Showing papers in "IEEE Transactions on Communications in 1968"

A Theoretical Study of Performance of an Orthogonal Multiplexing Data Transmission Scheme

Abstract: This paper presents a theoretical analysis of the performance of an orthogonal multiplexing data transmission scheme (parallel transmission scheme) subject to a number of degrading factors normally encountered by a practical operating system. The factors considered jointly are sampling time error, carrier phase offset, and nonideal phase characteristics of transmitting and receiving filters. Performance is measured by the familiar criterion of eye opening of the received data signal. A closed-form expression for the eye opening is obtained. It is shown that the lengthy nonlinear functions in the solution can be closely approximated by simple piecewise linear functions for parameter values of interest. The optimum settings of the sampling time and the carrier phase are determined for given filter phase distortion. Also, considering all factors, simple formulas are developed for computing interchannel interferences, intersymbol interference, and the resulting eye opening. Simple relationships between the eye opening and filter phase distortion are explored, and a concept of parametric eye is introduced to aid in filter design. An illustrative example is considered which shows that the impairment of the system performance is not excessive for a reasonable range of system design parameters.

Adaptive Feedback Communications

Abstract: 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.

Probabilities of Error for Adaptive Reception of M-Phase Signals

Abstract: Probabilities of error are derived for 2- and 4-phase signaling over an L -diversity branch, time-invariant, additive Gaussian noise channel and for M -phase signaling over an L diversity 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 all L -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 for M -phase signaling over an L -diversity branch, Rayleigh fading channel, 2) the explicit expressions for the probabilities of error for M -phase signaling systems operating over an L -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.

PCM Data Reliability Monitoring Through Estimation of Signal-to-Noise Ratio

Abstract: Reliability monitoring of digital pulse-code modulation (PCM) data in coherent transmission systems can be based upon online estimation of signal-to-noise ratio (SNR) at the receiver. Maximum likelihood SNR estimation is analyzed to determine the structure of the estimator and its ability to monitor error rates with a given degree of confidence. In particular, relationships between estimation confidence intervals, observation time, word error rates, and PCM word structure are developed.

Further Results on Nyquist's Problem in Pulse Transmission

Abstract: The Nyquist criterion specifies pulse shapes for a synchronous pulse amplitude modulation (PAM) signal which produce no intersymbol interference when the signal is sampled at the proper instants. For bandwidths in excess of one half the pulse repetition rate, the criterion is not uniquely satisfied. This paper considers the selection of pulse shapes for a single-sideband carrier signal which, in addition to satisfying the Nyquist criterion, will introduce minimal interference for small sampling time and carrier phase error at the receiver.

Cascade Decoding of Cyclic Product Codes

Abstract: In this paper, it is shown how to synthesize simple decoders for cyclic product codes. Cyclic product codes may be synthesized in the form of interlaced codes if the block lengths of the codes are relatively prime. In this case, the decoder can be synthesized in the form of a cascade of decoders, each of which operates on one of the subcodes forming the product code. The cascade decoders described differ from most of the decoders given in the coding literature in two major respects. First, they can be built. Second, instead of correcting all error patterns with weight less than some fixed value and no error patterns of greater weight, cascade decoders correct many error patterns beyond their guaranteed correction capability. Thus, the effective error-correction performance using a cascade decoder may be considerably beyond the usual Elias bound for product codes.

On the Probability of Error for Multichannel Reception of Binary Signals

Abstract: 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.

COMET--A New Meteor-Burst System Incorporating ARQ and Diversity Reception

Abstract: A system has been built to make efficient use of meteor-trail reflections for telegraph communications in the VHF band over distances up to 2000 kin. The information is transmitted over the circuit in both directions at an instantaneous rate of 2000 bauds whenever a radio path is established by reflection from a meteor trail. The control of the flow of information is not based on an assessment of the signal strength, as for instance in the JANET system, but on an error-detection scheme with automatic request (ARQ). This technique makes it possible to minimize starting delays and to carry on transmission until the end of the signal burst independently of the rate of decay. A special synchronization procedure has been developed to enable the system to cope with path length variations up to one character in length. The system also features frequency, space, and height diversity. For more than a year extensive measurements were carried out on a 1000-km path at frequencies in the 36- to 39-MHz band. The transmitter power was 200 watts and the antennas were five-element Yagis. Hourly average values of the traffic capacity were found to vary between four to eight 50-baud channels in the early morning and between one half to two channels in the late afternoon with the average over 24 hours approximately three channels. The error rate was less than one in 3000 characters for 90 percent of the time. Statistical data concerning bursts, interval between bursts, and delays in the transmission of messages are presented.

