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Communications and Signal Processing, 1994. COMSIG-94., Proceedings of the 1994 IEEE South African Symposium on

Date 4-4 Oct. 1994

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  • Proceedings of COMSIG '94 - 1994 South African Symposium on Communications and Signal Processing

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  • Classification of aerospace targets using superresolution ISAR images

    Page(s): 138 - 145
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    This paper describes investigations into a recognition system that takes the 2-D ISAR (inverse synthetic aperture radar) image of an aerospace target as input and classifies the target based on features calculated from the image. Four types of features were implemented, namely geometrical moments, invariant features based on moments, shape features, and quantized energy strips. Nearest-neighbour and neural-net classifiers are considered View full abstract»

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  • Phase-plane analysis of phase-locked loops used for clock recovery

    Page(s): 99 - 104
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    The transient analysis of phase-locked loops is not an easy analytical task. Although linearisation of these devices is common practice, the exact nature of the loop is not obtained. Using phase-plane techniques it is possible to determine both the transient and steady-state responses of phase-locked loops. We investigate the application of phase-plane techniques to a second-order phase-locked loop and to a more complex clock-recovery loop. A number of interesting results were obtained and are discussed including the effects on the phase-plane plot for a variety of loop filter bandwidths and loop gain values View full abstract»

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  • 2-D radar imaging

    Page(s): 146 - 151
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    Radar images generated with the 2-D Fourier transform of the scattered field data limited in frequency and/or angle have poor resolution. This has led to the introduction of the hybrid method and 2-D super resolution techniques for limited field data. However, the images contain no amplitude information of the various scattering centers. In this study, a modified hybrid method and 2-D AR technique are applied to generate high quality radar images using limited backscattered field data. The final image presents the scattering properties of the target in a quantitative way. The peaks in the image represent the positions of scattering centers contributing to the backscattered field. Furthermore, the amplitudes of the peaks correspond to the intensities of the scattering centers View full abstract»

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  • Security of Levenshtein synchronization error correcting codes

    Page(s): 81 - 84
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    A new cryptographic system using single insertion/deletion (synchronization) error correcting codes is described. The new system's security is evaluated according to three basic attacks. Supplementary practical results are used to demonstrate that the new system is computationally secure View full abstract»

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  • Deception in authentication channels with multiple usage

    Page(s): 60 - 67
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    By considering a simple but very general authentication code, we obtain some lower bounds on the probability of successful deception, through impersonation or substitution, if an authentication channel is used without a change of key after each message. In particular an authentication scheme proposed by Johanssen (see Designs, Codes and Cryptography, vol.4, p.69-81, 1994) is analysed, and found to be extremely vulnerable in multiple usage View full abstract»

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  • An implementation of an elliptic curve cryptosystem

    Page(s): 85 - 90
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    We discuss the implementation of the analog of the ElGamal (1984) public-key cryptosystem using elliptic curves over finite fields. More specifically, the kind of elliptic curve used is a supersingular curve over GF(2208). Furthermore, it is shown that this cryptosystem is cryptographically secure to the best of current knowledge View full abstract»

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  • A comparison of three families of spreading sequences for CDMA applications

    Page(s): 68 - 75
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    A comparison between three different families of spreading sequences for an all-digital Direct sequence spread spectrum code division multiple access (DSSS CDMA) modem is presented. The families considered are binary Gold and Kasami and polyphase generalised chirp-like (GCL) sequences. Comparisons are made with regard to the autocorrelation, cross-correlation, family size, length and ease of implementation of these sequences View full abstract»

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  • Rock recognition using feature classification

    Page(s): 152 - 157
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    A knowledge based approach to finding fixed size rocks within an image is described. It is intended to be a building block within a multiresolution system. Each point is hypothesised to be a rock and a region surrounding it is labelled using knowledge of rock characteristics. Twelve features are then measured and used to test the hypothesis by means of a combination of thresholding and k-nearest-neighbour classification View full abstract»

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  • A multi-oscillator approach to CPFSK signal creation

    Page(s): 20 - 23
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    The development of a MSK modulator/demodulator by Massey (1980) and the Hodgart and Schoones (see ibid p.211-216, 1992) implementation has prompted the development of a multi-oscillator multi-h modulator/demodulator. The approach taken has been an investigation and extension of basic CPFSK multi-oscillator modulator/demodulator design using digital logic. In this paper the multi-oscillator MSK modulator as developed by Massey is first re-examined before introducing the scheme developed by Crawford (see Master's thesis, Dept. Elec. Eng, University of Cape Town, Rondebosch, South Africa, June, 1993). Thereafter elements of the related digital logic demodulator design are discussed View full abstract»

