Showing papers on "Multipath propagation published in 1981"

Miniature Interferometer Terminals for Earth Surveying: Ambiguity And Multipath with Global Positioning System

Abstract: With the recent launching of several satellites of the global positioning system (GPS), a variety of schemes based on radio interferometry have been proposed for the accurate determination of relative positions of receiving terminals on the ground. Provided that the integer-cycle ambiguities of the interferometric phase observations can be correctly resolved, the baseline vector extending from the antenna of one terminal to that of another should be determinable with uncertainty much smaller than the 19-cm wavelength of the GPS transmissions. We propose a method of ambiguity resolution that is suitable for observations made with antennas of low directive gain. Such antennas are compact, but the feasibility of their use has been questioned because observations with them are susceptible to multipath interference. For short-baseline interferometric observations of GPS our method yields correct ambiguity resolution despite severe multipath interference and significant sky blockage, even when instability of the frequency standards governing the separate receiving terminals limits the time span of coherent integration to five minutes.

MMSE equalization of interference on fading diversity channels

Abstract: Adaptive equalization is used in digital transmission systems with parallel fading channels. The equalization combines the diversity channels and reduces intersymbol interference due to multipath returns. When interference is present and correlated from channel to channel, the equalizer can also reduce its effect on the quality of information transfer, important applications for interference cancellation occur in diversity troposcatter systems in the presence of jamming, diversity high frequency (HF) systems which must cope with interfering skywaves, and space diversity line-of-sight (LOS) radio systems where adjacent channel interference is a problem. In this paper we develop the general formulation for minimum mean square error (MMSE) equalization of interference in digital transmission diversity systems. The problem formulation includes the use of available receiver decisions to assist in MMSE processing. The effects of intersymhol interference are included in the analysis through a critical approximation which assumes sufficient processor capability to reduce ISI effects to levels small enough for satisfactory communication. The analysis also develops he concept of additional implicit or intrinsic diversity which results from channel multipath dispersion. It shows how the MMSE processor sacrifices diversity to suppress interference even when the interference arrives in the main beams of the receiver antenna patterns. The condition of near synchronous same-path interference is also addressed. Because the spatial angle of arrival of the interference may result in delay differences between interference signals in different antenna channels, interference delay compensation may be required. We show that this effect is compensated for with a small number of appropriately spaced equalizer taps.

Effects of Multipath Propagation on Digital Radio

Abstract: This paper summarizes experimental work at Bell Laboratories in the field of multipath propagation as it affects digital radio. Field and laboratory measurements are presented on commercial-quality high-speed digital radio systems working in the 4, 6, and 11 GHz common carrier bands. Circuits and equipment used in these systems are all state of the art, but no attempt has been made to describe them in any detail. Modulation types are offset-4 PSK, conventional 8 PSK, and 16 QAM. Outages (BER > 10^{-3}) vastly exceeding accepted objectives are measured on unprotected radio hops. A very effective method for reducing outages has been the use of a space diversity combiner followed by an amplitude slope equalizer. Results from single, as well as dual-polarized digital radio relay systems, are presented. The paper also describes the laboratory measurement of an equipment "signature" as a means to compare different digital radio designs in their sensitivity to multipath fading.

Single sideband receiver with pilot-based feed forward correction for motion-induced distortion

Abstract: Effects of multipath fading and Doppler frequency spread in a single sideband radio communication system are largely overcome by separating (17) from a received signal spectrum, a pilot frequency component including those effects and using that component to accomplish both phase (36) and gain (38) corrections on an information-containing frequency component. Signals essentially free of those effects are provided (10) for controlling necessary frequency translations in the receiver. One illustrative embodiment (FIG. 4) is conveniently arranged for facilitating diversity signal combining.

Minimization of multipath and doppler effects in radiant energy communication systems

Abstract: Apparatus is disclosed for reproducing transmitted information which is subject to multipath delay spread effects and doppler frequency-shift effects. The information is transmitted as a series of modulated information pulses of differing nominal frequencies. The nominal frequencies of successive pulses are repeatedly shifted through a predetermined sequence of frequencies. Arrivals of the transmitted pulses are detected at a receiver and are processed to determine the frequencies to which the transmitted pulses are doppler shifted and the multipath delays of each pulse. This frequency and delay information is then used to reproduce the transmitted information from the arrival pulses.

