In contrast, ORI members include several network operators. All the three interfaces support digitized data. Among the three, CPRI is dominating and by-default, followed by the fronthaul interface. Sometimes the fronthaul interface is also called a CPRI interface.

In [39], an overview of CPRI and its application in the LTE scenario is well explained. A CPRI can support many topology such as star, tree, chain, ring, and multi-hop option. It can support different radio standards such as 3GPP UTRA FDD, 3GPP GSM/EDGE, and WiMAX apart from 3GPP EUTRA (LTE), which is our point of concern. CPRI requires strict synchronization and time accuracy between RRH and BBU. It supports an operating range of at least 10 km and a maximum of 40 km. Apart from the BW requirement, CPRI needs a stringent delay, jitter, and BER requirement. It allows the maximum of $200 \mu s$ delay for round trip transmission. The CPRI link can contribute maximum $5 \mu s$ delay, excluding the propagation delay. The maximum allowed frequency deviation from the link to the base station is of 0.002 ppm.

CPRI supports digital data, and it follows either Time Division Multiplexing (TDD) or Frequency Division Multiplexing (FDD). TDD is widely used in CPRI, and it has a hierarchical frame structure, as shown in Figure. 3. An LTE frame structure, also known as the CPRI frame, starts from a Basic Frame (BF) of variable size. BF allows transmitting user plane data, control, management plane data, synchronization, and timing data in each frame. It consists of 16 words from which 15 words carry user plane data and the starting word ($W_{0}$ ) of a frame carry control, management, and synchronization data. Each word length of a BF depends on the number of bits required per I and Q sample and the number of antenna-carrier (A$\times$C). A$\times$C depends on the MIMO scheme and the number of antenna sectoring used. Basic frame duration ($T_{c}$ ) is just the reciprocal of the sampling frequency. In every $T_{c}$ , it is bound that a BF is created and transmitted. For the different channel, BW, and number of A$\times$C, the required CPRI data rate is different. The line coding used in CPRI also depends on the number of A$\times$C. A Collection of BFs equal to the number of FFT points is known as Hyper Frame. The duration of a Hyper frame is the summation OFDM symbol duration ($66.67 \mu s$) and the duration of Cyclic Prefix (CP). A Hyper frame contains 256 control words, one from each BF. These control words are organized into 64 sub-channels; each carries four control words. Total sub-channels divided into seven categories, and each sub-channel belongs to one category. A Grouping of 150 numbers of Hyper frame completes a 10ms of LTE frame (CPRI frame). All the existing fronthaul candidates follow the CPRI frame structure and its three mapping methods to accommodate different AxC

Existing Fronthaul candidates– A C-RAN system follows a full centralized structure for its smooth operation and maintenance. Due to this structure, RRH contains only the Radio Frequency (RF) section and has no processing power. So RRH collects the RF signal and sends sampled I/Q data (digital) to BBU for further processing. After processing, BBU sends the digitized data to RRH through the CPRI-based fronthaul link.

Figure 3: Hierarchical frame structure of CPRI for normal cyclic prefix [40].

There is a vast communication overhead between RRH and BBU due to the full centralized structure. For the above reason, the CPRI link demands a high fronthaul data rate. Fronthaul link with CPRI standard also follows other performance requirements such as maximum one-way delay, maximum jitter, and the allowable BER. These performance specifications are imposed by CPRI [41]. We can choose those fronthaul candidates who can satisfy the above CPRI requirements. List of existing fronthaul candidates are A) Optical fronthaul, B) Wireless fronthaul, and C) Ethernet fronthaul. From the above list of candidates, Ethernet fronthaul is a novel and emerging, fronthaul candidate. Optical fronthaul is widely used as compared to Ethernet and Wireless fronthaul.

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