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  • On the reduction of fractal image compression encoding time

    Page(s): 158 - 161
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    Lossy image coding by partitioned iterated function systems, popularly known as fractal image compression, has recently become an active area of research. An image is coded as a set of contractive transformations in a complete metric space. As a result of a well known theorem in metric space theory, the set of contractive transformations (subject to a few constraints) is guaranteed to produce an approximation to the original image, when iteratively applied to any initial image. While rapid decompression algorithms exist, the compression process is extremely time consuming; an exhaustive search for the optimum mappings is O(n4) for an n×n image. The most common solution involves classification of domain and range blocks according to features such as the presence of edges, after which matches across class boundaries are excluded. We propose a geometric construction, allowing clustering, as well as providing upper and lower bounds for the best match between domain and range blocks, allowing blocks to be excluded from the computationally costly matching process View full abstract»

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  • Some non-linear effects of quantization in phase-locked loops

    Page(s): 116 - 120
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    The combination of feedback and quantization causes special problems. (Each on its own has its problems, like instability and round-off errors.) The main problem is the likely existence of multiple equilibria of limit cycles. Another is possible coherence between the input signal and quantization error or limit cycle, making the error indistinguishable from whatever signal characteristic we want to measure. In this paper the conditions in which limit cycles occur are described, and the coherence between limit cycles and incoming signals, which can lead to spurious measurements, are examined. Particular mention is made of the fact that the phase-locked loop (PLL) exhibits a non-linear and phase-dependent relationship between amplitude quantization and the resultant phase quantization. A remedy for some of these problems, in the form of a design rule specifying desirable quantization resolution at various points in a typical phase-locked loop, is described View full abstract»

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  • Adaptive estimation of speech parameters

    Page(s): 177 - 182
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    Linear predictive coding (LPC), and transformations of it, is currently the most popular way of analysing speech signals. Major limitations of using a frame-based technique are that each frame is analysed in isolation of the rest while assuming the excitation source to be a white noise process. In order to reduce computation time, an all pole model is usually employed. In the present project an adaptive algorithm is proposed for speech signal analysis. The algorithm is based on the recursive least squares method with a variable forgetting factor. A pole-zero model is used to estimate the anti-formants present in certain sounds (i.e. nasals and nasalized vowels). This method offers better detection of poles and zeros in stationary environments and faster tracking of pole and zero frequencies in nonstationary signals than other sequential methods. An effective input estimation algorithm eliminates the influence of pitch on the parameter estimates by assuming the input to be a white noise process or a pulse sequence View full abstract»

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  • Interconnection strengths and injected currents of a Hopfield neural net applied to an adaptive linear combiner

    Page(s): 121 - 124
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    Mathematical expressions for the interconnection strengths and injected currents of a Hopfield type neural network are derived for the case where the network is expected to perform as an adaptive algorithm of an adaptive linear combiner (adaline). It is shown that the performance of the neural network is theoretically optimum. The problem is basically a quadratic programming one, and it is shown that the proposed neural network solution can be used for both cases where the variables are unconstrained and binary constrained. A representative application is given in the form of a simulation of a FIR filter View full abstract»

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  • Fuzzy modeling and prediction of network traffic fluctuations

    Page(s): 41 - 45
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    Network management is a real-time observation and control action to optimize the grade of service of a network. More emphasis is being placed on the intelligent automatic exchange for stored program controlled (SPC) systems that are capable of real-time network management. The modeling of traffic characteristics and the prediction of future traffic flow are the first steps to efficient network control and management. This paper presents a fuzzy logic self-learning model and adaptive predictor of traffic now. The results of an application of this predictor on real world telephone links, are shown, and are compared to applications of a math-model and a Kalman-filter predictor View full abstract»

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  • New cepstrum algorithms

    Page(s): 105 - 107
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    New algorithms are introduced for computing 2N-1 points of the complex cepstrum and N unique points of the real cepstrum while using only N-point FFT calculations. These methods are compared to existing methods, and some indication is given of their applicability View full abstract»

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  • Motion compensation for airborne SAR

