Showing papers in "Bell System Technical Journal in 1974"

Codes which detect deception

Abstract: We consider a new kind of coding problem, which has applications in a variety of situations. A message x is to be encoded using a key m to form an encrypted message y = Φ(x, m), which is then supplied to a user G. G knows m and so can calculate x. It is desired to choose Φ(.,.) so as to protect G against B, who knows x, y, and Φ(.,.) (but not m); B may substitute a false message y' for y. It is shown that if the key can take K values, then an optimal strategy for B secures him a probability of an undetected substitution ≪ K−1. Several encoding functions Φ(.,.) are given, some of which achieve this bound.

Some properties of the erlang loss function

Abstract: This paper develops the properties of the Erlang loss function, B (N, a), used in telephone traffic engineering. The extension to a transcendental function of two complex variables is constructed, thus permitting the methods of complex analysis to be employed for the further study of its properties. Exact representations, Rodrigues formulas, and addition theorems are given both for the loss function and for the related Poisson-Charlier polynomials. Asymptotic formulas and approximations are developed for the loss function and also for its derivatives. A table of coefficients is included which, together with one of the asymptotic formulas, permits computation of B (N, a) by simple means even when the number of trunks, N, is very large. This same table is used to obtain ∂B(x, a)/∂x.

Source coding for a simple network

Abstract: We consider the problem of source coding subject to a fidelity criterion for a simple network connecting a single source with two receivers via a common channel and two private channels The region of attainable rates is formulated as an information-theoretic minimization Several upper and lower bounds are developed and shown to actually yield a portion of the desired region in certain cases

Low-loss single-material fibers made from pure fused silica

Abstract: Low-loss single- and multimode optical fibers were fabricated solely from pure fused silica. Their spectral losses corresponded closely to those of unclad fibers drawn from the same material, provided the cores of the single-material fiber preform were redrawn under pure conditions. The lowest steady-state loss of about 3 dB/km at a wavelength of 1.1 μm was obtained with a fiber 130 meters long that had a Spectrosil WF core. Experimental numerical apertures agreed excellently with theoretical predictions.

Scattering of a plane electromagnetic wave by axisymmetric raindrops

Abstract: This paper gives details of the analytical and num$eLrical procedures used to solve the basic problem of the scattering of a plane electromagnetic wave by an axisymmetric raindrop. A nonperturbative solution is obtained by expanding the scattered and transmitted fields in terms of spherical vector wave functions, so that Maxwell's equations are satisfied exactly in the regions exterior and interior to the raindrop, and by combining point matching with least-squares fitting to satisfy the boundary conditions on the surface of the raindrop with sufficient accuracy. Numerical results are presented for scattering by oblate spheroidal raindrops, with eccentricity depending on (and increasing with) drop size, for two orthogonal polarizations of the incident wave. The calculations were made at 4, 11, 18.1, and 30 GHz, in the case in which the direction of propagation of the incident wave is perpendicular to the axis of symmetry of the raindrop, which is of interest for terrestrial microwave relay systems. At 30 GHz, the calculations were also made for the case in which the angle between the direction of propagation and the axis of symmetry is 70° and 50°, since different elevation angles are of interest for satellite systems. These basic results were summed earlier over the drop-size distribution to calculate the differential attenuation and differential phase shift caused by rain, which are of importance in the investigation of cross polarization in radio communication systems.

Slab-coupled waveguides

Abstract: The slab-coupled waveguide, consisting of a dielectric rod lying on a slab that in turn covers a substrate, is a multidielectric waveguide that includes such special cases as the single-material fiber, the rib waveguide, and the strip-loaded film guide. These guides have recently become known as potentially useful either for long-distance optical transmission or for integrated optics. Simple, closed-form, approximate solutions have been found to describe the following properties of the guide: number of modes, their field configurations and propagation constants, numerical aperture, requirements for single-mode operation, field penetration in the slab, tolerance to curvature of the guide axis, dispersion, and impulse response.

