## 6 Conclusions  
The continued evolution of mobile broadband networks relies on the exploitation of new spectra, and the creation of a robust service delivery platform, incorporating both terrestrial and satellite technologies. This points toward the likely proliferation of mm-wave frequency transmission systems for high throughout applications and use within 5G distribution networks, particularly considering recent efforts in standardization. The generation, processing and distribution  
Figure 6: Simulated system and theoretical predicted performance for varying mm-wave carrier filter bandwidths in terms of (**a**) EVM and (**b**) SNR, and (**c**) mm-wave spectral profiles of the un-filtered and filtered heterodyne mm-wave carrier, and PNC carrier band-pass filter.  
of mm-wave signals by photonic means offer a flexible and high-bandwidth solution compared to electronic technologies. Inherent compatibility with optical distribution networks and the continued advancement of chip-scale photonic circuits make the convergence of mm-wave and optical communications technologies inevitable. Through the development and evaluation of a detailed model of a hybrid photonic/mm-wave communications system employing optical heterodyning, this work has shown how such systems can be used to deliver the single and multi-carrier waveforms synonymous with both satellite and terrestrial broadband communications.  
Overall, due to its single-carrier implementation, the APSK format exhibits superior tolerance to both intensity noise and mm-wave phase noise generated through the optical heterodyne process, albeit without offering the subcarrier flexibility and single-tap frequency domain equalization provided by OFDM. The 5G compatible multi-carrier OFDM signal performs poorly in the presence of phase noise, due to the relatively low subcarrier spacing, imposing severe limitations on the optical source linewidths, ranging from kHz to sub-kHz levels.  
Extending the model to include an analog phase noise cancellation stage at the receiver reveals that these optical linewidth restrictions can be significantly alleviated for both waveforms. The evaluated system is shown to enable phase noise free operation for optical linewidth values up to \(\sim\)2 MHz, showcasing its potential to provide an efficient platform for photonic/mm-wave system deployment by enabling the use widely available optical sources, such as distributed feedback (DFB) lasers. This relaxed upper bound on optical linewidth is found to be determined by the receiver's carrier filter design. The related analysis leads to a direct relationship between the filter's bandwidth and the achievable system performance (in terms of EVM and SNR) in the presence of phase noise, with good matching exhibited for both theoretically predicted and simulated performances. This relationship serves as a critical design rule for mm-wave receiver optimization; by indicating the minimum required filter bandwidth for a target SNR/EVM and, in turn, facilitating optimum spectral usage.  
Ultimately, through the significant alleviation of optical linewidth requirements--enabling the use of cost-efficient laser diodes--and an optimized receiver design, the hybrid photonic/mm-wave system discussed and analyzed in the work has the potential to provide a flexible, low-cost and bandwidth efficient solution for next generation mm-wave applications, employing both single and multi-carrier transmission techniques.  
Conceptualization, C.B.; methodology, D.D. and C.B.; software, D.D., A.D. and C.B.; validation, D.D. and C.B.; formal analysis, D.D., C.B. and S.O.; investigation, D.D. and C.B.; writing--original draft preparation, D.D. and C.B.; writing--review and editing, D.D., S.O. and A.D.;g supervision, C.B.; project administration, C.B.; funding acquisition, C.B. All authors have read and agreed to the published version of the manuscript.  
This research was funded by Science Foundation Ireland under grant number 18/SIRG/5579.  
The authors declare no conflict of interest.