Unique Word Detection in Digital Burst Communications

Abstract: In a satellite time-division multiple-access (TDMA) system, the information is transmitted in bursts from the participating ground stations. In order to establish a time reference in each burst and to determine its origin, special code patterns called unique words (UW) are contained in the bursts. Reliable detection of the UW is the basis for keeping the stations in synchronism and providing high-quality communication links. The influence of miss and false detection of the UW on acquisition and the retention of a stable detection mode are investigated. The tolerable loss of information bursts provides criteria for the detector parameters, and for length and patterns of the UW codes. Practical results obtained during tests on an experimental system are compared with theory.

Cycle Slipping in a First-Order Phase-Locked Loop

Abstract: In this paper a model for a carrier plus narrowband noise is presented, and with its aid the expected number of cycle slipping events per second for a first-order phase-locked loop (PLL) driven by a narrowband noise corrupted carrier, both with and without a limiter, is determined. Experimental data are presented which indicate the extreme accuracy of the results obtained with the use of this model. The noise model, which is based on the hypothesis that steps of \pm2\pi in phase exist when a carrier is added to narrowband noise, yields not only computational simplicity in calculating the expected number of cycle slipping events per second in a PLL, but also a good deal of insight into the basic threshold improvement mechanism of the PLL. In particular it is shown that a cycle slipping event, or equivalently a PLL output "click" occurs not when the PLL "loses lock," but rather when it tracks a step of \pm2\pi appearing at its input.

Error Distribution and Diversity Performance of a Frequency-Differential PSK HF Modem

Abstract: A 4800-bit/s digital data modem is operated over a simulated HF channel to determine the bit-error rate and error distributions of the received serial binary data stream. The measured error distributions for multipath-limited conditions are compared with back-to-back operation and theoretically determined random distributions, and are also used to evaluate forward acting error correction assuming half-rate random error-correcting block codes. This improvement in bit-error rate is compared with that measured by operating the modem at 2400 bit/s with dual in-band frequency diversity. The error distributions for back-to-back operation approach random except for small block lengths where the systematic error resulting from the frequency-differential PSK coding appears. The error distributions for multipath-limited operation deviate from random especially for small multipath delay spreads (low biterror rates) where the correlation bandwidth is large. Forward error correction produces larger improvement factors as the code block length is increased. However, the improvement factor barely exceeds two orders of magnitude for the largest block considered. In fact, the burst nature of the errors is such that improvement factors actually decrease in most cases for multipath-limited operation as the raw bit-error rate decreases. On the other hand, inband diversity is more effective for small multipath delay spreads than for large ones. However, forward error correction appears to offer more potential especially if the bit errors can be spread over a greater number of bits.

Companded Delta Modulation for Telephone Transmission

Abstract: 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.

Performance Monitor Techniques for Digital Receivers Based on Extrapolation of Error Rate

Abstract: There is a need in many digital communication systems to estimate the current digital error rate of the receiver without the use of special transmissions and without interrupting traffic flow. Devices for performing this function have been termed performance monitor units (PMU). As an element of an adaptive communication system, the PMU could be used to determine when adaptive change is needed and, by comparing the error rates which would result from the various available choices of adaptation, to select the best change to be made at any time. This paper describes a PMU technique which is applicable to a wide class of digital modulation methods. Details are given for the cases of noncoherent frequency shift keying (FSK), differentially coherent phase shift keying (PSK), and coherent PSK receivers operating over a fading channel.

