# Fronthaul Design for Wireless Networks
## 2 Background on C-RAN and Fronthaul Communication Technologies and Related Work
### Related Work  
Since the fronthaul optimization can lead to a large decrease of CAPEX and OPEX of the C-RAN, it is of utmost importance to study and evaluate different approaches regarding the fronthaul architecture. The main objective of this paper is to evaluate and compare the use of MRT, FSO, and FO systems in the fronthaul, namely to study their influence in terms of cost-effective solutions for RRH-BBU links. \begin{table}
\begin{tabular}{c c c} \hline \hline  & **MRT** & **FSO** & **FO** \\ \hline
**Type** & Wireless (requires line-of-sight) & Wired \\ \hline
**Capacity** & Up to several Gbps & Up to several dozen Gbps & Up to several hundred Gbps \\ \hline
**Installation** & Easy to deploy and suitable for any terrain & Unsuitable for difficult terrain (e.g., mountains) \\ \hline
**Climate influence** & Notably rain & Notably fog & Not influenced by climate (except floods) \\ \hline
**Other** & MRT maximum range per hop is higher than FSO & Installation is usually more expensive \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparison of typical fronthaul communication technologies. There are few works, available in the related literature, that tackle topics similar to the one of this paper. The authors of [20] analyzed operational and deployment costs of C-RANs and proposed a mathematical formulation for fronthaul planning, taking into account the aforementioned costs. Solutions were also presented regarding their optimization problem, which aim at minimizing the C-RAN costs subject to traffic demand constraints. Unfortunately, this work was limited in the sense that it considers only a single technology (namely FO) for the fronthaul. In [21], different fronthaul technologies were discussed, including MRT, FSO, and FO, and potential solutions were suggested in order to achieve an efficient C-RAN. Nonetheless, the authors only provided a qualitative costs comparison of the different fronthaul technologies, without considering any specific scenario, as they focused their study on optical technologies (FSO and FO) in order to present feasible means of reducing the system complexity, costs, bandwidth requirement, and latency in the fronthaul. A comparative costs study regarding the technologies addressed herein (MRT, FSO, and FO) along with networked flying platforms providing FSO links (\"vertical FSO\") was performed in [22] for a fronthaul/backhaul scenario. However, only a very high dense deployment of mobile cells was considered, and the technologies were compared in terms of the whole network costs when a single technology was adopted. Nevertheless, it can be concluded from their work that although the \"vertical FSO\" was a valid approach, it was a very expensive option--in fact, not only \"vertical FSO\" was shown to be the most expensive option, but also the associated costs were estimated to be more than twice the second most expensive solution; furthermore, its implementation posed additional problems, including safety and regulatory ones. For these reasons, the \"vertical FSO\" approach was not considered in our study. Recently, other authors addressed rural connectivity and investigated the costs of solutions for the backhaul segment [23], namely by considering MRT, FSO (including the \"vertical FSO\" approach), and FO, in addition to satellite. In this case, the studied scenario is very specific--remote rural areas where the backhaul link needs to traverse long distances (e.g., 100 km); hence, its results cannot be generalized to the more common fronthaul scenario of RRH-BBU links separated by a few kilometers.