1. Introduction

Driven by the astounding development of smart phones, mobile applications and the Internet of Things (IoT), traffic demand grows exponentially in current mobile networks. Based on the recent statistics from Cisco, the global mobile data traffic has grown 18-fold over the past 5 years and is expected to increase sevenfold by 2021. Moreover, the number of mobile-connected devices, including smartphones, wearable devices, machine-to-machine modules, auto pilot cars, etc., is estimated to be 11.6 billion in 2021, which will be approximated to one and a half times of the world’s projected population at that time. The rapid proliferation of mobile devices, coupled with an abundance of new applications, will also bring a large variety of new service requirements. For instance, massive machine type communications require high connection density, videos require very high throughput per connection, auto pilot cars require low latency and ultra high reliability, augmented reality requires both high throughput and low latency, and so on.