In the study of deterministic sea waves, the predictable zone refers to the area where the phase-resolvable wave field can be fully reconstructed and accurately predicted based on measured data. ...Under certain marine environmental and radar measurement parameter conditions, expanding the predictable zone is of great significance for domains such as quiescent period prediction (QPP) for ships and optimizing wave energy harvesting. Prior research on the predictable zone mostly relied on either temporal or spatial measurements, significantly constraining the range of the predictable zone. In this article, based on coherent microwave radar, we present a novel method combining spatial and temporal measurements to expand the predictable zone. First, the boundary of the predictable zone under a single spatial measurement and multiple spatial measurements (spatial-temporal measurement) is analyzed, and the closed-form expressions for the predictable zone are derived for both the unidirectional and multidirectional wave fields. The quantitative relationship between the predicted time at the desired spatial location and the radar parameters and wave spectra is obtained. Subsequently, the theoretical analysis of the predictable zone is validated using the numerical simulation by comparing the theoretically predictable zone with the error distributions for the unidirectional and multidirectional wavefield cases. Finally, the analysis of experimental data from Weihai, China, in 2021 also indicates that the joint spatial-temporal measurements can effectively expand the predictable zone.
Visible light communication (VLC) and non-orthogonal multiple access (NOMA) are deemed two promising technologies in the next generation wireless communication systems in achieving high capacity and ...massive connectivity. In this paper we study the performance of a NOMA-enabled VLC system using different modulation schemes. In particular system level bit error rate (BER) is derived for different modulation schemes. Conventional methods used for analyzing the BER under orthogonal multiple access cannot be directly applied to NOMA. In order to obtain the closed-form BER expressions for the NOMA-enabled VLC systems, an analytical framework based on bitwise-decision axis and signal space is proposed.Moreover, the analysis method can be extended to the any wireless communication networks with NOMA. Simulation results demonstrate the accuracy of the theoretical analysis. The study shows that the BER gap among users decreases with the increase of the modulation order but at the cost of a higher power consumption in order to achieve a better signal to noise ratio. It is observed that 8-PSK modulation in NOMA-enabled VLC systems strikes a good tradeoff between the power cost and the achievable BER.
Abstract
Wavenumber–frequency spectra obtained with coherent microwave radar at upwind-grazing angle consist of energy along the ocean wave dispersion relation and additional features that lie above ...this relation labeled as “high-order harmonic” and below this relation known as “group line.” Due to these nonlinear features, low-frequency components appear in the radar-estimated wave spectrum and the energy and peak frequency of the dominant wave spectrum decrease, which are responsible for the overestimation of radar-measured wave period. According to the component distribution in the wavenumber–frequency spectrum, a mean wave period inversion method based on a dispersion relation filter for coherent S-band radar is proposed. The method filters out the “group line” and preserves the high-order harmonic to compensate for the energy loss caused by the decrease of peak frequency of the dominant wave spectrum. A two-dimensional inverse Fourier transform is applied to the filtered wavenumber–frequency spectrum. Then the radar-measured velocity sequence is selected to obtain the velocity spectrum via a one-dimension Fourier transform. The wave height spectrum is estimated from the one-dimensional velocity spectrum by the direct transform relationship between the one-dimensional velocity spectrum and the wave height spectrum. Later, mean wave periods can be derived by the first moment of the wave height spectrum. A 13-day dataset collected with a shore-based coherent S-band radar deployed at Zhelang, China, is reanalyzed and used to retrieve mean wave periods. Comparisons between the measurements of radar and wave buoy are conducted. The results indicate that the proposed method improves the wave period measurement for coherent S-band radar.
Significance Statement
This work provides a mean wave period inversion method for coherent S-band radar. The mean wave period is always overestimated due to the “group line” in the wavenumber–frequency spectrum and the energy loss caused by the decrease of peak frequency of the dominant wave spectrum. Therefore, dealing with these estimation errors is important.
