With the development of communication networks towards broadband and mobility, the optical fiber wireless communication system (ROF) integrates optical fiber communication and wireless communication, giving full play to the advantages of broadband and anti-interference of optical fiber lines, as well as wireless communication. Convenient and flexible features meet people’s demand for broadband. The early ROF technology was mainly dedicated to providing high-frequency wireless transmission services, such as millimeter wave optical fiber transmission. With the development and maturity of ROF technology, people began to study hybrid wired and wireless transmission networks, that is, optical fiber wireless communication (ROF) systems that provide wired and wireless services at the same time. With the rapid development of radio communications, the shortage of spectrum resources has become more and more prominent. How to improve spectrum utilization under the condition of limited wireless resources to alleviate the contradiction between supply and demand of spectrum resources has become a problem to be solved in the communication field. Cognitive radio (CR) is an intelligent spectrum sharing technology. It can effectively improve the utilization of spectrum resources through the “secondary use” of authorized spectrum, and has become a research hotspot in the field of communications. In 802.11 wireless local area network [1], 802.16 metropolitan area network [2] and 3G mobile communication network [3] have begun to study the application of cognitive radio technology to improve the capacity of the system, and began to study the application of ROF technology to achieve mixed transmission of different business signals[ 4]. Cognitive radio-based optical fiber wireless communication networks that transmit wired and wireless signals are the development trend of future communication networks. The hybrid transmission ROF system based on cognitive radio technology faces many new challenges, such as network architecture design, layer protocol design, generation of wired and wireless modulated signals based on multiple services, network management, and identification of modulated signals.
1 Cognitive radio technology
Cognitive radio is an effective way to solve the lack of spectrum and the underutilization of spectrum. Cognitive radio is an intelligent wireless communication system. It senses the spectrum utilization of the surrounding environment and adjusts its own parameters adaptively through learning to achieve effective utilization. Spectrum resources and reliable communication. The application of cognitive radio is a key technology to realize the spectrum resource from fixed allocation to dynamic allocation. In the cognitive radio system, in order to protect an authorized user (or become a master user) from interference from a slave user (or CR user), the function of spectrum sensing is to perceive whether an authorized user exists. Cognitive radio users can temporarily use the frequency band when it is monitored that the frequency band used by the authorized user is not being used. When it is monitored that the frequency band of the authorized user is in use, the CR user releases the channel to the authorized user, thus ensuring that the CR user does not interfere with the authorized user. Therefore, the cognitive wireless communication network has the following salient features: (1) The primary user has absolute priority to access the channel. On the one hand, when the authorized user does not occupy the channel, the secondary user has the opportunity to access the idle channel; when the primary user reappears, the secondary user should exit the channel in use in time and return the channel to the primary user. On the other hand, when the master user occupies the channel, the slave user can access the channel without affecting the service quality of the master user. (2) The CR communication terminal has the functions of perception, management and adjustment. First, the CR communication terminal can perceive the frequency spectrum and channel environment in the working environment, and determine the sharing and allocation of spectrum resources according to certain rules according to the detection results; on the other hand, the CR communication terminal has the ability to adjust the working parameters online, such as changing The transmission parameters such as carrier frequency and modulation method can adapt to changes in the environment. In cognitive wireless communication networks, spectrum sensing is a key technology. Commonly used spectrum sensing algorithms include energy detection, matched filter detection, and cyclostationary feature detection methods. These methods have their own advantages and disadvantages. The performance of these algorithms depends on the prior information obtained. The existing spectrum sensing algorithms are: matched filter, energy detector and feature detector methods. The matched filter can only be applied when the main signal is known. The energy detector can be applied to the situation where the main signal is unknown, but its performance deteriorates when a short sensing time is used. Because the main idea of the feature detector is to use the cyclostationarity of the signal to detect through the spectral correlation function. Noise is a broad stationary signal and has no correlation, while the modulated signal is correlated and cyclostationary. Therefore, the spectral correlation function can distinguish the energy of the noise and the energy of the modulated signal. In an environment with uncertain noise, the performance of the feature detector is better than that of the energy detector. The performance of the feature detector under low signal-to-noise ratio is limited, has high computational complexity, and requires long observation time. This reduces the data throughput of the CR system. With the development of wireless communication technology, spectrum resources are becoming more and more tense. Because CR technology can alleviate this problem, CR technology has been paid attention to in wireless communication networks, and many wireless communication network standards have introduced cognitive radio technology. Such as IEEE 802.11, IEEE 802.22 and IEEE 802.16h. In the 802.16h agreement, there is an important content of dynamic spectrum selection to facilitate WiMAX’s use of radio and television frequency bands, and its foundation is spectrum sensing technology. In the IEEE 802.11h international standard for wireless local area networks, two important concepts have been introduced: dynamic spectrum selection (DFS) and transmit power control (TPC), and cognitive radio has been applied to wireless local area networks. In the 802.11y standard, orthogonal frequency division multiplexing (OFDM) technology is used to provide a variety of bandwidth options, which can achieve rapid bandwidth switching. WLAN (wireless local area network) systems can take advantage of the characteristics of OFDM to avoid avoidance by adjusting bandwidth and transmit power parameters. Interfere with other users working in this frequency band. Because the optical fiber wireless system has the advantages of wide optical fiber communication bandwidth and the flexible characteristics of wireless communication, it has been widely used. In recent years, the transmission of radio frequency cognitive WLAN signals in optical fiber has attracted attention. The author of literature [5-6] proposed that the ROF system Cognitive radio signals are transmitted under the architecture, and simulation experiments show that the network performance has been improved.
2 ROF-based hybrid optical fiber wireless transmission system architecture
In order to meet the needs of multimedia services for video transmission, the emerging fiber-to-the-home (FFTH) will become the ultimate broadband access technology, and the passive optical network (PON) has become the focus of attention once it comes out. Since the devices used in the PON network are passive devices, they do not need power supply, can be immune to the influence of external electromagnetic interference and lightning, can achieve transparent transmission of services, and have high system reliability. PON networks mainly include time division multiplexing passive optical networks (TDM-PON) and wavelength division multiplexing passive optical networks (WDM-PON). Compared with TDM-PON, WDM-PON has the characteristics of user exclusive bandwidth and high security, becoming the most potential optical access network in the future. Figure 1 shows the block diagram of the WDM-PON system.