Performance of a frequency-hopped differentially modulated spread-spectrum receiver in a Rayleigh fading channel

Abstract: The performance of a spread-spectrum receiver previously described by the authors is analyzed in detail. Performance curves are given for a wide range of mobile radio channel conditions, including multipath distortion and correlated fading. The use of optimal filters to combat the former and space diversity to combat the latter are investigated. Some errors in the authors' earlier papers are corrected. The form of space diversity suggested has the unusual feature that it can be employed at the fixed station site only and provide diversity in both communication paths, i.e., to and from the mobile. Degradation due to fading and other aberrations is shown to be in the range 3-6 dB for typical mobile channel conditions. In a nonfading channel the receiver is suboptimum by about 3 dB.

Calculation of error probability for MSK and OQPSK systems operating in a fading multipath environment

Abstract: A technique is given for determining the error-rate performance of a class of digital communication systems operating with fading multipath interference. The approach uses a truncated series to represent the fading process and takes into account the effects of intersymbol interference caused by linear filtering in the receiver or channel. Application of the method to the cases of minimum shift keyed (MSK) and offset quadriphase shift key (OQPSK) systems reveals a slight performance advantage for MSK.

Cross polarization during clear‐air conditions on terrestrial links: A review

Abstract: This paper reviews experimental and theoretical research on cross polarization that occurs during clear-air conditions on terrestrial microwave links. After consideration of the various clear-air mechanisms that have been suggested, the conclusion is drawn that multipath mechanisms interacting with the cross-polarized patterns of the antennas are the most important. Theoretical models that explain these mechanisms and semi-empirical models that characterize the cross-polarized signal statistics are then discussed. Particular attention is given to the semi-empirical models since they can be used most readily for statistical predictions in design applications. Some new insight into the similarities and differences between models developed in France, Japan, and the United States is provided. Finally, application of the semi-empirical models in the design of digital systems is briefly considered, and the relative importance of cross polarization during clear-air and precipitation conditions is discussed.

Clear-air propagation on line-of-sight radio paths: A review

Abstract: Recent progress in theoretical and experimental work on clear air propagation on line-of-sight paths is reviewed. Descriptions of the physical mechanisms involved, illustrations of the variability in time and in space of the phenomena, and descriptions of the modeling of the propagation are addressed. Absorption by atmospheric gases is dealt with briefly. Refractive index structures are mentioned as background information. Theoretical and experimental studies of the physical modeling of multipath propagation conditions are dealt with in a detailed form. Various examples of multipath parameters modeling measured propagation conditions are presented. Aspects of statistical descriptions of multipath characteristics are also treated. Impacts on radio communications systems are mentioned. Attention is called to the difference between physical models of the propagation behavior and simplified models relating statistical characteristics. Duct theory and duct propagation characteristics are described. Examples of valuable statistical descriptions of radiometeorological data are mentioned. Other clear air effects are mentioned briefly. Theoretical and experimental progress in the description of low elevation angle propagation is also included. Finally, a number of areas for further work are pointed out. It should be noted that the aspects of cross polarization are not included, as they are dealt with elsewhere.

FM Receiver for use with motorcars

Abstract: An FM receiver provided with an array of antennas and suitable for use in a moving vehicle such as a motorcar, which is provided with means for automatically avoiding multipath noises. The means so operates as to connect one of the antennas in an instantaneously best sensing condition to an FM tuner.

Adaptive IF equalizer for digital transmission

Abstract: A radio signal will often suffer from amplitude and group delay distortion caused by multipath fading. An adaptive equalizer, which effectively cancels the amplitude distortion effects, is based upon the use of control voltages derived from selected portions of the IF band; the control voltages being selectively applied to bump and slope equalizer sections to cancel the effects of amplitude distortion. Cancellation of group delay distortion is predicated upon its being proportional to amplitude of the slope correction required. Thus, group delay correction is introduced in relation to the slope correction introduced to compensate for distortion introduced during normal multipath propagation conditions.

Modeling multlipath fading responses using multitone probing signals and polynomial approximation

Abstract: We show in quite a general way that highly accurate modeling of multipath fading responses is possible using low-order complex polynormals. This applies to all terrestrial radio systems in the channelized common carrier bands below 15 GHz, where channel widths are 40 MHz or less. The context of the study is a new multipath experiment being conducted in New Jersey over a 23-mile path at 11 GHz. The transmitted signal consists of up to nine tones in a 40-MHz bandwidth. These tones are coherently processed, sampled, and digitized in the receiver and recorded, during fading events, for later off-line reductions. Simple routines can be used to determine polynomial coefficients from these recorded data. This paper describes the signal processing and data reduction methods and analyzes them to assess the accuracy of polynomial fitting. The analysis uses a mean-square error measure and assumes a representative form for the underlying response function. Our results predict that the vast majority of multipath fading responses can be accurately approximated over bandwidths of 40 (62) MHz using first- (second-) order complex polynomials.

