摘要:

The weighted median filter (WMF) is a generalisation of the median filter. The WMF has been described elsewhere, where it was shown that a given WMF class operates in a finite number of ways. An expression for the number of modes of operation of any WMF class, in a given group of classes, has recently been derived. Here, an algorithm is described for constructing a complete set of representative members of any class in such a group. This algorithm, expressed as a Pascal program, is used to generate results for two useful classes of WMFs.

DOI:10.1049/ip-f-1.1986.0069

出版商:IEE    

年代:1986

数据来源: IET

摘要:

A receiver architecture is described that is ideally suited to UK paging applications. The receiver is similar to a direct conversion receiver, in that the absence of an ‘image’ response allows integration, but it only requires a single front-end mixer and so consumes less power. To achieve this the local oscillator frequency is offset slightly from the incoming carrier frequency, allowing the modulation to be recovered by a straightforward discriminator. Extensive measurements show that a bit error rate of 0.01 can be obtained with a 12 dB IF S/N ratio. Noise-generated DC is identified as having a major impact on the receiver performance, and is shown to be determined by the IF noise spectrum. The degradation of sensitivity caused by various tolerances is considered, and the best nominal parameter values selected.

DOI:10.1049/ip-f-1.1986.0070

出版商:IEE    

年代:1986

数据来源: IET

摘要:

Multipath propagation, and in particular the interference between the ordinary and the extraordinary waves, places a fundamental constraint on the performance of wideband HF skywave radio links. Furthermore, the dispersive nature of ionospheric propagation causes phase nonlinearity and hence distortion of narrow pulses. In the paper an FMCW wideband sounder built for the purposes of characterising the channel is described. Spectral analysis of the audio output of the sounder via the FFT algorithm is shown to permit measurement of the amplitude/frequency function, the polarisation bandwidth, the fade rate, the fade depth and the distortion of a narrow pulse, all for a desired isolated ionospheric propagation mode. The sounder was used to collect data over an oblique path in the UK. The results of applying the FFT processing technique to the experimental data are presented.

DOI:10.1049/ip-f-1.1986.0071

出版商:IEE    

年代:1986

数据来源: IET

摘要:

Receivers for the Decca Navigator system have recently been developed in which a single, frequency-agile receiving channel, operating under microprocessor control, replaces the five single-frequency channels of a conventional receiver. The paper describes techniques which allow such receivers to utilise the multipulse lane-identification and the zone-identification transmissions of the Decca chain which were designed for receivers with multiple channels. An algorithm is described by means of which the microprocessor calculates zones and zone fractions from samples of the phases of the transmissions received. Operational results, measured under conditions of night-time skywave propagation, are presented which demonstrate that the technique works successfully and that it has certain advantages over conventional methods of multipulse and zone-identification reception.

DOI:10.1049/ip-f-1.1986.0072

出版商:IEE    

年代:1986

数据来源: IET

摘要:

A digital modem is proposed which uses a frequency-hopping sixth-order frequency diversity signal, and which incorporates the automatic excision of interference from other HF users. The modem has the property of achieving low error rates at low data rates (75 bits/s), in the presence of severe interference from other HF users, frequency-selective fading and noise. The interference is defined by simple empirical parameters, which are used to determine the theoretical performance of the modem in a combination of the above channel disturbances. An example of experimental results is also presented.

DOI:10.1049/ip-f-1.1986.0073

出版商:IEE    

年代:1986

数据来源: IET

摘要:

The paper considers the stochastic modelling of radar returns. In particular, returns from a typical airport surveillance radar (ASR) system have been modelled as autoregressive-moving average (ARMA) processes. Both maximum-likelihood (ML)- and autocorrelation-based techniques have been used. Order selection algorithms were studied and modified to optimise their performance for short-data records necessitated by the nonstationary radar environment. Distinctively different models have been found for typical combinations of ground, weather and aircraft returns.

DOI:10.1049/ip-f-1.1986.0075

出版商:IEE    

年代:1986

数据来源: IET

摘要:

In the paper results on the use of vector quantisation for interframe image coding for compression are presented. The initial stage in vector quantisation is the formation/extraction of vectors from the data source. Two vector formation schemes, generalized from intraframe image coding, are proposed and evaluated on a test image sequence. The first, direct three-dimensional block vector quantisation, applies vector quantisation to small 3-dimensional blocks of pixels extracted from sequences of image. In the second, the 3-dimensional Hadamard transform is first calculated on a block basis and is followed by vector quantisation of a subset of the Hadamard coefficients. Significantly improved results are obtained in the second case, as the components in the vectors are decorrelated in both the spatial and temporal directions. Three training sets are used for the simulations, and results suggest that adaptively updating the codebook could result in improved performance.

DOI:10.1049/ip-f-1.1986.0077

出版商:IEE    

年代:1986

数据来源: IET

摘要:

The paper presents a maximum-likelihood consistent bounded distance decoding algorithm for convolutional codes. The algorithm correctly decodes all error sequences which fall within the error-correcting sphere. A class of codes is defined, in which the decoder exploits the fact that only certain error sequences need to be corrected. For these codes the decoding is based on a reduced encoder state diagram. Thus, only a subset of the trellis or tree has to be searched in order to find the error pattern. An exact characteristication of the reduced state diagram is given in the paper along with an example.

DOI:10.1049/ip-f-1.1986.0079

出版商:IEE    

年代:1986

数据来源: IET

摘要:

A theoretical model for the quantitative analysis of sequential acquisition by a tracking radar is presented. The relation between the acquisition performance and radar and designation system parameters is established. A method to optimise the acquisition process is suggested and some numerical examples are given.

DOI:10.1049/ip-f-1.1986.0080

出版商:IEE    

年代:1986

数据来源: IET