Showing papers on "Fading published in 1977"

Mobile fading—Rayleigh and lognormal superimposed

Abstract: A closed solution is presented showing the composite probability distribution of power levels derived from short term Rayleigh fading with superimposed long term lognormal variations of mean value. An example shows how the results can be applied to the prediction of bit error rates in a mobile radio data transmission channel, and how the error rate will vary with standard deviation of the lognormal distribution.

Radio propagation for vehicular communications

Abstract: Radio propagation is affected by many factors including the frequency, distance, antenna heights, curvature of the earth, atmospheric conditions, and the presence of hills and buildings. The influence of each of these factors at frequencies above about 30 MHz is discussed with most of the quantitative data being presented in the series of nomograms. By means of three or four of these charts an estimate of the received power and the received field intensity for a given point-to-point radio transmission path ordinarily can be obtained in a minute or less. The theory of propagation over a smooth spherical earth is presented in a simplified form that is made possible by restricting the frequency range to above about 30 MHz where variations in the electrical constants of the earth have only a secondary effect. The empirical methods used in estimating the effects of hills and buildings and of atmospheric refraction are compared with experimental data on shadow losses and on fading ranges.

Theoretical and Measured Performance of a DFE Modem on a Fading Multipath Channel

Abstract: A decision-feedback equalizer (DFE) is the basis of a recent development of a quadruple diversity troposcatter modem which can operate up to a data rate of 12.6 Mbit/s in a 99% bandwidth of 15 MHz. In this paper a theoretical approach is developed for the calculation of average bit error rate (ABER), including the effects of intersymbol interference due to multipath and the finiteness of the transversal filters used to realize the DFE. By omitting the intersymbol interference effect, the calculation provides a lower bound which can be used to assess the intersymbol interference penalty for a particular DFE structure. The paper includes calculations of a DFE configuration which has a three tap forward filter with tap spacing equal to one-half a symbol interval. Measured performance results from fading channel simulator tests of a three tap forward filter DFE are presented for data rates from 1.5 to 12.6 Mbit/s and for a wide range of multipath statistical conditions. The results for this DFE configuration show (1) excellent agreement between calculated and measured ABER, (2) a small intersymbol interference penalty when the 2σ multipath spread is less than approximnately one-half the data symbol interval, and (3) successful operation at values of multipath spread up to twice the data symbol interval. In a sequel to this paper, the results of a field test of the DFE modem are presented. These live links test results are consistent with both the calculated and simulator measured data presented here.

Theoretical and measured performance of a DFE modem on a fading multipath channel

Abstract: A decision-feedback equalizer (DFE) is the basis of a recent development of a quadruple diversity troposcatter modem which can operate up to a data rate of 12.6 Mbit/s in a 99% bandwidth of 15 MHz. In this paper a theoretical approach is developed for the calculation of average bit error rate (ABER), including the effects of intersymbol interference due to multipath and the finiteness of the transversal filters used to realize the DFE. By omitting the intersymbol interference effect, the calculation provides a lower bound which can be used to assess the intersymbol interference penalty for a particular DFE structure. The paper includes calculations of a DFE configuration which has a three tap forward filter with tap spacing equal to one-half a symbol interval. Measured performance results from fading channel simulator tests of a three tap forward filter DFE are presented for data rates from 1.5 to 12.6 Mbit/s and for a wide range of multipath statistical conditions. The results for this DFE configuration show (1) excellent agreement between calculated and measured ABER, (2) a small intersymbol interference penalty when the 2σ multipath spread is less than approximnately one-half the data symbol interval, and (3) successful operation at values of multipath spread up to twice the data symbol interval. In a sequel to this paper, the results of a field test of the DFE modem are presented. These live links test results are consistent with both the calculated and simulator measured data presented here.

Probability of Block Error for Very Slow Rayleigh Fading in Gaussian Noise

Abstract: Expressions for probability of block error, P_{f}(M,N) , the probability of more than M errors in a block of N digits, are derived for binary signaling over a channel with very slow nonselective Reyleigh fading and additive Gaussian noise. The analysis is applicable to noncoherent FSK, coherent FSK, and coherent PSK signaling. An asymptotic expression for the block error probability, suitable for high signal-to-noise ratio, is also derived. Computations of P_{f}(0,N) are presented for a broad range of error probabilities and block sizes, and the characteristics of block error probabilities for the fading and nonfading cases are compared.

