Showing papers on "Multipath propagation published in 1975"

A computer generated multipath fading simulation for mobile radio

Abstract: A brief description is presented of a computer simulation of the Rayleigh distributed fast fading encountered in mobile radio. This simulation should be of interest to all those whose studies involve parameters of a mobile system that interact strongly with the radio environment.

Spread spectrum correlation receiver

Abstract: A spread spectrum communication system having a correlation receiver for decoding an information signal transmitted on a pseudo-noise (PN) carrier. The receiver synchronizes to the transmitted signal by performing a continuous sequence of correlations until a correlation output exceeding a predetermined threshold level is detected. Thereafter, the receiver performs a plurality of correlations during a sampling interval which is timed to occur at approximately the time when high subsequent correlation output signals are likely to appear. After enhancement through adaptive filter processing, the correlation outputs generated during each sampling interval are accumulated (integrated and the summed output present at the end of the sampling interval is representative of the transmitted data message (a binary bit). The adaptive filter develops and stores weighting values representing the expected signal strength of the correlation outputs and the actual correlation outputs are multiplied by the weighting values. Received signal energy representing atmospheric and specular multipath signal components are detected and channeled to the accumulator along with the main signal component. The accumulator thus realigns the multipath signals with the main signal, resulting in a significant increase in processing gain. An audio version of the system is also disclosed.

Space-diversity engineering

Abstract: Vertically separated antennas are recommended to increase the transmission availability of line-of-sight microwave links by reducing the duration and frequency of multipath fading events. The emphasis is on application to the 4- and 6-GHz bands on links with negligible ground reflections. Necessary signal processing, links utilizing passive repeaters (reflectors), and overwater links are also treated. Some new experimental data are presented.

Mobile diversity radio communication system

Abstract: A diversity system in a mobile radio communication system provided with a plurality of antenna systems operating over a multipath medium in at least one of the sending path and the receiving path. At the sending path, a carrier wave frequency- or phase-modulated by a digital baseband signal is transmitted from a transmitting antenna. A plurality of receiving antenna systems are switched at a constant frequency higher than the signaling rate of the digital baseband signal but less than the frequency shift width of the frequency modulated wave or less than a product of the maximum phase shift of the phase modulated wave and the signaling rate, so that average-power dispersion in a signal element of the digital baseband signal received at the receiving antenna systems is effectively compressed. Alternatively, the plurality of antennas may be transmitting antennas, and switched at the same rate to achieve compression of average power dispersion in the baseband signal elements.

Differential Phase-Shift Keying Performance Under Time-Selective Multipath Fading

Abstract: Differential phase-shift keying (DPSK) performance analysis in the presence of multipath is extended to include cases in which the Doppler spread is on the order of the bit rate or greater. As the Doppler spread is increased from zero, the performance degrades below that predicted by Jones [1]. However, as the Doppler spread is increased beyond values on the order of the bit rate, performance improves. In the limit as the Doppler spread approaches infinity, with total multipath power held constant, performance approaches that of ideal DPSK as expected. Bit error rate performance in the presence of time-selective Rician fading is presented along with confirming experimental data obtained with a channel simulator.

The Calculation of Ionospheric Doppler Spread on HF Communication Channels

Abstract: Using some basic concepts of ionospheric propagation, multipath and Doppler spread calculations are obtained for the HF ionospheric communication channel. Because of sensitivity to the solar control mechanism, the Doppler spread results are found to demonstrate a pronounced dependence on time of day and operating frequency. The practical consequences of these calculations are discussed in detail. This includes the definition and generation of the "Doppler reduction factor," comparable to the "multipath reduction factor," which can be used to determine the appropriate operating frequency on a given HF link.

Anti-multipath digital signal detector

Abstract: The invention is a detector for radio signals which have been transmitted through a multipath medium. The device operates in conjunction with the radio frequency portion of a receiver to detect digital signals which have been transmitted in known modulation formats. The transmitted signal is constructed by assigning known and distinct modulation waveforms to a sequence of message symbols, or digits. The basic digital message, which the detector is to reconstruct consists of a sequence of digits, each lasting for a fixed time, say T seconds, and each picked from an alphabet of arbitrary, but fixed, size. For example, in a binary message, the alphabet consists of the digits 0 and 1. In a quarternary message, the alphabet consists of the digits 0, 1, 2 and 3.

