Showing papers on "Multipath propagation published in 1978"
TL;DR: A maximum‐likelihood procedure for estimating the amplitudes and arrival times of individual pulses in a multipath signal is derived and is shown to be significantly more accurate than either the matched or inverse filter.
Abstract: A maximum‐likelihood procedure for estimating the amplitudes and arrival times of individual pulses in a multipath signal is derived. Computer simulation results are presented that compare this procedure with the more traditional matched‐filter and inverse‐filter techniques. The maximum‐likelihood technique is shown to be significantly more accurate than either the matched or inverse filter. A computer algorithm for implementing the maximum‐likelihood estimation procedure is described.
103 citations
TL;DR: In this paper, a spread-spectrum waveform centered at 1370 MHz was used to measure urban and terrain conditions with high time-delay resolution, and a detailed analysis of the multipath structure was made for various terrain conditions.
Abstract: Measurements over a variety of urban and terrain conditions were made using a spread-spectrum waveform centered at 1370 MHz. Chip rates of 10 and 20 MHz were used, giving high time-delay resolution. The transmitter end of the measurement link was elevated and fixed while the receiver was mobile. A detailed analysis of the multipath structure was made for various terrain conditions. This included distributions of the number and spacing of individual multi-path components as a function of amplitude threshold and also distribution of total delay. Thresholds are referenced to both average signal level and the maximum level in each pulse interval. Some instances of spatial variation of the received signal are also presented.
49 citations
01 Jul 1978
TL;DR: Envelope detection, optimal for detecting Rayleigh and Rician signals, is shown to be optimal for the considered non-Rayleigh signals in additive white Gaussian noise.
Abstract: The first-order statistics of the vecter sum of a complex Gaussian variable and N complex exponentials with constant amplitudes but random independent phases are used to model a non-Rayleigh fading signal. Envelope detection, optimal for detecting Rayleigh (N = 0) and Rician (N = 1) signals, is also shown to be optimal for the considered non-Rayleigh signals in additive white Gaussian noise. However, the signal-to-noise ratio required to detect the latter signal for N = 2 may be substantially higher than for the former cases. Possible applications of this fading model to studies of multipath fading, non-Rayleigh sea echoes, microwave or laser radar detection and tracking are discussed.
46 citations
TL;DR: A model for the analysis of the bandwidth along an optical transmission line is suggested and tested and it is found that optical equalization is a real effect that has been observed in practice and is affected by only two major factors.
Abstract: A model for the analysis of the bandwidth along an optical transmission line is suggested and tested on the Post Office experimental link. It is found that optical equalization is a real effect that has been observed in practice and that the degree of equalization is affected by only two major factors. These are the degree of mode coupling in the route and the functional relationship between mode delay and mode number. Finally a procedure for the optimization of the bandwidth of a link is suggested and tested.
45 citations
TL;DR: Quantitative estimates are derived of the elevation angles, and hence, range, at which targets of specified height can be accurately tracked, and to indicate specific areas in which additional experimental data are critically needed.
Abstract: This paper investigates the problem of tracking targets at a low elevation angle in the presence of specular and diffuse multipath. Quantitative estimates are derived of the elevation angles, and hence, range, at which targets of specified height can be accurately tracked. A parametric approach is followed in which the long-standing uncertainty of how terrain forward-scatters at low grazing angles is recognized at the outset. Particular attention is given to the effects of target motion which permit rejection of multipath components falling outside the radar tracker's passband. The results are presented in a form which can be readily applied to a spectrum of radar trackers with differing requirements. The limited experimental ental data on the specular and diffuse scattering parameters for several generic types of terrain are applied to estimate the significance of multipath under different situations and to indicate specific areas in which additional experimental data are critically needed.
42 citations
TL;DR: It is shown that, in the channelized common carrier bands below 15 GHz, the first two (complex) terms of the power series are usually sufficient for characterizing multipath effects.
Abstract: The frequency transfer function of a multipath fading channel is examined in terms of its effects on digital radio signals. The transfer function is expanded into a power series about the channel center frequency and the coefficients are related to the multipath structure. It is then shown that, in the channelized common carrier bands below 15 GHz, the first two (complex) terms of the power series are usually sufficient for characterizing multipath effects. This demonstration is based on a mean-square error-of-fit measure which is applied to the multipath fading response and evaluated under some worst-case assumptions.
26 citations
TL;DR: The analytic and experimental results presented in this paper clearly indicate the usefulness of this adaptive spectral equalization technique for a single multipath channel, yielding a performance advantage of between 5 and 20 dB.