A Satellite Time-Division Multiple-Access Experiment

Abstract: An important inherent advantage of communications via satellite is that large numbers of earth stations, separated by long distances, can communicate with each other simultaneously. Thus a satellite provides a multiple-access capability by nature of its physical location. Earth stations can be multiplexed at the satellite by means of a variety of techniques. One technique is to use time-division multiple-access (TDMA) for satellite multiplexing. Field tests using the INTELSAT I (Early Bird) satellite have shown that TDMA is a practical method for using the satellite multiple-access capability efficiently. This paper describes the experimental system and the results of the field tests.

Space-Diversity Performance as a Function of Antenna Separation

Abstract: The reduction of multipath fading on line-of-sight microwave radio-relay links by dual space diversity is quantitatively described. One of the main results is a fade reduction factor F that specifies the improvement provided by space diversity as a function of the vertical separation of the receiving antennas, frequency, path length, and the relative size of the antennas in the diversity pair. The results are based on a theoretical analysis of recent experimental data.

Multichannel FSK and DPSK Reception with Three-Component Multipath

Abstract: The multichannel bit error probability for noncoherent FSK and differentially coherent PSK systems is obtained for a slow nonselective-fading multipath as a function of the multipath parameters and the order of diversity. The muitipath model involves three components consisting of two specular components and one scatter (diffuse) component. For diversity combining the multichannel noncoherent FSK receiver model employs square-law envelope addition and for DPSK employs linear baseband, postproduct-detection addition. These two diversity combining techniques are shown to be equivalent in terms of a DPSK noncoherent detector model that matches over two adjacent bits of the transmitted signal. As with Single-channel systems, the muitichannel DPSK performance is improved 3 dB over the multichannel FSK performance but otherwise equivalent. Results are given for muiltipath situations corresponding to a variety of differential path delays ranging from small to large delays and for mixed delays. Performance curves are presented for these multipath models and for several special cases representing simplified models.

On Optimum Digital Phase-Locked Loops

Abstract: This paper gives the design procedure of optimum digital filters for analog-digital phase-locked loops. The inputs considered are the step and ramp change in phase. The digital filters can easily be realized on the digital computer or otherwise. Design curves are given to choose proper noise bandwidth, sampling period, and loop parameters. It is shown that the optimum digitalanalog phase-locked loop is not the discrete version of the optimum continuous phase-locked loop.

Generation of Digital Signaling Waveforms

Abstract: 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.

Noise Loading Analysis of a Third-Order Nonlinear System with Memory

Abstract: This paper presents the noise loading analysis of a third-order nonlinear system characterized by Volterra kernels. The analysis is of practical interest since intermodulation distortion is often a limiting consideration in the design of solid-state long-haul broad-band frequency-division multiplexed transmission systems, and since speech on such systems is best represented by zero mean Gaussian noise. The analysis technique is amenable to digital computer calculation; as such, quantitive results are readily obtained. The analysis was applied to a single-stage transistor amplifier that was represented by a frequency-dependent nonlinear model. An excellent agreement between measured and calculated results was obtained. Portions of the program written for this example are readily applicable to the calculation of intermodulation distortion for any thirdorder nonlinear system (e.g., amplifier, repeater, or cascade of repeaters and cable sections) given the appropriate Volterra kernels.

Error Control Techniques Using Binary Symbol Burst Codes

Abstract: Much has been written on the theoretical description of error correcting codes but, due to a lack of actual channel error patterns, little has been said of practical performance. In this paper the performance of three types of error control is evaluated for the case of independent random errors and for an actual channel exhibiting dense bursts. The selected codes are burst codes with high probabilities of error detection and correction.

Companded Delta Modulation for Telephony

Abstract: 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.

Further Results Concerning the Effect of Frequency-Selective Fading on Differentially Coherent Matched Filter Receivers

Abstract: In this correspondence, error probabilities are calculated for binary DPSK communication through frequency-selective channels with predetection diversity matched filter reception. A method due to Bello and Nelin is used to carry out the calculations. Error probabilities are calculated for three different combinations of signaling pulse shape and channel frequency correlation function. One of these cases was investigated previously by Bello and Nelin, but some errors were made in their calculations. It is shown that for each of the three cases, an irreducible error probability exists that is proportional to the 2L power of the relative data rate (where L is the order of diversity) when the data rate is small. Also, when the results for the three cases are suitably compared, it is found that the error probabilities are more sensitive to the shape of the signaling pulse than to the shape of the channel frequency correlation function.