Capacity of the Gaussian channel with memory: The multivariate case

Abstract: A formula is derived for the capacity of a multi-input, multi-output linear channel with memory, and with additive Gaussian noise. The formula is justified by a coding theorem and converse. The channel model under consideration can represent multipair telephone cable including the effect of far-end crosstalk. For such cable under large signal-to-noise conditions, we show that channel capacity and cable length are linearly related; for small signal-to-noise ratio, capacity and length are logarithmically related. Crosstalk tends to reduce the dependence of capacity on cable length. Moreover, for any channel to which our capacity formula applies, and for large signal-to-noise ratio, there is an asymptotic linear relation between capacity and signal-to-noise ratio with slope independent of the channel transfer function. For small signal-to-noise ratio, capacity and signal-to-noise ratio are logarithmically related. Also provided is a numerical evaluation of the channel capacity formula, using measured parameters obtained from an experimental cable.

Strip-loaded film waveguide

Abstract: Low-loss strip-loaded guides, consisting of 7059 glass film on fused quartz substrate with sputtered SiO 2 as the loading strip, have been investigated. The number of modes supported by the strip-loaded structures were determined experimentally and compared with the values predicted by the application of an equivalent index analysis. Agreement between theory and experiment is good in the case of the smaller number of modes which result from small loading, with the 7059 film thickness far away from cutoff.

A statistical analysis of telephone noise

Abstract: The results of a statistical analysis of telephone noise are presented. The analysis consists of two stages: an exploratory data analysis stage, where the data are characterized through various nonparametric statistics and a model-building stage, where the data are matched to models. The exploratory data analysis stage involved examination of noise waveforms, power spectra, and covariance estimates. The results show that telephone noise consists of a deterministic component (sinusoids at various frequencies) and a stochastic component. It is assumed that these components add. The data are filtered to remove the deterministic component. Next, central moment estimates are presented, as well as first-order amplitude statistics (histograms and empirical cumulative distributions) for these filtered data. The results indicate that the filtered data appear wide-sense stationary over short periods of time (typically 1 second) and, although close to gaussian, are distinctly nongaussian. The model-building stage involved fitting the filtered data to two classes of models. The first class of models is based on symmetric stable distributions that arise from the central limit theorem. The second class of models assumes two different physical processes that contribute to the random component of telephone noise: The low-variance process is assumed to be gaussian, while the high-variance component is assumed to be a filtered Poisson process. Both classes of models agree intuitively with the physical processes generating telephone noise and are mathematically tractable. Based largely on graphical tests, both models appear to fit the filtered data reasonably well.

Rain-induced cross-polarization at centimeter and millimeter wavelengths

Abstract: Rain-induced cross-polarization is an important factor in design of dual-polarization microwave radio communication systems. We present current estimates of this effect based upon calculated differential characteristics of canted oblate raindrops and their relationship to experiments. Measured differential attenuation and cross-polarization, mainly at 18 GHz, are used to determine two empirical parameters: an effective average of the absolute value of the canting angle and a measure of the imbalance between positive and negative canting angles. We can then provide estimates for median values of cross-polarization discriminations at other frequencies; these are found to agree fairly well with available measured data. Differential phase shift is the dominant factor in the rain-induced cross-polarization at frequencies below about 10 GHz, and differential attenuation becomes increasingly important at higher frequencies. For a given rain fading, the cross-polarization decreases with increase in frequency and is relatively insensitive to the rain rate, whereas for a given amount of rain the cross-polarization increases with frequency up to about 35 GHz. The cross-polarization discrimination of circularly polarized waves is much poorer than that of linearly polarized waves. When the angle α between the direction of propagation and the axis of symmetry of oblate raindrops is not equal to π/2, as on earth-space paths in satellite communication systems, the differential attenuation and differential phase shift can be approximated by sin2 α times those for α = π/2, which is the condition for terrestrial paths.