The effect of having unequal branch gains practical predetection diversity systems for mobile radio

Abstract: A review of practical predetection diversity techniques for mobile radio is presented. Theoretical carrier-to-noise distributions for selection combining, equal-gain combining, and maximal ratio combining in two-branch predection diversity systems are derived as a function of gain unbalance between branches. Experimental results are also presented from an equal-gain system, including predetection level statistics and post detection baseband average noise measurements as a function of gain unbalance. It was found that unbalances as high as 10 dB can be tolerated with equivalent input signal-to-noise degradations less than 3 dB for equal-gain and maximal ratio systems. Selection diversity degrades much faster than equal-gain or maximal ratio, but significant diversity improvement is given for unbalances less than 6 dB.

Voice and data transmission in a mobile radio channel at 850 MHz

Abstract: This paper is a review of those properties of a communications channel in the 850 MHz band that have importance to the quality of voice and data transmission for frequency modulated land mobile radio. The presentation is made with emphasis on the simpler aspects and consequences of the propagation environment encountered. The distribution of fading amplitude and rate and the distribution of fade durations are described such that a qualitative impression is gained of the effect of fades on voice transmission, and a quantitative description is obtained of their effect on data transmission.

A spread spectrum technique for high capacity mobile communications

Abstract: A spread spectrum technique for cellular mobile communication systems is proposed and the results of an analytical study of this technique are summarized. It is found that degradation due to rapid fading is substantially reduced and that, with adequate power control, the user density in a high-quality small cell system may be significantly greater than with conventional FH systems. Furthermore, unique addresses may be permanently assigned to each user and no channel switching is required as the user moves from cell to cell.

Impact of microwave depolarization during multipath fading on digital Radio performance

Abstract: Experimental data describing the statistics of microwave depolarization during multipath fading have been obtained from a propagation experiment conducted near Atlanta, Georgia. The experiment included 6- and 11-GHz reception on a 26.4-mile path, and 11-GHz reception on a 15.9-mile path. A theoretical model, suggested by T. O. Mottl, indicates that the interference occasioned by depolarization for a given copolarized signal level is Rice-Nakagami distributed. The theoretically calculated distribution agrees well with the data. The cross-polarization interference consists of a signal-level-dependent component as well as a residual that is independent of the in-line signal level. The residual is Rayleigh distributed with an rms value about 40 dB below the nonfaded in-line signal level, and limits the multipath fade margin of a cochannel dual-polarized digital radio to approximately 30 dB. Calculated multipath outage probabilities for cochannel, dual-polarized, 11-GHz, quaternary-coherent-phase-shift-keyed digital radios with and without space-diversity protection are presented.

Performance of DPSK with Convolutional Encoding on Time-Varying Fading Channels

Abstract: The bit error probability performance of a differentiallycoherent phase-shift keyed (DPSK) modem with convolutional encoding and Viterbi decoding on time-varying fading channels is examined. We consider both the Rician and the lognormal channels. Bit error probability upper bounds on fully-interleaved (zero-memory) fading channels are derived and substantiated by computer simulation. It is shown that the resulting coded system performance is a relatively insensitive function of the choice of channel model provided that the channel parameters are related according to the correspondence developed as part of this paper. Finally, a comparison of DPSK with a number of other modulation strategies is provided.

Dual-polarized channel outages during multipath fading

Abstract: A statistical model for outages that occur in a dual-polarized-frequency radio channel during periods of multipath fading is proposed, and an analytical expression for outage time obtained. The model, which results in Rice-Nakagami statistics for the cochannel interference signal, describes channel outage time as a function of several environmental and radio-system parameters. The formulation obtained allows for efficient parameteric studies to evaluate the importance of these parameters to channel outage time and to examine parameter sensitivity questions. Results of practical significance relative to hardware xpd requirements, maximum hop length, system gain, dependence on geographic environment, and digital terminal performance characteristics are obtained for present 11-GHz QCPSK (quaternary-coherent-phase-shift-keyed) digital radio systems. Estimates of dual-polarized-frequency channel outage time are obtained for a variety of representative system parameter values and compared with expected outage times for a conventional channel. Particular attention is given to the mechanism of channel outages during multipath fading, and several potential means for control and/or reduction of channel outage time are discussed.

Tilting antennas to reduce line-of-sight microwave link fading

Abstract: Tilting the antennas up to obtain 2 dB loss (for each antenna) over maximum gain during steady signal conditions is shown to reduce fading over this particular path. The technique should also work on other paths where sufficient angular separation between the direct and reflected rays exists, and where antennas with sufficiently "sharp" main beams are used.

Influence of mounting media on the fading of basic aniline dyes in epoxy embedded tissues.

