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Communications, IEEE Transactions on

Issue 4 • Date April 1986

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Displaying Results 1 - 18 of 18
  • [Front cover and table of contents]

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  • Author's reply

    Page(s): 395
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    First Page of the Article
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  • [Back cover]

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  • Interframe Predictive Coding of Images Using Hybrid Vector Quantization

    Page(s): 411 - 415
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    The temporal and spatial redundancies of image sequences can be reduced by prediction and vector quantization, respectively. The robustness of the coder is increased through the use of a hybrid two-stage vector quantizer. The first stage is of the Linde-Buzo-Gray type. The second stage encodes the residual error of the first stage using a lattice vector quantizer. View full abstract»

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  • Gray-Scale Image Coding for Freeze-Frame Videoconferencing

    Page(s): 382 - 394
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    A new gray-scale image coding technique has been developed, in which an extended DPCM approach has been combined with entropy coding. This technique has been implemented in a freeze-frame videoconferencing system which is now operational at IBM sites throughout the world. Following image preprocessing, the two fields of the interlaced 512 x 480 pixel video frame are compressed sequentially with different algorithms. The reconstructed image quality is improved by subsequent image postprocessing, the final reconstructed image being almost indistinguishable from the original image. Typical gray-scale video images compress to about a half bit per pixel and transmit over 4.8 kbit/s dial-up telephone lines in about a half minute. The gray-scale image processing and compression algorithms are described in this paper. View full abstract»

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  • Evaluation of the Optimal SRPT Strategy with Overhead

    Page(s): 338 - 344
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    Considering queueing systems in data networks, one can identify a special feature of the communication system: the service time, which is proportional to message length, is known in advance. This is a precondition for queueing strategies such as SPT (shortest processing time first) or SRPT (shortest remaining processing time first), which offers the shortest mean delay time among all conceivable strategies. For a practical evaluation of these strategies it is essential to include the influence of overhead time, especially for the Preemptive SRPT strategy. This paper analyzes SRPT with constant overhead time CV modified by a constant preemption gapCP gcong CVforM/G/1systems. The comparison of strategies for FIFO, SPT, SRPT, and RR (round robin) using different service time distributions with coefficients of variation >1 and overhead shows, for SRPT and for a combination of SRPT and RR, a considerable reduction of the mean delay timeoverline{TD}, especially for service time distributions with high coefficients of variation. This result indicates the potential advantages of these strategies in data network applications. View full abstract»

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  • On the Throughput of Channel Access Algorithms with Limited Sensing

    Page(s): 345 - 347
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    We consider some access protocols for Aloha type multiaccess channels. We argue, and show in an important case, that they can be modified to allow new transmitters to join the system at arbitrary times. This feature, known as "limited sensing" or "continuous entry," need not reduce throughput performances. In the case presented, the modified algorithm is also robust with respect to feedback errors. View full abstract»

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  • Dynamic Flow Control Protocols for Packet-Switching Multiplexers Serving Real-Time Multipacket Messages

    Page(s): 348 - 356
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    The prospect of using packetized services for real-time data, particularly voice data, is generating considerable interest. This paper proposes three dynamic flow control protocols for a multiplexer handling packetized real-time data, in an attempt to provide flexibility to the multiplexer operation and at the same time to satisfy strict delay constraints imposed on real-time data. It is typical of real-time traffic that data lose their value as they age prior to the reception. The flow control protocols are based on this idea and selectively discard portions of a message when congestion develops within the multiplexer. Queueing analyses of the protocols as well as simulation results are presented. View full abstract»

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  • Dependency of Service Time on Waiting Time in Switchng Systems--A Queueing Analysis with Aspects of Overload Control

    Page(s): 357 - 364
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    Performance degradation of switching systems when the load increases above the engineered load can be caused by systemdependent and customer-dependent factors. In this paper the dependency of the service time and the call completion rate on the waiting time of a customer is investigated. The problem is modeled by means of a queueing system of typeM^{[x]}/G/1, where state-dependent batch size distributions are considered. Two analysis methods, the continuous Markov chain approach and the regenerative method, are used for Markovian and generally distributed service phases, respectively. Numerical results are given for system characteristics, in particular the call completion rate of the system. Finally, a basic overload control scheme is investigated which increases the throughput of completed calls at higher traffic levels. View full abstract»

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  • Spread-Spectrum Despreading Without the Code

    Page(s): 404 - 407
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    A new method for despreading direct-sequence spreadspectrum signals, without use of the spreading code, is evaluated by analysis and simulation. The new method exploits the cyclic autocorrelation in place of the conventional autocorrelation, which was used in a prior method. Both methods require that the period of the code be equal to a multiple of the data symbol interval. Broad-band noise and narrowband interference rejection capabilities of both methods are compared. It is shown that the new method, unlike the prior method, can provide substantial immunity to strong narrow-band interference. View full abstract»

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  • Channel Utilization and Blocking Probability in a Cellular Mobile Telephone System with Directed Retry

