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Điện thoại di động vô tuyến điện - Tuyên truyền Channel P5

Characterisation of Multipath Phenomena In Chapter 3 we described some methods for predicting path losses, concentrating on those applicable to mobile communication systems. The discussion centred around techniques that deal principally with radio propagation over irregular terrain; methods of predicting signal strength in urban areas or in other environments, e.g. inside buildings, were deliberately left until Chapter 4. These propagation models are extremely important since the vast majority of mobile communication systems operate in and around centres of population | The Mobile Radio Propagation Channel. Second Edition. J. D. Parsons Copyright 2000 John Wiley Sons Ltd Print ISBN 0-471-98857-X Online ISBN 0-470-84152-4 Chapter 5 Characterisation of Multipath Phenomena 5.1 INTRODUCTION In Chapter 3 we described some methods for predicting path losses concentrating on those applicable to mobile communication systems. The discussion centred around techniques that deal principally with radio propagation over irregular terrain methods of predicting signal strength in urban areas or in other environments e.g. inside buildings were deliberately left until Chapter 4. These propagation models are extremely important since the vast majority of mobile communication systems operate in and around centres of population. Having introduced them we can now go into more detail about the propagation mechanism in built-up areas not only qualitatively but also in terms of a mathematical model. In that way we can understand the full significance of the prediction techniques and indicate the ways forward towards a global model that includes the effects of topographic and environmental factors. The major problems in built-up areas occur because the mobile antenna is well below the surrounding buildings so there is no line-of-sight path to the transmitter. Propagation is therefore mainly by scattering from the surfaces of the buildings and by diffraction over and or around them. Figure 5.1 illustrates some possible mechanisms by which energy can arrive at a vehicle-borne antenna. In practice energy arrives via several paths simultaneously and a multipath situation is said to exist in which the various incoming radio waves arrive from different directions with different time delays. They combine vectorially at the receiver antenna to give a resultant signal which can be large or small depending on the distribution of phases among the component waves. Moving the receiver by a short distance can change the signal strength by several tens of decibels .