Adaptive array for elimination of multipath interference at HF

Abstract: A four element adaptive array using a modulated pilot signal added to the communication signal at the transmitter has been constructed and tested at high frequency (HF). The pilot signal modulation was designed to discriminate against undesired HF multipath. The array reduces the strength of undesired modes by spatial nulling, while maintaining response in the direction of the desired mode. The array was used to receive signals that propagated via a 150-mi over-ocean path. The antenna configuration and frequency selection were such that the ground wave and ionospheric modes of propagation were approximately equal. The pilot signal, used as a reference for the adaptive array, consisted of a single tone, phased reversed by a pseudorandom sequence. The bandwidth of the pilot signal was selectable at either 3 kHz or 6 kHz. A pulse sounder was used to measure the response of the array system to the arriving modes. During the test the ground wave 1E, 1F_{1} and 1F_{2} modes were observed, occasionally simultaneously. The array reference signal could be locked to any arriving mode and the array processor was able to discriminate against all other modes by directing spatial nulls. The reduction in strength of the undesired modes measured during the test was greater than 15 dB.

Frequency Diversity in Low-Angle Radar Tracking

Abstract: Elevation angle errors due to sea-reflected multipath are evaluated theoretically for a radar operating in an off-boresight monopulse tracking mode. The computer simulation accounts only for specular reflection. Angle estimates at three frequencies are assumed to be available. It is shown that one can improve upon a simple average of the three indicated angles by unequally weighting them according to rank (lowest, middle, or highest). Some sample computations show that there is an optimum difference between the three frequencies.

A Tracking and Telemetry System for Severe Multipath Acoustic Channels

Abstract: Underwater telemetry systems require special design when they are to be used in severe multipath acoustic channels. This paper describes a modulation/signal processing method, called Spaced-Frequency-Shift Keying (SFSK), that effectively penetrates such channels. SFSK includes features for reliable message validation, message identification, transfer of binary telemetry, and operation over Doppler ranges commensurate with maximum-speed underwater vehicles. Although SFSK was developed for the specific purpose of underwater vehicle tracking, it is easily adaptable to a wide range of underwater communications applications.

A stochastical model of the urban UHF radio channel

Abstract: A random model for urban ultrahigh frequency (UHF)- multipath propagation is proposed. The model uses an optical analogy, which can be well justified at these high frequencies. Several results are obtained. The transfer function and its autocorrelation properties are investigated. Also the time domain behavior is studied. The model is finally tested against experimental data from a field trial made in the town of Linkoping, Sweden.

Multipath depolarization theory combining antenna with atmospheric and ground reflection effects

Abstract: A formulation combining three sources of cross polarization in multipath propagation is given. The first relates to finite antenna isolation which becomes worse for off-axis multipath reception. Antenna amplitude and phase patterns are most important. The second, associated with atmospheric refractive index variations, requires asymmetric gradients for both linear and circular polarization. This by itself is the least signifiant source of depolarization but these rays cause variable interference between themselves and with other rays. The third is due to rays reflected once or twice from the ground. For linear polarization, inclined ground reflectors are required, but for circular polarization, polarization-dependent reflection coefficients are sufficient to give cross-polarization.

Antenna for vehicle

Abstract: PURPOSE:To reduce multipath noise, TV ghost, etc., by providing unidirectivity while improving sensitivity, by equipping side visors with antenna conductors for a vehicle which receives FM broadcasts and TV broadcasts. CONSTITUTION:Side visors 1 and 1' equipped on both sides of a vehicle 100 are made of insulating materials, such as plastics, and inside of them or on or to their external surfaces, antenna conductors 2 and 2' are formed or stuck. Further, a dipole antenna 2a is provided with a coil 7 and varacter diode 8 for impedance matching. A diode 8 is applied with a required DC voltage from the inside of the vehicle via a cable 10. With this constitution, the antennas 2 and 2' have unidirectivity through the reflecting effect of a vehicle body and multipath disturbance and TV ghost are therefore reduced. Furthermore, excellent diversity reception is performed by providing antenna elements 13-16.

Observation of Underwater Ultrasound Propagation in the Sea by Two-Period M-Sequence Signal Method

Abstract: The velocity of a current going in and out through a strait can be measured by the differences in underwater sound propagation times with and against the current. However, ambient pulsive noises often interfere with the measurements of sound propagation times when a pulsed sound is used. A two-period M-sequence signal method is therefore proposed which is free from ambient pulsive noises. A sound propagation test was carried out. Short term propagation stability and tidal effects on multipath propagation were observed.

