## 5 Experimental Demonstration of the Proposed Hybrid Transport Implementation  
Figure 8 illustrates the experimental layout of the implemented hybrid analog-digital optical network topology. The targeted architecture aims to integrate different transport technologies in a shared optical network segment, facilitating the transition from existing PtP links towards B5G connectivity. In detail, as it was explained in Section 2, the proposed approach relies on the coexistence of legacy CPRI connectivity (LTE/4G), broadband binary streams for D-RoF 5G deployments and B5G A-IFoF implementations in order to support extremely high capacities, such as hot-spot areas. The integration of the A-IFoF transport segment implementation that was discussed in the previous two Sections was a significant step for the demonstration of heterogeneous protocols' coexistence delivering real-world services. Network flexibility was provided using a WSS-based optical node which could handle optical wavelengths that were implementing different transport protocols. This SDN-compatible, dynamically controlled optical switching node allowed for wavelength routing, provided dynamicity to the network and facilitated the protocol-agnostic coexistence of heterogenous streams. In our proposed WDM-enabled deployment for the implementation of an optical switching node, an arrayed waveguide grating (AWG) followed by an erbium-doped fiber amplifier (EDFA) and a commercial \(1\times 4\) WSS (NISTICA FFLB-C04L000-4 UC) were utilized in order to multiplex and dynamically route four different optical streams. The WSS node was based on the digital light processing (DLP) technology of Texas Instruments [43], which is applied to commercial WSSs by optical component vendor NISTICA (Bridgewater, NJ, USA). The dynamic operation of this optical switching node was achieved by controlling the WSS, being the only active block of the node, through a custom MATLAB 2018a script that was developed for the interface between the SDN agent and the WSS. Its main elements were a \"listener\" and a \"controller\", as shown in the block diagram of Figure 9a. This script could recognize any changes to a common text file (.txt) where the SDN agent wrote the network commands. In this way, the WSS was configured to route the wavelengths to the proper destination output according to the commands in this file. For the purpose of this experiment, the.txt file was changed manually as the configuration script was running. The optical streams at \(\lambda_{1}\) and \(\lambda_{2}\) that were carrying A-IFoF and optical binary signals, respectively, were used for the physical layer performance evaluation, while the optical  
Figure 8: The 4\(\lambda\)–optical transport experimental layout. streams at \(\lambda\)3 and \(\lambda\)4 were used for the demonstration of true mobile services. Regarding the RoF transceivers' implementation, a signal generator (arbitrary waveform generator) was used for the generation of complex up-converted waveforms (A-IFoF) and of the NRZ signals (D-RoF) supporting both sub-carrier multiplexing (SCM) A-IFoF transmission at a center intermediate frequency (IF) of 1.6 GHz and an up to 10 GHz binary optical stream, respectively. The IF up-converted stream was fed to an externally modulated laser (EML) emitting at \(\lambda_{1}\). After propagation over SMF, the analog stream was detected via a 10 GHz avalanche photodiode (APD), transmitted over a wireless PtP link that was operating at 60 GHz and captured by a real-time oscilloscope (RTO). For the post-processing of the IF waveforms, vector signal analysis (VSA) software was used. The binary stream carrying the D-RoF traffic was modulated via a Mach-Zehnder modulator (MZM) at \(\lambda_{2}\). After the SMF transmission, the binary stream was detected by a 14 GHz photoreceiver and fed to the RTO. Offline processing involving a 21-tap feed-forward equalizer (FFE) was performed. For the demonstration of the services, the existing mobile infrastructure was exploited.