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Communication Technology, IEEE Transactions on

Issue 4 • Date August 1971

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Displaying Results 1 - 25 of 35
  • [Front cover and table of contents]

    Page(s): 0
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  • Dynamic Behavior of a System of Mutually Synchronized Oscillators

    Page(s): 373 - 395
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    An analysis is presented of the performance of a synchronous digital communications system in which the clock rate of each station is established as the average of the clock rates of all incoming signals to each station. In a previous paper the steady-state frequencies and phase relationships between stations were analyzed. Here the dynamic response of various frequency-averaging system configurations are analyzed using a time-incremental computer simulation of the system. Plots of the instantaneous frequency and phase variations in response to transmission-path changes ate provided. The frequency and phase response predicted by the computer simulation was correlated against a laboratory breadboard model. Also, the final steadystate conditions predicted by the computer simulation are correlated against the steady-state changes predicted by the previous static final-value analysis. View full abstract»

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  • On the use of Channel Introduced Redundancy for Error Correction

    Page(s): 396 - 402
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    The problem considered is that of reducing intersymbol interference caused by time dispersive channels. Similarity between the response of a time dispersive channel and that of a cyclic algebraic encoder has led to a method of treating the dispersive channel as a linear encoder. A tapped delay line equalizer is used to complete the encoding process so that the result can be decoded and errors corrected. The tap gain vector for the equalizer is developed, and an upper bound on the probability of error is derived. The analysis and computer simulation results show that under certain conditions the error rate can be reduced by orders of magnitude. View full abstract»

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  • Evaluation of Diver Communication Systems by a Diver-to-Diver Technique

    Page(s): 403 - 409
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    This investigation was conducted as part of a program of research designed 1) to develop several methodologies for the evaluation of diver communication systems and 2) to carry out these evaluations on available units. The major focus of this report is on a diver-to-diver procedure and data resulting from the evaluation of seven diver communication systems, viz: a) hard-lineAquaphone; b) acoustic-Bendix and Yack-Yack; and c) amplitude modulated-PQC-1a, PQC-2, Aquasonics 811, and Aquasonics U-42. All systems were used with a Nautilus muzzle and a double-hose regulator. Talkers were five divers experienced in such tasks; listeners were six to eight divers familiar with the talkers' speech; stimulus materials were the Clarke 50-word multiple choice lists. The following results were noted: 1) no single approach to diver communication (modulated, acoustic, hard-line) completely dominated the results; 2) the use of optimum (currently available, that is) muzzle-regulator combinations, a closed set of speech materials, and procedures where talkers' speech was familiar to the listeners, all operated to improve the intelligibility of communication; 3) the performance of the military systems was poorer than that of the commercial ones; and 4) these systems-even when used optimally-still do not provide adequate communication for divers. View full abstract»

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  • Optimal Power Transfer Through Atmospheric Turbulence Using State Knowledge

    Page(s): 410 - 414
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    The design of a laser transmitter that achieves maximum power transfer through the earth's turbulent atmosphere is considered. It is shown that optimum power transfer would be realized with an adaptive system that uses the appropriate beacon signal to probe the channel state. This result is applied to show that the average power gain of the optimal earth-to-space optical link is the diffraction-limited gain for the apertures in vacuum. For large transmitting apertures the percentage fluctuation about this mean gain goes to zero. Thus the fundamental limitation on power transfer imposed by the turbulence is considerably milder than had been thought. View full abstract»

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  • An Empirical Comparison of Two Sequential Decoding Algorithms

    Page(s): 415 - 419
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    The results of comparison of the conventional Fano algorithm and a new stack algorithm proposed by Zigangirov and Jelinek, by computer simulation of two sequential decoding algorithms are reported. The results indicate that for rates near Rcompthe stack algorithm offers a considerable improvement in decoder speed over the Fano algorithm, provided that fairly large storage capacity is available for use by the decoder. View full abstract»

