Microwave transmission

Radio relay systems are radio systems for the transmission of information between fixed locations. In honor of Heinrich Hertz, who succeeded in 1886, proving the existence of electromagnetic waves that carry the French radio links the name Faisceaux hertziens. In the English language radio links as point- to-point system and the service via these systems are referred to as Fixed Service. Today exclusively digital microwave radio systems are used. In Germany frequencies above 3.4 GHz are reserved for these systems.

Historical development of radio relay

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

After the end of World War II radio relay systems contributed significantly to the development of the national and international telecommunication networks. Radio relay systems were used almost exclusively in long-distance network. Radio field lengths between 30 km and 60 km were the rule. Important compounds in the telecommunications networks were performed in parallel both via coaxial cable lines and radio relay systems. The first transmission of a television program about the now built, international radio network took place in 1953 for the coronation of Elizabeth II

Until about 1980, analogue radio-relay systems with a transmission capacity of up to 2700 telephone channels and radio frequencies between 1.9 GHz and 11 GHz in use. The transmission of television took place almost exclusively via radio. The transmit power was 0.5 watts for systems with 120 voice channels and 20 W for systems with 2700 telephone channels. The modulation method sat down for multi-channel systems by frequency modulation.

From about 1970 step by step digital signal transmission systems were introduced in the networks. With optical transmission systems, it was now possible to transmit very high bit rates over long distances without repeaters. This meant that all metropolitan areas were cross-linked with optical transmission systems. The area of ​​application of microwave systems shifted as a result in the regional and local level network of the telecommunications network.

After the reunification of the two German states in 1990, the telecommunications network had to be expanded in the new eastern states and connected to the network of the western states in a short time. This problem has been successfully solved by the massive use of digital radio relay systems. In 1991, the construction of the digital mobile communication networks. Large parts of the fixed network to the mobile communication systems are realized with microwave systems for cost reasons. In particular, the network foothills are ideal for use on wireless systems. With the enactment of the Telecommunications Act, the former telecommunications monopoly of the covenant was in 1996 in Germany ended. Now, private companies were able to build and operate their own telecommunications networks. Many compounds in these new networks are created via radio. End of October 2013 were in Germany more than 125,000 radio links in operation, with annual growth rates of 10 %. Germany has probably the most extensive worldwide radio network.

For radio in Germany are frequency ranges between 3.8 GHz and 86 GHz with a bandwidth of 41 GHz are available. The allocation of frequencies for radio links is carried out in Germany by the Federal Network Agency ( FNA ). Most still contributes to the further development of the mobile radio networks. Radio stations are usually used as sites for mobile phone base stations ( see figure). Microwave radio systems are available as an alternative and complement to line-bound transmission systems continues to be an indispensable communication medium in the national and international telecommunication networks.

Structure of a microwave link

The emission and reception of electromagnetic waves is carried out at microwave links by satellite dishes with great directionality. There is line of sight between the transmitting and receiving antenna. Radio systems are point-to -point radio systems generally. The use of point-to- multipoint Systemem is limited to special cases. As used for transfer quality and availability are microwave links to the same subject requirements as transmission systems that use fiber cable as the transmission medium.

Figure 1: radio relay line

Figure 1 shows a radio link between the terminal stations A and B is shown in the scheme. Since there is no line of sight between the sites A and B, a repeater is required. The connection is in this example thus consists of two radio links. Locations of relay station in many cases are nodal points of the radio relay network ( see figure).

Radio relay systems are bidirectional transmission systems in most cases. In the lower part of Figure 1 shows the device configuration is specified for this purpose. In the modulator M to be transmitted digital data stream is a intermediate frequency carrier is impressed. A quadrature amplitude modulation method, the modulation method can be used with 4-2048 stages ( 4QAM to 2048QAM ). In the transmission assembly S of the intermediate frequency carrier is converted into the radio frequency plane and its power level is raised to the transmission level. The usual transmission level of radio relay systems are between 20 dBm (= 100 mW ) and 30 dBm (= 1 W). About a duplexer of the radio frequency carrier of the antenna is fed and radiated toward the peer. There, the carrier arrives at the receiver E, which amplifies the received signal and resets at the intermediate frequency level. In the demodulator of the intermediate frequency carrier is finally demodulated and the case recovered data signal regenerated.

The radio channel

Of the radio channel includes the radio field including the transmitting and the receiving antenna. The electromagnetic wave between the transmitting and receiving antenna propagates in the troposphere. The propagation of the electromagnetic wave are thus dependent on the weather conditions, and thus also on the geographical location of the radio field. They change with the year and time of day. These temporal and spatial dependencies of the propagation behavior are incorporated into the planning calculations for a radio link.

If the first Fresnel zone is clear of obstructions, applies to the path loss:

= Radio frequency, radio = field length = antenna gains in dB

This tailored size equation is derived from the formula for the free-space loss. The first Fresnel zone is an area around the line of sight. Its radius at the location is:

= radius of the earth

The line of sight is also referred to as radio beam. In most time, the refractive index of the troposphere decreases linearly with the height, so that the microwave beam is refracted towards the earth. To obtain a straight line of sight, is multiplied in the representation of the earth's radius to the so-called K- factor. In Europe, the k- factor is more than 50 % of the time, k = 1.33 ( standard atmosphere ). In small time percentages are formed in the troposphere inversion layers in which a microwave beam is refracted by differing conditions in the normal atmosphere. This broad, several radio beams over different distances from the overlap in the receiving antenna (see Figure 2). This propagation behavior is referred to as multipath propagation. As a result produced at the output of the receiving antenna in the spectrum of the microwave carrier wideband fading ( flat fading ) in combination with selective fading (selective fading ).

