Over the past few years, digital technologies have been growing in such a way that a new generation of audio broadcasting systems, fully digital, can now become cost effective. For this purpose Coded Orthogonal Frequency Division Multiplexing (COFDM) [1], a new transmission technique designed in Europe, is promising. In order to perform the evaluation of this system, we need an accurate channel model which takes into account the physical structure and the time varying nature of the propagation medium. The purpose of this work is to study the performances of COFDM using a wide band channel simulator based on the scattering function. 1.IntroductionThe realization of a high quality digital sound broadcasting system in a time varying selective frequency channel is possible with the development of good sound data compression algorithm and with the design of a high rate digital system. This system will have to resist to deep fades throughout the band and to offer a good spectrum efficiency. This work deals with performance evaluations of such a system. In order to analyze the performances of a communication system, we need to reproduce with precision the channel characteristics. In the first part of this paper, the multipath and time varying nature of the mobile radio propagation channel are briefly described. The influence of the physical channel structure on the UHF scattering electromagnetic waves viewed by the mobile receivers is discussed. This analysis leads to a mobile radio channel model based on the scattering function of the propagation medium.Digital technologies have been growing in such a way that more complex modulation systems are now feasible and can be cost effective. COFDM is such a technique. It combines a multicarrier modulator/demodulator, a convolutional coding system and a time-frequency interleaver. The purpose of the second part of this paper is to describe the COFDM system. The last part of this work presents the results obtained with our simulator. Several simulations have been carried out, each of them representing specific channel conditions. This gives a general idea of the system behavior. 2.Channel Modeling via the Scattering FunctionMultipath channels are an important class of channels which includes ionospheric, acoustic and mobile radio channels. In a multipath channel, the signal viewed at the receiver is a sum of the delayed paths of the transmitted signal (eq. 1). This sum can be constructive or destructive over all the band, which can result in severe deep fades.(1)As the receiver is moving, these paths are combining differently over time. This fluctuation is referred to as short time fast fading. Some paths appear and disappear with time. These variations lead to slow fading. In this work, we restrain our study to short time fading.When a receiver is moving with respect to the transmitter, the received signal is subjected to a frequency shift called Doppler shift. Assuming constant vehicle speed, the maximum Doppler shift is then given by the following relation:where v i...
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