Showing papers on "Fading published in 1968"

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.

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.

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.

Characteristics of small-area signal fading on mobile circuits in the 150 MHz band

Abstract: Transmission on mobile radio circuits is often affected by standlng-wave breakup" resulting from spatial variations in rf signal amplitude, which to the moving mobile unit appear as rapid fades. Measurements made in the 150 mHz band confirm previous reports that within any small area such signal variations tend to follow the Rayleigh distribution. Statistical distributions have also been determined for the depths and widths of fades, in several different environments where fading was severe; rural, suburban and urban situations were found to produce rather similar results in-these respects. Some information is also given on the relation between median signal and circuit quality for speech, under fading conditions.

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.

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.

A Survey of Microwave Fading Mechanisms Remedies and Applications

Abstract: After a brief description of the significance of the radio refractive index and its variation in the lower troposphere, a catalog of fading mechanisms is presented. Attention is directed to the supporting refractive index structure, the characteristics of the fading signal, and the available remedies. The phenomena of multipath fading are described, and the theoretical amplitude distributions are presented. Diversity reception (frequency or space) is outlined for reducing the fading due to multipath. Specific expressions are given for determining the frequency or space separations. Successful remedies for microwave fading that are keyed to specific fading mechanisms also constitute a means of measuring the characteristics of the prevailing meteorological conditions. Specific expressions are given to accomplish this for the multipath and diffraction fading mechanisms.

Fading Characteristics of Thermoluminescent Lithium Fluoride

Abstract: In view of conflicting results on the fading characteristics of lithium fluoride, a careful reinvestigation has been made on possible fading of the commonly used phosphor, lithium fluoride TLD-100. Measurements over a twelve week period on freshly irradiated powder from a new batch, and also on some old powder, indicate fading of 5 ± 3%, most of which may be due to changes in sensitivity after annealing. Recommendations are made for the proper use of the phosphor in radiation dosimetry applications.

Relationship between complex- and envelope-covariance for Rician fading communication channels

Abstract: Relationship between complex covariance and covariances of envelopes of Rician fading communication channels

Optical Communications Experiments at 6328 A and 10.6 micro.

Abstract: Diagnostic optical communication experiments were performed comparing noncoherent and coherent detection techniques. Three different receiver–transmitter configurations with variable apertures were used during the experiments that were performed over a 1-km real atmospheric path. In every case, it was found that the coherent system fading, due to atmospheric turbulence, was considerably greater than the noncoherent system fading. This result shows the greater sensitivity of the coherent system to the time-varying wavefront breakup produced by atmospheric turbulence. A coherent homodyne experiment at 10.6 μ over a 2-km round-trip path was also performed. Its results indicated that a coherent system at 10.6 μ is less susceptible to atmospheric turbulence than a coherent system at 6328 A.

Space DIversity on Line-of-Sight Microwave Systems

Abstract: 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.

Response of an FM Discriminator to a Digital FM Signal in Randomly Fading Channels

Abstract: The calculation of the error rate resulting from the detection of a binary FSK signal using an FM discriminator-lowpass filter detector is described for an FM signal undergoing nonselective fading in additive Gaussian noise. The results extend the error rate calculations of Schilling et al. [1], [2] which were performed for a nonfading channel. While the technique employed is quite general, examples considering Rayleigh fading and an alternating mark-space sequence are given. Following a CCIR recommendation for Rayleigh fading, the percentage of time that the signal envelope is below a minimum level is ignored, since in this case the circuit would be completely cut off or switched to a better path. Thus, the results also depend on the depth of fade or ratio of the minimum signal amplitude squared to the average carrier power. The use of diversity is studied, and the error rate for a discriminator using predetection combining diversity is found. In this case the possibility of infinitely deep fades is considered. The analysis is based on the work of Nelson[9], [14], who considered signal-to-noise ratio performance of an FM discriminator with an analog FM signal undergoing nonselective fading. Experimental results are shown to verify the theory.

