Showing papers on "Multipath propagation published in 1980"

Introduction to spread-spectrum antimultipath techniques and their application to urban digital radio

Abstract: In a combination tutorial and research paper, spread-spectrum techniques for combating the effects of multipath on high-rate data transmissions via radio are explored. The tutorial aspect of the paper presents: 1) a heuristic outline of the theory of spread-spectrum antimultipath radio receivers and 2) a summary of a statistical model of urban/suburban multipath. The research section of the paper presents results of analyses and simulations of various candidate receivers indicated by the theory, as they perform through urban/suburban multipath. A major result shows that megabit-per-second rates through urban multipath (which typically lasts up to 5 µs) are quite feasible.

Analysis of multipath propagation structure in urban mobile radio environments

Abstract: An analysis is made of the physical structure of multipath propagation in an urban area. The incident angle, path length, and field strength are measured for each principal component of the received multipath signal. The results of the analysis reveal that the observed principal waves compare fairly well with those propagating via geometrical optics. The experimental data indicate that a limited number of ray-theoretical wave components play an important role in propagation in urban mobile radio environments.

A polynomial model for multipath fading channel responses

Abstract: We present a channel model useful for analyzing the effects of multipath fading in digital radio systems. The frequency response of a fading channel is represented by the function A 0 — ωB 1 + jωA 1 , where ω (=2πf) is measured from the center of the channel, and A 0 , A 1 ,and B 1 are variable coefficients that change slowly with time. The model consists of this function, the joint probability density function (pdf) for the three coefficients, and the average number of seconds per heavy-fading month for which this response applies. The model is derived from a large base of multipath fading data, obtained on a 26.4-mile path in Georgia in June 1977. It consists of nearly 25,000 recorded measurements of received power vs frequency in a 26.3-MHz bandwidth at 6 GHz. In this paper, we present the methods of data reduction and statistical analysis used to derive the model; describe some assessments of Us validity; and discuss its limitations, virtues and possible uses. By all available measures, the model is highly accurate. It suffers from a potentially important phase ambiguity that can be resolved only via new, coherent measurements. The existing model should prove very useful in the design and planning of such measurements.

Time- and frequency-domain representation of multipath fading on line-of-sight microwave paths

Abstract: Propagation on a radio path experiencing multipath fading can be modeled in the time domain by an N-path transmission network with each path characterized by gain and delay. We examine the inter∗∗∗relations of such a representation with the more frequently observed frequency domain characteristics of the channel. We show that the decibel gain and envelope delay distortion of the transfer function may both be expressed as summations of elementary functions each associated with a zero in the complex frequency domain. We find that the average frequency spacing of the zeros, and of transmission minima, is determined by the maximum delay. Although published propagation data indicate an average spacing of 100 MHz or more for common carrier radio hops, we verify that closely spaced zeros can occur with nominal path gains and short delay spreads. This is demonstrated by developing multipath synthesis procedures which allow path gains and delays to be determined from a specification of gain and/or delay distortion in a finite frequency band. The resulting networks are not unique; for instance, more than 5000 three-path delay networks with delay spread less than 10 ns can be found to provide a match over a 60-MHz band near 6 GHz to a sample gain shape with two minima. These developments provide the basis for a qualitative comparison of the ability of a channel modeling function to statistically represent the state of the transmission channel. We describe the limitations of both delay and complex power series models of varying degrees of complexity.

Computer simulation of multipath fading in the land mobile radio environment

Abstract: This paper describes a radically new approach to the simulation of multipath fading in the civil land mobile radio environment. The simulation package described takes into account vehicle movement through a particular environment, signal shadowing and coherent waves. Statistical results obtained with the simulation are compared with those obtained by v.h.f. field trials for three areas within the City of Bath which exhibit differing fading characteristics, and are shown to be in excellent agreement.

Frequency-hopping, multiple-access, phase-shift-keying system performance in a Rayleigh fading environment

Abstract: Recently, various Frequency-Hopping, Multiple-Access (FHMA) schemes have been proposed as alternatives to frequency-division (FDMA) techniques for guarding against interference from other users and multipath fading in transmitting digitized speech in mobile radio. While the advantage of frequency diversity against fading is well known, the system degradation of frame-asynchronous FHMA-PSK due to interference from other users and fading has not been studied, except by modeling the interference as additive white Gaussian noise (AWGN). In this paper, we analyze the performance of two FHMA-PSK systems in a Rayleigh fading environment. We assume that the interferers' addresses are assigned at random and derive expressions for the union upper bound on the bit error rate (P b ) for U users, each assigned N frequencies, i.e., N chips. As an example, we consider transmitting digitized speech (R = 31.25 kb/s) over the mobile radio channel (bandwidth W = 20 MHz), using orthogonal coding of rate r = 5/32 (N = 32). For the differentially coherent FHMA-PSK case, the number of users in a two-way radio system is limited to 26 by the performance of the mobile-to-base link, which corresponds to an equivalent bandwidth of 770 kHz per user.

