# Ultrafast and Wideband Microwave Photonic Frequency-Hopping Systems: A Review
## 1 Introduction  
Frequency-hopping systems, which change the information carrier frequency between a series of frequencies determined by a unique hopping code sequence (Figure 1), has always attracted great interest in radar and satellite systems, wireless radio frequency (RF) communications, as well as emerging dynamic communication systems (i.e., 5G/6G) [1]. The history of frequency-hopping dates to 1941, when it was patented by a Hollywood actress [PERSON] and pianist [PERSON]. The technology was not taken seriously in electronic countermeasures worldwide until the 1980s. With the merit of frequency-hopping systems, such as mitigating the effects of inter-symbol interference and jamming, it significantly increases the communication capacity of emerging RF wireless systems--making it an essential tool to fulfil the high RF spectral resource demand in the commercial, defense, and civilian federal marketplaces [2; 3; 4]. Furthermore, the fast frequency-hopping capability greatly increases the security of wireless services--the lack of amplitude variations and fast-changing in carrier frequency makes it challenging for Eve to interpret the information. Moreover, signal interference resulting from multi-path effects could be largely reduced by fast-changing frequency carriers. Frequency-hopping systems have been widely implemented, e.g., in Bluetooth [5], Advanced Extremely High-Frequency (AEHF) communications satellites [6], and the Military Strategic and Tactical Relay (MILSTAR) communication satellites [7]. Although frequency-hopping systems have been proved successfully in various RF systems, the hopping speed is limited to milliseconds, and the hopping-frequency bandwidth is less than several GHz, which hinders its implementation in emerging RF wireless communications and sophisticated electronic warfare systems. Due to theinherent electronic bottleneck and available frequency bandwidth of electronic devices, it is very challenging for electronics-based frequency-hopping systems to achieve hopping-frequency bandwidth over several GHz in close to real-time hopping speed.  
Microwave photonics (MWP), which bridges RF signal and photonic signal processing, is capable of overcoming bottlenecks in electronics due to its large bandwidth, instantaneous response, as well as flexibility and reconfigurability [8]. Various RF signal processing tasks have been successfully demonstrated using photonic technologies [9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19], such as MWP analog-to-digital converters, MWP filters, MWP arbitrary waveform generators, MWP image rejection mixer, and MWP frequency measurement, to name a few. Although microwave photonic approaches are capable of generating various types of RF signals [20], it is not trivial to use them for generating frequency-hopping signals due to the limited frequency tunability and hopping speed in the existing signal generating approaches, as well as the time required to stabilize at the desired frequency.  
In this review paper, we will first introduce the operation principle and configuration of several frequency-hopping systems based on microwave photonics. Several recent breakthroughs in microwave photonic frequency-hopping signal-generation schemes that enable ultrafast and wideband operation are compared. In Section 3, a bio-inspired photonic circuit mimicking the jamming-avoidance system in a gene of electric fish is presented, which has great potential in facilitating an adaptive frequency-hopping system. Finally, the paper is concluded by a discussion of the existing frequency-hopping techniques and their potential in dynamic and wideband RF signal processing.