Showing papers on "Fading published in 1974"

Adaptive Equalization of the Slow Fading Channel

Abstract: This paper considers equalization of the slow fading channel for a serial data transmission application. Linear and decision-feedback adaptive equalization techniques are contrasted. The error propagation effect in decision-feedback equalizers is analyzed by a Markov process model. The error probability magnification is computed for both fixed and fading channels and for both binary and quaternary phase-shift-keying (PSK) transmission. The results show that the error propagation effect is small and in regions of practical error probabilities the decision-feedback equalizer is superior to its linear counterpart. Parameters of a practical decisionfeedback equalizer are estimated and a performance evaluation is performed. The implicit diversity gain is shown to be significant and the intersymbol interference penalty is found to be less than 1 dB. Because the intersymbol interference penalty is small, more complex nonlinear processors such as the Viterbi algorithm cannot be recommended for this application. Time jitter effects for the equalizer are included in a calculation of average error probability.

Digital communications system with immunity to frequency selective fading

Abstract: A digital communications system with increased resistance to frequency selective fading. Digital signals to be transmitted are phase shift modulated upon a spectrum spreaded carrier. The carrier is generated by mixing the output of a band spread oscillator with a locally generated IF oscillator signal. The signal thus obtained is divided into transmission periods, each of which is longer than the maximum spreading time of the nonlinear transmission medium. The increased bandwidth occupancy thus obtained increases the immunity of the transmitted signals to frequency selective fades moving across the allocated bandwidth. The signals are received by a differentially adaptive receiver which automatically adjusts its response to the envelope of received signals.

Error Performance for Adaptive Transmission on Fading Channels

Abstract: A communication system is analyzed in which the transmission power and/or rate are adjusted to the observed state (attenuation) of a slowly fading channel. The optimum control of power and rate with channel attenuation is determined. Some suboptimum control rules are also investigated. The average probability of error is evaluated for various types of adaptive transmission, imposing different constraints on power, rate, and bandwidth. It is shown that rate control is superior to power control and that simultaneous power and rate control further improves the performance. The most effective power and rate control presupposes a high peakpower-average-power ratio and a high peak-rate-peak-rate-average-rate ratio; with no peak power and bandwidth limitation, arbitrary small error probability is attainable for a given power and average rate.

On the Relationship Between Scintillation Index and Rician Fading

Abstract: The literature dealing with communication theory contains numerous analyses of the effect of fading on communications signals, while the literature dealing with ionospheric physics contains numerous models and measurements of ionosphericinduced scintillation. This correspondence relates the two; thus, the literature of each field can be applied to the other.

Importance of Amplitude Scintillations in Millimetric Radio Links

Abstract: Amplitude scintillations in millimetric radio propagation are not usually considered by systems engineers In this paper, the experimental measurements of these scintillations in a 36 GHz radio link have been fitted into a model suitable for the millimetric range. From it, the amplitude scintillations can be regarded as a noise component superimposed on the mean value, with some expected mean rate fading fo. The r.m.s. value of this noise is then used to determine the outages for the scintillation-type fading, as a function of the frequency and link distance. The implications of the results in the power budget are discussed and reference is made to the limiting performance of digital transmission.

Feedback Communications over Fading Channels

Abstract: A communication system using noiseless feedback for Rayleigh fading channels is proposed. Pilot tone signaling is used to provide channel amplitude and phase information for the detection of antipodal signals. Channel estimates generated in the receiver are fed back to control the transmitter power and rate. Power division and rate control rules are derived, and it is shown that definite performance improvement over diversity systems is obtained. Because of the requirement of small loop propagation delays, the technique is mainly applicable to low data rate multitone modems for short range HF channels.

Propagation characteristics on line-of-sight over-sea paths in Japan

Abstract: The results of line-of-sight over-sea propagation measurements over 37.2- and 35.5-km paths near Tokyo, Japan, are presented. Measurements of the fading (0.25, 3.99, 4.15, 6.72, and 18.0 GHz) of the cross polarization distortion (linear polarized wave at 4.15 GHz and circular polarized wave at 6.72 GHz) of the space diversity effect at 4.15 GHz, of the height-gain patterns at 6.72 GHz, and of the frequency-sweep patterns from 18 GHz to 22 GHz are included. The following statistical results are presented: the frequency, seasonal, and path dependence of fading; the effect of different fading occurrence mechanisms, such as K-type, duct-type, and distorted raindrops, for the cross polarization distortion; the diversity improvement factor; the effective reflection coefficient of sea surface; and the path-length difference between direct and sea reflected waves.