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  • Multiple Error Performance of PSK Systems with Cochannel Interference and Noise

    Page(s): 420 - 430
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    The multiple error performance of a phase-shift-keyed (PSK) communications system, when both cochannel interference (due possibly to other cochannel angle-modulated systems) and Gaussian noise additively perturb the transmitted signals, is considered. The results are fairly general: the main requirement is that the interference be circularly symmetric. All of our findings are also applicable to the case when only noise is present. The results indicate that one cannot approximate well the effect of interference on the performance of a PSK system by treating it as additional Gaussian noise. First, we derive the probability density function fAof the phase angle of a cosinusoid plus interference and Gaussian noise. We then obtain readily computable expressions (in terms of fA) for the probability of any number of consecutive errors in anm-phase system when either coherent or differential detection is utilized. For numerical results, the interference is assumed to be due to other cochannel angle-modulated communications systems, and the double error probability and conditional probability of error are given for 2- and 4-phase systems. View full abstract»

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  • Performance Evaluation of a New Modulation Technique

    Page(s): 431 - 445
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    The performance of a new modulation technique which has been called mixed-base modulation (MBM) is presented. This technique provides significant performance advantages over other common modulation techniques for applications where both power and bandwidth are limited. Its performance for bandspreading factors of two relative to single-sideband is within 4 dB of the ultimate Shannon bounds. Most available techniques are very poor in this region. The viewpoint which allowed synthesis of this technique will be given before the basic approach and its performance are discussed. A section comparing the theoretical performance of MBM and that of other common techniques is followed by a discussion of some of the practical considerations. View full abstract»

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  • Signal Design and Error Rate of an Impulse Noise Channel

    Page(s): 446 - 458
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    An optimal signal design of the classical smeardesmear technique for combating impulse noise is demonstrated. The signal design makes use of calculus of variations to derive an integral equation to which several solutions are given, along with a technique for generating additional solutions. A computer simulation and an evaluation of Rice's triple integral pdf for impulse noise are used to analyze the error probability of a smear-desmear data channel and a standard data channel. This is the first known publication of a general evaluation of Rice's integral for the intermediate repetition rate case. The error rates show that the smeardesmear channel is superior only for the lower repetition rate and/or lower level impulse noise cases. An analysis is then made of a clipper before the receiving filter. It is shown that this improves the system error rate to a degree, depending on the clipping level and the preclipper bandwidth-with the wider bandwidth giving greater improvement. The smear-desmear channel experienced a greater improvement than the conventional channel, so that the smear-desmear channel can be considered superior except for very high noise levels or repetition rates; practical considerations in the preclipper bandwidth and clipping level are the limiting factors. View full abstract»

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  • The Spectra of Digitally Encoded Video Signals

    Page(s): 459 - 466
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    Because of the nonlinear nature of quantizing and coding operations, it is not possible, in general, to derive the power spectrum for a digitally encoded signal from a knowledge of only the spectrum of the original analog signal. For video signals, however, the analog process can be modeled by the random step function. This leads to a convenient expression for the power spectral density of the digital signal with separate factors characterizing the effects of 1) digital pulse shape, 2) quantizing and coding operations, 3) scanning raster, and 4) the bandwidth of the analog signal. Experimental data from television and Picturephone® signals are presented. The results are of special significance to a hybrid analog-digital system. View full abstract»

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  • On Decoding of Correlative Level Coding Systems with Ambiguity Zone Detection

    Page(s): 467 - 477
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    Decoding of a correlative level coding or partialresponse signaling system is discussed in an algebraic framework. A correction scheme in which the quantizer Output includes ambiguity levels is proposed. The implementation and algorithm of error correction is discussed in some detail. An optimum design of the quantizer based on Chow's earlier work is discussed. Both analytical and simulation results on the performance of the proposed decoding scheme are presented. An asymptotic expression for the decoding error rate is derived in closed form as a function of the channel signal-to-noise ratio. This is also compared with the conventional bit-by-bit detection method and the maximumlikelihood decoding method recently studied. View full abstract»

