TAILIEUCHUNG - Điện thoại di động mạng lưới Radio P2

Fundamentals of Radio Transmission In mobile radio systems, unlike wired networks, electromagnetic signals are transmitted in free space (see Figure ). Therefore a total familiarity with the propagation characteristics of radio waves is a prerequisite in the development of mobile radio systems. In principle, the Maxwell equations explain all the phenomena of wave propagation. However, when used in the mobile radio area, this method can result in some complicated calculations or may not be applicable at all if the geometry or material constants are not known exactly | Mobile Radio Networks Networking and Protocols. Bernhard H. Walke Copyright 1999 John Wiley Sons Ltd ISBNs 0-471-97595-8 Hardback 0-470-84193-1 Electronic 2 System Aspects Fundamentals of Radio Transmission In mobile radio systems unlike wired networks electromagnetic signals are transmitted in free space see Figure . Therefore a total familiarity with the propagation characteristics of radio waves is a prerequisite in the development of mobile radio systems. In principle the Maxwell equations explain all the phenomena of wave propagation. However when used in the mobile radio area this method can result in some complicated calculations or may not be applicable at all if the geometry or material constants are not known exactly. Therefore special methods were developed to determine the characteristics of radio channels and these consider the key physical effects in different models. The choice of model depends on the frequency and range of the radio waves the characteristics of the propagation medium and the antenna arrangement. The propagation of electromagnetic waves in free space is extremely complex. Depending on the frequency and the corresponding wavelength electromagnetic waves propagate as ground waves surface waves space waves or direct waves. The type of propagation is correlated with the range or distance at which a signal can be received see Figure . The general rule is that the higher the frequency of the wave to be transmitted the shorter the range. Based on the curvature of the earth waves of a lower frequency . larger wavelength propagate as ground or surface waves. These waves can still be received from a great distance and even in tunnels. Figure Radio transmission path transmitter-receiver. Z0 and ZW are the radio wave resistances in free space and on the antenna feeder link 28 2 System Aspects 1000 km 100-150 km Radio horizon Geom. horizon Ground _ _ . surface waves Space waves Space waves Direct waves LF MF HF VHF UHF SHF EHF

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