## 1 Introduction  
Notwithstanding the constant evolution of telecommunication networks and the new set of features appearing at each new release of the 3 GPP (3 rd Generation Partnership Project) standard specifications [1], mobile terminal users still do not experience the promised stable, seamless, and high-throughput connectivity when moving around. That is why the telecommunication ecosystem is on the verge of launching the fifth generation of telecommunication networks (5G), which will deploy a series of notable innovations that will implement the edge network paradigm to support very high traffic volume, higher data rate, lower latency, and increased reliability in both heterogeneous and dense scenarios [2].  
As such, 5G represents an evolution of more classical network paradigms with new demands on backhaul networks [3] that must be able to carry several heterogeneous radio access technologies to power data centers. Notably, the concept of Cloud-Radio Access Networks (C-RAN) employs optical backhaul for transfer of radio signals for direct processing in data centers, with minimal processing at base stations. Therefore, C-RAN represents one of the most demanding use cases for backhauling in 5G, and is the focus of our work in this paper.  
Cheaper radio equipment composed solely by array of antennas, digital to analog converters, and usually an optical wired interface, also known as Remote Radio Head (RRH), will gradually replace current radio base stations in C-RAN. The RRHs have the ability to upload all their sampled signal workload to be processed at cloud data-centers, also known as Baseband Unit (BBU) pools. Far from the edge of the network, BBU pools centralize the processing of workload, thus allowing for several benefits such as better energy consumption, simpler synchronization, and less complicated implementations of recent features, e.g. coordinated multipoint (CoMP). Considering 5G and C-RAN literature, some authors make a clear distinction between the network interconnecting BBUs and RRHs, calling it fronthaul, and the network interconnecting BBUs and the core network, calling it backhaul, whereas other authors merge the fronthaul and the backhaul terms [4]. In this paper, we adopt this second terminology, considering the entire optical network that connects the C-RAN elements as the backhaul network.  
A C-RAN approach to backhaul design is hampered by four main challenges:  
* High link capacity demands: The workload transmitted from RRH to BBU requires a massive quantity of data to be represented, i.e. samples at rates from 0.6-24.3 Gbps for each RRH antenna using the Common Public Radio Interface (CPRI) protocol or even higher when considering its successor eCPRI (enhanced CPRI) [5].
* Scalability limitations: C-RAN systems are envisioned to accommodate the workload of high density RANs in BBU pools, ranging from 100 to 1000 RRHs, which demands intricate approaches to designing and managing a large number of data streams and processing points.
* Strict latency requirements: The workload of RRHs must be processed in time for any responses to be transmitted from the RRH, in accordance with the radio protocol definition [6]. For example, given the processing of the Hybrid Automatic Repeat reQuest (HARQ) protocol, the RRHs workload must be processed within 3 ms to meet protocol requirements [7].
* Survivability: C-RAN systems must provide guarantees about the degree to which operation can remain unaltered during element failure or unexpected events; that is, C-RAN must offer some level of survivability [8].