Showing papers on "Fading published in 1978"

A spread-spectrum technique for high-capacity mobile communications

Abstract: A spread-spectrum technique for cellular high-capacity mobile communications is described and some results from an analytic study are summarized. The technique uses a very large set of frequency-hopped signals which are designed for minimal mutual interference. No synchronization of the mobile units is required, and each user is permanently assigned his own signal, which serves as an identifying signal and as a carrier for the biphase-modulated digital message. The spectral efficiency of the spread-spectrum system is analyzed and compared with the efficiencies of developmental FM/channel reuse schemes currently under construction in the U.S. and Japan. It is concluded that even with relatively simple speech digitization schemes, the efficiency of the spread-spectrum scheme may exceed those of the narrow-band schemes by a factor of almost five. More ambitious bit-rate-reducing speech digitization methods could improve still further on these figures. Additional benefits of the spread-spectrum scheme include immunity from fading and interference, more consistent speech quality, simpler system control algorithms, and more flexible blocking properties under overload conditions.

Tropospheric scatter radio communication systems

Abstract: A tropospheric scatter radio communication system uses a multiple signal diversity system in order to improve reliability of reception. A dual time diversity is used in which adjacent bits in a digital word are interleaved with bits from adjacent digital words so that more than one bit from a particular digital word is unlikly to be adversely affected by short term fading.

Error Probability Characteristics for CPSK Signal Through m-Distributed Fading Channel

Abstract: The average error probabilities and outage rates of error probability for the M -phase CPSK signal through the Nakagami's m -distributed fading channel are exactly evaluated both for nondiversity reception and for diversity reception. The probability density functions of the composite phase of fading signal and noise and those of the diversity-combined signal envelopes are newly derived in this paper. The results are generally obtained including the digital phase modulation component, the fading figure as a measure of fading depth, the average carrier-to-noise power ratio, the power correlation coefficient between two diversity branches, etc. The diversity improvements are also verified. Additionally some useful approximate formulas are briefly shown. This study will add a newly widened view to the considerations of system performances and designs for digital radio communications via fading channels.

Radio propagation in urban areas

Abstract: This paper reviews earlier work on radio propagation in urban areas, including data, studies of multi-path fading, and empirical propagation models A model is described, for use with a digital computer, which provides a rapid means of calculating both the median attenuation and the location, or path-to-path, variaability

First-order statistics of a non-Rayleigh fading signal and its detection

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.

Error rate predictions and measurements in the mobile radio data channel

Abstract: A method of predicting the loss in error performance in mobile radio data transmission is given in which the variation in local mean signal level (shadowing) is included as well as fading. Also, the density function of the received signal envelope is found for the case of fading and shadowing. Field measurements are reported of error rates at VHF and UHF due to vehicle ignition noise, and the distribution of errors is plotted.

A Multipath Fading Channel Model for Terrestrial Digital Radio Systems

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.

COMSTAR experiment: The 19- and 28-GHz receiving electronics for the crawford hill COMSTAR beacon propagation experiment

Abstract: This paper describes the receiving electronics built at the Bell Laboratories Crawford Hill facility at Holmdel, New Jersey to use the 19-and 28-GHz beacons on the COMSTAR satellites for propagation measurements. The receiving system accurately determines attenuation, differential phase, depolarization, bandwidth limitations and angular scatter of these signals produced by rain. This highly reliable system operates continuously and unattended; it automatically reacquires the beacon signals after dropout due to severe attenuation or momentary power outage. Correlations among strong and weak signal components are used to permit detection of weak cross-polarized signals during severe fading. Receiver noise bandwidths as low as 1.6 Hz are used. A high degree of phase stability is achieved in all circuits and components.

A Receiver with Memory for Fading Channels

Abstract: In this paper, an adaptive receiver with memory for fading communication channels is considered. The receiver consists of an estimator and a detector. The estimator is a finite-memory MMSE estimator with decision-feedback which minimizes the probability of error of the receiver. Asymptotic approximations are employed to derive an adaptive decision role based on the estimate. An example is presented where the receiver with memory is shown to perform better than an optimum receiver without memory.