Abstract: A simple cytophotometric technique is used to quantitate stain fading of basic aniline dye-stained epoxy-embedded tissues mounted in six different commonly used mountants. Significant fading was detected with all six mountants, although rates varied. the lowest rate of fading was observed with immersion oil and the highest rate of fading with Canada balsam. No significant differences in fading rates of four synthetic mounting preparations were observed.

Measurements of low angle fading in the Canadian arctic

Abstract: In July 1974, an experiment was conducted at Eureka, Canada (80° N, 86° W), to measure the tropospheric fading of satellite signals at 4 and 6 GHz. At Eureka, the 4.6 m antenna was located on a hill 250 m above sea level providing a clear path at an elevation angle of 1° to the Anik II satellite. The 4 GHz downlink signal strength was recorded at Eureka. Fading of the 6 GHz uplink signal from Eureka was measured at the Communications Research Centre, Ottawa. The uplink and downlink fading contributions for the entire experimental period are nearly identical, indicating that the fading distributions are essentially frequencyindependent over the 4–6 GHz range. For the 18 days period of the experiment, the link margins are 10.8 dB and 20.3 dB for 99 % and 99.9 % reliability, respectively. For the worst twohour period, the margins are 18 dB and 28 dB, corresponding to those for a Rayleigh distribution. No marked diurnal effect is discernible. Distributions of the fade durations indicate, for example, that 90 % of all fades exceeding 6 dB do not exceed 36 seconds duration. Although fading at 4 GHz is always accompanied by fading at 6 GHz, their correlation is sometimes poor, yielding correlation coefficients as low as 0.34. The amplitude correlation of two 6 GHz signals separated by up to 10 MHz always exceeds 0.90, indicating minimal amplitude distortion across the band. The implications of these results on the design of a satellite link are discussed.

Compensation for signal fading in communication system - uses single antenna and compares signals received at different angles of incidence

Abstract: The method for the compensation of propagation fading in a communications system using angular diversity reception with a single aerial system. Signals arriving axially to the antenna exciter (Er) are arranged to excite a signal of basic wave type (H10) and those arriving at an angle to the axis excite a signal of a higher mode (H20), the two being separated in the mode coupler (M). The two separated signals are fed to a pair of receivers coupled to give diversity reception in the normal way.

Performance of a 400 Mbit/s System over a Line-of-Sight Path

Abstract: During November 1971, a 400 Mbit/s quadriphase-shift-keyed system was operated at 13.3 and 14.9 GHz over a 22.8-km line-of-sight path. This path was chosen because of its high incidence of meteorological conditions favorable to anomalous propagation. Performance of the system during fading periods is reported here. Additional information from extensive meteorological measurements near midpath is presented to identify the source of the fading. The dual frequency measurements for spaced receiving antennas indicate that diversity reception, properly implemented, would have provided an effective fading remedy.

Buffer Control for Transmission of Blocked Data Over Fading Channels

Abstract: Since fading channels are characterized by frequent signal dropouts which are long compared to packet duration, substantial buffer space must be provided at both ends of the link. Systems which rely on selective, on/off transmission to achieve low bit error rates are seriously affected by the special conditions which must be imposed on them to prevent buffer overflow or underflow. This effect can be reduced by a method of buffer control developed and analyzed in this paper, in which the average transmission rate is varied as a function of queue length in an attempt to keep the queue away from the ends of the buffer. It is shown that buffer control provides the same improvement in average error probability as does doubling the buffer size, but without the associated doubled storage cost and doubled delay. In order to simplify the analysis and keep the discussion relevant, the system is oriented to transmission of fixed length blocks or packets.

Frequency coherence and time coherence in random multipath channels

Abstract: The underwater acoustic propagation channel is dispersive in time, frequency, and space. This paper investigates frequency coherence and time coherence for simple multipath channel models with an emphasis on situations where there are a few paths. The propagation channel is postulated to consist of a finite set of paths with random amplitudes and lengths. The fluctuation characteristics of the frequency selective fading in a random time spread channel are investigated in terms of the statistics of the channel transfer function. The effects of the fluctuations in path length are clearly separated from the effects of nominal multipath structure in the channel statistics. The conditions under which coherence bandwidth and coherence time provide reasonable characterizations for the multipath channel are examined. All of the results obtained for the random time spread channel can be applied to the mathematically dual problem of a random frequency spread channel.

Latent image fading in nuclear emulsions

Abstract: The effect of dry N2 desiccation on fading of fast neutron personal film dosimeters was examined.

