Three Minutes, but You Understand! Resurgent Millimeter Wave Communication
With the rapid development of microelectronics, optoelectronic technology and nanotechnology, satellite communication, remote sensing technology and global positioning system, broadband high-speed digital integrated communication network, information compression and high-speed transmission technology, artificial intelligence technology, multimedia technology and virtual reality technology have made continuous progress. Mankind will gradually enter the information age.The 20th century is not only the most brilliant era of scientific and Technological Development in human history, but also the era of information technology. At the turn of the century, people have higher and higher requirements for information business. When making a phone call, you should hear the sound and see the shadow; Although the meeting participants are thousands of miles away, it is like a meeting room; When watching TV, the image should be clearer than wide screen movies... To develop such high-capacity information services, it is far from enough to rely on microwave relay communication, coaxial cable communication and other communication means in the field of electrical communication. How can we solve these contradictions? "Marching to a higher frequency" is a good strategy. Because only by increasing the transmission frequency can the working frequency band be wider and the communication capacity be larger. Millimeter wave radio communication is making a comeback and unique under this situation.The reason for its comeback is that as early as the 1940s, scientists began to study millimeter wave radio communication. In the 1950s, electronic tubes were successfully developed as radio millimeter wave generators and amplifiers. However, due to poor reliability, short service life and high cost, they entered the museum without leaving the laboratory. Millimeter wave radio communication has not been applied in practice.In the 1970s, due to the successful development and mass production of millimeter wave integrated circuits and millimeter wave solid-state devices, the production cost decreased day by day, which made millimeter wave radio communication take a turn for the better, just like a dead tree in spring.The wavelength of radio millimeter wave is 10mm 1mm, and its corresponding frequency is 30 GHz 300 GHz (1 GHz = 109). Scientific experiments show that when millimeter waves propagate along space, due to the influence of the atmosphere, some frequency attenuation is small and others are large. Due to the absorption of water vapor and oxygen molecules, the transmission attenuation near 60, 120 and 180 GHz appears a maximum, which is called "attenuation peak". In contrast, the transmission attenuation near 35, 95, 140 and 220 GHz is small, which is called "atmospheric window". Because the frequency of millimeter wave is very high, if it is used as transmission medium for communication, even if it is only a small part of it, its communication capacity will be very considerable. Assuming that a small part in front of it is selected in the frequency band of 30 GHz to 300 GHz, that is, 30 GHz to 100 GHz, its working frequency band has reached 70 GHz. This frequency range is 100 times wider than the sum of the working frequency bands of microwave relay communication and coaxial cable communication, which undoubtedly provides a broad world for the development of a variety of information services. Millimeter wave communication is a novel and unique communication mode emerging with the advent of the information age.According to the family division of electromagnetic wave family, millimeter wave originally belonged to microwave portal. However, because the working frequency band of microwave communication (microwave relay communication and satellite communication) is mostly used in the range of centimeter wave (frequency is 3 30 GHz, wavelength is 10 1 cm) and decimeter wave (frequency is 300 3000 MHz, wavelength is 10 1 decimeter), millimeter wave is usually separated from micro wave band, which has become a unique and promising communication means. Some people describe millimeter wave communication as follows: if microwave communication is the sun at 8:00 a.m. in the 1940s, millimeter wave communication is the sun at 8:00 a.m. in the 1990s. According to different transmission modes, millimeter wave communication can be divided into two categories: millimeter wave radio communication and millimeter wave wired communication. Millimeter wave radio communication can be divided into ground radio communication and space radio communication. Millimeter wave wired communication is the communication of transmitting 30 120 GHz electromagnetic wave by waveguide.Compared with the traditional radio short wave, ultrashort wave and microwave communication, millimeter wave radio communication has many unique features. Because millimeter wave is adjacent to microwave and light wave (its wavelength is between microwave and light wave), it has some advantages of both microwave and light wave. The volume of the communication equipment is very small, and the compact antenna can be used to obtain high directivity, which is convenient for the concealment and confidentiality of communication. Millimeter wave is less affected by clutter in the propagation process, has strong penetration ability to dust and other particles, and the communication is relatively stable.Millimeter wave belongs to the "extremely high frequency" (EHF) band. In the huge electromagnetic wave family, its frequency is second only to "to high frequency". From the working frequency band, most of them are selected at 38 GHz, 60 GHz and 90 GHz. Millimeter wave propagates in space as a very narrow direct wave, which has good radio wave masking. It is a rare communication with low interception probability.Millimeter wave has a wide transmission band, which is more than ten times the sum of radio short wave, ultrashort wave and microwave bands. Due to the high carrier frequency, the instantaneous RF band can be very wide. Therefore, the communication capacity is very