摘要:

Attenuation at 60 GHz is investigated on a 680 m vertically polarised link, with two fast-response raingauges and a dystrometer. Variations in attenuation for a given ground rainfall intensity are seen to be caused by drop-size variations. This conclusion is reached after conversion of point rainfall rates to path rainfall rates using a synthetic storm model, when the resultant variation of attenuation with path rainfall rate is seen to follow relationships predicted from the dystrometer. Nevertheless, on the average, the Laws/Parsons drop-size distribution is seen to be remarkably good for predicting attenuation from ground rainfall rates. Fading for one year was in excess of 15 dB for 10−3% of the time. By inference, radio paths of 2 to 3km should thus be possible at 60GHz in the UK.

DOI:10.1049/piee.1978.0025

出版商:IEE    

年代:1978

数据来源: IET

摘要:

Records of physical quantities often arise as continuous electrical signals. Spectral estimates may be formed either by analogue means or from digitised samples that are then processed on a computer, When the samples are provided at regularly spaced time instants, this can be achieved very quickly with the aid of the f.f.t (fast Fourier transform) algorithm. There are situations, however, where the data is known only at random time instants, and the paper is concerned with the computation of spectral estimates from such data. When the sample times are Poisson distributed, it has been shown, in previous papers, that unbiased alias-free estimates can be formed, either through the correlation function or by a direct Fourier transform of short blocks of data. Random sampling introduces additional variability in these spectral estimates, and it is consequently necessary to process a large amount of data in order to achieve stable results. Unfortunately, this is very time consuming, most of the computer effort being spent evaluating sine and cosine functions which are then multiplied by the data samples. Here, two methods that can be used to simplify this operation are discussed. It is shown that when the sine and cosine functions are replaced by their equivalent rectangular waveforms, the resulting estimates can be related to spectral estimates through the Fourier expansion for the rectangular waves. A second way of speeding up the processing of Gaussian signals can be achieved by quantising the data to a sign bit and using the ‘arc-sine’ rule to transform the autocorrelation function to that of the full signal. It is shown that when both techniques are used together, and the processing reduced to 1-bit logical operations, valid spectral estimates can indeed be formed. These ideas are tested on various simulated sets of data.

DOI:10.1049/piee.1978.0026

出版商:IEE    

年代:1978

数据来源: IET

摘要:

Minimum-distance decoding of convolutional codes has generally been considered impractical for other than relatively short constraint length codes, because of the exponential growth in complexity with increasing constraint length. The minimum-distance decoding algorithm proposed in the paper, however, uses a sequential decoding approach to avoid an exponential growth in complexity with increasing constraint length, and also utilises the distance and structural properties of convolutional codes to considerably reduce the amount of tree searching needed to find the minimum-distance path. In this way the algorithm achieves a complexity that does not grow exponentially with increasing constraint length, and is efficient for both long and short constraint length codes. The algorithm consists of two main processes. Firstly, a direct-mapping scheme, which automatically finds the minimum-distance path in a single mapping operation, is used to eliminate the need for all short back-up tree searches. Secondly, when a longer back-up search is required, an efficient tree-searching scheme is used to minimise the required search effort. The paper describes the complete algorithm and its theoretical basis, and examples of its operation are given.

DOI:10.1049/piee.1978.0027

出版商:IEE    

年代:1978

数据来源: IET

摘要:

An improvement on the Varshamov-Gilbert lower bound on the minimum Hamming distancedof linear block codes is proposed. The improved bound is based on the assumption that, for an (n, k) block code, the number of distinct vectors resulting from the linear combination of every (d−2) columns of the parity-check matrix is much less than the total number of vectors generated from such linear combinations. An expression for the largest possible number of distinct vectors obtainable for any (n, k) group code can therefore be introduced and shown to be a function of the weight distribution of the code.

DOI:10.1049/piee.1978.0028

出版商:IEE    

年代:1978

数据来源: IET

摘要:

The paper describes a means of control for fully controlled thyristor bridges which effects regeneration of energy to the a.c. supply source and still maintains the better power-factor and ripple characteristics usually associated with half-controlled rectifier arrangements. The technique is applicable to polyphase bridge circuits but the advantages are greatest in the handling of large single-phase powers such as occur in a.c.-fed traction systems. Although these advantages are applicable to d.c. motor control schemes the convertor bridge characteristics are naturally suited for use with current-source-type invertor-controlled a.c. motors. Such control techniques may thus prove vital in the consideration of a.c.-motor schemes for a.c.-supplied traction applications.

DOI:10.1049/piee.1978.0031

出版商:IEE    

年代:1978

数据来源: IET

摘要:

A drive system is described which uses two conventional thyristor convertors to provide a d.c. link between a.c. mains and the varying-frequency voltages of a synchronous-machine stator, both convertors being naturally commutated by line voltages; the result is a variable-speed drive. The paper describes a detailed analysis of machine behaviour in the absence of saturation, but taking account of winding resistances. This analysis is used to investigate the effects of assumptions made in deriving a simple analysis, which takes account of magnetic saturation. The validity of the simple analysis is demonstrated by comparison with results obtained from tests on a practical drive system.

DOI:10.1049/piee.1978.0032

出版商:IEE    

年代:1978

数据来源: IET

摘要:

In treating the phenomenon of the slew-region instability of the permanent magnet stepmotor the authors are aware of its strong relation to the problem of hunting in synchronous machines. The latter problem was treated by Rogowski and many others from 1915 onwards but the authors believe that their approach leads to a more detailed understanding of the phenomenon of spontaneous vibrations, as well as the fact that the results are obtained by way of simple and straightforward mathematics.

DOI:10.1049/piee.1978.0033

出版商:IEE    

年代:1978

数据来源: IET

摘要:

The paper indicates how the reconnection of the supply to a running induction motor can be effected in a manner that avoids excessive transient disturbances. An optimum condition is postulated for a matched reconnection and the paper indicates how this selected reclosure can be brought about. Typical results obtained by the sensed-switching technique are illustrated for a 3.75 kW (Shp) motor and are compared with results obtained during random switching of the same machine.

DOI:10.1049/piee.1978.0034

出版商:IEE    

年代:1978

数据来源: IET

摘要:

The complementary functions of the transient network analyser and the digital computer are examined and further consideration is given to the most appropriate spheres of application of the analyser. A description is given of the design of a new versatile solid-state analyser and its use in a specific investigation of overvoltages due to transmission-line energisation.

DOI:10.1049/piee.1978.0035

出版商:IEE    

年代:1978

数据来源: IET

摘要:

The paper provides a comparative study of three methods for transient security assessment of power systems: step-by-step simulation, Lyapunov's direct method and an empirical method based on the kinetic energy of the system. Different ways of formulating the Lyapunov algorithm are shown, and the basic concepts are summarised in a simple form. Particular emphasis is given to the application of the methods to relatively large practical problems with case studies and comparative results provided.

DOI:10.1049/piee.1978.0036

出版商:IEE    

年代:1978

数据来源: IET