# Millimetre-Wave Backhaul for 5G Networks: Challenges and Solutions
## 2 Millimetre-Wave Technology: State-of-the-Art and Trends  
The mm-wave technology has long been an attractive field for both academia and industry. Numerous research literature works have investigated the features of mm-wave channels and electronics, as well as sought for ways to fully utilize these features for achieving a higher data rate. Furthermore, standardization bodies devote great efforts to establish a norm for mm-wave communication applications. For example, European Computer Manufacturers Association (ECMA)-387, IEEE 802.15.3c, IEEE 802.11 ad and IEEE 802.11 aj are all existing 60-GHz industrial standards, among which IEEE 802.11 ad [5] is the most frequently- used one. The task group of the next generation of 60 GHz standards, IEEE 802.11 ay, has been recently formed, aiming to provide at least a 20-Gbps data rate. Accordingly, mm-wave products have begun to appear in the mass market, such as the 60-GHz chips manufactured by Qualcomm. Despite these supports, mm-wave bands have been deemed unattractive for cellular communications for a long period. The main concern is the physical barrier, including high path loss and other degradation losses, which confines mm-wave applications in an indoor environment. However, the works of New York University [6] demonstrated the feasibility of mm-wave transmission in the small cell of 4G communication under various urban environments at 28-GHz, 38-GHz and 73-GHz bands based on extensive measurement data. Similarly, the work [7] dispelled several myths on the 60 GHz band by measurements and simulations, showing the potential of mm-wave techniques in outdoor wireless networks. An overview of the above-mentioned results is summarized in Table 1. Clearly, the 28-GHz and 38-GHz bands suffer less total loss, but have a smaller bandwidth, while the 60-GHz and 73-GHz bands have larger loss, but the larger bandwidth and smaller antenna size may both contribute to compensate for this loss. Given the coverage, the effects of atmospheric and rain attenuation are small, compared to the path loss in mm-wave bands. Encouraged by these results, mm-wave communication has recently been highlighted as a promising, enabling and disruptive technology for the future 5G system. In February 2014, Samsung announced a mm-wave beamforming prototype, with a bandwidth in excess of 500 MHz at 28 GHz. Hybrid beamforming is adopted in the prototype, which can achieve a peak rate of 1.056 Gbps and guarantees more than 500 Mbps in an area with a-few-hundred-meter radius even in a non-line of sight (NLOS) environment [8]. In August 2014, Millimetre-Wave Small Cell Access and Backhauling (MiWaveS), a European collaborative project, was been released, the goal of which is to provide a gigabit data rate for 5G wireless communication in the frequency band of 60 GHz and the E-band [9]. A baseband modem for a mobile wireless backhaul has been introduced in [10]. In the survey and design of future 5G mm-wave backhaul, the large amount of previous databases and results may help as important references. Recently, there appeared numerical surveys and works that envisaged adopting mm-wave backhaul for 5G communications. [PERSON] _et al._[11] investigated joint scheduling of access and backhaul links in mm-wave small cells by considering device-to-device communications to further improve system performance. [PERSON] _et al._[12] proposed a promising mm-wave backhaul-based massive MIMO scheme for a 5G ultra-dense network. The proposed scheme enables multi-user MIMO with multiple streams for each user. [PERSON] _et al._[13] discussed the feasibility of adopting mm-wave wireless backhaul. Two business models using E-band and 60 GHz have been compared and analysed, as well as the choices for the backhaul transceiver have been discussed in terms of performance and cost. In [14], [PERSON] _et al._ pointed out several MAC and networking design challenges for mm-wave small cells.