## 4 FWA Design Considerations, Challenges, and Solutions
### Envisaged 5G Deployment Spectrum
#### 4.1.4 Penetration Loss due to Vegetation (Tree Foliage Density)  
The implementation of the mm-wave band brings about higher penetration loss while propagating through objects such as building walls, trees, as well as other vegetation [267; 290]. As depicted in Figure 14c, foliage medium presence along the communication link plays a substantial role in the achievable wireless communications QoS. It has been observed that tree parts such as leaves, branches, twigs, and trunks that are randomly distributed in the propagation environments can cause scattering, attenuation, absorption, and diffraction of the radiated waves [252]. These factors could have significant constraints on the wireless communication systems design. Consequently, there have been notable research efforts on the foliage effect on modern wireless communication systems such as high-speed PtP fixed link and cellular communication at mm-wave and microwave wave frequencies. In general, foliage-induced effects on the radio-wave propagation can be considered regarding the tree, forest, and a line or multiple lines of trees [291]. In addition, experimental and modeling results have shown that signal attenuation with foliage is significantly greater than without foliage. Furthermore, the attenuation level also depends on the foliage density and the operating frequency [267,292]. Nevertheless, due to the broad category of conditions such as snow, rain, wind, commonplace and moving objects, as well as foliage types, a generalized prediction method development is challenging. For instance, attenuation variations can occur because of the foliage movement in resonance to the wind [293]. Consequently, it is demanding to evaluate the sole influence of foliage in practice [294]. The probability of foliage-covered target recognition at mm-wave frequencies using a high-resolution, nadir-looking radar with a suitable model for facilitating data interpretation has been demonstrated [295,296,297]. The foliage production can also be modeled for a specific environment to help in the transmission process [298]. In addition, mitigation techniques such as spatial diversity, spatial multiplexing, depolarization diversity, or MIMO technique with foliage effect consideration can be employed to improve the link reliability [267,291]. Apart from the penetration losses, another important factor is the diffraction that reduces quickly at mm-wave frequencies. This brings about a reduction in the wave's ability to diffract around obstacles such as trees or structures at higher frequencies as usually experienced at lower frequencies. Moreover, diffuse scattering is another factor that can considerably affect wave propagation at higher frequency bands. For these reasons, mm-wave implementation is more suitable in situations where there is an LoS or close-to-LoS with barely small obstacles [95,267]. Note that the sub-6 GHz frequency spectrum can considerably aid in overcoming a number of challenges of mm-wave, especially ones that are a result of obstacles. However, it is limited due to the offered lower data rates [264]. In general, the fundamental effects of mm-wave implementation impairments are significant signal quality reduction and throughputs. This subsequently causes considerable poor QoE. As a result of this, for gigabit-wireless implementations, effective management of the envisaged high throughput density, as well as high path-loss at mm-wave frequencies, demands the adoption of innovative schemes. Cell densification is one of the technologies that are highly imperative for network efficiency and reliability. However, the smaller cell sizes required in the densification imply an increase in the infrastructure cost. The challenges can be alleviated with the implementation of antenna technologies such as active antenna systems (AAS) and massive MIMO that support BF techniques with advanced signal processing as illustrated in Figure 12. This will help in delivering the demanded capacity throughout the sectored cell coverage, resulting in appropriate indoor penetration.