1. Introduction

According to the Cisco visual networking index (Cisco, Cisco visual networking index: Global mobile data traffic forecast update, 2016–2021. Technical report, Feb 2017), mobile data traffic has grown 18-fold over the past 5 years to reach 7.2 exabytes per month at the end of 2016, of which 4G Long-Term Evolution (LTE) traffic accounted for 69%. This trend has largely been driven by the increased use of smart phones and tablets as well as by the emergence of services such as internet of things and machine-to-machine communications. As the mobile connections continue the rapid growth, which is accompanied by the demand of high-definition video streaming, augmented reality, and virtual reality, a rethink of the current architecture of wireless communications systems has become more important than ever.