# Fixed Wireless Access in Flexible Environment: Problem Definition and Feasibility Check
## 6 Radio Throughput Simulations and an Experimental Link  
For throughput estimation, it is assumed that advanced microwave and millimeter-wave digital radio links using adaptive modulation are deployed [43; 44; 45]. In case of strong fading events, adaptive modulation radios automatically switch to simpler modulation formats. Simpler modulation modes transmit fewer bits per symbol, making them less sensitive to attenuation. Thus, the radio connection remains operational, and transmission is maintained [6]. However, when fewer symbols are transmitted, the digital throughput is reduced to lower bitrates [7; 46]. The main factors causing fading in the radio link are intensive rainfall, atmospheric attenuation, and obstacle losses [47; 48; 49; 50]. When FWA links are designed, it is not sufficient to know only the maximum capacity of the radio equipment from the technical specifications, it is also necessary to determine the expected link throughput and its availability taking the fading events into account. The applicability of the different modulation modes (resulting in different link throughputs) can be estimated using local meteorological data for the rain intensities and atmospheric attenuation data from ITU-R recommendations [35; 47]. Starting with the system gain values (equipment specific for each modulation mode available in the selected RF channel) it is possible to determine the link Fading Margin (FM) values for each modulation mode [11]. The radio link operates at a given modulation mode and throughput, as long as its \(FM_{\text{n-mod}}\) is higher than the sum of the link losses [7; 14]:  
\[FM_{\text{n-mod}}^{\text{(dB)}}>A_{\text{rain}}^{\text{(dB)}}+A_{\text{atm}}^{ \text{(dB)}}+A_{\text{obst}}^{\text{(dB)}}, \tag{11}\]  
where  
\[n\text{-mod}\in\{\text{BPSK}\ \mathbb{1}\text{\text{\text{\text{\text{\text{ \text{\text{\text{\text{\text  
method is general, and applicable for other digital radios too, e.g., [39]). A 2 GHz RF bandwidth is selected (other bandwidth options are also supported, e.g., 62.5, 125, 250, 500, 750, 1000, 1250 and 1500 MHz). With the 2 GHz bandwidth, modulation modes range from BPSK % to 128-QAM, yielding throughputs from 350 Mbps to 9.3 Gbps. 2. Availability for each throughput is calculated iteratively. For a given antenna pair and link length, the simulation begins with the most advanced modulation mode (128-QAM in this example). Using the ITU-R P.837 defined \(R_{0.01\%}\), the value \(A_{0.01\%}\) (rain attenuation exceeded 0.01% of the time) is calculated [35]. ITU-R P.530 [51] extrapolates this to calculate \(A_{p\%}\) (rain attenuation exceeded \(p\%\) of the time). For _E_-band, atmospheric attenuation is not significant (\(A_{\rm atm}\) = 0.36 dB/km used). If the FM of the radio link (with selected antenna pair and distance) exceeds these losses, the link operates for more than (100-\(p\)) % of the time. The availability of the radio link with these antenna size, length, modulation mode parameters is found iteratively, where FM becomes equal to calculated losses. 3. To simulate the switchback to the subsequent simpler modulation mode (if not already at BPSK %), the calculation continues using the \(p\%\) value from Step 2. Step 2 is repeated by applying the same iterative method with simpler modulation mode to determine its availability. 4. Repeating this iterative method yields a throughput-availability series for an antenna pair at a specific radio link length. (e.g., the yellow dot series for a 65 cm antenna pair in Figures 1-3). Repeating the above steps for all applicable antenna pairs produces Figures 1-3. Nokia _E_-band Wavence radio outdoor units feature fiber-optical ports. With single polarization and a single RF channel, these units support baseband throughputs up to 10 Gbps (see Figure 7) [18]. Throughputs exceeding 10 Gbps are achievable by doubling the radio transceivers behind the antenna enabling cross-polarization or RF channel aggregation. The experimental verification is based on long-term radio hop measurements.