Wireless access technology of ROF-PON Based on Cognitive Radio

With the development of the communication network towards broadband and mobile, 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, at the same time, it makes full use of the convenient and flexible wireless communication to meet people's needs for broadband. Early ROF technology was designed to provide 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 to provide both wired and wireless services for optical fiber wireless communication systems.

With the rapid development of radio communication, the shortage of spectrum resources is becoming increasingly prominent. How can we increase the spectrum utilization rate under the conditions of limited wireless resources to alleviate the contradiction between supply and demand of spectrum resources, it 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 by making secondary use of the authorized spectrum. It has become a hot topic in the communication field. In the 802.11 Wireless LAN [1], 802.16 MAN [2] and 3G mobile communication networks [3], Cognitive Radio technology has been applied to increase the system capacity, and began to study the application of ROF technology to achieve Mixed Transmission of different business signals [4]. Fiber-optic wireless communication networks for hybrid transmission of wired and wireless signals based on Cognitive Radio are the development trend of communication networks in the future. The hybrid transmission ROF System Based on Cognitive Radio technology faces many new challenges, for example, network architecture design, protocol design at various layers, generation of wired and wireless modulation signals based on a variety of services, network management, and modulation signal recognition.

1. Cognitive Radio Technology

Cognitive Radio is an effective way to address the lack of spectrum and the inadequate use of spectrum. Cognitive Radio is an intelligent wireless communication system that perceives the spectrum utilization in the surrounding environment, in addition, you can learn to adjust your own parameters to effectively use spectrum resources and reliable communication.

Cognitive Radio is a key technology that enables the allocation of spectrum resources from fixed to dynamic.

In a cognitive radio system, in order to protect authorized users (or primary users) from interference from user (or user), the function of spectrum sensing is to detect whether authorized users exist. Cognitive Radio users can temporarily use it when they monitor that the band used by authorized users is not in use. When the band of the authorized user is monitored for use, the CR user releases the channel to the authorized user to ensure that the CR user does not interfere with the authorized user.

Therefore, cognitive wireless communication networks have the following notable features:

The primary user has the absolute priority of the access channel. On the one hand, when the authorized user does not occupy the channel, the user can access the idle channel from the user's opportunities; when the primary user appears again, the user must immediately exit the channel in use, return the channel to the primary user. On the other hand, when a primary user occupies a channel, the user can access the channel without affecting the service quality of the primary user.

Communication terminals have the functions of perception, management, and adjustment. First, the CR communication terminal can perceive the spectrum and channel environment in the working environment, and determine the sharing and allocation of spectrum resources according to the test results according to certain rules; on the other hand, CR communication terminal can adjust working parameters online, such as changing transmission parameters such as carrier frequency and modulation mode to adapt to environment changes.

In cognitive wireless communication networks, spectrum sensing is a key technology. Common spectrum sensing algorithms include energy detection, matching filtering detection, and cyclic smoothing Feature Detection. These methods have their own advantages and disadvantages. The performance of these algorithms depends on the obtained prior information. Existing spectrum sensing algorithms include the matching filter, energy detector, and Feature Detector.

The matching filter can only be applied to conditions where the primary signal is known. The energy detector can be applied to unknown primary signals, but its performance degrades when a short sensing time is used. Because the main idea of the feature detector is to use the cyclic stability of the signal, the function of spectrum correlation is used for detection. Noise is a wide and steady signal with no correlation, while modulation signals are correlated and cyclic. Therefore, the spectral correlation function can distinguish between the noise energy and the energy of the tuned 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 is limited in low signal-to-noise ratio. It has high computing complexity and requires a long observation time. This reduces the data throughput of the CR system.

With the development of wireless communication technology, spectrum resources are getting increasingly tight. Since CR technology can alleviate this problem, CR technology has been paid attention to in wireless communication networks, and many standards on wireless communication networks have introduced Cognitive Radio technology. For example, IEEE 802.11, IEEE 802.22, and. In the 802.16 h protocol, Dynamic Spectrum selection is an important part to facilitate WiMAX to use radio/TV frequencies. spectrum sensing technology is the basis of this technology. Two important concepts are introduced in the IEEE 802.11 h International Standard for wireless LAN: Dynamic Spectrum selection and transmission power control (TPC). Cognitive Radio is applied to wireless LAN. In the standard, orthogonal frequency division multiplexing technology is used to provide a variety of bandwidth options to achieve fast bandwidth switching, WLAN (Wireless LAN) the system can use the characteristics of OFDM to adjust bandwidth and transmit power parameters to avoid mutual interference with other users working in this band. Fiber-optic wireless systems have been widely used because of the advantages of fiber-optic communication bandwidth and the flexibility of wireless communication. In recent years, radio-frequency cognitive WLAN signal transmission in optical fibers has attracted more and more attention, the author of article [5-6] proposed to transmit Cognitive Radio signals in the system architecture. The simulation results show that the network performance has been improved.

