This paper investigates the relationships between friction velocity, 10 m drag coefficient, and 10 m wind speed using data collected at two offshore observation towers (one over the sea and the other on an island) from seven typhoon episodes in the South China Sea from 2008 to 2014. The two towers were placed in areas with different water depths along a shore-normal line. The depth of water at the tower over the sea averages about 15 m, and the depth of water near the island is about 10 m. The observed maximum 10 min average wind speed at a height of 10 m is about 32 m s . A comparison of the measurements from the two towers shows that the 10 m drag coefficient from the tower in 10 m water depth is about 40% larger than that from the tower in 15 m water depth when the 10 m wind speed is less than 10 m s À1. Above this, the difference in the 10 m drag coefficients of the two towers disappears.
The use of radial k-space trajectories has drawn strong interest from researchers for their potential in developing fast imaging methods in magnetic resonance imaging (MRI). Compared with conventional Cartesian trajectories, radial sampling collects more data from the central k-space region and the radially sampled data are more incoherent. These properties are very suitable for compressed sensing (CS)-based fast imaging. When reconstructing under-sampled radial data with CS, regridding and inverse-regridding are needed to transfer data between the image and frequency domains. In each CS iteration, two-dimensional interpolations are implemented twice in the regridding and inverse-regridding, introducing errors and undermining reconstruction quality. To overcome these problems, a radial-like pseudo-polar (PP) trajectory is proposed for the CS MRI applications. The PP trajectory preserves all the essential features of radial trajectory and allows an image reconstruction with PP fast Fourier transform (PPFFT) instead of interpolations. This paper attempts to investigate the performance of PP trajectory-based CS-MRI. In CS-based image reconstruction, the transformation of PP-sampled k-space data into the image domain is realized through PPFFT, which is based on the standard one-dimensional FFT and the fractional Fourier transform. To evaluate the effectiveness of the proposed methods, both numerical and experimental data are used to compare the new methods with conventional approaches. The proposed method provided high-quality reconstruction of the MR images with over 2-dB gain in peak signal-to-noise ratio while keeping structural similarity over 0.88 in different situations. Compared with the conventional radial sampling-based CS MRI methods, the proposed method achieves a more accurate reconstruction with respect to image detail/edge preservation and artifact suppression. The successful implementation of the PP subsampling-based CS scheme provides a practical and accurate CS-based rapid imaging method for clinical applications.
Directional modulation (DM) technique based on artificial noise (AN) enables the physical layer security (PLS) for wireless communications in free space. However, on one hand, the introduction of AN results in less power efficient for a DM system; on the other hand, the AN-aided DM technique is strongly dependent on the channel state information (CSI) of transceivers, which greatly limits its practical application. What is more, when the eavesdropper (EVE) is slowly approaching the legitimate user (LU) or the EVE and the LU are in the same direction, the transmission security cannot be guaranteed. To circumvent these problems, DM technique with multiple parameters weighted-type fractional Fourier transform (MP-WFRFT) and chaotic scrambling (CS) aided is proposed in this paper to realize the powerefficient and security-enhanced wireless transmissions. Then, the symbol error rate (SER), secrecy rate, robustness, and anti-interception performance of the proposed method are analyzed and simulated. The simulation results are presented to verify that the proposed method is more power-efficient than the traditional AN-aided DM schemes, and the PLS is guaranteed due to the proposed approach, albeit knowing the intended direction for an EVE.INDEX TERMS Directional modulation (DM), artificial noise (AN), physical layer security (PLS), multiple parameters weighted-type fractional Fourier transform (MP-WFRFT), chaotic scrambling (CS), symbol error rate (SER). I. INTRODUCTIONBesides connectivity and reliability, security is another significant crux for modern communication systems. For a wired communication system, communicating devices are physically connected through cables. An eavesdropper (Eve) is unable to access the system for illicit activities without direct association. Compared with wired communication systems, the information security has been a critical issue for wireless communication systems due to the broadcast nature of radio propagation. The air interface is open and accessible to all users authorized and unauthorized. The open environmentThe associate editor coordinating the review of this manuscript and approving it for publication was Huapeng Zhao.
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