## 2 Test-Bed  
We experimentally test a 400 Gbit/s fully coherent downstream transmission in a convergence between metro and access networks. We first test it in a real installed fiber-urban test-bed using transceivers with offline processing, and then, we repeat the measurements  
Figure 1: Comparison of telecommunication networks: current networks (yellow, green and blue) and the future merged network (teal dotted). with two pre-commercial transceivers using a laboratory test-bed (the overall system in the same structure). Let us begin by describing the first set-up. The upper part of Figure 2 depicts the utilized set-up, while the bottom part illustrates the corresponding path of 16.5 km urban span (each direction) in the city of Turin, thus, overall, representing a 33 km link of a real installed optical fiber (Single Mode Fiber following the standard ITU-T G.652) operated by a TIM telecom operator. The transmission begins at the Polito-PhotoNext lab (left in red in Figure 2), where the signal is generated. It then travels, by using the installed urban fiber, to the TIM laboratories (right in yellow/orange in Figure 2), located 16.5 km away, where it passes through a ROADM. Afterward, the signal returns through another 16.5 km of urban installed fiber links to reach the Polito-PhotoNext lab, where the receiver side is located. This set-up allows the transmission to be tested over a significant distance and its performance to be evaluated under real-installed urban conditions. The signal is generated using a Continuous Wave (CW) laser operating in the C-band and is then modulated to produce a 50 Gbaud PM-16 QAM fully coherent optical transmission, resulting in a bit rate of 400 Gbps. Following modulation, the signal is amplified and then attenuated to achieve power level of 11 dBm. After this signal generation at the Polito-PhotoNext lab, the data stream travels along the \"metro path\" (green in Figure 2) which covers a distance of 16.5 km of fiber length and concludes at the ROADM, placed at the TIM laboratory premises. By setting different values of the Variable Optical Attenuator (VOA) placed inside the ROADM, signals with different Optical Signal-to-Noise Ratio (OSNR) values can be achieved, ranging from 21 to 36 dB. Referring to Figure 2, the described VOA is inside the block labeled \"WDM DEMUX\", which represents the first of the five ROADM components. This first block consists of an optical tunable filter set  
Figure 2: Above: Set-up used for demonstrating the convergence between the metro and access network with 400 Gb/s fully coherent transmission and a ROADM between the boundary of the networks. Bottom: Fiber urban path inside the city of Turin used to perform the experiment. to 100 GHz bandwidth and the aforementioned VOA. After that, the signal crosses the ROADM second block, an Erbium-Doped Fiber Amplifier (EDFA), operating with fixed optical gain. Next, there is the third block: another VOA, set to 0 dB of attenuation. Then, the signal is amplified once again by the ROADM fourth block, this time operating at a fixed output power. Finally, the signal arrives at the fifth and final ROADM block, the \"WDM MUX\", where the signal is filtered again with an optical bandwidth of 100 GHz and then set to transmit 11 dBm of optical power on the access path of Figure 2, i.e., the same amount of optical power transmitted on the metro path. Overall, the ROADM is installed with the goal of filtering the signal, producing a different OSNR, and amplifying the signal. At this point, similarly to the \"metro path\", the \"access path\" (blue in Figure 2) also spans 16.5 km of fiber length, joining a VOA at the Polito-PhotoNext Lab. This VOA emulates a PON splitter, allowing for the generation of different-distance ranges and optical received power levels by varying its attenuation. In the end, the signal reaches the coherent receiver, where offline processing is conducted to perform the experimental analysis.