Showing papers on "Multipath propagation published in 1983"

A new approach to multipath correction of constant modulus signals

Abstract: An adaptive digital filtering algorithm that can compensate for both frequency-selective multipath and interference on constant envelope modulated signals is presented. The method exploits the fact that multipath reception and various interference sources generate incidental amplitude modulation on the received signal. A class of so-called constant modulus performance functions is developed which sense this AM term but are insensitive to the angle modulation. Simple adaptive algorithms for finite-impulse-response (FIR) digital filters are developed which employ a gradient search of the performance function. One of the resulting algorithms is simulated for the example of an FM signal degraded by specular multipath propagation. Substantial improvements in noise power ratio (NPR) are observed (e.g., 25 dB) with moderately rapid convergence time. These results are then extended to include tonal interference on a FM signal and intersymbol interference on a QPSK data signal.

Detection of non-Gaussian signals by frequency domain Kurtosis estimation

Abstract: The detection of signals by power spectrum density (PSD) estimation is a well known and often employed method. The PSD estimate is essentially a second order measure which is not sensitive to the statistical nature of the signals. For non-Gaussian signals, a frequency domain Kurtosis (FDK) estimate supplements the PSD estimate and in some cases of practical importance is superior as a detection statistic. Non-Gaussian signals occur in underwater acoustics due to multipath and frequency modulation effects. Specifically, we have considered sinusoidal and narrowband Gaussian signals which, when propagated through fading or multipath environments, are received as non-Gaussian in terms of a frequency domain Kurtosis estimate. The environment is modeled by introducing amplitude probability density distributions which are due to the fading or multipath conditions. Several distributions were considered previously which included Rayleigh and log-normal, both of which have been experimentally verified to exist in the ocean. The asymptotic probability of detection for a randomly occurring signal is derived for the PSD and FDK. A simulation comparing the probability of detection for the FDK and PSD is also included.

Frequency-Selective Fading Effects in Digital Mobile Radio with Diversity Combining

Abstract: we analyze the effects of frequency-selective fading in a cellular mobile radio system that uses 1) phase-shift keying (PSK) with cosine rolloff pulses, and 2) space diversity with maximal-radio combining. The distorting phenomena with which we deal are multipath fading (which produces the frequency selectivity), shadow fading, and cochannel interference. The relevant quality measure is defined to be the bit error rate averaged over the multipath fading, denoted by (BER). The relevant system performance characteristic is defined to be the probability distribution for (BER), taken over the ensemble of shadow fadings and locations of the desired and interfering mobiles. To obtain numerical results, we use a combination of analysis and Monte Carlo simulation, invoke widely accepted models for the multipath and shadow fadings, and assume a cellular system with seven channel sets and centrally located base stations. The outcome is a set of performance curves that reveal the influences of various system and channel parameters. These include: the number of modulation levels (two or four), the diversity order, the shape of the multipath delay spectrum, and the standard deviation (or delay spread, τ 0 ) of the multipath delay spectrum. Practical factors accounted for in these assessments include fading- and interference-related timing recovery errors and combiner imperfections. Our results highlight the importance of the ratio \tau_{0}/T , where T is the digital symbol period. They show that the delay spectrum shape is of no importance for \tau_{0}/T \leq 0.2 , but can have a profound influence for \tau_{0}/T \geq 0.3 . We also find that using 4-PSK leads to better detection performance, in certain cases, than using 2-PSK.

Antenna Diversity Performance in Mitigating the Effects of Portable Radiotelephone Orientation and Multipath Propagation

Abstract: Antenna diversity can mitigate signal impairments caused by random angular orientation and multipath radio propagation when using portable radiotelephones. Cumulative distributions of signal-to-noise ratio ( S/N ) were determined for antenna diversity using realistic orientation and multipath propagation models. In a random orientation and multipath propagation environment with -6 dB average crosspolarization coupling, two-branch selection diversity with two perpendicular antennas yields an S/N distribution with the same slope as two-branch selection diversity in the fixed-oriented mobile radio environment. The distribution for random orientation is about 4.5 dB worse, however, than the mobile radio distribution.

