Showing papers on "Multipath propagation published in 1984"

The Effects of Multipath and Fading on the Performance of Direct-Sequence CDMA Systems

Abstract: We show the following results concerning the maximum number N max of simultaneous users supportable by an asynchronous direct-sequence code-division multiple-access (CDMA) system using DPSK. (a) For a network of single-path nonfading links in which all users' signals arrive at any receiver with equal energies, N max is 10-20 percent of the number of chips M in the system's codes at bit error rates of 10-3to 10-5. (b) If typical urban/suburban multipath and fading phenomena occur and no power control ameliorates them, N max falls to 1-5 percent of M even under the best of circumstances (availability of multipath diversity and ideal multipath combining receivers). (c) If the links consist mostly of only single fading paths and no power control is available, direct-sequence CDMA becomes unusable. Power-control policies can be devised to overcome shadowing (slow fading) or near/far problems, thus partially restoring the N max of (a). It is unlikely that power control will effectively combat fast fading, especially in single-path situations.

Use of the kurtosis statistic in the frequency domain as an aid in detecting random signals

Abstract: Power spectral density estimation is often employed as a method for signal detection. For signals which occur randomly, a frequency domain kurtosis estimate supplements the power spectral density estimate and, in some cases, can be employed to detect their presence. This has been verified from experiments with real data of randomly occurring signals. In order to better understand the detection of randomly occurring signals, sinusoidal and narrow-band Gaussian signals are considered, which when modeled to represent a fading or multipath environment, are received as non-Ganssian in terms of a frequency domain kurtosis estimate. Several fading and muitipath propagation probability density distributions of practical interest are considered, including Rayleigh and log-normal. The model is generalized to handle transient and frequency modulated signals by taking into account the probability of the signal being in a specific frequency range over the total data interval. It is shown that this model produces kurtosis values consistent with real data measurements. The ability of the power spectral density estimate and the frequency domain kurtosis estimate to detect randomly occurring signals, generated from the model, is compared using the deflection criterion. It is shown, for the cases considered, that over a large range of conditions, the power spectral density estimate is a better statistic based on the deflection criterion. However, there is a small range of conditions over which it appears that the frequency domain kurtosis estimate has an advantage. The real data that initiated this analytical investigation are also presented.

TDMA spread-spectrum receiver with coherent detection

Abstract: To utilize indirect-path signals in a receiver for fixed and mobile transmitter-receiver stations of a TDMA spread-spectrum digital radio system, coherent detection is employed. At the same time, the signal imaging the multipath profile is continuously regenerated by a regeneration circuit. The regeneration circuit provides a phase-coherent signal employed for demodulating the received signal.

Time delay spread measurements of wideband radio signals within a building

Abstract: Measurements of time delay spread of wideband 850 MHz digital radio signals due to multipath propagation within a large building are described. These measurements show a median RMS time delay spread of 125 ns and a worst case of 250 ns. Consequently, signalling rates above 400 kHz may not be feasible.

Multipath Outage Performance of Digital Radio Receivers Using Finite-Tap Adaptive Equalizers

Abstract: Recent analysis/simulation studies have quantified the multipath outage statistics of digital radio systems using ideal adaptive equalization. In this paper, we consider the use of finite-tap delay line equalizers, with the aim of determining how many taps are needed to approximate ideal performance. To this end, we assume an M -level QAM system using cosine rolloff spectral shaping and an adaptive equalizer with either fractionally spaced or synchronously spaced taps. We invoke a widely used statistical model for the fading channel and computer-simulate thousands of responses from its ensemble. For each trial, we compute a detection signal-to-distortion measure, suitably maximized with respect to the tap gains. We can thereby obtain probability distributions of this measure for specified combinations of system parameters. These distributions, in turn, can be interpreted as outage probabilities (or outage seconds) versus the number of modulation levels. A major finding of this study is that, for the assumed multipath fading model, very few taps (the order of five) are needed to approximate the performance of an ideal infinite-tap equalizer. We also find that a simple, suboptimal form of timing recovery is generally quite adequate, and that fractionally spaced equalizers are more advantageous than synchronously spaced equalizers with the same number of taps. This advantage is minor for rolloff factors of 0.5 and larger but increases dramatically as the rolloff factor approaches zero.

