Directional radio

High alpine directional radio antenna on the Jungfraujoch

As a radio relay ( English transmission Microwave or directional (directive) radio ), a wireless communication (including data or information transmission) by means of radio waves (including radio or Hertzian waves ) denotes that from a starting point to a defined target point (English: point-to -point ). The terms directional radio , radio relay station , radio relay system or radio frequency are derived from this special feature in the German-speaking scope of this radio application . The German frequency administration decreed that directional radio stations are usually fixed radio stations and are assigned to the fixed radio service .

The directivity of this radio application results from the use of energy bundling antennas , which largely limit the electromagnetic energy transmission to the desired direction; in contrast to the round radiation of the transmission power when broadcasting service . By concentrating the transmission power in this direction, comparatively lower transmission powers are sufficient for directional radio than for omnidirectional radiation . This directional effect also results in multiple reusability of the same radio frequencies or radio frequency channels for several directional radio directions, lines or routes. According to the frequency plan, the frequency ranges with the designation "fixed radio service" are explicitly permitted for this radio application (message transmission using radio relay ) . Operators of this radio application can obtain a usage license with a corresponding frequency allocation from the relevant frequency administration . Directional radio stations contain devices for generating these radio frequencies and for modulating with the signals of the data, messages or information to be transmitted.

particularities

• Depending on the frequency range, directional radio is divided into digital point-to-point or digital point-to-multipoint directional radio and is mainly used for long-range connections in telecommunications. In addition, the digital radio relay serves as an infrastructure to and from height platforms or as an alternative radio connection of subscriber lines to wired subscriber lines.
• Military radio applications and BOS radio are usually for fixed service , although in addition to fixed radio relay points also tactically movable or mobile radio relay points are used. The users in question therefore receive frequency allocations from frequency ranges which contain the frequency range allocation of a fixed radio service and / or the mobile land radio service .

Historical development of radio relay

Directional antennas on the English Channel

Heinrich Hertz's mirror experiment was basically a radio relay system.

The first directional radio link for the transmission of an analogue telephone channel was put into operation in 1931 between Calais in France and St. Margaret's Bay near Dover in England . It worked at a radio frequency of 1.7 GHz with rotating parabolic antennas 3 m in diameter, the transmission power was 1 W and the radio field length was 40 km. The first multi-channel radio link that could transmit nine analogue telephone channels at a radio frequency of 65 MHz was established in 1936 between Scotland and Belfast in Northern Ireland .

After the end of the Second World War, radio relay systems made a significant contribution to the development of national and international telecommunications networks. Radio relay systems were used almost exclusively in the long-distance network. Radio field lengths between 30 km and 60 km were the rule. Important connections in the telecommunications networks were carried out in parallel both via coaxial cable lines and via radio relay systems. The first transmission of a television program via the international radio relay network, which has now been established, took place in 1953 on the occasion of Elizabeth II's coronation.

Until around 1980, analog radio relay systems with a transmission capacity of up to 2700 telephone channels and radio frequencies between 1.9 GHz and 11 GHz were in use. The transmission of television took place almost exclusively via radio relay. The transmission power was 0.5 watts for systems with 120 telephone channels and 20 W for systems with 2700 telephone channels. Frequency modulation has established itself as the modulation method for multi-channel systems.

Radio relay location with cellular base station in Spachbrücken, Hesse, Germany

From around 1970 digital transmission methods were gradually introduced into the networks. With optical transmission systems it was now possible to transmit very high bit rates over large distances without repeaters. As a result, all metropolitan areas were networked with optical transmission systems. As a result, the area of ​​application of radio relay systems shifted to the regional and local network level of the telecommunications network.

After the reunification of the two German states in 1990, the telecommunications network in the new eastern federal states had to be expanded and connected to the network of the western federal states. This task was successfully solved through the massive use of digital radio relay systems. In 1991 the development of the digital mobile communication networks started. For cost reasons, large parts of the fixed network of mobile communication systems are implemented with radio relay systems. The network extensions in particular are predestined for the use of radio relay systems. With the entry into force of the Telecommunications Act in 1996, Germany's previous telecommunications monopoly was ended. Private companies were now able to set up and operate their own telecommunications networks. Many connections in these newly created networks are also routed via radio relay. At the end of October 2013, there were more than 125,000 radio links in operation in Germany with annual growth rates of 10%. Germany probably has the densest radio relay network in the world.

