## 3 Communication in the 60 Ghz Band  
The use of the 60 GHz frequency for wireless communication goes back to 2001 when the US regulator (FCC) adopted rules for unlicensed operations in the 57 to 64 GHz band for commercial and public use. Radios operating in the 60 GHz band have some unique characteristics that make them different to radios operating in the traditional 2.4/5 GHz bands. Using 60 Ghz leads to smaller sizes of RF components, enabling a more compact realization of an array structure, which in turn offers larger antenna gain with high directivity. Furthermore, oxygen absorption attenuates 60 GHz signals such that they cannot travel far beyond their intended path. These properties can also help reduce interference among terminals, enhance data security and, very importantly, enable the spectrum to be re-used in dense deployment scenarios. The 60 GHz band also allows very high data rate communication in applications such as video conferencing, media streaming and gaming.  
[PERSON] et al. in [12] stated that mmWave is suitable for backhaul links in ultra-dense wireless networks due to several unique properties: having a high capacity, being inexpensive and small form factor equipment, and having an immunity to interference. More recently, research has been conducted on the use of this technology to support densely meshed wireless backhauls as shown in Figure 2. The work in [13] studied the performance of a mmWave meshed backhaul deployment in a district of Barcelona. The links between nodes in the network were based on IEEE 802.11 ad technology at 60 GHz. The work demonstrated the viability of the technology over multiple hops and showed the  
Figure 1: Overview of Liverpool 5G Backhaul Deployment.  
influence the number of gateways (POPs) and the number of radio channels available has on performance.  
Further work in [14] addresses the challenges and the properties of mmWave communications for 5G to support the re-design of protocols and architectures; specifically, interference management and spatial re-use. For a high-level view of 5G fronthaul and backhaul wireless transport over mmWave the reader is referred to [15]. To further promote the development of mmWave communications, many other projects are currently evaluating this technology in the UK including the Liverpool 5G Testbed [16], the 5G Smart Tourism project [17] and the Worcestershire 5G Consortium Overview project [18], to name only three.  
IEEE 802.11 ad, also known as WiGig, is an enhancement to the 802.11 standard that enables multi-gigabit wireless communication in the 60 GHz unlicensed band [19]. The current IEEE 802.11 ac standard can support a maximum of 2.5 Gbps with three 160 MHz channels and 256-QAM data rate. In contrast, the 60 GHz band could provide a maximum throughput of up to 7 Gbps. However, utilizing this spectrum resource comes with challenging propagation characteristics such as strong attenuation from obstacles and huge path loss. Furthermore, due to severe penetration loss and reflection due to the short wavelength, mmWave communications are generally only feasible in line of sight (LoS) environments. To overcome these drawbacks, IEEE 802.11 ad provides a robust modulation and coding scheme and link adaptation mechanism to optimize throughput and minimize packet and bit error rate. In addition, the standard defines a directional communication scheme that takes advantage of beamforming antenna gain to cope with the increased attenuation in the 60 GHz band [20].