Multipath cells for extending dynamic range of optical absorbance measurements

Abstract: La longueur de parcours effective d'une cellule a trajet multiple varie avec l'absorbance de la solution et est plus grande pour les solutions de faible absorbance

Characteristics of L-band multipath fading due to sea surface reflection

Abstract: For low G/T ship-based earth stations in future maritime satellite communications, the effect of multipath fading due to sea surface reflection will be very important at low elevation angles. However, a practical model of the L -band multipath fading for fade prediction has not been available so far. A practical fading model is presented based on the shadowed Kirchhoff approximation theory for the coherent and incoherent components of the scattered power as a function of sea surface conditions. Using this model, theoretical fading depths are presented as a function of elevation angle, wave height, and antenna gain. The results indicate that intense fading occurs for wave heights greater than 50 cm, and the dependence on wave height is small under these conditions at L -band frequencies. Theoretical results presented agree well with experimental results obtained by field experiments.

The effects of multipath and fading on the performance of direct-sequence CDMA systems

Abstract: We show the following results concerning the maximum number N max of simultaneous users supportable by an asynchronous direct-sequence code-division multiple-access (CDMA) system using DPSK. (a) For a network of single-path nonfading links in which all users' signals arrive at any receiver with equal energies, N max is 10-20 percent of the number of chips M in the system's codes at bit error rates of 10-3to 10-5. (b) If typical urban/suburban multipath and fading phenomena occur and no power control ameliorates them, N max falls to 1-5 percent of M even under the best of circumstances (availability of multipath diversity and ideal multipath combining receivers). (c) If the links consist mostly of only single fading paths and no power control is available, direct-sequence CDMA becomes unusable. Power-control policies can be devised to overcome shadowing (slow fading) or near/far problems, thus partially restoring the N max of (a). It is unlikely that power control will effectively combat fast fading, especially in single-path situations.

Commutation Signaling - An Antimultipath Technique

Abstract: Two types of commutation signaling for combating multipath are introduced: M \times N signaling, in which the signal set cycles successively among N different sets of M sigals each; and M + N signaling, in which the signal set at any time consists of M members of an alphabet of M + N signals, N of which are inactive at any epoch. In both cases, the objective is to remove from use any signal sent recently enough that it is still "ringing" in the multipath channel. We consider the problems of minimization of M and N , hence the total number signals used; analysis of 2 \times N sets ( M = 2 is almost uniformly optimal); analysis of the simplest M + N set, i.e., a 2 + 1 set; and presentation of the results of simulation of M \times N and M + N signaling through realistic urban/suburban radio channels. Comparison of these simulations to simulations of other antimultipath techniques is also made. The simulation results are due to Kamil [3].

Multipath Fading Models and Adaptive Equalizers in Microwave Digital Radio

Abstract: In the past few years, a great deal has been learned about the modeling and equalization of multipath fading in microwave digital radio systems. This paper adds a number of new results to this body of knowledge. We begin with a description of two published multipath fading models that were derived from the same collection of data, i.e., the threepath and polynomial models. We then derive outage probability characteristics for a digital radio link using M -level quadrature amplitude modulation, cosine rolloff spectral shaping, and some form of equalization. We obtain results for both models, several kinds of equalization, and various combinations of system parameters. We find that the theoretically optimal equalizer, which is nonlinear, is not measurably better in performance than the best of the linear equalizers, which is simpler. Also, the performance of a properly equalized receiver can be very accurately estimated by knowing just the probability distribution, over many fades, of the gain amplitude at a single frequency. Finally, we show the influences on performance of such parameters as cosine rolloff factor, symbol rate, and unfaded carrier-to-noise ratio.

