Showing papers on "Multipath propagation published in 1970"

Analysis of rayleigh-wave multipath propagation at lasa*

Abstract: An investigation has been made of the multipath propagation of Rayleigh waves by using data obtained from the large aperture seismic array (LASA). The use of the LASA in conjunction with a high-resolution analysis technique provides a greater angular resolution and accuracy than was previously possible for the analysis of the multipath propagation. Measurements have been made of this phenomenon for the Rayleigh waves of 26 events distributed at various azimuths and distances from LASA. On the bas of these measurements reasonably good conjectures are made concerning the actual propagation paths for groups in the 20- to 40-sec period range. It is shown that in almost all cases these propagation paths can be associated with refractions and reflections at the continental margins.

Mobile locator system for metropolitan areas

Abstract: A locator system is disclosed wherein the vehicle whose location is unknown transmits a short, modulated radio frequency pulse. Receiving stations about the periphery of the area being serviced phase compare the received signal with a standard time reference signal. The phase difference is used to determine the arrival time differences at the known locations of the receiving stations from which the location of the vehicle may be determined. The wavelength of the modulating signal is generally greater than the maximum distance encountered in the service area. The effects of multipath reflections are minimized by utilizing a space dispersed antenna.

Adaptive signal processor for diversity radio receivers

Abstract: A signal processor in a diversity receiver for digital data transmitted over dispersive and fading radio channels performs the functions of demodulation, diversity signal combining, delay equalization, multipath distortion equalization and timing jitter elimination. Transversal equalizers, one in each diversity channel, are made adaptive to a common, time-varying mean-square error signal derived from the combined postdetection output data.

Apparatus for reducing multipath distortion of signals utilizing cepstrum technique

Abstract: Distortion of an acoustic signal caused by multipath transmission is minimized by utilizing a cepstrum technique. Signals received at a plurality of microphones are processed to obtain signal representations of their complex cepstrums. The cepstrum signals are then summed. In summing, the portions of the cepstrum signals representative of the undistorted acoustic signal cohere while the portions of the cepstrum signals representative of the multipath transmitted signals do not. Selective clipping of the summed cepstrum signal eliminates the distortion components. Inverse transformation of the clipped summed cepstrum signal yields a replica of the original acoustic signal.

Microwave line-of-sight propagation with and without frequency diversity

Abstract: Amplitude measurements were made for 68 days in 1966 for seven 4-GHz and 6-GHz signals on a typical radio relay path. Identical measurements were also made for one 4-GHz signal on a second path having a common reception point with the first path. We present the results from an analysis centered on the fade-depth distribution for fades exceeding 20 dB. The more significant results are: (i) The fade-depth distribution for all single (nondiversity) channels in a 5–10 percent band on the same path are essentially the same. Further, the distribution has the Rayleigh slope. (ii) The single-channel fade-depth distributions differ for 4 and 6 GHz on the same path; the distributions also differ for the same 4-GHz frequency on adjacent paths with a common reception point. (iii) One-for-one frequency diversity can be characterized during multipath fading periods for either the 4- or 6-GHz bands by the ratio of two quantities. The first is the precent frequency separation between diversity components. The second is the nondiversity fade-depth distribution.

A prediction model for multipath propagation of pulse signals at VHF and UHF over irregular terrain

Abstract: A prediction model is presented that permits calculation of the probability of occurrence of distinct multipath propagation of pulse signals at VHF and UHF over irregular terrain. The model applies to terrain characterized by an irregular distribution of obstacles such as hills, buildings, trees, etc., so as to make it impractical to calculate the effect of multipath propagation by diffraction or bistatic-reflection theory. Statistical data on wave propagation over irregular terrain form the basis for the empirical model developed. Generally, the model predicts that 1) for constant transmitter-receiver separation, the amplitudes of the received echoes decrease with increasing echo delay, and 2) for constant echo delay, the occurrence of echo pulses increases as the transmitter-receiver distance increases. The results obtained from the model for rural, hilly terrain, and for a built-up metropolitan area are compared with available measured data.

The number of fades in space-diversity reception

Abstract: Experimental and theoretical results are presented which show that deep fades due to multipath propagation on line-of-sight microwave links are greatly reduced in number when two vertically separated receiving antennas are used and the stronger of the two received signals is selected as the diversity signal. The experimental results are based on 6 GHz propagation data obtained for a 72-day period on a 28.5 mile path in Ohio, with a 27.5 foot vertical separation of the receiving antennas. The theoretical results are obtained by treating the received signals as correlated Rayleigh distributed random variables. Theoretically predicted variations with fade depth agree with experimental observations. Combination of the theoretical results with experimentally determined parameters provides results which can be used, for example, to calculate the reduction in the number of fades as a function of the vertical separation of the receiving antennas, wavelength, path length, fade depth, and the gain difference (if any) of the receiving antennas.