Abstract: In this paper, an adaptive spectral equalization technique is developed for multipath cancellation, and more generally for any minimum-phase distortion, which is implemented by a constrained form of adaptive linear prediction. The technique does not use decisiondirected feedback, has guaranteed convergence and stability, and is simple to realize using conventional circuit technology. Binary amplitude-shift-keying (ASK) is considered as an example of the technique, and the theoretical performance characteristics are derived for a bandlimited pulse shape. Experimental results are also presented for an operational real-time processing system which has been developed for this application and are shown to conform favorably to theory. The analytic and experimental results presented in this paper clearly indicate the usefulness of this concept for a single multipath channel, yielding a performance advantage of between 5 and 20 dB. Particularly significant is the potential of this adaptive technique to acquire and track the multipath without any a priori knowledge of the specific parameters.
19 citations
01 Oct 1978
TL;DR: In this article, the authors investigated short-haul non-line-of-sight optical scatter communication links in the middle ultraviolet (UV) wavelength region, where the absence of solar background radiation makes wide-angle receivers practical.
Abstract: : This report discusses the work performed in the second year of a two year program to investigate short-haul non-line-of-sight optical scatter communication links. The wavelength region considered for this study is the middle ultraviolet (uv) (2000 A - 3000 A). This region is attractive for communication purposes because the absence of solar background radiation makes wide-angle receivers practical. Furthermore, the short wavelengths result in significant scatter from molecular atmospheric constituents (Rayleigh scatter), so that the availability of a non-line-of-sight link is extended into clear weather. The goal of the investigation has been to obtain a fundamental understanding of scatter propagation in low visibility atmospheric environments. In particular, the work has focused on the determination of parameters needed for optical communication system design, such as angular spread and multipath time spread.
19 citations
TL;DR: Multipath and angular spectrum measurements made under a variety of weather conditions over a 13.6-km line-of-sight propagation path are reported to examine the dependence of scattered plus unscattered optical transmission on optical thickness.
Abstract: An experimental program aimed at measuring critical channel parameters of atmospheric optical communication channels under low-visibility weather conditions is described. Multipath and angular spectrum measurements made under a variety of weather conditions over a 13.6-km line-of-sight propagation path are reported. The latter measurements are used to examine the dependence of scattered plus unscattered optical transmission on optical thickness.
18 citations
22 Mar 1978
TL;DR: A high resolution pseudo-random noise channel probe is applied for evaluating the transmission character of a land-mobile communication channel and examples of data are presented which characterize the frequency correlation of the channel transfer function, and provide additional information on the spectral distortions caused by multipath.
Abstract: This report describes the application of a high resolution (6 ns) pseudo-random noise (PN) channel probe for evaluating the transmission character of a land-mobile communication channel. Preliminary measurements were performed at a microwave frequency in a number of locations in Boulder, Colorado. Examples of data are presented which characterize the frequency correlation of the channel transfer function, and provide additional information on the spectral distortions caused by multipath.
8 citations
TL;DR: In this paper, three wholly refracted paths were observed in the return signals, and a discussion of the six-hour time series of arrival times and amplitudes for each path was presented.
Abstract: FM slide and pulsed‐tone signals at 2, 4, 8, and 16 kHz were transmitted from a source to a receiver at the same depth 1100 m distant. Three wholly refracted paths were observed in the return signals, and a discussion of the six‐hour time series of the arrival times and amplitudes for each path is presented. Evidence is given of acoustic frequency‐dependent scattering, and scattering from oceanic fine structure is suggested as the most likely physical mechanism to explain the observations.
01 Jan 1978
TL;DR: In this article, the authors proposed a novel idea of reducing multipath interference in underwater acoustic communication systems, based on the observation that the time separation between direct and multipath waveforms can be utilized to suppress the interference.
Abstract: This paper describes a novel idea of reducing multipath interference in underwater acoustic communication systems. The proposed idea is based on the observation that the time separation between direct and multipath waveforms can be utilized to suppress the multipath interference. A proposed system employs a carrier with instanteous frequency swept periodically in time over a broad frequency range. This frequency range is much larger than the bandwidth occupied by a message signal. On the other hand the reciprocal of sweeping rate is much smaller compared to the message bandwidth. The carrier is modulated by a message resulting in a time varying instantaneous narrow-band spectrum of the transmitted signal. The received signal is demodulated by a band-pass tracking receiver and suppression of the out-of-band multipath spectrum is accomplished. A simple model for the multipath channel has been proposed and related to the system operation and performance. Some results and conclusions from tests performed on the prototype are presented.
TL;DR: In this paper, both deterministic and random spatial and temporal variations of the ionosphere and the troposphere are included in determining the multipath delay and Doppler frequency spreading of a channel in which the signals are reflected off of a multilayered ionosphere.
Abstract: The time and frequency selective behavior of a fading HF ionospheric reflection channel is considered in this paper. Both deterministic and random spatial and temporal variations of the ionosphere and the troposphere are included in determining the multipath delay and Doppler frequency spreading of a channel in which the signals are reflected off of a multilayered ionosphere. An example which illustrates the diffuseness of the multipath and Doppler spectra under typical propagation conditions is given.
