How to combat long echoes in OFDM transmission schemes: sub-channel equalization or more powerful channel coding

The OFDM technique with guard-time, which has been proposed for terrestrial digital image and digital audio broadcasting applications in Europe, is an interesting approach to combat the frequency selectivity of the channel. However, it is sensitive to the interference caused by echoes longer than the guard-time. The aim of this article is twofold. First, the effects of the guard interval on a real case multipath channel are analyzed for an OFDM transmission system. The system performance is analytically derived as a function of the guard interval. The sensitivity of the OFDM system to the interference caused by echoes exceeding the guard-time is analyzed. The degradation due to the interference can be restored by applying two different techniques: using more powerful channel coding or employing sub-channel equalization. For channel coding, a combination of multilevel coding with multiresolution modulation, which is proposed for a digital terrestrial TV broadcasting, is used. The sub-channel equalizer is based on the DFE, where its adaptation calls-upon the LMS-algorithm, using the reference symbols sent periodically. Simulation results show that using powerful channel coding, rather than equalization, can partly restore the degradation caused by a shorter guard-time.