Results of the OV4-1 Dual Satellite Experiment on Guided Ionospheric Propagation

Abstract: The OV4-1 (orbiting vehicle 4, experiment 1) dual satellite guided ionospheric propagation experiment has confirmed the existence of long-range, low-loss, earth-detached propagation paths in the bottomside ionosphere with mean path losses smaller than equivalent multihop and, in some instances, smaller than free-space propagation. Satellite-to-satellite signals at 34.3 MHz were received from 56 to 100% of the five-minute samples taken in real time by the ground telemetry network while the orbits of the two satellites were decaying from 300 to 250 km. Measured values of path losses range from between 124 and 159 db. Multipath spread was never found larger than 550 μsec, and there is some evidence that Doppler spread was less than 0.012 Hz. Signals at 20.75 and 46.8 MHz in the satellite-tosatellite link were rarely received beyond the line of sight.

Multipath tolerant ranging and data transfer techniques for air-to-ground and ground-to-air links

Abstract: This work addresses data acquisition problems associated with an improved air traffic control system. A mathematical theory of scattering which describes the multipath process is developed. From this model, the optimum signal structure required for ranging and communicating data through the multipath propagation medium is derived. It is shown that the Reed-Solomon Codes have near optimum properties for ranging and data communications. A baseline data transmission system for air traffic control is designed which will accommodate 4000 aircraft per second at the rate of 100 bits per second in a 60 by 120 mile area. The baseline system combines coding with frequency hopping as the best means of combating severe multipath interference. Several coding systems are compared and curves relating error rate to E b /N 0 (signal-to-noise energy per bit) for a channel characterized by Rayleigh fading are presented. This paper concludes that a ground-to-air and air-to-ground high capacity data link at a reasonable cost will be feasible for an improved air traffic control system.

Wide-Band Detection of FSK Transmissions in a Three-Component-Two-Path Channel

Abstract: The multipath model evaluated is of a three-component-two-path type. In several space channels we have a directtransmission path and a reflected-transmission path. The direct component can be described as specular, while the reflection is a composition of both a specular component and one that is diffuse. The channel bit error probability for noncoherent wide-band frequency-shift keying (FSK) is obtained for such channels. It is shown that parameters such as rate of fade, Doppler, and data rate relative to the delay between the two transmission paths affects the system performance significantly.

On Optimizing Array Reception of Multipath

Abstract: This paper reviews system configuration requirements and analyzes detectability performance characteristics for maximum likelihood array reception of multipath. Performance is analyzed to determine the effects of channel multipath structure (multipath delay and signal power division among the paths), space-time correlation properties of the incident processes, and the array spacing. It is shown by a series of case studies, that for single element coupling, as well as array coupling, an increased multipath delay factor results in decreased system detectability for fixed signal and noise intensity levels. The performance capacity is degraded as the available signal power tends to distribute more uniformly between the paths. These effects are attributed to the loss of effective signal energy concentration, resulting in a lower effective pre-detection signal-to-noise ratio. An investigation of the effects upon system performance, due to array element spacing, shows that performance is enhanced by increasing the spacing relative to the multipath delay factor and the reciprocal signal bandwidth. The former is the result of a more directive detectability (beam) pattern arising from the increased spacing. In effect, with increased spacing, the main lobe of the pattern is narrowed, while the side lobes are optimally suppressed by the required noise related array element link, frequency filters (weights).

Single-RF Diversity Receiver for OFDM System Using ESPAR Antenna with Alternate Direction

Abstract: This paper presents a single-RF diversity scheme for orthogonal frequency division multiplexing (OFDM) receiver using Electronically Steerable Passive Array Radiator (ESPAR) antenna whose direction changes alternately at the OFDM symbol rate. OFDM is widely used for mobile communication systems because of its broadband wireless transmission capability in a severe time dispersive multipath propagation channel. OFDM is, however, not efficient for mitigating the performance degradation due to fading. Diversity is an efficient technique for solving this problem. Although maximal ratio combining diversity is the most efficient technique, it requires the same number of RF front-end circuitry and analog-todigital converters (ADC) as antennas. Although ESPAR antenna-based diversity technique requires only a single-RF and ADC, the convergence is not fast enough to track fast variation of the channel state. Furthermore, it is not efficient in a frequency selective channel. In this paper, we propose a new OFDM diversity scheme using ESPAR antenna. The proposed scheme is capable of obtaining the diversity gain in a frequency selective fading environment and avoids the slow convergence rate problem in the conventional technique using ESPAR antenna. Computer simulation results show that the proposed scheme gives diversity gain in a frequency selective fading channel.