    Page(s): 128 - 131
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    This paper reports on a motion compensation algorithm applicable for airborne SAR. The algorithm is described at an geometrical level to aid intuitive understanding and the equations used are given for easy implementation. The algorithm has been applied in the processing of ESAR data and found noticeably to improve image resolution View full abstract»

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  • Using correlation filters to classify aircraft down-range radar profiles

    Page(s): 132 - 137
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    This paper details a classifier which uses correlation filters to discriminate between down-range profiles of radar targets. Down-range profiles are formed via an inverse FFT from frequency data obtained with the MTI (Moving Target Identification) process from successive scans of the aircraft. MTI separates the reflected backscatter of a moving target from the backscatter of the stationary environment. Simulation is used to quantify the resolution of the down-range profiles obtained with MTI. Classification using correlation filters is performed on data from a compact range as well as MTI data from measurements of actual aircraft in flight View full abstract»

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  • A novel code phase synchronization network for direct sequence spread spectrum

    Page(s): 95 - 98
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    Direct sequence spread spectrum systems require that the receiver's replica of the spreading waveform be in code-phase and frequency lock with that of the transmitter's. Present methods of achieving synchronization involve correlation and energy detection. This paper presents the development of an alternative synchronization network. The network is a novel robust feedback loop where the input is the differentiated transmitter pseudo-noise sequence. The effects of these positive and negative going voltage spikes modulate the receiver clock until lock is achieved. The loop is further adapted to a data driven environment View full abstract»

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  • VLSI appropriate design of a trace-back Viterbi decoder

    Page(s): 76 - 80
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    This paper concerns the research into the implementation of an efficient Viterbi algorithm for the decoding of convolutional codes. The original algorithm, as presented by Truong (see IEEE Trans. Commun. Technol., vol.40, no.3, p.616-624, 1992), is described, with many new points being noted. A detailed discussion of the improvements made to the algorithm is included, along with a full example of the operation of the algorithm View full abstract»

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  • Maximum entropy and average error rates in digital communication systems

    Page(s): 11 - 15
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    In digital communication systems, the criteria of merit of system performance is usually average probability of error as function of signal-to-noise ratio. The Gauss quadrature rule (GQR) formulation and the maximum entropy method (MEM) have been proposed in the literature to determine and calculate an unknown distribution and average error rate from moments. We compare the accuracy of these two methods when a distribution is estimated and the average error rate is calculated. It is shown that the MEM needs significantly less moments when the distribution is estimated from its moments than the GQR formulation, and that the GQR formulation fails under certain conditions when average error rate is calculated. Specifically, the latter is encountered at high signal-to-noise ratios, where the MEM still delivers reliable results View full abstract»

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  • Investigation of data transmission over SST channel

    Page(s): 24 - 28
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    The paper concentrates on the effect of FEC on the supplementary signal transmission (SST) channel. The SST channel and its possible applications are discussed, as its uses are unexplored in South Africa. The noise properties of the channel are investigated, with and without forward error correction (FEC). A discussion of FEC is given to introduce the concept and functional operation of the system. Bit error rates are measured without the use of FEC, to demonstrate the problem and the effect of the FEC on the noise corrupted data is presented View full abstract»

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  • Power spectral density of duobinary-signalled minimum shift keying (MSK) with arbitrary pulse shaping

    Page(s): 16 - 19
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    We present a general method for computing the power spectral density (PSD) of a partial-response, continuous phase modulated (CPM), constant-envelope signal. The modulation scheme employed is minimum shift keying (MSK). The frequency modulating signals are a random baseband pulse train in which the arbitrary pulse is time limited. Random data symbols transmitted during different time slots are assumed to be identically distributed and statistically independent View full abstract»

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  • A universal homophonic coding algorithm based on arithmetic coding

    Page(s): 52 - 59
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    We present a practical algorithm for the homophonic coding of a message source, as required for cryptographic purposes. The purpose of homophonic coding is to transform the output of a non-uniformly distributed message source into a random, uniformly distributed source sequence. This is achieved by mapping each source symbol into one of a set of homophones in a random way. The selected homophones are then encoded by means of arithmetic coding. In this paper a new arithmetic coding algorithm is introduced, which relies on the fact that the homophones are chosen be dyadic. This leads to particularly simple implementation of homophonic coding, requiring no multiplications but only shifts and additions. The operation of the algorithm is demonstrated by the homophonic encoding of an ASCII file. The results show that homophonic does increase the entropy by less than 2 bits per symbol, and also provides source encoding (data compression) of the original file View full abstract»

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