scispace - formally typeset
Search or ask a question

Showing papers on "Fading published in 1972"


Journal ArticleDOI
TL;DR: An urban multipath propagation experiment, involving the simultaneous transmission from a fixed site of 100-ns pulses at 488, 1280, and 2920 MHz and their reception at a mobile van, is described, and a statistical analysis of the data in the resulting multipath responses is given.
Abstract: An urban multipath propagation experiment, involving the simultaneous transmission from a fixed site of 100-ns pulses at 488, 1280, and 2920 MHz and their reception at a mobile van, is described. A statistical analysis of the data in the resulting multipath responses is given and used as a basis for a statistical model of urban multipath propagation.

820 citations


Journal ArticleDOI
J. Cavers1
TL;DR: A modulation system is proposed that continuously adjusts its data rate in response to signal strength variations in a fading channel, revealing a reduction on the order of 14 dB in transmitter power for a typical duplex link.
Abstract: A modulation system is proposed that continuously adjusts its data rate in response to signal strength variations in a fading channel. The optimum variation of data rate with channel conditions is determined, and includes the effects of feedback channel time delay, the interval between rate changes, and restriction of the number of allowable rates. Application of these results to a full duplex situation allows determination of the optimum fraction of the data stream to be devoted to service information. Comparison of this scheme with diversity transmission on the basis of error probability and bandwidth utilization reveals a reduction on the order of 14 dB in transmitter power for a typical duplex link.

282 citations


Journal ArticleDOI
TL;DR: In this paper, a general relationship is given for the probability of deep multipath fading which is linear in frequency, cubic in path length, and varies with meteorological-geographical factors.
Abstract: Signals at 4, 6, and 11 GHz, transmitted over a 28.5-mile radio relay path in Ohio, were continuously monitored during the late summer of 1966. Previous publications have reported on the observed 4- and 6-GHz multipath fading statistics, and on the improvements available with space or frequency diversity. This paper presents data for the 11-GHz transmission, and, in combination with the earlier results, establishes an empirical frequency dependence for the amplitude statistics. A general treatment of the relationships between the factors underlying multipath propagation is intractable. However, based on the results in this and other papers, a general relationship is given for the probability of deep multipath fading which is linear in frequency, cubic in path length, and varies with meteorological-geographical factors. Temporal aspects of the Ohio data were also investigated at all frequencies, utilizing both a 1-hour and a 1-day clock time interval. It was found that the multipath fade time statistic can be described by a single parameter for either interval. A subset of the multipath fading hours was also analyzed using a 1-minute clock interval, with the result that the difference between the minute median fade and the hourly median fade is frequency independent, and normally distributed with a standard deviation of 5.5 dB.

141 citations


Journal ArticleDOI
TL;DR: In this article, the spectral and temporal characteristics of a narrowband radio channel subject to multipath fading were estimated from a detailed sampling of channel loss variations, which was obtained during a 59-day experiment in which the amplitudes of a set of coherent tones spanning a band of 33.55 MHz and centered at 6034.2 MHz were continuously monitored.
Abstract: The spectral and temporal characteristics of a narrowband radio channel subject to multipath fading were estimated from a detailed sampling of channel loss variations. The data base for this characterization was obtained during a 59-day experiment in which the amplitudes of a set of coherent tones spanning a band of 33.55 MHz and centered at 6034.2 MHz were continuously monitored. The more significant observations were: (i) For fade depths less than 30 dB the frequency selectivity is accurately described by linear and quadratic components (in frequency) of amplitude distortion. The derived statistical distributions of such distortion parameters exhibit slopes of a decade of decrease in probability of occurrence for each 10 dB increase in distortion, for bandwidths greater than 5 MHz. (ii) For fade depths greater than 30 dB and bandwidths in excess of 5 MHz the amplitude distortion exceeds second order. (iii) Maximum observed rates of change for the linear and quadratic distortion were 90 and 60 dB/second, respectively.

