## 1 Introduction  
An immense range of mobile and broadband services, such as Netflix, Spotify, etc., provoke an unprecedented growth of consumer-driven data consumption, which leads to an increase of bandwidth demands in fiber-optic access networks [1]. In this segment, passive optical networks (PON) together with a wavelength division multiplexing (WDM) and a time division multiplexing (TDM) technologies promote a pay-as-you-grow approach in the third generation fiber-to-the-x (FTTH)systems. As 10G PON systems are not capable support future services, especially in the context of the fifth generation (5G) mobile networks, 25G is the next evolution step that must be taken [2]. However, it is not clear which means will be used to make it. Recent ITU-T recommendations for passive optical networks (PONs), ITU-T G.9807.1 (symmetric 10G PON (XG(S)-PON)) and ITU-T G.989 (next-generation PON2 (NG-PON2)) describe transmission requirements for line rates of up to 40 Gbps per channel, while new recommendations for higher line rates (e.g., G.HSP.Req, G.HSP.comTC, G.HSP.50 Gpmd, G.HSP.TWDMpmd) are under active development [3; 4; 5]. The enhancements being discussed for the next-generation PON standards include the increase in channel line rates from 10 to 25 Gbps [6]. Now, 100G-EPON standard is still under development by the IEEE P802.3 ca Task Force. It is intended to scale up to 25 Gbps or 50 Gbps per single lane capacity while reusing the existing infrastructure of 10G-EPON [1; 5; 7]. Overall, the future PON is currently under development and standardization for future broadband network solutions beyond 10 Gbps [8; 9; 10]. The evolution towards higher line rates is mainly driven by point-to-point (P2P) connections and wireless fronthaul, e.g., the next generation of mobile cellular networks (i.e., 5G and beyond 5G), where a 25 Gbps could be needed soon for either backhauling or new interfaces supporting the functional splits. It is expected that the x-haul of these mobile cellar networks will be centralized and based on a point-to-point (PtP) WDM-PON architecture [3; 11; 12]. However, due to cost considerations and commercial viability, it is desired to re-use some of the components from 10 Gbps transceivers operating in 10G PONs [13]. Bandwidth limitations from electrical and electro-optical components increase significantly due to the utilization of signals with higher line rates. Nevertheless, such elements/components are crucial for component vendors as their re-use reduce development costs. For example, a cost of an optical line terminal (OLT) transmitter is about twice the cost of the receiver and modulator of the optical network unit (ONU), which represents the major part of the transceiver cost. Therefore, the operational capabilities of the bandwidth-limited transceivers have received considerable attention from component vendors, optical network operators, and research communities [9; 14; 15; 16]. The choice of a modulation format plays a critical role in the performance of HS-PON systems to provide the necessary line rate for the end-users [17]. It seems unreasonable that coherent technology for such relatively low rates could penetrate this market segment at this time, despite the potential sensitivity and optical power budget (OPB) improvements. Therefore, we assume that IM/DD formats will be used to deliver 25G or even 50G over a 20 km long single-mode fiber, i.e., the conventional PON configuration. For some applications, shorter fibers are also considered (e.g., 10-15 km when PON-based fronthauling solutions for 5G are on the table) but some require a substantially longer reach (up to 40 km) [6; 12]. Interestingly, when it comes to the physical (PHY) layer technology and its maturity, three abbreviations (except \"NRZ\") are met the most often: \"Duobinary\", \"PAM--pulse amplitude modulation\", and \"OFDM--orthogonal frequency division multiplexing\".