# Fronthaul Design for Wireless Networks
## 4 Fronthaul Design Study Results
### Individual Links Assessment
From here, it could be concluded that although the presence of rain makes it harder to select one of the wireless technologies considered herein, if the fog intensity is low, then FSO systems are still a good cost-effective option if the required bit rate is low  
Figure 7: Cost-effective solutions for S(r_h/f_h). to medium (lower than 1 Gbps) and the link distance is small (below 2.5 km); on the other hand, even in the case of high rain intensity, MRT solutions should not be disregarded, since they are cost-effective for medium link distances (2.5 to 14 km) and small bit rate requirements (lower than 400 Mbps). Turning now the attention to the scenario characterized by low rain and high fog intensities, S(r_l/f_h), Figure 6 shows that FSO systems were practically excluded from the cost-effective solutions due to harsh visibility conditions; in particular, FSO links were only feasible for very small distances (up to 200 m). With respect to MRT systems, their cost-effectiveness was similar to the one observed for S(r_l/f_l) (cf. Figure 4). Another noteworthy result is that, when compared to the previously analyzed scenarios, it was the FO technology (and not the MRT one) that replaced FSO as the cost-effective solution for small link distances (below 2 km) and low to medium bit rate requirements (lower than 1 Gbps); this result reinforces the previous conclusion that the feasibility of a wireless link does not necessarily mean that it is a cost-effective solution, namely when considering small link distances. With respect to S(r_h/f_h), which is a scenario characterized by high rain and high fog intensities, Figure 7 shows that the cost-effectiveness of MRT systems was similar to the one observed for S(r_h/f_l) (cf. Figure 5), whereas the cost-effectiveness of FSO systems was similar to the one observed for S(r_l/f_h) (cf. Figure 6), thus yielding analogous conclusions, respectively. Combining all the findings gathered so far, a system designer, when planning an RRH-BBU link, can somewhat foresee, without making computations, which technology is more likely to be adopted; this is useful to decide, especially in the first stage of link planning, which equipment should be thoroughly surveyed on the market, thus saving time and money. A summary of the mapping between the pair link distance/bit rate requirement and the respective cost-effective solutions is given in Figure 8, which led to the following rules of thumb:  
* For short link distances (under 4 km) and small to medium bit rate requirements (lower than 1 Gbps), an FSO system is the best option, followed by an FO one if the former is unfeasible, notably in foggy scenarios (MRT systems can also be used as fallback);
* For short link distances (under 3 km), but for a higher bit rate requirement (greater than 1 Gbps), FO systems should be adopted, followed by FSO ones;
* For medium to high link distances (5 to 11 km) and small to medium bit rate requirements, as well as some high link distances (14 to 16 km) and lower bit rate requirements (lower than 400 Mbps), the first choice corresponds to the MRT technology, followed by FO if the former is unfeasible, notably in rainy scenarios;
* For medium to high link distances (4 to 14 km) and lower bit rate requirements, as well as some medium link distances (4 to 5 km) and medium bit rate requirements (lower than 1 Gbps), not only MRT systems are the best option, but they also show a high resilience even regarding scenarios with high rain intensity;
* For all the remaining cases, FO systems are the only option, which means that if these systems cannot be deployed, then the link becomes unfeasible (unless wireless relay stations are considered).