systems

Traffic growth is anticipated to be 1000 times in future fifth generation (5G) networks, which necessitates dense deployment of small cells in a heterogeneous environment. Currently, heterogeneous networks (HetNets) are being considered as the most promising solution to improve coverage and capacity in both outdoor and indoor environments. However, to reap the benefits of HetNets, efficient spectrum sharing techniques are inevitable due to the scarcity of spectral resources. Traditionally, WiFi (2.4/5.0 GHz unlicensed spectrum) has been used to offload macrocells employing licensed bands in cellular networks. However, with the advent of Long Term Evolution in the unlicensed spectrum (LTE-U), offloading cellular networks has been more efficient. In this article, we describe LTE-U WiFi HetNet architecture along with deployment scenarios in detail. We outline the technical challenges that hinder the effective utilization of unlicensed bands in LTE-U WiFi HetNets. The primary challenge is to design an efficient spectrum sharing mechanism for the coexistence of different radio access technologies (i.e., LTE-U and WiFi). Continuous interference from LTE-U to WiFi results in starved WiFi users. We discuss potential solutions to this problem, and present a case study for a joint user association and power allocation method for LTE-U WiFi HetNets with the objective to maximize the sum rate.