A UHF channel simulator for digital mobile radio

Abstract: A device to simulate the channel propagation characteristics of the ground mobile environment at UHF has been designed, built, and characterized. This device provides a flexible, easily changed set of simulated channel characteristics, which allows the performance of a mobile radio unit to be evaluated in the laboratory under controlled conditions. The channel simulator was designed to simulate mobile platform speeds up to 675 mi/h and multipath components having differential delays approaching 10 µs. The channel simulator can provide up to four easily selectable, independently fading, multipath components, having calculated time delay spreads of up to 3.5 µs. The adjustable fading bandwidths and the exceptional long delays were implemented using the relative new signal processing technologies of charged-coupled devices and surface wave devices. Envelope statistics such as fading distributions and level crossing rates produced by the channel simulator show excellent agreement with theoretical prediction and documented experimental data.

Phase and amplitude dispersion for Earth-satellite propagation in the 20- to 30-GHz frequency range

Abstract: Amplitude and phase dispersion have been measured for over a year on a 19- and 28-GHz earth-space propagation path. In the experiment amplitude and phase differences were compared for a 28-GHz carrier with \pm264 -MHz sidebands and a 19-GHz carrier, all transmitted from a COMSTAR satellite. No dispersion (frequency selective fading) was found of the type caused by multipath propagation with a large spread in time delay or by resonances in the propagation medium. The only frequency dependences evident were due to the bulk properties of water in rain. The conclusion from this investigation is that amplitude and phase dispersion should not pose a problem for wideband (on the order of 1 GHz) satellite communication systems operating at frequencies greater than 10 GHz with elevation angles from the earth terminals of greater than 15\deg .

Diversity receiver for car

Abstract: PURPOSE:To minimize the generation of multipath noise, by taking the received electromagnetic waves of an antenna connected to a receiver as the reference and switching the receiver to an antenna over the reference through the sequential comparison with the received electromagnetic waves of other antennas. CONSTITUTION:When an antenna switching circuit 7 connects an FM radio receiver 9 to a received antenna A1, a switching scanning circuit 8 switches antennas A2-A4 to a subreceiver 12 sequentially every given period with the scanning signal ep from a switching control circuit 15, the received input of the antennas A2-A4 is detected at a reception state detecting circuit 13, and a detection signal eo is given to a comparator 14. To the comparator 14, a reception state detecting signal er of the antenna A1 from a reception state detecting circuit 11 is inputted and it is compared with the signal eo. When any of the reception state detecting signal eo of the antennas A2-A4 is over the signal er, the H level signal is fed to a switching control circuit 15 at the circuit 14, and the circuit 15 feeds a switching control signal ee to the circuit 7, where the receiver 9 is switched to an antenna over the reception state of the antenna A1.

Theoretical studies of a simulcast digital radio paging system using a carrier frequency offset strategy

Abstract: In a multistation simulcast digital radio paging system, each base station transmits the same RF signal simultaneously with the resulting efficient frequency utilization and simplified receiver design. In this system a paging receiver in the overlapping area receives several RF signals transmitted from different base stations. When frequency-shift keying (FSK) is used as a modulation method, experimental test results have already shown that the timing of each RF signal should be synchronized as closely as possible, but that the carrier frequency of each transmitter should be set following a certain offset assignment. The signal transmission performance in a multipath fading environment can then be markedly improved. The cause of this improvement effect is theoretically analyzed. It is clarified that the improvement effect is caused by transforming the probability distribution of time-averaged signal power.

Extraction of multipath parameters from swept measurements on a line-of-sight path

Abstract: A computerized method of obtaining the multipath parameters from swept measurements is described, and some results are discussed. The accuracy of the multipath parameters determined is about \pm0.3 ns in delay and \pm0.1 in amplitude. The time variation of the propagation medium during the 331-ms sweep time is included in the muitipath model. This has a significant impact on the analysis and on the results obtained.

Tracking radar system

Abstract: In a tracking radar system target glint and multipath sometimes produce signals which falsely indicate high acceleration rates of the target. These signals cause the boresight of the antenna to be driven away from the direction in which the target actually lies. To avoid this problem, received signals from a receiver are fed through a switch which is opened whenever the target appears to have an acceleration or velocity above a threshold value. The threshold value is varied so that the proportion of the time during which the switch is open is kept approximately constant. Thus, if there is a period during which a large number of high acceleration or high velocity signals appear from the receiver (for example is there is a very high noise level) the switch still remains closed for a certain proportion of the time period. This ensures that the tracking system never loses the target while also ensuring that false tracking due to target glint and multipath is minimized. A circuit is included so that, when the switch is open the antenna is not brought to a complete halt but rather is allowed to continue to move in the direction that it was moving before the switch was operated.