Electroanesthesia studies: a new current pattern and control of fading.

Abstract: HIS STUDY is part of a continuing inquiry T into the possibility of producing clinical anesthesia solely by the application of electrical current or currents. Every form of current we have used to date has presented some disadvantage in terms of its effects on test subjects. Unidirectional current-direct current and unidirectional square pulses causes iontophoresis and tissue injury. Bidirectional current in reasonable intensities does not injure tissue. Sine wave current, a bidirectional form, does not injure tissue but is severely limited by its constant ratio of average-to-peak current. This relationship is serious in electroanesthesia (EA ) studies; the peak current is critical, and with sine wave, high average current must be applied to achieve the required peak current.

Rough Surface Scattering on a Communication Link

Abstract: On a communication link between two terminals using essentially isotropic antennas, where at least one of the terminals is in the general vicinity of an extended rough surface, scattering from that surface introduces multipath fading which must be taken into account in assessing the performance of the link. Fading, of course, occurs only if the terminal or the surface changes position with time. This paper discusses the characteristics of fading under the assumption that the surface is rigid but the terminal is moving relative to the rough surface. The important systems parameters, including the depth and rapidity of fading, associated with a given communication link are evaluated. The extension to other links is indicated.

A Sequential Logic for Improving Signal Detectability in Frequency-Agile Search Radars

Abstract: Certain fading models have in the past become standard for taking into account the effects of fluctuations in target echoing area in the theoretical prediction of radar range performance. In the application of these models, fading has always been treated as an entirely random phenomenon. In this paper it is noted that this is not necessarily true for radars which have the transmission frequency as a controlled variable, since in that case the fading characteristics of the target may be modified by properly controlling the transmission frequency. A strategy for taking advantage of this possibility is developed and theoretical results obtained by Monte Carlo simulation for a specific logic indicate that gains in detectability of several decibels (relative to what is usually considered optimum) may be obtained when certain realistic assumptions are made for the target fading characteristics.

Design of Truncated Sequential Tests for Rapidly Fading Radar Targets

Abstract: Curves and equations are presented from which the exact performance of truncated sequential tests can be determined for one important case: the biased square-law detector for the detection of rapidly fading targets. The method of generating functions is used to derive probability distributions for sample size. It is shown how these probability distributions can be used to determine truncation errors and the effects of multiple-resolution elements. Sample calculations are performed to determine the effects of a particular truncation procedure.

Performance of selected block and convolutional codes on a fading HF channel

Abstract: This paper describes the application of selected block and convolutional coding techniques to digital transmission over an HF radio channel. Performance results are presented for interleaved binary Bose-Chaudhuri-Hocquenghem (BCH) codes, interleaved two-stage concatenated codes, and diffuse convolutional codes. The performance results are based upon raw error data recorded in transmission over a 640-km HF path, with various data runs representing the typical effects of frequency-selective and non-selective fading, atmospheric impulse noise, and interference from other users of the HF band. The performance of binary BCH codes with bit interleaving is presented and the selection of a code to meet a specified performance criterion is described. The sensitivity of specific codes to changing channel conditions is presented, along with the cost, in terms of interleaving, of designing for one set of channel conditions rather than others. Concatenated codes are discussed as a means of effective error control on channels with clustered errors. Performance data are presented for selected two-stage codes used with inner-stage word interleaving and the effects of varying certain of the code parameters are shown. Emphasis is placed on code designs that use the inner-stage code only for error detection and the outer-stage code mainly for erasure filling with only a small amount of symbol error correction. The impact of error clustering on the performance of concatenated codes is discussed. Two diffuse convolutional codes are evaluated with the use of recorded error data in computer simulations of their threshold-decoding algorithms. Performance of various configurations of these convolutional codes is given and a comparison is made with that of a simple block coding scheme having the same decoder storage requirements. The various coding techniques studied are compared on the basis of achievable performance as well as certain factors affecting the feasibility of implementation.

