## 3 Short-Reach Systems Classification and Enabling Technologies
### Pulse Amplitude Modulation
DMT is a category of the OFDM scheme, and it is known as DD-OFDM. In DMT, the frequency spectrum is divided into orthogonally modulated subcarriers. Then, the input data sequence are electrically encoded onto each subcarrier. Furthermore, being a multicarrier modulation scheme, it presents attractive features such as flexible multilevel coding, high SE, and high tolerance to channel impairments. As previously mentioned, DMT is being implemented in the ITU-T ADSL standard, and it was later adopted for the ITU-T very-high-speed DSL 2 (VDSL2) standard. Furthermore, it has been attracting considerable attention for the low-cost and bandwidth-limited short-reach communication applications. This is because of its inherent flexibility to shape the transmitted signal's frequency spectrum [42; 76]. Nonetheless, as other multicarrier schemes such as OFDM, DMT presents a high peak-to-average power ratio (PAPR) issue. The high PAPR issue usually has a considerable influence on the DMT's tolerance to the relative intensity noise (RIN), as well as the ADC and DAC resolution requirements [42]. Furthermore, the efficiency of ADC and DAC with a limited effective number of bits (ENoB) can be optimized by clipping the related signal waveform. The employed optical modulators can be driven in the nonlinear region. This is to ensure a high modulation index and, consequently, that the achievable system signal-to-noise ratio (SNR) is maximized. It is noteworthy that this approach is essential when the SNR is the limiting factor for highly spectrally efficient signals. Nevertheless, the approach presents an issue regarding the enhanced system nonlinearities. This occurs with the interplay between the transmitter-induced nonlinearities and the fiber chromatic dispersion, as well as the nonlinear square law detection at the receiver. This issue can be mitigated with the use of nonlinear equalizers at the receiver [42]. In addition, with a number of advanced optical modulations that have been presented, the capacity of optical transmission systems has been pushed towards the limit. Consequently, in an attempt to squeeze out the last few bits from bandwidth-limited optical channels, multicarrier entropy loading (EL) has been proposed. EL is based on probabilistic constellation shaping (PCS) and goes beyond bit loading (BL) through continuous source entropy adaptation. This can significantly help enhance the achievable information rate. In this context, EL is an effective solution for realizing an optimum AIR in bandwidth-limited channels under frequency-selective fading. This makes it a promising scheme for short-reach interconnects [81]. Optical Single-Sideband Modulation  
Another attractive modulation scheme that has been gaining significant recognition for different applications is the single-sideband (SSB) modulation. For instance, the SSB-DMT, SSB-PAM4, and SSB-CAP schemes have been gaining significant attention [53; 82; 83; 84; 85]. This is mainly due to the notable offered advantages such as the improved SE and significant resistance to chromatic dispersion because of the EDC capability [55; 56]. Comprehensive information on the SSB signal generation and associated concepts is given in Section 4.