Communication system performance model for VHF and higher frequencies

Abstract: : Communication system designers and engineers often must decide whether to use design-aid models not well suited to the problem at hand or to perform many tedious and laborious hand calculations which, coupled with certain 'rules of thumb,' provide an estimate of system performance. The model documented in this report provides relief from such problems. For communication systems which operate at VHF and higher frequencies, the character of the terrain between the transmitting and receiving antennas, as well as the heights of the antennas above immediate terrain, will affect significantly the attenuation of the radio signals. The model reported herein considers these terrain influences, along with other commonly considered influences, by automatically accessing digitized topographic data files to develop the path data needed to computer the basic transmission loss. Computation of the basic transmission loss is accomplished using the Longley-Rice formulation for point-to-point paths. The statistical character of the basic transmission loss results from long-term fading (time availability), path to path differences (location variability), and prediction confidence considered in the model. The type of model output data is selected by the user. Options range from simple path terrain information to basic transmission losses to utilizations of the basic transmission loss in computing power density or received signal level.

Simultaneous measurements of radio refractivity and multipath fading on 2 July 1975 at 11, 19 and 36 GHz on a 7.5 km path

Abstract: A change of about 20 N-units in radio refractivity between 30 and 40 m above the ground is reported, together with simultaneous radio-propagation measurements at 11, 19 and 36 GHz. Abnoraml radio-propagation effects, e.g. divergence and multipath, are discussed, using examples from the refractivity and propagation measurements.

Analysis of fading due to rain, snow and multipath propagation for 11 GHz radio links variously located in England

Abstract: The propagation performance of six microwave links operating at frequencies near 11 GHz were monitored for the years 1973-75. Fades were classified as due to rain, snow or multipath (including other refractive effects). It was found that, on average, the contributions of each of the three types of fade to the total statistics were comparable, although there were considerable variations in the statistics, both from year to year and at different locations.

Effect of Fading on FM Reception with Cochannel Interference

Abstract: The combined effect of cochannel interference and nonselective Rayleigh fading on the reception of analog FM signals in additive Gaussian noise is analyzed. Rice's click model for the breaking region is used. Analysis in the absence of interference is performed first and the effect of modulation on the click rate is found to be nearly negligible. For the case with cochannel interference the click rate is computed in the absence of modulation. Curves are given for detected signal-to-noise ratio (SNR) versus carrier-to-noise ratio (CNR) with signal-to-interference ratio (SIR) as a parameter and SNR versus SIR with CNR as a parameter. The results agree well with recently published results of a simulation study.

A digital high-frequency multipath propagation simulator

Abstract: A digital h.f. multipath propagation simulator is described which can be applied to communications receiver and modulation techniques evaluation and operator training. It can also be applied to a variety of applications in the high-frequency field where programmable and repeatable ionospheric conditions require simulation. Simultaneous communications paths are simulated, four of which exhibit independent delays, Doppler shifts, Rayleigh fade and phase variations and attenuation consistent with ionospheric reflection (skywave) paths.

Multipath over LOS Channels Study.

Abstract: : A major source of propagation outages on microwave LOS links is multipath fading caused by steep negative refractive index gradients. The advent of highspeed digital transmission over the DCS microwave LOS links brings into concern the possible degrading effects of frequency-selective fading associated with such multipath. The present study extends previous results by CNR on channel modeling for multipath fading on LOS links and the consequent degrading effect of such distortion on digital modems and diversity combining. Two 2 bits/sec/Hz modems were examined. One technique used a baseband modem utilizing the conventional radio with a frequency modulator and frequency discriminator in the receiver. The other technique used an IF modem and involved coherent receiver processing. While the theory developed is applicable to all the DCS links, two particularly difficult links were singled out for study: the Hohenstadt-Zugspitze link in Germany and the Swingate-Houtem link across the English Channel. Considerable SNR degradation was found to be possible for some propagation conditions. In addition, it is shown that conventional diversity switching can produce loss of bit count integrity, but a modified diversity switching plan can alleviate this problem. (Author)

Effects of fading on a multi-user spread spectrum system using direct sequence modulation

Abstract: Worcester Polytechnic Institute: Provost ad interim, March, 2007-present. Dean of Undergraduate Studies, July, 2006-present. Electrical and Computer Engineering Department Head, July, 1988-June, 2003. Professor of Electrical and Computer Engineering, July 1986-present. Associate Professor of Electrical Engineering, July, 1981 – July 1986. Satin Distinguished Fellow in Electrical Engineering, academic year 1983-84. Assistant Professor of Electrical Engineering, September, 1977-July 1981.