Showing papers on "Multipath propagation published in 1979"
Patent•
24 Oct 1979TL;DR: In this article, a multipath diversity spread spectrum communications receiver is proposed to enhance system performance over communication channels with distortion caused by multipath and fading effects, which is based on decision directed coherent integration with post detection correlation techniques for performance enhancement.
Abstract: A multipath diversity spread spectrum communications receiver will enhance system performance over communication channels with distortion caused by multipath and fading effects. The multipath diversity receiver utilizes decision directed coherent integration with post detection correlation techniques for performance enhancement. The receiver also incorporates a fast attack, fast release automatic gain control to process noncontinuous wideband spread spectrum signals which are subject to fading and multipath effects.
106 citations
TL;DR: In this article, a statistical model of fading on a radio path is used with laboratory measurements on a digital radio system to estimate the outage due to multipath fading, where outage is the time that the bit error rate (ber) exceeds a threshold.
Abstract: A statistical model (introduced in a companion paper) of fading on a radio path is used with laboratory measurements on a digital radio system to estimate the outage due to multipath fading, where outage is the time that the bit error rate (ber) exceeds a threshold. Over the range of ber of interest (10−6 to 10−3), the calculated outage agrees favorably with the outage observed during the period for which the fading model was developed. It is further shown that the calculated outage, when scaled to a heavy fading month on the basis of single-frequency, time-faded statistics, agrees equally well with the outage observed on the same path during a heavy fading month The agreement between measured and predicted outage substantiates the selective fading model. The prescribed laboratory measurements characterize the sensitivity of the radio system to selective fading. Thus, the methodology provides a useful basis for comparing the outage of alternative realizations of digital radio systems.
69 citations
TL;DR: In this article, the spectrum efficiency of a frequency-hopped-DPSK spread-spectrum modulation technique proposed for use in urban mobile radio systems is estimated based on simple models for the factors influencing system performance, and requires no computer simulation.
Abstract: The spectrum efficiency (bits/s per unit bandwidth) of a frequency-hopped-DPSK spread-spectrum modulation technique proposed for use in urban mobile radio systems is estimated. The analysis is based on simple models for the factors influencing system performance, and requires no computer simulation. It permits straightforward evaluation of spectrum efficiency over a broad range of system parameters. For an isolated service area (cell) with no propagation impairments, the efficiency is 0.12 at a bit error rate of 10-3. When interference from nearby cells is considered (assuming centrally located base stations with omni-directional antennas), the efficiency falls to 0.03 - 0.06, depending on how much performance degradation can be tolerated near the cell boundaries. Finally, the effects of multipath (Rayleigh) fading reduce system efficiency to 0.02 - 0.05.
45 citations
TL;DR: An expression is derived for critical pairvalues of fade depth and path delay for a digital signal transmitted over a two-path medium for microwave transmission on line-of-sight paths.
Abstract: Microwave transmission on line-of-sight paths occasionally experiences severe fading associated with multiple transmission paths of differing propagation delays. An expression for the envelope delay distortion caused by two such paths is presented. Assuming that envelope delay distortions comparable to the symbol length of a digital signal could be intolerable, an expression is derived for critical pairvalues of fade depth and path delay for a digital signal transmitted over a two-path medium. Good agreement of the principle was obtained with laboratory measurements made on 4- and 8-level systems. Curves are given showing the relationship between critical fade depths and relative delays, and estimates are made of channel outages based on assumed distributions of fades and time delays. The outage times are extemely sensitive to hop length, and exceed a commonly used objective for hops mote than 25 mi long.
44 citations
TL;DR: An experimental study of multipath fading effects on an 8-PSK 6 GHz digital radio system was conducted on a 26.4 mi test link near Atlanta, Georgia, and found the digitalRadio system was fragile by comparison to conventional analog FM radio.
