# A Review of DSP-Based Enabling Technologies for Cloud Access Networks
## 2 Cloud Access Network Concept
### CAN Architecture  
An example CAN architecture is illustrated in Figure 3. The CAN connects multiple end-points to an optical line terminal (OLT) in the operator's central office (CO), where the connectivity to the core network is provisioned. The data connections within the CAN can support fixed data, mobile backhaul or mobile fronthaul, thus the end-points would be optical network units (ONUs) for fixed data links, baseband units (BBUs) for backhaul links and remote radio heads (RRHs) for front-haul links. The CAN can also support direct connectivity between end-points within the CAN, thus completely bypassing the OLT. For example, fronthaul links can be established between BBUs and RRHs at different locations in the CAN and dedicated fixed data links can be established between two end users connected to the CAN, such as a private network between two different offices of a business. There are three key networking devices that achieve the dynamic interconnectivity of the CAN, reconfigurable optical transceivers (ROTs), soft-reconfigurable optical add/drop multiplexer (soft-ROADMs) and spectral converters (SCs). The ROTs are embedded within the ONUs/BBUs/RRHs and within the OLT, key characteristics of the ROTs are:  
* They form the link terminations and are the only points at which optical-electrical (OE) and electrical-optical (EO) conversion occurs. * They incorporate the embedded DSP which realises the digital orthogonal filtering-based channel multiplexing. * The only point at which signals are modulated and demodulated as the CAN is transparent to signal modulation formats. * Provide variably/adaptively modulated signals with variable bandwidth located within the digital orthogonal filtering space. * Support SDN control via a centralised SDN controller, to manage channel allocation and avoid possible channel contention. Figure 2: Example electrical spectra of combinations of digital orthogonal filter multiplexed channels. Multiple soft-ROADMs would be interconnected, for example in a ring architecture, as shown in Figure 3, to allow channels to be added and dropped as required to allow dynamic distribution of the total CAN capacity to the various end-points in the network. Wavelength division multiplexing (WDM) is fully supported by the soft-ROADMs thus allowing high-level channel multiplexing at the wavelength level. Each wavelength supports the aforementioned DSP-enabled sub-wavelength and orthogonal sub-band level channels, which can be dynamically switched by the soft-ROADMs channel add/drop functions. Key features of the soft-ROADMs are:  
* Operation completely in the optical domain, thus avoiding the need for OE and EO conversions. * Supports WDM-multiplexed optical signals. * Channel add/drop functions operating at the wavelength, sub-wavelength and orthogonal sub-band levels. * Provides connectivity to one or more PONs with dynamically variable capacity. * Provision of mobile backhaul links to BBUs with dynamically variable capacity. * Provision of mobile fronthaul links between BBUs and RRHs at different locations in the CAN and with dynamically variable capacity. * Transparent to the underlying signal modulation formats. * Fully compatible with SDN-based network control. * Virtually negligible latency for compliance with the ultra-low latency 5G requirements. The soft-ROADM thus provides channel level switching at the wavelength, sub-wavelength and orthogonal sub-band levels, thus offering dynamic bandwidth allocation with a granularity below the wavelength level. To achieve even finer bandwidth granularity, a single channel can be shared by multiple end users by employing multiple access modulation formats. For example, orthogonal frequency division multiplexing (OFDM) supports multiple subcarriers which can be independently allocated to different end users. Thus a channel supporting an OFDM modulated signal can be added/dropped by a soft-ROADM to support an OFDM-multiple access (OFDMA) PON.