## 4 SSB Signal Generation and SSBN Mitigation Techniques
### Optical SSBN Mitigation
#### 4.2.2 Optical SSBN Mitigation Approaches  
This subsection considers the performance of the Tx-EDC and Rx-EDC systems as illustrated in Figure 7. In this context, their implementation in conjunction with different receiver-based digital linearization approaches is discussed.  
Figure 6: Direct detection scheme with the (**a**) Tx-EDC and (**b**) Rx-EDC, together with SSBN cancellation (adapted from [29]).  
Single-Stage Linearization Filter Approach  
The Rx-DSP configuration that is based on a single-stage linearization filter (SSLF) and the Rx-EDC is illustrated in Figure 7a. In the SSLF block, a digital SSB signal is acquired from the detected electrical DSB signal by means of a sideband filter (SF) [29]. Furthermore, in this scheme, the SSBN terms are treated as a perturbation to the signal [22]. Then, the SSBN terms' waveform approximation is estimated in accordance with the SSB signal and subtracted from the SSB signal that has been filtered. This process helps the partial compensation of the SSBN [31]. Therefore, the performance difference between the Tx-EDC and Rx-EDC can be reduced with the implementation. Moreover, for both EDC schemes, the transmission performance over a range of launch powers can be enhanced. A notable benefit of this scheme is the offered DSP structure, which is very simple, resulting in a significantly low DSP complexity implementation [29]. In view of the fact that the SSBN terms' calculation is realized right from the detected signal, as opposed to its reconstruction based on the symbol decisions, as usually done in the SSBN estimation and cancellation algorithm, it reduces the DSP complexity considerably. It averts the requirement for precise symbol decision-making in the course of the beating terms' reconstruction [22]. Nevertheless, since the SSBN terms estimation is in line with the received distorted signal, extra beating interference is presented by the scheme. This considerably limits the compensation performance [29].  
The SSLF approach can be improved by adopting a two-stage linearization filter and the Rx-EDC in the Rx-DSP design as depicted in Figure 7b. The main limitation of the SSLF  
Figure 7: The Rx-EDC and Rx-DSP with (**a**) a single-stage linearization filter, (**b**) a two-stage linearization filter, (**c**) an iterative linearization filter, (**d**) SSBN estimation and cancellation, and (**e**) the Kramers–Kronig algorithm. SF: sideband filter, DS: detected signal, \(\eta_{(\bullet)}\): scaling parameter that controls the compensation gain for the related stage (adapted from [21; 22; 29; 30; 54; 55; 57; 58; 59]).  
approach is the introduced beating interference, which is largely the signal-SSBN beating products. This issue can be addressed by employing another linearization stage in addition to a stage being employed in the SSLF approach. This will facilitate the elimination of the unwanted beating interference that emanates from the first linearization stage. Therefore, the second stage helps further improve the compensation gain. Furthermore, it can help the Rx-EDC to have an enhanced BER performance and transmission performance comparable to those of the Tx-EDC. The small difference that could occur between the BER Rx-EDC and Tx-EDC can be attributed to the uncompensated residual beating terms being caused by the linearization filter in the second stage [29].