Encyclopedia of radio electronics and electrical engineering / Civil radio communications

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In our time, there is a rapid process of development of systems and means of communication, the development of traditional and non-traditional radio wave bands, including microwave frequencies, including millimeter waves (MMW). And although this range is relatively young compared to others that have long been mastered, it is now generally recognized that the frequency band occupied by the MMW far exceeds those that have hitherto been at the disposal of mankind.

For a long time, IMFs were considered unsuitable for practical use, since there were no technically advanced means of generating, receiving, sewering microwave oscillations, there was no necessary element base, and the laws of IMF propagation in the inhomogeneous earth's atmosphere were not well studied. Moreover, it is of undoubted interest to consider the trend in the development and application of millimetric communication systems for various purposes, which have been reflected in numerous domestic and foreign publications.

The creation of communication systems in the millimeter wave range is based on scientific research on the propagation of these waves and the development of principles and means for generating and receiving microwave signals at frequencies above 30 GHz.

Prominent scientists and specialists from many countries of the world, including Russia, have made a significant contribution to theoretical and experimental research in the field of MMW propagation. And even today, theory and practice reveal more and more new advantages of using MMW, in particular in communication systems. These, first of all, include an increase in the volume and speed of information transmission, the propagation of these waves in an unfavorable state of the environment, high antenna gain with a small aperture, and increased noise immunity.

However, when the IMW propagates, the signal is weakened in atmospheric gases and hydrometeors, as well as radiation depolarization, amplitude and phase changes. Moreover, the attenuation of the signal in the atmosphere tends to increase with increasing frequency and depends on weather conditions. In the atmosphere, there are also constant bands of intense absorption of radio waves due to the presence of oxygen and water vapor. These phenomena are observed at 22,2 GHz (H2O), 60 GHz (O2), 118,8 GHz (O2) and 180 GHz (H2O). Under conditions of moderate humidity of the atmosphere (~7,5 g/m3 at the Earth's surface), complete attenuation of radio waves in certain parts of the spectrum (even exceeding 200 dB) can be observed during their single vertical propagation.

Of practical interest for communication are the "transparency windows" identified by science at frequencies of about 35, 94, 140 and 220 GHz, in which there is minimal attenuation compared to neighboring IMW sections. At mid-latitudes with moderate humidity and temperature near the earth's surface (20 ° C) in the "transparency windows" the total attenuation is small and with a single vertical propagation through the atmosphere, for example, at frequencies of 94 GHz, it is 1,3 dB.

We note that, until recently, there was no statistics of various absorption levels in experimental studies of molecular absorption. The accumulation of these statistics is a very laborious task due to the strong variability of humidity values ​​and its dependence on climatic conditions.

Due to the relatively large absorption in the atmosphere, SMWs are referred to as short-range waves.

At present, the problem of IMW propagation has been largely studied, and the results of studies and theoretical calculations of molecular absorption in atmospheric hydrometeors agree quite satisfactorily. The emerging trend towards the use of the MMW range for solving various applied problems has now acquired a stable character. The possibility of their application in satellite communication systems, radio relay lines, microcellular communication, on-board communication lines and automated control systems, as well as in measuring equipment has opened up. This is due to the success in the development of the MMW element base and the creation of technically advanced devices based on it, the need to transmit large amounts of information, where the advantages of radio waves of this range are especially manifested.

MMV in satellite communications. Satellite communication systems are developing at a very fast pace. For example, in 1982, in the US satellite communications, there were about 150 repeater trunks with a bandwidth of 36 MHz each, and by the beginning of the 90s, the launch rate of satellites had increased so much that the frequency bands allocated for communication were 6/4 and 14/12 GHz were almost completely occupied.

Therefore, the task of mastering the MMW range for satellite communications is very urgent. This explains why in the last decade only the United States launched 15 IC3s with equipment operating in the 16...40 GHz frequency range. Their on-board repeaters have largely confirmed all the advantages of using MMW for satellite communications. The narrow radiation patterns of the MMW antennas contributed to the secrecy of communication and the weakening of interference interference, and the large gain led to a decrease in the power of the transmitters and reduced the weight and size characteristics of the satellite equipment.

But that's not all. The use of narrowly directed multibeam onboard antennas made it possible to switch beams to expand coverage areas, as well as to increase communication reliability in bad weather conditions due to diversity reception.

Among the highest priority IC3, the repeaters of which were developed abroad in the late 80s and early 90s to operate at frequencies above 20 GHz, are the following.

The L-SAT/OL YMPUS satellite (Western Europe) has a total operating bandwidth in the 14/11 and 30/20 GHz bands of about 6,8 GHz. The bandwidth of the trunk is 240 MHz, which provides information transmission at a speed of 360 Mbps, sufficient to organize 5500 telephone channels.

