scispace - formally typeset
Search or ask a question

Showing papers on "Multipath propagation published in 1968"


Journal ArticleDOI
J. Hayes1
TL;DR: Results that show the improvement in performance of the system under the assumptions of perfect receiver learning and noise-free feedback are presented are presented.
Abstract: A scheme for transmitting binary signals through a Rayleigh fading multipath medium is analyzed. The scheme envisions an adaptive receiver and a feedback channel. The feedback channel conveys information to the transmitter on the forward channel state learned at the receiver. The transmitter uses this information to modify transmission. Results that show the improvement in performance of the system under the assumptions of perfect receiver learning and noise-free feedback are presented. In this way, upper bounds on system performance are established.

313 citations


Journal ArticleDOI
01 Oct 1968
TL;DR: In high-speed communication through a medium with time-frequency spread (such as in HF ionospheric, sonic underwater, and voice-quality telephone line transmission), the transmission speed and system errors are determined by an overall system variance which comprises the intersymbol interference variance from time spread, as well as the variance from frequency spread and the variance arising from channel noise.
Abstract: In high-speed communication through a medium with time-frequency spread (such as in HF ionospheric, sonic underwater, and voice-quality telephone line transmission), the transmission speed and system errors are determined by an overall system variance (or equivalent noise). This overall variance comprises: 1) the intersymbol interference variance from time spread (or dispersion in the unit impulse response of the medium); 2) the variance from frequency spread (or dispersion it the CW response of the medium); and 3) the variance from channel noise at the receiver. In order to mitigate intersymbol variance from time spread, after discussing some early and limited attempts, a review is made of the general synthesis of the infinite and finite Wiener optimal networks or equalizers using delay lines with feedforward and/or feedback taps (or their shift register digital network counterpart). Also considered are optimal finite feedforward delay line approximations to the Wiener networks, synthesized nonsequentially and sequentially using a steepest descent evolutionary network synthesis, resulting in a monotonically decreasing and convergent overall variance. Applications of these networks are shown in the transmission of speeded-up analog facsimile pictures over unconditioned voice-quality telephone lines. It is shown that the received smeared picture can be refocused or de-smeared, or its time spread mitigated, to allow overall satisfactory high-speed picture transmission. Moreover, in over-the-horizon HF ionospheric analog and/or data transmission, time spread occurs in the form of discrete multi-path reception, such as the familiar ghosts observed in television. A particularly stringent multipath comprises two equal or nearly equal magnitude paths, a situation which is actually observed in HF ionospheric communication between two ships at sea. For this case it is shown that serial data transmission with pulses thin enough to resolve the multipath achieves, with optimal equalization, an irreducible bit error ratio (BER) which is a number of orders of magnitude less than the irreducible BER of the parallel data transmission method used in all contemporary HF modems (modulators and demodulators). Design curves are given showing the intersymbol interference variance from time spread as a function of the number of taps it the delay line correction networks, along with the variance arising from channel noise, and the optimal allocation of both variances for minimal overall BER with a given number of taps for the finite realizable correction networks. Because of frequency spread, the ever-changing unit or impulse response of the medium (e.g., HF ionospheric) causes the correction or equalization networks to become aged, giving rise to frequency spread variance. This is formulated both for determinate and for random changes of the medium unit response. For transmission media having simultaneous time spread and frequency spread, the equivalent overall variance is a simple function of the time-frequency spread product of the medium. These formulas are applied to find the overall minimal BER as a function of the time-frequency product for a contemporary parallel data modem and for a new serial adaptive data transmission system (ADAPTICOM) which periodically in real time and with digital techniques rejuvenates the time spread digital correction networks. It is shown that the transmission limit of communication of the parallel data modem is for a time-frequency spread product of about 1/2000, while that for the new serial adaptive data modem is about ten times larger, or 1/200. As a result, new communication channels are opened up for serial adaptive transmission, such as the HF spectrum below the maximum usable frequency (MUF).

39 citations


Journal ArticleDOI
A. Vigants1
TL;DR: In this article, the authors quantitatively described the reduction of multipath fading on line-of-sight microwave radio-relay links by dual space diversity and provided a fade reduction factor that specifies the improvement provided by space diversity as a function of the vertical separation of the receiving antennas, frequency, path length, and the relative size of the antennas in the diversity pair.
Abstract: The reduction of multipath fading on line-of-sight microwave radio-relay links by dual space diversity is quantitatively described. One of the main results is a fade reduction factor F that specifies the improvement provided by space diversity as a function of the vertical separation of the receiving antennas, frequency, path length, and the relative size of the antennas in the diversity pair. The results are based on a theoretical analysis of recent experimental data.

