Quote:
Originally Posted by wa2ise
Tall towers, that the UHF DTV transmitting antennas are mounted at the top of, tend to sway in the wind. This sway can be as much as a good fraction of a wavelength of the carrier frequency up on UHF. That, combined with some significant ghosts, can make the conditions at a receiver be constantly changing. Which adds extra fun in receiver decoder design...
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Doppler performance in COFDM (DVB) is limited by inter-carrier interference between the many closely-spaced carriers. Single carrier (ATSC) Doppler performance is limited by the tracking ability of the receiver equalizer, which is greatly affected by the frequency of transmission of a reference signal. In ATSC for fixed terrestrial reception, the reference sync segment is transmitted approximately 40 times per second, too slow for mobile reception. The mobile adaptation of ATSC transmits a reference approximately 800 times per second during the mobile portion of the data. COFDM achieves mobile performance by using modes with fewer, more widely spaced carriers, and through the use of reference "pilot" carriers that are inserted with the necessary density in time and frequency.