ABSTRACT
For orthogonal frequency division multiplexing (OFDM) systems, trellis coding and soft-decision decoding of each carrier will not work because they depend on the carrier being received at a sufficient level to decode most of the data, with only occasional symbols that are misinterpreted. In OFDM systems, carriers disappear completely because of the selective filtering of the channel caused by echoes. The technique developed to correct for the loss of carriers through selective filtering/fading involves coding of the data before it is separated by the first parallel-to-serial converter in the modulator so that the data appears at two places in the data stream. This coding — the 'C' in Coded Orthogonal Frequency Division Multiplex (COFDM) — effectively makes the data appear on two (or more) carriers that are positioned in different parts of the channel but that are linked by the data they carry. With COFDM, in theory, just the opposite is true: the additional echoes decrease the threshold level.
