# Fronthaul Design for Wireless Networks
## 4 Fronthaul Design Study Results
### Network Assessment
#### 4.2.1 Rural Environment  
Figure 9 presents, for each of the considered test scenarios, the obtained results for the rural environment, namely the average total network costs along with the average technology usage percentage; the colocated percentage corresponds to the average percentage of RRHs that were colocated with the placed BBUs; thus, the respective RRH-BBU links did not make use of MRT, FSO, or FO individual links. Additionally, the average number of BBUs required for each test scenario is also indicated in this figure.  
Figure 8: Summary of the cost-effective solutions for individual link deployment.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Environment** & **RRH Density** & **Test Area** & **Total No. of RRHs** \\ \hline Rural & 0.015 RRHs/km\({}^{2}\) & 20 km \(\times\) 20 km & 6 \\ Urban & 0.3 RRHs/km\({}^{2}\) & 120 \\ \hline \hline \end{tabular}
\end{table}
Table 8: Characteristics of the test environments for network assessment.  
The results showed that even though the average number of BBUs required for a cost-effective network design was similar in all cases (ranging from 1.6 to 2.8), the distribution of technology usage regarding the RRH-BBU links clearly varied according to the considered scenario. Nevertheless, it is important to notice that, in all cases, cost-effective solutions were obtained (on average) when some BBUs were colocated with RRHs. Moreover, the FO technology was always adopted for RRH-BBUs links, and its usage percentage increased as the weather conditions deteriorated.  
As expected, the scenario characterized by low rain and low fog intensities, S(r_1/f_1), yielded the lowest average network costs, whereas the high rain and high fog intensities of S(r_h/f_h) led to the highest average network costs. However, these scenarios did not give rise to the lowest or highest average number of required BBUs, respectively, concerning all four test scenarios. This shows that the number of required BBUs on its own is not sufficient to have an estimate regarding the final network costs in rural environments.  
Another noteworthy result is that regardless of the rain intensity, MRT systems were always adopted for RRH-BBUs links, albeit with lower usage percentages when the rain intensity was high. On the contrary, an equivalent behavior was not observed by the FSO technology; more specifically, although this technology was the most used when the fog intensity was low, FSO systems were not adopted in the scenarios with high fog intensity. This particular outcome in the rural environment enables to highlight an advantage of considering the FSO technology when planning fronthaul networks: if the visibility conditions are favorable, then the use of FSO systems entails significant savings in the network cost; for instance, there was a cost reduction of 6% in rainy scenarios (cf. S(r_h/f_h) vs. S(r_h/f_1)), whereas a cost reduction of 13% was verified under non-rainy scenarios (cf. S(r_1/f_h) vs. S(r_1/f_1)). Furthermore and besides the cost savings, the use of FSO systems leads to an increase of the number of BBUs required for a cost-effective network, which may represent an advantage in future network expansions, especially in rural environments, as new RRHs have a higher likelihood of being served with lower costs by the existing BBUs.