The 5th generation of mobile communication systems (5th generaTIon mobile networks, 5G for short) is getting closer to formal commercial use (2020). 5G should achieve more than ten times faster than 4G in transmission rate, that is, 5G transmission rate can achieve 1Gb/s. There are basically two ways to increase the transmission rate of wireless transmission. One is to increase the spectrum utilization, and the other is to increase the spectrum bandwidth. Compared to improving spectrum utilization, the method of increasing the spectrum bandwidth is simpler and more straightforward. The commonly used frequency bands below 5 GHz are already very crowded. In order to find new spectrum resources, the method that the major manufacturers think of is to use millimeter wave technology.
Millimeter wave definition
The microwave band includes: decimeter wave, centimeter wave, millimeter wave and submillimeter wave. Among them, millimeter wave (millimeterwave), usually refers to the electromagnetic wave with a frequency range of 30~300GHz and corresponding wavelength of 1~10mm. Its working frequency is between microwave and far infrared wave, so it has the characteristics of two kinds of spectrum. The theory and technology of millimeter wave are the extension of microwave to high frequency and the development of light wave to low frequency.
Millimeter wave development
Since Maxwell published "General Theory of Electromagnetics" in 1873, people have made extensive use of electromagnetic resources to expand the tiling. The study of millimeter waves has been proposed as early as 1889 and has been a long period of a century. The development of millimeter waves has been ups and downs, but the study of millimeter waves has always attracted many scholars, and thus gained a lot of basic knowledge. Research on millimeter waves must be supported by corresponding technologies, so research in this field has been slow, and it can be said that it is full of twists and turns. However, with the development of the corresponding technology and the inability of the infrared and visible light technologies to provide the best solution in some important occasions, the millimeter wave is increasingly distinguished by its potential research and application value due to its distinguishing characteristics from ordinary microwaves.
Until the 1970s, due to the successful development of millimeter-wave integrated circuits and millimeter-wave solid devices and mass production, the production cost was declining, and millimeter-wave communication was just like a dead spring. It can be expected that with the advancement of technology, millimeter wave communication will certainly have broad application prospects.
Millimeter wave propagation characteristics
Generally, the millimeter wave band refers to 30 GHz to 300 GHz, and the corresponding wavelength is 1 mm to 10 mm. Millimeter wave communication refers to communication using millimeter waves as a carrier for transmitting information. Most of the current application research focuses on several "atmospheric window" frequencies and three "attenuation peak" frequencies.
1, is a typical line of sight transmission
The millimeter wave belongs to the very high frequency band, and it propagates in space in the form of direct waves. The beam is narrow and has good directivity. On the one hand, since the millimeter wave is heavily affected by atmospheric absorption and rainfall fading, the single-hop communication distance is short; on the other hand, because the frequency band is high and the interference source is small, the propagation is stable and reliable. Therefore, millimeter wave communication is a typical communication technology with a high quality, constant parameter wireless transmission channel.
2, with "atmospheric window" and "attenuation peak"
The “atmospheric window†refers to the 35 GHz, 45 GHz, 94 GHz, 140 GHz, and 220 GHz bands where millimeter wave propagation is less attenuated near these special frequency bands. In general, the “Atmospheric Window†band is more suitable for point-to-point communication and has been adopted by low-altitude air-to-ground missiles and ground-based radars. The attenuation near the 60 GHz, 120 GHz, and 180 GHz bands has a maximum value of about 15 dB / km or more, which is called an "attenuation peak." Often these "attenuation peak" bands are preferred by multi-channel concealed networks and systems to meet the network safety factor requirements.
3, the attenuation is severe during rainfall
Compared with microwaves, millimeter-wave signals are much more attenuated under harsh climatic conditions, especially during rainfall, which seriously affects the propagation effect. The conclusion of the study is that the attenuation of millimeter wave signal rainfall is closely related to the instantaneous intensity of rainfall, the length of the distance and the shape of the raindrop. Further verification shows that: Generally, the greater the instantaneous intensity of rainfall, the farther the distance, and the larger the raindrops, the more severe the attenuation. Therefore, the most effective way to deal with rainfall attenuation is to leave enough level attenuation margin when designing a millimeter-wave communication system or communication line.
