_Article_  
Dimensioning Models of Optical WDM Rings in  
Xhaul Access Architectures for the Transport of  
Ethermet/CPRI Traffic  
[PERSON] 1,*  
network connectivity, reduce the costs of the entire network and enhance energy and transport efficiency. Cloud Radio Access Network (C-RAN) or centralized RAN is a technology in which most of the functionalities of a classical e-node of a Long Term Evolution (LTE) network, except the radio ones are centralized and shared in a pool [2]. Traditional C-RANs are organized as a three elements network that contains Base Band Unit (BBU) pool, Radio Remote Unit (RRU) and the network interconnecting BBUs and RRUs called fronthaul [3, 4]. The BBU provides baseband signal processing functions and the RRU provides Radio Frequency signal transmission and reception functions. The Common Public Rate Interface (CPRI) [2] has been defined to transport the radio samples between the BBUs and the RRUs through the fronthaul network. The main proposed solutions are: Dark fiber [1, 5, 6], Passive WDM [1, 5], WDM/OTN [7, 8], WDM/PON [9, 10], and Ethernet [11, 12].  
The high extension of the fronthaul network leads to two problems: (i) difficulty supporting the real time functionalities of an e-node; and (ii) the high bandwidth cost when pure CPRI solutions are implemented. For this reason, the C-RAN functions are re-split [1]. Instead of two, three functional entities are defined: Centralized Unit (CU), Distributed Unit (DU) and RRU. The CU provides the non-real-time functionalities while the DU provides physical layer functions and the real-time HARQ and ARQ ones. The new fronthaul network is the one between DU and RRU. It is less extensive and that leads to solve the problems before mentioned.  
Probably the Ethernet-based technology is the more promising and efficient one but it is not mature yet and needs many investigations. For this reason, we propose and investigate an OTN/WDM-based solution, which is mature and allows for good performance from the delay and jitter point of view.  
The objective of this paper is to propose a transport solution for the fronthaul segment based on optical technology that allows for a dynamic bandwidth resource allocation needed to carry the CPRI flows with a consequent saving of bandwidth with respect to a static solution in which the network is provided with bandwidth dimensioned in the worst case when the radio stations are working at full load. The main contributions of this paper are the following:  
* We propose a low cost and modular WDM/OTN architecture based on a ring topology; because, in the early developments, C-RAN solutions will coexist with traditional mobile solutions and Radio Base Station (RBS), we define the network architecture so as to support both CPRI and Ethernet traffic.
* We propose a reconfiguration policy of RRUs equipped with smart antennas and implementing Multiple-Input-Multiple-Output (MIMO) techniques; the policy determines the number of antennas to be switched on to support the current traffic.
* An analytical model is introduced for the dimensioning of the number of wavelengths of the optical rings.
* We evaluate the best network configurations (number of optical rings, number of wavelengths, number of RRUs, and \(\cdots\)) that lead to a low cost solution and supported by the current Dense Wavelength Division Multiplexing (DWDM) technology.
* We evaluate the bandwidth advantages that the introduced transport solution allows us to achieve with respect to the one in which the bandwidth resources are statically allocated in a worst case in which the radio stations are working at full load.  
The evolution of the fronthaul network technologies in C-RAN architectures is illustrated in Section 2. The reference scenario considered in our analysis is reported in Section 3. Meanwhile, the dimensioning analytical model of the optical rings is reported in Section 4. The main results are illustrated in Section 5. Finally conclusions and future research items are illustrated in Section 7.