Examples of Microwave Systems
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Examples of Microwave Systems
This chapter presents examples of radiated communications services that depend on the material developed in earlier chapters. Since many modern services use VHF and higher, the emphasis is on space wave – i.e. line-of-sight – services.
6.1 The Design of Open Microwave Repeater Systems Although now only a relatively small part of the set of radio wave communications systems encountered in practice, it is instructive to consider how a broadband radiated system operating at microwave frequencies might be designed. While useful in itself, it will also help us understand some of the propagation and signal aspects of satellite communications, and of wide band wireless applications more generally. Carriers at VHF and above are necessary to support the bandwidths required for high speed digital and video applications. Typically, bandwidths of 10 MHz and above would demand carriers of about 1 GHz so that several services could be carried simultaneously. At those frequencies unguided propagation will be line-of-sight space wave. Thus long transmission paths will require means by which the signal can be carried around the curvature of the earth. There are three simple means for doing that. First, we can use terrestrial repeaters, which is largely the focus of this material. Generally those repeaters would be sited on mountains or other regions of high relief to maximise range and to clear topographic features as depicted in Fig. 6.1. The alternative is to use coaxial cables or optical fibres for that purpose. While they have the benefit of larger potential bandwidths and relative freedom from interference, they present installation challenges owing to the need to secure land easements for laying the fibres or cables. In contrast, radiated systems have more flexibility. A second method for overcoming the limitations of earth curvature would be to use the mechanism of troposcatter as depicted in Fig. 4.7. Thirdly, we could use communications satellites; that is the topic of Sect. 6.2.
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6 Examples of Microwave Systems
Fig. 6.1 The use of microwave repeaters to clear topographic features and to cope with earth curvature
Consider the simple single link of Fig. 6.2. Essentially, what we want to know is how much power do we need to transmit from one repeater to the next. Based on the example of Fig. 5.7, it is clear that that will depend on the signal to noise ratio we must have at the output of any RF amplifier in the receiver of the next repeater, so that the signal can be successfully demodulated or detected. Figure 6.3 shows a simplified version of the RF stages in the receiver along with some example system parameters. In designing such a link, we start with the signal to noise ratio we need at the RF output and then work back to the necessary transmitter power from the previous repeater. That requires a knowledge of the free space distance between the transmitter and receiver. There are several ways for determining the signal to noise ratio required at the output of the RF stage; for
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