Previous review
    In front of us, we unveiled the RoF (radio over fiber) system's mystery and introduced its working principle and technical advantages. ROF system broke through the "electronic bottleneck" in the field of microwave communication, fully integrated the benefits of optical fiber transmission, and showed incomparable advantages in anti-electromagnetic interference, volume, weight, cost, and stability. In recent years, it has been studied by researchers. The focus of attention, and widely used, today small braid with you to disk it.
The development of information technology has promoted the rapid growth of microwave communication and optical fiber communication. The defects of microwave communication in long-distance transmission and large capacity demand have contributed to the optical fiber transmission technology of microwave signal, namely radio over fiber (RoF) technology. Therefore, the first and most extensive application of RoF technology is in optical fiber communication, such as indoor coverage, mobile base station inn, broadband wireless access, cable television, etc.; Telescope and other fields have also been widely used.
 1. Indoor coverage
 in the second generation mobile communication system, ROF is mainly used for indoor range; typical examples are large shopping centers, airport halls, exhibition centers, and other large buildings. These places need better signal coverage and signal quality, and the traditional indoor and outdoor channel sharing will affect the signal quality. Therefore, the indoor establishment of a wireless base station and distributed antenna systems (DAS) can effectively improve the coverage and increase the system capacity. Das uses a high-frequency analog transmission system to transmit microwave signals from the central station to each distributed antenna. As ROF has the advantages of low additional loss, small size, and light weight, it is ideal for the DAS transmission system.

  Figure 1 airport hall

  2. Base station Inn
One of RoF's applications is to transmit data between a central base station and a certain number of cell antennas. Operators widely use microcell technology to obtain high system capacity. The range of micro area is generally 100-200m, and the technology of micro place based on ROF is used in areas requiring a higher degree (such as in densely populated cities).

  Figure 2 mobile base station

3. Broadband wireless access
     In recent years, the research, implementation, and application of broadband wireless access technology have attracted more attention. Although there are still many problems in the application of broadband wireless access technology, various signs show that this situation is changing rapidly. The ROF system's adoption can provide high-speed, high bandwidth wireless access services, greatly simplifying the design of access end, reducing the cost of installation and maintenance. With the popularization of broadband access services, ROF will become a widely used broadband wireless access technology.
 4. Promote intelligent driving
     Intelligent transport system (ITS) has been widely used to provide information services such as traffic conditions, road conditions, and the best route According to the different services offered. Its communication system has a diverse network structure. At present, there are two main types of research: road to vehicle communication (RVC) and inter-vehicle communication (IVC). With the development of research, its communication system operating in microwave band has been widely concerned. Panasonic has developed a 36GHz RoF and RVC access system, which can provide broadband access and various services. Simultaneously, it has the characteristics of simple installation and low cost, showing the advantages of the RoF access network.

  Figure 3 intelligent driving

 5. Assist rail transit system
   The application of RoF technology in high-speed railway transportation has been reported for a long time. For example, in Japan, ROF technology is not only used in rail transportation but also the intelligent transportation system of expressway Using ROF technology in rail transit and antennas distributed along the road can form a wireless network with seamless coverage along the railway In the specific implementation, 

  Figure 4 rail transit

6. Integration with a cable TV system
     CATV business has been a powerful promoter to promote the application of optical fiber in the local ring. CATV network began to use optical fiber to replace coaxial cable before the development of digital TV technology. With the development and popularization of digital TV technology and the fixed CATV service, mobile CATV service is also proposed in many areas. Simultaneously, other broadband broadcasting services and convenient communication services, such as voice and Internet access, are continuously carried out. Combining the ROF technology with the existing CATV network, based on using the original optical fiber resources, ROF technology can achieve access to a variety of services and has the characteristics of simple access equipment and fast networking.
7. Microwave photon radar
     Radar is the primary means of all-weather target detection and recognition. Multi-functional, high-precision, and real-time detection has always been the target of radar researchers. These characteristics are based on the high-speed control of broadband microwave signals, but limited by the "electronic bottleneck," the generation, management, and processing of broadband signals are overly complicated or even impossible to complete in traditional electronics. The inherent characteristics of photon technology, such as large bandwidth, low transmission loss, and anti-electromagnetic interference, make it a key enabling technology to break through the bottleneck of radar bandwidth "illuminate the future of radar." Simultaneously, the photon system is light in weight, small in size, and integrated, which can reduce the volume and weight of radar systems dozens of times, thus significantly reducing the load of aircraft, satellite, ship, etc. Therefore, the introduction of photon technology is likely to change the existing radar system and give the radar system more vigorous vitality

 Figure 5 progress of DARPA microwave photon radar in the United States

8. Astronomical radio telescope
     With the continuous improvement of bandwidth, dynamic range, and noise coefficient of optical transceivers, optical fiber transmission links in radio telescope are common. Some high-performance radio telescopes at home and abroad have adopted optical fiber transmission systems to achieve high-quality microwave signal transmissions, such as ovens Valley array in the United States, GBT, EVLA, Nobeyama in Japan, even in South Korea, etc., 

 Figure 6 "SkyEye" radio telescope
    They can see that ROF technology has played an essential role in both military and civil fields. With the continuous efforts and promotion of relevant researchers, more and more areas will introduce ROF technology.