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.

In this paper, we propose a closed-form expression of Doppler power spectrum model for underwater acoustic (UWA) channels. Using the geometry model for shallow UWA channels, we analyze the theoretical background of Doppler effects generated by the transmitter/receiver (Tx/Rx) movement, or by the motion of sea-surface. As a result, the Doppler power spectrum can be modeled as a summation of the Spike-shape and the Gaussian-shape. The Spike-shape presents the Doppler component from the Tx/Rx movement, while the Gaussian-shape presents the Doppler component from the sea-surface motion. The proposed model is validated through curve fitting with the Doppler power spectrum measurement results of a real shallow UWA channel in Halong bay, Vietnam. The optimal parameters of the proposed model are derived from the measurement results by applying an optimization algorithm called the Lp-norm method. The curve fitting results show that our proposed model matches well with the measurements. Therefore, the proposed Doppler model can accurately describe the Doppler effects for shallow UWA channels. The proposed model can be used to design UWA channel simulators for the performance evaluation of UWA communication systems.

In this paper, we propose an algorithm for estimating the beam aperture of the acoustic transducers by using the geometric backscattering model for side-scan sonar systems. The geometric backscattering model is developed to describe the propagation paths of the signal transmitted from the transducers towards the seabed and backscatters to the hydrophones. To evaluate our proposed algorithm, we have developed a side-scan sonar system. The side-scan sonar system uses two transducers, operating on two different frequencies and focusing on two different wave beams, to scan the images of the seabed. The proposed algorithm provides the estimated beam apertures of each transducer. Our obtained results agree quite well with the parameters provided by the manufacturers. Moreover, these results are used to calibrate the scanned images. We provide the scanned sonar 3D images of the Dong Do lakebed, Vietnam, to justify our proposal.

Due to the strong influence of the ambient noise and the time variation of underwater acoustic (UWA) channels with a large Doppler frequency shift, the time synchronization is a challenging issue in UWA-OFDM systems. In addition, the signal bandwidth of UWA communication systems is very limited. Thus, optimizing the bandwidth efficiency of the system is also an important target in designing synchronization methods for UWA communications. For combating the above-mentioned problems as well as to increase the spectrum efficiency of the system, this paper proposes a time synchronization method for OFDM-based underwater acoustic communication systems. Instead of the using preamble, we use only the guard interval (GI) for the time synchronization. Hence, there is no additional redundancy for synchronization that is requested for the proposed method. Experimental and simulation results show that the proposed time synchronization method outperforms some conventional algorithms used for UWA communication systems.

In this paper, we propose a new method to compensate Doppler frequency shift for OFDM-based underwater acoustic communication systems. In order to save bandwidth, we do not use additional signal header (preamble) in each OFDM frame as proposed in many conventional approaches. Instead, the central sub-carrier is reserved for pilot transmission. This subcarrier is so-called as the carrier frequency pilot (CFP), which is used to detect the Doppler frequency. At the receiver, two synchronization steps are deployed. The first step performs coarse synchronization. In this step, the Doppler frequency is roughly estimated on the basic of the detected carrier frequency. In the second step, we use the CFP to regulate the estimated Doppler frequency. This regulation is called as fine synchronization. The advantage of the proposed method is to reduce the OFDM frame length. Therefore, the system bandwidth can be increased. The proposed method is able to track the fast time-varriation of the Doppler frequency, which is a typical characteristic of the underwater channel. To verify the perfomance of the proposed method, we have tested our approach in real underwater channel with the speed of the relative Rx movement of 3m/s. Experimental results show that the estimated Doppler frequency matches well with the theoretical calculation.

This paper proposes a novel channel allocation method for multi-cells OFDMA-FDD networks with a reused frequency factor (RFF) of one. The proposed algorithm predicts the co-channel interference (CCI) of active mobile stations (MSs) before allocating suitable subcarriers to a new MS. In order to maintain a desired quality of service (QoS) of the network, only subcarriers having signal-to-interference-plus-noise ratio (SINR) higher than a given threshold SINR _min are selected. The simulation results show that the number of sub-carriers which are assigned to MSs increases when decreasing the threshold SINR _min , or increasing the number of MSs.