01 Jan 1978
01 Jan 1978
TL;DR: In this paper, a design for a system providing highly reliable command and control acoustic communications between a mother ship and a number of small, fast submersibles is presented for underwater mining, exploration, bottom mapping, or military surveillance.
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 initial doppler and message doppler variation problems inherent in a scenario with mobile end points. A Figure-of-Merit 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 extreme end point motion.
TL;DR: In this article, the authors present time-delay/Doppler scattering functions for two different suburban locations where multipath propagation is evident, and explain the major features of the scattering functions in terms of the surrounding environment and the street orientation with respect to the transmitter.
Abstract: Time-delay/Doppler scattering functions are presented for two different suburban locations where multipath propagation is evident. The major features of the scattering functions are explained in terms of the surrounding environment and the street orientation with respect to the transmitter.
TL;DR: A new mode of diversity transmission is described which largely overcomes problems of multipath fading for the outgoing path in land mobile radio.
Abstract: A new mode of diversity transmission is described which largely overcomes problems of multipath fading for the outgoing path in land mobile radio. The technique can be applied in many forms, depending on the type of modulation used. Trials with data transmissions have shown two orders of magnitude reduction in error rates as compared with conventional transmission. The system is compatible with quasi-synchronous working.
TL;DR: The performance of a noncoherent frequency-shift keyed (FSK) system is analyzed in the presence of multipath fading by using a sinusoidal series expansion technique for the representation of a bandlimited Gasussian process.
Abstract: The performance of a noncoherent frequency-shift keyed (FSK) system is analyzed in the presence of multipath fading by using a sinusoidal series expansion technique for the representation of a bandlimited Gasussian process. The probability of error is calculated as a function of the system parameter BT , signal-to-noise ratio, fading bandwidth and direct to reflected signal power ratio.
TL;DR: Qualitative and quantitative agreement between experimental and theoretical results for binary error probability for a coherent QPSK system degraded by intersymbol interference is given.
Abstract: Qualitative and quantitative agreement between experimental and theoretical results for binary error probability for a coherent QPSK system degraded by intersymbol interference is given. The source of intersymbol interference is a delayed and attenuated replica of the transmitted signal which may represent several forms of observed multipath. Although the experimental results pertain to a particular 1 Gbit/s modem, the theory is applicable for any data rate.
TL;DR: The analysis shows that the improvement offered by delayed encoding relative to ordinary delta modulation has its basic origin in diminished overload distortion.
Abstract: A study of the asymptotic performance of delta coding with delayed decision is presented for the cases of first and second order integration assuming a stationary, bandlimited Gaussian source. The resulting signal-to-distortion-ratio serves as an upper bound for any delayed (multipath) decision scheme provided that the sampling rate exceeds the Nyquist rate by a factor greater than about two. The analysis shows that the improvement offered by delayed encoding relative to ordinary delta modulation has its basic origin in diminished overload distortion. Some insight is given into how the improvement is related to the decoding filter. Finally, numerical results are given for some sources with speech-like spectra
TL;DR: In this article, the authors derived power spectra for a large number of equal energy paths by appealing to the central limit theorem, which states that the Cartesian components for the multipath field are Gaussian distributed.
Abstract: Statistics of fluctuations of low‐frequency sound in the ocean are analyzed for an arbitrary number of paths. Covariances are derived for the intensity, phase rate (time derivative of the phase), and related parameters. Power spectra are derived from the covariances. The multipath results are based on weakly fluctuating single paths whose phase is perturbed by the internal‐wave field. The theory is illustrated by deriving some results for a single internal wave. We specialize to a large number of equal energy paths by appealing to the central‐limit theorem which states that the Cartesian components for the multipath field are Gaussian distributed. Some results available in the literature are shown to be valid for the special case of a large number of paths. Power spectra for data obtained in a recent experiment are shown to follow the predictions of this analysis.
01 Aug 1978
TL;DR: In this paper, the upper and lower lobes of two conically scanning capture antennas were simulated using 50.8mm and 152.4mm horn-lens antennas at a range of 100 m over high weeds, mowed grass and asphalt.
Abstract: : As part of a study of the feasibility of a 140-GHz beamrider, measurements were made of the effect of multipath on the antenna power patterns of simulated missile capture antennas. The upper and lower lobes of two conically scanning capture antennas were simulated using 50.8-mm and 152.4-mm horn-lens antennas. Vertical field probes were made at a range of 100 m over high weeds, mowed grass and asphalt. A theoretical model for specular ground reflection compared favorably with the experimental results. On the basis of this model, it is concluded that the capture of a beamrider missile would be difficult at ranges greater than 200 m over some terrain. (Author)
01 Jun 1978
TL;DR: In this paper, the authors describe the theoretical background, methods of implementation, and utility of a multi-purpose wideband LOS (line-of-sight) channel simulation system developed by CNR, Inc. for Rome Air Development Center as a means for evaluating wideband digital modems.