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  • Distortion Analysis of Binary FSK

    Page(s): 478 - 486
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    The performance of the binary frequency-shift keying (FSK) data system employing a conventional limiterdiscriminator detector is analyzed. Both additive noise and distortion produced by the transmitter and receiver bandpass filters are taken into account. Performance tradeoffs with respect to transmitter frequency deviation, bandpass filter shape, and bandpass filter bandwidth are investigated. Intersymbol interference caused by distortion is taken into account for each bit sequence by calculating two distortion factors. The first factor is related to the actual baseband signal distortion produced by the bandpass filters. The second factor is related to the power distortion caused by the bandpass filters. It is shown that both factors must be considered in determining the detrimental effect of distortion on system error probability. Three particular bandpass filter shapes are considered. They are the first-, second-, and third-order Butterworth filters. For each of these filter shapes the binary FSK data system is optimized over transmitter frequency deviation and bandpass filter bandwidth. It is found that the third-order Butterworth filter operating at a 3-dB bandwidth of1.1/THz, whereTis the bit interval, coupled with a transmitter frequency deviation of0.36/THz yields the best performance for the binary FSK system with limiterdiscriminator detection. This bandwidth (64 percent of the Carson's rule bandwidth) optimizes a tradeoff between additive noise and distortion effects. This frequency deviation optimizes a tradeoff between the spike and nonspike noise components of the discriminator output. It is shown that the performance of the optimized binary FSKlimiter-discriminator system closely approaches the performance obtained by using an optimum coherent detector on the FSK signal set. View full abstract»

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  • Statistical Bit Synchronization in Digital Communications

    Page(s): 487 - 491
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    A Kalman filtering algorithm has been applied to the problem of bit synchronization in anM-ary communication system. No synchronizing signal is assumed present; the approach is arranged to determine timing information from the transitions occurring in a pseudorandom sequence of symbols, generated with a fixed but initially unknown bit rate. Procedural steps subdivide naturally into an acquisition phase (block data processing) and a track mode (recursion). The method is supported by sample binary frequency-shift keying (FSK) simulation results, obtained from the output of a 4-pole Butterworth digital filter fed by a random MARK/SPACE sequence plus additive Gaussian noise. Results demonstrate accurate determination of both the bit phase reference time and the bit period. The scope is restricted to high SNR digital communication systems, for which acceptable error rates are obtainable without sophisticated decoding schemes. View full abstract»

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  • The Effect of Gaussian Error in Maximal Ratio Combiners

    Page(s): 492 - 500
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    In a fading channel, maximal ratio diversity combilling improves the average signal-to-noise ratio over thatof a single branch in proportion to the number of diversity branches combined. However, its main advantage is the reduction of the probability of deep fades. The effect of Gaussian errors in the combiner weighting factors on the probability distribution of the output signal-to-noise ratio is computed. The limits on allowable error for a specified probability of fades below any given level are indicated. The results are applied to a mobile radio example in which the weighting factor is determined from a pilot transmitted along with the signal. To keep the pilot from overlapping the signal, they are separated either in frequency or in time. In this case the Gaussian error is due to decorrelation of the pilot from the signal either because their frequency separation or their time separation is too large. View full abstract»

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  • Computer-Aided Analysis and Design of Negative Impedance Boosted Transmission Lines

    Page(s): 501 - 516
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    Negative impedance boosting using activeRCcircuits series connected at intervals along a cable pair has been shown to minimize loss and provide nearly distortionless bilateral transmission of either digital or analog signals. Results from computation and field measurements are in good agreement. Recently, on-line plotting with a time-shared computer has made it feasible to find the circuit parameters which minimize the remaining distortion for many commonly used cables and spacings. Parameter adjustments are shown which permit a tradeoff between bandwidth and either amplitude or phase distortion. Factors affecting pulse transmission have been investigated. In general, bandwidth is inversely related to booster spacing. Bandwidth-spacing relations are given for two cable gauges and compared with the upper bound where propagation is at light velocity. Bandwidth increases with wire size, while the bandwidthspacing product improves with reduced spacing. View full abstract»

