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To accurately predict the performance of broadcast teletext systems, one must know the characteristics of typical over-the-air broadcast television channels. Consequently, ensembles of typical complex impulse responses of broadcast teletext channels in the VHF and UHF frequency ranges are required for computer performance simulations. To circumvent the lack of comprehensive measured channel data, a theoretical multipath propagation model for suburban and urban environments was developed to gain insight into the nature of typical television multipath channels. In this model, both the direct and scatter paths consist of a line-of-sight and ground-reflected component. The basis of the propagation model is an expression for the ratio of the scattered path to direct path field strength caused by vertically oriented dielectric slabs of specified thickness, height, and width. The reflector location, orientation, width, height, and thickness are randomized for each Monte Carlo simulation cycle. A scatter table of relative amplitude versus multipath delay is maintained for subsequent analysis and formation of typical multipath channel impulse responses. A perceived desired-to-undesired ratio (PDUR), which is essentially a signal-to-clutter ratio, is computed to eliminate channels of unacceptable video quality, as these would be of no interest to the teletext application. A modification to account for near-field scattering is described. Finally, mobile channel applications are discussed.
Date of Publication: Feb 1987