Abstract: : This report describes the theoretical background, methods of implementation, and utility of a multi-purpose wideband LOS (line-of-sight) channel simulation system developed by CNR, Inc. for Rome Air Development Center as a means for evaluating wideband digital modems. The system provides both a channel probing/channel playback capability and a synthetic stochastic channel simulation. Propagation media effects simulated include both multipath due to refractive anomalies (refractive multipath) and scattering off the surface (surface multipath). The types of LOS channels handled by the system include airplane-airplane, ground-airplane, ground-ground, and airplane-satellite. In addition, the simulator allows the introduction of controlled amounts of nonlinearity, phase jitter, and frequency offset. The simulator operates at selectable IF frequencies of 70, 300, or 700 MHz. At the two high IF frequencies, signal bandwidths up to 100 MHz may be accommodated, while at 70 MHz, bandwidths up to 25 MHz may be handled. (Author)
01 Nov 1978
TL;DR: An initial investigation into the frequency and time characteristics of interfering signals within aeronautical h.f. voice channels is described and modem proposals that can improve the performance of slow-rate data transmissions in the presence of such interference are considered.
Abstract: Interference from other users can be an important source of errors in aeromobile h.f. data transmission and is often more detrimental than the effects of multipath propagation and atmospheric and receiver noise. This paper describes briefly an initial investigation into the frequency and time characteristics of interfering signals within aeronautical h.f. voice channels and considers modem proposals that can improve the performance of slow-rate data transmissions in the presence of such interference.
TL;DR: A suboptimum scheme is developed which is less sensitive to inaccurate knowledge of the multipath parameters and derives a performance bound.
Abstract: In many mobile satellite communication systems specular multipath exists and acts as a distortion to the transmitted message stream. In most cases there is a singular specular return which can be characterized by a delay and reflection coefficient. If this return is viewed as intersymbol interference on the channel, then it is possible to consider the resulting channel as modeled by a finite state machine and process the received data stream optimally using the Viterbi algorithm. This paper develops such a scheme and derives a performance bound. A suboptimum scheme is also developed which is less sensitive to inaccurate knowledge of the multipath parameters. Performance bounds for the suboptimum scheme are also developed.
01 Apr 1978
TL;DR: A useful technique for displaying the complex envelope of the band limited impulse response of a time-varying channel is presented and this impulse response measurement is also the approximate delay spread function of the channel.
Abstract: A useful technique for displaying the complex envelope of the band limited impulse response of a time-varying channel is presented. This impulse response measurement is also the approximate delay spread function of the channel. This is an illustration of a display of a complex-valued function of two independent variables (e.g. time and time-delay). The utility of the display is emphasized by propagation examples from the MIMI experiment [5] in the Straits of Florida. Channel characteristics readily observed from a twelve hour display are delay spread, multipath structure, individual path propagation time, and correlation of channel structure with scalar measurements of the channel.
Patent•
17 Nov 1978
TL;DR: In this article, a method for encoding signals for transmission to provide signals which simultaneously have specifiable autocorrelation properties and low predictability is presented, where the expected time delay is equal to N-bits.
Abstract: A method is disclosed for encoding signals for transmission to provide signals which simultaneously have specifiable autocorrelation properties and low predictability. To enhance the accuracy of time of arrival measurements in the presence of either noise or multipath due to skywaves in systems such as LORAN, the coding method provides for low autocorrelation function values at time offsets approximately equal to the expected delay between direct waves and skywaves. This is accomplished by constructing the codeword which is used to encode the signal, from an Nth difference constant ratio codeword, where the expected time delay is equal to N-bits. The constant ratio codewords and the transmitted codewords are both pseudo-randomly generated, resulting in a transmitted signal having very low predictability to an unintended recipient, thereby considerably reducing the effectiveness of attempts at jamming or other electronic countermeasures attacks.
Patent•
31 Mar 1978
TL;DR: In this paper, an electronic navigation system ground station, which may be either an omni-range beacon (such as VOR) or a passive direction finder, has a feed arrangement which includes switched programming of at least one discrete set of phase shifters to effect successively changed phase-rotation fields for minimizing the adverse effects of multipath signals.
Abstract: An electronic navigation system ground station, which may be either an omni-range beacon (such as VOR) or a passive direction finder. A circular array of antenna elements has a feed arrangement which includes switched programming of at least one discrete set of phase shifters to effect successively changed phase-rotation fields for minimizing the adverse effects of multipath signals between the ground station and a remote station, aboard an aircraft for example.