Intruder detection radar system and automatic nulling antenna array

Abstract: A bistatic VHF Doppler radar is utilized to detect and track one or more intruders crossing a large annular ring surveilance zone surrounding a central high security installation. A central omni-directional transmitter radiates the surveillance zone with a continuous wave (CW) signal. Predetermined ones of a plurality of receiver antennas circumferentially spaced around the surveillance zone receive a portion of the signal which is reflected by an intruder crossing the surveillance zone and a portion of the signal which is directly radiated from the transmitter antenna. The reflected signal portion may be received by antenna arrays each of which is controlled to form a null in its pattern toward the transmitter antenna to overcome direct signal overload. The direct signal portion may be received by stub antennas. Each antenna array and each stub antenna is connected to a separate channel of a dual channel receiver. An intruder moving across the surveillance zone causes signals representative of Doppler cycles of phase change to be generated. The outputs of the two separate channels are compared and their relative phase changes measured. A detected intruder causes an alarm signal at a central processing station and the Doppler information is processed to generate tracking data which is shown on a display device. An automatic null control circuit may also be utilized to accommodate multipath conditions and static clutter due to environmental factors.

A transmitter diversity for MSK with two-bit differential detection

Abstract: Transmitter diversity is a technique that is effective for mitigating signal transmission degradation caused by multipath fading which is one of the most serious problems in land mobile radio. A frequency-offset transmitter diversity is proposed for a land mobile radio system that employs minimum-shift keying (MSK) and two-bit differential detection. It is shown that high transmission efficiency can be obtained in comparison with the other frequency-offset transmitter diversity. In addition, the diversity effect on the bit error rate (BER) performance is equivalent to that of postdetection equal gain combining diversity. The BER performance improvement was confirmed by the experimental test results.

Simultaneous Adaptive Estimation of Carrier Phase, Symbol Timing, and Data for a 49-QPRS DFE Radio Receiver

Abstract: The use of adaptive decision feedback techniques in the joint estimation of data, carrier phase, and symbol timing is investigated for a 49-QPRS 90 Mbit/s digital radio. The performance of the carrier and timing recovery loops are analyzed, first in a multipathfree environment and then for the case of a multipath distorted signal and a fixed decision feedback equalizer. Finally, the joint problem of data recovery, estimation of carrier phase, and symbol timing for a

Data transmission over UHF fading mobile radio channels

Abstract: A combination of transparent tone-in-band (TTIB) and feedforward signal regeneration (FFSR) audio signal-processing techniques enable the efficient transmission of speech and data communications over mobile radio links, and amplitude companded pilot tone single sideband (SSB) in particular The contribution is concerned primarily with data communications, coherent and noncoherent, over such networks in the presence of multipath induced amplitude and phase distortion A detailed consideration of the properties of the combined TTIB/FFSR system show it to have several significant advantages over data-derived reference schemes

High performance digital communications in mobile channels

Abstract: Random amplitude and phase modulations experienced in mobile channels result in a number of system limitations and link impairments. In this paper, a novel approach named "tone calibrated technique" which mitigates multipath fading induced distortions in a digital link is presented. It is shown that a low power pilot tone can be used to continuously calibrate a mobile link against time varying impairments. Coherent demodulation of data is possible by means of the pilot tone without using a conventional tracking loop. Since very little power is consumed by the pilot tone, the performance of the tone calibrated technique is only marginally below BPSK in an unfaded environment.

The generalized phase spectrum method for time delay estimation

Abstract: The concept of the Generalized Phase Spectrum (GPS) TDE is put forward. The relation between the GPS TDE and the GCC TDE is derived. A multipath signal model is considered. The method of Amplitude Square (AS) weighting for TDE is proposed and the comparison between the AS TDE and the Phase Data (PD) TDE is made in the multipath environment. The results of theoretical calculation and computer simulation experiment show that the performance of the AS weighting TDE is superior to that of the PD TDE.