33 citations


Journal ArticleDOI
L. Schiff1
TL;DR: In this article, three different switch diversity techniques are considered, and the improvements produced by each are calculated, and it is also shown that some are effective in combating an additional difficulty of the mobile-radio environment; this difficulty is that while the signal received in a moving vehicle is locally Rayleigh, the mean of this Rayleigh process changes slowly with time.
Abstract: In a small-cell mobile-radio system different base stations transmit different signals simultaneously, and at the same carrier frequency, to mobile vehicles in their respective cells. It is desirable that these cochannel stations be placed as close to one another as possible without creating undue interference. However, as a vehicle moves within a cell, both the desired and undesired signals exhibit Rayleigh (local) fading, creating a relatively high probability that the interference exceeds the signal. On the other hand, at the frequencies contemplated it is relatively simple to construct additional antennas that provide independently fading signals, thereby offering the use of diversity techniques. Three different switch diversity techniques are considered, and the improvements produced by each are calculated. It is also shown that some are effective in combating an additional difficulty of the mobile-radio environment; this difficulty is that while the signal received in a moving vehicle is locally Rayleigh, the mean of this Rayleigh process changes slowly with time.

31 citations


Journal ArticleDOI
TL;DR: Use of optimum channel encoding was shown to reduce considerably the channel capacity required to obtain a given speech quality, and the effect of the overload level of the quantizer on the system signal-to-noise ratio was determined.
Abstract: Isopreference tests were used to study the subjective effects of channel transmission errors on computer-simulated PCM and previous-sample feedback DPCM voice systems. Numerically scaled isopreference contours were obtained and plotted on planes having as axes the number of quantization bits N and the bit error probability p of the binary symmetric transmission channel. For any N and p considered, the quality of DPCM speech was found to exceed that of PCM speech. With N fixed and p decreasing, the improvement of DPCM relative to PCM increased to the point where quantization noise limited the performance of both systems. Two nonredundant codes, natural binary and folded binary, were used to encode the quantizer output. The two codes yielded comparable speech quality. The maximum subjective quality obtainable for white Gaussian and Rayleigh fading channels was determined as a function of channel capacity. Use of optimum channel encoding was shown to reduce considerably the channel capacity required to obtain a given speech quality. The subjective ratings were compared with measured system signal-to-noise ratios. Finally, the effect of the overload level of the quantizer on the system signal-to-noise ratio was determined.

14 citations


Patent
18 Apr 1972
TL;DR: A data diversity combining technique for a system for data transmission of blocks of data each made up of a combination of M distinct and mutually exclusive types of signal elements occurring at a fixed rate is presented in this paper.
Abstract: A data diversity combining technique for a system for data transmission of blocks of data each made up of a combination of M distinct and mutually exclusive types of signal elements occurring at a fixed rate, characterized in that an error indication signal is produced during any data signaling element interval during which the detection circuitry is incapable owing to noise, fading or other signal perturbation, to determine which of the M types of signal elements is present during the signaling element interval. The same data block is transmitted two or more times with each transmitted data block being stored separately, and the signaling elements and accompanying error signals from the various data blocks is combined in a logic circuit to form a new data block consisting of selected signaling elements. Each element is selected from those received during each transmission as the ones with no indicated error; if all have indicated errors an arbitrary choice is made so that any one of the M types of signal elements is selected whenever error signals are associated with all blocks of data.

13 citations


Patent
23 Aug 1972
TL;DR: In this article, a space diversity communication system is presented in which the effects of signal fading are reduced through the cooperative receipt by closely spaced antenna sensor elements and the processing in a hybrid network of signals originally transmitted as a single intelligence-modulated carrier, though susceptible of multiple path propagation.
Abstract: A space diversity communication system is presented in which the effects of signal fading are reduced through the cooperative receipt by closely spaced antenna sensor elements and the processing in a hybrid network of signals originally transmitted as a single intelligence-modulated carrier, though susceptible of multiple path propagation. Sum and difference signals are developed for analysis in a novel diversity signal selector or combiner. In one form, the latter may develop signals for electronically steering the system receptivity pattern so as to maximize the reception signal-to-noise ratio.

