Showing papers on "Multipath propagation published in 1976"

Acoustoelectric convolvers for programmable matched filtering in spread-spectrum systems

Abstract: Acoustoelectric convolvers for a spread-spectrum communication application age described with a 100-MHz bandwidth. The use of convolvers as programmable matched filters provides the ability to change the coding waveform from bit-to-bit, thus offering improved multipath performance, security against decoding, and protection against repeat jamming. In this paper, we describe design and performance details of a prototype convolver which processes signals of 10-µs duration with a 100-MHz bandwidth. A dynamic range of 50 dB is obtained, and error signals are 30 dB below the output signal with input signal levels of +14 dBm. A test circuit is described which creates typical spread-spectrum signals in which each bit is encoded into 512 chips and data is encoded by inverting the phase of an entire bit at a data rate of 100 kbits/s.

Multipath acoustic propagation with a moving source in a bounded deep ocean channel

Abstract: An analysis is presented for coherent multipath propagation between a moving source and a fixed receiver The major mulitpath effects of source motion are frequency spreading range or emission‐coordinate spreading, and travel‐time spreading of the multipath arrival set These are analytically related to the signal propgation as functions of time and velocity Computer simulations with a shallow moving source and a fixed receiver below the SOFAR channel are presented which clearly illustrate the contributions of source motion developed in the analysisSubject Classification: [43]3020 [43]2020

Multipath Parameter Computations for the MLS Simulation Computer Program.

Abstract: : A set of mathematical models and computer programs have been developed to characterize multipath propagation in an airport environment. When combined with system mathematical models, these models are intended to provide a firm technical basis for assessing the performance of candidate Microwave Landing Systems (MLS) in realistic airport environments. The two paramount issues in developing these models have been (1) validation based on actual field test data and (2) computer running time. The obstacles modeled include buildings and aircraft, as well as the ground which can cause both specular reflections and diffuse scattering. In addition, the shadowing effects due to runway humps, and aircraft, buildings approaching the line of sight between transmitter and receiver are included. Computational procedures are presented for obtaining the salient multipath parameters, i.e., relative magnitude, phase, directional angles, Doppler frequency, and time delay. Computer programs have been written for these algorithms using the Fortan programming language, with structured programming methods, such as Iftran, employed whenever possible. A presentation is given of computer validation data for the computational procedures. A comparison of these computer validation results with experimental field data demonstrates good agreement in all cases of interest. The computer running time for these computer programs is quite reasonable, e.g., it takes about five times longer than actual flight time to run a model of a typical airport environment on an IBM 370 model 168. (Author)

On the design of a small zone land mobile radio system in UHF band

Abstract: For designing a land mobile radio system, particularly a small zone high capacity system in the UHF band, various performance degradation causes, such as signal fading, cochannel interference, intermodulation disturbance, and sensitivity suppression, should be inclusively taken into account. The magnitudes of these degradations vary with time, because of field strength variation due to multipath fading, environmental shadowing effect, and the distance of mobile station from the base. Hence the circuit performance objective given in a probablistic form must be allotted for in each one of these causes. Variabilities of these degradations are estimated in terms of desired and undesired signal field strength variations. In the estimation, path diversity effect is considered and a root square sum approximation technique is employed. Also, the process used to determine the allottment is proposed from the viewpoints of efficient frequency utilization and realizability of base and mobile radio equipment.

On the Performance of Underwater Data Transmission Systems Using Amplitude-Shift-Keying Techniques

Abstract: As part of a general investigation into various aspects relating to underwater data transmission, an experimental amplitude- shift-keyed (ASK) system has been developed and tested. The system is described and the results from tests carried out with the system are presented. The tests, which were carried out under shallow-water transmission conditions, were designed to provide information relating to the limits of achievable performance. The results obtained indicate that with a simple system, it is possible, when using a peak transmitted power of only 50 mwatts, to transmit data at a rate of approximately 600 bits/sec over ranges up to 650 meters, and to do so with a probability of bit-error that is consistantly of the order of The results indicate that if the probability of bit-error is to be maintained at approximately le3, then significant improvements in system performance in the form of increases in range and data-rate should be possible provided greater transmitter power is used. However, in order to do so it would be necessary, in addition to using increased transmitter power, to employ a much more sophisticated system capable of overcoming the effects of pulse dispersion and multipath. 1. INTRODUCTIOS

Antenna Performance in the Presence of Diffuse Multipath

Abstract: Diffuse multipath is a random phenomenon that arises out of the diffraction of waves from rough surfaces. This form of multipath can in turn affect the angle estimation performance of monopulse antenna systems. In order to evaluate its effect, it is necessary to obtain the channel spread function that results. This function is basically the wave number spectrum of the resulting random field generated by the scattering. To do this, use is made of the Kodis-Barrick arrick scattering cross section and Wagner's shadowing function. The evaluation is performed for the specific geometry of a spherical earth. Evaluation of the system performance degradation is made for various angle estimation system configurations.

