## 4 Simulation Modeling
### Reconfigurable Optical Add-Drop Multiplexer (ROADM) Simulation Model  
In this sub-section, we describe the ROADM simulation model used for the evaluation of the proposed hotspot IFoF fronthaul link. This simulation model was matched with the estimated response of a PIC-based MZI-based ROADM design that is currently being fabricated on the Si\({}_{3}\)N\({}_{4}\)/SiO\({}_{2}\) TriPleX  
Figure 3: (a) Simulated setup of the reference analog IFoF optical link using the matched EML and SMF models. Matching of the experimental and simulated EVM performance of the SC modulated A-RoF link for: (b) 4-IF bands and (c) 6-IF bands after 7 km fiber transmission. platform [32]. The employed ROADM was designed by Lionix International B.V. [61] to add/drop four wavelengths in the C-band with a channel spacing of 100 GHz. The choice of the ROADM's operational band and the maximum number of supported WDM channels was made in such a way that the proposed fronthaul infrastructure will meet the KPI targeted bandwidth for hotspots [5]. The ROADM design can be certainly extended to support a higher number of WDM channels by following an expanded MZI-based layout similar to the work reported in [62]. Figure 4a depicts the ROADM circuit-layer design which consisted of the identical drop-enabling and the add-enabling circuitries, both based on a cascaded configuration of MZI-channel interleavers [62]. For the sake of simplicity the description of the drop operation will follow, whereas the add operation is considered to be regulated by the same principle, being performed on the reverse direction. Initially, the four wavelengths (\(\lambda_{1}\), \(\lambda_{2}\), \(\lambda_{3}\), \(\lambda_{4}\)), were fed into the input port of the first (green) cascade of MZIs. This block of MZIs had a free spectral range (FSR) of 200 GHz in order to split the 200 GHz spaced \(\lambda_{1}\), \(\lambda_{3}\) and \(\lambda_{2}\), \(\lambda_{4}\) wavelengths to its output port 1 and port 2, respectively. Each WDM output signal was inserted to the brown cascade of MZIs which had an FSR equal to 400 GHz. Focusing on the upper arm of the ROADM, the first brown block of MZIs dropped \(\lambda_{1}\), while the second cascade of MZIs dropped \(\lambda_{3}\). Similarly, \(\lambda_{4}\) and \(\lambda_{2}\) were exiting the output port 1 of the first and second cascade of MZIs at the lower arm. Figure 4b illustrates the building block of the cascaded MZIs comprising two MZIs connected to each other via tunable couplers (TCs). In this way, it was possible to control the coupling coefficients of the device, and thus, obtain the desired transfer functions at its outputs. The first MZI had an arm difference of 2-dL while the arm difference of the second one was equal to dL. Due to the dispersion, it is crucial that the 2-dL and dL are not in the same path, but on the opposite parts of the waveguides. Phase Shifters (PS1 and PS2) were placed in each MZI in order to adjust the maximum and minimum power spectral positions of the filter of the MZIs for dropping or passing the proper wavelength. The required arm difference for the block of MZIs with FSR = 200 GHz (green) was equal to dL\({}_{1}\) = 872.1 um while the respective difference for the block of MZIs with FSR = 400 GHz (brown) was equal to 436 um. The coupling coefficients of TC1, TC2 and TC3 can be set anywhere between 0 and 1. When the add-drop operation was required, the TC1, TC2, and TC3 coefficients were adjusted to be equal to 0.9248, 0.28 and 0.50, respectively, in order to achieve the proper filtering of the four  
Figure 4: (a) ROADM physical layer design and implementation comprising the identical drop-enabling and the add-enabling circuitries, both based on cascaded Mach-Zehnder Interferometers (MZIs), (b) building block of the cascaded MZIs. wavelengths, while all coupling coefficients were set to 0 when through operation of the ROADM was needed. In the second case, the filter structure was circumvented and the all the incoming light passed at the output of the ROADM.