Abstract: An experimental study of multipath fading effects on an 8-PSK 6 GHz digital radio system was conducted on a 26.4 mi test link near Atlanta, Georgia. Results were obtained for a non-diversity arrangement and a space-diversity arrangement employing a cophasing combiner operating at 70 MHz and fed by two antennas separated by 30 ft. Multipath fading was found to have a significant impact on bit error rate performance, i.e., the digital radio system was fragile by comparison to conventional analog FM radio. Modest in-band linear amplitude dispersion, 0.2 dB/MHz for example, was sufficient to cause a BER\geq10^{-3} . Such dispersion occurred at (average power) fade depths as shallow as 20 dB. However, average fade depth was a poor indicator of BER performance. The measured hop missed short haul outage objectives by an order of magnitude without space-diversity, and was close to acceptable when diversity was activated.
41 citations
TL;DR: The preconditioning effect of phase adaptive space diversity combining is necessary to reduce distortions sufficiently that they can be dealt with by a relatively simple adaptive equalizer.
Abstract: In the course of development of the DRS-8 Digital Radio System a series of digital propagation trials were conducted over a 51 km path at 8 GHz to determine the effect of multipath propagation on a 40 MHz bandwidth digital radio system. It was found that the effects of frequency selective fading result in unacceptably high system unavailability unless adaptive equalization and space diversity are employed. Specifically the results are: 1) Multipath induced outage is much higher than would be predicted from the measured flat fade margin of the equipment. For a nondiversity system, the probability of outage for a single hop in the worst fading month is approximately 1 \times 10^{-3} . This is 700 times the objective for a long haul system and corresponds to an effective fade margin of only 27 dB. 2) The primary cause of outage is inband distortion caused by the frequency selectivity of the multipath fading process. 3) Phase adaptive space diversity combining is very effective in reducing the amount of fading. In addition it affords some increase in the effective fade margin of the radio, i.e., it reduces the severity of inband distortion for a given fade depth. Outage for the System with space diversity combining was 2.6 \times 10^{-4} which is about 18 times the long haul objective. 4) A simple adaptive linear amplitude equalizer in conjunction with the phase adaptive space diversity combining provides an additional improvement of approximately a factor of 20, reducing the multipath outage to levels compatible with long haul availability objectives. 5) This same equalizer when applied to a non-diversity channel provides an improvement of about 2. Thus the preconditioning effect of phase adaptive space diversity combining is necessary to reduce distortions sufficiently that they can be dealt with by a relatively simple adaptive equalizer. These results apply to wideband digital radio which operates over long hops using a two dimensional modulation system such as QAM or multiphase PSK.
41 citations
TL;DR: An analytical study to predict multipath fading outages in terrestrial digital radio systems is described, finding that the choice of operating band has only a minor effect on the results; that OffsetQPSK systems suffer somewhat more multipath outage than conventional Q PSK systems; and that 8-level systems suffer more multipATH outage than 4-level Systems (although this difference may be offset by cross-polarization effects in the latter).
Abstract: An analytical study to predict multipath fading outages in terrestrial digital radio systems is described. The method, which is quite general, makes specific use of previously reported statistics on multipath fading. It is applied, in this study, to 6- and 11-GHz systems carrying 90 Mbits/s per RF channel. For purposes of comparing alternate approaches, three different modulations are considered, namely, (i) conventional QPSK at 45 Mbits/s, using two polarizations per channel; (ii) Offset-QPSK at 45 Mbits/s using two polarizations per channel; and (iii) 8-level PSK at 90 Mbits/s, using one polarization per channel. In all cases, a fourthorder Butterworth transmit filter is assumed which enforces FCC spectral emission requirements. The channel is treated as a two-path propagation medium, the receiver is assumed to be coherent, and various practical assumptions are invoked regarding the recovered carrier phase, timing phase, receive filter response, bit error rate objectives and fade margin. Results are given in terms of expected yearly fraction of multipath outage per repeater hop vs. hop length. We find that the choice of operating band has only a minor effect on the results; that OffsetQPSK systems suffer somewhat more multipath outage than conventional QPSK systems; that 8-level systems suffer more multipath outage than 4-level systems (although this difference may be offset by cross-polarization effects in the latter); and that all the approaches considered require repeater spacings of 15 mi or less in order to meet short-haul outage objectives over average terrain.
39 citations
TL;DR: It was found that the outage probability can be accurately estimated using only the in-band frequency response without knowledge of the delay difference between two incident waves, and the significance of noticing the delay dispersion sign was pointed out when attaining sufficient equalization effect.