The effect of Doppler on long‐range sound propagation

Abstract: In this sound propagation study of the effect of Doppler on source and/or receiver motion has been developed. It is based on modifying the inputs to existing normal‐mode software so that the source and/or receiver trajectory and consequent Doppler effects of the given ocean experiment are properly modeled without requiring direct changes in the static softward. From this the effects of the received signal after long‐range, multipath sound propagation from a moving source can be modeled and interpreted.

Multipath communications system

Abstract: A communications system utilizing a noisy reference signal and a noisy information signal wherein the information signal is delayed from the reference signal as a function of the information modulation and then the two signals are combined for simultaneous transmission. The receiver amplifies the combined signal and delays it by a predetermined amount corresponding to the delay in the transmitter. The delayed and undelayed signals are autocorrelated to reconstruct the information signal. The system provides for autocorrelation of signals which have traveled a plurality of signal paths from transmitter to receiver.

Multipath and ducting tolerant location techniques for automatic vehicle location systems

Abstract: This paper discusses various methods of improving the accuracy of a linear least squares solution algorithm which is commonly used in Time Difference of Arrival (TDOA), hyperbolic location systems. Topics considered include methods of generating a useful weighting matrix and a residue examination scheme as the basis for rejecting data contaminated by large propagation delay errors. The performance of two linear schemes is compared to that of the nonlinear least squares solution using some 2000 sets of actual TDOA measurements.

Sonar design in the real ocean: Multipath limitations on sonar performance

Abstract: Traditionally, the nost important military use of underwater sound has been the detection of subnarines at as great a distance as technology and costs will allow (sonar). Nowadays, these distances are as great as hundreds of miles for passive sonars, utilizing the noise radiated by the target, and tens of niles for active sonars, utilizing echoes from their targets. Detection ranges of these nagnitudes require the naxinun state-of-the-art enployment of techniques in signal processing, array designs and displays. The purpose of this paper is to show that these techniques are limited - in their possible gains and their degree of sophistication - by the peculiar and disadvantageous peculiarities of the ocean medium. These characteristics are the result of the presence of multipaths in the ocean - that is, the fact that there is always nore than one ray path between source and receiver. Here, some of the consequences of multipath propagation in deep water will be pointed out, and the limitations imposed on processing and the gain of arrays will be mentioned.

Time Reference Microwave Landing System Multipath Control Techniques

Abstract: The Time Reference Scanning Beam (TRSB) landing system is an air- derived data system which operates at microwave frequencies to provide guidance information proportional to an aircraft’s lateral and vertical displacement from a selected flight profile. Angular position of an aircraft is measured in the aircraft by receiving ground generated fan beams which scan “to” and “fro” across the coverage sector in both azimuth and elevation. The airborne subsystem measures the elapsed time between a given “to’‘-“fro” scan. The time difference is controlled to be an indication of the aircraft’s angular position. Thii indicated angle could be corrupted by multipath. Basic to the TRSB concept is the azimuth and elevation controlled antenna radiation patterns (both in the scan and non-scan dimensions) which, in conjunction with their high scan rates, control the multipath to an operationally acceptable level. Since the multipath is controlled primarily on the ground, there is no special impact on the cost of the airborne equipment because simple, conventional RF and signal processing techniques need only be implemented into the airborne receiver hardware.

MLS Multipath Studies. Volume II. Application of Multipath Model to Key MLS Performance Issues.

Abstract: This report summarizes MLS multipath work carried out at Lincoln Laboratory from March 1974 to Sept. 30, 1975. The focus of the program is the development of realistic models for (1) the multipath in representative real world environments and (2) the multipath characteristics of candidate MLS techniques. These multipath and system models are used in a comprehensive computer simulation to predict the strengths and weaknesses of major MLS systems when subjected to representative real world environments. The report is organized into two volumes. Volume I describes the algorithms and validation of various portions of the program. In Volume II, the simulation (or selected portions thereof) is applied to key multipath related MLS issues.