Impairment mechanisms for SSB mobile communications at UHF with pilot-based doppler/fading correction

Abstract: Several impairments are associated with the pilot-based correction of a single sideband (SSB) voice signal for the effects of vehicle motion in a Rayleigh multipath field. These include the distortion caused by a limit on the available correction gain, the distortion resulting from frequency selective fading, the degradation of the signal-to-interference (S/I) [noise (S/N)] ratio, and the distortion induced because of interference to the pilot from a cochannel pilot or from noise. We have investigated these impairments for a specific feedforward, phase- and gain-correcting, ideal receiver. For this receiver, the S/I ratio is degraded at baseband because of the time-varying nature of the gain correction signal and the assumption that the correction signal is correlated with the desired voice signal fading envelope but is independent of the interferer fading envelope. A limit on correction gain appears necessary to limit the magnitude of several of the impairments, and has been considered as a parameter in the analysis of each impairment type. The results are here extended to characterize an “equal-gain” space diversity receiver. Diversity combining is applied after phase correction, but before gain correction to maintain proper voice correction while retaining the benefits of the improved envelope statistics. This paper deals only with the SSB idealized channel, not with a system employing SSB modulation; the authors recognize that many complicated systems problems (eg., antenna combining, channel generation, and cost reduction) must be solved before a working system using the channel analyzed herein can become a reality.

Tropospheric microwave propagation--An X band diagnostic system

Abstract: A microwave system for direct measurement of angles of arrival and multipath delay times is described. Based on the utilization of interferometer and very wide frequency sweeping (1- GHz range) techniques, high accuracy measurements on these parameters are possible. Some experimental results are presented which suggest that movements in the angle of arrival of a single ray path (as opposed to multipath) may be responsible for much of the fading experienced on light-of-sight microwave links in southwestern Ontario.

Ultrasonic inspecting device for junction part

Abstract: PURPOSE:To automatically inspect the area and strength of a junction part with good accuracy by detecting the information mecessary for automatic discrimination from the multipath reflected waves which are actually obtained and performing data processing. CONSTITUTION:Only the multipath reflected wave signals reflected from the junction part of an object S to be measured are detected from the ultrasonic multipath reflected wave signals outputted from a pulser receiver U by a multipath reflected wave detecting circuit 31, and the quantity of the multipath reflected wave signals from the junction part and the prescribed value corresponding to a specified junction area are compared by a discriminating circuit 33, and whether the area of the junction part of the object S is above the specified value or not is discriminated. Thereby, the state of the junction part of the object S is automatically inspected.

A Spread Spectrum System with Frequency Hopping and Sequentially Balanced Modulation--Part II: Operation in Jamming and Multipath

Abstract: The broad-band noise performance of a spread spectrum system with frequency hopping and sequentially balanced modulation was analyzed in Part I of this paper. Here, the analysis is extended to include operation In jamming environments and in specular multipath. Bit-error probability and ranging accuracy curves are presented in each case.

Effects of Intersymbol Interference on a Rayleigh Fast-Fading Channel

Abstract: It is shown in this paper that the performance of certain Rayleigh fast-fading channels can be accurately and efficiently evaluated using the elementary trapezoidal integration rule. The effects of intersymbol interference on such channels are analyzed.

A new adaptive antenna for elevation angle estimation in the presence of multipath

Abstract: A new adaptive antenna is described for elevation angle estimation as encountered in low angle tracking radar. The antenna is used to sense the directions of arrival of both the direct and multipath signal in a symmetric manner. A priori knowledge of the reflection coefficient of the reflecting surface is not necessary. Computer simulation results are included to illustrate the effects of (1) signal-to-noise ratio, and (2) the phase difference between the two signals at the centre of the array, on the performance of the system.

Multipath fade equalizer

Abstract: An adaptive multipath fade equalizer is disclosed comprising differencing means (24) for forming a difference between an input equal to said equalizer and a component of the equalizer output signal and a feedback loop (21) for coupling said component of output signal to the differencing means. The feedback loop includes a fixed delay T (22), a variable attenuator (23) and a variable phase shifter (25). A phase control network (30) and an attenuation control network (31) compare selected portions of the frequency spectrum of the signal and adjust the variable controls to minimize the effects of multipath fades upon the signal. It is an advantage of the equalizer that it automatically adjusts for changes in the frequency and depth of the fade notch.

Multipath Doppler shift vertical speed measurement system

Abstract: A radar system for measuring the vertical component of velocity of a target uses the differences in Doppler frequency between the direct and multipath radar returns. The system filters from this difference a component caused by the targets relative radial speed and then determines the vertical speed from the remaining component.

