## 8 RAN Decomposition Scheme
### Fronthaul Interface  
The C-RANs development presents a new digital radio interface for connecting the RRHs to the BBUs. The arising connectivity segment is known as the fronthaul link [47]. Based on the function decomposition point, apart from the fronthaul, terms such as midhaul and backhaul links are widely used. Note that for seamless connectivity, the C-RAN architecture imposes strict requirements on the MFH link. The low-latency and high-speed digital links are responsible not only for the transmission of wireless digitized I/Q waveforms but also for conveying the control words that are intended for equipment control and management (C&M) purposes. The radio equipment (RE) manufacturers have defined CPRI and OBSAI specifications in which full interoperability cannot be guaranteed due to the associated vendor-specific elements. Additionally, the European Telecommunications Standards Institute (ETSI) presents ORI with the intention of having an interface specification for multi-vendor interoperability. Nonetheless, CPRI is the predominant interface that has been extensively adopted by the vendors in the C-RAN due to its efficient mapping techniques [47]. Based on this, we focus on the CPRI specification as a case study. The advent of CPRI specification facilitates efficient splitting of the radio BS into RE and radio equipment controller (REC). It helps in ensuring a simplified cellular BS architecture by offering a digitized and serial internal interface for connecting the REC and RE. In CPRI, signals are multiplexed in low-latency timeslot-like fashions in which the required maximum latency, as well as near zero-bit error rate and jitter, are specified. Furthermore,information like the C&M data, digitized radio signals, signaling, and synchronization demanded are defined in the CPRI specifications for various radio access technologies (RATs). However, the CPRI specification demands constant connections between the elements for continuous transmission of overhead signals such as pilot tones and cell broadcast information. Irrespective of whether there is an active user or not, these signals have to be transmitted. The achievable data rate for effective connection between the BBU and RRH depends mainly on factors such as the implemented RAT, number of sectors, antennas per sector, and radio frequency (RF) carrier bandwidth. Therefore, the CPRI data-rate for multi-antenna and multi-sector configurations can be defined as [29; 47; 365]  
\[\mathrm{R_{DCFRI}}=N_{s}\,M\,R_{s}\,N_{res}\,v\mathrm{C_{w}}\,C, \tag{13}\]  
where \(C_{w}\) represents factor of the CPRI codewords, \(N_{res}\) is the sample width (bits/sample), \(R_{s}\) represents the sampling used rate for digitization (sample/s/carrier), \(N_{s}\) denotes the number of sectors, \(M\) denotes the number of antennas per sector, \(C\) is the coding factor, and a factor \(v=2\) is employed to describe the complex nature of the IQ samples [47]. The required CPRI-based MFH transmission rate for supporting different RF bandwidths and antenna ports in the 5G wireless network, considering typical transmission parameters presented in Table 13 as specified by the 3 GPP, is depicted in Figure 26. It can be inferred that the required MFH transmission rate increases not only with an increase in the number of antennas per sector, but also with an increase in the RF bandwidth. For instance, the required transmission rate for a 20 MHz radio bandwidth and 16 antenna ports in a sector is 19.66 Gbps; this eventually increases to 78.64 Gbps for an 80 MHz of radio bandwidth with the same number of antenna ports and sector. Furthermore, in a scenario where more than one sector is considered, the required MFH transmission rate even increases significantly. For instance, as illustrated in Figure 27 when three sectors are considered for the aforementioned 80 GHz radio bandwidth, the required MFH transmission rate increases from 78.64 Gbps to 235.9 Gbps. These huge MFH bandwidths and the envisaged massive connections with subsequent increase in data