摘要:

A method is presented to integrate high-resolution line graphics in the UK Teletext system. Differential Chain Coding (DCC) enables an attractive combination of efficient graphics representation and a low-cost consumer decoder. Separate graphics overlay pages are linked to the text page by means of a ‘ghost row’, a concealed row of the text page. This separation of text and graphics guarantees compatibility with the existing teletext format. One major problem of DCC is its sensitivity to errors. Two fundamentally different approaches have been considered to solve this problem: error-correcting codes and protective measures to decrease the impact of errors. Experimental results indicate the feasibility of including DCC graphics in public Teletext systems, for bit error rates up to 5 × 10-3. For the higher BERs, repeated reception of the cyclic transmissions of teletext pages appears to be essential. This leads to better results than most error-correcting codes, and the decoder can remain simple and inexpensive.

DOI:10.1049/ip-i-2.1990.0045

出版商:IEE    

年代:1990

数据来源: IET

摘要:

We propose a systematic trellis encoding technique based on trellis partitioning and on newly defined branch distances. This encoding technique avoids the usual exhaustive search for finding good trellis codes. It gives the lower and upper bounds on free distance which depend only on the encoder memory and the signal space. We also discuss the optimal combination of signal set and trellis and give the sufficient condition for avoiding catastrophic codes.

DOI:10.1049/ip-i-2.1990.0046

出版商:IEE    

年代:1990

数据来源: IET

摘要:

The spectral characteristics of minimum-phase signals and their detection process are outlined in this paper. Investigation of the resulting envelope distortion, as a consequence of reducing the channel bandwidth of minimum-phase signals to one half, is the prime objective of the study reported here. This bandwidth reduction is important in applications such as mobile radio services, where the available bandwidth is tightly controlled and compatibility with simple envelope detection will be advantageous. Some interesting results of computer simulation of the reduced bandwidth scheme, and its performance under different fading channel conditions are presented. A novel correction method at the receiver, which gives distortionless envelope demodulation, is described.

DOI:10.1049/ip-i-2.1990.0047

出版商:IEE    

年代:1990

数据来源: IET

摘要:

The paper presents four schemes for the adaptive adjustment of a linear feedforward filter placed ahead of a conventional digital data detector. The filter is broadly adjusted according to two criteria, those of maximisation of the signal-tonoise ratio at the input of the detector where perfect equalisation of the detector input signal results in zero intersymbol interference, or minimising the mean-square error present in the equalised signal at the detector input. The paper presents the results of extensive computer simultions performed upon models of the four schemes when operating over four different voice-frequency bandpass channels. The results are arranged to show the relative accuracies and complexities (in terms of the number of arithmetic operations required to implement the algorithms) of the tested systems. Furthermore, the results have been arranged to show the behaviour of the four systems in the presence of additive white Gaussian noise and various amounts of distortion introduced by the channels, for different lengths of the linear pre-filter.

DOI:10.1049/ip-i-2.1990.0048

出版商:IEE    

年代:1990

数据来源: IET

摘要:

This paper describes and analyses a new priority CSMA/CD protocol for multichannel local area networks. In this scheme, low priority messages can be transmitted by using the CSMA/CD protocol only when the number of idle channels exceeds a certain value. Following a general description of the proposed priority protocol, this paper analyses the protocol by first formulating the various state transition probabilities of the network. The delay and throughput characteristics of the various priority messages are then calculated and presented. It is found that the performance of this simple scheme is comparable to the priority schemes studies in Reference 8 for CSMA/CD networks. Also, in order not to starve off low priority stations, the largest number of idle channels when the transmission of low priority packets becomes inhibited should not be more than about one third of the total number of channels.

DOI:10.1049/ip-i-2.1990.0049

出版商:IEE    

年代:1990

数据来源: IET

摘要:

The paper describes the implementation of a robust 75 baud adaptive frequency hopping (FH) model, and its evaluation on an 800 km HF link in which its performance is compared with fixed frequency signal formats. The aim has been to achieve effective data transmission, in very severe channel conditions, comprising interference from other users, noise, and frequency selective fading. The modem uses sixth order frequency diversity, with automatic excision of interference at the receiver, but without the use of a control link from the receiver to the transmitter. The open loop system approach has been used to avoid the difficulty of establishing the control link under difficult transmission conditions. The error structure resulting from the FH modem is such that forward error correcting codes are particularly effective, and their application is investigated extensively. Also, an interleaving technique which is especially relevant to the FH signal is described and used to advantage.

