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Communication Technology, IEEE Transactions on

Issue 1 • Date February 1968

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Displaying Results 1 - 25 of 30
  • [Front cover and table of contents]

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    Freely Available from IEEE
  • Synchronization of a PCM Integrated Telephone Network

    Page(s): 1 - 11
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    To compose a telephone network with low transmission loss without cumulative increase of quantizing noise, it is desirable to accomplish both transmission and switching with PCM modulation. Such a system, called an integrated telephone network, requires synchronization of incoming signals arriving at each exchange to the local timing pulse generator controlling a time division switch of the exchange. In the synchronization method described in this paper-the equational timing system a local oscillator of controllable frequency that drives the local timing pulse generator in each exchange is controlled by making reference to the phase differences between incoming signals arriving at the exchange from the other exchanges and incoming signals arriving at the other exchanges from the exchange. This synchronization method permits the minimun storage capacity of the buffer memory indispensable for compensation of the junction-delay variation, indefinite enlargement of the network and multiplication of its complexity, and invulnerability to the consequence of failure of one or more junctions or timing pulse generators in the network. The phase differences between the local timing pulse generators of exchanges in the network, the stability of the network applied with this synchronization method, and an example of the equational timing system are discussed. View full abstract»

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  • Communications for Time Sharing at Bell Telephone Laboratories

    Page(s): 12 - 18
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    The communications system proposed for provision of remote access to computers for users at Bell Telephone Laboratories is described with emphasis on the dial terminal arrangements. It is planned to offer time-sharing services from four large-scale computers located at the four major Bell Telephone Laboratories installations: three in New Jersey and one in Illinois. Provision of full flexibility of access is planned by utilizing the voice communications arrangements at each location to provide switching for the remote terminals and computer ports involved. It is proposed that the Bell Telephone Laboratories-Western Electric common control switching arrangement network (CORNET), which provides interlocation communication for users at the major locations as well as users at the various branch laboratory installations throughout the country, be utilized for this purpose. Areas of primary consideration in planning communications for the system are discussed, and proposed implementations reflecting these considerations are presented. The considerations largely concern the use of a single communications facility to carry a heterogeneous load resulting from the integration of traffic with two distinctly different sets of characteristics. A number of proposals toward making the most effective use of the telephone communications facilities to carry the two types of traffic simultaneously are described. These proposals are directed at specific equipment and program configurations utilized at the four Bell Telephone Laboratories locations studied. Specific implementations proposed herein include new central office arrangements, as well as new PBX configurations. View full abstract»

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  • Effects of Sampling Contact Design on TDM System Tranmission Performance

    Page(s): 19 - 25
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    Performance requirements are established for practical time-division multiplex (TDM) sampling and distribution contacts for use in high speed TDM systems employing the resonant transfer technique. Various contact designs and their shortcomings are discussed. Actual system transmission performance of a 128 time-slot TDM switching system is presented. View full abstract»

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  • TONE-DIAL® Telephones Applied to Direct Control Exchanges

    Page(s): 26 - 29
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    This paper describes the method by which direct control telephone systems may be adapted to receive tone signals from the subscriber instrument. The methods of association of the common equipment and control of connections during the signaling period are outlined together with the intermediate storage techniques that are used. The guards employed to assure receipt of only legitimate signals are indicated. A description of digit examination facilities and equipment mounting space requirements is included. View full abstract»

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  • Adaptive Feedback Communications

    Page(s): 29 - 34
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    A scheme for transmitting binary signals through a Rayleigh fading multipath medium is analyzed. The scheme envisions an adaptive receiver and a feedback channel. The feedback channel conveys information to the transmitter on the forward channel state learned at the receiver. The transmitter uses this information to modify transmission. Results that show the improvement in performance of the system under the assumptions of perfect receiver learning and noise-free feedback are presented. In this way, upper bounds on system performance are established. View full abstract»

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  • Error Rate for Peak-Limited Coherent Binary Channels