The Bit Error Rate analysis of Direct Sequence Ultra Wide Band System in Mine Channel

Abstract: The necessity for wireless communications in underground mines is well understood. Some companies have started to deploy modern wireless networks in mine galleries with the objective of increasing safety and productivity. This paper presents the Bit Error Rate (BER) analysis of Direct Sequence Pulse Amplitude Modulation (PAM) Ultra Wide Band (UWB) system for underground mine environment. Discrete time channel impulse response is used to build up revised channel model for underground mine which is based on the UWB channel model proposed by IEEE802.15.3a.With the revised channel model, we compare the performance of IEEE channel and mine channel by considering Mean Excess Delay, Root Mean Square (RMS) delay and Number of significant Paths with in 10 dB of peak (NP 10dB) and observe the significant increase in the parameter. We evaluated the BER performance using RAKE receiver employing maximal ratio combining (MRC) for different data rate, repeat bit and number of RAKE figure in underground mine channel. Simulation results shows that DS-PAM-UWB system can sustain in the dense multipath environment of underground mine and provide acceptable BER.

Analysis of satellite communications in a multipath environment

Abstract: Satellite communications performance improvement by use of multipath signal for channel models including noncoherent FSK

Multipath dispersion for the atmosphere laser channel

Abstract: Consideration is given to multipath dispersion due to atmospheric inhomogeneities for point and nonzero-area apertures under clear weather conditions. Arguments are given which show that the time dispersion is primarily due to unscattered ray paths for large apertures. Using this fact an expression is derived for the structure function of the propagation time for large receiver apertures. This expression applies for large dispersions, i.e., beyond the uninteresting case where one has coherence over the receiver aperture. The expression indicates that during medium turbulence conditions, the dispersion for a 6-m aperture receiver system is 0.07 ps for a vertical path through the whole atmosphere and 0.695 ps for a 15-km horizontal path. In contrast, when using a point receiver aperture, the dispersion is expected to be considerably less than 0.001 ps for a vertical path through the whole atmosphere. Finally, a multipath equivalent circuit model is presented for the atmospheric channel.

State-Space Estimation for OFDM and SC-FDE Schemes with Strongly Varying Carrier Frequency Offset

Abstract: Parametric algorithms for the estimation of rapidly-varying Carrier Frequency Offset (CFO) usually employ pilot symbols multiplexed with the data transmission. As the CFO variation rate increases so has to increase the density of pilot symbols transmitted, thus impairing the bandwidth efficiency. In order to reduce the number of pilot symbols used in the estimation of rapidly-varying CFO it was proposed to use a truncated Taylors series to predict the CFO, where the derivatives up to order d-1 are recursively estimated with a d-order Kalman filter (KF).We propose to compare the performance of a fourth-order KF predictor in the most popular block transmission systems: Orthogonal Frequency Division Multiplexing (OFDM) and Single Carrier Frequency-Domain Equalization (SC-FDE). Simulating different transmission scenarios, e.g., channel coding and spatial diversity, our results show that for static multipath fading channels the proposed receiver for the SC-FDE scheme exhibits better Bit Error Rate (BER) performance than that of OFDM.

Measurement of time delays in multipath systems

Abstract: In a recent letter, Gouriet proposed a method of measuring time delay, which uses the fact that the time delay of a linear system is the group delay at zero frequency. Based on the same method, a system is proposed in this letter to measure the time delays in a linear system which provides more than one transmission path between a single output and a single input. The system assumes that a rough knowledge of the magnitudes of time delays is available and that the individual multipath responses do not overlap in the time domain.

Medium Disturbance Sensing Techniques for FSK Modulation

Abstract: This paper describes measurement techniques for rapidly characterizing frequency selective fading channels. The approach used involves estimating each of the parameters describing the transmitted signal and channel disturbances. This includes signal level, noise level, and multipath spread. While specifically applied to FSK modulation and HF channels, the techniques are applicable to other fading media such as troposcatter. The advantages of the proposed techniques are several: the measurement is performed by operating on the transmitted information bits and special test signals or redundancy are not required. Instrumentation is relatively simple and the techniques are applicable to practical communication media. Calculations are provided to determine the measurement time for stated accuracies and confidence levels. Results are obtained for several multipath distributions and as a function of the multipath-to-noise energy ratio.

