# Ultrafast and Wideband Microwave Photonic Frequency-Hopping Systems: A Review
## 3 Adaptive Frequency-Hopping System Bio-Inspired by a Gene of Electric Fish--Eigenmania  
The above frequency-hopping systems are solely depending on a hopping code sequence to determine what frequency the system should be using. The applied pseudorandom hopping code sequence (corresponding to channel assignment) is pre-set to achieve a pre-determined frequency-hopping sequence and is only known by the transmitter and target receiver. Frequency-hopping system reduces the possibility of jamming by hopping to different pre-set frequency at different time, resulting in extremely short dwelling time at a particular frequency. To facilitate a dynamic usage of the RF spectrum, it is desirable to have an adaptive frequency-hopping scheme that responses to the population in the RF spectrum, and hops its frequency to the available spot to avoid jamming and increases the use of the whole spectrum effectively. Inspired by the jamming-avoidance response (JAR) in a gene of electric fish--Eigenmania, an adaptive frequency-hopping photonic circuit that is capable of detecting potential jamming and then spectrally moves away from the potential jamming frequency is experimentally demonstrated [60]. By mimicking the JAR neural algorithm with photonics [61; 62; 63], the resultant dynamic frequency-hopping system has wideband and instantaneous response benefiting from photonic systems. The dynamic frequency-hopping system is efficient and have adaptive capability thanks to the unique design of the neural algorithm. Figure 14 shows the block diagram of the bio-inspired photonics-based adaptive frequency-hopping system. As shown in Figure 14, there are four building blocks that enables the JAR, including (1) Zero-crossing point detection unit (ZeroX unit), (2) Phase detection unit, (3) Amplitude unit, and (4) Logic unit. In the scheme, the reference signal is the current transmitting frequency, while the jamming signal could be the transmitting frequency occupied by another user. All signals are modulated onto optical carriers such that they can be processed in the optical domain using the photonic JAR. The photonic ZeroX unit uses self-phase modulation and offset spectral filtering to generate spikes at the positive zero-crossing points of the reference signal. A beat signal, \(S_{B}(t)=\sin(2\pi f_{R}t)+\sin(2\pi f_{f}t)\), between the reference signal and the jamming signal is generated and will be used in the phase unit. The phase unit takes both the zero-crossing spikes and the beat signal as the input, and use cross-gain modulation in SOA to identify the phase relationship between them. The beat signal is the pump that tries to deplete the carriers in the SOA to induce cross-gain modulation, while the zero-crossing point spikes are the probe signal that could experience the cross-gain modulation effect. Each peak in the beat signal and the zero-crossing spikes are a pair, which is either able or not able to have cross-gain modulation, depending on the phase relationship within the pair. If the phase of the beat signal is leading, the peak of the beat signal enters the SOA before the zero-crossing spike and causes cross-gain modulation to the zero-crossing spike. On the other hand, if the phase of the beat signal is lagging, the peak of the beat signal enters the SOA after the zero-crossing spike, making it unable to cause cross-gain modulation to the zero-crossing spike. Therefore, a weak zero-crossing spike is resulted if  
Figure 14: Illustration of the JAR design for adaptive frequency-hopping system. The four functional units are ZeroX unit, Phase unit, Amplitude unit, and Logic unit. the phase of the beat signal is leading, while a strong zero-crossing spike is resulted if the phase of the beat signal is lagging. Then, the amplitude unit takes the envelope of the beat signal and make an inverted copy using cross-gain modulation at an SOA.