Hua Lee

An underwater acoustic telemetry modem for eco-sensing

An underwater acoustic telemetry modem is presented for ecological research (eco-sensing) applications. The modem is intended for the physical layer (PHY) in an ad hoc network of AquaNodes. Each AquaNode will incorporate an acoustic modem, router and host processor with standardized interfaces to oceanographic sensors. The transmitted waveforms use a composite Walsh/m-sequence format in which each bit in an 8-bit long Walsh function is spread by a 7-chip m-sequence. The resulting waveform has a 5 kHz bandwidth for robustness to multipath and achieves a 133 bps data rate. An 11 msec, time-guard band for channel clearing eliminates the need for equalization. The receiver employs a generalized multiple hypothesis test with a matching pursuits (GMHT-MP) inner loop for symbol-by-symbol channel estimation. Symbol-error rate (SER) results demonstrate that the GMHT-MP is superior to the noncoherent RAKE receiver at usable SERs (<10-2) and can accommodate up to 10 msec, multipath spreads with Doppler spreads on the order of 1 Hz

Resolution analysis of tomographic reconstruction of electron density profiles in the Ionosphere

Traditionally, knowledge of the ionospheric electron density is obtained using Faraday rotation or differential Doppler techniques which measure total electron content in columns of the ionosphere. Conventional data processing can only image the electron density in the direction perpendicular to these columns, thereby forming one‐dimensional images. Because this data is proportional to line integrals through the region of interest, tomographic techniques may be used to reconstruct two‐dimensional electron density images. In this paper, the resolution limit of the image reconstruction process is analyzed in terms of limited‐angle tomography.

Orthogonal decomposition technique for ionospheric tomography

The possibility of reconstructing two‐dimensional electron‐density profiles in the ionosphere with ionospheric tomography is significant. However, due to the nature of the imaging system, there are several resolution degradation parameters. In order to compensate for these degradation parameters, a priori information must be used. This article introduces the orthogonal decomposition algorithm for image reconstruction, which uses the a priori information to generate a set of orthogonal basis functions for the source domain. This algorithm consists of two simple steps: orthogonal decomposition and recombination. In the development of the algorithm, it is shown that the degradation parameters of the imaging system result in correlations among projections of orthogonal functions. Gram–Schmidt orthogonalization is used to compensate for these correlations, producing a matrix that measures the degradation of the system. Any set of basis functions can be used, and depending upon this choice, the nature of the algorithm varies greatly. Choosing the basis functions of the source domain to be the Fourier kernels produces an algorithm capable of isolating individual frequency components of individual projections. This particular choice of basis functions also results in an algorithm that strongly resembles the direct Fourier method, but without requiring the use of inverse Fourier transforms.

Novel robust watermarking technique in dithering halftone images

In this letter, we present a novel robust method for embedding watermarks into dithered halftone images. The method is named paired sub-image matching ordered dithering (PSMOD), of which the decoder is provided with a priori information of the original watermark. The method utilizes the bit and sub-subimage interleaving preprocesses. The experiments show that the technique is sufficiently robust to guard against the cropping, tampering, and printed-and-scanned degradation processes, in either B/W or color dithered images. This technique is also sufficiently flexible for various levels of embedded capacities.

MP core: algorithm and design techniques for efficient channel estimation in wireless applications

Channel estimation and multiuser detection are enabling technologies for future generations of wireless applications. However, sophisticated algorithms are required for accurate channel estimation and multiuser detection, and real-time implementation of these algorithms is difficult. This paper presents architectural design methods for wireless channel estimation which can be leveraged to enable real-time multiuser detection. We redesign the matching pursuit (MP) channel estimation algorithm to reduce the complexity while maintaining the estimation accuracy. Furthermore, we develop a parameterized intellectual property (IP) core, which provides a hardware implementation of the MP algorithm. Experimental results demonstrate the effectiveness and efficiency of the new algorithm and IP core for channel estimation. The implementation of our MP core on a modern, high performance reconfigurable system is about 216 times faster than running the algorithm on a state of the art microprocessor. The MP core possesses the speed required for performing true multiuser detection, enabling future generations of wireless communication applications.

