Cyclic prefix extension in DMT systems

In this paper we describe a novel method of reducing ISI in DMT systems. The standard way of combating ISI is to add the so-called cyclic prefix (CP) to every symbol being transmitted. If the cyclic prefix is longer than the equalized channel impulse response, it will absorb the effects of ISI and the rest of the symbol will be ISI-free. The purpose of the time-domain equalizer (TEQ) is do shorten channel impulse response to the length of CP. The main idea of the new technique is to make the task of the TEQ easier by allowing for the equalized channel response to be longer than the CP; which will cause the first several samples of every symbol to be corrupted. We use the knowledge of the position of corrupted samples and the fact that not all subchannels are being used for data transmission to correctly demodulate these partially corrupted symbols. We illustrate the theory with examples of performance improvements on test loops where gains of 6-10 dB in SNR are achievable.