# Millimetre-Wave Backhaul for 5G Networks: Challenges and Solutions
## 1 Introduction  
The commercial operation of 5G is expected to start globally by 2020. The new generation mobile communication is estimated to provide 1000-times more capacity and support 100-times more smart devices than current cellular networks [1]. Among all traffic demands, services requiring a high transmission rate, such as video streaming, are predicted to be the dominant service type according to the statistics obtained by Cisco. Inevitable dense deployment of mobile terminals and infrastructures makes a heterogeneous network (HetNet) a crucial solution to meet the capacity growth [2]. Cost-effective small cells are connected with macrocells in HetNet to extend coverage in crowded environments [3]. Densification of small cells produces massive backhaul traffic in the core network, which inevitably becomes an important, but somewhat less addressed bottleneck in the system. Employing fibre or cable in dense small cell backhaul would result in prohibitively high cost and practical difficulty in implementation [4]. Wireless backhaul may offer a scalable and cost-effective solution. However, traditional microwave frequency bands, e.g., sub-5 GHz, may be limited in achievable gains due to the existing spectrum crunch. Despite emerging mechanisms to enhance spectrum efficiency, it remainshard to achieve data rates exceeding 1 Gbps or even 10 Gbps. Therefore, it is difficult to meet the rapid increase of 5G traffic demands in such relatively low frequency bands. Moreover, interference becomes a limiting factor, especially in densely-populated HetNets. Millimetre-wave (mm-wave) techniques ranging from 30 to 300 GHz have become feasible and promising means to overcome the above-mentioned issues [5]. Currently, the three most potential mm-wave bands include 28 GHz, 60 GHz and the E-band, which are mostly unlicensed. With a huge available bandwidth, the gigabit data rate is practically achievable, which solves the capacity problem that exists in lower frequency backhaul systems. In mm-wave networks, directional links are commonly established to compensate for the high path loss. The use of highly steerable antennas greatly reduces interference, and high penetration losses due to walls and other obstacles also mitigates several interfering signals. Recently, the feasibility of mm-wave transmission within a range of a few hundred meters is proven by systematic outdoor measurements [6; 7], which offer sufficient coverage for small cell backhaul. Although mm-wave backhaul seems to offer a promising solution, increasing demands for 5G communication bring new challenges, which must be overcome in order to make mm-wave backhaul applicable to various scenarios. Traditional mm-wave techniques are mostly focused on direct single user link, but supporting multi-streams for multi-user scenarios is likely to be a trend for future 5G backhaul with growing traffic demands. Directional communication paves the way for spatial reuse, which should be fully exploited to improve overall backhaul performance. Moreover, high distance loss and blockage severely affect mm-wave network capacity, which requires suitable routing schemes to establish relaying links dynamically. In this article, we establish a novel framework for 5G mm-wave backhaul based on a general and tractable system architecture. The proposed framework combines promising physical layer techniques, such as hybrid beamforming and full-duplex transmission, with routing and scheduling schemes in higher layers. Specifically, we solve the problems of transmission path selection and time allocation in mm-wave backhaul by proposing an optimization model, and we carry out the corresponding algorithmic analysis. Extensive simulations with realistic assumptions have shown that new features of physical layer techniques are capable of bringing great performance enhancement, in terms of capacity and latency in a mm-wave backhaul network.