Angles-of-arrival and delay times on terrestrial line-of-sight microwave links

Abstract: A ray-tracing approach is used to generate quantitative information on the ranges of angles-of-arrival, delay times, and amplitudes associated with individual paths in a multipath propagation situation as expected on typical terrestrial microwave links. Some experimental evidence relating to, and confirming, the predicted values is considered.

Ultrasonic imaging system with correction for velocity inhomogeneity and multipath interference, using an ultrasonic imaging array

Abstract: An ultrasonic imaging system (50) has a means (70) connected to compare the signals received by the transducer elements (54-1 through 54-21). A means (130) is connected to inhibit or otherwise modify gain of selected ones of the signals based on the comparison to reduce multipath ultrasonic wave interference, refraction or obstruction image distortion or degradation. A transducer array (52) is formed as sectors of circles with substantially equal arc lengths. A digital signal processing circuit for the system (50) has a plurality of dual port RAMs (66-1 through 66-21) as delay lines and a cross-correlation means (70) including output addressing circuit (130) controlled by a microcomputer (128). The output addressing circuit (130) generates addresses for output of information from the dual port RAM delay lines (66-1 through 66-21) to accomplish time delay correction and elimination or other gain interference, refraction or obstruction.

HF avalanche relay communication technique

Abstract: An "avalanche" relay communication network is configured of a plurality of transceiver stations spread out over a geographic area of interest to establish multipath communication diversity among the stations. The transceiver equipment at each station has the capability of simultaneous transmission over the same frequency through a "common knowledge" network timing scheme such as TDMA and has the capability of taking advantage of received multipath signals. Communications between an originating station and an intended recipient station are achieved by the originating station modulating onto an HF carrier a digital packet formatted to contain the number of times the message is to be repeated and a means of establishing the quality of the received message. All stations which have correctly received the packet repeat that same message on the same carrier frequency at the same preestablished future absolute time (or times, based on the number of repeats).

Theoretical and experimental investigation of feedforward signal regeneration as a means of combating multipath propagation effects in pilot-based SSB mobile radio systems

Abstract: A technique is described, feedforward signal regeneration (FFSR), to combat the effects of multipath propagation on VHF and UHF pilot tone single sideband (SSB) mobile radio systems. Unlike feedforward automatic gain control (FFAGC), FFSR suppresses both the random amplitude and phase fluctuations in the received signal. Extensive laboratory and field tests have shown that the operation of SSB at UHF frequencies is a viable proposition for both speech and data communication.

Signal Processing Equipment and Techniques for Use in Measuring Ocean Acoustic Multipath Structures.

Abstract: : This work describes a set of digital signal processing techniques and associated hardware that were developed at the Cooley Electronics Laboratory (CEL) of The University of Michigan for use in measuring the multipath structure of acoustic propagation channels in the ocean. The design and construction of this hardware, its programming and its use to measure multipaths were done as part of CEL's support of the 1981 Ocean Acoustic Tomography Demonstration Experiment. The purpose of the Tomography experiment was to use acoustic probe signals to determine the thermal structure of a volume of the ocean which, at the surface, corresponded to a square 300 km per side.

Technique for digital radio space diversity combining

Abstract: The present invention relates to a space diversity combiner which includes two branches wherein separate multipath signals received from a remote source are propagated. The branch signals are combined and the combiner output signal is used for obtaining noncoherent spectrum measurements which are in turn used to account for both dispersion and noise in controlling the relative amplitudes and phases of the two branch signals to provide a maximum performance measure for the combiner.

Characterization of multipath parameters for line-of-sight microwave propagation

Abstract: Ray equations are derived for microwave multipath propagation in a two-layer atmosphere, and the connection between the different solutions is demonstrated by numerical examples. Analytical expressions are developed to characterize the condition for multiple rays, the variation in angle-of-arrival (AOA), and the delay spread. The results suggest that more than three rays will be rare in practice and that the three-ray model (obtained from the zero-order rays defined in the paper) may provide a good representation of selective fading on microwave links.

A Stochastic Simulation of Low Grazing Angle, Forward Scatter, Over-Water Multipath Effects

Abstract: : This report describes a stochastic model and a simulation of low grazing angle, forward scatter, over-water multipath applicable to microwave frequencies. The model is based primarily on the work of Charles Beard, using a coherent and an incoherent vector summation to describe the reflected signal. A second order stochastic model and an associated stochastic simulation are described. Controlling parameters are sea state, radar frequency, transmitter/receiver geometry, and radar antenna patterns. A model is suggested for the incoherent spectrum when one or both radars are moving.