A Modified Monopulse Technique for Radar Tracking with Low-Angle Multipath

Abstract: In this paper a new monopluse technique for reducing radar multipath angle-tracking errors is proposed. The solution is achieved by using two monopulse antennas at different heights. The available signals allow one to control the antenna-height diversity so that the elevation angle control signal is not affected by reflected-wave components. The size of the antenna system may be reduced by properly using phase shifters. A possible configuration of such a system is considered and some practical data on the antenna-height diversity are discussed.

Experimental validation of palm - a system for precise aircraft location

Abstract: : A ground based system to precisely locate aircraft in three dimensions based on transponder replies has been designed. Field tests have been conducted to validate the design of the elevation performance. This document reports on the principle of operation, experimental hardware and field test of the Precision Altitude and Landing Monitor (PALM). The key features incorporated in the PALM design include: (a) no new avionics required, i.e., uses standard aircraft transponder; (b) high accuracy position data, i.e., a 1- mrad (0.06 degrees) rms error in elevation and in azimuth; (c) broad airspace coverage, e.g., 40 degrees in elevation, 120 degrees in azimuth (expandable to 360 degrees) and several 10s of miles in range; and (d) low life cycle equipment cost. The high accuracy in the difficult airport multipath environment is a direct result of (a) a newly developed antenna synthesis procedure, (b) the development of adaptive multipath suppression techniques, and (c) the use of digital signal processing. The program, to date, has focused on an experimental evaluation of the elevation performance. The electronics required for the tests have been built into a self-powered van to facilitate experiments at remote airports. The actual flight test data demonstrated experimental errors on the order of 1 mrad; moreover, as predicted by theory, the resulting errors were essentially independent of elevation angle. Potential applications for the PALM position data include parallel approach monitoring, independent altitude monitoring, and/or performance assurance monitoring of landing guidance systems.

The Logan MLS Multipath Experiment.

Abstract: This document is disseminated under the sponsorship of the Department of Transportation in the interest of information exchange. The United States Government assumes no liability for its contents or use thereof. The National Plan for a Microwave Landing System (MLS) has specified a carrier frequency for the system in the vicinity of 5.1 GHz. At that frequency, no multipath data taken at a major civilian airport eXisted. The purpose of this experiment was to obtain such data at Logan International Airport in order to ascertain: 1) which objects are the major causes of measurable multipath reflections and their levels relative to the direct signal (MID level), 2) whether or not the reflections from these objects can be satisfactorily simulated by the Lincoln computer model and, if so, how complicated must that model be, and 3) if the. characteristics of multipath provide a significant discriminant between the Doppler and scanning beam techniques. It was found in the experiment that regions where reflections were noted could be predicted from ray optics and diffraction. No measurable reflections were noted elsewhere. For the purpose of modeling for multipath, building surfaces could be characterized as a flat plate with a reflection coefficient determined by measurement if it were a complicated surface, or by the dielectric properties of the surface material, if a simple surface. The airplane reflection model was also found to agree well with measurements. ReproJuction of completeJ page authorized ACKNOWLEDGMENT The author is indebted to James E. Evans for his many useful suggestions and comments on both the experiment and this document and to Massport for its cooperation! without which the experiment would not have been possible.

An application of Poisson process models to multipath sound propagation of sinusoidal signals

Abstract: A theory of multipath sound propagation is developed in which the number of rays between a source and receiver is modeled as a Poisson distributed random variable. Using probability distributions obtainable from environmental measurements and acoustic ray tracing programs, moments of the received sound field are derived as functions of space and time. An example is presented in which spatial and temporal coherence are estimated for a horizontal line array. It is shown that spatial coherence decreases with increasing angle from broadside, with increasing frequency, with increasing source speed, and with increasing ray vertical arrival angle at the receiver. Temporal coherence decreases with increasing source speed, with increasing ray vertical angle at the source, and with increasing frequency.Subject Classification: 20.20; 30.20; 60.20.