On the behavior of minimax fir digital Hilbert transformers

Abstract: Optimum (in a minimax sense) linear phase FIR Hilbert transformers can be designed efficiently using a Remez optimization procedure. This paper presents useful design data on wideband Hilbert transformers with even and odd values of N, the impulse response duration (in samples) of the filter. Based on these data, the following observations can be made: (i) In the case of equal lower and upper transition regions, Hilbert transformers with odd values of N can be realized more efficiently than those with even values of N, assuming the same peak errors of approximation for both cases. This is because every other impulse response sample is exactly zero for odd values of N. (ii) The peak approximation error for Hilbert transformers with odd values of N is determined primarily by the minimum of the values of the lower and upper transition widths. (iii) The peak approximation error for Hilbert transformers with even values of N is determined primarily by the lower transition width of the filter. (iv) The smaller the bandwidth of the Hilbert transformer, the faster the decrease of peak error of approximation with decreasing bandwidth of the Hilbert transformer. (v) The larger the value of N, the faster the decrease of peak approximation error with decreasing bandwidth of the Hilbert transformer. These observations lead to the conclusion that the bandwidth of the Hilbert transformer should be made as small as possible, and that odd values of N should be used, whenever possible, for efficient direct form realizations. A set of tables of values of the impulse response coefficients is included for several different bandwidth Hilbert transformers and for both even and odd values of N.

Some comparisons between fir and iir digital filters

Abstract: The purpose of this paper is to make comparisons between optimum, linear phase, finite impulse response (FIR) digital filters and infinite impulse response (IIR) digital filters which meet equivalent frequency domain specifications. The basis of comparison is, for the most part, the number of multiplications per sample required in the usual realizations of these filters — i.e., the cascade form for IIR filters, and the direct form for FIR filters. Comparisons are also made between group-delay equalized filters and linear phase FIR filters. Considerations dealing with finite word-length effects are discussed for both these filter types. A set of design charts is also presented for determining the minimum filter order required to meet given low-pass filter specifications for both digital and analog filters.

Outage of the l4 system and the geomagnetic disturbances of 4 august 1972

Abstract: An outage of the Plano, Illinois, to Cascade, Iowa, link of the L4 coaxial cable occurred at about 2240 UT on 4 August 1972 during a large geomagnetic storm. The available geomagnetic data measured in North America, as well as data received from two satellite instruments, are analyzed. These data show that, at the time of the L4 outage, the boundary of the magnetosphere was pushed to unusually low altitudes by a greatly enhanced solar wind. As a result, large, rapid changes of the earth's magnetic field strength were observed over North America. It is demonstrated that the field changes at about 2241 to 2242 UT were of such magnitude as to induce earth currents of sufficient strength to produce the L4 outage by causing a high-current shutdown of the system link. The geomagnetic disturbances that produced the shutdown were not of the auroral-electrojet type normally associated with disruptions of power systems.

On the behavior of minimax relative error fir digital differentiators

Abstract: Optimum (in a minimax relative error sense) linear phase FIR digital differentiators can be designed in an efficient manner using a Remez optimization procedure. This paper presents data on wideband differentiators designed with even and odd values of N, the filter impulse response duration in samples. Based on these data, several interesting observations can be made, including: (i) Differentiators with even values of N have peak relative errors which are approximately one to two orders of magnitude smaller than identical bandwidth differentiators with odd values of N, and with the same number of multiplications per sample in a direct convolution realization. (ii) The smaller the bandwidth of the differentiator, the faster the decrease of the peak relative error with increasing N. (iii) The larger the value of N, the faster the decrease of the peak relative error with decreasing bandwidth. These observations lead to the conclusions that the bandwidth of a differentiator should be made as small as possible, and that even values of N should be used whenever possible. Complete tables of values of the impulse response coefficients are included for several wideband differentiators for even and odd values of N.

B.S.T.J. brief: Optical waveguides with very low losses

Abstract: Low-loss optical fibers may be necessary for economical optical tranamieeion sy.stema. We have developed fibers that exhibit losses of lese than 2 dB/km. at 1.06 µm. The fibers were made by a chemical vapor deposition (CVD) technique that employs simultaneous reaction and fwdon to a clear glassy core material.

