摘要:

The compoundK-distribution model for sea clutter previously described in the literature provides the foundation for a quantitative treatment of the spatial and temporal correlation characteristics of the envelope of clutter returns. In the paper the compoundK-distribution model is extended to cover the spatial characteristics of sea clutter. The relationship of these characteristics to the physical basis of the model is described, and methods for their simulation are discussed. The effects of different radar range resolutions are analysed theoretically and these results compared with observations on real data. Finally, quantitative techniques are derived for optimising radar range resolution to give the best target detection performance in different clutter conditions, and some practical examples are presented.

DOI:10.1049/ip-f-1.1987.0090

出版商:IEE    

年代:1987

数据来源: IET

摘要:

Noiseless encoding strategies based on conditional entropy encoding are proposed for 8-bit μ-law PCM speech, with μ = 255. Specifically, we formulate the conditional probabilities of the current and previous PCM samples to generate the codebook employed in the noiseless encoding process. In addition, we consider a second-order conditional entropy encoding scheme which, when coupled with a state-reduction technique, reduces the size of the codebook. The entries in the codebooks are established with the aid of a training speech sequence, and different sentences are used to test the noiseless encoding performance. For first- and second-order conditional entropy encoding we obtain average transmission rates of 50kbit/s and 40kbit/s, respectively, compared to the original bit rate of 64 kbit/s. These reductions in bit rate are achieved without affecting the recovered speech quality.

DOI:10.1049/ip-f-1.1987.0092

出版商:IEE    

年代:1987

数据来源: IET

摘要:

The digital noise power due to transmission errors is determined for 8-bit μ-law PCM, μ = 255, signals transmitted viaM-level QAM,M= 16, 64 and 256, over Gaussian and Rayleigh fading channels. The analytical results apply for various laws that map the 8-bit μ-law PCM signal into the QAM format, and for different binary Gray codes used to represent the QAM signal points. In addition to the theoretical results expressed as overall speech signal/noise ratio (SNR) as a function of channel SNR for different input speech signal levels, simulations for Gaussian and mobile radio channels using speech signals are also presented. From our deliberations we can conclude that the 256-level QAM system employing the best mapping law and Gray code has a gain of about 6dB in overall speech SNR compared to transmission without co-ordination of the speech bits over a Gray-coded link. Corresponding gains for 16-level and 64-level QAM are marginally smaller. By increasing the number of QAM levels from 4 to 256, the required increase in channel SNR per bit to maintain an overall speech SNR of 30 dB is approximately 12 dB. The bandwidth efficiency increases by a factor of four. The theoretical analysis in the paper is approximate. Only the effect of the most likely channel errors (single errors) has been taken into account. The theoretical results coincides well with the simulations, where such approximations were made.

DOI:10.1049/ip-f-1.1987.0093

出版商:IEE    

年代:1987

数据来源: IET

摘要:

The transmission of 8-bit μ-law PCM signals usingM-level weighted QAM (WQAM),M=16, 64 and 256, over Gaussian and Rayleigh fading channels is examined. The weighting process modifies the positions of the QAM constellation points so that the overall distortion in the recovered information-bearing source signal is reduced. The PCM bits are mapped to the WQAM points such that the most significant bits have a lower probability of being in error than the least significant bits. Gray coding of the constellation points is also used. The WQAM systems have been optimised for the same average signal energy per transmitted symbol as for unweighted QAM. Optimised systems have also been derived for the same peak signal energy per symbol. The theoretical and simulation results using speech indicate that 16-level and 256-level WQAM have a gain of up to 3 and 5dB, respectively, over unweighted QAM for the Gaussian channel. The gains due to weighting for 16-level QAM operating over an ideal slow Rayleigh fading channel are up to 5 dB.

DOI:10.1049/ip-f-1.1987.0094

出版商:IEE    

年代:1987

数据来源: IET

摘要:

The paper presents an approach by which absolute error bounds may be derived for the continuous channel, thus determining the limits of the performance of a modulation and coding scheme. The code is considered as a set of points inn-dimensional space, and the concept of a sphere-packing inndimensions based on the regular simplex is used to determine the maximum packing density. The effect of code points on the edges of the signal space is also considered. Upper and lower bounds on the error rate are derived for various code rates and various block lengths (numbers of dimensions). The results are of relevance to combined coding/modulation schemes, andn-dimensional coding schemes, and show that a coding advantage of approximately 2dB is obtained for every doubling of the number of dimensions.

