摘要:

Based on the fact that the performance of random error-correction decoding is insensitive to code symbols erased before its decoding process, a new approach to mitigate the effects of interference is derived. This approach consists of three basic functions: it modifies conventional interleaving, detects and deletes burst errors, and corrects random errors. Therefore this approach minimises the effects of jamming, antenna switching and nuclear deep fading. In the paper we first describe a tactic for integrating coding and interleaving. The derivation and application of a special rate-1/4 convolutional code are then illustrated, and, finally, some preliminary computer simulation results are utilised to demonstrate the effectiveness of the proposed technique.

DOI:10.1049/ip-f-1.1987.0022

出版商:IEE    

年代:1987

数据来源: IET

摘要:

Constant false-alarm rate (CFAR) techniques are compared in an airborne pulse-Doppler detection application. Standard methods such as CA-CFAR, GO-CFAR and OS-CFAR prove to be inadequate in dealing with the extreme variations of power between radar cells, resulting in large CFAR losses. Two new techniques, nadir-blanked CFAR and gradient CFAR, were specifically designed to accommodate these variations. Nadir-blanked CFAR simply does not attempt detection in the first few gates that include or exceed the altitude range. Gradient CFAR does not perform as well as nadir blanking, but is still considerably better than standard CFAR methods. The method is similar to GO-CFAR, but the GO-threshold is boosted by the gradient between two groups of reference cells. Both methods allow for large reductions in the CFAR multiplying constant and therefore substantial improvements in detectability over the standard methods.

DOI:10.1049/ip-f-1.1987.0025

出版商:IEE    

年代:1987

数据来源: IET

摘要:

A new maximum-likelihood algorithm for Toeplitz covariance matrix estimation is introduced. The new algorithm requires substantially less computational cost than the generalised procedure proposed by Burget al. It solves for the reflection coefficients rather than the covariance lags, and it calculates these coefficients successively starting from filter length one: thekth reflection coefficientakkis computed by maximising the associated likelihood function and assuming that the reflection coefficientsann,n= 1, …,k− 1, have been previously calculated. The algorithm does not assume that the noise data are expressed by an autoregressive (AR) filter. This assumption is incorrect since noise is always present in real signals and usually has serious consequences at low signal/noise ratios where the approximation due to the assumption is significant and the true spectrum is required for reliable detection. The optimisation associated with thekth-order recursion specifying thekth reflection coefficientakk, produces a cubic equation whose root with absolute value less than one is taken equal toakk; if more than one root satisfies this condition, the one with the higher likelihood is chosen. The rest of thekth-order filter coefficients are computed using the Levinson recursion. The stability of the algorithm is guaranteed as the reflection coefficients are constrained to have absolute values less than one during the recursion. As is verified by simulation results, the spectrum of the new algorithm exhibits no line splitting and is not affected by the initial phase.

DOI:10.1049/ip-f-1.1987.0027

出版商:IEE    

年代:1987

数据来源: IET

摘要:

A unified matrix treatment which is defined for binary Walsh matrices is presented. This unified treatment, based on the concept of dyadic symmetry, defines Walsh matrices of different orderings using a simple equation. This equation in turn provides a straightforward derivation of various reordering schemes and Walsh matrix properties. Various fast computational algorithms can also be derived within the same framework using dyadic decompositions. It is hoped that this unified treatment of Walsh matrices using dyadic symmetry will provide a better understanding of Walsh transforms, and will lead to the discovery of more useful transforms.

DOI:10.1049/ip-f-1.1987.0028

出版商:IEE    

年代:1987

数据来源: IET

摘要:

In the paper the idea of computing a projection matrix for bearing estimations by using anyMrows of the covariance matrix, whereMis the number of sources, is extended to the cases of low signal/noise ratio (SNR) and totally correlated sources. For low SNR, the projection matrix can be obtained by using off-diagonal terms ofMrows of the covariance matrix, and a reduced search vector can be used. For totally correlated sources, the idea of spatial averaging is adapted to compute the projection matrix. However, no averaging is actually carried out.

DOI:10.1049/ip-f-1.1987.0030

出版商:IEE    

年代:1987

数据来源: IET

摘要:

The paper presents a relatively simple method for analysing the effect of IF filtering on the performance of multilevel FM signals. Using this method, the error rate performance of narrowband FM signals is analysed for three different detection techniques, namely limiter-discriminator detection, differential detection and coherent detection followed by differential decoding. The symbol error probabilities are computed for a Gaussian IF filter and a second-order Butterworth IF filter. It is shown that coherent detection and differential decoding yields better performance than limiter-discriminator detection and differential detection, whereas two noncoherent detectors yield approximately identical performance.