Frequency ranges between 3.8 GHz and 86 GHz with a bandwidth of 41 GHz are available for directional radio in Germany. In Germany, frequencies for radio link connections are allocated by the Federal Network Agency (BNetzA). Mobile communications continue to contribute most to the further expansion of directional radio networks. Directional radio stations are mostly used as locations for cellular radio base stations (see figure). As an alternative and supplement to wire-based transmission systems, radio relay systems are still an indispensable transmission medium in national and international telecommunications networks.

Structure of a radio link

The emission and reception of the electromagnetic waves takes place with directional radio links by parabolic antennas with a high directional effect. There is a line of sight between the transmitting and receiving antennas. Radio relay systems are usually point-to-point radio systems. The use of point-to-multipoint systems is limited to special cases. In terms of transmission quality and availability, microwave links are subject to the same requirements as transmission systems that use fiber optic cables as the transmission medium.

Figure 1: Directional radio line

Figure 1 shows a radio link between terminals A and B in the scheme. Since there is no line of sight between locations A and B, a relay station is required. In this example, the connection consists of two directional radio links. In many cases, the locations of relay stations are nodes of the radio relay network (see figure).

Relay station with lattice mast in Weiterstadt, Hesse, Germany

Radio relay systems are usually bidirectional transmission systems. The device configuration for this is given in the lower part of Figure 1. In the modulator M, the digital data stream to be transmitted is impressed on an intermediate frequency carrier. Quadrature amplitude modulation methods with 4 to 2048 levels (4QAM to 2048QAM) are used as the modulation method. In the transmission assembly S, the intermediate frequency carrier is converted into the radio frequency level and its power level is raised to the transmission level. The usual transmission levels of radio relay systems are between 20  dBm (= 100 mW) and 30 dBm (= 1 W). The radio frequency carrier is fed to the antenna via a transceiver switch and emitted in the direction of the opposite station. There the carrier arrives at receiver E, which amplifies the received signal and resets it to the intermediate frequency level. The intermediate frequency carrier is finally demodulated in the demodulator and the data signal recovered in the process is regenerated.

Belt position rule

Directional radio links usually work bidirectionally and are therefore symmetrical. Internationally, fixed frequency bands with radio channels with fixed bandwidth and center frequency are defined for the best possible use of the frequency range . For bidirectional transmission, the frequency bands are divided into a lower and an upper band, each of which includes an equal number of radio channels.

For example, the 13 GHz frequency range used for radio link connections according to ITU-R F.497 comprises eight radio channels in the lower and upper bands with a bandwidth of 28 MHz per channel. One channel from the lower and one upper belt each form a channel pair, one channel from it being operated in one direction and the second channel operating in the opposite direction. The so-called diplex spacing of these two channels between the lower and upper band is 266 MHz in this frequency range and, thanks to the large frequency spacing, allows interference-free operation between transmitter and receiver.

Since directional radio links over longer distances generally consist of several partial radio links, so-called radio fields, the so-called band position rule is applied for optimal use of frequencies. This rule states that on a radio mast, all radio links in a certain frequency range may only transmit in the upper band position or only in the lower band position, but never mixed. This requirement avoids mutual interference between the transmitters and receivers at the same location. Each directional radio station along the route is either a so-called upper band location (all directional radio units at this location only transmit in the upper band) or a so-called lower band location (all directional radio units at this location only transmit in the lower band). This requirement applies across all operators for all transmission systems at one location. In order for a radio link consisting of several radio fields to manage with only one pair of channels, there is a requirement within the framework of the band position rule that upper or lower band locations alternate along the radio link. This means that several successive sections can be operated with just one pair of channels, which should practically never be in a straight line. In exceptional cases, for example when setting up meshed radio relay networks or branches, there may be a deviation from the band position rule. In such cases, for example, channels from other frequency ranges are used for individual routes.

The directional radio channel

The directional radio channel comprises the radio field including the transmitting and receiving antenna. The electromagnetic wave between the transmitting and receiving antenna spreads in the troposphere . The propagation conditions are therefore dependent on the weather and thus also on the geographic location of the radio field. They change with the time of year and day. These temporal and local dependencies of the propagation behavior flow into the planning calculations for a radio link.

