## 1 Introduction
Additionally, the coherent transceivers used in long-haul networks utilize the same wavelength for downstream and upstream transmission, a characteristic that is not applicable in PONs, which are bidirectional. However, the scientific community has already begun to discuss and try to overcome these critical issues. Indeed, regarding the cost and power consumption, it is expected that they will decrease over time [9], making them more affordable. Regarding bidirectional transmission, the community has already started to discuss whether, for future coherent PONs, transmission with multi-sub-carrier single-center wavelength (choosing, for instance, even sub-carriers for the downstream and odd for the upstream) [10] is better or a double-wavelength solution (one for each direction) is preferable, trying to find the best solution in terms of cost and complexity. The high complexity introduced by coherent transmission may be tolerated thanks to the huge increase in its performances. Additionally, it seems that coherent transmission may find application primarily in front-haul use cases rather than residential cases, reducing the number of implementations. This targeted deployment could help mitigate some of the challenges associated with cost and complexity while still leveraging the benefits of coherent transmission where it is most needed. The purpose of this paper (a follow-up to our conference papers [11; 12]) is to suggest the introduction of coherent transmission in a PON access network, show the performance of this choice and also to study the possibility of merging a metro network and an access network together by providing a joint design between its segments. Metro-access network merging could be achieved by introducing Reconfigurable Optical Add-Drop Multiplexers (ROADMs) between the two networks boundaries; in this paper, we demonstrate its feasibility and the performance results obtained. Figure 1 shows these concepts graphically, visualizing the current division between networks and the proposed network aggregation. The core network is depicted in yellow, the metro network in green and the access network in blue. The central office (CO), situated between the metro and access networks (at the top of the figure), illustrates the current division between networks, where coherent transmission is used inside the metro network, and intensity modulation with direct detection is used inside the access network. At the center of the figure, the sector represented by a teal dotted line, which includes the metro network, the access network and the ROADM between them, shows the proposed network configuration resulting from the merger of metro and access network domains. In our work, we extend previous studies [11; 12], considering different transceivers, also including some pre-commercials ones. In particular, we experimentally analyze the merger between metro and access networks, by means of a fully coherent 400 Gbit/s downstream transmission system (50 Gbaud PM-16 QAM, where the PM acronym stands for polarization multiplexing) in the C-band. We demonstrate the metro-access convergence by experimental characterizations in the following two different scenarios:  
* A real installed urban test-bed with a fully coherent transceiver and offline processing. * A laboratory test-bed using pre-commercial fully coherent transceivers. In both cases, we perform experimental characterizations in terms of BER curves for different OSNR and received optical powers. The collected data have been analyzed in post processing by fixing the BER target to \(10^{-2}\), thus investigating the metro-access convergence optical power budget. The paper is organized as follows: In Section 2, we present the two test-beds used in our study, the real in-field set-up within Turin city, performing offline processing at the optical receiver side, and the laboratory set-up using the pre-commercial transceivers. Next, in Section 3.1 we present the transceivers' experimental characterization results for both test-beds.