Showing papers on "Fading published in 1970"

A Gaussian channel with slow fading (Corresp.)

Abstract: An interleaved fading channel whose state is known to the receiver is analyzed. The reliability function E(R) is obtained for rates R in the range R_c \leq R \leq C . The capacity is shown to be C = E_A \{ \frac{1}{2} \ln (1 + A^2 n)\} where A is a factor describing the fading mechanism and u is the signal-to-noise ratio per dimension.

The Photochemistry of Dyes II—Some Aspects of the Fading Process

Abstract: The effect of various factors (oxygen, temperature and humidity, atmospheric contaminants and concentration of dye) on the fading of dyes is discussed. The phototropic behaviour of azo and thioindigoid dyes is described. The fading products resulting from the prolonged irradiation of dyes are discussed, as is the relation between the light-fastness properties of a dye and its chemical constitution.

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.

Diversity system for mobile radio using fade rate switching

Abstract: A mobile radio system having a plurality of base station transmitting antennas for each channel. When the mobile receiver encounters a fading signal level from the antenna then in use, it transmits an out of message band signal back to the base station which commands the base to switch to a different antenna thus providing diversity on the base to mobile link. Particular circuits are provided to discriminate between rapid Rayleigh fades and slow shadowing fades and to transmit a command for an antenna switch based on the Rayleigh fades.

Comparison of simultaneous line-of-sight signals at 9.6 and 34.5 GHz

Abstract: Signals at 9.6 and 34.52 GHz, propagated simultaneously over a slant line-of-sight overwater path, have been analyzed to compare the power spectra of phase-of-arrival variations and fading and to determine the coherence of these signals with regard to both phase variations and fading. The phase data at the two radio frequencies exhibited nearly identical power spectra from 0.01 to 5 Hz and very high coherence from 0.01 to 0.1 Hz. The coherence dropped rapidly above 0.1 Hz and was in most cases less than 0.4 above 0.5 Hz. The power spectra of fading were similar in shape at the two frequencies, but the fading spectral density was consistently higher at 34.52 GHz than at 9.6 GHz from 0.1 to 5 Hz. The shape of the coherence function for fading was similar to that of the corresponding phase coherence function, but the fading coherence was lower at the low spectral frequencies. The possible effect of the small spatial separation of the propagation path on the coherence analysis is discussed.

Approximations for lognormally fading optical signals

Abstract: A lognormally distributed random variable can be used to approximate accurately a noncentral chi-square random variable, This letter uses this result to approximate the photoelectron count distribution of a lognormally fading optical signal.

An analysis of adaptive retransmission arrays in a fading environment

Abstract: We analyze in this paper the performance of adaptive retransmission for improving two-way communication between antenna arrays in a randomly fading environment. For a stationary environment, S. P. Morgan has shown that complex conjugate retransmission reaches a stable state and maximizes the signal-to-noise ratio of a maximal ratio diversity reception system. We show that a simpler system using phase conjugate retransmission will also stabilize and maximize the signal-to-noise ratio of an equal gain diversity reception system. Where the fading is slow in comparison to the system settling-down time, both systems provide a significant improvement in transmission. Subject to Rayleigh fading, we have obtained the average signal strength and its cumulative probability distribution for various combinations of numbers of antennas in the two arrays for each of the above mentioned systems. This information is useful in choosing an optimal division of diversity branches for the two antenna arrays. It is further observed that although the phase conjugate retransmission system is much simpler to implement, its performance is only slightly inferior to the corresponding complex conjugate system.

Optical Communication in Turbulent Media

Abstract: A study is made of the error rate for binary single-path fading channels using photocount detection. First, noiseless channels are examined. The photocount-distribution change caused by fading is proved to be a linear transformation representable by a matrix containing both positive and negative terms. Second, the performance of noisy channels with log-normal fading is discussed by giving the error exponent as a function of noise photocounts, signal-to-noise ratio, and amount of turbulence.

Performance of the Analog Moving Window Detector

Abstract: A type of analog integrating moving window detector for use with a scanning pulse radar is examined. A performance analysis is carried out, which takes into account both the radiation pattern of the antenna and the dynamic character of the detection process due to the angular scanning of the antenna. An expression for the false alarm rate of the detector is first derived and evaluated numerically. The detection performance and angular accuracy are next determined in a direct Monte Carlo simulation of the detector on a digital computer for both no fading and pulse-to-pulse Rayleigh fading. Finally the influence on detection performance of the width used for the moving window is investigated.

The Effect of Fading on the Performance of a Multihop PCM Radio System

Abstract: The effects of fading on the error-rate performance of a multihop line-of-sight PCM radio system, operated with or without diversity, are theoretically analyzed. A set of approximate formulas for the distribution of error probability has been established. These formulas can be used to determine the necessary carrier-to-noise power ratio in a noncoherent frequency-shift keying or coherent phase-shift keying system in order to meet the specified error probability and percent outage time.

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.

Overwater line-of-sight fade and diversity measurements at 37 GHz

Abstract: The results of line-of-sight propagation measurements at 37 GHz over a 10.0-mile overwater test site are presented. Measurements of the short-term fading statistics, and of the time, space, and frequency diversity parameters are included. Strong negative correlation was observed during both the space and frequency diversity measurements; negative correlation coefficients were as large as -0.8. The diversity improvement obtained was greater than the value expected from correlation calculations, which is attributed to the consistent occurrence of negative correlation in the vicinity of the interference nulls rather than the random occurrence assumed in the calculations. For the optimum case where maximum negative correlation occurs, sufficient diversity improvement is achieved with either space or frequency diversity to permit the elimination of almost the entire fade margin that is needed to overcome the multipath fading associated with reflections from the water surface.