If the first Fresnel zone is free of obstacles, the following applies to the radio field attenuation : ${\displaystyle a_{F}}$

${\displaystyle a_{F}=92{,}4+20\cdot \log {\frac {f}{\mathrm {GHz} }}+20\cdot \log {\frac {d}{\mathrm {km} }}-g_{1}-g_{2}}$

${\displaystyle f}$= Radio frequency, = radio field length, = antenna gain in dB ${\displaystyle d}$ ${\displaystyle g_{1},g_{2}}$

This tailored size equation is derived from the formula for the free space attenuation . The first Fresnel zone is an area around the line of sight. Your radius at this point is: ${\displaystyle r_{x}}$ ${\displaystyle x}$

${\displaystyle r_{x}={\frac {{\frac {x}{\mathrm {km} }}\cdot {\frac {d-x}{\mathrm {km} }}}{2\cdot {\frac {R_{\mathrm {E} }}{\mathrm {km} }}}}\cdot 1000\,\mathrm {m} }$

${\displaystyle R_{\mathrm {E} }}$ = Radius of the earth

The line of sight is also known as a directional radio beam. Most of the time, the refractive index of the troposphere decreases linearly with altitude, so that the directional radio beam is refracted towards the earth, i.e. it is curved. In order to get a straight line of sight in the representation, the earth radius is multiplied by the so-called k-factor. In Europe, the k-factor is k = 1.33 (normal atmosphere) more than 50% of the time.

Sometimes, however, inversion layers form in the troposphere, so that the directional radio beam is refracted differently. For example, several directional radio beams propagate over paths of different lengths that are superimposed in the receiving antenna (see Figure 2). The result is multi-way reception . As a result, broadband fading ( English flat fading ) in combination with selective fading ( English selective fading ) occurs at the output of the receiving antenna in the spectrum of the radio relay .

Figure 2: Multipath propagation

Multipath propagation is the predominant disruptive propagation phenomenon for radio field lengths greater than 30 km, as is the rule for frequencies below 10 GHz. At frequencies above 10 GHz, rain attenuation is the predominant interference factor. Rain creates broadband fading. The rain attenuation increases with increasing frequency, so that the realizable radio field lengths decrease with increasing radio frequency. From around 20 GHz, in addition to the damping of rain, the absorption by atmospheric gases gains influence. The main absorption occurs through the water vapor and the oxygen molecules in the air. The specific absorption attenuation has a maximum at 23 GHz and at 60 GHz.

Radio relay systems

The system assemblies modulator and demodulator are connected to the assemblies for power supply and the interfaces in an indoor unit (IDU short, of English indoor unit condensed). The transmitter and receiver form the radio . It is in many cases as an external unit (ODU short of English Outdoor Unit ) is executed and installed near the antenna. The outdoor unit is connected to the indoor unit via a coaxial cable, via which the power supply for the outdoor unit also runs (see illustrations).

Indoor unit (IDU) and antenna with flange-mounted outdoor unit (ODU)

Installation of an antenna including two external units (ODUs) in Frankfurt am Main (source: CBL)

The modulator and demodulator are implemented in the form of complex, programmable integrated circuits for digital signal processing. This means that the modulation method and bandwidth can be configured according to the different requirements. The most important system parameters are the transmission level , the system threshold (reception level with a bit error frequency of ), bit rate and modulation method. Bit rate and modulation method determine the required bandwidth in the electromagnetic spectrum. In a radio frequency channel with a bandwidth of 28 MHz, a net bit rate of 193 Mbit / s can be transmitted using the 256QAM modulation method. If a higher bit rate is required, the number of stages in the modulation process can be increased. When using both polarization directions (horizontal and vertical) in the same radio frequency channel, the bit rate doubles. In addition, several systems on the route can be operated in parallel in different radio frequency channels. A net bit rate of 1 Gbit / s can be transmitted in a bandwidth of 56 MHz and 1024QAM. ${\displaystyle L_{S}}$ ${\displaystyle L_{\text{Schw}}}$ ${\displaystyle 10^{\text{-6}}}$

To counteract the effects of multipath propagation, the demodulator contains an adaptive time domain equalizer (ATDE for short, from English Adaptive Time Domain Equalizer ). When using both polarization directions in the same radio frequency channel, the two differently polarized carriers influence each other. The demodulators of the receivers for vertical and horizontal polarization are therefore coupled to one another via a cross polarization compensator (XPIC for short, from English Cross Polarization Interference Compensator ), which compensates for these influences. The transmission level of radio relay systems is lowered by automatic level control (ATPC for short, from English Adaptive Transmitter Power Control ) to a value that is around 10 dB above the system threshold in a non-fading time. This minimizes interference to neighboring radio links. In the event of fading events in the radio field, the transmission level is increased according to the fading depth until a so-called operational transmission level is reached.