large. At present, the uplink / downlink frequency of satellite communication system (radio wave from satellite communication earth station to communication satellite is called "uplink"; radio wave from communication satellite to satellite communication earth station is called "downlink") mostly adopts 4 / 6 or 12 / 14 GHz frequency band. The use of these frequency bands is close to saturation, and there are only thousands of communication channels for each satellite. If millimeter wave band is adopted, Then it can suddenly increase to tens of thousands or even hundreds of thousands of channels. Millimeter wave is not only used for ground and satellite communication, but also suitable for satellite to satellite communication. It is an ideal interstellar link. In the vast outer space, millimeter wave energy is hardly attenuated, so long-distance communication can be realized with only a small power, which is very beneficial to military space communication.Millimeter wave is not only a good transmission medium for space communication, but also can be used in marine communication. For a long time, submarine command and communication has been a problem perplexing scientists. Submarine communication is most afraid of exposing the target. If the submarine floats to the sea for communication, it is easy to be found by enemy anti submarine weapons. Because the millimeter wave communication equipment (including antenna) is very small, it can be directly installed on the submarine periscope. As long as the periscope is exposed to the sea, it can carry out high-speed communication through satellite without affecting the concealment of the submarine.Millimeter wave is not only suitable for communication between two fixed points, but also useful in mobile communication. Recently, Japan has tried millimeter wave communication on the train for the first time and achieved success. They set millimeter wave communication equipment (small transmitter and receiver) at the front end of the train to connect with millimeter wave receivers in ground communication bureaus at stations and along the line. Using this interconnection system, they can not only transmit voice information, but also increase the transmission volume of information by more than 10 times than in the past. In addition, the train millimeter wave communication system can also play a role in large-capacity transmission of information such as image and high-speed digital communication.Millimeter wave radio communication has naturally won the favor of the designers of "information superhighway" because of its advantages of broadband, large capacity and high speed. In this regard, millimeter wave guided communication is not weak, and it is better than millimeter wave radio communication in some aspects.When millimeter wave propagates in the atmosphere, its energy will be attenuated due to the absorption of water vapor and oxygen molecules. Among them, the attenuation caused by rainfall is the most serious. Sometimes a rainstorm may interrupt the communication, so the transmission reliability is poor. In addition, rain, snow and fog also scatter millimeter waves, which destroys the directional transmission characteristics of millimeter waves to varying degrees. Therefore, millimeter wave communication is usually not an all-weather communication mode. It is mostly used for short-range point-to-point secure communication and satellite communication. Japan and other countries have sent test satellites working in the millimeter wave band into synchronous orbit.Is there any way to prevent millimeter wave from being affected by atmospheric attenuation and use it for long-distance communication trunk lines? As early as the early 20th century, scientists theoretically inferred that electromagnetic waves could be transmitted in metal tubes through the phenomenon that "tubes can transmit sound" (sound waves). In 1936, scientists used a cylindrical metal tube with an inner diameter of 12.5 cm to transmit electromagnetic waves with a wavelength of 9 cm 260 meters away. Although it was not a millimeter wave at that time (the electromagnetic wave with a wavelength of 9 cm was "centimeter wave") and the propagation distance was not far, it proved for the first time the assumption that metal tubes could indeed transmit electromagnetic waves. This experiment lays a foundation for the realization of millimeter wave guided communication in theory and practice. This kind of metal tube that can propagate electromagnetic waves is called "waveguide", or waveguide for short. Using waveguide to transmit information in the form of electromagnetic wave is called "waveguide communication". If the frequency band of electromagnetic wave is millimeter wave, it is "millimeter wave guided communication".Millimeter wave guided communication is a special communication mode, which has the characteristics of wired communication and radio communication. Because waveguide is a tangible object, it is no different from wire, cable and other transmission media in function. It can be regarded as wired communication; Millimeter wave is an invisible material, which is essentially no different from ordinary radio communication. Therefore, millimeter wave guided communication can be regarded as the "marriage" between wired electric communication and radio communication, and "wired radio communication".In millimeter wave guided communication, because the electromagnetic wave is bound to propagate in a specific space in the waveguide, it is relatively safe, confidential, reliable, strong anti-interference ability, and not easy to be affected by atmospheric attenuation; Millimeter wave propagates along the waveguide with small attenuation and long transmission distance. Generally, a relay station is set every 40 or 50 kilometers, which is close to the microwave relay distance; In addition, it has high transmission frequency and large communication capacity, and can be used as a high-capacity communication trunk line. With the construction of "information superhighway", millimeter wave guided communication, a new technology, will be rapidly applied and developed.