摘要:

The participation of five European countries in preliminary experiments with methods of television subjective testing, intended for application to a 2 Mbit/s codec, has provided the first opportunity for comparing results obtained by different organisations using a single-stimulus full-range quality-rating method under fairly well standardised conditions. The range in standards of subjective judgment was about 2 dB, as expressed in terms of the equivalent level of wideband random noise. This small range, coupled with simplicity, encourages the continued use of quality grading by the single-stimulus method for television subjective studies involving a substantial range of subjective magnitudes.

DOI:10.1049/ip-f-1.1983.0048

出版商:IEE    

年代:1983

数据来源: IET

摘要:

The equations of Milsom are evaluated, giving the ground range and group delay of radio waves propagated via the horizontally stratified model ionosphere proposed by Bradley and Dudeney. Expressions for the ground range which allow for the effects of the underlying E- and F1-regions are used to evaluate the basic maximum usable frequency orM-factors for single F-layer hops. An algorithm for the rapid calculation of theM-factor at a given range is developed, and shown to be accurate to within 5%. The results reveal that theM(3000)F2-factor scaled from vertical-incidence ionograms using the standard URSI procedure can be up to 7.5% in error. A simple addition to the algorithm effects a correction to ionogram values to make these accurate to 0.5%.

DOI:10.1049/ip-f-1.1983.0049

出版商:IEE    

年代:1983

数据来源: IET

摘要:

The paper describes a new adaptive canceller for estimating the elevation angle in a low-angle tracking radar environment in the presence of specular and diffuse multipath. The adaptation is achieved using the complex least-mean-squares (LMS) algorithm. Computer-simulation results are included to demonstrate the usefulness of this scheme for estimating the elevation angle when it is a small fraction of a standard beamwidth.

DOI:10.1049/ip-f-1.1983.0050

出版商:IEE    

年代:1983

数据来源: IET

摘要:

It is becoming increasingly popular, when considering transform coding schemes for images, to attempt to take the spatial frequency response of the human observer into account by performing a classical one- or two-dimensional filtering (pre-emphasis) operation upon the coefficients of the Fourier transformed image, and it would be advantageous if such a procedure could be carried out with more commonly used image transforms, notably the discrete cosine and Walsh-Hadamard transforms. It is demonstrated here that, notwithstanding the theoretical difficulties associated with the convolution/multiplication operation where the discrete cosine transform is concerned, its spectral response and the nature of the appropriate filtering characteristic are such that an operation of the above mentioned kind may be carried out, and the benefits of psychovisual coding obtained. On the other hand, the results obtained in the case of the Walsh-Hadamard transform show that it is unlikely that its performance will be found satisfactory in such an application.

DOI:10.1049/ip-f-1.1983.0051

出版商:IEE    

年代:1983

数据来源: IET

摘要:

The paper illustrates a computer modelling method for the generation of a highly correlated pseudorandom process with a desired power density spectrum. The samples of the pseudorandom process are generated as the output of a cascade connection of a correlation shaping transversal filter and a reconstruction filter with statistically independent samples fed to the input. Such a generation method needs a much smaller number of mathematical operations compared with the traditional transversal filtering method, and does not require any special or careful filter design for each process parameter, as is necessary in the application of an IIR filter.

DOI:10.1049/ip-f-1.1983.0052

出版商:IEE    

年代:1983

数据来源: IET

摘要:

In the paper we investigate one aspect of the validity of models currently used to calculate rainscatter interference in radio and satellite communication systems. Exact bistatic scattering cross-sections from spheroidal shaped raindrops are compared with those obtained from spheroids in the Rayleigh approximation and from spheres using the Mie theory at 11.7, 20 and 30 GHz. Comparisons are also made between bistatic radar reflectivities for rain populations at these frequencies. It is found that both the Rayleigh and Mie (sphere) approximations to the spheroidal raindrop yield errors in the bistatic scattering reflectivities. These errors can be significant in the case of the Rayleigh theory, but the Mie theory, in general, yields much closer results to the exact.

DOI:10.1049/ip-f-1.1983.0053

出版商:IEE    

年代:1983

数据来源: IET

摘要:

Using the concept of net coding gain (NCG), we measure the benefits of using a class of rate 1/2 codes in error control systems. A bounded distance decoder for group codes is assumed. The NCG can be compared with other expedients such as increasing transmitted signal power. Although the codes are used in conjunction with phase and frequency modulation systems, the technique is valid for any binary modulation system to which the binomial distribution of errors applies. Using the weight distribution of the codes considered, the net coding gain is computed for (8, 4), (16, 8), (24, 12), (32, 16), (40, 20), (48, 24), (56, 28), (64, 32) and (72, 36) codes. These are self-dual codes with weights divisible by four. All codes except the (72, 36) are known to exist. It is shown that significant coding gains can be obtained with these codes. Decoding algorithms have been described previously in the literature.

DOI:10.1049/ip-f-1.1983.0054

出版商:IEE    

年代:1983

数据来源: IET

摘要:

A two-stage four-level conditional detector is described. The detector is a constant-false-alarm-rate detector. Its performance is compared with that of a single-stage (fixed-sample-size) four-level conditional detector. In a finite-sample comparison the noise is assumed to have a Rician probability density function, which is the probability density function of the output of an envelope detector in a radar receiver when the background noise is white Gaussian noise. In an asymptotic comparison the asymptotic relative efficiency (ARE) is defined and expressions are given for calculating the ARE. Both finite and asymptotic results show significant reduction in the average sample size in the no-signal case. Therefore the proposed two-stage detector is suitable for signal detection in a situation where the no-signal hypothesis has a higha prioriprobability.

DOI:10.1049/ip-f-1.1983.0055

出版商:IEE    

年代:1983

数据来源: IET

摘要:

Catastrophe theory is used to analyse the behaviour of a slotted ALOHA data communication system with the aim of demonstrating the power of catastrophe theory for the study of communication systems in general. A brief introduction to the required aspects of catastrophe theory is given, followed by detailed numerical and qualitative results of their application to slotted ALOHA.

DOI:10.1049/ip-f-1.1983.0056

出版商:IEE    

年代:1983

数据来源: IET

摘要:

The paper deals with a satellite channel reservation system with variable frame format. The operating scheme of the system is as follows. The current queue length at each user is reported through minislots in the up-link header to a satellite having processing capability. The satellite broadcasts slot assignments for transmission in the next frame to users through the down-link header, based on the users' reported queue status. The number of data slots per frame varies, depending on the users' reported queue status. A Markov-chain model is presented to analyse the average total delay of the system. Numerical results are given to compare the delay of the system with the delay of TDMA and reservation systems with fixed frame format. It is shown that the system provides a better delay/throughput performance than these other systems over a wide range of traffic intensities. Moreover, the delay curve of the system is relatively flat up to a traffic intensity of about 0.8.

DOI:10.1049/ip-f-1.1983.0057

出版商:IEE    

年代:1983

数据来源: IET