MILSTART satellite (USA) with a broadband transponder in the 44/20 GHz frequency range. The use of noise-like signals, pseudo-random frequency tuning in the 2 GHz band and signal switching on board are provided. Inter-satellite communication in the MILSTART system is carried out in the 60 GHz frequency range, in which a large attenuation in the atmosphere makes it practically impossible to create active intentional radio interference from the Earth for the operation of onboard equipment.

ECS-2 and ACTS-E satellites (Japan). The equipment operates in the frequency ranges of 30/20 and 50/40 GHz with a bandwidth of 250 MHz, with a data transfer rate of at least 400 Mbps. For this type of satellite, NTT has developed ultra-high capacity systems (not less than 7920 Gbps per IC3). It is believed that the inclusion of 15 large communication IC3 in the future system will make it possible to obtain a total throughput of satellite communication systems up to 119 Gbit / s. According to Japanese experts, the experience accumulated in the course of experiments makes it possible to start creating inter-satellite communication links operating in the MMW range.

One of the possible applications of such inter-satellite links is international communications. At the same time, the presence of a direct connection between two IC3 eliminates the need for the use of intermediate earth stations. With the help of inter-satellite links, it is also possible to communicate between several IS3 located at a distance of several tens of kilometers from each other in any one region of outer space.

There are a number of domestic satellite communication systems with spacecraft in geostationary, elliptical and low circular orbits, similar to foreign systems.

Until now, radio frequencies in the range of 0,3 ... 0,4 GHz have been allocated for low-orbit systems. But since various radio-electronic services operate here on a primary basis, it is hardly possible to obtain bands for new satellite communication networks in the future. Therefore, in the repeaters of low-orbit IC3 it is supposed to use broadband pseudo-random signals, which make it possible to avoid interference from other transmitters and, in turn, not interfere with their work. With this method of transmission, the speed in the partial channel can be 4,8 kbps, and taking into account noise-correcting coding - 2,4 kbps. The application of the MMW range in such systems is considered.

Thus, the need to increase the throughput and overall efficiency of communication systems was one of the reasons for the development of the frequency range above 30 GHz. The potential capabilities of systems in the specified frequency range are estimated at 10 thousand communication channels with a minimum information transfer rate in each channel of at least 2 Mbps. It is assumed that in 2000 the Intelsat satellite communications network alone will provide operation for about 750 telephone channels, which is 15 times higher than the system's capabilities in the 6...4 and 14...12 GHz bands.

The technical problems of using the MMW range in satellite communications include the study of methods for organizing diversity reception at ground stations when transmitting digital information at a rate of 1 Gbit / s, the development of reliable ferrite switches and switching matrices for onboard repeaters, as well as the creation of improved multi-beam antennas with increased accuracy in manufacturing elements. designs. The solution of these problems will make it possible to achieve high efficiency of satellite systems when operating in the range of 50...40 GHz, and in the organization of inter-satellite communications also in the frequency range up to 60 GHz. In the future, it is possible to use even higher frequency parts of the spectrum.

Of considerable interest are on-board radio links for communication and information transmission, designed to operate in the millimeter range. In the future, they will provide a bandwidth of 3...5 Gbps, high reliability (about 0,99998). So, for an inclined radio link with a bandwidth of 3 Gbit / s, a range of 20 km, with the dimensions of parabolic antennas on board an aircraft of 0,2 ... 0,5 m and on Earth at a reception point of 1 m, with a noise figure of a ground receiver ~15 dB, small weight and volume of the onboard equipment, the power of the onboard transmitter will be in the range of 0,1...100 W. Energy indicators, requirements for the equipment of such a radio link, with the current state of MMW technology, are quite realizable.

The use of MMW on cellular networks. In recent years, in the developed countries of the world, there has been significant progress in the creation and application of mobile communication systems in urban and rural areas. An unprecedented growth in the volume, speed and quality of the transfer of various information has been achieved on a scale not only of a single country, but also of countries located on different continents. This became possible due to the development of solid-state electronics, microelectronics, photonics, acoustoelectronics, and satellite communication systems.

However, the massive use of decimeter and even more so meter radio waves in urban communication systems creates a number of difficulties in designing transceiver and antenna-waveguide systems, increases the level of mutual electromagnetic interference and limits the transmitted frequency band, which leads to an increase in distortion during information transmission.

Further expansion of the deployment of cellular communication networks in cities is obviously impossible without the use of millimeter waves. The expediency of switching to MMW in cellular systems is confirmed by the results of studies carried out in the laboratories of the Institute of Radio Engineering and Electronics of the Russian Academy of Sciences.

Systematization and analysis of research results lead to an optimistic conclusion that in difficult urban conditions it is possible to predict the most important characteristics of the electromagnetic field at distances from several hundred meters to tens of kilometers from the radiation source. Such a forecast can be performed by statistical methods on a topographic map of the city based on data on building densities, heights and horizontal dimensions of buildings, building materials from which the walls are made, as well as taking into account the layout of urban areas, terrain and the location of antenna systems. Techniques have also been developed for calculating field characteristics when designing communication lines in urban conditions using computer databases. They make it possible to calculate energy characteristics, distributions of polarization parameters of the field, as well as classify the statistical characteristics of radio interference in urban mobile communication channels.

