## 3 Short-Reach Systems Classification and Enabling Technologies
### Pulse Amplitude Modulation  
The simplest high-order modulation format to achieve a higher bit rate with limited bandwidth components is pulse amplitude modulation (PAM). Compared to the non-return-to-zero on-off-keying (NRZ-OOK) format, information bits can be encodedon multiple optical amplitude levels such as PAM-\(M\) (e.g., PAM-4, PAM-8, or PAM-16). The NRZ modulation format is more tolerant of ISI compared to multilevel intensity signaling such as PAM-\(M\)[17; 42]. Furthermore, it is noteworthy that when the level number doubles, the receiver sensitivity will be significantly reduced by a factor of four. Therefore, a higher optical signal-to-noise ratio (OSNR) is usually demanded by the higher-order PAM signals. Besides being more susceptible to channel impairments such as the chromatic dispersion, they demand DSP schemes that are more complex [36; 42]. Furthermore, it is noteworthy that in view of the tradeoff between the achievable bit rate and system performance, PAM-4 is a more practical and promising alternative compared with higher-order PAM signals [35]. Therefore, regarding the implementation complexity and system performance, it has been considered as a more attractive format for short-reach applications [76]. Remarkably, PAM-4 has been adopted by the IEEE 400 GbE P802.3 bs task force for the DCIs [35]. Furthermore, PAM-4 has been adopted by the 100 GBASE-KP4 for electrical backplane interconnects [16]. Carrierless Amplitude and Phase ModulationBell Laboratories proposed carrierless amplitude and phase (CAP) for asymmetric digital subscriber line (ADSL) applications in order to improve the transmission AIR by means of orthogonal multiplexing method [36; 77]. It facilitates the quadrature amplitude modulation (QAM)-type signal generation with a relatively simpler implementation [35]. Unlike the QAM scheme where individual data streams are modulated, a digital filtering process is employed in CAP for the generation of quadrature signals without additional carriers. To achieve this, the CAP transmitter generates a QAM-like signal through the combination of the Hilbert pair of two filtered multilevel signals [16; 36]. In this regard, the CAP system implementation is relatively simple. With the rapid advancement regarding the capability and the processing speed of integrated circuits, CAP modulation has been finding application in the IM-DD schemes and has been considered as one of the viable solutions for realizing optical transmission in the range beyond the usual capacity (i.e., over 100 Gb/s can be realized) with enhanced performance regarding the SE, complexity, and cost [77]. However, CAP is highly sensitive to nonflat channel frequency responses. Based on this, the ITU G.992.1 G.DMT (discrete multitone) has been adopted as the predominant standard. The DMT-based ITU-T G.992.1 expands the usable bandwidth of telephone lines [36]. Discrete Multitone ModulationIn a distributed radio access network (D-RAN) or centralized/cloud radio access network (C-RAN) where massive-MIMO techniques and CPRI/eCPRI interfaces are implemented, radio-over-fiber (RoF) schemes are normally employed for the transmission between the BBU and RRHs [1]. In this network, the RoF signal is usually impaired by the fiber link chromatic dispersion (when SMF is employed) or multimode dispersion (when MMF is employed). The multipath fading of the wireless link and imperfection of the BS components contribute to the signal impairment as well. One of the promising solutions to simultaneously attend to the fiber link dispersion and wireless link multipath fading effects is optical orthogonal frequency division multiplexing (OFDM) [78; 79; 80]. In this context, if the guard interval is sufficiently longer than the total delay spread caused by chromatic dispersion and maximum differential group delay, the impairment can be compensated using an optical OFDM. To achieve this, the employed cyclic prefix has to be longer than the total delay spread owing to dispersion and multipath spreading effects [78].