38 citations


Journal ArticleDOI
J. Jones1
TL;DR: In this article, the multichannel bit error probability for non-coherent FSK and differentially coherent PSK systems is obtained for a slow nonselective-fading multipath as a function of the multipath parameters and the order of diversity.
Abstract: The multichannel bit error probability for noncoherent FSK and differentially coherent PSK systems is obtained for a slow nonselective-fading multipath as a function of the multipath parameters and the order of diversity. The muitipath model involves three components consisting of two specular components and one scatter (diffuse) component. For diversity combining the multichannel noncoherent FSK receiver model employs square-law envelope addition and for DPSK employs linear baseband, postproduct-detection addition. These two diversity combining techniques are shown to be equivalent in terms of a DPSK noncoherent detector model that matches over two adjacent bits of the transmitted signal. As with Single-channel systems, the muitichannel DPSK performance is improved 3 dB over the multichannel FSK performance but otherwise equivalent. Results are given for muiltipath situations corresponding to a variety of differential path delays ranging from small to large delays and for mixed delays. Performance curves are presented for these multipath models and for several special cases representing simplified models.

37 citations


01 Mar 1968
TL;DR: After a brief description of the significance of the radio refractive index and its variation in the lower troposphere, a catalog of fading mechanisms is presented and specific expressions are given for determining the frequency or space separations.
Abstract: After a brief description of the significance of the radio refractive index and its variation in the lower troposphere, a catalog of fading mechanisms is presented. Attention is directed to the supporting refractive index structure, the characteristics of the fading signal, and the available remedies. The phenomena of multipath fading are described, and the theoretical amplitude distributions are presented. Diversity reception (frequency or space) is outlined for reducing the fading due to multipath. Specific expressions are given for determining the frequency or space separations. Successful remedies for microwave fading that are keyed to specific fading mechanisms also constitute a means of measuring the characteristics of the prevailing meteorological conditions. Specific expressions are given to accomplish this for the multipath and diffraction fading mechanisms.

17 citations


Journal ArticleDOI
TL;DR: It is concluded that vertical space diversity provides a very effective tool for combating multipath fading on line-of-sight paths, regardless of the source of the fading.
Abstract: Deep fading on line-of-sight microwave paths is usually a result of multipath interference phenomena. Frequency diversity has been the most common method of protecting against such fading. In the past few years, vertical space diversity has also come into use. This paper discusses its characteristics, methods of determining spacings, and describes some results achieved. It is concluded that vertical space diversity provides a very effective tool for combating multipath fading on line-of-sight paths, regardless of the source of the fading.

10 citations


Journal ArticleDOI
TL;DR: In this article, an air-ground, ground-air communications system that utilizes a satellite and a central control facility is described, and the double-hop pseudo-noise system advantages, including equal sharing of power, use of frequency translating repeaters, optimum multipath rejection, and acceptable power densities on the ground are discussed.
Abstract: An air-ground, ground-air communications system that utilizes a satellite and a central control facility is described. The double-hop pseudo-noise system advantages, including equal sharing of power, use of frequency translating repeaters, optimum multipath rejection, and acceptable power densities on the ground are discussed. The selection of code division is presented together with an analysis of signal-to-noise ratios with a varied input. Modulation schemes are discussed, pro and con, and the effective radiated power versus antenna sizes for several combinations are analyzed.

10 citations


Journal ArticleDOI
TL;DR: The important systems parameters, including the depth and rapidity of fading, associated with a given communication link are evaluated and the extension to other links is indicated.
Abstract: On a communication link between two terminals using essentially isotropic antennas, where at least one of the terminals is in the general vicinity of an extended rough surface, scattering from that surface introduces multipath fading which must be taken into account in assessing the performance of the link. Fading, of course, occurs only if the terminal or the surface changes position with time. This paper discusses the characteristics of fading under the assumption that the surface is rigid but the terminal is moving relative to the rough surface. The important systems parameters, including the depth and rapidity of fading, associated with a given communication link are evaluated. The extension to other links is indicated.

9 citations


Journal ArticleDOI
TL;DR: The value of chirp modulation derives from its efficient use of available bandwidth and transmitter power in combating multipath and other forms of interference, including the "clutter" of other transmissions in multiple-access systems.
Abstract: The white Gaussian noise-limited channel so often assumed by system designers is encountered less and less in practice as the density and sophistication of modem communications increase. Long-range radio communications in particular tend to be limited by interference rather than by receiver noise; for that reason, the design of the signal waveform has great impact on system performance. A waveform that shows particular promise for channels such as satellite relay and HF radio is swept FM or "chirp" modulation. The chirp waveform is characterized in Fig. 1. A carrier is swept in frequency over a band W during a sweep interval T. The sweeps shown are linear and assume a square amplitude envelope of unity value, although nonlinear sweeps and shaped envelopes may be used. The sweep slope W/T may be used to convey information in a variety of ways, the simplest being +W/T for binary 1 and -W/T for binary 0 (slope-shift keying SSK). Higher order alphabets can be represented by different slopes, and amplitude can be added if desired. SSK will usually be most efficient, reserving the distinguishability of different slopes for the purpose of defining channels or addresses in multiple-access applications. The value of chirp modulation derives from its efficient use of available bandwidth and transmitter power in combating multipath and other forms of interference, including the "clutter" of other transmissions in multiple-access systems.

