radio relay system
Directional radio is a radio link in the fixed radio service in which highly focused parabolic 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 and microwave directional radio, Free Space Radio Link (FSRL), are defined by the International Telecommunication Union( ITU) in the F. specifications. Microwaves are transmitted at frequencies between 2 GHz and 60 GHz. The carrier frequencies used in Germany for licensed microwave directional radio are 7, 13, 18, 23, 26 and 38 GHz. The frequencies that can be used without a license are at 5, 24 and 60 GHz. Transmission rates range from 1,000 Mbps to 20,000 Mbps.
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 4096, to two- phase shift keying(BPSK) or to quadrature phase shift keying( QPSK), depending on the weather conditions. This ensures uninterrupted transmission.
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). Any interference that occurs is compensated for by Cross Polarization Interference Cancellation(XPIC).
The stations for relaying the signals are called microwave relay stations. They receive the microwave signals, amplify them and transmit 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 microwave radio relay protocols, with Plesiochronous Digital Hierarchy( PDH) using asynchronous transmission and Synchronous Digital Hierarchy( SDH) using synchronous transmission. In addition, the 802.16working group of the IEEE has also taken up the topic and is developing corresponding protocols.
The transmission types of radio relay systems
Radio relay systems can basically be divided into point-to-point links( P2P) and point-to-multipoint links( P2MP). Point-to-point microwave radio relay systems connect two outdoor units via a microwave radio relay link and are used in particular in wide- area networks and for bridging larger distances. In the case of licensed microwave directional radio, data rates of 10 Gbit/s in duplex can be achieved.
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 usefully employed when 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.