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radio relay system

Directional radio is a radio link of the fixed radio service in which highly focused parabolic antennas, so-called pencil beam antennas, transmit and receive signals in the microwave range. The radio range between the transmitting and receiving equipment is formed by the Fresnel zones, which is decisive for the radio transmission quality.

The transmission frequencies and channel bandwidths for directional radio are defined by the International Telecommunication Union (ITU) in the F. specifications. Microwaves with frequencies between 2 GHz and 60 GHz are used for transmission. The carrier frequencies used in Germany for microwave directional radio are 7, 13, 15, 23, 26 and 38 GHz, the channel bandwidths between 3.5 MHz and 80 MHz and the transmission rates between 2 Mbit/s and 34 Mbit/s, allowing scaling up to 622 Mbit/s.

Fresnel zones

Fresnel zones

Weather influences and modulation methods

The maximum bridgeable distance ranges of microwave directional radio are frequency and weather dependent and can be between 40 km and 100 km at frequencies of 2 GHz. Since the bridgeable distances are affected by weather conditions, microwave radio relay systems change their modulation methods from quadrature amplitude mod ulation with QAM 1024 to QAM 16, to two-phase shift keying (BPSK) or quadrature phase shift keying (QPSK), depending on the weather conditions. This ensures uninterrupted transmission.

Typical ranges of directional radio depending on the radio frequency

Typical ranges of directional radio depending on the radio frequency

At higher frequencies, the range is reduced and is a maximum of 30 km at 10 GHz and about 1 km at 60 GHz. Orthogonal polarization is also used to increase transmission capacity. In this case, the vertically and horizontally polarized frequencies transmit different signals. This mode of operation is called Co-Channel Dual Polarization (CCDP). Occurring interferences are compensated by Cross Polarization Interference Cancellation (XPIC).

Antenna mirrors for directional radio, Photo: Rolf Heine High-frequency technology

Antenna mirrors for directional radio, Photo: Rolf Heine High-frequency technology

The stations for forwarding the signals are called directional radio relays. They receive the microwave signals, amplify them and send them to the next station. Whereas frequency-modulated analog signals were transmitted in the past, phase shift keying is increasingly used today.

Data transmission is governed by directional radio protocols, with Plesiochronous Digital Hierarchy (PDH) using asynchronous transmission and Synchronous Digital Hierarchy (SDH) using synchronous transmission. In addition, the IEEE802.16working group has also addressed the issue and is developing appropriate protocols.

The transmission types of radio relay systems

Radio relay systems can basically be divided into point-to-point (P2P) and point-to-multipoint (P2MP) links. Point-to-point radio relay systems connect two stations via a radio relay link and are used in particular in wide-area networks and for bridging larger distances. Transmission capacities of over 155 Mbit/s can be achieved. In addition, the RegTP has released the license-free frequency bands around 2.4 GHz and 5 GHz for PP radio relay.

Radiation head of an optical directional radio link, Photo: axess-pro networks GmbH

Radiation head of an optical directional radio link, Photo: axess-pro networks GmbH

In addition to microwave directional radio (FSRL), there is also optical directional radio, Free Space Optic (FSO), known as free space transmission. These optical systems can be used if there is a line of sight between the transmitting and receiving points and these are not affected by atmospheric or other factors. The optical systems operate with nfrared and can bridge distances of several kilometers at transmission rates of several Gbit/s.

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Englisch: radio relay system
Updated at: 06.10.2019
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