# A Review of DSP-Based Enabling Technologies for Cloud Access Networks
## 3 CAN Network Elements
### Soft-ROADMs  
#### 3.2.1 Operating Principle  
A schematic diagram of the proposed soft-ROADM architecture is shown in Figure 7, which includes the channel add and drop functional elements. The solution fully supports SDN controllability, as shown in Figure 7. In the soft-ROADM, the input and output optical signals of each wavelength (in both east and west directions) consist of multiple sub-wavelength bands at different RF central frequencies generated within the ROTs, where, as described in Section 2.2, each individual sub-wavelength band can support one or two digital orthogonal filtering-enabled spectrally-overlapped sub-bands (I and Q) or a single sub-band (I or Q). For a specific optical carrier frequency, \(f_{o}\), the optical signal containing \(n\) sub-wavelength bands is expressed as:  
\[s(t)=\sum_{i=1}^{n}\sum_{w=\{I,Q\}}y_{i}^{w}(t)e^{j2\pi f_{o}t} \tag{1}\]  
where \(y_{i}^{w}(t)\), (\(w=I\) or \(Q\)) is the generated sub-band signal when the original base-band signal is up-sampled and subsequently filtered by the corresponding \(I\) or \(Q\) component of the \(i\)-th orthogonal digital filters in the ROTs. As shown in Figure 7, to drop a targeted sub-band signal conveyed by a sub-wavelength band of a selected wavelength, the wavelength containing the targeted sub-band is directly inputted into an intensity modulator (IM) driven by a SDN-controlled drop RF signal having a sinusoidal waveform. The drop RF signal for the \(w\)-th sub-band and \(i\)-th sub-wavelength is expressed as:  
\[RF_{i}^{w}(t)=\left\{ \begin{array}{c}Re\Big{\{}V_{bias}+Ae^{j2\pi f_{oi}t} \Big{\}},\ w=I\\ Re\Big{\{}V_{bias}+Ae^{j(2\pi f_{oi}t-\frac{\pi}{2})}\Big{\}},\ w=Q\end{array} \right. \tag{2}\]  
where \(V_{bias}\) is the required IM bias voltage, \(A\) is the amplitude of the \(RF\) component of the drop \(RF\) signal and the digital filters' central frequencies are positioned at \(f_{ci}\) (\(i\) = 1, 2, 3, 4,..., \(n\)). As the \(I\) and \(Q\) component of the \(w\)-th sub-band is orthogonal, they effectively have a phase difference of \(-\pi/2\), this \(-\pi/2\) phase shift should be applied to the drop \(RF\) signal to select either the \(I\) or \(Q\) component in the drop operation, as shown in Equation (2). When both the frequency and phase of the drop \(RF\) signal match the targeted sub-band signal, the drop operation shifts the targeted sub-band to the  
Figure 7: Architecture of DSP-enabled soft-reconfigurable optical add/drop multiplexer (soft-ROADM). baseband spectral region. At the IM output, the dropped sub-band with a reversed spectrum is shifted to the baseband spectral region, while all other sub-bands of the same wavelength are ruined and unrecoverable. After the direct detection of the dropped optical signal in a ROT, the receiving digital filter is configured as a simple baseband low-pass filter (suitable for use in all the drop operations) and is employed to select the dropped signal and remove the ruined and unrecoverable sub-bands. The values of \(V_{bias}\) and \(A\) can be adjusted for optimum transmission performance. By simultaneously using \(I\) and \(Q\) drop \(RF\) signals in two parallel drop function elements, as indicated in Equation (2), a sub-wavelength level drop operation can also be performed. Furthermore, to drop an entire wavelength, the IM can be configured as an optical passway if the drop \(RF\) signal amplitude is set to zero (\(A=0\)). Here it is worth addressing that, an alternative approach for the soft-ROADM drop operation could involve splitting part of the signal power from the required optical wavelength, direct detection and A-D conversion of the entire band of channels and finally digital demultiplexing via a matching filter and down-sampling. To compare these two approaches, an IM is not needed in the alternative approach, however the big advantage of the IM-based approach over the alternative one is that the ROT only needs to operate at the bandwidth of the dropped sub-band which is usually only \(1/100\) to \(1/10\) of the aggregated wavelength channel in the CAN.