Planning of microwave links

In the first step of planning directional radio links, a terrain section is created. This checks whether the first Fresnel zone is free of obstacles. If the first Fresnel zone is partially shaded, this can be taken into account by additional attenuation . In the next step, the reception level is calculated in non-fading time: ${\displaystyle a_{\text{zus}}}$ ${\displaystyle L_{E}}$

${\displaystyle L_{E}=L_{S}-a_{F}-a_{\text{zul}}-a_{\text{zus}}-a_{\text{abs}}}$

( = Attenuation of the antenna feed lines, = absorption attenuation ) ${\displaystyle a_{\text{zul}}}$ ${\displaystyle a_{\text{abs}}}$

The difference between the reception level and the threshold system is the flat fading margin (FFM short of English flat fade margin ): ${\displaystyle L_{E}}$ ${\displaystyle L_{\text{Schw}}}$

${\displaystyle FFM=L_{E}-L_{\text{Schw}}}$

Various empirical and semi-empirical forecasting methods can be used to predict whether, under the influence of multipath propagation or precipitation (rain), the calculated flat shrinkage reserve is sufficient to guarantee the required transmission quality or availability of the radio link. If the shallow fading reserve is insufficient and it cannot be increased by increasing the transmission level or by using antennas with a higher gain, there is the option of significantly reducing the fading probability in the frequency range below 13 GHz by using spatial diversity . With spatial diversity, two receiving antennas are used that are set up at a sufficient vertical distance from one another so that the reception conditions of the two antennas are poorly correlated. The output signals of both reception branches are combined with one another. Either it is only switched to the better reception in each case or the two signals are combined after an amplitude and phase adjustment.

The allocation of the radio frequency channel for a planned route is carried out by national regulatory authorities, in Germany by the Federal Network Agency (BNetzA), in Switzerland by the Federal Office of Communications (OFCOM), in Austria by the Federal Ministry for Transport, Innovation and Technology (bmvit). Among other things, it is the responsibility of the regulatory authority to ensure that the planned route in the intended radio frequency channel works without interference and that the new route does not inadmissibly affect other routes that are received in the same radio frequency channel or in the adjacent channels. For this purpose, interference calculations are required that take into account all the existing routes within the area of ​​influence of the planned radio link. These calculations include a. the transmission level and the directional diagram of the antennas at the affected radio relay locations.

Use of directional radio links

Radio relay system Berlin-Frohnau

Until the beginning of the 1990s, radio relay was used almost exclusively in Germany for the transmission of information from TF networks over long distances (approx. 100–120 km). The former Deutsche Bundespost as a monopoly in the telecommunications sector built a close-knit network of telecommunications towers and repeater offices through which connections between individual switching devices were established in the 1950s and 1960s . Noteworthy were microwave links to West Berlin , which had to be set up and operated on the edge of technical feasibility due to the great distance between the federal territory and Berlin. In addition to the telephone network, the post office also built directional radio links to distribute public radio programs. This included connections from the studios to the broadcasting systems spread across the country as well as between the broadcasting houses, for example for program exchange.

With the availability of fiber optic connections with very high capacities, however, the importance of radio relay for these applications quickly declined, although it offers advantages in real-time data transmission . When exchanging data via communications satellite, time delays must be expected due to the signal propagation time and, in the case of optical fibers, due to the buffering of the data. So z. For example, radio relay continues to be used between the London and Frankfurt am Main stock exchanges to ensure real-time prices.

The radio relay technology gained importance again with the emergence of the mobile radio networks . Here, directional radio is very often used to connect the individual mobile radio base stations to their higher-level units. Advantages compared to a leased fixed line are lower operating costs, faster installation and direct access to the hardware by the mobile network operator. Directional radio links are more susceptible to interference (e.g. from heavy rain), but can be suppressed more quickly than leased lines (e.g. if a buried amplifier fails), resulting in a higher overall availability.

