## 4 FWA Design Considerations, Challenges, and Solutions
### Beamforming Approach
#### 4.2.1 Digital Baseband Beamforming  
The digital baseband BF approach is significantly evolving and is being aided with the growing improvement in digital integrated circuits. The digital BF (DBF) scheme offers significantly high performance concerning the precision, bandwidth, number of beams, as well as output data rate usage. In relation to RF BF that is comparatively cheaper and demands lower power, DBF has to sacrifice a certain bandwidth and power for digitization and processing purposes. However, the digital circuit nature and the probability of embedding complex algorithms enable the system to be considerably flexible and viable for capacity enhancement [306; 307]. For instance, an all-digital BF massive MIMO scheme being employed for sub-6 GHz frequencies can be extended to mm-wave applications. Nevertheless, this approach presents design limitations regarding the required beam scanning range and power consumption [264]. Moreover, it has been observed that the DBF employs a number of low-resolution analog-to-digital converters (ADCs) that tend to high-power consumption [264]. Besides, this may become one of the possible technical bottlenecks at the Rx. In a scenario with a large AAS with massive bandwidth, there will be substantial concerns when an all-DBF solution is employed, as the associated power consumption will present design limitation [264]. This is because the array size is influenced by the scanning requirements, as well as the desired effective isotropic radiated power (EIRP). Note that EIRP is the product of the active channel numbers, intrinsic antenna element gains, per-channel transmit power, and BF gains [12]. In addition, note that the required scanning range is contingent on the specific deployment scenario. For instance, dense urban settings will require wide scan ranges both in azimuth and elevation planes. In the azimuth, a typical value of \(\sim\)120\({}^{\circ}\) is demanded, while \(\sim\)90\({}^{\circ}\) is the required usual value in the elevation. Consequently, the requirement for two-dimensional scanning in the environment is demanding. On the other hand, suburban deployment scenarios may demand a fixed or limited scan in the elevation (i.e., with a typical value of \(<\)20\({}^{\circ}\)) plane. This helps in exploiting higher intrinsic antenna element gain in this scenario. Consequently, suburban deployment scenarios could employ arrays with about 6 dB higher intrinsic antennas element gain than the urban scenarios. Therefore, a suburban phased array demands half the active channels required by the urban array, to realize the same EIRP. Note that the fact that suburban deployments demand a limited scan range to realize the equivalent EIRP helps in alleviating the associated power and cost constraints [12; 264]. Figure 17 presents an illustration of the required scanning ranges for both settings. Figure 17: Scanning range for different deployment scenario (**a**) suburban and (**b**) dense urban landscapes (adapted from [264]). Moreover, an all-digital approach will require some transceivers to realize BF gain and the target EIRP. This will be challenging regarding aggregate power consumption. Therefore, innovative technological solutions are required to address the drawback. The required margin can be achieved by the aids of gallium-nitride (GaN)-based Doherty Power Amplifiers (PAs) that are capable of maximizing the power-added efficiency for multicarrier applications. Compared with other efficiency enhancement methods like dynamic bias switching, envelope tracking, linear amplification with nonlinear components, as well as envelope elimination and restoration that demand additional control and/or pre-processing circuits, in principle, Doherty PAs do not require additional control circuitry for improving the system efficiency. Besides, they offer many benefits concerning cost-effectiveness, circuit complexity, and fabrication. Likewise, further linearization can be achieved using digital pre-distortion (DPD) [308; 309; 310; 311; 312].