Abstract: Multipath fading effects on a 200 Mbit/s 16 QAM digital radio system have been experimentally investigated. It was found that the outage probability can be accurately estimated using only the in-band frequency response without knowledge of the delay difference between two incident waves. New methods to evaluate the improvement factors for both space diversity reception and dynamic equalizing, which are required to combat multipath fading effects, were also proposed. In particular, the significance of noticing the delay dispersion sign was pointed out when attaining sufficient equalization effect. The validity of these evaluations was confirmed by simulation experiments and a field trial.
35 citations
TL;DR: In this article, practical design considerations for a low-altitude radar-guided air defense missile are presented. Butts et al. present a quantitative miss distance example showing the role of missile lateral acceleration capability and sensor design factors in the context of a complete missile guidance and control system.
Abstract: Practical design considerations for a low-altitude radar-guided air defense missile are presented. Low-altitude target signals return to the receiver mixed with large clutter signals from ground and large multipath signals from smooth sea. The doppler effect and the Brewster angle effect are used to separate true target returns from clutter and multipath contaminations. Sensor design factors, including clutter and multipath rejection, doppler resolution, and sensor stabilization, are discussed in the context of a complete missile guidance and control system. Basic contributors to miss distance are discussed with a quantitative miss distance example showing the role of missile lateral acceleration capability.
31 citations
TL;DR: Some techniques for evaluating the bandwidth characteristics of adaptive nulling systems are presented and the effects of feed-reflector multipath inherent in this geometry on the cancellation bandwidth is evaluated in detail.
Abstract: Some techniques for evaluating the bandwidth characteristics of adaptive nulling systems are presented. The system performance is categorized according to its antenna characteristics and the post-antenna channel tracking characteristics. The former tend to be radio-frequency (RF) percentage bandwidth dependent and the latter on the achievable component tolerances used to implement the channel (whether at RF or some lower intermediate frequency (IF)). Measured results are also presented for a seven-beam paraboloid reflector-type multiple-beam antenna (MBA), and the effects of feed-reflector multipath inherent in this geometry on the cancellation bandwidth is evaluated in detail.
27 citations
TL;DR: A complete transfer function characterization of refractive multipath on LOS links is developed, showing the dependence of discrete path amplitudes and delays on receiver height, transmitter height, refractive index gradient, layer height and ground range.
Abstract: A major source of propagation outages on microwave line-of-sight (LOS) links is multipath fading caused by steep negative refractive index gradients. This paper develops a complete transfer function characterization of refractive multipath on LOS links. The results clearly show the dependence of discrete path amplitudes and delays on receiver height, transmitter height, refractive index gradient, layer height and ground range.
TL;DR: In this paper, the underlying mathematical model of frequency diversity operation and atmospheric multipath fading was summarized and used to study frequency diversity behavior, and a computer program that produces such estimates was made available for general Bell System use in 1977.
Abstract: Estimates of transmission unavailability caused by multipath fading are needed to determine the adequacy of diversity protection arrangements. A computer program producing such estimates was made available for general Bell System use in 1977. In this paper, we summarize the underlying mathematical model of frequency diversity operation and atmospheric multipath fading and use it to study frequency diversity behavior.
TL;DR: Through theoretical and experimental analyses, a number of wave components in multipath propagation, the probability density function of multipath delay, the maximum delay and the relation between multipathdelay and radio duct parameters are examined.
Abstract: Multipath properties on microwave line-of-sight links for the purpose of applying to microwave systems design are discussed. The theoretical analyses of the fading mechanism and the relation between multipath delay and radio duct parameters are carried out using the ray-optical method. Multipath delay have been measured with the frequency-sweep method and the three-frequency method on several paths in Japan since 1954. These paths are distributed in length from 20 km to 80 km, and include horizontal and slant paths. Through these theoretical and experimental analyses, a number of wave components in multipath propagation, the probability density function of multipath delay, the maximum delay and the relation between multipath delay and radio duct parameters are examined. The estimation method for path-length difference statistics, needed for evaluation of propagation distortion, is quantitatively given.