A note on correlation distance of vhf fading from irregularities in the equatorial ionosphere

Abstract: Correlation distance of VHF fading from irregularities in equatorial ionosphere, using NASA STADAN in Chile

Multiple ground reflection effects on fading behavior of VHF/UHF satellite transmissions

Abstract: A computerized method to predict fading occurrence in VHF/UHF satellite transmissions, originating from multiple ground reflections is described. Based on ray optics the effective antenna gain is computed as a function of the elevation angle and ground parameters of unidealized irregular terrain.

Proposal of Channel Estimation Method for ITS systems by using STBC MIMO-OFDM

Abstract: This paper proposes a new road-to-vehicle communication system for the future ITS by using the STBC MIMO-OFDM technique which can provide the safety and comfortable driving, and collection of variable information from the network in the realtime to the users on the vehicle. To realize the proposed STBC MIMO-OFDM system, it is required to estimate the channel frequency response at every symbol precisely in the time varying fading channel which is the typical operation conditions for the roadto-vehicle communications system. In this paper, we propose a novel channel estimation method by using the scattered pilots and null sub-carriers inserted into the data sub-carriers both in the frequency and time axes which enables the accurate channel estimation even in the higher time varying fading channel. From the computer simulation results, this paper demonstrates the effectiveness of proposed system which can achieve the higher transmission data rate with keeping the higher signal quality even under the higher mobile ITS environments.

Evidence for severe microwave signal fading in the atmosphere of Venus due to turbulence

Abstract: Microwave signal fading in Venus atmosphere due to turbulence, discussing atmospheric dielectric permittivity variance

Transmission of signals containing harmonically related signals to overcome effects of fading

Abstract: To ensure that signals may be recovered after transmission through a fading medium, each pulse of a coded signal is represented by a carefully formulated collection of harmonically related components For example, if a sequence of regularly spaced pulses in which the repetition rate determines harmonic spacing is used to represent a binary ''''one,'''' the absence of pulses or a sequence of pulses with a different repetition rate then represents a ''''zero'''' A harmonic detector is used to recover the fundamental component of each group The fundamental frequency identifies the transmitted code signal Superior signal recovery despite fading, noise, or phase dispersion results

Intermodulation distortion in f.m. f.d.m. trunk-radio systems in 2-path fading situations

Abstract: The intermodulation-noise performance of fm fdm trunk-radio systems is examined in conditions of 2-path fading By using a convolution series expansion for the demodulated-baseband output corresponding to the CCIR noise-test-signal input modulation, it is possible to calculat both the noise/power ratio and the baseband enhancements to be expected in systems of 960 and 1800 audio-channel capacity Numerical results are presented representative of calculations made for a range of echo-delay times from 1 to 10ns, and relative echo amplitudes up to 099 (40dB maximum fade depth), showing the effect of intermodulation on the total system noise in this type of fading condition

Finding the statistical properties of the median values of a fading signal directly from its decibel values

Abstract: A method is introduced by which the statistical properties of the long-term fading of a signal can be found from a section of decibel-scale data. First the median of a subsection of signal fading in decibels is found directly from the average of its decibel values over each given time interval; then the long-term fading can be plotted. From such fading data, the statistical properties can be found directly and quickly from the measured level-crossing-rate (LCR) curve.

Change of the fading characteristics of reputed steady targets as a function of sea state in a marine-radar environment

Abstract: Analysis of recent radar-detection trials carried out on non-fading targets has revealed a transition in performance corresponding to that of a fading target, as the sea over which propagation took place became rough.

Probability-density function of a specularly fading signal-plus-noise minus noise

Abstract: A general expression for the probability-density function of a specularly fading signal-plus-noise minus noise has been obtained. Such an expression is frequently required for the analysis of different types of communication systems such as f.s.k. systems or pulse radar systems.

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.

Spectral Location of Rayleigh and "m" Fading Signals in White Gaussian Noise

Abstract: The classical problem is considered of locating a fading sinusoidal signal known to be present in one of several frequency " cells," each of which contains additive white Gaussian noise. The signal fading is assumed to follow the popular Rayleigh distribution, but generalizations tions to non-Rayleigh fading are included in terms of the " in" distribution due to Nakagami. The channel observation time is allowed to be either predetermined or variable (corresponding, respectively, to " fixed sample size" and " sequential" reception), and the practically important situation of intermittent signal transmissions is also examined. Results are in the form of optimal and near-optimal receiver structures, and of measures of performance.

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.

RAIN ATTENUATION STUDY FOR 15 GHz RELAY DESIGN.

Abstract: : A comparison was made between 10 GHz and 14.43 GHz microwave relay performance in northeastern Mississippi from February to May 1969. Interference fading caused by atmospheric multipath did not exceed 15 dB on either system, but as much as 48 dB attenuation was observed at 14.43 GHz during heavy rain. Rainfall rate on the 4.67 km (3 mi) test path was obtained from 20 tipping-bucket gages. A method of estimating rain attenuation for relay design and reliability problems is presented and general storm rainfall information is included. (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.

Bounded approximations for averaging multitrial detection probabilities over slowly fading targets (Corresp.)

Abstract: The averaging of target-detection probability over random target strength is more difficult when multiple-trial detection logic is used. This correspondence examines an approximation method sometimes used, and determines the theoretical bounds on the true average in terms of the estimated average. These are derived for a two-trial detection scheme and then generalized to N trials. An improved method of approximation is suggested and a specific example is given for a Swerling Type-I target.