    Page(s): 35 - 44
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    An exponential-type bound on error rate Pefor peaklimited binary coherent channels operated at low signal-to-noise ratio (SNR) is presented. The bound depends exponentially only on the first and second moments of the channel output and serves to justify, in part, the use of SNR calculations for error-rate performance. It is assumed that the receiver outputVis given by a simple sum ofn( = [TW])identically distributed, independent random variables wi, each of which is decomposable into the sum of two independent random variables ziand ηi, i.e.,w_{i} = z_{i}+eta_{i}. The ziare peak limited byB_{z},|z_{i}| leq B_{z}, whereas ηiare normal (0, sigma^{2}). The zirepresent the output of a peak-limited channel and the ηirepresent any post channel receiver thermal noise (which may be zero,sigma^{2} = 0). For example, the zimay represent the output of a bandpass-limited satellite repeater, with an interference input in addition to the desired signal, and ηithe front-end noise in a receiving ground station. No assumption as to the channel-limiting characteristic or interference model, other than stated above, is made. Defining α as the ratio[Ew]^{2}/var w(i.e., twice the receiver input average SNR) and β as the ratioB_{z}^{2}/var w(i.e., twice the receiver input peak SNR), then foralpha < 1,P_{e} < e^{-nmu}mu=frac{alpha}{2}(1-alpha)-frac {gamma^{3}}{6} e^{gamma}gamma= sqrt{alphabeta}, i. e., twice the geometric mean of average and peak SNR. If all odd moments ofzhave the same sign then a larger μ is obtained:mu=frac{alpha}{2}(1-alpha)-frac{gamma^{4}}{24}e^{gamma^{2}}/28Upper bounds on the size of α and β are provided to guarantee- mu > 0. An example of a captured limiter that exhibits SNR suppression effects on the error rate is presented. View full abstract»

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  • Performance of Analog Demodulators in a Fading Environment

    Page(s): 45 - 51
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    Various structures have recently been obtained for optimally demodulating analog signals transmitted over a noisy, randomly fading channel. Their performance, in a least mean-signal error sense, is evaluated in this paper. In the case of the optimum linear filter for AM (or equivalent baseband) signals, as the carrier-to-noise ratio increases indefinitely, an irreducible error is found. The Cramèr-Rao (lower) bound on the error is found and indicates, however, that the fading error can be reduced to zero by use of a nonlinear receiver. An analysis of the maximum a posteriori receiver, utilizing estimates of the channel fading characteristics, does in fact indicate that for this nonlinear receiver, the fading error can be made to approach zero. Similar considerations are found to apply in the case of PM signals. View full abstract»

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  • Spectral Properties of Single-Sideband Angle Modulation

    Page(s): 52 - 62
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    The generation of single-sideband angle-modulated (SSB-phiM) waves compatible with conventional detection methods requires involved processing. The result of these operations yields a signal whose spectral properties are more complicated than simply being one-sided versions of the analogous double-sideband spectrum, and hence it is not obvious whether or not a saving in bandwidth results. Here the spectral properties of SSB-phiMwaves are investigated in detail when the modulating signal is a sample function of a stationary Gaussian process. It is shown that the power spectrum of SSB-phiMis related to the modulating baseband spectrum through an integral equation. This equation is solved explicitly for small and large modulating indices, and it is shown how simple numerical techniques provide solutions for arbitrary index. It is concluded that SSB-phiMexhibits properties very similar to SSB-AM with a strong carrier component When the modulation index is much less than unity. Therefore, if minimum bandwidth plus the attributes of incoherent demodulation are desired, and a Strong carrier can be tolerated, SSB-phiMcould find an application. On the other hand, for modulation indices larger than about 3, no advantage is gained since the SSB spectrum has a bandwidth equal to or larger than the bandwidth of conventional angle-modulated waves. View full abstract»

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  • Signal-to-Crosstalk Power Radio in Smoothly Limited Multichannel FDM Signals