DOI:10.1049/ip-i-2.1990.0050

出版商:IEE    

年代:1990

数据来源: IET

摘要:

Classified vector quantisation (CVQ) of images is a vector quantisation-based coding method for preserving perceptual features while retaining simple vector quantiser distortion measures during codebook design and encoding process. In the paper, a new algorithm for CVQ codebook design the ‘classified nearest neighbour clustering’ (CNNC) algorith, is presented. The CNNC algorithm is based on a classification process of small image blocks and on an agglomerative clustering algorithm, and is used to design simultaneously M codebooks for M different classes, defined for a CVQ system. The CNNC algorithm can be used with squared error and weighted squared error distortion measures employing one of two optimisation criteria which are presented and tested in the paper. In addition, fast search algorithm is presented aimed at reducing computational efforts encountered during codebook design. The CNNC algorithm is shown to provide a systematic and effective method for CVQ codebook design making CVQ more feasible and easy to implement.

DOI:10.1049/ip-i-2.1990.0051

出版商:IEE    

年代:1990

数据来源: IET

摘要:

The paper considers a novel technique for the measurement and correction of the Doppler shift (frequency offset) in a received QPSK signal, where the Doppler shift may be very high, the signal/noise ratio very low, and the receiver has no prior knowledge of the received data symbols. The technique is designed for synchronous data and is to operate prior to the instigation of a phase locking algorithm necessary for coherent demodulation. It reduces the Doppler shift to a value within the capture range of the phase correction loop, and does not operate during the phase acquisition period nor during data detection. The residual Doppler shift in the coherently demodulated baseband signal at the receiver is measured by correlating this signal with two components (complex exponentials) of equal frequency but one positive and the other negative, where the frequency is an appropriate fraction (typically around one half) of the highest frequency component of the baseband signal. Depending on which of the two correlators gives the greater output signal, the frequency of the reference carriers in the coherent demodulator is increased or decreased to reduce the residual Doppler shift in the demodulated baseband signal. The paper first describes various arrangements of the new Doppler-shift corrector, together with the original DFT Doppler-shift corrector, from which these have been derived, and it then presents the results of computer-simulation tests to assess the performances of the different systems.

DOI:10.1049/ip-i-2.1990.0052

出版商:IEE    

年代:1990

数据来源: IET

摘要:

A simple four channel receiver for 2- FSK/8-PSK modulated signals is proposed and its performance evaluated. Using Gray-Code mapping and direct detection of individual bits, the overall receiver performance in terms of bit error rate (BER), is evaluated as a function ofEb/N0and a variable phase parameter α, under an assumed additive white Gaussian noise interference model. The variable phase parameter can be adjusted according to the intended application, as three out of four bits that constitute a symbol have individual error rates that depend on the value of α as well asEb/N0. Angle settings for the minimisation of the overall BER can be obtained for different values ofEb/N0. This represents a slight system disadvantage, but it is partly offset by the simplicity with which the receiver is able to deliver deinterleaved data to the intended users, by performing summing and squaring operations on the correlator outputs prior to binary comparisons against zero thresholds. The performance of this scheme can be shown to be superior to equivalent schemes such as 16-PSK and 16-QAM with a small loss in bandwidth efficiency, yet inferior to orthogonal signalling schemes such as 16-FSK, albeit without the corresponding bandwidth utilisation penalties of this latter scheme.

DOI:10.1049/ip-i-2.1990.0053

出版商:IEE    

年代:1990

数据来源: IET

摘要:

Onboard digital transmultiplexers and regenerative transponders in satellite systems employing multiple spot-beam antennas will play an important role in realising future cost-effective communications networks for use in developing countries and where isolated user terminals require access to the global telecommunications network. In the paper, a new fast algorithm for the realisation of a complex input/complex output fast transform based digital transmultiplexer (TMUX) is presented with application to a quadrature multicarrier modulation and demodulation system. The novel approach adopts a splitting of the transform processor into two conjugate operations, and a decomposition of the phase offsets is performed leading to a saving in the number of arithmetic operations required. A reduction in the amount of control circuitry required for implementation is also facilitated. An experimental four-channel QPSK modulator based upon the new technique has been implemented for hardware simulation purposes using a TMS320C25 DSP based system, and performance evaluation results are presented where 16 bit integer DSP processing is employed.

DOI:10.1049/ip-i-2.1990.0054

出版商:IEE    

年代:1990

数据来源: IET