Mitigation of Multipath E ects We have seen in Chapters 4 and 5 that buildings and other obstacles in built-up areas act as scatterers of the signal, and because of the interaction between the various incoming component waves, the resultant signal at the mobile antenna is subject to rapid and deep fading. The fading is most severe in heavily built-up areas such as city centres, and the signal envelope often follows a Rayleigh distribution over short distances in these heavily cluttered regions. As the degree of urbanisation decreases, the fading becomes less severe; in rural areas it is often only serious. | The Mobile Radio Propagation Channel. Second Edition. J. D. Parsons Copyright 2000 John Wiley Sons Print ISBN 0-471-98857-X Online ISBN 0-470-84152-4 Chapter 10 Mitigation of Multipath Effects INTRODUCTION We have seen in Chapters 4 and 5 that buildings and other obstacles in built-up areas act as scatterers of the signal and because of the interaction between the various incoming component waves the resultant signal at the mobile antenna is subject to rapid and deep fading. The fading is most severe in heavily built-up areas such as city centres and the signal envelope often follows a Rayleigh distribution over short distances in these heavily cluttered regions. As the degree of urlxmiSiiiuw deereases the fading becomes less severe in rural areas it is often only serious when there ire obstacles such as trees close to the vehicle. A receiver moving through this spatially varying field experiences a fading rate which is proportional to its speed and the frequency of transmission and because the various component waves arrive from different directions there is a Doppler spread in the received spectrum. It has been pointed out that the fading and the Doppler spread are not separable since they are both manifestations one in the lime domain and the other in the frequency domain of the same phenomenon. In add i ti on there is the delay spread which leads to frequency-selective fading. This causes distortion in wideband analogue signals and intersymbol interference ISI in digital signals. These multipath effects can cause severe problems and particularly in urban a reais multipath is p ro bab ly die single most destructive influence on mobile radio systems. Much attention has been devoted to techniques aimed at mitigating the deleterious effects it causes and this chapter reviews some of the available approaches to the problem. DIVERSITY RECEPTION One well-known method of reducing the effects of fading is to use diversity recepiion techniques. In .