12 citations


Journal ArticleDOI
Sing-Hsiung Lin1
TL;DR: A new formulation and analysis is used to show that the major results of previous theoretical work are actually valid without the restrictive assumption of a joint Rayleigh distribution of diversity combining systems.
Abstract: This paper presents the results of a statistical analysis of diversity combining systems. Previous theoretical work on this topic often assumes that the input signals are jointly Rayleigh distributed, which may not hold for a practical fading environment. In this paper, we use a new formulation and analysis to show that the major results of previous theoretical work are actually valid without the restrictive assumption of a joint Rayleigh distribution. The statistics include the probability of fade, the expected number of fades per unit time, and the average fade duration. We also extend the analysis to include the effect of a dominant interfering signal such as the water-reflected ray of an overwater radio link. The statistics of diversity signals on these radio links are different and are not treated in the literature.

9 citations


Proceedings ArticleDOI
01 Jan 1972
TL;DR: The effects which multipath propagation, Gaussian noise, and implusive noise have on digital errors are analyzed and recommendations for improving the noisy, fading digital channel are made.
Abstract: This thesis is aimed at the problem of quickly and accurately communicating digital data between fixed and mobile (either vehicle-mounted or hand-held) radio stations. Digital error is of prime concern and the effects which multipath propagation, Gaussian noise, and implusive noise have on digital errors are analyzed. The importance of other analyses such as economic factors, interrogation procedures, or analog transmission might be peculiar to land mobile communication, but the recommendations for improving the noisy, fading digital channel should have broader application and a more general appeal.

6 citations


Journal ArticleDOI
TL;DR: An intermittent on-off noiseless feedback scheme for binary communication over the slow- and fast-fading Rayleigh channels is proposed and analyzed and upper and lower bounds on the error probability for block orthogonal M -ary communication are presented.
Abstract: An intermittent on-off noiseless feedback scheme for binary communication over the slow- and fast-fading Rayleigh channels is proposed and analyzed. At high energy-to-noise ratios, doubling the number of feedback iterations yields a 3-dB power saving for the slowly fading channel. Power savings ranging from 1 dB for one feedback iteration to 9 dB for 16 iterations are typical for the fast-fading model. Also for the fast-fading model, by picking the optimum number of forward transmissions for each value of energy-to-noise ratio, the best achievable performance requires approximately 7.5 dB more energy than the minimum predicted by the rate-distortion bound. Also presented is a feedback communication system for wide-sense stationary, uncorrelated-scatterer, fading, and dispersive forward and feedback channels. The model used for both forward and feedback channels is Kennedy's. Upper and lower bounds on the error probability for block orthogonal M -ary communication are presented for this system.

ReportDOI
01 Sep 1972
TL;DR: In this article, a first order mathematical model of ground reflection multipath is used to predict the relationship between the depth of fading and environmental parameters such as surface roughness and the terminal positions relative to earth.
Abstract: : C-0002AF-649LESDTR-72-216(*microwave communications, *diversity reception), multipath transmission, attenuation, l band, mathematical models, propagation, air-to-air, air-to-surface, airbornefrequencyThe signal power reduction due to multipath fading is an important design consideration in the development of air-air and ground-air communications links at L-band. A first order mathematical model of ground reflection multipath is used to predict the relationship between the depth of fading and environmental parameters such as surface roughness and the terminal positions relative to earth. The model is then used to investigate two techniques for reducing the loss in received signal power: frequency diversity and antenna height diversity. A measurement program to experimentally evaluate the applicability of antenna height diversity is outlined.

