# A Review of DSP-Based Enabling Technologies for Cloud Access Networks
## 3 CAN Network Elements
### Spectral Converters  
#### 3.3.1 Functionality and Operating Principle  
The basic signal spectral repositioning function of the spectral converter was outlined in Section 2.3, however, it is worth illustrating the basic functionality with a practical example of its application in a CAN. Figure 10 depicts an example of how the spectral converter can be utilized in conjunction with a soft-ROADM to increase traffic switching flexibility and reduce traffic blocking probability. The soft-ROADM is incorporated in the CAN and provides flexible connectivity to several network tributaries, such as PON connections, dedicated fixed data connections or mobile backhaul/fronthaul connections to a BBU/RRH. In the example, an upstream tributary is to be added to an aggregated signal in the CAN, however the spectral location of the tributary is occupied in the aggregated CAN signal, so it blocks the tributary from being directly added. Therefore, to avoid the traffic-blocking situation, which restricts the network's flexibility, a spectral converter is employed to relocate the tributary to a free spectral location in the aggregated signal spectrum. The soft-ROADM can now perform a contention free add operation. Figure 9: BER curves for (**a**,**b**) each Sub-B before/after Sub-\(\lambda\) add; (**c**,**d**) each Sub-B before/after Sub-B add; (**e**) before/after the drop operation; Example subcarrier constellations at \(-18\) dBm for the BB-I (upper) and PB-I (lower) after (**f**) Sub-\(\lambda\) add; (**g**) after Sub-B add; example subcarrier constellations at \(-14\) dBm after the drop operation for (**h**) PB-I (upper) and PB-Q (lower). In Figure 11, the basic structure of the DSP-enabled spectral converter is illustrated. The operation of the spectral converter is based on a single standard dual-parallel Mach-Zehnder modulator (DP-MZM), comprising two parallel dual-drive Mach-Zehnder modulators (DD-MZMs), marked as MZ-a and MZ-b each driven by two RF driving signals and one DC bias voltage, and a phase modulator marked as MZ-c driven by a single DC bias voltage. The optical signal power input to the DP-MZM is equally split to feed MZ-a and MZ-b, each of the DD-MZMs converts its input signal to a set of optical components, which have a frequency spacing equal to the RF driving signal frequency, and with amplitudes and phases are governed by the Bessel function of the first kind. MZ-c is then used to control the phase difference between the two generated sets of frequency components, the two sets are then combined to generate an output signal containing the same set of frequency components, as shown in the output signal spectrum on the right of Figure 11. The generated set of optical components contains the same information within each optical component, however the power of each frequency components is dependent on the four RF driving signals and three DC bias voltages, which can be manipulated to control the distribution of power amongst the generated frequency components allowing maximisation of power at the desired frequency. The output optical signal is filtered by a tuneable optical filter to select the desired converted optical component and reject the unwanted optical components. Mathematical descriptions of the abovementioned processes are fully detailed in [11]. Two important characteristics of the spectral converter are spectral conversion efficiency and frequency detuning range. The conversion efficiency is defined as the optical power ratio between the desired output optical component and the input optical signal.