Comparison of OFDM and single-carrier schemes for Doppler Tolerant Acoustic Communications

Abstract

Orthogonal frequency division multiplexing (OFDM) implicitly assumes that the channel used for signal transmission is linear and time-invariant for the length of each OFDM symbol. This is necessary for the channel matrix to be diagonalizable via the DFT, under cyclic prefix. The underwater acoustic channel is highly time-variant due to platform motion and environmental fluctuations. As a result, the application of standard OFDM leads to communication algorithms that break down when the transmitter or receiver are mobile. Several modifications to standard OFDM have been suggested and tested in the field. These have demonstrated that OFDM can perform reasonably well when the Doppler spread is minor. Unfortunately, Doppler can be highly time-varying in practice, with different wave propagation paths experiencing different amounts of Doppler. In this paper, we simulate various scenarios of platform mobility such as a receiver moving at constant speed or acceleration, a transmitter whose position is tracing out an oscillating pattern near a nominal transmission point, and basic multipath with significant Doppler spread. We compare the performance of published OFDM and single-carrier methods under these conditions. The single-carrier adaptive resampling equalizer presented here demonstrates significant improvement over OFDM in a variety of scenarios.

Multibranch Autocorrelation Method for Doppler Estimation in Underwater Acoustic Channels

“…Previously, the term a 2 (1 − µ 2 )t 2 has been ignored for t ∈ [−Θ/2, Θ/2] in (32) and (38); we now justify this step in our derivation. In many applications, it can be assumed that a < 1 m/s 2 [22], [29], [35]. Assuming also that ∆ is the time-correlation interval of the signal s 0 (t), which is given by ∆ ≈ 1/F , the term a 2 t 2 (1 − µ 2 ) can be ignored if…”

“…However, this component can be described by a few parameters [4], [5]. Therefore, it can be accurately estimated using a Doppler estimator, such as the estimator based on calculation of the ambiguity function [29]- [31] or the autocorrelation [12], [32]- [35], and further equalized using resampling. The other component of the channel representation, described by the impulse responseh(t, τ ), still contains most of the parameters to be estimated, such as the multipath delays and complex amplitudes.…”

“…Such Doppler estimation techniques have been specifically developed for different single-carrier modulation schemes [7], [26]- [28]. These techniques however cannot be directly applied to multicarrier transmission, such as the orthogonal frequency-division multiplexing (OFDM); besides, multicarrier schemes are more sensitive to Doppler distortions and require more accurate Doppler estimation [29].…”

“…One efficient method of Doppler estimation in multipath channels is based on computing the cross-ambiguity function (CAF) between received and transmitted signals [29]- [31]. The CAF is computed on a two-dimensional (2D) grid of channel delays and Doppler compression factors.…”

Low-Complexity UAC Modem and Data Packet Structure

“…One of most accurate methods of Doppler estimation in UWA multipath channels is based on computing the crossambiguity function (CAF) between received and transmitted signals [10]- [13]. The CAF is computed on a two-dimensional (2D) grid of channel delays and Doppler compression factors.…”