Mu-King Tsay

Note on the multiple scattering in an IEM model

The authors derive the multiple scattering expression within the framework of an IEM model for rough surface scattering. The complementary field coefficients are rederived based on a new surface slope expressions which are dependent on spatial variables. This leads to a more complete expression of the multiple scattering terms, thus allowing the authors to account for multiple effects more accurately. Numerical calculations and comparisons with numerical simulation are provided to demonstrate the results.

An Improved Iem Model for Bistatic Scattering From Rough Surfaces

In the development of the IEM model for surface single scattering a simplifying assumption was applied to the phase of the Greens function in the average power calculation leading to a surface scattering model in algebraic form. In this paper we remove this simplifying assumption yielding a more complex model but still in algebraic form. We shall show that the assumption used in the past does not cause an appreciable difference when the incident and scattered angles are close to each other. However, for different incident and scatter angles there is a noticeable difference especially when the surface roughness is small and scattering is weak. For surfaces with large scale roughness the difference between the original IEM and the improved IEM is generally negligible. Bistatically, the improved IEM is in good agreement with the standard small perturbation model in the low frequency region and with the standard Kirchhoff model in the high frequency region. A recent publication extending the original IEM model along a different direction is shown to be a more restrictive surface scattering model in terms of applicability to moist rough surfaces.

Fuzzy Power Control for Downlink CDMA-Based LMDS Network

This paper proposes a novel fuzzy power control (FPC) scheme to effectively improve the performance of the downlink code-division multiple-access (CDMA)-based local multipoint distribution service (LMDS) system. Conventional power control scheme is taken to reduce rain fading effects and intercell interference, and then increase the capacity of this LMDS system. To more effectively improve the system performance, three major factors (i.e., rain rate, path loss exponent depending on channel fading conditions, and number of subscribers in the interfering cell sectors) are taken as the input variables of fuzzy logic control in our proposed FPC scheme to obtain the channel quality condition and distance threshold for power control region adjustments. Simulation results show that our proposed FPC scheme performs better than the conventional scheme in terms of outage probability.

Data compression on multifont Chinese character patterns

The problems arising in the modeling and coding of digital Chinese character patterns for noiseless compression purposes are discussed. The modeling is intended to capture the maximum redundancies of the source under the consideration of relevant parameters. In this study, a redundancy-gathering algorithm is proposed in which a 2-D parsing tree is used, and the nodes to extend the tree are ranked according to the maximum redundancy gathered. Hence, the modeling represented by the tree can achieve great performance in the small number of nodes. The algorithm is then applied to evaluate the performance of arithmetic coding for digital Chinese character patterns. The results are compared to those of the traditional compression methods. In the same complexity, the authors find that their algorithm can improve the coding efficiency as high as 25.54%, 30.23%, 32.17%, 15.90%, and 27.58%, for five Chinese fonts, respectively.

Secure Communications System Through Concurrent AJ and LPD Evaluation

In this paper, a systematic approach and their corresponding metrics for concurrent anti-jamming (AJ) and low probability of detection (LPD) performance evaluations of a secure communications system are investigated. In general, it is necessary to deal simultaneously with adversary threats of both active jamming and passive detection for a secure communications system concern. For independent or concurrent AJ and LPD performance evaluations, a single varying paired and multiple paired fixed collocated jammers/interceptors scenario is explored with real sinc type (sin(x)/x) antenna patterns being considered for the communicators. The error probability with both AJ and LPD being considered simultaneously can be derived as system- and geometry-dependent factors and categorized as system metrics related to jammers, interceptors, and communicators themselves only. Whenever multiple collocated jammers/interceptors are considered, we can observe interesting “smoothed” effects for bit energy to jamming density ratio (Eb/Jo), when one of these varying jammers/interceptors is approaching or receding. In addition, by means of these system metrics, it is intuitive and straightforward to enhance system performance for the victim or to deteriorate system performance for the adversary, respectively. Moreover, the proposed approach and metrics have also paved one practical way for the developments of the latest emphasized cognitive radio (CR) communications systems, if all the collocated jammers/interceptors on the proposed scenario are replaced with “cooperative” communicators.

Image compression using VQ and fuzzy classified algorithm

A fuzzy clustering algorithm is used for the image tree structure vector quantization (TSVQ). First, a digital image is divided into subblocks of fixed size, which consists of 4/spl times/4 blocks of pixels. By performing a 2-D discrete cosine transform (DCT), we select six DCT coefficients to form the feature vector, and use the fuzzy c-means algorithm in constructing the TSVQ codebook. By doing so, the algorithm can preserve the edge of image, make good image quality, and reduce the processing time while constructing the tree structured codebook, and reduce coding and decoding time.

An efficient repetition finder for improving dynamic Huffman coding

In this paper, a new repetition finder to be used with dynamic Huffman (1952) coding is proposed to improve the compression efficiency by reducing the redundancy due to string repetitions. Compared to the repetition finder proposed by Yokoo (1991), the proposed scheme effectively increases the numbers of consecutive symbols in the repetition mode and the total number of symbols in the repetition mode. Experimental results show that the proposed method outperforms the repetition finder of Yokoo by 14-40% in compression ratios with about the same memory requirement and running time.

A re‐examination of the IEM model for microwave scattering from randomly rough boundary

Abstract An IEM surface scattering model was examined in terms of its applicability to simulations and laboratory measurements. New expressions for both single scattering and multiple scattering were obtained by re‐deriving the scattering coefficient to keep all the phase terms in the spectral representation of Greens function. After quite intricate mathematical manipulations, a fairly compact form for an advanced IEM model (AIEM) is obtained for the scattering coefficients. In addition, the Fresnel reflection coefficients used in the model were replaced by a transition function. The comparisons in this paper were concentrated in the case of backscattering with both numerical simulations and measurement data. The results indicate that the IEM is improved, becoming more accurate and practical to use.

Simultaneous AJ and LPD Evaluations for Secure Communication

In this paper, a systematic approach for evaluating the interactions of the link geometry for anti-jamming, low probability of detection, and their combined applications are investigated. In many typical cases, communication designs have to deal simultaneously with adversary threats of both active jamming and passive detection. And by increasing communication power to counter jammer and enhancing anti-jamming (AJ) capability must be weighted carefully against the increased threats of deteriorating low probability of detection (LPD) capability and being detected by an unintended interceptor receiver. A qualitative and quantitative approach with sinc-type antenna patterns being included for evaluating both AJ and LPD concurrently is reached. And it is intuitive to see that by spreading signal spectrum, complicating signal waveforms, and lowering power control uncertainty, respectively, will enhance system performance accordingly.

Complete LC Ladder Bessel Filter Tabulation

Bessel filter has been widely used in industry for analog filter circuit design with its maximally flat time-delay or least group delay ripple performance. Nevertheless, there has been only one set of tabulation per order (LC element number) available for the realization of a specific Bessel filter through LC ladder architecture in conventional literature. To synthesize a LC ladder Bessel filter thoroughly, especially for higher order and more tabulations concerns, the problem of calculation accuracy should be precisely concerned to avoid neglecting the relatively significant but smaller coefficient values. In this paper we have obtained a complete set of tabulation per order, which can be generalized as 2[(n+2)/2] for an nth order LC ladder Bessel filter network with duality being included.