In the late 1990s, the global communications network began to expand on a large scale, first with the continuous expansion of the wide area network, followed by the massive construction of the metropolitan area network. At the same time, the intra-user LAN has grown rapidly. To connect these high-speed LANs to the operator's communication network, you must rely on a huge-capacity access network. Although optical fiber can solve the problem of transmission rate, the expensive and time-consuming laying has limited its development. Many wireless communication technologies can also solve the "last mile" problem. However, there are crowded frequency channels, difficulty in applying and safety issues of electromagnetic radiation. How to provide fast, low-cost, and safe broadband access has become an urgent problem to be solved.
Optical wireless broadband transmission is a newly developed access method and a product of the combination of optical communication and wireless communication. Optical wireless communication technology has become more mature with the development and utilization of laser devices. It uses a highly concentrated light beam to penetrate the atmospheric space as a transmission carrier for information, rather than transmitting signals through optical fibers. The access system of this technology is very similar in structure to the optical fiber transmission system, and the physical composition is also very simple. The user does not need to apply for wireless frequency, and the initial investment is low, the operating cost is low, it can be quickly installed, and can be provided with optical fiber. The transmission bandwidth of the system is similar. Because the system has the characteristics of fast transmission rate, simple structure, flexible networking, etc., it has high use value in occasions that do not have wired access conditions and need to quickly establish a communication network, such as across rivers and rivers, and is also receiving more and more telecommunications. Operators' concerns and the scope of applications continue to expand. Such systems already in commercial use
The amount ranges from 100 Mbit / s to 2.5 Gbit / s. There are also some experiments using wavelength division multiplexing technology to achieve a rate of 160 Gbit / s. Although it is restricted by weather conditions, it can only be used within a short distance, generally limited to less than 2 km. But in many occasions, as a unique way, optical wireless access can play a very good role, and it has become a good supplementary means for several existing broadband access methods.
1. Basic structure and technology of optical wireless communication network
Optical wireless communication is a line-of-sight transmission technology, the basis of which is electrical-optical and optical-electrical conversion, which can realize the transmission of data, images, etc., using the atmosphere as a medium. In fact, the optical wireless communication system was first developed after the laser appeared. The advantages of optical wireless communication are long transmission distance, large channel capacity, small transmitting antenna, good confidentiality and anti-electromagnetic interference. In addition, optical wireless communication does not require a license, no cable laying, no trenching, no leased lines, no spectrum planning, short construction period, and no impact on the environment. The electronic spectrum of broadband optical wireless communication is located in an extremely high optical frequency band, and there is no electromagnetic interference such as microwave. Optical wireless communication is gaining more and more attention because of these advantages.
An optical wireless communication system is composed as shown in Figure 1, including three parts: transmitter, channel and receiver. The light source of the optical transmitter is modulated by the electrical signal, and the optical signal is sent to the receiving end of the telescope through the optical telescope as the antenna. A high-sensitivity optical receiver converts the optical signal received by the telescope into an electrical signal. It is worth noting that the sending end and the receiving end must be visible from each other, and there must be no obstruction between the two terminals. Since the transmittance of the atmospheric space to signals of different optical wavelengths is quite different, a band window with a better transmittance can be selected. Optical wireless systems usually use 0.85 μm or 1.5 μm infrared bands. The 0.85μm equipment is relatively cheap and is generally used in applications where the transmission distance is not too far. The price of 1.5μm equipment is higher, but it has better performance in terms of power, transmission distance and visual safety. Most of the 1.5μm infrared light waves are absorbed by the cornea and cannot reach the retina. Therefore, the relevant safety regulations allow the power of 1.5μm wavelength devices to be 2 levels higher than that of 0.85μm devices. 1.5μm wavelength optical wireless communication has a wider range Prospects for use.
Figure 1 Schematic diagram of optical wireless communication system
According to the structure of the networking, the optical wireless communication network. Can be composed of point-to-point, star (point to multi-point) and grid (mesh network) three structures. The point-to-point structure is the simplest network topology. Most of the currently used systems adopt this structure. The reason is that most systems are only used to connect various buildings within the enterprise as high-bandwidth dedicated line connections. The advantages of the point-to-point structure are independent links and simple network planning. There are also many disadvantages. For example, it cannot be effectively expanded at low cost. There is no protection for the optical link. If a point fails, the link will be interrupted. It is not suitable for carrier-grade systems.
The advantage of the star (point-to-multipoint) structure is that the service can be concentrated to one point (hub or central node) and then connected to the core network, which is more efficient and more economical; its disadvantage is that it can provide less bandwidth and each chain The road still has no redundant protection, and its reliability is poor. In order to connect as many buildings as possible within line of sight, the location of the hub is critical.
The main advantage of the grid (mesh network) structure is that multiple network nodes can provide almost real-time detour links, so that the service is protected, that is, it has the characteristics of service recovery or service redundancy. The mesh network structure can also concentrate the business to some specific points: then access the network effectively, which is more in line with the requirements of the carrier level. The disadvantages are short transmission distance and high cost (each building has multiple links), and complicated network planning.
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