Microwave Doppler radar has been used to make noncontact river flow measurements and offers a high resolution and a wide range. However, discharge estimation with microwave Doppler radar is ...challenging due to the inability to measure flow velocity below the river surface based on the Doppler shift. To address this problem, a velocity distribution inversion method based on the Reynolds-Averaged Navier-Stokes (RANS) equations is proposed. A classical two-region model which divides the flow into the inner region and the outer region is applied. Velocity at the inner-outer boundary is estimated using surface velocity and the log law is used to estimate velocity distribution of the inner region. Taking velocities at the surface and the inner-outer boundary as boundary conditions, the velocity distribution in the outer region is derived by solving the RANS equations. Discharge is calculated by combining the proposed method with the velocity-area method. The results illustrated that the velocity distribution obtained by the proposed method and measured by an acoustic instrument are in reasonable agreement. Discharge estimated using the obtained velocity distribution has an error less than 10%. The work indicates the potential of microwave Doppler radar to retrieve the river cross section velocity distribution and estimate discharge.
For coherent microwave radar, the Bragg scattering from the broken-short waves generated after wave breaking usually introduces extra low-frequency components in the estimated wave height spectrum, ...and thus leads to inaccurate retrievals of wave parameters, especially the overestimation of wave period. In order to eliminate the impacts of wave breaking, some methods based on the removal of "group-line" in the spatial-temporal domain are proposed to estimate wave parameters. However, these methods are not suitable for the case that the spatial-temporal data are not available. To address this problem, a method is proposed to invert wave parameters only from the time-Doppler spectrum. Temporal velocity series are derived from the time-Doppler spectrum in which breaking components are removed. Then the wave height spectrum from which wave parameters can be obtained is estimated from the velocity series by the direct transform relationship based on the linear wave theory. Without spatial-temporal data, the "group-line" can be removed using the proposed method, and the method is validated by simulation. In addition, an approximately eleven-day dataset collected with a shore-based coherent S-band radar deployed along the coast of Zhejiang province in China is reanalyzed and used to retrieve significant wave height and mean wave period. Compared with the buoy-measured data, the significant wave heights and mean wave periods retrieved by the proposed method have root-mean-square differences (RMSDs) of 0.25 m and 0.60 s, respectively, and also have correlation coefficients (CCs) of 0.94 and 0.81, respectively. The results indicate that the proposed method can invert wave parameters from the time-Doppler spectrum with a reasonable performance.
The reconstruction and prediction of deterministic sea waves are important for increasing the safety of offshore operations and improving the efficiency of wave energy conversion. Coherent microwave ...radar, as a commonly used instrument for sea surface observation, can directly reconstruct deterministic sea waves based on the Doppler echo. However, its reconstruction performance relies heavily on the accuracy of the velocity measurements and is severely limited in a high sea state or in the presence of hard target clutter. In this work, we propose a robust deterministic wave reconstruction and prediction scheme to reduce the influence of velocity measurement anomalies on the reconstruction and prediction results. First, the spatial-temporal velocities are obtained from the coherent microwave radar, followed by the least absolute deviation (LAD) method for solving the parameters of the wave motion model, and the deterministic wave is reconstructed and predicted based on the obtained model parameters. The deterministic wave reconstruction and prediction under long-crested and short-crested sea states with broken waves are simulated, and the results confirm the effectiveness of the scheme. Simultaneously, the scheme is verified using the experimental data collected with a coherent S-band radar in Weihai in December 2021. First, the significant wave height is reconstructed using the scheme, and the correlation with the in situ data reaches 0.97. Then, the prediction performance of deterministic sea waves is compared with the velocity potential function method, and these results confirm the effectiveness of the scheme.
Shipboard coherent microwave radar is an emerging tool for ocean observation, which utilizes the direct relationship between the orbital wave velocity and the wave height spectrum to retrieve wave ...parameters. However, the ship's motion and broken waves would introduce extra Doppler components into the Doppler spectrum of the sea echo, and accordingly degrade the performance of wave measurements. Consequently, the Doppler components should be estimated before the inversion of ocean wave parameters. To address this problem, a Doppler frequency components estimation method, which describes the problem as an optimization problem with constraints to fit the raw Doppler spectrum, is proposed. First, the Doppler spectrum model for shipboard coherent microwave radar is parameterized and simplified. Then, an objective function and the constraints are established based on the generation mechanism of frequency components in the Doppler spectrum to make the reconstructed Doppler spectrum fit the raw Doppler spectrum. Subsequently, the particle swarm optimization (PSO) algorithm is used to find the optimal coefficient to solve the optimal solution. At last, frequency components, which are produced by ship motion, broken waves, and orbital modulation of gravity waves, are estimated. To validate the proposed method, the simulation data and the experimental data collected with a shipboard S-band radar in the South China Sea in December 2020 are analyzed. The estimated frequency components from radar data are compared with the MTi-G-measured and buoy-measured data. The results indicate that the proposed method is effective for estimating the frequency components and could improve the performance of wave measurements.