Computation of Bit-Error Probability Using the Trapezoidal Integration Rule

Abstract: It is shown in this paper that bit-error probabilities for some typical digital communication systems can be accurately and efficiently evaluated using the elementary trapezoidal integration rule. Examples involving intersymbol interference and a Rayleigh fading channel are worked.

The Theory of Operation of an Equal-Gain Predetection Regenerative Diversity Combiner with Rayleigh Fading Channels

Abstract: The theory of operation of an equal-gain predetection diversity combiner employing the Granlund technique of utilizing a combination of feedback and feedforward is presented. It is shown that the time delays of the filters and the loop phase shifts uniquely determine the operating frequencies within the system. The random FM at the discriminator output is shown to be a weighted sum of that on the input signals, and an upper limit for additional random FM caused by phase-shift error between the loops is derived. At the combiner output, the noise is found to be the sum of the uncorrelated noise from each input channel while the carrier amplitude of the diversity signal is the sum of the carrier amplitudes of the input signals after being cophased.

Negatively Correlated Branches in Frequency Diversity Systems to Overcome Multipath Fading

Abstract: In multipath environments in which there is a compact concentration of the values of time-delay-difference (TDD) between signals arriving over each of the indirect paths and the direct path, it is possible, by suitably choosing frequency differences, to design a frequency diversity system in which the diversity branches are negatively correlated. This results in a reduced probability of simultaneous deep fades on all branches over that obtained with branches designed to be independent. In the first part of the paper the magnitude of the negative correlation is shown to depend on the number of indirect reflectors contributing to the total indirect signal and that it is maximum when the indirect signal arises from a single reflector. Also shown is how the magnitude of the negative correlation can be maximized using the probability density function of the TDD to establish the frequency assignment. This is a precursor to the second part of the paper in which an investigation of the improvement of the error probability by a judicious choice of frequency differences is reported. The error probability as a function of frequency differences is found for the specific case of noncoherent frequency-shift keying (FSK) in a specular multipath channel using square-law combining. A general result for M th order diversity is obtained but a detailed study is made for two and three order diversity only. Results are presented for uniformly distributed TDD's with various ranges and mean values, the parameters being suggested by the physical conditions encountered when low flying aircraft are the multipath source or are themselves trying to communicate. Results for other densities of TDD, the normal and the gamma, are cited. For the second order case, it is found that an advantageous frequency spacing can be determined in terms of the average TDD. For the third order case, two frequency differences are available as design parameters. Results are presented based on a rule for selecting frequency differences similar to that used in the second order case and substantial improvements are found. Another rule investigated that gave even better results is cited. Finally, error probability comparisons are made with systems having no multipath, nondiversity systems with multipath, and with diversity systems in multipath with independent branches. For the conditions assumed, and for reasonable levels of transmitted signal power and receiver noise, order of magnitude improvements are found to be obtained over independent branches.

The fading of stabilized images: Eye movements and information processing

Abstract: Two experiments are reported which indicate that neural and photochemical satiation processes are inadequate to explain the fading of the stabilized retinal image. When eye movements are attempted over the stimulus, fading occurs more rapidly, indicating an information processing component in the disappearance of the image.

Performance of Digital FM Systems in a Fading Environment

Abstract: This paper investigates the effects of fading and predetection diversity on the error-rate performance of digital FM discriminator systems. Results are presented in graphs showing the relative improvement of the error rate for several types of diversity, and are compared with the well-known FSK systems. It is also shown that an optimum modulation index giving the minimum attainable probability of error for narrowband digital FM is not affected by the effect of fading, carrier-to-noise ratio, or the types and order of diversity.

