What does wireless optical communication mean and what are its applications?
Optical communication is divided into wired optical communication and wireless optical communication. Among them, wired optical communication, that is, optical fiber communication, has become one of the main transmission methods of wide area networks and metropolitan area networks; wireless optical communication is also known as free space optical communication (FSO, Free Space OpTIcal communicaTIon). In recent years, with the urgent need for high-bandwidth, low-cost access technology in the "last mile", FSO has new development opportunities in line-of-sight transmission and broadband access. At the same time, due to the rapid development of optical communication device manufacturing technology The manufacturing cost of wireless optical communication equipment has dropped significantly, and FSO has been used more and more. Although the current use of FSO does not need to pass the government's frequency license (the current radio frequency is divided into 300GHz, and the light wave is far beyond this frequency), but for the radio administration, understanding the characteristics and development trends of this new communication technology is a big Helpful.
1 Composition of wireless optical communication system
The wireless optical communication system uses the atmosphere as a transmission medium to transmit optical signals. As long as there is an unobstructed line-of-sight path and sufficient optical transmission power between the two transceivers at appropriate distances, wireless optical communication can be achieved.
The general principle of the wireless optical communication system is shown in Figure 1. It consists of a laser source, an erbium-doped fiber amplifier, a transmitting optical system, a receiving optical system, and a receiver. The specific instruments include a dedicated telephoto objective, a standard optical transceiver, and high power. The Er / Yb optical amplifier, etc., in which the telephoto objective lens and the optical transceiver are combined together. The key technologies are multipath emission and the use of amplifiers to compensate for optical channel losses.
In the case of point-to-point transmission, each end is equipped with an optical transmitter and an optical receiver, which can realize full-duplex communication. The basic technology used in the system is photoelectric conversion. The light source of the optical transmitter is modulated by the electrical signal, and the optical signal is transmitted to the receiver telescope through the atmospheric channel through the transmitting optical system as the antenna; the receiver telescope collects the received optical signal and focuses it on the photodetector. The detector converts the optical signal into an electrical signal.
Figure 1 Principle of wireless optical communication system
Due to the large difference in the transmittance of signals in different light wavelengths in the atmospheric space, FSO systems generally use band windows with better transmittance. The most commonly used optical wavelength is 850nm in the near infrared spectrum; some FSO systems use The 1500nm wavelength band can support greater system power.
2 Advantages of wireless optical communication system
At present, although the wireless optical communication technology has yet to be matured, it has huge market potential in a unique way and with significant advantages.
(1) Frequency bandwidth, high rate
In theory, the transmission bandwidth of wireless optical communication is the same as the transmission bandwidth of optical fiber communication. Optical signals in optical fiber communications are transmitted in optical fiber media, while optical signals of FSO are transmitted in air media. At present, foreign wireless optical communication systems generally use the 1550nm wavelength (frequency is about 1.935 × 105GHz) frequency band, and the transmission rate can reach 10Gbit / s (4 × 2.5Gbit / s), which can complete 120,000 voice channels, and the transmission distance can reach 5km. Domestic wireless optical communication systems generally use 850nm wavelength (frequency is about 3.529 × 105GHz) technology, the rate is 10Mbit / s ~ 155Mbit / s, and the transmission distance can reach 4km.
(2) Abundant spectrum resources
Compared with microwave technology, FSO equipment mostly uses infrared light transmission, which has very rich spectrum resources. There is no need to apply for a frequency license and pay a frequency occupancy fee from the radio management department, and it will not interfere with wireless communication systems such as microwave.
(3) Apply multiple communication protocols
As a physical layer transmission device, wireless optical communication products can be used in common communication networks such as SDH, ATM, Ethernet, and Fast Ethernet, and can support a transmission rate of 2.5 G bit / s, which is suitable for transmitting data, Information such as sound and video.
(4) Quick link deployment
FSO equipment can be directly erected on the roof of the building, and can even be deployed on the waters. It can complete communication tasks that cannot be completed by various fiber-optic communications such as ground-to-air and air-to-air. Its construction period is short, and communication can be established within a few hours. Link, and the construction cost is only about one-fifth of the underground fiber.
(5) Good transmission confidentiality
The security of wireless optical communication is very significant. Wireless optical communication has very good directivity and a very narrow beam, so it is almost impossible to eavesdrop and interfere with it.
3 Application of wireless optical communication
The main applications of wireless optical communication can be attributed to the following aspects.
(1) Provide efficient access solutions when there are no wired access conditions or the original bandwidth is insufficient
Wireless optical communication can quickly realize broadband digital communication between buildings without breaking roads and burying lines in cities. It can also realize broadband data communication transmission between the banks of large rivers in areas where it is inconvenient to lay optical cables and have no bridges. In 1994, ThermoTrex of California successfully conducted a transmission experiment between two mountain peaks at a distance of 42km and an altitude of 2133m, with a transmission rate of 1.2Gbit / s.
