## 1 Introduction  
An access network is the final part of a telecommunication network, delivering data to end users. For the past 15 years, access networks have been based on the passive optical network (PON) infrastructure, which consists of point-to-multipoint bidirectional single-fiber links between a central office and the end users. The PON architecture has achieved significant success due to its simplicity and relatively low cost compared to other network schemes. Each PON tree consists of the following main elements: one Optical Line Terminal (OLT) located inside the central office at the boundary between the metro and the access network, one or more cascade passive splitters and a pool of Optical Network Units (ONUs), one for each end user. Over the years, the capacity of PON systems has increased to meet the end users' requirements [1], according to specifications published by the two main standardization organizations, the IEEE and the ITU-T. In September 2021, the ITU-T released the 50G-PON standard [2], setting a downstream capacity of 50 Gbit/s. Despite remaining based on direct detection and intensity modulation, the capacity of PON structures has increased from a few Gbit/s (first G-PON standard) to several tens of Gbit/s (actually 50 Gbit/s), thanks to various strategies [2]. One key strategy has been the implementation of Low-Density Parity-Check (LDPC) codes. LPDC codes, performing error corrections until a \(10^{-2}\) Bit Error Rate (BER) [2], enhance optical receiver sensitivity, thereby increasing the overall optical power budget performance. Furthermore, advancements in photodetector technology have led to the availability of high-bandwidth Avalanche Photodiodes (APDs) capable of detecting high capacities in current PON structures. Finally, starting from the 50G-PON standard, the ITU-T introduced a digital signal equalization process at the receiver side to compensate for the bandwidth limitations of the optoelectronic components. The standardization bodies are currently actively discussing the next-generation PON. The most probable scenario will be based on a 100G-PON with a downstream transmission rate of 100 Gbit/s [3]. This architecture still uses direct detection at the receiver side, but the capacity increase is achieved through a change in modulation format from PAM-2 to PAM-4 and the introduction of semiconductor optical amplifiers [4]. Despite the associated little increases in complexity and cost, these seem tolerable for telecom operators. Subsequently, the standards released with even higher transmission rates (such as 200G-PON and beyond), based on direct detection receivers, become very challenging. Even if there are some studies [5; 6] in which high-capacity PON systems are still using the direct detection technique, the trend seems to be moving toward coherent detection [7]. This is primarily due to the limitations of the direct detection technique caused by chromatic dispersion above 50 Gbaud/s, bandwidth limitations of optoelectronic components and the very high optical power budget required by PONs, which is currently fixed by [2] at 29-35 dB depending on the class and may increase over the years. Coherent transmission can overcome all these issues with higher levels of modulation (intensity, phase and polarization multiplexing) and better-quality transmission thanks to digital signal processing. The result is an increased transmission rate with a higher power budget, offering greater coverage distance and more reachable end users. For this reason, the coherent technology already used in core and metro networks is under study for the access segment of PONs as well [8]. Unfortunately, this choice also has some drawbacks that need to be addressed. First, implementing coherent technology in PONs can lead to increased costs and complexity, which are currently not acceptable for telecom operators.