# Frequency Comb-Based WDM Transmission Systems Enabling Joint Signal Processing
## 5 Conclusions and Outlook
### Future Outlook  
There are several prospective future research areas related to joint processing in comb-based systems. The first one deals with the limitations of joint processing in terms of transmission distance. So far, the transmission experiments have been limited to single spans or back-to-back. Fiber transmission comes with the additional challenges of dispersion that will cause a walk-off between the subchannels, as well as fiber nonlinearities that will cause additional phase noise. To fully understand how these effects affect joint processing, long-haul transmission experiments need to be performed. A second interesting area is the possibility for co-integration of comb-sources and transceivers. The demonstrations of joint processing described in this work were based on discrete-component EO-combs. While EO-combs can be integrated on a single chip [110], other promising comb-sources are microring resonators [52] and quantum-dash mode-locked lasers [36]. Furthermore, quantum-dot mode-locked lasers show high phase coherence [111; 112]. For these comb-sources, joint processing remains to be demonstrated. The third area is understanding the full implementation aspects of joint processing. The benefits of frequency combs as light sources combined with joint processing depend on the cost and power consumption of several parts of the transceiver. Thus, a wide tradeoff analysis including the power consumption of lasers, comb generation and DSP ASICs is needed. Fourth, the potential for combining SDM, frequency combs and joint processing should be further investigated. Utilizing frequency combs in SDM transmission has been demonstrated [28], as well as joint processing in SDM systems [100; 101]. In [91], the receiver comb was regenerated using a set of pilot tones transmitted in a separate core. Combining pilot tones and data channels in the same core would lower the overhead further. In addition, also joint-DSP methods could potentially be extended to utilize both spectral and spatial phase coherence. [PERSON] and [PERSON] planned and wrote Section 1. [PERSON] and [PERSON] planned and wrote Section 2. [PERSON]-[PERSON] and [PERSON] planned and wrote Section 3. [PERSON] planned and wrote Sections 4 and 5. The experiments reported in Section 4 were planned and performed by [PERSON] with support from [PERSON] and [PERSON]-R. [PERSON] took the overall responsibility in managing the manuscript. [PERSON], [PERSON] and [PERSON] supervised the work and provided technical leadership. All co-authors contributed to the final version with suggestions and critical comments. This work was financially supported by the Swedish research council and the [PERSON] and [PERSON]. The authors declare no conflict of interest. * [PERSON] _et al._ (1972) [PERSON]; [PERSON]; [PERSON]; [PERSON] High-repetition-rate optical pulse generator using a Fabry-Perot electro-optic modulator. _Appl. Phys. Lett._**1972**, _21_, 341-343. [CrossRef]
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