    Page(s): 63 - 67
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    The passage of multichannel frequency division multiplexed (FDM) signals through an amplitude limiting nonlinear device produces crosstalk in each channel output due to intermodulations among signals in all channels. The signal power S0to crosstalk power Ncratio in any particular channel determines signal degradation. This paper considers multichannel FDM signals passing through a saturated bandpass amplifier with transfer characteristic of smoothly limiting error function curve, and determines a lower bound ofS_{0}/N_{c}as a function ofq, which is related to the limiting level. By invoking the central limit theorem, the FDM signals are represented by a Gaussian noise, excepting the signal in the channel whereS_{0}/N_{c}is to be determined. The signal in the channel under consideration is represented by a constant amplitude sine wave. When an assumption that the input noise power spectral density is symmetric about the center band is made, such as a Gaussian power spectrum considered in the analysis, the computation of a lower bound forS_{0}/N_{c}was equivalent to the consideration of the center channel, where the crosstalk power is maximum. The problem is an extension of an earlier work by Cahn, who considered an idealized saturation amplifier. The result is presented in a graph where it is seen that an infinite clipping case of the present analysis agrees with that of Cahn. View full abstract»

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  • On the Probability of Error for Multichannel Reception of Binary Signals

    Page(s): 68 - 71
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    In this paper a derivation of the probability of error which arises in 1) adaptive multichannel reception of binary signals and 2) multichannel communication with binary signaling over channels that are characterized by both a specular (nonfading or constant) component and a Rayleigh fading component is presented. View full abstract»

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  • Probabilities of Error for Adaptive Reception of M-Phase Signals

    Page(s): 71 - 81
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    Probabilities of error are derived for 2- and 4-phase signaling over anL-diversity branch, time-invariant, additive Gaussian noise channel and forM-phase signaling over anLdiversity branch, Rayleigh fading, additive Gaussian noise channel. Both channels corrupt the signaling waveforms transmitted through them by introducing an additive Gaussian noise and an unknown or random multiplicative gain and phase shift in the transmitted signal. The receiver processing consists of cross correlating the signal plus noise received over each diversity branch by a noisy reference signal, which is derived either from the previously received informationbearing signals or from the transmission and reception of a pilot signal, and adding the outputs from allL-diversity branches to form the decision variable. The novel aspects of this paper are: 1) the result for the phase distribution of the decision variable forM-phase signaling over anL-diversity branch, Rayleigh fading channel, 2) the explicit expressions for the probabilities of error forM-phase signaling systems operating over anL-diversity branch, Rayleigh fading channel, and 3) the expression for the probability of error for 4-phase signaling over a time-invariant channel. Previously published error probabilities for 2- and 4-phase signaling over a Rayleigh fading channel are obtained as special cases. View full abstract»

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  • Generation of Digital Signaling Waveforms

    Page(s): 81 - 93
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    Data transmission (digital signaling) systems generally transmit continuous, essentially bandlimited signals that represent alphanumeric sequences. The transmitting apparatus in such systems must therefore convert discrete symbols-often binary pulses-into prescribed analog waveforms. Current commercial systems do this via simple filtering, gating, and conventional modulation. These practices limit the types of waveforms that can be used and they are not always amenable to variable rate signaling. This paper describes a hybrid digital-analog method for approximating closely almost any reasonable digital signaling waveform. The basic idea is very simple: a "staircase" approximation of the desired signal can be generated via a shift-register version of a transversal filter. Variations on this idea, such as derivative approximation and multirate shifting, enhance its accuracy without destroying its simplicity and time-frequency scalability. View full abstract»

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  • The Prevention of Transmission Buffer Overflow in Telemetry Data Compressors

    Page(s): 94 - 107
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    A comparative evaluation of several feedback methods of telemetry data compressor output buffer overflow prevention is described. The study was made using a digital computersimulated multichannel data compressor, operating on recorded Explorer XVII satellite telemetry data. The performances of control methods based on precompression digital filtering and sample deletion are evaluated and compared with that of a system which varies the maximum allowable reconstructed data error, or tolerance, to control the queue length. The tolerance-control system is also examined in detail, and the desirability of tolerance control, based on the time integral of the queue length in addition to the queue length itself, is determined. Test results indicate that the tolerance-control system exhibits the highest performance with the Explorer XVII data, followed by the sample deletion and the filtering methods, in that order. Results also show that the addition of integral feedback enhances the performance of the adaptive tolerance-control system. Ranges of tolerance-control parameter values yielding the highest performance, according to the adopted criteria, are presented. View full abstract»