In particular, assuming that the transmitter power (Rizl) is 5 ... 10 mW, the receiver sensitivity is ~ 10 W in a 1 MHz band, the antenna gain is about 15 dB at a wave of 5 mm, and assuming a signal-to-noise ratio of ~ 10, it is possible to estimate the minimum range the effect of the bond on the MMW, taking into account the centers of resonant absorption in water and oxygen vapors (Fig. 1). Even under the worst propagation conditions, the length of such a link is always more than 0,5 km, which meets the requirements for such communication systems.

Taking into account the current level of development of semiconductor technology and the state of development of microelectronic circuits, there is a real opportunity to use various domestic transceivers, as well as antenna-waveguide systems for short information transmission lines in urban areas.

They can become reliable components of cellular communication systems with base stations in certain regions. With mass production, the cost of such systems on the MMW could be quite comparable with those existing on decimeter and meter waves.

In addition, in the conditions of the city they will completely solve the problem of overcrowding on the air and create a real opportunity to increase the volume of transmitted messages, at least by an order of magnitude or more. This is, for example, the use of the same frequencies for relaying messages through the so-called microcellular and picocellular systems in urban and suburban areas. Studies have shown another important advantage of using MMW. They do not have a harmful effect on a person in the premises where the transceivers are installed, as is noted during the operation of decimeter and meter wave equipment.

On fig. 2 shows an application of microcellular and picocellular communication systems in urban and suburban areas. Base station A communicates via macrocellular networks B, C, D, D, E, which provide information exchange with mobile communication objects. At the same time, microcells b and c available in the city are intended for communication with stationary objects, and pixots 1, 2, 3 ... 9 in the industrial building G function on its separate floors.

Laboratory and industrial transceivers and the state of the element base inspire confidence in the possibility of practical use of MMW in the considered cellular systems in the city.

Radio relay single-span lines on MMV. Recently, there has been a need to organize highly reliable single-span communication lines designed for the transmission of multi-channel telephony, as well as data exchange between computers and peripheral devices. For these purposes, radio relay lines of the MMW range are most suitable. They have high noise immunity, small size and weight, high bandwidth and low power consumption. Such systems include a duplex transceiver station (PPS) operating in the 42,5 ... 43,5 GHz band and designed to organize single-span digital radio relay lines up to 5 km long with an information transfer rate of 8,448 Mbps (129 telephone channels). To transmit information, frequency modulation with a modulation index equal to one is selected. The frequency spacing between the receiving and transmitting channels, as well as the value of the intermediate frequency, is 480 MHz, which allows, on the one hand, to provide the necessary isolation between the channels, and on the other hand, to organize automatic frequency adjustment relative to the stabilized local oscillator of the receiver.

With a total attenuation of 170 dB on a radio link 5 km long, the station will function normally if the gain of the transceiver antenna is at least 40 dB, the transmitter power is 30 ... 50 mW, the receiver noise figure is not more than 13 dB.

The block diagram of such a PPS is shown in fig. 3. It consists of the following functional units: a parabolic two-mirror antenna 1 with a diameter of 300 mm; waveguide bandpass receiving 2 and transmitting 4 microwave filters; polarization separator 3 (horizontal E and vertical H); mixers of receiving channel 5 and AFC channel 6 on diodes with a Schottky barrier, operating at the fourth harmonic of the local oscillator; Microwave generator based on the Gunn diode 7 with varakora frequency tuning; preliminary IF on silicon bipolar transistors 8; transistor microwave generator 9, stabilized by a dielectric resonator; frequency detector channel AFC 10; transmitter modulator video amplifier 11 and frequency detector module 12. This module is made on a single fiberglass printed circuit board and consists of a main IF with automatic gain control 13, a frequency detector on detuned circuits 14 and a video amplifier 15. The secondary power supply 16 provides a DC voltage conversion of +60 V into stabilized voltages of +12 V, -12 V and +5 V, necessary to power the functional units of the station.

Parabolic antenna, transceivers and secondary power source are structurally placed in a sealed cylindrical container with a diameter of 300 mm and a length of 250 mm. The small weight and size characteristics of the PPS make it possible in most cases to abandon the construction of special mast structures.

These examples of the use of MMW in communication systems do not exhaust the problem of their practical use. They certainly have a great future in the field of broadband communications and applications, at ground stations for communications with IC3, and in inter-satellite and airborne communications systems, as well as for organizing broadband communications in cities and towns, including pico-cell data transmission lines.

Authors: R.Bystrov, Doctor of Engineering. sciences, prof., A.Sokolov, doctor of tech. sciences, prof., Moscow

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