Input amplifiers for optical PCM receivers

Abstract: This paper describes the noise performance of input amplifiers for optical pulse-code-modulation repeaters. The noise is treated in terms of an effective noise generator in parallel with the photocurrent induced in the detector and the effective noise, in turn, is related to error performance. The analysis applies to both conventional and integrating front ends. Both field effect and bipolar transistor amplifiers are treated. For the latter, an optimum bias current that minimizes the effect of thermal noise is derived. Finally, predicted and measured performance are compared for silicon field-effect transistor input amplifiers at 6.3 Mb/s and 50 Mb/s, and for bipolar transistor and GaAs field-effect transistor input amplifiers at 274 Mb/s.

Lamp: System description

Abstract: A general description of the Logic Analyzer for Maintenance Planning (LAMP) system is presented. LAMP is a digital-logic simulation and analysis system used for logic-design verification, for generation and evaluation of fault-detection and diagnostic tests, and for generation of the trouble-location manual (or fault dictionary) data. It is implemented on the IBM 360/370 TSS and OS machines (for both interactive and batch operations), and has been in active use at Bell Laboratories in the development of electronic switching systems, data set facilities, transmission equipment, and advanced integrated circuit technologies.

An optical repeater with high-impedance input amplifier

Abstract: A 6.3-Mb/s repeater for fiber optic communication systems is described which incorporates a high-impedance input amplifier. It is shown that by utilizing an input circuit with a time constant which is long compared to the bit interval and equalizing after the signal has been sufficiently amplified to set the signal-to-noise ratio, thermal noise can be decreased. As a result, a reduction can be realized in the required signal and, with an avalanche detector, in the optimum gain. The repeater, which was realized in a compact form employing standard integrated circuits, utilizes a GaAs light-emitting diode as its optical source. Other features include automatic gain and threshold controls and recovered timing.

Lamp: Logic-circuit simulators

Abstract: The algorithms used for logic-circuit simulation in the Logic Analyzer for Maintenance Planning (LAMP) system are described. Several simulators are available to allow a cost-effective tradeoff between simulation cost and the level of detail needed for a particular application. The true value simulator provides efficient simulation of fault-free logic circuits. Two fault simulators simulate the classical stuck-at faults as well as shorted-gate-output faults. Hyperactive faults, those faults which cause an inordinate amount of simulation activity, are discussed along with their impact on simulation time. A four-value simulation logic is described which simplifies circuit initialization procedures.

Transverse coupling in fiber optics part III: Bending losses

Abstract: A general method is proposed to evaluate the radiation loss of bent open waveguides. This method consists in evaluating the coupling between the waveguide mode and the whispering-gallery modes that can be associated with the surrounding medium. The expression obtained for a reactive surface coincides with a previous result by Miller and Talanov.1 We investigate in detail the radiation loss of the fundamental (HE 11 ) mode of a dielectric rod coupled to a slab. This arrangement, described in Part II of this article series,2 provides a useful mode-selection technique. The radiation loss is given by a simple closed-form expression. We find that for a wavelength of 1 µm and a rod radius of 5 µm in physical contact with the slab, the bending loss is less than 1 dB/km if the radius of curvature, in the plane of the slab, exceeds 22 mm.

Rain attenuation on short Radio paths: Theory, experiment, and design

Abstract: The outage time on short radio paths at frequencies above 10 GHz can be estimated from distributions of point rain rates derived from U. S. Weather Service rain gauge charts. In this paper, a previous theory is extended to include the effects of path length and frequency. Experimental corroboration is presented for three locations on the east coast of the United States, and the design of short radio paths is illustrated by examples. One-minute rain rate distributions for 20 locations are also included.

Combining intraframe and frame-to-frame coding for television

Abstract: A method of frame-to-frame coding is proposed in which the changes from one frame to the next are first detected and then transmitted as an intraframe coded signal rather than as frame-to-frame differences. A coder was constructed to test the proposal using DPCM for the intraframe encoding. Three aspects of the coder design presented particular problems. They were: (i) Movement detection (as a result of the increase in frame-to-frame noise caused by the intraframe coding). (ii) Smooth reduction of bit-rate and picture quality so as to take advantage of the reduction in spatial quality that a viewer tolerates when areas are moving fast. (iii) Control strategy for linking the operation of the buffer, the movement detector, and the operating state of the coder. The coder gave good picture quality at a transmission rate of 1.5 megabits per second (0.75 bit per picture element), except in extreme situations where the moving area covered almost the entire screen. The performance is described in detail at bit rates of 2.0, 1.5, and 0.5 megabits per second. The experimental coder has a number of desirable properties from an overall systems point of view when compared with transmission of frame differences. These include high tolerance to transmission errors and small frame storage requirements.