4, has a strong penetration of dust and smoke
Atmospheric lasers and infrared light have poor penetration into sand and smoke, and millimeter waves have a clear advantage at this point. A large number of field tests have shown that millimeter waves have a strong penetrating power for dust and smoke, and can pass sand and smoke almost without attenuation. Even under the conditions of higher intensity scattering caused by explosions and metal foil strips, even if fading occurs, it is short-lived and will recover quickly. As the ions diffuse and fall, they do not cause severe disruption of millimeter wave communication.
Advantages of millimeter wave communication
1, extremely wide bandwidth
The millimeter wave frequency range is generally considered to be 26.5 to 300 GHz, and the bandwidth is as high as 273.5 GHz. More than 10 times the total bandwidth from DC to microwave. Even considering atmospheric absorption, only four main windows can be used for propagation in the atmosphere, but the total bandwidth of the four windows is also up to 135 GHz, which is five times the sum of the bandwidths of the bands below the microwave. This is undoubtedly very attractive today when the frequency resources are tight.
2, the beam is narrow
The millimeter wave beam is much narrower than the microwave beam at the same antenna size. For example, a 12 cm antenna has a beam width of 18 degrees at 9.4 GHz and a wave speed width of only 1.8 degrees at 94 GHz. It is therefore possible to distinguish small targets that are closer together or to see the details of the target more clearly.
3, strong detection ability
Wideband broad spectrum capabilities can be utilized to suppress multipath effects and clutter echoes. A large number of frequencies are available to effectively eliminate mutual interference. A large Doppler shift can be obtained at the target radial velocity, thereby improving the ability to detect and recognize low-speed moving objects or vibrating objects.
4, security and confidentiality is good
The advantages of millimeter-wave communication come from two aspects: a) Since the millimeter wave propagates in the atmosphere, the absorption of oxygen, water vapor and rainfall is greatly attenuated, and the point-to-point straight-through distance is very short, and the signal becomes very weak beyond this distance. This increases the difficulty of the enemy in eavesdropping and interference. b) The millimeter wave beam is narrow and the side lobes are low, which further reduces the probability of its interception.
5, high transmission quality
Since the frequency band high millimeter wave communication basically has no interference source, the electromagnetic spectrum is extremely clean. Therefore, the millimeter wave channel is very stable and reliable, and its bit error rate can be maintained on the order of 10-12 for a long time, which is comparable to the transmission quality of the optical cable. .
6, all-weather communication
Millimeter waves are more resistant to rain, sand, smoke, and plasma than atmospheric lasers and infrared. This makes millimeter wave communication have better all-weather communication capability, ensuring continuous and reliable operation.
7, the component size is small
Millimeter wave components are much smaller in size than microwaves. Therefore, the millimeter wave system is easier to miniaturize.
Unique application of millimeter waves
Potential applications for millimeter waves include mm-wave imaging, sub-THz chemical detectors, and applications in astronomy, chemistry, physics, medicine, and safety.
Important frequencies include 90 GHz, 140 GHz, and above 300 GHz or THz regions. The 60 GHz band is suitable for short-range network applications due to the absorption of oxygen. Other frequency bands, such as the 90 GHz, are ideal for long-distance imaging.
1, car radar
A very important application in the imaging field is automotive radar operating at 24 GHz and 77 GHz. Today only very extravagant cars are equipped with millimeter wave radar technology. This technology can help car driving in low visibility situations, especially in foggy weather, as well as automatic cruise control and even automatic driving on future highways.
2. Millimeter wave imaging for medical applications
Another potential application for millimeter wave technology is passive mm-wave imaging. By detecting the heat radiation of an object in the millimeter wave band, the image of the object can be presented like an optical system. It is necessary to either be a set of receivers or a mobile terminal antenna to continuously scan the area of ​​interest.
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