(2) Effectively solve the "last mile" problem
Wireless optical communication can solve the "last mile" problem of various service access, improve the transmission capacity and speed of the user access terminal, and can better meet the requirements of the triple-bandwidth of the telecommunications network, cable TV network and IP network. .
(3) Help LAN interconnection
FSO provides an option for interconnection between neighboring LANs, which not only solves the problem of high-speed transmission of user access in LANs, but also facilitates the connection between LANs to form a larger metropolitan area network and wide area network.
(4) Emergency backup plan
Wireless optical communication can be used as an emergency backup link in case of a wired communication line failure or emergency rescue, or as a communication solution for large-scale temporary activities.
(5) Quickly build a telecommunications network
For emerging telecommunications network operators, wireless optical communication networks can help them quickly build local networks and complete metropolitan area network construction with less capital, manpower, and time; for traditional telecommunications network operators, wireless optical communications The network system can be used as a supplement to its optical cable transmission system and is used in areas where it is inconvenient to lay optical cables. The construction period is short and the required cost is low. The wireless optical communication network system can realize the business model of networking first and then selling.
In addition, FSO has important applications between satellites and between satellites and ground stations. For example, in the joint experiment conducted by the United States and Japan in 1995, the two-way optical communication between the Japanese Chrysanthemum-6 satellite and the American atmospheric observation satellite was separated by 39,000 km. This is a long-distance communication application, which is still under development. However, inter-satellite optical communication has the advantages of large capacity and no need for ITU international coordination. It will become one of the important satellite communication methods.
4 Problems
Although the wireless optical communication network system is unique and has a large number of potential application markets, there are still many areas to be improved. At present, its main problems are:
(1) The alignment problem of the transceiver
FSO is a line-of-sight broadband communication technology, and strict line-of-sight transmission conditions are required between the transmitter and the receiver to achieve communication. When the communication device is installed on the top of a tall building, the device will swing under the influence of wind, which will affect the alignment accuracy of the laser. Factors such as thermal expansion or slight earthquakes in certain parts of the building structure can sometimes cause the transmitter and receiver to be out of alignment.
(2) Influence of atmospheric media
Severe weather conditions will attenuate the FSO propagation signal. Scattering particles in the air will cause different degrees of light deviation in space, time and angle. Particles in the atmosphere may also absorb the energy of the laser, attenuating the power of the signal. One of the indicators of measuring the reliability of FSO signal-LINK MARGIN, the unit is dB, which means the maximum power loss allowed by the normal operation of FSO equipment. The LINK MARGIN value of a typical FSO system is 20dB, that is, under good weather conditions, the power loss per kilometer of the optical signal is about 1dB, that is, the maximum working distance of the general wireless optical communication system is 20 kilometers. The infrared light used by FSO is susceptible to various climatic factors when it travels in the air, such as rain and strong sunlight. In foggy weather, the signal attenuation can reach 400dB per kilometer, which makes the effective working distance of the FSO system less than 50 meters, even shorter than the wireless LAN transmission distance. Therefore, FSO needs to seek an optimal wavelength frequency and find the optimal combination of wavelength and performance in the communication link.
(3) The contradiction between transmission distance and signal quality is prominent
The larger the FSO transmission distance, the wider the beam, and the worse the quality of the optical signal received by the receiving end. At present, there is no breakthrough in the research of atmospheric laser communication at a long distance.
(4) Laser safety issues
The safety of the laser beam is an issue that the FSO system must consider. The optical signal transmission power must be limited to the power range that guarantees the safety of human eyes, which also limits the communication distance of the FSO.
5 Summary
As a fast broadband network implementation, wireless optical communication has gradually become a reality. This article briefly introduces the basic concepts of wireless optical communication, the composition, characteristics and advantages of the system, as well as its application and research in the field of communication, and also analyzes its technical problems.
Future research on wireless optical communication systems will focus on increasing transmission capacity, extending transmission distance, automatic direction alignment, and reducing equipment cost. If these problems can be effectively solved, then FSO will play a huge potential and advantages, becoming a new bright spot in the field of wireless communications.
references
[1] Chen Jinying, "Application and Discussion of Wireless Optical Communication Technology", School of Information Engineering, Chengdu University of Technology.
[2] "Introduction to FSO Technology for Wireless Optical Communication", People's Post and Telegraph.
[3] Chen Cong, "Wireless Optical Communication (FSO) Technology", China Computer News, Network and Communication Edition.
[4] Dong Xinhui, "Wireless Optical Communication Technology", Hebei Shijiazhuang Electric Power Bureau.
[5] Yin Lu, "Wireless Optical Communication Rising Quietly", School of Continuing Education, Beijing Institute of Technology.
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