8 citations


01 Mar 1968
TL;DR: Multipath communication between low altitude spacecraft and synchronous satellites with interference reduction methods is discussed in this paper, where the interference reduction method is used to reduce the interference in the communication.
Abstract: Multipath communication between low altitude spacecraft and synchronous satellites with interference reduction methods

6 citations



Journal ArticleDOI
TL;DR: It is found that under certain constraints this ratio is approximately independent of the delay statistics of the medium, and the duration and frequency of the input for the three filtering schemes, and is √2 times greater for frequency‐inverse filtering.
Abstract: Three filtering techniques that can be employed in the recovery of a signal after it has propagated through a nonrandom, distortionless, multipath channel have been studied. The generalization of these studies to the random medium situation is the objective of this paper. A fixed, omnidirectional source emits a signal which propagates through a multipath medium and is detected by an array of point receiving elements located at some arbitrary distance from the source. The medium is characterized by a set of attenuation and delay random variables associated with each of the multiple paths. In addition, the additive noise contributed by the medium is assumed to be negligible. The detected signals are then processed by delay filtering, matched filtering, or frequency‐inverse filtering. The criterion for comparison of these three schemes is the ratio of the standard deviation of the filtering scheme’s output time‐average power to the expected value of this power. It is found that under certain constraints this ratio is approximately independent of the delay statistics of the medium, and the duration and frequency of the input for the three filtering schemes. In addition, this ratio is approximately equal for delay and matched filtering and is √2 times greater for frequency‐inverse filtering.

14 Jun 1968
TL;DR: In this paper, the authors investigated system requirements, configurations, and associated performance characteristics of maximum-likelihood space-diversity receiving systems from the maximumlikelihood principle, and the multidimensional Karhunen-Loeve expansion.
Abstract: : The research effort investigates system requirements, configurations, and associated performance characteristics of maximum-likelihood space-diversity receiving systems From the maximum-likelihood principle, and the multidimensional Karhunen-Loeve expansion, the continuous decision equations for space-time detection of noisy, random multipath signals are developed Interpretation of these equations yields the optimum space-time receiving system configuration requirements

Journal ArticleDOI
TL;DR: In this paper, a periodic fading of ionospheric echoes from E, E 8 and F -regions is described, sometimes present on one magneto-ionic component alone.

01 Jan 1968
TL;DR: In this article, a phase-locked receiver in a diffuse multipath fading environment with band-limited thermal noise is simulated with a phase lock receiver and phase-free phase-locking receiver.
Abstract: Digital simulation of phase locked receiver in diffuse multipath fading environment with band limited thermal noise

P. M. Seal1
01 Apr 1968
TL;DR: In this paper, the authors proposed a proposed orbiting data relay network using retrodirective antennas and multipath transmission problems associated with use on proposed orbiting Data Relay Network (DRN).
Abstract: Retrodirective antennas and multipath transmission problems associated with use on proposed orbiting data relay network

01 Jan 1968
TL;DR: In this paper, the authors concluded that vertical space diversity provides a very effective tool for combating multipath fading on line-of-sight paths, regard- less of the source of the fading.
Abstract: Deep fading on line-of-sight microwave paths is usually a result of multipath interference phenomena. Frequency diversity has been the most common method of protecting against such fading. In the past few years, vertical space diversity has also come into use. This paper discusses its characteristics, methods of determining spacings, and describes some results achieved. It is concluded that vertical space diversity provides a very effective tool for combating multipath fading on line-of-sight paths, regard- less of the source of the fading.

Journal ArticleDOI
TL;DR: A method for the simulation of initially unrelaxed, linear third-order type-zero and type-one systems having nonzero initial conditions using a minimum number of elements in an a priori assumed network configuration.
Abstract: —This note discusses a method for the simulation of initially unrelaxed, linear third-order type-zero and type-one systems having nonzero initial conditions. The simulation is accomplished with one operational amplifier and 2-terminal multipath feedback RC networks using a minimum number of elements in an a priori assumed network configuration.

Journal ArticleDOI
Maurice Diamond1
TL;DR: In this article, a pair of frequency-swept transmitters were used to suppress multipath reflections and provide flight information particularly useful in a high-density environment, and the system was concluded that the system can suppress multi-path reflections.
Abstract: Many electronic positioning systems, such as hyperbolic navigation and terminal guidance, suffer large errors from reflection and diffraction of their transmitted waves. The hyperbolic system described herein is free of these errors. Utilizing a pair of frequency-swept transmitters, it discriminates against reflections by separating the direct and reflected received signals into distinct sets of beat-frequency components. It is concluded that the system can suppress multipath reflections and provide flight information particularly useful in a high-density environment.

Journal ArticleDOI
TL;DR: It is shown that both the depth and rate of fading are reduced by using narrow-band filters, and curves demonstrate the saving in fading margin, which may be crucial in an emergency.
Abstract: Earth-reflected multipath interference can be serious for communication between near-earth spacecraft and stationary satellites. It is shown that both the depth and rate of fading are reduced by using narrow-band filters. Curves demonstrate the saving in fading margin; this saving may be crucial in an emergency.