An adaptive low-angle tracking system

Abstract: A system for source tracking down to a fraction of a beamwidth of a surface with unknown multipath reflecting properties is described The system has three major features: 1) the multipath returns are decorrelated by frequency diversity; 2) an efficient broad-band multiple beam antenna design is employed such that the data are characterized by a highly symmetrical (Toeplitz) correlation matrix; 3) Burg's reflection coefficient technique is employed to obtain rapid convergence to the maximum entropy spectrum from which the target location is determined Realistic simulations including imperfect decorrelation, antenna errors, and transient results show that the system can provide accurate low-angle estimates using a practical antenna and reasonable data-processing time

Modulation techniques for land mobile radio

Abstract: PM systems with 12.5 kHz and 25 kHz channel spacing have been compared with 12.5kHz AM and 5kHz SSB systems, and the effect of applying companding techniques to SSB as well as to PM has been investigated. The systems have been compared with respect to audio signal quality, speech intelligibility, spectrum utilization and applicational flexibility. The traditional - uncontrollable - field trials have been replaced by laboratory tests using advanced propagation simulation techniques, and the analysis has included pulse noise susceptibility and performance with multipath fading. The overall conclusion is that 12.5 kHz PM provides the best balance between transmission quality, spectral economy and applicational flexibility.

Effective range derivative for acoustic propagation loss in a horizontally stratified ocean

Abstract: A method for predicting acoustic propagation loss in a horizontally stratified ocean had been developed. We begin by comparing two widely used models, and demonstrate that unresolved discrepancies exist. Then classical ray tracing equations are developed by evaluating a multipath integral by stationary phase. The inaccuracy of ray theory is attributed to deficiencies in the stationary phase approach. In order to correct these shortcomings, we express multipaths in terms of Fresnel integrals and effective range derivatives. The approach is generalized to shadow zone propagation. Finally, we introduce a computer model that is based on the effective range derivative evaluation of the multipath expansion. The main emphasis of this paper, however, is not model comparison, but the effective range derivative technique, itself.

A computer simulation of XPD degradation due to multipath propagation

Abstract: A computer program has been constructed to study degradation inXpd(cross-polarization discrimination) due to multipath propagation. The geometric optics limit is used. The program will accept any analytic index of refraction including those with discontinuities in n or ∇n. Antenna effects are included and all reflected, refracted, direct and ground reflected rays can be considered. Using a realistic index of refraction and realistic discontinuity, we have found that, after the antenna pattern, the major influence onXpdis interference between the ground-reflected wave and the direct wave. There is also significant interference due to the waves reflected from the refractive index discontinuity which may arrive without ground reflection or may be reflected once or twice from the ground.

Multipath and interference effects in secondary surveillance radar systems

Abstract: Secondary surveillance radar (SSR) offers many advantages for air traffic control (ATC) compared to normal (or primary) radar including substantially lower cost and improved performance. Because of the increasing reliance being placed on SSR by ATC a number of the system problems associated with SSR are becoming less easily tolerated. The paper discusses these problems, which mainly arise from multipath and mutual interference effects, and proposes solutions.

Effect of the sound field structure on array signal gain in a multipath environment

Abstract: A wide‐aperture, horizontal line array receiving a low‐frequency, long‐range signal is considered. It is shown that the sound field structure as manifested by amplitude nonhomogeneity and the wave‐front corrugation can reduce the performance of a conventional linear beamformer. Beamformer expressions that explicitly show the effects of amplitude nonhomogeneity and wave‐front corrugation in a multipath environment are given. The limitations of some array algorithms that are inappropriate for a multipath environment are indicated. Array signal gain and side‐lobe suppression relations for an ocean multipath environment are generalized from similar relations that are not valid in an ocean multipath environment.

Multipath distortion detecting circuit of fm receiver

Abstract: PURPOSE:To improve the detection precision of multipath distortion, by causing the output of the FM demodulating circuit to pass through a high-pass filter and by using the output as a multipath distortion detecting signal and by subjecting this signal to DC conversion to output the converted signal as the multipath distortion detection output. CONSTITUTION:The demodulation signal including distortion input to pulse noise eliminating circuit 9 is output to terminal 9a through a high-pass filter. The output is given to the base of transistor 203 through capacitor 201 and is amplified. Amplified noise and distortion components are rectified by voltage multiplying rectifier 207 and are given to the base of switching transistor 217. When the charging voltage of capacitor 223 reaches a prescribed value by the output, switching transistors 229 and 231 become conductive simultaneously. Consequently, capacitor 233 is inserted between the left signal output of the output of stereo demodulating circuit 11 and the earth, and capacitor 235 is inserted between the right signal output and the earth. As a result, high frequency components of the output of circuit 11 are reduced.