Radar Resolution and Multipath Effects

Abstract: Radar resolution and multipath effects , Radar resolution and multipath effects , مرکز فناوری اطلاعات و اطلاع رسانی کشاورزی

Airport survey for mls multipath issues

Abstract: This document is disseminated under the sponsorship of the Department of Transportation in the interest of information exchange. The United States Government assumes no liability for its contents or use thereof. Eight majorU..S. civilian tiwofls werevisi~d anddata onthemrfacematefial ofau siz~le buildings vistile: fromtbemnwaysw= obttimd.Thisitiormation iscatdoWedherein. Itis otiy with tbeaid. ofsuchi~omaion that we canadtie ssissues suchasthelike~lboud ofa sys=m perfomace chan&sdw topolarization, pattern comrol andcovers@control. A total of93buildinp and123sutiaces areincluded andthebreakdowhtmen ttivarious surfaces isasfollows: 74sutiaces wre corm~ted 17surfaces werecintir block 16surfaces wre brick 9 surfaces werecomrete 5 sutiaces weresmooth metal. Oftbe74cormga~dsurfaces 18wre ofthe " fit " variety, M wereoneoffive suh-cate~ries and theremaining 22me~d 15sti-categories forclassification.

The ARIES Program: Analysis and Generation of Simulated Radar Measurements

Abstract: : The report describes the modeling of measurement errors in precision, long range tracking radars due to the combination of environmental effects and variations in target cross section. The environmental effects considered are tropospheric and ionospheric refraction and time delay, ionospheric scintillation, and multipath through the sidelobes at low tracking angles. In addition, radar errors due to thermal and galactic noise, equipment jitter, and bias are included. Six-degree-of-freedom vehicle dynamics are modeled to realistically represent the changing radar cross section due to target dynamics such as tumbling and precession.

Theory of Scalloping Amplitude Errors in Standard VOR Bearing Indications

Abstract: A rigorous theory is developed for the course scalloping amplitudes observed in the bearing indications of an airborne standard VOR receiver in the presence of multipath signals. The VOR transmitting antenna pattern characteristics, the scattering properties of the multipath source, and the effects of ground reflection are all built into the theory so that their combined effects on the scalloping may be quantitatively investigated. The analysis assumes that the VOR receiver characteristics are ideal.

Spectrum of phase fluctuations caused by multipath interference

Abstract: Fluctuations of CW phase caused by time‐varying multipath interference are simulated with fully coherent ray propagation models Fixed‐system measurements of fluctuations of CW transmission are presented and compared for three ocean ranges When the sound‐speed perturbation used for model inputs are adjusted to give the same statistics of transmission‐loss fluctuations (decorrelation time, variance, and mean‐square bandwidth) as observed, then the associated fluctuations of CW phase also have the same statistics as the experimental results Phase fluctuations are shown to be strongly influenced by the nonlinear multipath process For low‐frequency internal waves (less than 1 cycle/h), and especially for internal tides, linear or direct fluctuations influence the phase spectrum, but for higher frequencies it seems likely that nonlinear effects dominate Therefore, the possibility of a direct linear relationship between the internal‐wave spectrum and the spectrum of phase fluctuations is unlikelySubject Cl

A Statistical Model for the Fluctuation of Sound Transmission in the Sea

Abstract: : The fluctuations in amplitude of a narrow-band signal in the sea obey a simple mathematical relationship. Under the premise that fluctuations are caused by random multi-path contamination of an otherwise steady signal, it follows that they are distributed in amplitude in the same way as is the resultant of a constant vector plus the random vector representing the sum of the various multipath contributions. This distribution function was obtained many years ago by S. O. Rice and is sometimes called the Rician distribution. It can be represented by a family of curves having as a parameter the relative power in the multipath additions. In this report the validity of this distribution for real signals in the sea is demonstrated by examples of fluctuations observed under a wide range of frequencies and propagation conditions. If it is indeed valid, the model permits one to predict the fluctuations of a transmitted signal in the sea around its mean intensity better, in many circumstances, than one can predict the mean intensity itself.

