## 4 SSB Signal Generation and SSBN Mitigation Techniques
### Optical SSBN Mitigation
#### 4.2.1 Optical SSBN Mitigation Concepts  
As previously mentioned, the direct detection transceivers' performance is severely degraded by the SSBN penalty. To address this, DSP-based linearization techniques that facilitate a narrow (or zero) spectral guard band employment have been presented. In addition to their ability to address the SSBN penalty, they are also capable of enhancing the SE with a low optical hardware complexity [22]. Furthermore, the robustness to the polarization mode dispersion and the phase noise can be enhanced as well [110]. As illustrated in Figure 5, the linearization schemes are majorly based on using an optical ASE filter to remove the dominating noise. Then, the received signal can be split into two branches through a 1 \(\times\) 2 optical coupler. One section (the upper branch) is fed directly to the PD to generate both the SSBN and desired data sideband. Likewise, in the lower branch, a carrier suppression filter (CSF) can be employed to suppress the optical carrier, which brings about just the SSBN after the PD. Therefore, the outputs of the two branches can be subtracted to achieve the desired signal. It should be noted that the linearization schemes are also capable of alleviating the beat noise power effect. This subsequently results in an enhanced receiver sensitivity [22; 110]. \begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline  & & & & & & & & & & & \\ \hline
**Size-induced carrier-** & **SSB subcarrier modulation** & off null & SSB & IQM & * & simplest & & & \\ generation scheme & with IQM & & & & & & & \\  & & & & & & & & \\  & & & & & & & & \\  & & & & & & & \\  & & & & & & & \\  & & & & & & & \\  & & & & & & & \\  & & & & & & & \\  & & & & & & & \\  & & & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 4: A summary of carrier- and SSB signal-generation techniques. In addition, the optical transmission systems for short-to-medium-reach applications can extend from quite a few tens to a few hundred kilometers. Consequently, dispersion compensation is highly imperative in such links [111]. Furthermore, it has been observed that digital compensation of the fiber link dispersion is highly desirable due to the associated cost and complexity motives. Unlike the optical compensation scheme such as a dispersion-compensating fiber (DCF), the digital compensation scheme is known as an EDC [112; 113; 114; 115; 116]. Furthermore, apart from its potential to be implemented effectively with a suitable performance at the transmitter, it can also be employed at the receiver. The Tx-EDC can be realized by means of a complex modulator [29; 117]. Nonetheless, the nonlinearity due to square law photodetection is one of the notable impairments of the Rx-based EDC (Rx-EDC) direct detection systems. This limitation can be addressed with the aid of optical SSB signaling. The implementation aims at maintaining the optical signal phase waveform in the electrical domain after detection, to ensure an Rx-EDC employment [29]. Nevertheless, when SSB signaling and Rx-EDC are employed, the limiting factor to the system performance is the SSBN. Similarly, with SSB OFDM and Nyquist-SCM implementations, dispersion is usually compensated either by employing a Tx-EDC or by appending a cyclic prefix (CP) for dispersion tolerance. Nonetheless, the expense of the implementations can be an increase in system complexity. This is owed to the prerequisite knowledge of the cumulative dispersion of the link at the transmitter that necessitates feedback from the receiver. Another notable drawback that is as a result of the employed cyclic prefix is the reduction in the achievable SE. Furthermore, the implementation of the Tx-EDC can also result in an increase in the PAPR. This can eventually bring about modulation nonlinearities and larger quantization noise by the DACs [29]. In addition, effective linearization schemes can be employed to perform linear optical effect (dispersion) compensation at the receiver. This can offer comparable performance to the systems with precompensation [18].