Showing papers on "Multipath propagation published in 1977"

A Statistical Model for Urban Radio Propogation

Abstract: A statistical model, based on extensive experimental data, was established to characterize the urban radio propagation medium in various urban environments. Describing the medium by a linear filter, the peaks of the multipath response were analyzed statistically concerning the distribution of the path strength and the path arrival time. The statistical properties of these quantities depend on the modulation delay time. The resulting model can be used for simulation experiments in order to avoid costly hardware tests of ad hoc systems.

The Spread Spectrum Concept

Abstract: This paper describes an idealized spread-spectrum communication system. The processing gain concept is developed as a measure of a well-designed system's robust performance against independent wide-sense stationary interference. Multipath and repeater jammer rejection, partial correlation problems, and security requirements are related to spread-spectrum code properties.

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.

Radio wave loss deviation and shadow loss at 900 MHz

Abstract: Radio wave propagation between base and mobile stations is normally described as being Rayleigh distributed due to multipath radio wave combining. When the number of radio wave paths are limited the variation in received signal amplitude frequently follows a more general case given by a Weibull distribution. A significant portion of the Weibull distribution is defined as the transmission loss deviation. Other definitions have been included with the object of standardizing methods of measuring and reporting propagation data. Data will be presented showing loss deviation between 3 and 30 dB. Shadow loss over hills and around buildings are usually assumed to be knife-edge or rounded knife-edge. Shadow loss based on these assumptions is generally found to be less than the true measured value. Data will be presented comparing calculated shadow loss with measured value.

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.

Clutter and multipath suppressing sidelobe canceller antenna system

Abstract: An improved antenna and interference-cancelling system for improving signalampling, particularly in side-lobe canceller system operating in a multiple-interference-source environment An antenna is constructed as a circularly symmetric lens having loosely coupled feed elements disposed around the periphery of the lens Each feed element acts as a high-azimuth-gain antenna which permits signal reception from all directions The lens, when connected to couple the receiving feed elements to a side-lobe canceller, permits the canceller to discriminate against clutter and scattered signals and resolve interference sources in angle so that all canceller loops do not receive signals from all other interference sources, while still providing interference samples from all directions

Models for the amplitude fluctuations of narrow‐band signals and noise in the sea

Abstract: The sound from a steady distant sinusoidal source in the sea fluctuates in amplitude because of multipath interferences, surface motion, internal waves, microstructure, and other propagation effects. Such fluctuations are found to follow a Rician or modified‐Rayleigh distribution having as a parameter the fraction of random power in the received signal. At the output of conventional sonar processor—namely, a narrow‐band filter, a squarer, and an integrator—the fluctuation statistics are determined by the propagation processes occurring in the sea between source and receiver. On the other hand, ambient sea noise is found, from analyses of field recordings, to have fluctuation statistics determined by the processor itself; ambient noise samples at the processor output obey a chi‐square distribution having a number of degrees of freedom equal to twice the bandwidth‐time product of the processor, as would be expected from a Gaussian input. The two distributions—Rician power for signals and chi‐square for noise—while formally different, have remarkable similarities in the limits. In short, the output fluctuation statistics of narrow‐band signals and Gaussian ambient noise can apparently be predicted from estimates of the degree of randomness introduced by the prevailing propagation conditions, and from a knowledge of the processor, respectively, provided these statistics remain stationary during the analysis period.

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.

Optimization of Coded Spread Spectrum System Performance

Abstract: Error correction coding techniques significantly improve performance of spread spectrum communication systems in environments containing jamming, multipath, and unregulated multiple access. This paper investigates the optimization of spread spectrum system performance for time-varying unknown interference. Noncoherent frequency hopping (FH) spread spectrum modulation, and hybrid FH-PN incorporating a direct sequence PN modulation on each hopped frequency are studied. For FH or FH-PN, the data modulations considered are differential phase-shift-keying (DPSK), differential quadriphase-quadriphase-shift-keying (DQPSK), and multiple-frequency-multiple-frequency-shift-keying (MFSK). Both block and convolutional error correction coding techniques are studied as a means of improving the spread spectrum performance.