Time Domain Equalization Method for DFTS-OFDM Signal without GI under Highly Mobile Environments

Abstract: In highly time-varying fading channel, the Discrete Fourier Transform Spreading Orthogonal Frequency Division Multiplexing (DFTS-OFDM) signal would be damaged significantly by the inter-channel interference (ICI) due to the loss of orthogonality among subcarriers which leads a fatal degradation of bit error rate (BER) performance. To solve this problem, this paper proposes a time domain equalization (TDE) technique in conjunction with a time domain channel impulse response (CIR) estimation method for the DFTS-OFDM signal without using a guard interval (GI). The features of proposed method is to employ a time domain training sequence (TS) both for the estimation of time domain CIR at every sampling time and for removing the inter-symbol interference (ISI) incurred in the multipath fading channel. The proposed method also employs the TDE with a maximum likelihood (ML) estimation method in the demodulation of received time domain signal at every symbol instead of using the conventional Minimum Mean Square Error-Frequency Domain Equalization (MMSE-FDE) method. This paper presents various simulation results to demonstrate the effectiveness of proposed demodulation method for the DFTSOFDM signal without GI as comparing with the conventional MMSE-FDE and TDE methods both of using GI under highly mobile environments.

Multipath Reflections from a Random Surface

Abstract: A method for counting the expected number of multipath reflections from a random surface between a source with finite beamwidth and a target or receiver is presented. The analysis is carried out for both one‐ and two‐dimensional roughness and for multiple bounces between grossly parallel boundaries of the transmitting medium. As asides, a comparison is made between theoretical results and a graphical experiment, and a computer methodology for the case of linear phase velocity gradient is presented. The results of this paper could be useful in estimating numbers of multipaths for transmissions between earthbound or airborne high‐frequency radio source and receiver via earth and/or electron overdense ionosphere; between underwater sound source and receiver via ocean bottom and surface; and, in curvilinear coordinates, between earth station and space station via moon surface.

Troposcatter multipath analyzer.

Abstract: : The use of Rake instrumentation as a sounding technique for the troposcatter multipath channel is discussed and the implementation of the Troposcatter Multipath Analyzer is described. The transmitter emits a carrier which is biphase modulated by a pseudorandom code sequence at a selectable rate between 10 and 1.25 MHz. The corresponding multipath resolution is 0.1 to 0.8 microsec. The receiver embodies a unique implementation in which video is detected corresponding to both quadrature components of the incoming signal followed by single-bit polarity sensing. Noise is added to linearize the single-bit sampling. The receiver generates ten time-shifted local copies of the transmitter pseudo-random code and cross correlates these with the sampled video to realize ten Rake taps. The integration period or data sampling rate is selectable as 1.25, 10 or 80 ms. The resultant output of the analyzer consists of samples of the quadrature Rake tape components recorded on magnetic tape for later computer processing. The Multipath Analyzer was exercised over a test path in upper New York State. The test results are presented in the form of an extensive set of three-dimensional scattering functions. These show the distribution of received power as a function of multipath delay time and Doppler frequency. The data records leading to the calculated results were collected during the late Fall of 1969. The highlights of the results are discussed. The scattering functions calculated from data recorded during movement of a cold front through the test path provide a particularly unusual set of results. (Author)

Diversity Performance of the Wide-Band FSK System in a Three-Component Two-Path Channel

Abstract: In several space relay communication channels signal energy can be described as following two transmission paths. The first is the direct path from the transmitter to the receiver in which the signal is corrupted only by additive Gaussian noise. The second is the reflected path involving a reflection boundary, which may distort and delay the signal in that path. At the receiver the combined signal from the two paths exhibits multipath effects (primarily signal fading). In addition, substantial Doppler is frequently encountered so that the carrier frequency can be treated as a random variable. This paper describes a detailed mathematical model of this channel. This allows one to study the effects of such important parameters as Doppler, rate of fade, diversity, and the degree of correlation between the direct and reflected path transmission. The envelope of the reflect signal is assumed to be statistically described as a Rician random process and the modulation is assumed to be frequency shift keying (FSK). The model is used to derive performance equations for this system. It was found that a priori knowledge of the nature of the fade is necessary to determine whether diyersity is advantageous. Thus, for example, reflections from extremely smooth boundaries (specular reflections) show small degradations in performance with the use of diversity, while for most diffuse reflection cases, a significant improvement can be achieved through the use of diversity.

Multipath study for a low altitude satellite utilizing a data relay satellite system

Abstract: Technical considerations associated with a low altitude satellite operating in conjuction with a data relay satellite system are reported. Emphasis was placed on the quantitative characterization of multipath phenomenon and determination of power received via both the direct and earth reflection paths. Attempts were made to develop a means for estimating the magnitude and nature of the reflected power.

A co-channel filter for CATV

Abstract: A rigorous approach permits a co-channel filter design which effectively removes the beat interference of a weaker, but still noticeable, TV station on the same channel as a desired station. Unlike previous designs, the present filter does not rely on the strength or direction of the interfering station. Rather, it uses a novel multi-frequency, multipath, phase reversal technique to accomplish cancellation of the bandpass containing the beat interference without accompanying phase distortion, as occurs in conventional filters. The co-channel filter, designed to be used after demodulation to base-band, is equipped with a meter at the beat frequency for proper adjustment of the frequency and notch depth controls.