## 11 Further Research Directions and Future Considerations
### Performance Optimization Issues of Short-Reach Transceivers  
Usually, for the performance optimization of a transmission system, the employment of high-speed and high-order signals is desirable. This is also applicable to the short-reach systems that are based on low-cost and direct-detection-based transmitters. However, in the short-reach transmission system, the bandwidth of the transmitter and receiver is low and comparatively less than the Nyquist bandwidth of the signal. In this regard, the application of high-speed and high-order signals to low-cost optical transmitters and receivers will result in performance degradation. The degradation can be owed to factors such as bandwidth limitation and chromatic dispersion-induced channel fading. Based on this, advanced DSP schemes can be exploited for impairment compensation and performance improvement [165]. As previously mentioned, viable schemes such as SSB signaling and the KK method can be employed for chromatic dispersion mitigation by means of Rx-DSP. Furthermore, the nonideal performance of low-cost components regarding insufficient bandwidth can be addressed by different advanced DSP algorithms. For instance, the direct detection-faster than Nyquist (DD-FTN) scheme can attend to the narrowband filtering effects and can offer considerable performance improvement compared with the traditional linear equalization that is typically employed for the bandwidth limitation compensation of optical systems. This is owed to the related enhancement of in-band noise in the bandwidth-limited optical systems by the linear equalization, which can be mitigated through the post filter of direct detection-faster than Nyquist [165]. Furthermore, another viable approach for the narrowband filtering effect mitigation is spectrally efficient frequency division multiplexing (SEFDM). In SEFDM, the subcarriers are packed closer beyond the Nyquist limit in the frequency domain (compared with OFDM) to accomplish SE gains, while the orthogonality and receiver complexity are compromised [165]. Furthermore, the differentially modulated or index modulation concept can also be applied to the SEFDM scheme for mitigating intercarrier interference effects and realizing higher bandwidth efficiency [166; 167]. In addition, the Volterra series-based nonlinear equalization (VNLE) can also be employed to address the nonideal features of low-cost components. There is a linear term and nonlinear terms in the Volterra series expansion. Therefore, the linear and nonlinear impairments can be compensated simultaneously. However, as a result of the tradeoff between the equalization performance and computation complexity, VNLEs are usually designed just for the equalization of the first linear, as well as second-order nonlinear impairments [165]. Furthermore, with the progress in parallel computation, machine learning (ML) has been gaining significant attention and has been employed for nonlinearity mitigation. Furthermore, a number of ML-based algorithms such as support vector machines (SVMs) and artificial neural networks (ANNs) have been implemented in optical communication [168; 169]. In optical systems, the algorithms can be used for several purposes such as estimation, classification, performance monitoring, and failure prediction. An ANN has been considered as a suitable solution for mitigating nonlinear impairments in an optical communication system based on its capability to establish a complex nonlinear boundary between different classifications (e.g., input and output spaces) [169]. In this context, ANN-based nonlinear equalizer (NLE) can offer better system performance than the VNLE-based NLE [170]. Nevertheless, the critical issue of an ANN implementation is the associated computational complexity [168]. A transfer-learning-aided NN-based nonlinear equalization can be employed to alleviate the associated issue of the ANN-based equalization [171].