Adaptive Multipath Equalization for T.V. Broadcasting

Abstract: In recent years the growing use of vertical interval data transmission systems in television broadcasting, such as the Captioning for the Deaf system1 developed by the Public Broadcasting Service (PBS)*, has created a renewed interest in the problems of multipath distortion.2 This interest is heightened by the fact that new devices, such as microcomputers and charge coupled delay devices (CCD's), make possible an economic solution to this problem for the first time, not only for data but for video as well3, although the work described herein has concentrated primarily on data applications.

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.

Impulse Response of a Meteor Burst Communications Channel Determined by Ray-Tracing Techniques

Abstract: Ray-tracing techniques are used to derive the impulse response at VHF of a meteor burst communications channel. An approach to analyze the effects of multipath arising from wind shear is also presented and is applied to a simplified model of trail distortion. Within the limitations of the models adopted, we have found that a single path with a spread of approximately 0.2 μs adequately describes the channel behavior within roughly half a second from trail formation, and that a multipath structure with several paths and a total multipath spread of approximately 2 μs is typically associated with the channel at later times, when the wind shear substantially affects the trail shape. These results are consistent with experimental findings according to which a meteor trail communications channel is capable of transmitting beyond-the-horizon bursts of television images of broadcast quality typically for a fraction of a second after trail formation.

Mobile antenna gain in the multipath environment at 900 MHz

Abstract: A program has been completed that investigated the effect of the urban multipath environment on mobile antennas at 900 MHz. The program involved the construction of several mobile antennas, careful measurement of these antennas on an antenna range to determine their characteristics in a controlled environment, and measurement of their characteristics in the multipath environment of cities. Measured results were compared to computed results and pattern range results were compared to field data to permit generalized conclusions to be reached. The results of the second phase of the program, measurements made in the mobile multipath environment, are presented. Analysis of this data shows that gain antennas have significantly less gain in areas where multipath propagation predominates and that the increased aperture of the gain antennas provides no significant smoothing of the received signals in the multipath environment.

Low angle radar

Abstract: A receiving arrangement for a pulse radar tracker facilitating low elevation angle tracking. The received signals are sampled along the focal plane of the receiving antenna system aperture. Range gating at IF and delay line serialization are provided to generate a simulated (synthetic) scan of the aforementioned aperture plane in the form of a coherent signal waveform. The main lobe of a multipath (ground reflection in the elevation case) is substantially eliminated by the time gating and filtering is provided to greatly reduce the corresponding sidebands. Desired (direct) received signals are substantially unaffected.

Scanning-beam microwave landing system--Multipath-errors and antenna-design philosophy

Abstract: Multipath reception can cause guidance-angle errors in a microwave landing system (MLS). The antenna radiation-control problem for a scanning-beam MLS is defined and analyzed. "Centerline emphasis" is presented as a helpful design philosophy for the ground antennas. It is shown that the maximum guidance-angle error is proportional to 1) the amplitude of the indirect signal, 2) the antenna beamwidth, and 3) the time derivative of the indirect signal as the direct beam-peak scans by the receiver. This result is used in developing a rationale for the selection of the antenna beamwidth and sidelobe level (aperture size and excitation) for the azimuth and elevation guidance functions.

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.

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.

Multipath Dispersion in Low Visibility Optical Communication Channels

Abstract: : The potential for optical communication through low-visibility scattering atmospheres is the underlying concern of this investigation. To access that potential, the effects of the channel upon the optical signal propagating through it must be determined. The work reported here is primarily concerned with one of those effects: the time dispersion, or multipath spread, of the transmitted signal. The investigation was primarily an experimental one, utilizing the existing propagation facility operating over a thirteen kilometer experimental path between the MIT campus and a field station located at an Air Force site in suburban Boston. Experiments were performed at visible and near infrared wavelengths, with highly collimated transmitting beams. Measurements of the multipath spread for various receiver fields of view and at various offaxis angles were made. Little multipath and angular spread was observed within the realm of atmospheric conditions for which a detectable signal could be obtained. For optical thicknesses less than ten, the on-axis measurements are consistent with the hypothesis that the received signal is dominated by unscattered radiation; however, the off-axis measrements suggest that the scattered field itself may be only slightly spread in time and angle. Guided by the observed narrowness of the angular spectrum, an approximation to the linear transport equation was developed. The resulting equation is more amenable to solution than is the full transport equation. Here it is solved for isotropic scatter. For such scattering it is found that substantial spreading in both time and angle will occur when the optical thickness becomes appreciably greater than one.