The use of dispersion to eliminate multipath in planetary radio occultations

Abstract: Occultation geometry is analyzed with a view toward eliminating all multipath resulting from layer effects. It is assumed that the spacecraft moves behind the planet with a constant velocity perpendicular to the direction from the spacecraft to the earth, and that for radio frequencies of 1 to 2 GHz the angle of refraction of a radio ray passing through the ionosphere is so small as to be imperceptible if drawn to scale. It is shown that if a particular amount of dispersion is inserted into the process at any stage prior to signal detection, the frequency vs time curve will be converted to a frequency vs ray asymptote height curve. Because there can be only one mode from any one ray asymptote height in a layered medium, frequency must be a single-valued function of this parameter. The introduction of an appropriate dispersion will therefore eliminate all multipath insofar as it is caused by spherically symmetric planetary layers. The degree of accuracy to be provided in flight and during postflight analysis is discussed.

Design for system reliability in personal radio signaling

Abstract: Innovative techniques and attention to frequently-ignored factors have been combined in a new personal radio signaling system. Accessed by means of Touch-Tone®service, customer dialing errors are effectively controlled through the use of self-checking numbers, eliminating a common cause of missed or false signals. A multiplicity of radio transmitters are arranged to provide adequate building penetration for the entire service area. Simultaneous transmitter operation and low speed, digital receivers work together to make use of multipath radio propagation. Redundancy of equipment and terminal functions assure that electronic failures result in minimal impairment of service. Combining all of the above design features has resulted in a service that is significantly more reliable than that which is provided by systems of earlier design.

Mls multipath studies. volume i. mathematical models and validation

Abstract: This report summarizes MLS multipath work carried out at Lincoln Laboratory from March 1974 to Sept. 30, 1975. The focus of the program is the development of realistic models for 1) the multipath in representative real world environments and 2) the multipath characteristics of candidate MLS techniques. These multipath and system models are used in a comprehensive computer simulation to predict the strengths and weaknesses of major MLS systems when subjected to representative real world environments. The report is organized into two volumes. Volume I describes the algorithms and validation of various portions of the program. In volume II, the simulation )or selected protions thereof) is applied to key multipath related MLS issues. Mathematical models are given for the major MLS multipath sources (ground reflections, building and aircraft reflections, and shadowing by objects and humped runways), and it is shown that they agree well with field data (including the Lincoln measurements at Logan Airport). Models for the techniques (Doppler and scanning beam) considered in phase II of the U.S. MLS program are presented together with validation by comparison with theory and bench tests. Also persented are the results of a general study in motion averaging. The (validated) computer simulation (and portions thereof) is then applied to studying 1) the critical areas required by the TRSB system to avoid excessive relfection effects, 2) the expected TRSB performance with vertical polarization and benefits that might be derived with an alternative polarization and 3) siting of a specific TRSB system at Friendship International Airport (MD). (Author)

Air traffic control experimentation and evaluation with the nasa ats-6 satellite. volume v. multipath channel characterization test

Abstract: : Results of aeronautical L-band multipath channel characterization tests are given. All tests were conducted between September 1974 and April 1975 as part of the U.S. DOT aeronautical technology test program. These tests were part of the international Integrated ATS-6 L-Band Experiment coordinated by the NASA/Goddard Space Flight Center. Wideband PN-coded test signals transmitted from an FAA KC-135 jet aircraft were relayed by the ATS-6 satellite for reception by the satellite aeronautical channel prober (SACP) receiver located at the NASA/Rosman ground station. Both oceanic and overland multipath data are analyzed to provide delay-Doppler characterizations of the channel. Specific anlaysis outputs presented include the delay-Doppler scatter function, delay spectra, Doppler spectra, frequency and time autocorrelation functions, spread parameter measures, total scattered intensity and time-domain statistics. Oceanic results are compared to expectation through the use of physical optics surface integration vector scatter model. Results are presented for a variety of aircraft/satellite geometries, signal polarizations, and terrain states. The report consists of seven volumes: I-Executive Summary; II-Demonstration of Satellite-Supported Communications and Surveillance for Oceanic Air Traffic Control; III-Summary of U.S. Aeronautical Technology Test Programs; IV-Data Reduction and Analysis Software; V-Multipath Channel Characterization Test; VI-Modem Evaluation Test; VII-Aircraft Antenna Evaluation Test. (Author)