Coherent microwave radar utilizes the direct relationship between the orbital wave velocity and the wave height spectrum to retrieve wave parameters. However, due to the broken wave and its ...evolution, “Group-Line” is introduced in the radar-obtained wavenumber–frequency spectrum, and the dominant wave energy is reduced. Many methods have proposed to remove “Group-Line” in the wavenumber–frequency spectrum, but these methods can only remove low-frequency energy and fail to compensate for the dominant wave energy. To solve this problem, a method for wave parameter inversion using quasi-binary variational mode decomposition (QB-VMD) is proposed. First, QB-VMD decomposes the spatial–temporal radial velocity series, and a series of modes is obtained, including “Group-Line” and dominant wave modes. Second, the “Group-Line” mode is discarded, and the dominant wave mode is compensated appropriately to reconstruct the spatial–temporal radial velocity series. Finally, the wave parameters are retrieved according to the reconstructed spatial–temporal radial velocity series. The proposed method is verified by simulation. In addition, this article uses an eight-day dataset collected by the coherent Formula Omitted-band radar deployed at Beishuang island for analysis. The significant wave height and mean wave period are retrieved from the dataset. The wave parameters estimated by the proposed method are compared with the buoy results. The correlation coefficients are 0.97 and 0.80, and the root-mean-square errors are 0.12 m and 0.49 s, respectively. The results show that the proposed method can invert wave parameters using the coherent Formula Omitted-band radar with reasonable performance.
The forecasting of ship's roll motion is the key to ensuring the safety of ship surface operations and improving operations efficiency. A new hybrid multi-step forecasting model is proposed in this ...paper. The proposed model combines three methodologies, including adaptive empirical wavelet transform (EWT), multi-step forecasting under the multi-input multi-output (MIMO) strategy of bidirectional long short-term memory (BiLSTM) model, and hybrid particle swarm optimization and gravitational search algorithm (PSOGSA) hyperparameter optimization. The three sets of ship roll datasets in the South China Sea are selected to verify the performance of the hybrid multi-step prediction model. In the end, the results of the research indicate that: (a) The proposed model has a superior prediction accuracy in multi-step prediction, taking dataset #1 as an example, the root mean square error (RMSE) of the prediction result is 0.0934°, the mean average error (MAE) is 0.0742°, and the mean absolute percentage error (MAPE) is 2.9878%; (b) The proposed hybrid multi-step forecasting model is suitable for different datasets and has strong robustness. Taking the 3-step prediction of dataset #1 to #3 as examples, the RMSEs of the proposed model are 0.0879°, 0.0742°, and 0.0991°, respectively.
•A new multi-step forecasting model for ship's roll motion.•Reduce the non-stationary characteristics of the ship's roll data by an adaptive decomposition algorithm.•Reduce prediction error by the BiLSTM model under the MIMO strategy.•Obtain the optimal hyperparameters by the hybrid PSOGSA algorithm.
Accurate prediction of the ship motion attitude in the future period is important to ensure the safety of offshore operations and sea navigation. Coherent S-band radar is a novel wave monitoring ...device that can be applied to advance the development of ship motion attitude prediction. In this study, we propose a method for realizing ship motion attitude prediction from radial velocity of water particle using coherent S-band radar. First, the parameters of the wave motion model (WMM) are estimated using the regularized least squares (RLS) method, which can solve the ill-conditioning matrix on the estimation accuracy under the short-crested wave sea state. Then, a predictable zone model considering radar dwell time is proposed based on coherent S-band radar, which can improve the traditional predictable zone. Finally, the deterministic sea waves in the predictable zone are input into the ship response amplitude operator (RAO) to realize the ship motion attitude prediction. The performance of the proposed method for ship motion attitude prediction in short-crested wave condition is analyzed through simulation. Meanwhile, the proposed method is validated using real data from the sea trial experiment in December 2019 in the South China Sea. The prediction results obtained by the proposed method are compared with the measured data from the MTi-G-710 instrument, and the two trends are consistent with a correlation coefficient of 0.7564. The results show that the proposed method can effectively realize ship motion attitude prediction using coherent S-band radar.