New realisation of maximum likelihood receiver for low-angle tracking radar

Abstract: A new realisation of the maximum likelihood receiver is presented for resolving the effect of multipath in a low-angle tracking radar Results are included to confirm the validity of this receiver structure

Multiple terminal acoustic communications system design

Abstract: A design is presented for a system providing highly reliable command and control acoustic communications between a mother ship and a number of small fast submersibles The small submersibles may be employed for underwater mining, exploration, bottom mapping, or military surveillance Modulation and coding design is presented; the techniques discussed provide multiple protection against multipath and fading, high reliability, acceptable transmitted signal total time duration, simplicity, and economy The required decision point signal-to-noise ratio (SNR) for Rayleigh fading conditions is derived for the modulation and coding design Particular attention is paid in the receive signal processing to the Doppler (relative velocity) and Doppler variation (relative acceleration) problems inherent in a scenario with mobile endpoints A Figure-of-Merit (FOM) calculation is provided for typical geometrical and environmental parameters It is shown for a realistic source level that the required SNR can be achieved at long range with considerable endpoint relative motion

Source location with an adaptively focused array

Abstract: The maximum likelihood procedure for estimation of source location parameters in a multipath medium using a passive array of point sensors is presented. The maximum likelihood estimation procedure is explained in terms of combined focused beamformer operations on each multipath arrival. An approach to the adaptive realization of the multipath focused beamformer is given which exploits an orthogonal decomposition of the estimated spatial cross-spectral density matrix (CSDM) of the array. An example from passive sonar array processing is given.

FM Multipath Distortion in Automobile Receivers has been Significantly Reduced by a New Antenna System

Abstract: Multipath interference is one of the most serious problems for mobile receivers. To reduce interference many methods and systems have been developed. Two most effective methods are space diversity and electrical cancellation of the interference. The latter method needs not only some complex circuitry such as microprocessor but also a relatively long adjusting time. On the other hand space diversity has comparatively simple electrical structure and is easily realized by means of adequate antennas and their control circuits. The problem of space diversity especially for automobile reception is that a large space is required for a good diversity effect. Therefore, these two methods are not suitable for mobile FM receivers.

Analytical method to evaluate the effect of multipath fading on long-haul high-capacity digital radio-relay systems

Abstract: The outage probability is analytically evaluated for a digital transmissionCPSKsystem (four and eight phases) operating at 140 Mbit/s

Display system for interference distortion value caused by multipath disturbance of fm radio wave

Abstract: PURPOSE:To obtain a reproduction state of high fidelity by obtaining an indication value corresponding to a sense of hearing distortion by extracting only a signal component of approximate 19kHz in a signal outputted by a FM demodulator and then by displaying a multipath distortion value by its variation value. CONSTITUTION:A demodulated signal from FM demodulator 4 is supplied to narrow-band-pass filter BPF1 of 19kHz in center frequency, the output signal of which is supplied to high-gain amplitude limiting amplifier HAL to obtain a square wave of constant amplitude by converting. The output signal of it is applied to narrow-band-pass BPF2 of 19kHz in the center frequency of its passing band, thereby obtaining a sine-wave signal. The output signals of both BPF1 and BPF2 are processed by arithmetic circuit SUB to obtain an interference distortion component, which is converted by signal converter circuit SCC into a signal adequate for driving an indicator, so that indicator M will display its value. Consequently, the interference distortion can quantitatively be displayed which is caused by multipath disturbance of a FM radio wave.

Multipath interference nulls in long‐range, low‐frequency, acoustic propagation by normal modes

Abstract: Several aspects of multipath interference beats are here investigated by analyzing long‐range propagation losses computed by normal mode theory. Three typical deep water sound‐speed profiles are used at frequencies of 100 Hz and below. The phase shifts which accompany the nulls of interference beats are shown to be organized into patterns resembling certain patterns in the phase of ocean tides. The average distance between nulls of a specified depth is determined by counting and is found to differ widely over the three profiles. These differences arise from the convergence zone structure, which in each of the three profiles is different and leads to different multipath combinations. Plots of interference length between adjacent modes versus mode phase velocity are shown to give a result similar to ray theory loop length and are useful in determining which multipaths are present at a given point. Certain features of the mode theory results can be identified with ray theory caustics, but at sufficiently low...

Multipath discriminating antenna system

Abstract: To reduce guidance errors in a microwave landing system from false signalseflected by objects along runways, the transmitting antenna radiates an elliptically polarized beam. At the airborne receiver an antenna, such as a pyramidal horn, maintains the separation of the horizontal and vertical components. These components are separated and then split on an equal power basis. One part of the vertical field component is shifted by 90 degrees and added to one part of the horizontal component in a detector, whose output is in effect like that of a circularly polarized receiver. The other parts of the horizontal and vertical components are separately detected. The outputs of the three detectors are combined in a multiplier, to yield a resultant output. For a very broad angular distribution the undesired reflected signals are strongly attenuated.