Figure 2: multipath propagation

Multipath propagation is greater than 30 km in the radio field lengths, as they are typical of radio frequencies below 10 GHz, the dominant propagation phenomenon. For radio frequencies above 10 GHz the rain attenuation is the predominant influence. Rain produces broadband fading. The rain attenuation increases with increasing radio frequency so that the realizable radio field lengths decrease with increasing radio frequency. From about 20 GHz wins besides the rain attenuation, the absorption by atmospheric gases influence. The substantial absorption takes place by the water vapor and the oxygen molecules of the air. At 23 GHz and at 60 GHz, the attenuation in each case has a specific absorption maximum.

Radio-relay systems

The system modules modulator and demodulator are summarized with the modules for power supply and the interfaces in an indoor unit (English Indoor Umit, IDU ). Transmitter and receiver constituting the radio. In many cases, as an external unit (English Outdoor Unit, ODU ) carried and mounted in the vicinity of the antenna. To the indoor unit, the outdoor unit is connected via a coaxial cable, over which is also the power supply of the outdoor unit is running (see pictures).

Modulator and demodulator are implemented in the form of complex, programmable, integrated circuits for digital signal processing. This modulation scheme and bandwidth can be configured to meet different requirements. The main system parameters are the transmission level, the system level ( reception level at the bit error rate of a ), bit rate and modulation method. Bit rate and modulation method to determine the required bandwidth in the radio frequency band. In a radio frequency channel having a bandwidth of 28 MHz can be transmitted to the modulation method 256QAM a net bit rate of 193 Mbit / s. A higher bit rate is required, the number of stages of the modulation method can be increased. When using both polarization directions ( horizontal and vertical) in the same radio frequency channel, the bit rate is doubled. In addition, several systems can be operated in parallel in different radio frequency channels on the track. In a bandwidth of 56 MHz and 1024QAM can thus be transferred a net bit rate of 1 Gbit / s.

To counter the effects of multipath propagation, the demodulator includes an adaptive time domain equalizer (English: Adaptive Time Domain Equalizer, ATDE ). When using both polarization directions in the same radio frequency channel, the two differently polarized carrier influence. The demodulators the receiver for vertical and horizontal polarization are therefore a cross-polarization compensator (English Cross Polarization Interference Compensator XPIC ) coupled to one another, which compensates for these influences. The transmission level of radio relay systems is an automatic level control (English:, ATPC Adaptive Transmitter Power Control) in -shrink time, lowered to a value which is about 10 dB above the system threshold. This minimizes the effect of interference from adjacent radio links. When loss events in the radio field of the transmit level is increased according to the fading depth to the operating transmission level is reached.

Planning of radio links

In the first step of the planning of radio links a site section is created. This is checked whether the first Fresnel zone is clear of obstructions. For creating terrain sections planning tools are available that use digital terrain models. If the first Fresnel zone partially shadowed, this can be taken into account by an additional damping. In the next step, the reception level is calculated in -shrink time:

( = Loss of the antenna feed lines, = absorption loss )

The difference between the reception level and the system threshold is the flat fade margin FFM (English: flat fade margin ):

Using various empirical and semi-empirical prediction methods can be predicted whether under the influence of multipath propagation or precipitation in the form of rain, the calculated flat fade margin sufficient to ensure the required transmission quality or availability of the radio link. If flat fading margin is not sufficient and it can not be raised by increasing the transmission level or antenna with higher gain, is in the frequency range below 13 GHz, the possibility of the loss probability of the use of space diversity to reduce decisively. For space diversity reception, two antennas are used, which are placed to each other at a sufficient vertical distance so that the reception conditions of both antennas are less correlated. The output signals of the two receiving branches are combined in a suitable manner.

The allocation of the radio frequency channel for a planned path is accomplished by the national regulatory authority in Germany by the Federal Network Agency ( FNA ), in Switzerland by the Federal Office of Communications ( OFCOM), in Austria by the Federal Ministry for Transport, Innovation and Technology ( bmvit ). It is, among other things, the task of the regulator to ensure that the planned route in designated radio frequency channel without interference and operates the new route no other routes that receive the same radio frequency channel or in the adjacent channels, strongly influenced inadmissible. To this end, extensive interference calculations are required, taking into account all paths lying in the sphere of influence of the proposed radio link and existing. In these calculations assume, for example, the transmission levels and the directivity patterns of the antennas on the affected radio relay sites.

Application of microwave links

Until the beginning of the 1990s almost exclusively radio for the transmission of information from TF networks over large distances ( 100-120 km ) was used in Germany. The former German Federal Post Office as a monopoly in the telecommunications sector to built in the 1950s and 1960s an extensive network of telecommunications towers and amplifiers offices, were used to establish connections between individual switches. It is worth noting were microwave links to West Berlin, which had to be built and operated because of the large distance between the Federal Republic and Berlin on the edge of technical feasibility. In addition to the telephone network of microwaves for dissemination of public broadcasting programs were at that time by the post office built. This included both compounds from the studios to the transmission facilities spread across the country, as well as between the radio houses, for example, to program exchange.

With the availability of low-cost optical fiber links with very large capacities in the 1990s, the importance of the radio relay for these applications, however, fell quickly. New application areas, however, found the technique in about the same time emerging mobile networks. This radio is frequently used to connect the individual mobile phone base stations to their parent units. Beneficial for such use are across from a rented fixed line mainly due to lower operating costs, faster construction and direct access to the hardware for the mobile operator. In addition, radio links are indeed more prone to disturbances, eg from heavy rain, but also faster than entstörbar leased lines ( eg if a buried amplifier ), so that in the aggregate results in a higher overall availability.