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Volume 138
Issue 1
IEE Proceedings I (Communications, Speech and Vision)
Volume 138, Issue 1, February 1991
Volumes & issues:
Volume 138, Issue 1
February 1991
Performance of direct-sequence spread-spectrum multiple-access systems in mobile radio
- Author(s): W.H. Lam and R. Steele
- Source: IEE Proceedings I (Communications, Speech and Vision), Volume 138, Issue 1, p. 1 –14
- DOI: 10.1049/ip-i-2.1991.0001
- Type: Article
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The performance of coherent directsequence spread-spectrum multiple-access (DS/SSMA) communications over different mobile radio channels is investigated. Analytical techniques and numerical methods were used for a channel having a number of resolvable, discrete Rayleigh fading paths, while simulations were employed for transmissions over urban mobile radio channels. We found that even a flat Rayleigh fading channel caused a severe degradation in system performance compared with the additive white Gaussian noise channel, and that the system was unusable even for a few users when a wideband urban mobile radio channel was used. By transmitting a sounding sequence of chip length Ns = 511 to allow the receiver to estimate the channel impulse response, spreading the symbol over N = 127 chips, and on deploying fifth-order combinative space diversity equipped with adaptive correlation diversity of Kβ = 1.0 (CD/ACD), we were able to obtain a residual probability of bit error PR of less than 10−3 for 10 users, in spite of the urban mobile radio channel having 19 resolvable fading paths. By increasing N to 511 and deploying fifth-order maximal-ratio-combining (MRC) space diversity, a PR of 4 × 10−5 for 20 users was achieved when the radio channel consisted of 10 resolvable independent Rayleigh fading paths.
Performance evaluation of a composite microscopic plus macroscopic diversity system
- Author(s): A.M.D. Turkmani
- Source: IEE Proceedings I (Communications, Speech and Vision), Volume 138, Issue 1, p. 15 –20
- DOI: 10.1049/ip-i-2.1991.0002
- Type: Article
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Closed-form analytical solutions are obtained for the performance of M-branch maximal-ratio and selection microscopic diversity systems with a class of continuous phase and constant amplitude modulation schemes for Rayleigh and log-normal fading. The results indicate that diversity improvement is related to the standard deviation of the log-normal component and the improvement decreases as the standard deviation increases. Since microscopic diversity only mitigates the effect of fast (Rayleigh) fading, other means are needed to mitigate the effect of the slow (log-normal) component. Further development of the theory indicates that a composite microscopic plus macroscopic diversity system can substantially improve the performance by counteracting the two fading components simultaneously. Analytical results are produced for two-branch microscopic selection or maximal-ratio plus N-branch macroscopic selection diversity systems.
Application of multiple path sequential decoding for intersymbol interference reduction problem
- Author(s): D. Haccoun and S. Kallel
- Source: IEE Proceedings I (Communications, Speech and Vision), Volume 138, Issue 1, p. 21 –31
- DOI: 10.1049/ip-i-2.1991.0003
- Type: Article
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The application of the stack algorithm of sequential decoding and some of its multiplepath variants to the ISI problem, especially over long memory channels are presented. The proper metric to use is analysed and computer simulation with channels having memory lengths up to nine symbols show that the achieved error performances fall within the theoretical upper and lower bounds of an optimal decoder at only a small fraction of the computational effort of an optimal decoder. As for the computational variability, it can be totally circumvented by simultaneously extending a sufficient number of paths. Finally with a simplified multiple path sequential decoder, all the drawbacks of sequential decoding may be avoided at a cost of a very slight error performance degradation.
Performance of a carrier-recovery loop in a satellite modem
- Author(s): S.W. Cheung and A.P. Clark
- Source: IEE Proceedings I (Communications, Speech and Vision), Volume 138, Issue 1, p. 32 –42
- DOI: 10.1049/ip-i-2.1991.0004
- Type: Article
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The paper describes a carrier recovery loop for the coherent demodulation of a digital satellite modem for mobile systems. The loop is data aided, employing decision feedback. The modem transmits a band-limited differentially encoded quaternary phase-shift-keyed (DEQPSK) signal over a satellite link. Several different earth stations are assumed to have simultaneous access to a given transponder, in an arrangement of frequency-division multiple access (FDMA). Thus the DEQPSK signals that occupy the immediately adjacent frequency bands can introduce adjacentchannel interference (ACI) into the transmitted DEQPSK signal. The high-power amplifier (HPA) at the earth station transmitter introduces AM-AM and AM-PM conversion effects into the DEQPSK signal, but the satellite transponder is linear. A baseband predistorter operates on the baseband signals at the input to the modulator and assumes a prior knowledge of the HPA characteristics. A carrier-recovery loop is used at the receiver to generate the in-phase and quadrature reference carriers employed by the coherent demodulator. The performance of the system is studied by computer simulation, over a range of different conditions and with different arrangements of the modem, and the results are used to determine the preferred modem design.