Mobile antenna gain at 900 MHz

Abstract: A program has been initiated to investigate the effect of the Urban multipath environment on mobile antennas at 900 MHz. The program involves the construction of several mobile antennas, careful measurement of these antennas on an antenna range to determine their characteristics in a controlled environment, then measurement of their characteristics in the multipath environment of cities. Measured results are compared to computed results to permit generalized conclusions to be reached. This paper presents the results of the first phase of the program; measurements made in the controlled pattern range environment.

Multipath expansion for computing acoustic propagation loss in horizontally stratified media

Abstract: The multipath expansion for horizontally stratified media is a generalization of acoustic ray tracing that is, in theory, exact and, in practice, accurate at the sonar frequencies. A variation of the expansion includes normal modelike terms along with the multipaths. Computed propagation loss for a shadow zone to the right of a caustic shows the well‐known exponential decay. However, the acoustic pressure appears to increase when the shadow zone is at the left of the caustic, at least in the example studied. Finally, computed propagation loss in the Pacific Ocean is compared with a wave model at 50, 500, and 2000 Hz. The multipath expansion is as accurate as the wave model, but far more practical.

The Relation Between the Specular-to-Interference Ratio and the Scintillation Index

Abstract: This concise paper points out evidence that in many cases the fluctuations in a signal transmitted through ionospheric irregularities do not obey Rician statistics and that therefore, the Rician relation between the specular-to-interference ratio and the scintillation index, recently discussed by Shaft, may not apply to ionospheric communications studies in these cases.

Use of the Cepstrum for Processing Multipath Signals. II

Abstract: : The report summarizes an investigation into the use of cepstrum analysis in connection with the processing of communication signals which have been corrupted by multipath effects. Following up on an earlier phase of work, the primary objective consisted of the development and performance evaluation of an experimental setup, incorporating a minicomputer, for on-line cepstrum analysis of baseband communication signals. The purpose of the cepstrum analysis is the extraction of multipath information which can then be used to maintain a multipath cancellation filter in correct adjustment. Another objective was the design of signal waveforms whose cepstra have desirable properties for this type of processing. The basic experimental setup was completed and is described in this report. Test data is presented which was obtained with signals from various modems and with simulated simple discrete multipath interference. Various sources of error are discussed and analyzed.

Polarization Effect on Multipath for Extended Targets

Abstract: The results of a theoretical investigation of the influence of polarization on multipath are presented. First, the case of a point target is examined and it is confirmed that the multipath effect tends to vanish for rough sea and small grazing angles. Models of a buoy and of a large ship are then introduced and it is shown that, for targets that are extended in height, the multipath effect for vertical and horizontal polarization is substantially the same.

Effect of Feedback Error on Two Rate Data Transmission over a Rayleigh Fading Channel

Abstract: Two rate or intermittent data transmission with perfect channel state feedback has been shown to be extremely effective at combatting the effects of fading due to multipath effects. However, such a scheme usually requires a two state decision to be sent back to the transmitter over a similar fading channel. Possible feedback errors result in incorrect transmitter decisions which greatly increase the number of errors in the main data stream. However, these decisions can be weighted by a function of the feedback channel signal-to-noise ratio so as to minimise the effect of unreliable decisions. In fact, a simple sub-optimal threshold strategy, which is only marginally inferior to the optimum one, can reduce the effect of feedback error by several orders of magnitude. This simple strategy is also used to assure the receiver follows the transmitter decisions to allow negligible performance deterioration due to feedback or feedforward error.

Rician distribution and the amplitude fluctuation of transmitted signals in the sea

Abstract: In 1945, S.O. Rice, in a classic paper, derived the amplitude distribution of the sum of a sine wave and a narrow‐band Gaussian random process. This function, called the Rician distribution, may be expected to apply to the fluctuation of transmitted sound in the sea whenever the received signal consists of a steady component plus a component resulting from scattering or multipath interference. In the limit when the steady component vanishes, the result is the well‐known Rayleigh distribution. In the present context, it is physically meaningful for the Rician distribution to have as its parameter the ratio of the power in the random component to the total power. This quantity, which we may call the Rayleicity ratio, ranges from zero for a steady signal to unity for a signal consisting entirely of numerous scattered or multipath contributions. The validity of the Rician distribution is illustrated by examples from field‐recorded data.