# Optical Technology for NFV Converged Networks
## 2 Reference Network Scenario and Main Requirements
### WDM Overlay in PON  
In most cases, network operators would like to reuse installed PON infrastructure to carry mobile traffic. This allows to exploit the existing infrastructure, saving the high cost of new installations. The topology of a PON is point-to-multipoint, i.e., a \"tree\" that connects the CO with end user sites through a power splitter. A WDM overlay on a PON [8] is a good solution to support several radio splits, especially in the case of low latency requirements and high bandwidth traffic, such as CPRI and eCPRI [9]. With the WDM overlay, the same fiber transmits the PON upstream and downstream channels and the WDM channels over a separate spectrum [8]. Figure 2 shows a PON with WDM overlay. The distance between CO and antenna site depends on the operator infrastructure: it is typically 5-10 km in densely populated areas.  
Bidirectional operation (i.e., using a single fiber for both propagation directions) is another important feature of the network scenario in Figure 2. It simplifies the network installation and configuration, that can be done \"plug and play\", without taking care of the different fibers used in uplink and downlink.  
Figure 1: Reference scenario for access network segment.  
Figure 2: PON with WDM overlay [8].  
The key device to provide a first level of automation in such a network is a full tunable Transceiver (TRX), at least at the PON termination site. As shown in Figure 2, the WDM comb is transmitted from the CO to the antenna site, where a tunable optical filter at the receiver selects the right wavelength. Today, most WDM networks make use of fixed optical filters, which impacts inventory and network configuration complexity. To reduce the TRX cost, direct detection is preferable to coherent transmission [10]. The bit rate of interest in an Xhaul network to carry CPRI and eCPRI channels is 25 Gb/s [11]. Tunable direct detection TRX at 25 Gb/s are commercially available but they support tunability only in the transmission function, by means of tunable lasers. All current technologies to realize tunable filters at the receiver, such as Micro Electro-Mechanical Systems (MEMS) [12] or liquid crystals [13], are not complaint with the TRX requirements in terms of size and cost. Tunable TRXs developed for the New Second Generation PON (NG-PON2) only support 4-8 WDM channels [14; 15] while the number of channels should be much higher in a fully automated Xhaul network, for example 20 WDM channels with 100 GHz frequency spacing, according to the ITU-T standard frequency grid. As shown in Figure 3a, a conventional TRX, where only the transmitter is tunable, works only if it receives one wavelength that has been previously selected (e.g., from a MUX in the Figure 3a). This is due to the fact that it cannot select the wavelength from an ingress WDM channel comb. Hence a conventional TRX does not work in a PON infrastructure based on power splitters, where no wavelength selective optical devices are present (Figure 2). Figure 3 shows the compatibility of a fully tunable TRX with a power splitter. In this case a tunable filter is integrated in the TRX module in front of the receiver.  
Integrated silicon photonics is a promising technology for realizing cost effective tunable filters for fully tunable TRX. Details about this technology, design options and technological challenges, will be discussed in Sections 4 and 5.