Performance of Analog Demodulators in a Fading Environment

Abstract: 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 Cramer-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.

Reception of pulse-amplitude-modulated signals transmitted over a dispersive fading channel with nonstationary statistics

Abstract: The optimum linear receiver is described for the reception of PAM signals over a dispersive, randomly time-varying channel which causes intersymbol interference. For a wide-sense stationary channel the receiver is relatively simple. The results also apply to the reception of multiplexed signals where the modulation scheme is linear.

A Performance Monitoring Scheme for FSK Transmission over Fading Channels

Abstract: A scheme for monitoring performance of a frequency-shift-keyed (FSK) transmission over a selectively fading channel with a nonfading component is treated. The design of the monitoring scheme is based heavily on the assumption that the received signal and additive noise are Gaussian. The monitoring requires no special test signal. An asymptotically unbiased estimate of error probability is obtained in terms of four measurements of the received signal. The measurement time required for a reliable estimate of error probability is determined.

Study of Scintillation Fading Effects on Satellite Communications in the UHF Band

Abstract: SATELLITE communications systems using the UHF range are subject to fading along the transionospheric path. These fadings are of the same nature as the amplitude scintillations which were first known from radio star observations. They are caused by irregularities of the electron distribution which varies in space and time.

Periodic Fading of VLF Signals Received over a Polar Path During Sunset and Sunrise

Abstract: VLF radio signals from the transmitter NPM/261 kHz have been recorded in Kiruna, Sweden The signals show very strong mode interference with frequent cycle slippage From the data it is concluded that by night the first and second order modes have almost exactly the same attenuation and a phase velocity difference of 051%, which is in agreement with Wait and Spies’ calculations for a nighttime ionosphere of height 90 km and a lower waveguide boundary of infinite conductivity (sea water) When the same model is used for the excitation factors the mode conversion factor is determined and is found to depend strongly on the angle between the shadow line and the great circle path

A Performance Monitoring Technique for Arbitrary Noise Statistics

Abstract: A technique for monitoring performance of a pilottone phase-shift-keyed system over fading channels is treated. An important feature of this technique is that it requires no prior knowledge of the statistics of the additive noise or interference. The error probability is estimated without interrupting the data transmission or sending a special test signal. The measurement time required for a reliable estimate of error probability is determined for both Gaussian and lognormal additive noise. It is shown that the necessary measurement time depends strongly on the statistics of the noise, the order of diversity, and the actual error probability.

Morphology of fading of radio waves traversing the auroral ionosphere.

Abstract: The morphology of scintillation fading is developed for HF and VHF radio waves which have traversed the ionosphere at high latitudes. Forms are indicated for the variation of the depth of fading with sub-ionospheric latitude, geomagnetic conditions, seasonal and diurnal time and solar cycle. Techniques are suggested for normalizing data obtained by different observatories with varying radio frequency or geometric parameters. Interdependence of several parameters indicates that a common cause controls the details of their behaviour.

Gain control for diversity receivers

Abstract: Previous work on optimum gain control is extended to an important class of diversity receivers used for digital data transmission through fading media and for radar. As in the single diversity case the optimum gain (which yields minimum average cost of receiver saturation) is extremely insensitive to relative costs of saturation at the upper and lower dynamic range bounds. The sensitivity to relative cost decreases as the order of diversity increases. Optimum gain and performance characteristics are given from which dynamic range requirements for diversity receivers can be deduced.

Fading from Irregular Surfaces for Line-of-Sight Communications

Abstract: This paper is concerned with the communications channel between a planetary flyby or orbiting spacecraft and an ejected probe that is traveling toward the planet. Since the mission requires that a significant part of the probe's transmitted energy be reflected from the irregular planet's surface, we will be concerned with the effect of the scattered signal for line-of-sight communications. The statistical distribution of the received field and the fading rate are considered so that the fading margin may be determined for some required probability of satisfactory performance. Typical examples are given for a Martian atmospheric probe.