Interfacial dopants for dual-dielectric, charge-storage cells

Abstract: When a suitable interfacial dopant, such as W, is introduced at the interface between the dielectrics of a DDC cell, the write-erase characteristics of the cell are greatly improved The useful range of the dopant concentration is determined to lie between about 1014 to 1015 atoms/cm2 The interfacial dopant allows the fabrication of a DDC cell with relatively thick SiO 2 layers (> 50 A) The result is a substantially permanent memory cell that can still be subjected to electrical write-erase at reasonable gate-voltage conditions

Spectra of digital phase modulation by matrix methods

Abstract: We derive the spectral density of a sinusoidal carrier phase modulated by a random baseband pulse train in which the signaling pulse duration is finite and the signaling pulses may have different shapes. The spectral density is expressed as a compact Hermitian form in which the Hermitian matrix is a function of only the symbol probability distribution, and the associated column vector is a function of only the signal pulse shapes. If the baseband pulse duration is longer than one signaling interval, we assume that the symbols transmitted during different time slots are statistically independent. The applicability of the method to compute the spectral density is illustrated by examples for binary, quaternary, octonary, and 16-ary PSK systems with different pulse overlap. Similar methods yield the spectral density of the output of a nonlinear device whose input is a random baseband pulse train with overlapping pulses.

Transverse coupling in fiber optics part I: Coupling between trapped modes

Abstract: Two perturbation formulas have been proposed to evaluate the coupling between parallel optical waveguides, one involving a line integral and the other a surface integral. They are shown to be identical. The former expression is preferred because of its greater simplicity. The case of two parallel lossy dielectric slabs is discussed as an example.

Theory of noise in charge-transfer devices

Abstract: The noise introduced into charge packets transferred through and stored in charge-transfer devices is calculated in a manner that includes all important relaxation, suppression, and correlation effects. First, the noise induced into each packet during each transfer phase from thermal, trapping, emission-current, and leakage-current fluctuations, whose statistics are nonstationary, and from clock-voltage fluctuations, whose statistics are stationary, is determined. Relaxation of the transferring charge to these fluctuations is found to suppress their size. Second, the accumulation (collecting) of the noise as each packet is transferred through the device is calculated neglecting the role of incomplete charge transfer. Attention is drawn to the significant differences between the collecting of storage-process noise, which is unsuppressed, transfer-process noise, whose spectral density is nearly totally suppressed at low frequencies, and modulation noise, which is nearly totally suppressed for digital and analog signals. Third, the role of incomplete charge transfer in suppressing the collecting of the noise is shown for digital signals and indicated for analog signals. We conclude with a numerical calculation of the maximum possible signal-to-noise ratio that can be expected from charge-transfer devices. The presentation is sufficiently general and detailed that, with a minimum of background in formal noise theory, one can use the approach to evaluate noise in many novel, solid-state devices.

Statistical properties of a sum of sinusoids and Gaussian noise and its generalization to higher dimensions

Abstract: This paper investigates the statistical properties of the sum, S, of an n-dimensional Gaussian random vector, N, plus the sum of M vectors, X 1 , …, X M , having random amplitudes and independent arbitrary orientations in n-dimensional space. We derive expressions for the probability density function (p.d.f.) and distribution function (d.f.) of S and of its length, |S|, as series expansions involving only the moments of |X i |, i = 1, …, M. In addition, we find the p.d.f. and d.f. of the projection of S onto 1-dimensional space. Our results are generalizations of the n = 2-dimensional problem of finding the statistical properties of a sum of constant-amplitude sinusoids having independent uniformly distributed phase angles plus Gaussian noise. The latter problem has been treated by Rice1 and Esposito and Wilson,2 but our results can also deal with sinusoids having random amplitudes. When n = 3, our findings treat, in the presence of a Gaussian vector, the classical problem of “random flights” dating back to Rayleigh. Some calculations for the 2- and 3-dimensional problem are presented, and an application to coherent phase-shift-keying communications systems is discussed.