Combined Effects of Up- and Down-Link Fading Through a Power-Limiting Satellite Repeater

Abstract: Evaluating the capacity of a satellite communication system requires accurate prediction of the combined effects of up and down-path fading. This correspondence derives an expression for the resultant channel fade probability distribution as a function of up- and down-path fade distributions and the satellite TW input-output (I/O) power transfer characteristic, and demonstrate that power limiting in the satellite has a substantial effect.

Models of multipath propagation effects in a ground-to-air surveillance system

Abstract: : Models are developed for the electromagnetic propagation of diffuse and specular multipath for a surface-to-air ATC radar beacon link. The models take into account geometrical, topographical, electrical, and surface characteristics of the multipath propagation environment. The models are used to evaluate signal fading and azimuth estimation performance of a monopulse processor in a ground-based interrogator. The results are applied to assessing the performance of the DABS sensor. Conclusions and recommendations for experimental work are presented.

On Finite-Lag Receivers for Fading Channels

Abstract: This paper reports some results on the development of finite-lag receivers for reception via fading channels. The receivers are given in discrete form and clearly show the additional complexity due to introduction of delay between the instants of reception and estimation of the signal. The channel noise has been assumed to have a multiplicative component due to fading. This results in the optimal demodulators being nonlinear and unrealizable. However, the receivers proposed are suboptimal, but realizable. Simulation studies evaluating the performance of the various receivers are also discussed.

Long‐period fading of a LF radio signal in relation to geomagnetic activity

Abstract: In an earlier work it was shown that the integrated field intensity of VLF atmospherics exhibits long-period fading on certain nights and that the period of fading depends on international magnetic character figure C, being maximum around C=1.1 [Sen, 1967]. We were prompted to examine our nighttime records of the field strength of a radio navigational signal at 280 kHz (call sign CB) transmitted from China Bay in Ceylon (9° 11′N, 81°11′E) at a distance of about 1800 km from Calcutta (22°34′N, 88°24′E; geomagnetic latitude, 11 °54′N) to investigate the long-period fading. It was revealed that the signal strength shows long-period fading on thegreat majority of magnetically disturbed nights and that the period of fading appears to depend on the value of the geomagnetic activity indices. Some of the interesting results obtained from an analysis of the records during the 1-year period from June 1972 to May 1973 are presented in this communication.

Convolutional code performance in fading channels

Abstract: The performance of convolutional codes in fading channels typical of the planetary entry channel is examined in detail Short constraint length codes are considered in conjunction with binary phase-shift-keyed (BPSK) modulation and Viterbi maximum likelihood decoding while for longer constraint length codes we consider sequential decoding utilizing both the Fano and Zigangirov-Jelinek (ZJ) algorithms For short constraint length codes we are primarily interested in the bit error probability performance parameterized by the fading channel parameters For longer constraint length codes interest will center on the effect of the fading channel parameters on the computational requirements of both the Fano and ZJ algorithms In either case the effects of simple block interleaving in combatting the memory of the channel is thoroughly explored The approach is analytic where possible otherwise resort is made to digital computer simulation

Coherence of harmonically related CW signals

Abstract: Results of measurement of the coherence of harmonically related CW signals at long ranges are presented, together with a theoretical analysis. In a nondispersive medium, fluctuation in the relative phase angle caused by linear frequency‐dependent parameters are relatively low for harmonically related signals. In an experiment conducted with a moving source 500 NM away from the receiver, the relative phase angle stayed unchanged or varied quite slowly in most cases, except around a fading zone. The theoretical analysis of the effect on the phase angle relationship of receiving array configurations, multipath interference, and signal‐to‐noise conditions is given to explain the observed phenomena. Based on this study, a method of processing the received signal is described which permits the determination of the relative phase angle of two harmonically related CW signals propagating at a distance from the source.

Some Observations of Satellite/Aircraft Multipath Properties at 1600 MHz

Abstract: In reducing the data from a recent series of high altitude balloon to aircraft measurements at L band, some qualitative results were reported concerning the negative correlation between the vertical and horizontal polarizations. Some of these data are presented in strip chart form to display this effect. In addition, a plot of the sum of the squares of the two polarizations is also presented to indicate the level of the received power and the possible benefits of power combining. Finally, the joint envelope density is examined in the 1imit of deep fading and the power fades are compared with those for independent combining.

