# Performance Evaluation of Direct-Link Backhaul for UAV-Aided Emergency Networks
## 3 The Architecture of Backhaul for UABS in Emergency Networks
### Frequency Selection  
Backhaul frequency usage serving international mobile telephony (IMT) is divided into out-of-band or in-band, having different frequencies or sharing the same frequency band respectively. The first one is the most commonly used due to the versatility of the network and the independence between the access and the backhaul network, which leads to minimal interference [27]. On the other hand, the in-band perspective is included in LTE Release 10 as a relay mechanism to support evolved NodeB (eNBs) that have suffered from lost backhaul connectivity and should rely on a Donor eNB (DeNB) to have access to the CN [29]. In-band backhauling is an attractive option due to the fast reconfiguration time and low operational cost but imposes interference management and resource allocation challenges. [PERSON] et al. present a comparison of the different frequencies used in the backhaul and the benefits of using sub-6 GHz frequencies for UABS arrangements [27]. The authors conclude that licensed sub-6 GHz frequencies are the best compromise between coverage and capacity, allowing easy operation and management even in non-line-of-sight (NLoS) conditions. However, limited and expensive spectrum and the need for interference management are considered as significant drawbacks for traditional IMT sub-6 GHz bands.  
To overcome the issue of the limited spectrum, we proposed the use of the 3.5 GHz band, which is still not broadly occupied, as it is the case for the 800, 900, 1800, 1900 and 2600 MHz bands. In the range of the 3400 to 3600 MHz the band 22 is available for frequency division duplexing (FDD) services, the 42 or 43 bands for TDD and the n78 band for 5G New Radio (5G_NR) [30; 31]. The main reason for choosing this band is the ability to work with either an out-of-band or in-band configuration, as well as the low occupancy of this band, the licensed protection and the capacity-coverage trade-off [32]. Recent 5G trials have shown data rate peaks of 10.4 Gbps using 200 MHz of the 3.5 GHz band, proving to have the potential to fulfill backhauling requirements [33].  
Millimeter-wave frequencies have also been studied for 5G radio access networks, due to the ability to support large bandwidth (several GHz) but with high path loss attenuations. There are mainly two groups for mmWave frequencies studied for IMT: The 50 GHz group (45.5-47; 47.0-47.2; 47.2-50.2; 50.4-52.6) and the 80 GHz Group (66-76; 71-76; 81-86). However, most of them are studied for the access network. Some research is being done for fixed wireless access (FWA) and backhaul in the mmWave V-band. The 60 GHz band (57-66 GHz) is a promising candidate for backhauling, due to unlicensed or light-licensed operations, no spectrum shortage, high capacity, interference resistance, frequency reuse ability thanks to its narrow beam directivity and the small size of antennas that can be easily mounted in the UAV [34]. Recent trials in Hungary using 60 GHz for fixed wireless broadband have demonstrated that V-Band could support up to 1 Gbps connectivity [35].  
We proposed to use the 60 GHz band (57-66 GHz) band for the backhaul link because it supports 9 GHz of unlicensed bandwidth that could be used even with low spectrum efficiency to tolerate the high attenuation of this band and still provide the data rates needed in disaster scenarios.