Performance models of an experimental computer communication network

Abstract: This paper reports the results of a performance study of an experimental computer communication network. The network is currently being designed and built in order to test concepts and techniques that may find future application. The network consists of synchronous digital transmission lines connected in loops to a Central Switch. User traffic enters the system through multiplexers connected to the synchronous lines. The Central Switch has the two-fold function of routing and controlling traffic. Two multiplexing techniques were examined, Demand Multiplexing (DM) and Synchronous Time Division Multiplexing (STDM). In both techniques, user messages are blocked into fixed size packets, prior to multiplexing on the line. The synchronous line can carry these packets at a maximum rate of 4000 packet slots per second. In STDM each terminal is assigned a packet slot which recurs periodically. In contrast, for DM, packets are multiplexed on the line asynchronously into unoccupied packet slots. Alternative implementations of the DM technique were studied, one where each terminal transmits and receives at a maximum rate of 4000 packets per second and another where the maximum rate is 2000 packets per second. As part of its message-handling function, the Central Switch buffers messages in transit. This allows User Terminals to transmit and receive messages with a degree of independence from one another. However, the terminals' strategy affects the amount of storage required in the Central Switch. In order to prevent the loss of information when there is insufficient buffering, there is a mechanism to inhibit traffic from User Terminals when the Central Switch buffer is near overflow. Due to this control of traffic, there is a relationship between the amount of data that flows through the switch and the amount of buffering in the switch.

Coupling of nearly degenerate modes in parallel asymmetric dielectric waveguides

Abstract: The coupling of modes in two parallel dielectric waveguides is studied. The individual waveguides are assumed to be asymmetric and unlike each other. If the individual waveguides support modes with nearly equal propagation constants β 2 and β 4 = β 2 + 2Δ, then the double waveguide system will support two new modes with propagation constants β_ = β 2 − Δbar; and β + = β 4 + Δbar;. The shift Δbar; is related to Δ and to the shift Δ which would occur if the original modes were degenerate; Δ is expressed in terms of the parameters describing the asymmetric double waveguide system. The field distributions of the new modes are approximately even and odd combinations of those of the original modes in the isolated waveguides; the relative amplitudes with which they are combined depend upon the amount of mismatching Δ. As the modes travel down the waveguide system, they partially cancel and add, thus transferring power. A power transfer ratio F is defined and is shown to decrease rapidly as Δ/Δ increases. The beat length L depends upon both Δ and Δ/Δ it also decreases as Δ/Δ increases. A numerical example is given to illustrate the effects of mismatching and to demonstrate the feasibility of constructing a mode-coupling device. Possibilities of tuning the device to reduce mismatching are discussed.

Spectrum of a binary signal block coded for DC suppression

Abstract: This paper analyzes a block-coding scheme designed to suppress spectral energy near f = 0 for any binary message sequence. In this scheme, the polarity of each block is either maintained or reversed, depending on which decision drives the accumulated digit sum toward zero. The polarity of the block's last digit informs the decoder as to which decision was made. Our objective is to derive the average power spectrum of the coded signal when the message is a random sequence of + 1's and −1's and the block length (M) is odd. The derivation uses a mixture of theoretical analysis and computer simulation. The theoretical analysis leads to a spectrum description in terms of a set of correlation coefficients, {p q } = q = 1, 2, etc., with the p q 's functions of M. The computer simulation uses FFT algorithms to estimate the power spectrum and autocorrelation function of the block-coded signal. From these results, {p q } is estimated for various M. A mathematical approximation to p q in terms of q and M is then found which permits a closed-form evaluation of the power spectrum. Comparisons between the final formula and simulation results indicate an accuracy of ±5 percent (±0.2 dB) or better. The block-coding scheme treated here is of particular interest because of its practical simplicity and relative efficiency. The methods used to analyze it can be applied to other block-coding schemes as well, some of which are discussed here for purposes of comparison.