Doppler Scaling Correction in OFDM

This paper introduces the use of non-uniform fast Fourier transform (NUFFT) in detection of orthogonal frequency division multiplexing (OFDM) in a situation when the received signal samples are unevenly spaced. The application of interest to us is underwater acoustic communications, where the relatively low speed of (acoustic) signal propagation in water combined with movement of communicating vehicles introduces a time-varying time-scale to the received signal. This is called Doppler scaling. In the presence of Doppler scaling, when the received signal is sampled at the nominal rate, the samples will be seen as unevenly spaced within the domain of transmit signal. This leads to an undetectable OFDM signal, if not corrected. We develop the necessary equations that show how NUFFT should be applied to this application. We also discuss a method of Doppler scaling estimation by using of null subcarriers that we assume are inserted in the OFDM signal. Moreover, we examine our development on real signals obtained from an at-sea experiment (ACOMM10) and confirm its correct operation.