## Reference Scenario  
The reference scenario is shown in Figure 3. RRUs and RBSs are connected to both Ethernet switches and a DU pool through a WDM/OTN transport network with a given number of optical rings. The Ethernet switches and the DU pool are located in a Central Office (CO). The main components of the network architecture are:  
* Access Switch (AS): It is composed of one Reconfigurable Optical Add Drop Multiplexing (ROADM) and a WDM/OTN switch; the WDM/OTN switch frames/de-frames in OTN frames the CPRI and Ethernet flows coming from/directed to the RRUs and RBSs, respectively. The OTN/WDM switch is also equipped with transceivers able to generate/receive colored wavelengths that are added/dropped by an ROADMs. The use of an ROADM guarantees transparently routing wavelengths that are only generated from/received by other ASs and directed to/coming from the WDM/OTN HUB.
* WDM/OTN HUB: It handles the generated/received traffic by the Ethernet switches and the DU pool; it performs framing/de-framing of the Ethernet and CPRI flows in/from OTN frames; it is equipped with WDM interfaces to generate the colored wavelengths in which to carry the OTN frames; and it forwards/receives the WDM signal to/from the right optical ring.  
Recently proposed architectural and technological solutions help to realize low delay and cost-effective WDM/OTN transport networks. Among them:  
Figure 3: Reference scenario. RRUs and RBSs are connected to ethernet switches and a DU poll through a WDM/OTN transport network.  
* The definition of scalable WDM/OTN HUB architectures organized in three stages: the first and third stage are equipped with small size OTN switches while the second one is equipped with a spatial crossbar matrix [16; 17].
* The definition of a light OTN framing [8] allows for a reduction of the overhead introduced to perform the Forward Error Code (FEC) operation [8]; some experiments have shown a FEC latency lower than 4 \(\upmu\)s with no appreciable degradation of the FEC gain with respect to the traditional OTN case.
* A photonic integration technology allows for cost-effective realization of the ROADMs and transceivers [8]; in particular, silicon photonics are most suitable for large-scale integrated switching devices due to its characteristics of easy integration with control electronics, high miniaturization, mass producibility and low cost due to the use of a well-established Complementary Metal Oxide Semiconductor (CMOS) production infrastructure. Concerning integrated multiwavelength transceivers, relevant technological advances have been made with two different technological approaches that lead to InP monolithic-integrated DWDM transceivers and silicon photonics-based transceivers [18; 19].  
To reduce as much as possible the bandwidth consumption, we employ the reconfiguration capability of the WDM/OTN transport network and adapt the bandwidth resources to the current traffic; in particular, we employ the capacity of changing the CPRI flow rate according to the current traffic handled by any RRU. That could be realized in the case in which the RRUs are equipped with a given number of MIMO smart antennas [15] that are activated according to the traffic amount to be handled. In such a way, the bit rates of the CPRI flows are changed and the needed bandwidth is re-allocated.  
The objective of this paper is to evaluate the best configuration of the proposed OTN/WDM architecture in terms of number of optical rings, number of ASs, number of wavelengths, and percentage of used RRUs with the aim at getting a low solution and supported with the current DWDM technology. We also evaluate the advantages of the proposed solution with respect to a static resource allocation solution in which the network resources are dimensioned when the radio stations are working at full load.  
To perform the evaluation, we need to introduce an analytical model for the dimensioning of the number of wavelengths needed in the optical rings as a function of the main traffic and network parameters.