Pulse‐echo microwave imaging for NDE of civil structures: Image reconstruction, enhancement, and object recognition

This article presents an overview of the application of microwave imaging in nondestructive evaluation (NDE) of civil structures. The presentation includes the image formation algorithm operating in monostatic multiple‐frequency mode, resolution enhancement by wavefield statistics, and object recognition procedures. Results from theoretical analysis, laboratory experiments, and full‐scale field tests are discussed sequentially.

Resolution degradation parameters of ionospheric tomography

Tomographie reconstruction of the electron density structure in the ionosphere has generated much interest due to the possibility of high-resolution two-dimensional image reconstruction using total electron content (TEC) data. An algorithm based upon the filtered backprojection algorithm of X ray tomography that provides such two-dimensional reconstructions has been developed; however, the resolving capability is limited by the nature of the ionospheric system. In ionospheric tomography, each piece of data contains information similar to a sample on a projection in X ray tomography. Collectively, however, the information content of ionospheric data sets is significantly different from a set of tomographic projections. These differences are the result of characteristics of the data acquisition system and are directly related to the resolution limits of the reconstructed images. In this paper the data acquisition system is analyzed to determine the sources of this degradation. Two key parameters of nonuniform sample spacing and angular reindexing error are analyzed to determine their effect upon the reconstruction process. A method of evaluating sampling schemes to measure the information content of the resulting projections is presented. Finally, a relationship between the angular reindexing error and the point spread function of the imaging system is derived.

Synthetic-aperture sonar imaging with a multiple-element receiver array

The application of synthetic aperture techniques in sonar imaging of the ocean floor has been limited by sonar platform instability. The authors present a robust algorithm for motion estimation and compensation to perform synthetic aperture image formation. First, the algorithm uses the acoustic returns from a receiver array to form a complex-valued physical-aperture image for each transmit burst. Perturbations of the sonar platform velocity or attitude manifest themselves as registration errors and phase errors between successive images. The registration errors are estimated by cross-correlation-based techniques, and the images are aligned spatially. The phase errors are then removed. Finally, after phase correction, the registered images are coherently superimposed to form a high-resolution synthetic aperture sonar image. Results of experiments at sea with a prototype synthetic aperture sonar system are presented. A small point reflector on a sandy bottom at 70 m range was imaged using a 400 wavelength physical aperture and the synthetic aperture technique to compare the performance in detecting small targets near the maximum operating range of the sonar. The synthetic aperture image exhibits superior cross-range resolution and higher signal-to-noise ratio compared with the conventional techniques.<<ETX>>

Synthetic aperture active sonar imaging

The main problems involved in synthetic aperture sonar imaging are medium instability, slow mapping rate, and platform motion. Proposed solutions to these problems are reviewed, and a new technique for synthetic aperture sonar imaging is presented which allows a fast mapping rate and improved motion compensation. This technique involves using multiple receivers to form a complex-valued preliminary image for each pulse. The preliminary images are used to estimate translation and rotation, and then they are registered, phase corrected, and superimposed to form high-resolution synthetic aperture sonar images.<<ETX>>

Geolocation in ad hoc networks using DS-CDMA and generalized successive interference cancellation

Channel estimation and distributed positioning algorithms are presented for geolocation in a wireless ad hoc network. The network uses a direct-sequence code-division multiple-access-based handshaking protocol, in which nodes receive multiple acknowledgment packets in response to a request-to-send waveform. Round-trip travel time (RTT) and angle-of-arrival (AOA) measurements are obtained using the generalized successive interference cancellation/matching pursuits (GSIC/MP) algorithm. The performance of GSIC/MP is evaluated via simulation and comparison to the Crame/spl acute/r-Rao bound. Position estimates are initialized using linearized least-squares and updated by an extended Kalman filter-based algorithm that includes measurement validation for nonline-of-sight error mitigation. The method is generalized for distributed estimation in sparsely connected networks: at each node, position estimates from connected nodes are incorporated via a fusion algorithm and updated using locally processed RTT/AOA measurements. Finally, comprehensive ad hoc network simulations are presented including channel ray tracing, RTT/AOA estimation and validation, and distributed positioning.