Journal ArticleDOI
TL;DR: In this paper, the first three cumulants of the phototelectric counts for a single detector and for an array of detectors are obtained for arbitrary ratios of counting time to source coherence time, β, while assuming the fading to be fully time correlated over the detection interval.
Abstract: In this paper we investigate the photocount statistics of mixed coherent plus Gaussian light that has suffered lognormal fading, such as produced by the turbulent atmosphere. The Gaussian component need not be stationary, may have an arbitrary spectral distribution, and the mean frequency of the coherent and Gaussian components need not coincide. The first three cumulants of the phototelectric counts for a single detector and for an array of detectors are obtained for arbitrary ratios of counting time to source coherence time, β, while assuming the fading to be fully time correlated over the detection interval. In particular, the cumulants are evaluated for Gaussian light of Lorentzian spectrum. The variation of the cumulants with degree of turbulence and detector separation is exhibited graphically, for several values of β, and ratio of coherent to chaotic component, y. The effect of increasing the degree of turbulence is shown to cause the ratio of the second‐order cumulant for Gaussian light to the same cumulant for coherent light to approach a nominal value of 2, indicating the extent to which the fading dominates the counting statistics. As the detector separation is varied, the twofold cumulant exhibits the spatial correlations of the turbulence, when the source radiation alone is assumed to be approximately spatially coherent at the detector array. Furthermore, the cumulants are shown to increase exponentially with the turbulence level σ, the log‐intensity standard deviation.


Journal ArticleDOI
TL;DR: In this paper, the authors used a bank of narrow filters to improve the signal-to-noise ratio (S2R) of radio signals from satellite Beacon-B when it was below the optical horizon.

Patent
29 Dec 1972
TL;DR: A DC analog signal fader employing a single operational amplifier permits fading onto a common output the level of a selected one of a plurality of input signals in a manner which does not introduce lag or step level changes at the output as discussed by the authors.
Abstract: A DC analog signal fader employing a single operational amplifier permits fading onto a common output the level of a selected one of a plurality of input signals. A newly selected signal is faded from the level of a next previously selected input and in a manner which does not introduce lag or step level changes at the output.

Journal ArticleDOI
TL;DR: Several expressions are derived for the probability of error of a binary coherent phase-shift keyed communication system in the presence of interference due to discrete multipath propagation.
Abstract: Several expressions are derived for the probability of error of a binary coherent phase-shift keyed communication system in the presence of interference due to discrete multipath propagation.

C. H. Chen1
01 Jan 1972
TL;DR: While the error performance of the receiver is worse than that of the ideal MFSK receiver, it is interesting to note that the performance degradation due to the multipath fading diminishes as the product of bit duration and IF filter bandwidth becomes large.
Abstract: Studies have shown that there is a severe signal fading at the turbulent atmosphere of a distant planet such as Venus. The data may be lost during deep fade. It is the objective of this paper to examine the effects of multipath fading on the performance of the noncoherent coded system which is used for the low data-rate space communications especially with the small probes. An exact error probability of the wideband noncoherent MFSK receiver is derived from using the 'Rician' channel model. While the error performance of the receiver is worse than that of the ideal MFSK receiver, as expected, it is interesting to note that the performance degradation due to the multipath fading diminishes as the product of bit duration and IF filter bandwidth becomes large.

