# Evolution of Access Network Sharing and Its Role in 5G Networks
## 3 Sharing of Optical Access Networks
### Twdm-Pon  
Time and Wavelength Division Multiplexed Passive Optical Network (TWDM-PON) was designed to increase the TDM-PON capacity by providing multiple wavelengths over a power-split architecture. This was standardized in 2012 by FSAN as the Next-Generation PON Stage 2 (NG-PON2) architecture [43]. The main advantage of this architecture is that it increases the overall PON  
Figure 3: DBA Virtualization.  
capacity while maintaining the flexibility of a TDM-PON, as end-users can, in principle, be moved across wavelength channels for load balancing. In addition, this technology opens up a number of additional possibilities for network sharing, as discussed in [14]. For example, OLOs could be assigned a wavelength, to which the end-users' CNTs can tune according to their preference. Or the ONTs can be assigned dynamically to different OLTs for online load balancing purpose, while the multi-tenancy is addressed at the capacity scheduling level.  
The flexibility provided by TWDM-PON has spurred a number of novel ideas for network sharing.  
In [44], the authors have proposed a Multi-OLT and multi-wavelength optical access network which enables each ONT to be shared among all operators. However, their solution does not support simultaneous time and wavelength allocation, and in the upstream, each service operator uses an individual wavelength. Therefore it causes a channel separation among the operators, which leads to low utilization of the channel. Since operators might have different data rates and packet lengths, the advantage of the multi-OLT architecture proposed in [44] relies on reducing the burden of data processing and management compared to the case of using a single OLT for all operators.  
In [45], the authors propose a dynamic TWDM-PON to bring flexibility to mobile Radio Access Network (RAN). The proposed technology enables a virtual PON to be configured with a wavelength so that it can be effectively applied to a RAN scenario. The virtual PON dynamically responds to the changes in the capacity requirement, which improves bandwidth efficiency. Their numerical simulations verify that the proposed dynamic TWDM-PON can accommodate twice as many user terminals as a fixed PON.  
Today, a number of practical issues are slowing down the deployment of NG-PON2 technology. One is its elevated cost, due to the need for tunable transmitters and receivers both at the OLT and ONTs. While cost might be a lesser issue for 5G use cases where ONTs might be used for business-related services, other technical hurdles also need to be taken into consideration. These are mostly caused by cross-talk problems that are accentuated by the use of burst mode transmission in closely spaced Dense Wavelength Division Multiplexed (DWDM) channels. The authors in [46; 47] present a thorough analysis of these problems, which relate to: channel isolation at the Arrayed Waveguide Grating (AWG) multiplexer on the OLT side, causing leakage across channels; burst extinction ratio, where an ONT can add background noise to the PON while not transmitting information; side-mode suppression ratio of the laser, which can inject unwanted noise into other channels, while transmitting information; frequency drift of the ONT lasers due to self-heating during burst-mode transmission, which causes unwanted transients; etc. However, solutions are being proposed over time, such as the development of lasers with reduced thermal frequency shift [48], which are progressively increasing performance.