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  • Correlative Data Transmission with Coherent Recovery Using Absolute Reference

    Page(s): 108 - 115
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    A new data-transmission method that has twice the speed capability of the duobinary technique over an identical bandpass transmission medium is presented. Advantages of this method include promising performance and completely digital manner of signal generation, thus considerably reducing the circuit complexity. The system is based on correlative techniques in which the digital encoding forms an integral part of the carrier modulation. The process is sufficiently general to be applicable to both voice anti broadband communication channels. View full abstract»

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  • A High-Performance Digital Data-Transmission System

    Page(s): 115 - 119
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    A family of practical data-transmission systems, whose design is motivated by a desire to reduce the required energy per bit (for a specified noise power density) while operating in a well-defined part of the frequency spectrum, is described. The blocks ofjinformation bits that are periodically fed to the transmitter are encoded into equal energy waveforms whose design is based upon the use of truncatedsin(kt + b)/(kt + b)impulse response networks, together with ann-dimensional unit hypersphere code. The same waveform generation equipment is used for matched filtering at the receiver terminal. This design approach leads to equipment that performs well in the presence of impulse noise as well as Gaussian noise. View full abstract»

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  • Space DIversity on Line-of-Sight Microwave Systems

    Page(s): 119 - 133
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    Deep fading on line-of-sight microwave paths is usually a result of multipath interference phenomena. Frequency diversity has been the most common method of protecting against such fading. In the past few years, vertical space diversity has also come into use. This paper discusses its characteristics, methods of determining spacings, and describes some results achieved. It is concluded that vertical space diversity provides a very effective tool for combating multipath fading on line-of-sight paths, regardless of the source of the fading. View full abstract»

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  • Multiplex Telemetry Modulation and Demodulation Methods

    Page(s): 133 - 141
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    There are a wide variety of modulation methods among the recently used telemetry systems. Characteristics and capabilities of some of these methods are briefly described. There is no generally accepted "best" method. Requirements for conservation of radio spectrum have a strong influence on the choice of modulation parameters. New Inter-Range Instrumentation Group (IRIG) modulation standards have been adopted. Some parameter variations within the new FM-FM standards are presented. The spectrum and data capacity of IRIG PCM-FM are discussed. Various forms of phase-shift keying (PSK) are described. Needs for increased data bandwidth have led to the development of single-sideband subcarriers, used as SSB-FM. Coded transmission with coherent demodulation can provide better carrier signal energy efficiency, but has been rarely used. Noncoherent orthogonal codes appear worthy of further exploitation, being almost as efficient as coherent orthogonal but simpler to produce and detect. Literature references are included for further study. View full abstract»

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  • Construction and Performace of a PCM Frame Synchronizer with Self-Varying Threshold

    Page(s): 142 - 148
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    In the decommutation of PCM data, synchronization is accomplished by the detection of a coded word located at a known position in the message frame. It is the detection of the code word that is of interest here. This may be accomplished in a more efficient manner with a synchronizer using a self-varying threshold. This device is interesting in that no manual setting by the operator is necessary, and tedious calculation of the correct threshold setting is eliminated. Such a synchronizer has been built and is currently being tested. The object of the paper is two-fold. First, a description of the logic adopted to implement the synchronizer is given. This includes the solutions adopted to provide the window centered about the sync code position and the realization of the equivalent to a double threshold in a fixed-threshold synchronizer. Second, a description and summary of the results of tests, which provide a direct comparison of performance for fixed- and variable-threshold synchronizers and which verify the superiority of the latter, are given. View full abstract»

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  • Companded Delta Modulation for Telephone Transmission