Arrival time determination using iterative signal reconstruction from the phase of the cross spectrum

Abstract: A situation encountered in such applications as seismics, ocean acoustics, radar, sonar and others, is that of an unknown wavelet propagating nondispersively in a reverberatory or multipath environment. Two receivers placed in such an environment will each record the arrival of this wavelet numerous times and each time with a different attenuation factor. In this paper, we demonstrate that if 1) there is no noise, 2) the interreceiver delays are distinct, and 3) the arrival sequence at each receiver has no zero phase convolutional component, then the arrival times and attenuation factors at each receiver can be determined as well as the wavelet itself. We show this via the construction of a finite length sequence whose phase equals that of the cross spectrum of the two received signals. This reconstructed signal, under the conditions above, can then be "inverted" to recover the desired arrival time and attenuation information at each receiver. An example using synthetically generated data is provided.

Intelligent teletext decoder

Abstract: An equalizer and decoder system compensating for multipath distortion in digital format signals sent by teletext, for example. The system loads demodulated data signals and a received reference signal into a charge coupled device clocked in real time by a clock circuit. Later, the reference pulse is clocked to a microprocessor which determines tap coefficients for a transversal filter to correct the multipath distortion in the reference signal. The data signals are then applied to a transversal filter weighted with the calculated tap weights. After filtering, signals are decoded by the microprocessor. In another embodiment a charge coupled device with tapped stages is used at one time for receiving the data signals in real time, and at a later time for part of a transversal filter. In a simplest embodiment, distortions are removed by the microprocessor itself, without any separate transversal filter.

Time division adaptive retransmission for reducing signal impairments in portable radiotelephones

Abstract: Multiple-antenna receiving diversity was shown previously to be effective in mitigating the effects of random angular orientation and multipath radio propagation for portable radiotelephones. It is shown that time-division adaptive retransmission used with appropriate antenna configurations can also mitigate these effects. The retransmission configurations require fewer antennas than the receiving diversity configurations for a given improvement in relative signal-to-noise ratio (S/N). Cumulative distributions of S/N were determined for adaptive retransmission and diversity using random orientation and multipath propagation models. Distributions of S/N for systems with two antennas at the portable set and two appropriately polarized antennas at the portable radiotelephone terminal (PORT) are similar to distributions for two-branch selection diversity in the fixed-orientation mobile radio environment. Systems with one portable antenna and two PORT antennas have distributions with slopes similiar to two-branch mobile radio distributions but the distributions for the portables range from 3 to 7 dB worse.

A Simplified Method for Prediction of Multipath Fading Outage of Digital Radio

Abstract: We propose a simplified method based on the correlated Rayleigh probability distribution of in-band radio signals. It can be efficiently applied to the calculation of the outage for various radio path length, propagation terrain, radio frequency, bandwidth, and digital modulation techniques. Predicted results are compared to various propagation data.

Maximum-power and amplitude-equalizing algorithms for phase control in space diversity combining

Abstract: Space diversity receivers equipped with continuous combiners have, in recent years, found wide use in terrestrial microwave radio systems as a means of mitigating the effects of multipath fading. An analysis of two phase-control algorithms for space diversity combining, comparing the performance capabilities of a maximum-power and an amplitude-equalizing algorithm, is presented in this paper. The extended capabilities of the equalizing algorithm in mitigating linear and quadratic channel distortions are investigated through computer simulations. A reduction in digital radio outage time afforded by the equalizing algorithm (relative to that of the maximum-power algorithm) is approximated for over-water-paths where it is assumed that fading can be described in terms of a two-ray fading model. For the model chosen, we show that the additional outage reduction owing to the equalizing phase-control algorithm is highly dependent on the statistics of the multipath delay parameter, τ, and can vary from unity to a factor of five for different probability-density functions of τ.