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  • Surface Transfer Impedance of Cable Shields Having a Longitudinal Seam

    Page(s): 517 - 522
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    The magnitude of noise induced into communication cable from electromagnetic influences is reduced by the shielding properties of the metallic shield. One of these properties is the surface transfer impedance. The surface transfer impedance relates the current induced on one side of a shield to the longitudinal voltage appearing on the other side due to that current. At low frequencies the surface transfer impedance for nonpermeable materials is equal to the dc resistance of the shield. At high frequencies it decreases rapidly. The frequency at which the decrease begins is a function of the thickness and conductivity of the metal. For cylindrical shields having longitudinal seams, the transfer impedance increases at somewhat higher frequencies. The size of the seam opening determines the frequency at which the increase begins. Although the size of the seam opening is difficult to control, experimental results are in relatively good agreement with theoretical calculations. View full abstract»

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  • Subscriber Loop Multiplexer--A High Pair Gain System for Upgrading and Growth in Rural Areas

    Page(s): 523 - 527
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    The subscriber loop multiplexer is a combined carrier and switching system for serving 80 main stations on 24 channels. The system has been designed for use in rural areas to handle growth and upgrading from multiparty service toward individual service. The system consists of a control terminal and up to six remote terminals located near the customers being served. The system is digital and uses a bit rate of 1.544 Mbit/s for the repeatered line interconnecting the terminals. The system has many maintenance provisions and alarms that have been economically achieved through digital integrated circuits. The test desk can initiate a series of tests on the voice frequency loop extending from the remote terminal to the customer. Delta modulation provides a simple and economic conversion between analog and digital forms. At the remote terminals each main station Served has an individual modem allowing equipment to be added as required to serve customers. The system utilizes 14 codes of beam-lead integrated circuits of which 10 are bipolar and 4 are insulated-gate field-effect transistor (IGFET). The IGFETs provide the adaptive delta modulation control and also perform the time division switching function at the remote terminals. View full abstract»

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  • A New Store and Forward Message Switching System

    Page(s): 528 - 529
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    A store and forward message switching system, made up of duplexed 16-bit computers, fixed head discs (FHDs), moving head discs (MHDs), and multiline controllers (MLCs) is described. The paper discusses the area of application for a system of this type, and the equipment configuration and operation of the subject system. The paper concludes with a discussion of the redundancyswitchover aspects of the system and the system size. View full abstract»

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  • A Method for Computer Control Transfer in a Communication Switching System

    Page(s): 529 - 532
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    A method is outlined for maintaining digital processor control of a switching system in the event of a processor failure. The solution chosen uses a minimum of equipment consistent with high system availability. Transfer mechanisms, logic, and required software processing are discussed. View full abstract»

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  • Optimum Prefiltering and Postfiltering of Sampled Waveforms

    Page(s): 532 - 535
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    Communication systems that transmit samples from analog waveforms are considered. Optimum pre- and postfilters of these samples are derived. The criterion of optimality is minimization of the mean-square error in reproducing the waveforms subject to a constraint on the signal-to-noise ratio per transmitted sample. The results of this filtering are evaluated and compared to other filtering schemes. View full abstract»

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  • A Derivation of the Spectra of N-Ary Orthogonal Comtinuous-Phase FSK Waveforms for ELF/VLF Communications