Subcarrier Tracking Methods and Communication System Design

Abstract: It is advantageous from power considerations to allow suppressed-carrier coherently-detected communications. Two methods for generating a coherent reference for the demodulation of a suppressed-carrier signal, namely, the squaring loop and the mathematically equivalent Costas loop, are analyzed, including at some points the effects of VCO noise and initial frequency detuning. The steady-state phase error probability distribution is presented, as is the expected time to first loss of lock in the first-order loop. Probabilities of error in coded or uncoded telemetry systems which use a squaring loop to generate a coherent subcarrier reference are investigated, allowing appropriate values of system parameters to be chosen by system designers.

Synchronization of a PCM Integrated Telephone Network

Abstract: 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.

Error Control in Duobinary Data Systems by Means of Null Zone Detection

Abstract: It is shown that duobinary data system performance can be improved at least 0.7 dB in the presence of Gaussian noise by using a null zone detection procedure. Most nulls can be replaced by correct values using the analog signal redundancy information. Nulls which cannot be replaced using the analog redundancy are restored to their most probable values on the basis of the received sample levels.

Maximum-Likelihood Receiver for Digital Data Transmission

Abstract: The maximum-likelihood (ML) receiver for binary data, additive Gaussian noise, and nearest-neighbor intersymbol interference (ISI) is shown to be a matched filter followed by a feedback loop and a tapped delay line. The loop and the tap connections contain nonlinear amplifiers, each saturating at the level of the ISI. This receiver minimizes the persymbol probability of error P e ; upper and lower bounds on P e are obtained. The mathematical receiver specifications for multilevel data and for extensive ISI are given; an approximate structure for the latter receiver is presented as an intuitive extension of the binary, limited ISI structure.

Intermodulation Distortion in Freqency-Division-Multiplex FM Systems--A Tutorial Summary

Abstract: In the design of FDM-FM communication systems, a great deal of importance is attached to an appreciation of those transmission irregularities that yield second- and third-order intermodulation spectra. Such spectra are readily generated by amplitude, phase, and AM/PM conversion anomalies within the FM transmission path. In this paper the results of the available journal literature have been reworked for presentation in a generalized form appropriate for use by equipment designers and system analysts. Intermodulation noise magnitudes and spectral distributions are expressed in terms of the transmission network and system modulation parameters with the AM/PM conversion characteristic extended to that of a second-order power series in radian frequency. To fully exploit the usefulness of the foregoing, the coherency between the intermodulation spectra of all distortion mechanisms is considered. The effects of this correlation is then exemplified for a tandemly interconnected set of distorters. It is found that all secondand third-order distortion mechanisms may be grouped under the classifications of: amplitude, delay, equivalent amplitude/delay, and quasi-equivalent amplitude/delay distortion. It is concluded that, as a result of the correlative structure of the distortion processes, an overall system measurement of intermodulation noise may exhibit an unexpected level of magnitude and/or have a spectral distribution significantly different from any of the inherent system distortions.

On the Selection of Signals for Phase-Locked Loops

Abstract: The performance of phase-locked loops and related signal tracking devices is dependent upon the cross-correlation function between the input signal and a locally generated signal. This paper is concerned with the problem of selecting both the input and local signals so as to minimize the tracking error due to additive noise.

Correlative Data Transmission with Coherent Recovery Using Absolute Reference

Abstract: 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.

The Improvement of Digital HF Communication Through Coding: II--Tandem Interleaved Cyclic Coding

Abstract: In previous work the technique of error correction of digital data through the use of interleaved cyclic codes and a set of probability functions for the evaluation of error patterns have been presented. In Part I of this paper [1] the performance of a wide range of Bose-Chaudhuri-Hocquenghem (BCH) codes and PMsymbol codes was evaluated on representative portions of the data. Here a method (identified as tandem interleaved cyclic coding) will be presented which allows for a significant increase in error-rate improvement at a reduction in the delay time introduced into the channel. It is demonstrated that it is possible to get almost I00 percent error correction for delays under three seconds for all channel conditions measured.