What is the meaning of wireless optical communication, 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 network and metropolitan area network; wireless optical communication is also called free space optical communication (FSO, Free Space Optical communication).

In recent years, with the urgent demand for high-bandwidth and 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 use of FSO does not require the government's frequency license (the current radio frequency is allocated to 300GHz, and light waves far exceed this frequency), it is important for the radio management department to understand the characteristics and development trends of this new communication technology. helpful.

1 Composition of wireless optical communication system

The wireless optical communication system uses the atmosphere as the transmission medium to transmit optical signals. Wireless optical communication can be realized as long as there is an unobstructed line-of-sight path and sufficient optical transmission power between two transceivers at an appropriate distance.

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 special telescopic objective lens, a standard optical transceiver and a high-power Er/Yb optical amplifiers, etc., in which the telescopic objective lens and the optical transceiver are combined. Its key technologies are multipath emission and the use of amplifiers to compensate optical channel loss.

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 through the transmitting optical system as the antenna, the optical signal is transmitted to the receiver telescope through the atmospheric channel; the receiver telescope collects the received optical signal and focuses it in the photoelectric detector, the photoelectric The detector converts the optical signal into an electrical signal.

Due to the large difference in the transmittance of different optical wavelength signals in the atmospheric space, the FSO system generally selects the band window 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 is still to be mature, it has a huge market potential in a unique way and significant advantages.

(1) Bandwidth, high speed

Theoretically, the transmission bandwidth of wireless optical communication is the same as that of optical fiber communication. The optical signal in optical fiber communication is transmitted in the optical fiber medium, while the optical signal of FSO is transmitted in the air medium. At present, foreign wireless optical communication systems generally use the 1550nm wavelength (frequency is about 1.935×105GHz) frequency band, the transmission rate can reach 10Gbit/s (4×2.5Gbit/s), and 120,000 voice channels can be completed, 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 adopts infrared light transmission mode, has very rich spectrum resources, does not need to apply for frequency license and pay frequency occupation fee to the radio management department, and will not interfere with wireless communication systems such as microwave.

(3) Applicable to various communication protocols

As a physical layer transmission device, wireless optical communication products can be used in common communication networks such as SDH, ATM, Ethernet, Fast Ethernet, etc., and can support a transmission rate of 2.5 G bit/s, suitable for transmitting data, audio and video information.

(4) Fast deployment of links

FSO equipment can be directly erected on the roof of a building, or even deployed on waters, and can complete communication tasks that cannot be completed by various optical fiber 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 that of underground optical fiber.

(5) Good transmission confidentiality

The security of wireless optical communication is very significant. Wireless optical communication has very good directivity and very narrow beam, so it is almost impossible to eavesdrop and interfere with it.

3 Applications of wireless optical communication

The main application of wireless optical communication can be summed up in the following aspects.

(1) Provide efficient access solutions when wired access conditions are not available or the original bandwidth is insufficient

Wireless optical communication can quickly realize broadband digital communication between buildings without breaking roads and embedding wires in the city, and can also realize broadband data communication transmission in areas where it is inconvenient to lay optical cables and between the banks of large rivers without bridges. In 1994, the ThermoTrex company in California successfully carried out a transmission experiment between two peaks with a distance of 42km and an altitude of 2133m, and the transmission rate was 1.2Gbit/s.

(2) Effectively solve the "last mile" problem

Wireless optical communication can solve the "last mile" problem of various business access, improve the transmission capacity and speed of user access terminals, and can better meet the bandwidth requirements of the telecommunications network, cable TV network and IP network. .

(3) Assisting LAN interconnection

FSO provides an option for the interconnection and intercommunication between adjacent LANs. It can not only solve the high-speed transmission problem of user access in the LAN, but also easily realize the connection between LANs to form a wider 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 cable communication line failure or emergency rescue, and can also be used as a communication solution for large-scale temporary activities.

(5) Quickly build a telecommunication network

For emerging telecom 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 telecom network operators, wireless optical communication The network system can be used as a supplement to its optical cable transmission system for areas where laying optical cables is inconvenient. 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, between satellites and ground stations. For example, in the joint test 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 at a distance of 39,000km was realized. This is a long-distance communication application, which is still under research and development, but inter-satellite optical communication has the advantages of large capacity and no need for ITU international coordination, and will become one of the important means of satellite communication.

4 Existing problems

Although the wireless optical communication network system is uniquely attractive and has a large number of potential application markets, there are still many areas to be improved. Currently, its main problems are:

(1) Transceiver alignment problem

FSO is a line-of-sight broadband communication technology, and strict line-of-sight transmission conditions are required between the transmitter and receiver to achieve communication. When the communication equipment is installed on the top of a tall building, the equipment will swing under the action of wind, which will affect the alignment accuracy of the laser. Factors such as thermal expansion of certain parts of the building structure or minor earthquakes can also sometimes cause the transmitter and receiver to misalign.

(2) Influence of atmospheric media

Severe weather conditions will attenuate the FSO's propagation signal. Scattering particles in the air will cause the light to have different degrees of deviation in space, time and angle. Particles in the atmosphere can also absorb the laser's energy, attenuating the power of the signal. One of the indicators to measure the reliability of FSO signals - 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 fine weather conditions, the power loss per kilometer of the optical signal is about 1dB, that is to say, the maximum working distance of a general wireless optical communication system is 20 kilometers. The infrared light used by FSO is easily affected by various climatic factors, such as rain, strong sunlight, etc., when it propagates in the air. 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 will be, and the poorer the quality of the optical signal received at the receiving end. At present, there is no breakthrough in the research of long-distance atmospheric laser communication.

(4) Laser safety issues

The safety of the laser beam is an issue that must be considered in the FSO system. Optical signal transmission power must be limited within the power range to ensure the safety of human eyes, which also limits the communication distance of FSO.

5 summary

Wireless optical communication, as a fast broadband network implementation, has gradually become a reality. This paper briefly introduces the basic concept, system composition, characteristics and advantages of wireless optical communication, 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 costs. If these problems can be effectively solved, then FSO will play a huge potential and advantages, and become a new bright spot in the field of wireless communication.

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