Acoustic Telemetry Of Video Information

Abstract: Increasing use of underwater instrumentation packages and remotely operated submersibles is giving rise more frequently to applications where availability of an acoustic link for telemetry of video information would be useful. Such links are very sensitive to multipath interference which is an often present property of underwater sound propagation. Successful operation is normally limited to certain ideal situations which minimize multipath such as near vertical links in deep water. However, by using narrow beam transducers, alignment of the link with the most direct or robust path between two underwater points will reduce or eliminate any multipath interference common in coastal waters. This paper documents the use of a narrow beam ( \simeq 1\deg ) acoustic link to transmit slow-scan video frames underwater for distances up to 300 metres. The transudcer projected the beam at an angle of 10\deg down from horizontal with a source level of 234.5 dB re 1 \mu P at 1m. Very good quality frames could be received continuously at 8.5s intervals when the receiving hydrophone was properly located in the beam. Attempts were made to relate the video quality to pulse modulated carrier measurements.

Adaptive canceller for elevation angle estimation in the presence of multipath

Abstract: The paper describes a new adaptive canceller for estimating the elevation angle in a low-angle tracking radar environment in the presence of specular and diffuse multipath. The adaptation is achieved using the complex least-mean-squares (LMS) algorithm. Computer-simulation results are included to demonstrate the usefulness of this scheme for estimating the elevation angle when it is a small fraction of a standard beamwidth.

Characterization of the Underwater Channel Application to Communication

Abstract: This paper is a synthesis of the CEPHAG’s last years studies on the submarine acoustic channel characterization and its application to optimal communication. The methodology used for the characterization is recalled and some experimental results are given. More recently we have used some of these models to construct and value an optimal binary communication scheme. The more advanced results concern the fading multipath case and the frequency dispersive channel. Other results, concerning the estimation of data transmitted in a multipath channel, are also mentioned.

A Diagnostic Probe to Investigate .Propagation at Millimeter Wavelengths

Abstract: A diagnostic probe used to fully describe the propagation characteristics of a millimeter wave channel by nearly simultaneous recording of an impulse response, frequency spectra, amplitude response, and bit error rate is discussed. A 30.3 GHz carrier accommodates the subcarriers and baseband modulation modes in a fully coherent network. Signal–to–noise determining components will permit BER performance of better than 10-8 at a 500 Mb/s rate with a 25 dB fade margin through a clear air 50 km distortion free path. Back to back operation of the terminals and a short atmospheric path is used to establish the reference performance level of the hardware. Controlled multipath tests are reported to demonstrate probe capabilities and to obtain reference data to better classify the fades and resulting distortion which occur on terrestrial links at millimeter wavelengths. With dual reflectors, multipath data sets were compiled to aid in predicting bit error rate performance resulting from a combination of signal–to–noise ratio change and intersymbol interference. Included with the above data are corresponding measurements of the impulse response and amplitude distortion on the channel. Two additional coherent cw channels, 11.4, 28.8, and a Soon to be added 96.1 GHz channel, are included with the 30.3 GHz probe to aid in analyzing selective fades which fall outside the bandpass and to evaluate frequency dependent properties of a link.

An experimental study of atmospheric optical transmission

Abstract: This paper reports measurements made on a 23-mile, experimental, atmospheric, optical-transmission link for possible use as a standby substitute for microwave radio when the radio suffers severe multipath or obstruction fading. To allow comparison of transmission on a microwave and on an optical path, we used two parallel systems. One, a microwave system at 11 GHz, allowed frequency-selective fading to be measured, and the other, an optical system at 6328A, allowed amplitude changes of the received optical signal to be obtained. The measured clear-air loss on the optical path is 27 dB. This measurement is made up of 17 dB of atmospheric scattering and 10 dB due to the receiving antennas intercepting only 10 percent of the beam at the receiver. The signal-to-noise ratio, calculated using measured background sky-noise and measured received power, is about 60 dB for a 100-MHz band. The beam diameter was measured to be 32 feet where the signal is down 20 dB. On the single occasion when frequency-selective microwave fading was observed, there was no fading of the optical signal. We find that it is necessary to control the transmitter elevation angle with a servo error signal from the receiver; the azimuth angle needs only occasional manual correction. The optical beam can be automatically reacquired after severe atmospheric attenuation, and that scintillation is usually several decibels, and occasionally as much as 10 dB.

