## 2 Evolution of the Fronthaul Network Technologies in C-RAN Architecture  
The C-RAN architecture consists of a considerable number of distributed low-power and low-cost RRUs which are randomly deployed in the network. The RRUs consist of RF transceiver components such as power amplifier, duplexer, and low noise amplifier. Besides, the RRUs help in the conversion of baseband digital signal into RF signal and corresponding up-conversion to the required frequency of operation. Another component of the C-RAN is the centralized BBUs which are clustered as a BBU pool. It consists of high performance processors as well as real-time (RT) virtualization and management technologies [5]. In addition, the pool aggregates all computational resources and manages a huge number of RRUs located at different antenna sites. A significant advantage of the BBU pooling is that it supports dynamic allocation of the BBU resources to the RRUs to minimize the processing and power consumption. Consequently, it helps significantly in CAPEX reduction. The network interconnecting BBU pool and RRUs is referred to as fronthaul network. A variety of fronthaul technologies can be used in the C-RAN architecture, each with advantages and disadvantages. The main proposed solutions are:  
* Dark fiber [1; 5; 6] illustrated in Figure 1a: It has the advantages of using transmission equipment between BBU and RRU only and provides low latency and simple deployment; the disadvantage is the high use of fiber resources because point-to-point connections have to be established between BBU and RRU. * Passive WDM [1; 5] illustrated in Figure 1b: It uses passive optical multiplexer/demultiplexer to multiplex many wavelengths on an optical fiber. Its advantages are the fiber resource saving and the low power consumption. Its disadvantages are both the cost of colored interfaces in the BBU and RRU and the high accumulated insertion loss that limits the transmission distance. Furthermore, Operation, Administration and Maintenance (OAM) functions are not supported. * WDM/OTN [7] illustrated in Figure 1c: It allows for the time multiplexing of the CPRI signals on a same wavelength; the solution can save fiber resources, provide OAM functions as well as optical layer and electrical layer performance management and fault detection, provide network protection and ensure high service reliability. The optical bypass of multiplexed CPRI flows allows for low delay that can be even lowered with the introduction of a light OTN [8]. The solution does not need colored interfaces for the wireless equipment. The disadvantage is that the equipment cost is relatively higher and a low cost solution needs to be developed. * WDM/PON [9; 10; 13] illustrated in Figure 1d: This technology uses star networks, the fiber resources deployed on the Passive Optical Network (PON) network access layer can be used, and the equipment cost is low. * Ethernet [11; 12] illustrated in Figure 1e: This technology is based on the transport of CPRI flows in Ethernet circuits. the new specification eCPRI [14] has been defining some functional decomposition options and its scope is to enable efficient and flexible radio data transmission via a packet based fronthaul transport network like IP or Ethernet. When RRU switching off [2] and/or CPRI rate reconfigurability [15] techniques are used, this technology allows for resource saving due to the statistical multiplexing advantages. Unfortunately, it does not support the high-precision synchronization needed in CPRI technology and for this reason the IEEE has set up the 802.1 Time-Sensitive Networking (TSN) task group to study the latency-sensitive Ethernet forwarding technology and set up the 1914 Next Generation Fronthaul Interface (NGFI) working group to study CPRI over Ethernet and new Ethernet-based next generation fronthaul interfaces. Some interesting Ethernet-based solutions have been implemented in the Horizon2020 iCirrus project [12] but delay and jitter performance are not provided by the authors.