# Fixed Wireless Access in Flexible Environment: Problem Definition and Feasibility Check
## 4 Feasibility Check  
The feasibility check comprises three main steps. Step 1. Identify the feasible neighborhood, i.e., the potential connection points that are technically viable. Details are provided below. Let \(A_{\rm max}\) represent the highest availability, \(L_{\rm min}\) the lowest latency, and \(TP_{\rm max}\) the maximum throughput required at the PL by any service traffic. Formally:  
\[A_{\rm max}=\max\{A_{\rm PL}(st_{\rm i},t_{\rm j})\,|\,\text{for all $st_{\rm i}$ and for all $t_{\rm j}$}\}, \tag{3}\]  
\[L_{\rm min}=\min\{L_{\rm PL}(st_{\rm i},t_{\rm j})\,|\,\text{for all $st_{\rm i}$ and for all $t_{\rm j}$}\}, \tag{4}\]  
\[TP_{\rm ULmax}=\max\Bigl{\{}\sum_{k=1}^{m}GBR_{\rm UL}(\rm k,j)\Bigr{\}}\Bigl{|} \text{for all $t_{j}$}\Bigr{\}}, \tag{5}\]  
\[TP_{\rm DLmax}=\max\Bigl{\{}\sum_{k=1}^{m}GBR_{\rm DL}(\rm k,j)\Bigr{\}}\Bigl{|} \text{for all $t_{j}$}\Bigr{\}}, \tag{6}\]  
\[TP_{\rm max}=\max(TP_{\rm DLmax},TP_{\rm ULmax}). \tag{7}\]  
Note that (7) assumes a symmetric connection for the new radio link (i.e., equal uplink and downlink bandwidths), as typically mandated by local or national regulators (e.g., based on ETSI [38]). Therefore, the required maximum capacity is the maximum of the overall uplink and downlink GBR parameters at any given time. Since these represent the extreme values for e2e connectivity, the radio link itself must offer superior performance (higher availability, lower latency). These values serve as an upper limit for the length of feasible radio links. A link throughput estimation \(LT_{\rm est}\) can be calculated for any link length based on the parameters of the available antennas and the rain intensity statistics of the planned location, as shown in Figures 1-3. \(LT_{\text{est}}\) is a function that determines the maximum throughput, \(LT_{\text{est}}(d,A)\), achievable for a distance \(d\) with availability \(A\). The maximum length, \(d_{\text{max}}\), where the link throughput estimation meets the maximum availability and throughput requirement is defined as:  
\[LT_{\text{est}}(d_{\text{max}},A_{\text{max}})=TP_{\text{max}}. \tag{8}\]  
Details of link throughput estimation are provided in Section 6, including throughput simulations for various antenna size combinations (i.e., TX and RX), modulation modes, availability results, and experimental results. Figures 1-3 illustrate this; the red dot represents the availability and throughput requirements. In Figure 1 (\(d\) = 2 km), the red dot is beyond the capabilities of the 2 km long radio link; therefore, a 2 km radio link at PL cannot meet availability and throughput requirements. In Figure 2 (\(d\) = 1.5 km), the red dot is at the limit of the 1.5 km long radio link's capabilities. While this link fulfills the requirements, it is unlikely to be part of a feasible e2e solution, because the requirements are e2e requirements. (However, if the \(pCP\) is the \(pCT\) for all services, the 1.5 km radio link should be considered. See Figure 4, which simplifies to a single connection target,  
Figure 1: Link throughput estimation, \(LT_{\text{est}}\), for 2 km radio hop length. Figure 2: Link throughput estimation, \(LT_{\text{est}}\), for 1.5 km radio hop length. but any \(pCP\) could host a service and thus could be a \(pCT\).) In Figure 3 (\(d=1\) km), the red dot falls within the capabilities of the \(1\) km link, exceeding the requirements and making it easily part of a feasible e2e solution. Figure 3 also shows that different antenna pairs (e.g., 1- or 2-foot diameters) can satisfy the requirements; larger antennas may provide higher throughput. Besides combined availability/throughput limitations, more relaxed criteria (e.g., latency only) can be used if a single connection point cannot meet all service connectivity requirements. Figure 4 illustrates these limitations, the darker grey area shows where both availability and throughput requirements can be met for all services. The lighter grey area represents where latency requirements are met. Obstacles in the LoS further reduce these feasible regions.