Figure 2: Benefits of C-RAN compare to traditional RAN. (a) Shows the football ground, (b) shows the airport area

## 4  Fronthaul Architecture

In C-RAN, the interface that connects RRH and BBU is known as fronthaul. The concept of the fronthaul is not newly introduced in C-RAN. It was already present in traditional RAN architecture, but unnoticed. In traditional RAN, the fronthaul interface was used to allow the antenna function to be moved to the rooftops and mastheads, keep away from the baseband processing present inside the cabinet. It was unnoticed due to its short distance in the case of traditional RAN. But in C-RAN, the separation distance between the antenna head and BBU unit can be of several kilometers. The interface used for connecting RRH and BBU follows some dedicated transport standards, which is set by industry cooperation. The name of these standards are Open Base Station Architecture Initiative (OBSAI) [34], Common Public Radio Interface (CPRI) [35], Open Radio Interface (ORI) [36].

OBSAI was first introduced in 2002 by a common agreement among BS vendors: Hyundai, Nokia, LG Electronics, Samsung, and ZTE. The main aim of OBSAI was to open the market for creating cellular base station products by which the cost and development effort will be reduced. OBSAI-based fronthaul is a packet-based interface. It can achieve the data rate from 728 Mbps to 6.8 Gbps, and the BER requirement is 10-15. At the end of 2003, CPRI came to the picture as a result of cooperation among five radio equipment vendors: Ericsson, Huawei, NEC, Nortel Networks, and Siemens Mobile [37]. The interface that follows CPRI standard requires stringent performance requirements such as a maximum of $100\mu s$ one-way latency, 65ns jitter, and 10-12 BER. CPRI supports a constant bit rate. It deals with layer 1 (physical layer) and layer 2 (MAC, RLC, and PDCP) protocol stack for defining a frame that carries I and Q samples. The mapping methods of CPRI are more efficient compared to OBSAI [38], and it is widely used by most of the global vendors.

CPRI has restricted compatibility as compared to OBSAI, due to its closed-based architecture. To address this problem and to provide better compatibility, the European Telecommunication Standards Institute (ETSI) has initiated a new industry specification group (ISG) known as ORI. ISG is also agreed by telecom operators like SK Telecom, Japanese telecommunications operator KDDI and vendors like Samsung, Huawei, ZTE. ORI specification is a modification on CPRI standards by the removal of some options and the addition of other functions. The main difference between CPRI and OBSAI on one side and ORI on other sides is that the first two technologies are formed by cooperation among equipment markers.

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