# Ultrafast and Wideband Microwave Photonic Frequency-Hopping Systems: A Review
## 4 Discussions and Conclusions  
This paper reviews several state-of-the-art microwave photonic frequency-hopping systems that enable fast frequency-hopping for wireless RF systems. Frequency-hopping systems has the potential to improve information security, mitigate jamming and interference, and increase spectral efficiency, which are of great interest in military, commercial, and consumer applications. Microwave photonics have been promising in providing improved hopping speed and frequency-hopping bandwidth compared to its electronic counterparts, which essentially increases the capability of data transmission, improves information security as well as enhanced signal fidelity. Recent advancement in microwave photonic frequency-hopping schemes enable hopping speed ranging from picoseconds to nanoseconds and the hopping-frequency bandwidth over tens of GHz with hopping stability. Moreover, some of the microwave photonic approaches could be used for multi-level frequency-hopping, including flexible OEOs, tunable MWP filters, and nonlinear period-one dynamics in lasers. To achieve both multi-level frequency-hopping and fast switching speed, multiple frequency-hopping approaches with fast switching time could be incorporated together to increase the level of frequency-hopping while maintaining the high hopping speed.  
The rapid development of real-time frequency-hopping speed as well as the transparency of data formats, frequency, phase and amplitude in the frequency-hopping system could be potentially used in the future THz wireless communication and be used in any wireless communication  
Figure 15: Spectral waterfall measurement of the photonic JAR in action with sinusoidal reference signal \(f_{R}\) and jamming signals \(f_{J}\).(**a**) The jamming signal \(f_{j}\) is approaching \(f_{R}\) from the low frequency side and triggers the JAR, (**b**) The jamming signal \(f_{j}\) is approaching \(f_{R}\) from the low frequency side and triggers the JAR, and then is moved away, (**c**) The jamming signal \(f_{j}\) is approaching \(f_{R}\) from the high-frequency side and triggers the JAR, (**d**) The jamming signal \(f_{j}\) is approaching \(f_{R}\) from the high-frequency side and triggers the JAR and then is moved away.  
transmission scenario. Furthermore, the development of bio-inspired adaptive frequency-hopping system could identify the available hopping frequency and simultaneously allow the jamming-free frequency-hopping data transmission. Looking forward, the practical application of microwave photonic frequency-hopping system can be further improved and optimized by miniature, integration, and incorporating intelligence to adapt to more sophisticated wireless communications.  
Resources and Review, Q.L.; Project Administration and Supervision, M.P.F.; All authors contributed to data analysis and manuscript writing. All authors have read and agreed to the published version of the manuscript.  
The research work is supported by research grant from National Science Foundation, grant numbers ECCS 1653525 and ECCS 1917043.  
The authors declare no conflict of interest.