ABSTRACT Underwater acoustic (UWA) communication systems are often subject to severe Doppler distortion due to the relative motion between transmitter and receiver. While conventional methods typically assume constant relative velocity, practical UWA environments frequently involve non‐zero acceleration arising from autonomous underwater vehicle (AUV) maneuvers, drifting platforms, or environmental dynamics, which introduces additional time‐varying distortion. This acceleration‐induced Doppler effect results in nonlinear time scaling and frequency offset that significantly degrade receiver performance if left unaddressed. In this paper, a novel method to estimate and compensate for the Doppler scale caused by acceleration in orthogonal frequency division multiplexing (OFDM)‐based UWA systems is proposed. The proposed estimation algorithm leverages the inherent redundancy between the cyclic prefix (CP) and its replica in each OFDM symbol to extract phase‐based measurements without requiring grid search. A baseband‐domain compensation scheme is also proposed to mitigate both phase and timing distortions. Simulation results under additive white Gaussian noise (AWGN), synthetic multipath and realistic waymark channel conditions demonstrate that the proposed approach achieves accurate estimation and reliable compensation performance across a broad range of acceleration scenarios. Furthermore, the proposed method introduces no additional signaling overhead and operates with low computational complexity, offering a practical solution for mobile UWA communication in dynamic underwater environments.

This paper describes an iterative learning framework consisting of multi-layer prediction processes for underwater link adaptation. To obtain a dataset in real underwater environments, we implemented OFDM (Orthogonal Frequency Division Multiplexing)-based acoustic communications testbeds for the first time. Actual underwater data measured in Yellow Sea, South Korea, were used for training the iterative learning model. Remarkably, the iterative learning model achieves up to 25% performance improvement over the conventional benchmark model.

As a promising candidate to replace cyclic prefixed orthogonal frequency division multiplexing (CP-OFDM), a new quadrature amplitude modulation filter-bank multi-carrier (QAM-FBMC) has been proposed for enabling spectral efficiency increase and spectrum confinement while keeping near orthogonality. QAM transmission can combat intrinsic interference of conventional offset-QAM (OQAM)-FBMC, which is main bottleneck in channel estimator and multiple antenna transceiver design. Compared to existing single filter based QAM-FBMC, the new QAM-FBMC has more optimization freedom in multiple prototype filters to increase self signal-to-interference ratio (SIR). In this paper, the possibility of per-tone multiple input multiple output (MIMO) reception in the new QAM-FBMC system is exploited. Our MIMO receiver is constructed with per-tone MIMO equalization in oversampled signal domain and its soft-demapper in symbol domain. Not only the separate log likelihood ratio (LLR) calculation per stream but also joint LLR calculation (i.e., soft maximum likelihood (ML) detection) is implemented with pre-processing. Simulation results show that the per-tone MIMO reception in QAM-FBMC is comparable to that of CP-OFDM.

In this paper, a general channel modeling for beamforming MIMO system in LOS environments is introduced. In order to validate the channel model, prototype hardware that supports 2×2 beamforming MIMO transmission is developed and tested in outdoor LOS environment. This prototype operates at millimeter-wave band, in which occurrence of LOS channel is relatively high compared with the low legacy frequency band due to short wavelength. Numerical evaluation reveals that performance of the beamforming MIMO system in LOS channel is highly dependant on the number of arrays and the spacing between the adjacent arrays. Experimental results shows a good accordance to the numerical results, thereby validating our beamforming MIMO channel model for LOS environments.

In this paper, a new preamble which is constructed with only one OFDM symbol is proposed. Moreover, novel synchronization and cell searching techniques based on the proposed preamble are developed. In proposed algorithm, frame timing and fractional frequency offset estimation are performed in the time domain while the integer frequency offset and cell identification (ID) are estimated in the frequency domain. Each algorithm is designed under a consideration of the estimation error such as the timing error and the residual frequency offset. From the simulation results, it is verified that the proposed synchronization and cell searching schemes are robust to the estimation error and show reliable performance even in a harsh environment.

We introduce a new synchronization scheme to improve the synchronization performance in a ultra-wideband (UWB) non-coherent energy collection receiver. Our proposed scheme exploits the mean normalized energy profile, which is known at the receiver, to acquire synchronization. The synchronization process is performed through comparing the energy collected from the received signal with the mean normalized energy profile and correcting the timing error. The synchronization performance is investigated in terms of the normalized mean squared synchronization error (NMSSE) in IEEE 802.15.3a UWB indoor channel environment. Through simulation results, we show that the proposed scheme enhances the synchronization performance.