Performance of 16 kbit/s GMSK Transmission with Postdetection Selection Diversity in Land Mobile Radio

Abstract: Through laboratory simulation tests and field experiments in the Tokyo metropolitan area, 16 kbit/s Gaussian filtered minimum shift keying (GMSK) transmission performance has been experimentally clarified in the 920 MHz land mobile radio environment. The experimental results agree closely with theory, and they show that fast multipath fading severely degrades average bit error rate (BER) performance in GMSK transmission. However, a space diversity reception technique using a postdetection selection combining scheme is able to efficiently mitigate the fast multipath fading.

Adaptive transversal equalization of multipath propagation for 16-QAM, 90-Mb/s digital radio

Abstract: Adaptive transversal equalization is an effective and relatively new countermeasure for dispersive multipath propagation in terrestrial digital radio networks. In this paper we describe the design and performance of a five-tap baseband analog equalizer developed for a family of 16-QAM, 90-Mb/s radio systems. Laboratory measurements and field evaluation during a five-month fading season in Palmetto, Georgia, indicate that the use of this adaptive transversal equalizer can significantly reduce the need for costly space-diversity equipment.

Apparatus for and method of adaptively processing sonar data

Abstract: The invention discloses an apparatus for and a method of adaptively processing a sonar power spectrum for improving the display of narrowband line structure characteristic of a target in a background of broadband interference which is rippled due to multipath propagation arrival structure. The processor employs a first Fourier transformer to convert the sonar frequency data into a complex correlation format. The autocorrelation is then time difference sampled to separate the broadband ripple interference from the broadband trend interference. The separate time difference samples are then transformed back to their real frequency format to form a broadband ripple estimate and a broadband trend estimate of the interference background. The ripple estimate is subtracted from the sonar signal to remove the ripple component and the resulting difference is normalized by a division by the interference trend estimate. The result is improved detectability of the narrowband lines in the presence of widely varying multipath interference.

The monopole slot as a two-port diversity antenna for UHF land-mobile radio systems

Abstract: Urban land-mobile communication systems designed to operate over the upper portion of the UHF band are very susceptible to frequent wide signal variations. Deep fades arise due to the presence of many obstructions in the propagation path between the transmitter and receiver. One way to combat the effects of Rayleigh fading due to multipath propagation is to provide a diversity combiner in the mobile receiving system with two signals, each containing the same information, which fade independently of one another. Such a system can virtually eliminate deep fades from the receiver output. Described herein is a method of obtaining two diversity branches, with independent signals if so desired, from a single two-port antenna which is both well matched to the transponder and efficient over the land-mobile frequency band from 820- 890 MHz. A directive pattern is associated with each port. When operating with a direct wave, signal gain over an omnidirectional antenna is therefore possible.

Performance of 16 kbit/s GMSK transmission with postdetection selection diversity in land mobile radio

Abstract: Through laboratory simulation tests and field experiments in the Tokyo metropolitan area, 16 kbit/s Gaussian filtered minimum shift keying (GMSK) transmission performance has been experimentally clarified in the 920 MHz land mobile radio environment. The experimental results agree closely with theory, and they show that fast multipath fading severely degrades average bit error rate (BER) performance in GMSK transmission. However, a space diversity reception technique using a postdetection selection combining scheme is able to efficiently mitigate the fast multipath fading.

Multipath distortion reducing circuit

Abstract: A multipath distortion reducing circuit particularly adapted for use with stereophonic receiving circuits in which the amount of mixing between right and left channels is reduced. In accordance with the invention, the envelope of the received FM signal is AM detected, and the envelope detected circuit is multiplied by the FM detected signal differentiated to provide a distortion cancelling signal. The distortion cancelling signal is summed with the detected FM signal to provide an output having a reduced multipath distortion component.

High‐resolution multipath measurements of the meteor scatter channel

Abstract: An experimental method for determining the multipath spread of the meteor scatter communications channel is discussed. The technique measures the spreading of equalization pulses contained in commercial television signals received via meteor scatter. Resolution of spread to within 0.5 μs is possible with this technique.

