Qualitative analysis of the impact of clock timing error and/or frequency offsets on an OFDM waveform demodulator

Abstract

Orthogonal frequency division multiplexing (OFDM) is a discrete multitone (DMT) waveform that is used on dispersive channels such as the subscriber twisted pair associated with asymmetric digital subscriber line (ADSL) or with digital television terrestrial broadcasting (DTTB). Implementation is based upon the IFFT for modulation and the FFT for demodulation. Orthogonality is obtained by having the DMT carrier spacing the same as the baud rate, and having the FFT demodulation process accumulate over exactly one baud symbol. However, orthogonality can only occur if the demodulator clock is synchronized to the modulator and no frequency offsets are present. This paper discusses, in a qualitative manner, the impact of attempting to FFT demodulate the OFDM waveform if clock synchronization has not occurred and/or if frequency offsets have accumulated on the channel.