TL;DR: In this article, a theoretical model of diffuse multipath reflections from rough surfaces is applied to the prediction of multipath power distribu tions in radar coordinates: elevation angle, time delay, and Dop pler frequency.
Abstract: A theoretical model of diffuse multipath reflections from rough surfaces is applied to the prediction of multipath power distribu tions in radar coordinates: elevation angle, time delay, and Dop pler frequency. These distributions are used to predict radar tracking errors in elevation angle, for both monopulse and scan ning antenna systems, and typical results are presented. These show a small increase in tracking error for scanning systems, on radially approaching targets, caused by sensitivity of these trackers to amplitude scintillation of the composite direct-plus multipath signal. Effects of knife-edge diffraction and of vegetation ion are briefly considered.
TL;DR: Using computer simulation, the transmitted signal and encoded teletext data (as defined in Broadcast Teletext Specifications') will be examined for different multipath conditions.
Abstract: Information for data broadcasting services such as Teletext is transmitted as digital signals inserted in the vertical interval of the television video waveform. The specifications for these signals are drawn very carefully to ensure compatibility with the existing television system. This paper examines some of these specifications such as data rate and waveform shape, which must be selected to maintain the spectral energy within the television bandwidth. Using computer simulation, the transmitted signal and encoded teletext data (as defined in Broadcast Teletext Specifications') will be examined for different multipath conditions. For all these experiments, echoes have been generated and evaluated at the baseband, ignoring system amplitude and group delay characteristics. These, in general, will have additional influence on decoding reliability.
27 Mar 1979
TL;DR: In this article, the authors estimate the spectrum efficiency of a Frequency-Hopped-DPSK modulation technique proposed for use in urban mobile radio systems, based on simple models for the factors influencing system performance, and require no computer simulations.
Abstract: We estimate the spectrum efficiency (bits/sec. per unit bandwidth) of a Frequency-Hopped-DPSK modulation technique proposed for use in urban mobile radio systems. The analysis is based on simple models for the factors influencing system performance, and requires no computer simulations. It permits straightforward evaluation of spectrum efficiency over a broad range of system parameters. For an isolated service area (cell) with no propagation impairments, the efficiency is 0.12 at a bit error rate of 10-3. When interference from nearby cells is considered (assuming centrally located base stations with omnidirectional antennas), the efficiency falls to 0.03 - 0.05, depending on how much performance degradation can be tolerated near the cell boundaries. Finally, the effects of multipath (Rayleigh) fading reduce system efficiency to 0.02 - 0.04.
TL;DR: It is shown that the same action obtained with the well-known selection diversity can be achieved when the difference between the modulation indices is equal to two, and the diversity effect on the bit-error-rate (BER) performance is theoretically analyzed.
Abstract: Multipath fading is one of the most serious problems for the signal transmission in mobile radio. To combat this problem, the application of transmitter diversity to a digital FM paging system is described. The method is to transmit two digital FM paging signals with different modulation indices, each of which is modulated by an identical binary signal, from separate antennas. It is shown that the same action obtained with the well-known selection diversity can be achieved when the difference between the modulation indices is equal to two. The diversity effect on the bit-error-rate (BER) performance is theoretically analyzed. The diversity effect on the BER performance in a 600-bit/s Manchester-coded digital FM system is verified by the laboratory simulation tests using a Rayleigh fading simulator.
TL;DR: Three main results are found; that is 1) echo delay decreases exponentially with fade depth, 2) the worst case fade distribution follows a Rayleigh distribution, and the unavailability of a typical 6 GHz path during multipath propagation is unacceptably high without the use of an adaptive amplitude slope equalizer.
Abstract: Analysis is presented to evaluate the worst case availability of 90 Mbit/s, 8 PSK digital microwave systems during multipath propagation. The model used in the analysis is based on the assumption that during multipath fading, the microwave energy propagates from the transmitter to the receiver via two paths, thus generating a single echo. Based on this model, three main results are found; that is 1) echo delay decreases exponentially with fade depth, 2) the worst case fade distribution follows a Rayleigh distribution, 3) the unavailability of a typical 6 GHz path during multipath propagation is unacceptably high without the use of an adaptive amplitude slope equalizer.