    Page(s): 536 - 539
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    The power spectral density of continuous-phase minimum-bandwidth waveforms composed from a set ofNmutuallyorthogonal chips (pulsed sinusoids of equal amplitude and duration and constant frequency) is derived. The spectrum and its derivation for this class of signals are of particular interest in highpower extremely low frequency/very low frequency (ELF/VLF) communications where noise immunity is a prime consideration, but the signal bandwidth and transients at the transmitting antenna must be minimized. Exact formulas are also given without derivation for wider bandwidth signals in the same class but of less practical importance. Although the power spectra of more general binary signals are already known, the special case presented here permits a much simpler derivation than that given in an earlier paper [1]. In theN-ary case a previously derived formula for the spectra of continuous-phase multilevel FM signals [2] is augmented and evaluated to explicitly yield new spectral formulas useful in describing the frequency-hopping mode. View full abstract»

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  • Distribution of Click Amplitudes

    Page(s): 539 - 543
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    The amplitude of a click is defined as the output of an ideal FM demodulator at the time of occurrence of the click. It is shown that the amplitude of a click (when the carrier is unmodulated) is a function of two random variables whose join distribution function is derived under the condition that a positive click has occurred. The distribution of positive click amplitudes is derived with the carrier-to-noise power ratio (CNR) at the output of the intermediate frequency (IF) filter as a parameter The results are extended to include negative clicks. The probability density and distribution functions of click amplitudes are plotted for the special case of a rectangular IF and CNRs of 4 and 9 The effect of modulation on the distribution of click amplitudes is determined. Results are given for the special case of a sinusoidal modulating signal fully deviating the carrier and a rectangular IF View full abstract»

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  • Implementation of Reed-Solomon Erasure-Correcting Decoder for Hybrid Coding Scheme

    Page(s): 543 - 546
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    Implementation of the algebraic section of a relatively simple sequential-algebraic error-correcting scheme for very noisy channels is detailed, and its complexity evaluated. Since one of the properties of this hybrid is that the sequential portion can be any standard sequential decoder, the description here is sufficient to specify the modifications which must be performed on a convolutional-encoder-sequential-decoder to convert it to a scheme with improved performance. The algebraic section consists of a Reed-Solomon erasure-correcting code. View full abstract»

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  • Adaptive Delta Modulator for Telephony and Its Application to the Adaptifon System--An Alternative Implementation of the Lincompex Concept

    Page(s): 547 - 551
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    An adaptive version of the basic delta modulator employing full-width pulses andRCintegration is described. A digital technique is used to sense the slope of the input signal and to control the amplitude of the pulses supplied to theRCnetwork in the feedback loop. Subjective testing with speech signals and a modulator clock rate of 56 kbit/s has indicated a useful volume range of 40 dB for commercial telephony-grade performance. At a clock rate of 19.2 kbit/s a signal-quantization noise ratio of 16 dB has been obtained over a dynamic input range of 20 dB for an 800-Hz sine wave. Also described is an application of the adaptive delta modulator known as the Adaptifon system in which the compression and expansion circuits of Lincompex are realized by the delta modulation technique. Speech is transmitted in analog form at constant amplitude, which, together with an FM syllable rate channel, occupies the conventional 3-kHz bandwidth. The receiving system has the capability of removing fading from signals transmitted over an HF path. View full abstract»

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  • Storage and Delay Estimates for Asynchronous Multiplexing of Data in Speech

    Page(s): 551 - 555
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    The communication system considered here interpolates data in the pauses in speech. Speech is gated on and off by comparing its own envelope to a threshold. At the same time a source is generating data at a fixed rate. The data are stored in a buffer, awaiting transmission during one of the gaps in speech. This paper presents a theoretical analysis of a model in which successive speech durations are assumed to be independent exponentially distributed random variables. The successive durations of silence are modeled by independent random variables that are obtained by forming the mixture distribution of two independent exponentially distributed random variables. On the basis of this model for alternating speech and silences, and an infinite buffer that is assumed to take on a continuum of states, we solve for the stationary probability density of nonzero states of the peak buffer occupancy. This enables us to determine buffer allocation as well as average delay using typical speech parameters. View full abstract»

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Aims & Scope

This Transactions ceased publication in 1971. The current retitled publication is  IEEE Transactions on Communications.

Full Aims & Scope