A Minimum Dispersion Combiner for High Capacity Digital Microwave Radio

Abstract: A new minimum-dispersion (MID) combiner, which reduces multipath degradation in a high capacity digital microwave radio, is proposed. A high capacity digital microwave radio is inherently very sensitive to waveform distortion caused by multipath in-band delay dispersion and in-band amplitude dispersion. To minimize the in-band dispersion, the combined-signal spectrum shape from the two antennas is monitored before and after a small change takes place in the combining phase. The phase shifter rotates in the direction of the flatter of the spectrum shapes, either the one before or the one after monitoring. Performance evaluations through simulation calculation and theoretical estimation using in-band amplitude dispersion probability density are given. More improvement can be obtained when the MID combiner is used instead of the maximum power (MAP) combiner currently in use. Laboratory and field experiments, using a 200 Mbit/ s 16-QAM signal, verify these analyses and show an additional outage reduction factor of more than 5.

The Combined Effects of Noise and Multipath Propagation in Multilevel PSK Radio Links

Abstract: A general analysis of the behavior of multilevel PSK radio links is carried out under multipath propagation conditions. A method is proposed to evaluate system unavailability due to the combined effects of noise and faded channel distortion. The "outage domain," having fixed the transmission channel, is calculated in the space of the variables which characterize any anomalous propagation condition. In particular, with reference to a "twopath model" (a "direct ray" and an "echo"), variations of carrier and symbol synchronisms due to the echo are taken into account and their importance is emphasized in realizing the numerical results. These are shown to be in very good agreement with the experimental ones which have appeared in the literature. An analytical realization of the results obtained is developed and leads to a simple asymptotic formula explaining outage probability as a function of some parameters of the echo statistics, the transmission channel adopted, and the number of modulation levels. Rapid evaluation of this formula allows us to plot outage probability as a function of spectral efficiency, measured in bits/s/Hz.

Adaptive Multiplication-Free Transversal Equalizers with Application to Digital Radio Systems

Abstract: Recently, multiplication-free digital filters have been proposed for different applications to digital radio systems and signal processing. The absence of multiplication is obtained by constraining each coefficient to be sum of powers of two. In this way, multipliers are substituted by shift registers and, if appropriate, some adders. The optimal power-of-two coefficients must be computed through a nonlinear optimization procedure. Until now the problem of adapting the power-of-two coefficients of base band transversal equalizers in the case of time-varying channels had not been considered. In this paper two algorithms are proposed to cope with this problem and are compared from the viewpoint of complexity, accuracy, and convergence speed. As an application example, transmission of a biphase PSK digital radio system over a multipath fading channel is considered.

A Computer Simulation of the Effect of Fading on a Quasi-Synchronous Sideband Diversity AM Mobile Radio Scheme

Abstract: In this contribution, a software multipath fading simulation package (previously applied to the statistical analysis of the fading statistics of single transmitter schemes for three very different mobile radio environments) is used to analyze the performance of a quasi-synchronous, multitransmitter, amplitude modulated sideband diversity area coverage system. By conducting field trials in an area known to obey Rayleigh fading statistics, it has been possible to compare the simulated statistical results to those obtained theoretically and experimentally, and to show that good agreement exists. The fading simulator is capable of being extended to other types of multitransmitter radio scheme employing different forms of modulation.

Decision Feedback Equalization for Multipath Induced Interference in Digital Microwave LOS Links

Abstract: The performance of a 49-QPRS, 90 Mbit/s digital radio receiver equipped with a decision feedback equalizer (DFE) to counter multipath fading is investigated via computer simulation. The simulation includes the transmitted data, multipath fade model, receiver model, and DFE. The results indicate that a DFE equipped with five forward and five feedback taps can adequately compensate a 40 dB minimum-phase fade anywhere in the receiver passband. The study is extended to other receiver configurations including the use of space diversity and/or slope equalizers and the use of a transversal equalizer (TE) with the same delay-span in place of the DFE. The results indicate that the DFE equipped receiver outperforms the TE receiver and that still better performance may be achieved using a combination of space diversity and DFE.