## 11 Further Research Directions and Future Considerations
### Advancement towards High-Baud-Rate Transmission  
The future optical networks are envisaged to support a diverse bandwidth demand due to the persistent growth in the DCN traffic. In this regard, for the associated large traffic demand between the main nodes, the deployment of Tb/s transponders is anticipated. Furthermore, the smaller nodes can exploit a fraction of the Tb/s traffic without employing expensive Tb/s receivers. It should be noted that there has been significant improvement in the COH systems that are capable of supporting a single-channel 1 Tb/s transmission [131]. Moreover, the increase in the production and advancement in photonics integration has helped alleviate the associated high cost of COH transceivers [131; 161]. With advanced DSP, these have contributed immensely to the improvement in the SE, AIR, speed, and flexibility of optical transmission [161; 162]. Based on this, the COH detection system is now becoming an attractive solution for short-reach use cases that have been dominated by the direct detection counterpart. In addition, for the direct-detection-based schemes to offer comparable performance to the COH system, there have been a number of innovative direct detection schemes that have been presented. However, as previously mentioned, the schemes usually demand relatively complex optical hardware just as the COH schemes. For instance, an \(IQM\) is required for the KK detection and a Stokes vector receiver for a polarization-diversity direct detection [14; 23; 131]. In this context, only marginal transceiver cost differences are anticipated between both systems in the future. As previously mentioned, there are various proposed direct-detection-based schemes such as the KK and SSBN iterative cancellation receivers that can perform optical field recovery. Nevertheless, these SCOH schemes are limited to the SSB modulation format. Consequently, they conspicuously lose half of the electrical spectrum efficiency compared with the DSB modulation. The generation of the SSB is always inefficient either in transmitter bandwidth or in power. For instance, it is susceptible to the noise folding issue, which demands a precise optical filter. This requirement considerably complicates the receiver design. Therefore, it is noteworthy that for the SCOH to be competitive with the COH, it needs to handle the DSB. In this regard, all the SSB formats could potentially become just bookish discussions. Just as the heterodyne that has been extensively discussed in the lab, it has never been deployed largely in the field, due to the inferior electrical spectrum efficiency. Consequently, one research direction for the SCOH would be schemes that can detect the DSB rather than the well-studied and focused SSB. In this context, a notable and novel receiver scheme that can accomplish optical field recovery of complex-valued DSB signals through direct detection is carrier-assisted differential detection (CADD). Compared with the KK and SSBN iterative cancellation receivers, CADD doubles the electrical spectrum efficiency through the adoption of the DSB modulation without sacrificing the receiver sensitivity. By leveraging the direct detection in which a precise receiver optical filter is unnecessary, CADD can adopt cost-effective uncooled lasers in contrast to the expensive temperature-controlled lasers in the COH systems. As a result, the CADD receiver architecture initiates a new category of direct-detection-based schemes that are appropriate for photonic integration. This is comparable to the homodyne receivers in the COH detection [163; 164]. In general, one of the effective means of minimizing the operational cost of a direct-detection-based scheme is to maximize the related reception rate per channel under the receiver bandwidth constraint. Therefore, an essential factor to be considered is the receiver bandwidth efficiency which is the ratio of the achievable rate to the receiver bandwidth limit.