    Page(s): 329 - 337
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    A directed retry facility, which enables subscribers in a mobile telephone system to look for free radio channels in more than one cell, is investigated with respect to blocking probability and channel utilization. An iterative procedure is devised, by which the dependencies among cells can be illustrated. This procedure makes use of theories developed for overflow systems in classical telephony, and proves to be very accurate for the situations under study. Analytical results are compared with simulations and good agreement is observed. Results show that a substantial improvement, compared with systems without a directed retry facility, can be achieved as far as carried traffic is concerned. The improvement is accomplished at the expense of those subscribers who cannot make use of the directed retry facility due to variations in radio coverage. View full abstract»

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  • Linear Direct-Detection Fiber-Optic Receiver Optimization in the Presence of Intersymbol Interference

    Page(s): 365 - 374
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    A technique is developed for optimizing communication receivers by minimizing a Chernoff bound (CB) on the Bayes cost function. Being couched in terms of moment generating functions, the procedure is well suited to optical systems. Optimal CB receivers are derived and shown to coincide exactly with known minimum cost Bayes receivers for certain special cases of both electrical and optical communications for which closed form analytic solutions are possible. This provides endorsement for the method, which is then applied to more complex practical cases of linear optical fiber receivers in which both colored Gaussian noise and intersymbol interference play a part. The CB receivers are shown to offer some improvement compared with a zero forcing raised cosine equalizing receiver such as is commonly adopted, and all the evidence suggests that CB receivers achieve performance very close to the best possible with a linear receiver. View full abstract»

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  • Performance Analysis of the Delay-Line Clock Regenerator

    Page(s): 321 - 328
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    A general analysis is provided for the jitter performance of a delay clock regenerator as a function of various system parameters such as modulation format, channel filtering, signal-to-noise ratio, and line delay. The novelty with respect to previous works on the same subject is that we find an expression for the jitter variance of the regenerated clock, while other studies have dealt with an upper bound to such variance. Numerical results have been checked by computer simulations. View full abstract»

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  • A Method of Examining Orchard Codes for Minimum Hamming Distance Five

    Page(s): 399 - 404
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    Orchard codes are linear, systematic tree codes of rate(n - 1)/nand infinite block length. Calculation of parity bits is over prior parity bits, as well as prior information bits. The memory needed to encode is about half that of comparable convolutional self-orthogonal codes. After a brief review of recent work involving two-error-correcting orchard codes [1], [2], the authors present a method of analysis of orchard codes to establish whether minimal distance criteria are met. It is assumed that parity is taken over three bits per track. The code introduced by Scott and Goetschel [1], and a truncated version of the code designed by Shiozaki [2], are then analyzed. New orchard codes, designed on the basis of the analysis method, are presented. The method is extendible to codes designed to correct more than two errors, although extension beyond three errors is computationally intensive. View full abstract»

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  • M-ary FSK Performance for Coherent Optical Communication Systems Using Semiconductor Lasers

    Page(s): 375 - 381
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    This paper describes the design and performance of anMary frequency shift keyed (FSK) signaling and demodulation scheme for an optical communication system using semiconductor lasers and heterodyne detection. Frequency or phase noise in semiconductor lasers causes spectral spreading, producing a nonzero linewidth laser signal. This degrades communication performance when compared to a system using an ideal laser with zero linewidth. We present estimates of the bit error rate (BER) performance ofM-ary frequency shift keying (FSK) with noncoherent demodulation in the presence of white Gaussian frequency noise and additive channel noise. This is typical of an optical system using semiconductor lasers and heterodyne detection. Estimates use the union-Chernoff bound with a simplified channel model to predict the effects of frequency noise. Two effects of frequency noise are identified: signal attenuation or suppression, and crosstalk. These cause an offset in the BER curve from the BER in the absence of frequency noise, and an error rate floor, respectively. The error rate floor is lower than previously predicted. When performance is not crosstalk limited,M-ary FSK is found to perform better than binary FSK with the same system bandwidth constraints, as would be predicted if ideal lasers are used. Theoretical results are compared with Monte Carlo simulations of the system. View full abstract»

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  • Futher Results on Rate 1/N Convolutional Code Constructions with Minimum Required SNR Criterion

    Page(s): 395 - 399
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    New good(K, 1/N)convolutional codes for8 leq K leq 13and2 leq N leq 8were found and tabulated which require minimum signal-to-noise ratio (SNR) for given desired bit error rates (BER) with Viterbi decoding. The transfer function bound was used for the BER evaluations. View full abstract»

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  • An Echo Canceller with Controlled Power for Frequency Offset Correction

    Page(s): 408 - 411
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    In an echo canceller (EC) compensating far-end echos affected by frequency offset, the phase loop gain is highly dependent on the echo and signal powers. Adoption of a normalized gain multiplied by the echo power yields an improved robustness. It is shown that the latter gain depends only on the echo-to-signal ratio. Practical implementation of this idea is realized by generating a first unitary echo replica, on which the DPLL will act, and then controlling the output power. The system has the additional advantage of a reduced binary size for the canceller taps. View full abstract»

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Aims & Scope

IEEE Transactions on Communications focuses on all telecommunications including telephone, telegraphy, facsimile, and point-to-point television by electromagnetic propagation.

 

 

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Meet Our Editors

Editor-in-Chief
Robert Schober
University of British Columbia