TL;DR: A microwave line-of-sight propagation experiment is carried out in Denmark at frequencies around 14 GHz to enable an improved description of the effects of multipath propagation.
Abstract: A microwave line-of-sight propagation experiment is carried out in Denmark at frequencies around 14 GHz. Results from long term measurements of multipath propagation are presented. The multipath fade durations are shown to be log-normally distributed. The level dependence of the probability of fading, P , the average duration of fades, \bar{t} , and the number of fades, N 0 , is investigated. Results show that P \propto L^{2} , \bar{t} \propto L^{2/3} and N_{0} \propto L^{4/3} . This differs from previously published results, where proportionalities of L2, L and L , respectively, have been proposed. Statistical results on enhancements above free space level are also presented. The results presented are believed to enable an improved description of the effects of multipath propagation.
TL;DR: The outage reductions made possible by using appropriate receiver responses are found to be quite large, possibly two or more orders of magnitude, and can be used to predict the probability of multipath outage.
Abstract: Frequency-selective fading in a radio channel, caused by multipath propagation or any other phenomenon, can seriously degrade the effectiveness of digital transmission. Receiver processing that adapts in an appropriate way to the prevailing channel response can strongly reduce the harmful effects of such fading. We examine here the theoretical possibilities of adaptive processing by deriving and analyzing receiver responses for three different criteria. The criteria used permit an analytical approach that is simple and exact. The performance characteristics derived for the three receivers bracket the "best" performance that is possible in practice and reveal possible tradeoffs between performance and practicality. The analysis applies generally to the broad class of quadrature amplitude modulation (QAM) signals, which are assumed to be filtered in the transmitter to enforce spectral emission requirements. For each receiver response we derive a formula for the "power penalty", defined as the increase in transmitter power (over some theoretical minimum) needed to compensate for transmit filtering and channel fading. The power penalty formulas are evaluated for a two-ray multipath fading channel and for each of two common forms of modulation/transmit filtering. Graphical results are given for numerous combinations of the multipath parameters and the transmitter bandwidth-to-symbol rate ratio. The results of this study can be used to predict the probability (time fraction) of multipath outage, provided that the statistics of the channel parameters are known. Some cursory comparisons are made between the new results and those of a previous analysis for fixed receivers. The outage reductions made possible by using appropriate receiver responses are found to be quite large, possibly two or more orders of magnitude.
Patent•
07 Nov 1979
TL;DR: In this article, a multipath distortion compensation circuit comprises an FM detector, an AGC circuit, an envelope detector, a capacitor, a function generator, an analog multiplier, first and second operational circuits, first-and second variable delay circuits, and a plurality of switching circuits for producing a demodulated signal having no distortion components caused by multipath interference.
Abstract: A multipath distortion compensation circuit comprises an FM detector, an AGC circuit, an envelope detector, a capacitor, a function generator, an analog multiplier, first and second operational circuits, first and second variable delay circuits, and a plurality of switching circuits for producing a demodulated signal having no distortion components caused by multipath interference. The switching circuits are used to selectively constitute one of first and second arrangements by changing the connections among the circuits. The first and second arrangements are respectively used depending on whether the magnitude of a direct wave received by a receiving antenna is greater than that of an indirect wave, such as a reflected wave, also received by the antenna. A suitable compensation signal is produced and is subtracted from the demodulated signal to cancel the distortion components.
01 Dec 1979
TL;DR: In this article, the Cramer-Rao bound for an unbiased estimate of the elevation angle of a target in the presence of multipath is calculated for the symmetric (target and image symmetric about the elevation symmetry plane of antenna) and nonsymmetric cases for an antenna consisting of 21 elements.
Abstract: The Cramer-Rao bound for an unbiased estimate of the elevation angle of a target in the presence of multipath is calculated for the symmetric (target and image symmetric about the elevation symmetry plane of antenna) and nonsymmetric cases for an antenna consisting of 21 elements. These bounds are compared to the maximum likelihood estimates and it is found that the rms error of the maximum likelihood estimate (which has a bias) is below the Cramer-Rao bound for unbiased estimates.