DOI:10.1049/ip-f-1.1987.0095

出版商:IEE    

年代:1987

数据来源: IET

摘要:

A new method for predicting attenuation on satellite-earth links is applied throughout the European region. Sources of long-term point rainfall data required for the method are identified, and the approach to compiling a rainfall intensity database is described (including conversion techniques for integration time, which are developed and tested). A technique for predicting effective rain height from available meteorological data, which takes into account seasonal and climatic dependencies, is also described and evaluated. Finally, the method is applied to the prediction of attenuation at arbitrary sites in the European region and to the production of contour maps of attenuation for specific satellites. The values predicted at 33 European sites with available attenuation data from the COST 205 programme show excellent agreement, and give lower errors than other methods.

DOI:10.1049/ip-f-1.1987.0096

出版商:IEE    

年代:1987

数据来源: IET

摘要:

Three-subaperture methods for estimating the elevation angles of two targets within the 3 dB beamwidth for the main aperture of a linear array are discussed and applied to the case of a radar target in the presence of multipath over a smooth surface. Simulation results and comparison with the three-subaperture maximum-likelihood estimator by Cantrellet al. are presented. A four-subaperture technique to solve the in-phase and antiphase signal cases is suggested.

DOI:10.1049/ip-f-1.1987.0097

出版商:IEE    

年代:1987

数据来源: IET

摘要:

The design of asymptotically optimum detectors (AODs) is considered for a class of fluctuating targets in non-Gaussian interference, as a reasonable alternative to optimum (Neyman-Pearson) detectors, which prove hopelessly complex. The useful signal is modelled as an incoherent train with constant amplitude or with fluctuating amplitude, either from scan to scan or from pulse to pulse. The AOD structure is based on an asymptotical expression of the log-likelihood ratio, which has been derived by a suitable modification of the classical AOD approach; it results in a conventional detector (CD) but for the replacement of the quadratic envelope detection with another zero-memory nonlinearity, depending only on the noise statistics. The invariance properties of the CD with respect to the signal fluctuation model are shown to be preserved in the more general AOD class. As the test statistics are asymptotically normal, the asymptotical AOD operating characteristics are expressed in terms of asymptotical relative efficiency (ARE), thus allowing direct comparison with the CD under the same interference. The ARE for log-normal interference is calculated, showing a potential improvement on the order of at least 20 dB.

DOI:10.1049/ip-f-1.1987.0098

出版商:IEE    

年代:1987

数据来源: IET

摘要:

The situation of shipboard DF antennas is briefly analysed. It is shown that DF errors are worst within the HF range, and then especially to port and starboard. It is shown that this region can be covered by a parallel loop arrangement which must be kept ‘blind’ to the main reradiating structures in addition to the conventional crossed loop DF. Theoretical considerations prove that all structures in a ship's symmetry plane and an idealised ship's hull no longer create bearing errors. Unsymmetrically placed components, which continue to affect the parallel loop, turn out to produce merely a very smooth error curve, which can easily be compensated by computer-stored correction tables during the evaluation process. Polarisation errors, which appear when direction-finding with a crossed loop, are shown to be at a much lower level when using a parallel loop.

DOI:10.1049/ip-f-1.1987.0100

出版商:IEE    

年代:1987

数据来源: IET

摘要:

A centroiding algorithm for the refinement of coarse noisy measurements (e.g. spectral or range samples) is proposed and analysed. In contrast to the current algorithms which use constant centroiding weights, in our method the coefficients are chosen usinga prioriinformation about the noise level. The criterion for selection of the optimal weights is the minimisation of the estimate error for the given signal/noise ratio. The numerical results presented demonstrate that the method provides a more effective refinement of the coarse measurements than existing methods.

DOI:10.1049/ip-f-1.1987.0102

出版商:IEE    

年代:1987

数据来源: IET