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.

L-Band Air-to-Air Multipath Measurements

Abstract: : A series of air-to-air earth-scattered L-Band multipath measurements are described and experimental results presented. During these measurements RF pulses were transmitted between two instrumented general aviation aircraft flying coaltitude, diverging paths over a variety of terrain and water surfaces. Multipath data was collected over grazing angles ranging from approx 5 deg. to approx 75 deg. The objectives of these measurements were the: (1) Characterization of the multipath environment in which beacon based airborne collision avoidance (BCAS) equipment would operate. (2) Investigation of the merits and limitations of various degrees of antenna diversity in the rejection of multipath. (Author)

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)

The Measurement of Microwave Multipath in an Airport Environment.

Abstract: : Multipath in an operating airport, and its impact on the performance of a Microwave Landing System (MLS) is an important aspect of the development of these systems. Test programs on the candidate MLS systems developed in the U.S. were conducted in areas that do not emulate large commercial airports. In order to better evaluate multipath in a realistic environment, measurements of reflected signals at the MLS operating frequency were performed, and the results used to develop or modify a computer simulation program. Both a cw system and a pseudorandom noise (PN) channel probe were used in the measurement program. This report presents the results of multipath measurements made on (1) airport terminal buildings, (2) large maintenance hangars, and (3) aircraft on the surface of the airport. Results indicate that significant reflection levels are prevalent from these sources, and could produce a multipath reception problem at the receiver of an aircraft approaching the runway. (Author)

Multipath signal generating method

Abstract: PURPOSE:To obtain an FM signal having certain delay time in the way of easy in engineering and low in cost.

Multipath Measurements over Troposcatter Paths

Abstract: : In support of a program to develop a new family of digital troposcatter radio terminals for tactical application, The MITRE Corporation and Rome Air Development Center jointly conducted an extended series of multipath measurements over two troposcatter paths of 86 and 168 miles at 4.7 GHz. The results of these tests include long-term statistics for and correlation among RMS multipath spread, path loss, fade rate, surface refractivity, and effective earth radius. The data obtained on these parameters is currently being used in the evaluation of various high-speed digital modulation/detection techniques and in the specification of radio terminal performance. This paper describes the test program, reports test results, and discusses related test programs, troposcatter multipath prediction, and the impact of dispersion on digital communication.

The Effects of Multipath Propagation on the Range Measurement System at Fort Hunter Liggett.

Abstract: : Instrumentation was developed for investigating the radio frequency link between the A station and the B unit in the Range Measurement System. Propagation effects are characterized using a multipath simulation in a laboratory environment. Field experiments were conducted to investigate the presence of a multipath environment at Ft. Hunter Liggett. The effects of varying the ambient temperature on the A station and the B unit were measured. (Author)

Operator formulation for maximum‐likelihood estimation of acoustic emitter delay and Doppler

Abstract: A general, compact, and quickly derived solution to maximum‐likelihood estimation of acoustic emitter delay and delay rate can be achieved by representing the signal propagation to each of M sensors as a matrix operator containing the unknown parameters. A general solution is first derived in which the emitted signal is unknown but nonrandom. This solution can incorporate the combined effects of propagation parameters such as wide‐band Doppler, frequency‐dependent attenuation, multipath time delays, and signal phase inversion due to some types of reflection. The result is simplified if only wide‐band Doppler, single‐path delays, and frequency‐independent attenuation are present and, in the case of two sensors, it reduces to simple cross correlation. The matrix approach is also applicable to casses where the emitted signal is modeled as a random process.