Low Complexity Non-Uniform FFT for Doppler Compensation in OFDM-Based Underwater Acoustic Communication Systems

Abstract

The Doppler effect critically degrades the performance of orthogonal frequency division multiplexing (OFDM) systems in general. This problem is significantly worse for underwater acoustic (UWA) communication systems due to the distinct characteristics of the underwater channel, resulting in the loss of orthogonality among sub-carriers. In order to compensate Doppler shifts, including phase noise and multipath channels in realistic communication scenarios, the joint of channel estimation and ICI reduction is often performed. However, the accuracy depends on the channel estimation and the FFT size, while this leads to increased computational complexity at the receiver. To achieve this dual goal in the actual underwater communication environment, a novel pilot structure in the frequency domain has been applied to overcome the channel impulse response (CIR) variation in a block period. The coarse Doppler shift is firstly estimated by using the received pilot signal. Afterward, the study takes advantage of the flexibility provided by non-uniform fast Fourier transform (NFFT) in choosing the sampling points to construct a fast and stable Doppler frequency Compensation Matrix-based NFFT (DCMN) to fine compensate the Doppler phase shift. Finally, this study shows the improvement of the proposed method's performance by actual experimental measurements and simulations.

Comparison of Doppler Effect Estimation Methods for MFSK Transmission in Multipath Hydroacoustic Channel

“…In [23] the authors propose and investigate a multibranch autocorrelation (MBA) Doppler estimation method, which can be used in communication systems with periodically transmitted pilot signals or repetitive data transmission. Another very popular solution is the use of pilots as a mean of estimating the Doppler effect [9], [24]. There are also propositions to use only direct paths during signal receptions which is possible in DSSS techniques [25].…”

“…However, it should be noted that most of the publications deal with hydroacoustic signal propagation in conditions where multipath effect does not occur or is negligible [8], [16]- [19], [21], [24]. Some research has been carried out under simulation conditions [13], [26] - [28].…”

“…In this case, there are made assumptions that greatly simplify the models used, but at the same time result in significant deviations from conditions encountered in reality. There has also been some research conducted under real conditions [8], [18], [21], [24].…”

Proposal for a Doppler Shift Compensation Method Using Non-Uniform FFT with Pilot Carrier Frequency for OFDM-Based Underwater Acoustic Communication Systems

“…5b, the OFDM symbol is affected by the CFO, the ortho-gonality of subcarriers is damaged. Therefore, the mathematical description of an OFDM symbol affected by the CFO is written by: To remove the ICI, we use the non-uniform FFT matrix [ ] RS G with the size of FFT FFT x N N as described in [19]. The non-uniform FFT matrix is a matrix where ( ) k − ∈ is not an integer due to the influence by the CFO according to (12).…”

“…The novelty of our proposed method lies in the second step, where we estimate the residual CFO using nonuniform FFT and mitigate the ICI that was not fully handled in step 1. The study in [19] provides a method compensation Doppler frequency. In this study, the relative doppler frequency iscalculated by the rotate phase of the PCF pilot.…”