ISSN:1124-318X    

DOI:10.1002/ett.4460020302

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1991

数据来源: WILEY

摘要:

AbstractThe asynchronous transfer mode (ATM) technique will probably increase the demand for sophisticated video services. The availability of wide bandwidth and fast rerouting at affordable rates will influence the design of new video codecs. Firstly, variable bit rate coding algorithms can efficiently be employed in order to achieve a uniform picture quality at the receiver. Secondly, the impairment due to cell loss that may occur in the ATM environment demands specialized error concealment strategy. In this paper the segmentation and reassembling of the bit stream generated by a hybrid DCT broadcast quality video codec is addressed. A VIDEO PACKET (V.P.), composed of a variable number of ATM cells, is associated to a defined portion of the television signal. Predefined patterns and layered protection included in the V.P. allow the detection of lost cells and consequently the application of a simple error concealment technique.

ISSN:1124-318X    

DOI:10.1002/ett.4460020303

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1991

数据来源: WILEY

摘要:

AbstractVideocoding algorithms possess a high degree of computation concurrency that can be exploited by parallel architectures. This paper presents the status of an applied research aimed at the implementation of a Transputer based videocoder for professional workstations. Basic algorithms are reviewed in terms of computation requirements and the ones specifically developed for ATM systems up to 2Mbit/s are quantitatively analyzed. Architectural tradeoffs are presented so that the demonstrator architecture can be defined in its hardware and software aspects. A video transputer module is defined and designed down to the logical level. Different partitioning of the algorithm execution is possible using a newly proposed «Farmer» Operating Syste

ISSN:1124-318X    

DOI:10.1002/ett.4460020304

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1991

数据来源: WILEY

摘要:

AbstractStarting from the principle that the distorsion measure, representing the cost payed by the users for lossy reproducing an image, takes its maxima in correspondence of local minima of Shannon's Entropy, a space variant resolution coding method matched to the local content of the image is presented. A fidelity criterion based on the estimated kurtosis function of the image is then applied to drive the quantization phase of an ADCT coder. Finally, an enhanced version of the ISO/CCITT hierarchical progressive transmission mode is proposed and its perfomance evaluated on the basis of reference images.

ISSN:1124-318X    

DOI:10.1002/ett.4460020305

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1991

数据来源: WILEY

摘要:

AbstractMany approaches have been used to the purpose of image data compression. Each of them combines various techniques to exploit the redundancy of the image signal.In this paper two different approaches will be presented, analysed and compared. They have been developed by the CNR‐CSTV and the RAI Research Centre(1) respectively, in the framework of subproject 3.2 of the CNR «Telecomunicazioni» project. The goal is to define a coding scheme suitable for the bit‐rate compression of digital HDTV signals in order to cope with the characteristics of two kinds of channels: a synchronous channel at 70 Mbit/s and an asynchronous one with peak bit‐rate of 140 Mbit/s. The coded images must have a good quality for distribution networks.Both algorithms are based on the hybrid transform image coding using the 2D‐DCT to reduce the spatial correlation. In order to further decrease die temporal correlation a motion compensation technique is also applied. The aim of this paper is to describe the two algorithms and to investigate the main di

ISSN:1124-318X    

DOI:10.1002/ett.4460020306

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1991

数据来源: WILEY

摘要:

AbstractThe capacity of packet radio channels is investigated using the interference model for the coherent and incoherent cumulations in the presence of Rayleigh fading and log‐normal shadowing, assuming a nonpersistent multiple access protocol with inhibit bits to avoid collisions. Expressions for the throughput are derived for uncorrelated packet signals. The combined propagation effects yield higher channel capacity than each individual effec

ISSN:1124-318X    

DOI:10.1002/ett.4460020307

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1991

数据来源: WILEY

摘要:

AbstractDiversity techniques are widely used when the communication channel is affected.by heavy noise and disturbances. In the diversity techniques the same information is transmitted overm>1 channels. At the receiver side, the signals are combined together in order to reduce the bit error probability. In this paper a new diversity technique, using a suitable channel coding algorithm, is described. In this technique the information transmitted differs from one channel to the other one. The performance of this technique is obtained through a theoretical analysis. The application both to forward error correcting codes and automatic repeat request systems is described. The proposed technique is applied also to automatic repeat request schemes without diversity. The proposed technique permits to achieve in many cases higher performance than the classical diversity schemes.

ISSN:1124-318X    

DOI:10.1002/ett.4460020308

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1991

数据来源: WILEY

摘要:

AbstractA central topic in active microwave remote sensing is the measurement of polarimet‐ric scattering matrix time series. At least two successive measurements, employing orthogonal transmitter polarisations, are necessary to determine the full S‐matrix of an object. In many remote sensing applications, however, the observed targets experience small but non‐negligible variations even in the small time interval which is required to perform the S‐matrix measurement. In order to cope with this problem, a Doppler phase correction scheme can be applied which compensates for the deterministic mean target motion. The possible internal variation of a composite target due to the random motion of the target's individual scattering centres of particles, however, results in decorrelation of the consecutively measured S‐matrix columns and leaves significant statistical errors uncorrected. In the present paper the mean Doppler phase correction method and the related estimation errors are studied in greater detail, employing a quite general model of time‐variant random targets. It turns out that the critical quantity which determines the statistical accuracy of the Doppler correction is given by the ratio of the pulse repetition interval and the decorrelation time of the observed random target. The pulse repetition frequency of the radar measurement, hence, has to be chosen carefully in order to avoid large estimation errors in the S‐matrix determination. Matching the pulse repetition frequency properly to the decorrelation time of the observed target, on the other hand, allows the measurement and processing of polarimetric S‐matrix series with maximum information content on the target. The present statistical studies, this way, are of fundamental interest in many polarimetric remote sensi

ISSN:1124-318X    

DOI:10.1002/ett.4460020309

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1991

数据来源: WILEY

摘要:

AbstractThis work presents the results of an analysis regarding propagation in single‐mode fibers conducted by means of finite element method. A particular form of the functional is taken, in order to avoid the presence of nonphysical spurious solutions. Special attention is then given to the choice of the mesh when profile geometry is not circularly simmetric, like in the case of Fibers able to preserve the state of polarization or nominally single‐polarization. This situation has been carefully analysed, showing that particular new kinds of index profiles, such as elliptical azimuthally modulated W profiles, are able to produce extended intervals of single‐polarization operation, to the point where their practical use appears fea

ISSN:1124-318X    

DOI:10.1002/ett.4460020310

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1991

数据来源: WILEY

摘要:

AbstractDigital computer simulation is a powerful tool for the design, the mission planning and the image quality analysis of advanced SAR Systems.«End‐to‐end» simulators describe the whole process of the SAR imaging including the generation of the coherent echoes and their processing and allow, unlike the «product simulators», to evaluate the effects of the various impairments on the final image.The main disadvantage of the «end‐to‐end» approach, as described in this paper, is the heavy computation burden; therefore, a new type of simulator is presented, attempting to reduce the burden but presenting a greater degree of completeness and realism than the SAR product simulators, already existing.An «end‐to‐end» simulator and an «intermediate» simulator are described and analyzed, with emphasis on their archite

ISSN:1124-318X    

DOI:10.1002/ett.4460020311

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1991

数据来源: WILEY