Abstract
Terrestrial television broadcasting stations have been asked to use frequency bands more efficiently because of the shortage of frequencies. Digital transmission systems are therefore being extensively studied, particularly those using OFDM. We have developed an OFDM modem for use in digital terrestrial television broadcasting. Its characteristics are described in this paper. The modem has a function called graceful degradation to compensate for the cliff effect which is commonly seen in digital radio transmission systems. Of several graceful degradation technologies, we chose a method that uses multiple constellations. Computer simulations confirmed that the graceful degradation system made up of different constellations can compensate effectively for an insufficient D/U ratio and an extremely long excess delay time.
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