Proceedings ArticleDOI
01 Jan 1972
TL;DR: In this paper, the effects of ionospheric irregularities on NASA satellite tracking and data acquisition operations are discussed. But the results of these observations are presented in both the auroral and equatorial regions.
Abstract: Discussion of some results of the effects of ionospheric irregularities on NASA satellite tracking and data acquisition operations. Ionospheric scintillation fading produced by irregularities has been observed at 136 MHz (vhf), 400 MHz (uhf), 1550 MHz (L-band) and 1700 to 2200 MHz (S-band). Details of these observations are presented. Vhf scintillation effects are evident in both auroral and equatorial regions. Fading effects decrease with increasing radio frequency in the auroral region. The same frequency dependence for fading is not observed in the equatorial region. Although there is a seasonal and diurnal character to scintillation in the equatorial region, fading effects are usually more severe than in the auroral region for a given radio frequency. Space diversity measurements indicate that reasonable solutions for vhf telemetry problems are available for either region. Space diversity should provide a solution for microwave frequencies as well. Ionospheric fading amplitude for 1700 MHz is relatively small in the auroral region. In the equatorial region amplitude fading levels for 1550-MHz signals from ATS-5 are often much larger than expected. Observations of the Apollo Lunar Surface Experiment Package (ALSEP) operating at 2300 MHz observed near the geomagnetic equator show fading peaks in excess of 15 dB.

01 Jan 1972
TL;DR: In this article, the authors compared the performance of computer simulated mobile radio systems with several demand profiies and selected the profiles to give an indication of the effects on system performance of either uncom- pensated changes in average traffic or normal fluctuations which occur in the randomly offered traffic even when the longterm average value does not change.
Abstract: signal output from the energy-density antenna system and that from a single unipole antenna are obtained as expressions which depend on the angle of incidence and the magnitude and the direction of vehicle velocity in a general way. From these general expressions the fading frequency and the maximum range of output variation are found. The extent of amplitude fluctuation for the energy-density antenna system is always smaller than that for the single antenna. When the incident Abstruct-The design of future mobile radio systems will be based upon an estimated user demand. Systems probably will be engineered to Serve the "average business day" spatial distribution of call attempts, which for the purpose of this study was assumed to be uniform. There will be day-to-day fluctuations about this long-term average, the magni- tudes of which are only to be conjectured at this time. This paper com- pares the performance of computer simulated mobile radio systems op- erating with several demand profiies. The profiles were selected to give an indication of the effects on system performance of either uncom- pensated changes in average traffic or normal fluctuations which occur in the randomly offered traffic even when the long-term average value does not change. The dynamic channel assignment systems increased the traffic capacity at low blocking levels even over a perfectly designed fiied channel assignment system. In addition, it was found that the dynamic channel assignment systems were relatively insensitive to the periodic spatial demand distributions studied by this computer simula- tion. Even though new call attempts were made to fluctuate markedly between adjacent base stations within a reuse interval, the blocking rate

Proceedings ArticleDOI
A. Vigants1
01 Dec 1972

Journal ArticleDOI
TL;DR: It is seen that limiting does not necessarily improve receiver performance, as is often thought, when the input is a fading binary frequency-shift-keyed signal.
Abstract: In this paper the performance of a radiometric receiver preceded by a bandpass limiter is studied when the input is a fading binary frequency-shift-keyed signal. An expression for the probability of error versus signal-to-noise ratio, with receiver bandwidth as a parameter is derived. It is seen that limiting does not necessarily improve receiver performance, as is often thought.

Journal ArticleDOI
Carson K. H. Tsao1
TL;DR: The successful application of space diversity to achieve a high propagation reliability over line-of-sight microwave links requires proper separation between diversity antennas, whereby the signals received at the antennas are sufficiently decorrelated.
Abstract: The successful application of space diversity to achieve a high propagation reliability over line-of-sight microwave links requires proper separation between diversity antennas, whereby the signals received at the antennas are sufficiently decorrelated. The multipath-propagation model is examined, and en expression is derived for the envelope correlation coefficient which is a function of antenna spacing and is dependent on the parameters of the troposphere.


Journal ArticleDOI
TL;DR: In this paper, the variation of drift velocity with fading frequency is examined by suitably analysing the fading records of three "closely spaced receivers" and the dispersion is found to be related to the random changes of irregularities in the ground pattern.
Abstract: Dispersion in drifts, i.e. the variation of drift velocity with fading frequency is examined by suitably analysing the fading records of three ‘closely spaced receivers’. The dispersion is found to be related to the random changes of irregularities in the ground pattern.