    Page(s): 149 - 157
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    A new method of improving delta modulation with syllabic companding is presented. Companded delta modulation has a sort of syllabic companding function in its local decoder without depending on any nonlinear elements such as a variolosser. Analyses are made of the stationary companding law, dynamic response, overload characteristics, signal-to-noise ratio, overall transmission characteristics, etc. These results have been experimentally verified. The signal-to-noise ratio of the proposed companded delta modulation method is almost equivalent to that of a conventional seven-bit compandored PCM. According to the present analysis, even better performance can be obtained with the same clock frequency used for PCM: 56 kilobits per second. View full abstract»

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  • Companded Delta Modulation for Telephony

    Page(s): 157 - 162
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    An experimental delta modulation system that appears to satisfy Bell System telephone requirements in the telephone plant environment is described. A significant improvement in signal-to-noise ratio over a large dynamic range has been achieved with a modification that is akin to syllabic companding. It consists in adapting the modulator quantizing step size to the signal level over part of the dynamic range. The development was stimulated by the suitability of delta modulators for low-cost integrated circuits. The implementation is based on repeated use of two standard monolithic integrated circuits: an RTL logic gate and an operational amplifier. Several laboratory models have consistently satisfied Bell System transmission requirements with a bit rate of 96.5 kHz and a companding range of 26 dB. A series of subjective evaluation tests are being carried out at this time to assess listener reactions and to optimize system parameters accordingly. Indications are that a reduction of the bit rate will result from these tests. The delta modulation scheme is economically more attractive than pulse-code modulation (PCM) in telephone applications that require digital coding of the analog signal close to its source and with a minimum of common equipment. View full abstract»

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  • General Transmission Considerations in Telephone Conference Systems

    Page(s): 163 - 167
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    A general discussion of the transmission problems involved in telephone conference systems due to noise, echo, and singing is presented. The application of voice switched loss seems to be the best way to alleviate these difficulties. New problems must now be considered due to weak talkers and occasional simultaneous talking. Two types of conference connections are discussed: First, when three or more telephones are connected together, and second, when groups of people at two or more locations talk back and forth using microphones and loudspeakers. Recent work discussed here provides substantial transmission improvement for this type of service. View full abstract»

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  • A Multiport Telephone Conference Bridge

    Page(s): 168 - 172
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    Conference bridges allow a number of remotely located telephone subscribers to hold a conference. A new bridge using solid-state circuitry and a plug-in module-type equipment arrangement has been designed. The number of participants in a conference call varies. The capability and flexibility required have been achieved by designing a 30-port bridge that can be subdivided into five separate 6-port groups. These groups can then be combined to service large or small groups of subscribers. When large numbers of two-wire subscribers are connected together, the sum of the returning echoes is substantial. In the new bridge, each port is equipped with a voice activated gain switch that keeps a fixed amount of loss in each of the two-wire loops. View full abstract»

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  • A Six-Channel Station Carrier System

    Page(s): 172 - 184
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    There is an increasing need for a low-cost, reliable station carrier system for use in the outside telephone plant. The system design constraints of cable attenuation, including bridge taps, cable crosstalk, switching noise, and remote power feed considerations are discussed. The particular design choices that have been made with respect to the modulation plan, carrier transmission levels, line impedance matching, system power, and signaling are described for a six-channel station carrier system. A repeater is used with this system with nominal span loss of 35 dB at 112 kHz, but the loss may be as little as 20 dB to as much as 40 dB. No adjustments or building-out networks are required. The system is installed and maintained in operation without adjustment or alignment, because automatic level regulation and coordination is provided. View full abstract»

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  • Phase-Locked Loop Pull-In Range

    Page(s): 184 - 186
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    In this paper the derivation of a general equation for the pull-in range of a phase-locked loop is outlined. The equation is applied to a typical third-order loop, and algebraic results are derived in terms of the loop dc gain and network break frequencies. Expressions are given for the second-order terms which are usually neglected, so that an evaluation of the accuracy of results can be made. 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