High-capacity digital microwave radio systems

Abstract: The design problems associated with high-capacity digital microwave radio systems and their present status are reviewed Nonmilitary systems having transmission capabilities of about 90 Mbit/s or greater per radio bearer have been considered, those known to be under development operating within the approximate frequency range 8–21 GHz At the lower-frequency end, repeater spacings are limited by multipath effects to about 60 km, whereas at the higher-frequency end, spacings are dominated by rain attenuation and are limited to a few kilometres The effects of both types of fading and the calculation of repeater spacings are considered Bandwidth utilisation is very important for digital systems, and the design criteria for suitable filtering schemes are discussed leading to possible overall radio-frequency-channel arrangements Constraints imposed by the system design are discussed, and the consideration of available components leads to outlines of possible repeater arrangements

Performance of an M-ary p.s.k. system in the presence of additive noise, intersymbol interference and fading

Abstract: The results of an investigation of the evaluation of energy signal degradation of an M-ary p.s.k. system due to intersymbol interference and fading are analysed. Attention is concentrated upon the influence of intersymbol interference caused by the limited band of the receiver filter as well as the transmitter one. As far as the channel is concerned, it has been assumed there is nonselective fading in it. Part 1 presents the analysis method being applied; Part 2 analyses the energy degradation characteristics of M-ary signals resulting from intersymbol interference in the case when no fading occurs in the channel; Part 3 analyses the influence of intersymbol interference and fading jointly.

Dispersive properties of the underwater acoustic channel

Abstract: The underwater acoustic propagation channel is dispersive in time, frequency, and space. The channel connecting a sound source and receiver can be modeled as a finite set of sparsely distributed scatterers with fluctuating reflection parameters. This paper examines the fluctuation characteristics of the frequency selective fading in terms of the channel transfer function for a time spread channel with random path lengths. The first and second moments of the channel transfer function are determined. The effect of path length randomness on the first and second moments of the channel transfer function is clearly separated from the effect of the nominal multipath structure. The conditions under which the coherence bandwidth of the multi-path channel is a meaningful concept are examined. All of the results can be used for the mathematically dual problem of a frequency spread channel.

Multipath scintillation spectra

Abstract: In a recent paper by Dyson, Munk, and Zetler [’’Interpretation of multipath scintillations Eleuthera to Bermuda in terms of internal waves and tides,’’ J Acoust Soc Am 59, 1121–1133 (1976)], approximate expressions were derived for the spectrum of the phase and intensity scintillations of a sinusoidal signal after propagation through a certain random multipath channel From the multipath structure specified in that paper, exact results are obtained for the phase‐derivative fluctuation spectrum This expression can be easily evaluated and compared to the approximation In addition, amplitude and square‐amplitude fluctuation spectra are derivedSubject Classification: [43]3020, [43]3030

A simulator to produce narrowband multipath effects on l-band aircraft-to-satellite signals

Abstract: The purpose of the program was to study the aircraft-to-satellite communications channel and to develop instrumentation to accurately simulate the effects of the multipath, Doppler and additive noise effects of such channels Such a device which simulates aircraft-to-satellite communication channels has been designed and fabricated The simulator provides capability for test and evaluation of communications and navigation equipment under controlled and repeatable conditions without the need for extensive, costly, time-consuming and nonrepeatable field experiments The basic approach to channel simulation is to split up the signal into several parts, delay each path differently then multiply the delayed signals by a set of complex noise waveforms and sum the results Both additive and multiplicative noise signals are exactly reproducible; thus the channel conditions may be reset and repeated The channel bandwidth is 10 MHz; the relative delay between direct path and multipath signals may be selected to be 5, 30 or 55 microsec The multipath delay spread is 8 microsec with a 2 microsec resolution

Array processors for simultaneous noise and interference suppression or simultaneous signal‐to‐noise and resolution enhancement

Abstract: A symmetrical treatment is described of signal and interference arrivals, both spatially coherent across the aperture of a receiving array and appearing in a generalized ambient‐noise background. By separating the coherent interference from the rest of the noise background, it is possible to implement a simple form of adaptive suppression of the interference, particularly when the latter is time varying and the remainder of the noise is stationary. Optimal processors are derived for simultaneous suppression of noise and interference, and for enhancement of the signal‐to‐noise ratio after elimination of interference. The processor for the latter case is shown to be a suboptimum version of the former. Both deterministic and stochastic interference are considered, as well as interference whose arrival time is synchronized with that of the desired signal (e.g., unwanted signal multipath arrivals, reverberation, etc.). Processors for two or more simultaneous and independent interference sources are described. ...