Practical equalization and synchronization strategies for use in serial data transmission over h.f. channels

Abstract: This paper describes methods for the serial transmission of data which have been developed and subsequently tested in practice during the course of experiments carried out over a 1000 km h.f. radio link. The methods include the use of adaptive equalization to combat the severe effects of channel-induced intersymbol interference and techniques for securing carrier and bit synchronization in the presence of intersymbol interference. In the system described, a probe pulse is imbedded into the data stream and use is made of the fact that the shortest signal path arising from multipath ionospheric reflections in the h.f. channel is on average the most stable of the multipaths. The presence of the probe signal reduces the data rate by 25%, but enables synchronization to be ensured in the presence of multipath and, furthermore, it enables the channel to be adaptively monitored for equalization purposes. In addition, it can be used to overcome system failure which is inherent in the use of decision-directed equalization operating in the presence of high levels of noise and multipath.

The AR6A Single-Sideband Microwave Radio System: Equalization for multipath fading

Abstract: Multipath fading can introduce severe amplitude distortion and level changes in a radio channel. These must be dynamically equalized to meet the toll transmission requirements of AR6A. This article describes equalizer circuitry at intermediate frequencies, which continuously senses the level as a function of frequency in the transmission band and dynamically corrects the effects of multipath fading.

Time division adaptive retransmission for reducing signal impairments in portable radiotelephones

Abstract: Multiple-antenna receiving-diversity was shown previously to be effective in mitigating the effects of random angular orientation and multipath radio propagation for portable radiotelephones. This paper shows that time-division adaptive retransmission used with appropriate antenna configurations can also mitigate these effects. The retransmission configurations require fewer antennas than the receiving diversity configurations for a given improvement in relative signal-to-noise ratio (S/N). Cumulative distributions of S/N were determined for adaptive retransmission and diversity using random orientation and multipath propagation models. Distributions of S/N for systems with two antennas at the portable set and two appropriately polarized antennas at the Portable Radiotelephone Terminal (PORT) are similar to distributions for two branch selection diversity in the fixed-oriented mobile radio environment. Systems with one portable antenna and two PORT antennas have distributions with slopes similar to two branch mobile radio distributions but the distributions for the portables range from 3 to 7 dB worse.

Digital Microwave Transmission Tests at the Pacific Missile Test Center, Pt. Mugu, California

Abstract: This report describes an experiment designed to measure the performance of digital microwave radio systems in a line-of-sight (LOS) link that is subject to strong atmospheric multipath. It outlines the measurements made to characterize the propagation medium, the status of space-diversity receiver systems equipped with switched combiners, and the burst-error statistics caused by the multi path environment. The results presented in this report compare the performance of two radios, operating simultaneously in the same transmission channel. Detailed error distribution data have not yet been analyzed.

Urban Millimeter Wave Propagation Studies.

Abstract: : Measurements of millimeter wave propagation in urban areas at 9.6, 28.8 and 57.6 GHz, and a determination of signal levels reflected from several building surfaces, were made to study wave propagation characteristics in a city environment. Principal emphasis was on the evaluation of communications link reliability, detectability and usable bandwidth as a function of position of terminals. Reflected signal level measurements were performed on building surfaces of concrete aggregate, painted smooth concrete with protruding ribs, brick, and metal siding. Multipath measurements were recorded as a function of distance for several runs over paths of about 1 kilometer in the center of the Denver metro area. Non-line-of-sight observations showed a large number of substantial signals arriving from a wide range of angles. This report contains the results and analysis from single-path and multipath and multipath reflections recorded for the study. Included are an evaluation of multipath effects for both urban and non-urban line-of-sight path and measurements of signals received on non-line-of-sight paths, using edge diffraction effects. (Author)

Frequency diversity receiver

Abstract: PURPOSE:To reduce multipath disturbance by supplying the detection output of a broadcast radio wave selected by a deciding device and an electric field intensity detecting cicuit to an acoustic device when multipath disturbance appears in the detection output of the acoustic device. CONSTITUTION:When a main receiver 3 receives multipath disturbance, distortion appears in the detection output of the main receiver 3. This distortion is detected by a multipath detecting circuit 5, whose output is sent to a control circuit 14. On receiving this signal, the control circuit 14 reads frequency data on a station having the 2nd electric field intensity out of a storage circuit 15 and sends this data as a tuning frequency signal C1 to the main receiver 3. Consequently, multipath disturbance is evaded and the same program is listened to repeatedly.