The requirements of modulation for digital TV are significantly different from that of a single carrier QPSK modulation specification for satellite and single carrier QAM modulation for cable. The terrestrial channel may be impaired by severe multipath, due to terrain and buildings, electrical noise, environmental conditions, and the congested frequency spectrum. The Europeans have chosen COFDM. The US has chosen 8-VSB.
http://web-star.com/hdtv/cofdmvs8vsb.htm