Estimation of unreliable packets in subband coding of speech
- Author(s): W.C. Wong ; N. Seshadri ; C.-E.W. Sundberg
- Source: IEE Proceedings I (Communications, Speech and Vision), Volume 138, Issue 1, p. 43 –49
- DOI: 10.1049/ip-i-2.1991.0005
- Type: Article
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A robust speech communications system over a digital mobile radio channel is considered. We examine a system using 12 Kbit/s embedded subband coding of speech and rate compatible punctured convolutional (RCPC) codes with generalised Viterbi decoding for combined error detection and correction. System performance is improved by exploiting the frame-toframe redundancy in the subband energy profile used for adaptive bit allocation in the subband coder. Unreliable subband side information about the energy profile is identified through a two-stage process comprising the channel error detector and a source-based subband energy profile error detector. The subband side information deemed unreliable is replaced with the estimates based on frame-to-frame redundancy. This improved error detection capability in the subband side information allows a judicious reassignment of channel error protection bits to the subband main information, leading to an overall improvement in system performance. Using a realistic simulation model of a digital mobile radio communication system, an improvement of up to 4.4 dB in recovered speech segmental SNR is obtained at a channel SNR of 6 dB, compared with a system with only channel error protection.
Maximum likelihood synchronisers with arbitrary prior symbol probabilities
- Author(s): J.H. Chiu and L.S. Lee
- Source: IEE Proceedings I (Communications, Speech and Vision), Volume 138, Issue 1, p. 50 –52
- DOI: 10.1049/ip-i-2.1991.0006
- Type: Article
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The likelihood function for carrier or symbol synchronisation problems with arbitrary prior symbol probabilities is derived using a new approach. The analysis indicates that the general structure of such a maximum likelihood (MAXL) synchroniser is the same as that of the conventional one with equal prior probabilities except for biasing the tanh operator input with a transition offset value v, v = (1/2) In (q/p); here p is the prior probability of symbol ‘1’, and q = 1 – p. For the case p = 1/2, the general synchroniser is reduced to a conventional MAXL synchroniser, for instance the Costas loop carrier synchroniser or MAXL symbol synchroniser. But when there are no data transitions, i.e., p = 1, or 0, the general synchroniser is shown to reduce to a phase-locked loop (PLL) for carrier synchroniser, or a PLLtype symbol synchroniser.
Mini-max algorithm for image adaptive vector quantisation
- Author(s): S. Panchanathan and M. Goldberg
- Source: IEE Proceedings I (Communications, Speech and Vision), Volume 138, Issue 1, p. 53 –60
- DOI: 10.1049/ip-i-2.1991.0007
- Type: Article
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The paper presents a technique which employs the mini-max error criterion for image compression using adaptive vector quantisation. Vector quantisation is a promising method for low bit rate image compression. In vector quantisation (VQ), the image vectors are usually coded using a ‘universal codebook’ generated from a set of training images. The coding performance using this codebook is potentially poor for images outside the training set. A number of inter- and intra-image techniques have been proposed to adapt the codewords to the input image. However, these techniques do not guarantee the closest codewords to be within a prespecified bound of the input vectors. This can result in large errors which give rise to artifacts. We propose an intraimage adaptive technique which employs a criterion that minimises the maximum error. Here, the codebook is generated on the fly from the input vectors to be coded. A primary codebook of size two, four or eight is typically used to store the frequently used codewords. A larger secondary codebook is used to store the less frequently used codewords. Both the transmitter and receiver maintain identical codebooks and hence keep track of any changes without any overhead information. As it is a single-pass technique, real-time implementation is possible.
Competitive training: a connectionist approach to the discriminative training of hidden Markov models
- Author(s): S.J. Young
- Source: IEE Proceedings I (Communications, Speech and Vision), Volume 138, Issue 1, p. 61 –68
- DOI: 10.1049/ip-i-2.1991.0008
- Type: Article
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The paper presents hidden Markov models (HMMs) within a connectionist framework and shows how error back propagation can be used to discriminatively train HMM parameters. The relationship between this competitive training approach and conventional Baum–Welch re-estimation is explored and experimental results presented for its application in ergodic HMM architectures.
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