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DAV Nguyên tắc và các ứng dụng P2

Tham khảo tài liệu 'dav nguyên tắc và các ứng dụng p2', kỹ thuật - công nghệ, kĩ thuật viễn thông phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Digital Audio Broadcasting Principles and Applications. Edited by Wolfgang Hoeg Thomas Lauterbach Copyright 2001 John Wiley Sons Ltd ISBNs 0-471-85894-3 Hardback 0-470-84170-2 Electronic 2 System Concept THOMAS LAUTERBACH HENRIK SCHULZE and HERMAN VAN VELTHOVEN 2.1 The Physical Channel Mobile reception without disturbance was the basic requirement for the development of the DAB system. The special problems of mobile reception are caused by multipath propagation the electromagnetic wave will be scattered diffracted reflected and reaches the antenna in various ways as an incoherent superposition of many signals with different travel times. This leads to an interference pattern that depends on the frequency and the location or - for a mobile receiver - the time. The mobile receiver moves through an interference pattern that changes within microseconds and that varies over the transmission bandwidth. One says that the mobile radio channel is characterised by time variance and frequency selectivity. The time variance is determined by the vehicle speed v and the wavelength . c f0 where 0 is the transmission frequency and c the velocity of light. The relevant physical quantity is the maximum Doppler frequency shift A - 21 Om x c70 1080 MHz km h Table 2.1 shows some practically relevant figures for Omax. Digital Audio Broadcasting Principles and Applications edited by W. Hoeg and T. Lauterbach 2001 John Wiley Sons Ltd. 24 Digital Audio Broadcasting Principles and Applications Table 2.1 Examples for Doppler frequencies bmax v 48 km h v 96 km h v 192 km h 0 225 MHz 10 Hz 20 Hz 40 Hz o 9OO MHz 40 Hz 80 Hz 160 Hz q 1500 MHz 67 Hz 133 Hz 267 Hz The actual Doppler shift of a wave with angle a relative to the vector of the speed of the vehicle is given by fo fo cos . 2.2 Typically the received signal is a superposition of many scattered and reflected signals from different directions so that we may speak not of a Doppler shift but of a Doppler spectrum. Figure 2.1 shows an .