DOI:10.1049/ip-f-1.1987.0031

出版商:IEE    

年代:1987

数据来源: IET

摘要:

The paper presents a subspace iteration based eigensystem solution algorithm for solving the minimum eigenpair (eigenvalue and associated eigenvector) of a Hermitian matrix. Specifically, the focus is on the class of covariance matrices which have near-Toeplitz structures. First, a modified Rayleigh quotient iteration (MRQI) method developed earlier is generalised to handle the near-Toeplitz structures. Next, a classical Rayleigh-Ritz (RR) subspace approximation procedure is employed to further enhance the performance. Extensive simulation is carried out to compare the new RR method, the (generalised) MRQI method and the classical bisection method. Favourable results are observed. With parallel processing taken into account, it is estimated that this novel covariance eigensystem solver, withO(N) processors, is able to solve the minimum eigenpair of a covariance matrix inO(kN) time units. It is also observed that the number of iterationskis relatively independent of the dimension of the covariance matrix, and thus may be considered as a constant.

DOI:10.1049/ip-f-1.1987.0032

出版商:IEE    

年代:1987

数据来源: IET

摘要:

It is known that convolutional coding and Viterbi decoding together with high-level modulation can provide error-control protection without sacrificing bandwidth efficiency on additive white Gaussian noise (AWGN) channels. Bearing this in mind, the paper is concerned with the performance of short constraint length convolutional codes in conjunction withM-ary phase-shift-keyed modulation and maximum-likelihood Viterbi decoding on the fully interleaved Rician fading channels. The primary interest is in the bit error probability performance as a function of system parameters and channel parameters. The generating function union bounding approach is applied, and fairly general analytic upper bounds on the event and bit error probability performance are developed. The bit error probability performance of selected rate-1/2 optimum codes with QPSK and of selected rate-2/3 Ungerbock codes and the Hui code with 8PSK are evaluated using the developed bound. The tightness of the analytic upper bound on bit error probability is also checked by means of computer simulation.

DOI:10.1049/ip-f-1.1987.0033

出版商:IEE    

年代:1987

数据来源: IET

摘要:

The paper deals with the problem of radar detection of a target echo embedded in Weibull clutter and white Gaussian noise (WGN). Relevant features of the paper, with respect to previous papers on the same subject, refer to the coherent nature of the Weibull process (that modelling the clutter) and of the processing chain. In more detail, the in-phase and quadrature components of the clutter echoes have been modelled to give a Weibull probability density function (PDF) of the amplitude and a uniform PDF of the phase. Any shape of the correlation function among consecutive clutter samples is also allowed in the model. The so called ‘coherent Weibull clutter’ (CWC) introduced in the paper represents a suitable generalisation of the conventional ‘coherent Gaussian clutter’ (CGC). The processing chain is also coherent, i.e. it operates on the in-phase and quadrature components of the signals. To derive a suitable processing scheme, we resort to the general theory of radar detection which applies to any type of PDF and autocorrelation function (ACF) of the target and clutter. Briefly, it is found that the processing scheme is based on two nonlinear estimators of the clutter samples in the two alternative hypotheses (i.e. H0and H1the fully fledged architecture being discussed in the paper. The detection processor turns out to be a suitable generalisation of that concerning the CGC case. A new family of processing schemes is derived in accordance with the statistical model assumed for the useful target. The following cases are covered: target knowna priori, Swerling 0, 1 and 2 models, and partially fluctuating target. Attention is also paid to the interesting case of a target modelled as a coherent Weibull process. The detection of such a target against white Gaussian noise is worked out. The detection performance of the afore-mentioned processing schemes has been evaluated in a number of operational cases of interest. Another novelty of the paper refers to the conception of detection schemes having adaptive features. In more detail, methods are suggested for the on-line estimation of the parameters of the two nonlinear estimators of the clutter. Results are presented concerning the detection loss against the number of range cells along which the average of parameters estimate is performed. Another adaptive feature explored refers to the on-line setting of a constant false-alarm rate (CFAR) detection threshold. Also in this case, an evaluation of the corresponding detection loss is made. To summarise, the paper represents a suitable generalisation of the detection theory of a coherent Gaussian target embedded in a CGC. This new theory proves its usefulness in advanced radar systems that should have high performance, notwithstanding challenging environments including spiky clutter and/or spiky target (e.g. stealth).

DOI:10.1049/ip-f-1.1987.0034

出版商:IEE    

年代:1987

数据来源: IET

摘要:

A statistical characterisation of clutter as a complex random process is needed in the design of optimum detection schemes. The paper considers modelling complex clutter as a spherically invariant random process (SIRP), namely assuming that its PDFs can be expressed as non-negative definite quadratic forms, a generalisation of a Gaussian process. Relevant properties of SIRPs are summarised, and shown to comply with basic requirements such as circular symmetry of the joint PDF of the in-quadrature components or, equivalently, the uniformity of the phase distribution. A constraint of admissibility must be imposed on the envelope distribution, but most commonly used envelope distributions, including Weibull, contaminated Rayleigh andK-distribution are shown to be admissible. Although a general SIRP is not ergodic, a characterisation of the clutter process as an SIRP scanned in the ensemble is finally proposed, which restores ergodicity. The interpretation of this model in the light of already proposed composite scattering models is also discussed.

DOI:10.1049/ip-f-1.1987.0035

出版商:IEE    

年代:1987

数据来源: IET