TL;DR: In this paper, the first-order probability densities for the signal amplitude, and amplitude rate are derived for the short time average mean square pressure and the time rate of change of the level in decibels.
Abstract: The phase‐random model of multipath acoustic propagation is used to derive the first‐order probability densities for the time rate‐of‐change of the short time average mean‐square pressure ?, and the time rate‐of‐change of the level in decibels of the short time average mean‐square pressure ?. It is shown that the probability densities for the signal amplitude, and amplitude rate are insensitive to frequency or phase modulation of the signal by the source, but that the density for the time rate‐of‐change of the multipath phase ?, is sensitive to such modulation. Because the finite bandwidths of acoustic signals can be modeled by uniform frequency modulation, the analysis presented applies to this problem as well. It is shown that bandwidth effects can be neglected only if B≪2ν, where B is the signal bandwidth, and ν2 is the single path mean‐square phase rate. This inequality provides a useful definition of what is meant by ’’narrow band’’ as it applies to phase‐random multipath propagation. It is also shown that the density for ? depends only on ν, while the density for ? depends on ν and parameters of the modulation. A potentially powerful technique is developed for determining source bandwidth and parameters of the source modulation, minus the effects of oceanic fluctuations, from the received multipath signal. The analytical results are compared with a computer simulation, and data from experiments in the ocean with extremely favorable results.
TL;DR: Using a classical model, and adding two complementary propositions, it is possible to obtain a general method for calculating the fading distribution laws on line-of-sight links of indifferent lengths undergoing multipath propagation.
Abstract: Using a classical model, and adding two complementary propositions, it is possible to obtain a general method for calculating the fading distribution laws on line-of-sight links of indifferent lengths undergoing multipath propagation. In particular, it is shown that the conventional empirical d3.5 power law gives a rough approximation in a very limited domain, but is not a general law.
TL;DR: In this paper, a 13.6 to 14.9 GHz sweep measurement equipment designed to measure amplitude, differential phase, and differential gain of a propagation path is briefly described, and the qualities of the data are discussed.
Abstract: A 13.57–15.0 GHz sweep measurement equipment designed to measure amplitude, differential phase, and differential gain of a propagation path is briefly described, and the qualities of the data are discussed. Statistics of fadings and intermodulation noise in the frequency band of 13.6 to 14.9 GHz from a 45 km l.o.s propagation path in Denmark are presented. The statistics cover the period July 1974 to July 1975. Both monthly and diurnal variations of the fading are given as well as the distribution of the fading durations. Separate statistics are given for multipath propagation and for precipitation attenuation. Distributions of the 1 minute and the 1 hour mean values of total noise power for the whole period, and for the worst month are presented. The monthly variations of these values are also presented. Based on the data it is concluded that this propagation path will satisfy the relevant part of the CCIR Recommendation 395–1, concerning the transmission quality in the worst month—provided the antennas are protected against wet snow and the system uses some type of diversity to cope with multipath propagation.
TL;DR: In this paper, a computer-based measurement system has been developed for the measurement of acoustic propagation loss in multipath arrivals of signals from explosive sources, which involves both signal measurement operations and careful theoretical representations of the measured quantities.
Abstract: A computer‐based measurement system has been developed for the measurement of acoustic propagation loss in multipath arrivals of signals from explosive sources. The system involves both signal measurement operations and careful theoretical representations of the measured quantities. Initial applications have been for measurement of bottom reflection losses using signals which reflect many times from the bottom. It has been found that the levels of arrival pulses can be strongly affected by predictable surface reflection effects. Measurements demonstrate consistent bottom reflection loss values across homogeneous basin areas. [Work supported by Naval Ocean Systems Center.]
TL;DR: A two-path model is used to simulate propagation during multipath fading and to calculate the quality of digital radio links to lead to bit error-rate statistics which are in good agreement with experimental results.
Abstract: A two-path model is used to simulate propagation during multipath fading and to calculate the quality of digital radio links. The characteristics of the model are deduced from measurements made on the level of the pilot in f.m. radio links. Its use leads to bit error-rate statistics which are in good agreement with experimental results.