2 ROF-based hybrid optical fiber Wireless Transmission System Architecture

To meet the needs of multimedia services for video transmission, the emerging fiber-to-the-Home (FFTH) technology will become the ultimate broadband access technology, and the advent of Passive Optical Networks (PON) has become the focus of attention. Because all devices used in PON networks are passive devices, no power supply is required, and the devices can be immune from external electromagnetic interference and lightning effects, thus achieving transparent business transmission and high system reliability.

The main networks are time-division multiplexing Passive Optical Network (TDM-PON) and wavelength division multiplexing passive optical network.

Compared with the TDM-PON, it has the characteristics of dedicated bandwidth and high security, and has become the most promising optical access network in the future. Figure 1 shows the WDM-PON system diagram.

In Figure 1, the OLT is an optical line terminal and an optical network unit. Wireless Access Networks, WiMAX, and mobile communication networks have their own characteristics and application scenarios, so they coexist. There are other networks in the actual working environment, but the main network should be mainly optical network, we adopt the architecture based on WDM-PON. In Figure 1, only three typical wireless access networks are drawn, and other wired networks can also be connected to the system through PON-ONU. In the system architecture shown in figure 1, the wireless network and PON access units provide optical/electrical and electrical/Optical (E/O) functions without the need for modulation and demodulation, transmit Wireless RF signals directly in WDM-PON.

3 structure of ROF-PON Optical Wireless Access Network System Based on Cognitive Radio

With the development of network technology, optical networks are also evolving towards intelligence, and Cognitive Radio is applied to optical networks. Because different services are provided in optical networks, and different services have different QoS requirements, it brings new challenges to network management. Cognitive networks can provide cross-layer design and cross-layer optimization, so they can effectively monitor and optimize optical networks. In addition, the use of cognitive technology in optical networks can provide greater flexibility for telecom operators, and adjust the characteristics of various physical layer components (such as modulation format, forward correction, and wavelength capacity) According to application and service requirements) and network layer parameters (bandwidth, number of Optical Paths transmitted simultaneously, and QoS ). Figure 2 shows the architecture of the optical fiber Wireless Access Network System Based on Cognitive Radio. RF signals are directly transmitted in the optical fiber. The optical layer can adopt WDM-PON technology or be a custom optical access layer ). The sensing information obtained from the remote ONU is sent back to the Central Bureau (CO) and processed by Cognitive Radio technology. Spectrum sensing and channel access are all being processed. Therefore, the complexity of the entire system is reduced, ROF's advantages are utilized, and Cognitive Radio's expertise in spectrum management is also used.

The main features of this system structure are:

The cost of access point (AP) nodes is reduced, and the cost of the central base station is increased. In a conventional network, AP has modulation and demodulation and signal processing functions. In this solution, AP only converts light to electricity (O/E) or electricity to light (E/0) the modulation, demodulation, and management functions of AP are all transferred to the central base station, reducing the cost of Wireless Access to nodes, but increasing the cost of the central base station.

Diversity technology can be used to improve the system's link performance. Due to the use of the ROF system, different Fiber Links can receive the same user signal in the central base station to achieve macro diversity processing, while in the wireless network, collaborative diversity technology can be used to improve the link performance.

Channel Access and spectrum sensing processing are completed by the central base station, and the network front-end AP provides data collected by RF.

Although this cognitive network architecture can increase the system capacity, it also increases the complexity of network management and faces many challenges [7].

The biggest challenge is the design of the access control mechanism at the MAC layer. That is, how to propose a more optimized MAC layer protocol based on the features of the MAC layer of the network and the MAC layer of the Wireless Access Network. In the network, the optical layer and the RF layer are mutually dependent. In this case, the interaction of multiple media access control mechanisms will bring about design problems. For example, if the spectrum sensing function of the RF layer is moved to the optical layer for processing, the function should not interfere with the channel sensing capability of the RF layer when it is enabled.

In a Cognitive Radio Network, the MAC protocol is used to determine the channel access policies adopted by CR users. It is the basic premise for realizing the spectrum sharing of the entire cognitive network. MAC is designed to reduce the ratio of node transmission conflicts and improve the spectrum resource utilization.

4 Conclusion

With the development of communication networks towards broadband and mobile, the optical fiber wireless communication system integrates Optical Fiber Communication and wireless communication, giving full play to the advantages of broadband and anti-interference of optical fiber lines, at the same time, it makes full use of the convenient and flexible wireless communication to meet people's needs for broadband. Cognitive Radio can not only dynamically access the authorized spectrum, but also access other non-authorized spectrum. Therefore, the combination of Cognitive Radio and ROF can increase the system capacity and improve the system performance.