Delay and Doppler power spectra of a fading ionospheric reflection channel

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.

The Fading Effect of Government on Inequality

Abstract: (1978). The Fading Effect of Government on Inequality. Challenge: Vol. 21, No. 3, pp. 32-37.

Small Submersible Acoustic Communications System Design

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.

A Technique for Measuring Fading Microwave Signals from the ATS-5 Spacecraft

Abstract: A description of a technique for measuring fading microwave signals from the NASA ATS-5 spacecraft is presented. A ground station was used to transmit to and receive from the ATS-5 via its L-band transponder. The experiment included an L-band transmitter, three independent L-band receiving systems, and a semiautomatic data acquisition and analysis system. Sample data are presented.

Processing circuit for irregular signal

Abstract: PURPOSE:To make constant the width of fluctuation of detected output, by changing the response time constant of the center value detection circuit according to the speed variation of irregular variation signal. CONSTITUTION:In a mobile radio, the mobile speed v and the fading pitch have a correlativity. The comparator 2 compares the input signal with the output of the integrator 3, i.e., the output of the center value detection circuit, and it outputs -E or +E depending on the magnitude. The variable attenuator 5 varies the attenuation inversely proportional to the control signal 9. The control signal gives a voltage in which the attenuation is greater with slower mobile speed v. Accordingly, since the gain is in proportional to fading even with the integrator 3 constant, the output variation width 11 can be constant.

On the estimation of noise covariances in linear discrete-time systems

Abstract: This paper is concerned with an approach for estimating or tracking the time-varying input and measurement noise covariances in time-varying discrete-time linear systems. The approach is firstly to introduce the estimators for the case where the noise co-variances are unknown constants. (The estimators are defined as the mean squares of the estimators of noises based on all the available measurement data.) They arc then transformed in sequential form, and are subsequently modified by incorporating a fading memory to yield estimates for time-varying noise covariances. The time-varying noise covariance estimates are evaluated as the fading mean squares of the estimates of noises based on all the measurement data up to present time. A numerical example for a simple system indicates acceptable performance of the proposed method.

Learning, Transfer, and Retention of Errorless Fading versus Trial-and-Error Teaching:

Abstract: Although several investigators have used prompting and fading techniques to teach tasks with few or no errors, there has been disagreement about subsequent transfer and retention as compared with trial-and-error learning. Fourth grade students in an errorless fading condition learned a symbol discrimination task by a prompting and fading program in which relevant characteristics of the line drawings were emphasized. Another group learned the same discrimination by trial-and-error with right-and-wrong feedback. Findings indicated that percentage of errors was less for errorless fading than trial-and-error in initial learning but did not differ during transfer or retention. However, in terms of time, a history of prompting-fading learning did not transfer to trial-and-error learning as well as one of trial-and-error learning.

Noncoherent Detection of FSK Signals in the Presence of Multipath Fading

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.

Flight Tests of Digital Data Transmission

Abstract: A series of flights were made to measure the characteristics of air-to-ground digital transmission in the VHF aeronautical mobile frequency band. Digital transmission rates of 2400 and 4800 bit/s were used with minimum shift keying (MSK) as the baseband modulation format. Signal level, signal fading, and radio horizon characteristics are described. Bit error rates and the causes of the errors also are discussed.

Mean power of the components of a Rice fading signal determined from measurements

Abstract: A method is provided for estimating the mean power of each of the two components of a Rice fading signal, given measurements of the mean and standard deviation of the total signal power.

A One-bit Memory Receiver for Channels with Memory

Abstract: A receiver is investigated with one-bit memory for communication channels with memory. The memory of the channel is characterized by the statistical correlation of the received bits due to fading. An optimal solution to the problem is considered but is found to be quite complex to implement. Therefore, a suboptimal receiver having a decision-feedback structure is examined. The performance of the receiver is shown to be better than a receiver without memory. Binary on-off keying system is considered as an example and numerical results are obtained.