Mobile radio relay station of the Danish army

Energy supply companies also use directional radio links and have built their own towers for this purpose, see the list of directional radio towers of energy supply companies in Germany .

Since data transmission with high bandwidths is desired, but the required fiber optic dedicated line is not available everywhere, a radio link is often set up. This is set up either as a peer-to-peer connection or a directional wireless Internet connection in order to enable backups at large locations as well as a location coupling of different branches. So that such directional radio links can be used as efficiently and without interference as possible, they should not be longer than 20 km and within sight.

The Swiss pay TV provider Teleclub has been a regular victim of piracy . Since 2001, the Teleclub signal has only been fed to the cable television networks in Switzerland via the Swisscom radio relay network . Previously, the insecure delivery was via satellite (first via Astra , then via DFS-Kopernikus ).

Surveillance and espionage

"Interception" of microwave links by satellites.

During the Cold War , American intelligence agencies like the NSA were able to use satellites to monitor the Warsaw Pact radio links . A part of the beam of a radio link goes sideways past the receiving antenna and radiates in the direction of the horizon into space, which can be used to intercept the link with a satellite located on this geosynchronous orbit. Like other radio links, the connections to and from West Berlin and within West Germany along the inner German border were monitored by listening posts from the Ministry for State Security and the East German NVA . For this reason, encrypted transmission is preferred for radio links used for military purposes . Even today, radio links are monitored and spied on by secret services .

Examples of microwave radio stations

• Radio link to West Berlin
• Directional radio station Ansfelden
• Directional radio station Exelberg
• Jauerling transmitter
• Transmitter Anninger
• Signal de Botrange - radio link between the London and Frankfurt stock exchanges

literature

• Jürgen Donnevert: Digital radio relay basics - system technology - planning of routes and networks. Springer Fachmedien and Vieweg, Wiesbaden 2013, ISBN 978-3-8348-1782-2 .
• Wolfgang Heinrich (Hrsg.): Radio technology . R. v. Decker's Verlag G. Schenk, 1988, ISBN 3-7685-2087-0 .
• Helmut Carl: microwave links . Verlag Berliner Union, Stuttgart 1964, ISBN 3-408-53036-X (SEL series of books).

Web links

Commons : Radio relay  - collection of images, videos and audio files

Individual evidence

1. ↑ Peter-Klaus Budig (Ed.): Langenscheidts specialist dictionary electrical engineering and electronics (D – E) . 1999, ISBN 3-86117-109-0 .
2. ^ VO radio of the ITU , 2012 edition, Article 1.66, definition: fixed radio station ; Article 1.20, definition: fixed radio service .
3. ↑ Radio relay technology (system, planning, structure, measurement) . Fachverlag Schiele & Schön, Berlin 1970, ISBN 3-7949-0170-3 .
4. ↑ Height platform = radio station on a platform at a certain height in the stratosphere with a fixed position ( English High Altitude Platform Station , acronym: HAPS)
5. ^ Helmut Carl: directional radio links. SEL series of books. Verlag Berliner Union, Stuttgart 1964, ISBN 3-408-53036-X .
6. ↑ ITU-R F.497: Radio-frequency channel arrangements for fixed wireless systems operating in the 13 GHz (12.75-13.25 GHz) frequency band. Retrieved June 24, 2017 .  
7. ↑ ITU-R Rec. P.383-3: Specific attenuation for rain for use in prediction methods on itu.int
8. ↑ ITU-R Rec. P.676-9: Attenuation by atmospheric gases on itu.int
9. ↑ ITU-R Rec. P.530-13: Propagation data and prediction methods for the design of terrestrial line-of-sight systems 2009. on itu.int
10. ↑ M. Glauner: Outage prediction in modern digital radio-relay systems. 1st European Conference on Radio Relay Systems (ECRR); November 1986, Munich, pp. 90-96.
11. ↑ New tower for Botrange. Retrieved December 24, 2019 . 
12. ↑ Radio link: fast data transmission with high bandwidths. Retrieved January 3, 2018 . 
13. ^ The main department III of the MfS