# Machine Learning Applications for Short Reach Optical Communication
## 1 Introduction
OMCN: optical metro-core node, OAN: optical aggregation node, ROADM: reconfigurable optical add-drop node, ONU: optical network unit, RN: remote node, OLT: optical line termination, OWC: optical wireless communication. for the fiber to premises connections [10]. A PON consists of an optical line terminal (OLT) located at the central office of the service provider and several optical network units (ONUs) located close to customers. High-speed PON can increase the network capacity and improve QoS metrics [11]. ITU-T and IEEE have boosted the nominal line rate of PON systems to more than 10 Gbps (XG(S)-PON) in recent years [12]. Following XG-PON, 40 Gbps-capable PON (NG-PON2) have added solutions to meet higher speed, and larger bandwidth demands [13]. There are three types of higher-speed PON systems: time division multiplexing PON (TDM-PON), time and wavelength division multiplexing PON (TWDM-PON), and WDM-PON [12]. TDM-PON uses time division multiple access (TDMA) to connect multiple ONUs to the same OLT. The commercially widely used TDM-PON technology can provide a symmetrical bit rate of up to 10 Gbps [14]. According to IEEE and ITU definitions, the next-generation TDM-PON will deliver bit rates of 25 Gbps and 50 Gbp, which can cost-effectively accommodate xhaul traffic [14]. To further increase the capacity, WDM-PON has been proposed and studied. The ONUs coexist in the same fiber with light sources operated at various wavelengths, boosting the overall network bandwidth and the number of users serviced in the optical access network [15]. In terms of network connections, the WDM-PON may employ point-to-point, point-to-multipoint, or hybrid systems, which is widely used for FTTx (Fiber to the x, x = home, building, curb or node) [16]. The spectral efficiency of WDM-PON would also have to be improved in order to enhance system capacity and the number of users serviced, where narrowing the inter-channel spacing and reusing WDM channels are two possible strategies [17]. On the other hand, different from TDM-PON and WDM-PON, TWDM-PON is a multi-wavelength PON that uses TDM/TDMA to share each wavelength channel across multiple ONUs. The details of 50G TWDM-PON such as the wavelength plan and channel information are gradually being standardized under the research of ITU-T [12]. Recently, several studies have focused on the energy efficiency of TWDM-PON equipment such as the protocol design for energy efficient OLT [18; 19]. The radio access network (RAN) adds a new network section known as fronthaul, which is the link between a radio digital unit (DU; also called a baseband unit, BBU) and a remote radio unit (RRU) [20]. Various functional splits are being specified in the recently proposed next-generation fronthaul interface (NGFI) to allow varied trade-offs between the RRU complexity, fronthaul bandwidth-efficiency, and system performance [20]. Mobile-PON, a high bandwidth efficiency, low latency mobile fronthaul architecture based on functional split and TDM-PON with combined mobile and PON scheduler, has also been presented [21]. In addition, ref. [22] presents a type of mobile fronthaul architecture design plan that uses WDM-PON in addition to the wireless multi-hop and point-to-point optical links to lower fronthaul deployment costs. Furthermore, the 5G RAN architecture of a optical fronthaul network using ultra dense WDM-PON (UDWDM-PON) combined with phased array fed (PAF) reflector antenna systems for radio frequency transmission and reception is also under active research [23]. With broadband access delivered to the doorstep, high-speed in-building and indoor communications have attracted intensive interests. Compared with wired connections, the wireless in-building and indoor communications are more attractive due to the mobility feature. The optical wireless communication (OWC) has been widely used in indoor and in-building communications.