Dispersive motions in the equatorial ionosphere

Abstract: The results of the dispersion analysis of the spaced receiver records of fading for an equatorial station in the Indian zone are discussed. It is shown that the apparent drift speed increases almost linearly with the frequency of fading in all the cases studied for both the E and F regions. The dispersion of drift speed is larger for the F region than for the E region in general. Furthermore, the F region shows a larger dispersion during the daytime whereas it is of the same order in the day and nighttimes for the E region. The drift speed corresponding to the highest Fourier frequency present in a particular record is found to be close to the apparent drift speed Va as determined from the shift of the peak of cross correllograms while the total frequency spread of the speed is linearly related to the random drift speed Vc derived by the full correlation analysis. Several possible causes of dispersion in the ionosphere such as gravity waves, independently moving reflecting screens, height gradient of the horizontal drift and size of irregularities, the dispersive nature of the vertical drift velocity, and the variability of the drift during the course of record are discussed. Interference of the signals reflected from two or more highly aspect-sensitive areas in the ionosphere is suggested as a possible mechanism resulting in the positive and linear dispersion observed on the ground at an equatorial station.

Performance of convolutional codes on fading channels typical of planetary entry missions

Abstract: The performance of convolutional codes in fading channels typical of the planetary entry channel is examined in detail. The signal fading is due primarily to turbulent atmospheric scattering of the RF signal transmitted from an entry probe through a planetary atmosphere. Short constraint length convolutional codes are considered in conjunction with binary phase-shift keyed modulation and Viterbi maximum likelihood decoding, and for longer constraint length codes sequential decoding utilizing both the Fano and Zigangirov-Jelinek (ZJ) algorithms are considered. Careful consideration is given to the modeling of the channel in terms of a few meaningful parameters which can be correlated closely with theoretical propagation studies. For short constraint length codes the bit error probability performance was investigated as a function of E sub b/N sub o parameterized by the fading channel parameters. For longer constraint length codes the effect was examined of the fading channel parameters on the computational requirements of both the Fano and ZJ algorithms. The effects of simple block interleaving in combatting the memory of the channel is explored, using the analytic approach or digital computer simulation.

Simultaneous observation of fading rates on two transequatorial radio paths

Abstract: Fading rates of transequatorial h.f. signals are evaluated from observations on the paths LindauRoma and Tsumeb-Lindau which cross the equatorial zone of Africa. The fading characteristics on both paths are shown to be correlated. A comparison with transequatorial pulse transmission, which indicates off-great-circIe propagation due to spread-F irregularities, gives evidence for a high correlation between the fading of c.w. signals propagating on transequatorial paths and the occurrence of off-great-circIe propagation due to equatorial spread-F irregularities.

Channel simulation for optical communication systems.

Abstract: A technique is reported for simulating the signal fading that will be experienced by typical optical communication systems. The desired irradiance or amplitude fading statistics can be simulated by incorporating a linearized optical modulator subsystem between the transmitter and receiver. This technique has been implemented in the design and construction of a laboratory channel simulator. The design of the processing electronics is discussed along with the results of tests performed for each mode of operation.

Transfer of training after fading and standard discrimination training

Abstract: Summary.-Standard discrimination and fading techniques were utilized to teach preschool children a two-choice discrimination task in which their nondominant dimension (color or form) was relevant for solution. The fading group performed better than the standard discriminacion group on both an initial and a later generalization-discrimination task. On the generalization task, the superiority of the fading technique was limited to form-dominant Ss. When retested for dimensional dominance after 1 wk. fading significantly decreased Ss' choices of their previously dominant dimensions while the standard discrimination trained Ss showed no change in preference. Learning rate on discrimination tasks has been shown to increase as a function of the number and saliency of the relevant stimulus dimensions and decrease with the number and saliency of the irrelevant dimensions (Trabasso & Bower, 1968). Ultimately, the probability of solution hinges upon S's attending to the relevant dimensions, and his attention to these dimensions is in turn a function of their saliency relative to that of the irrelevant dimensions. Dimensional dominance or preference may be viewed as an example of dimensional saliency. Dimensional preferences influence the accuracy and probability of solution in a wide variety of discrimination-learning and concept-attainment tasks (Caron, 1969; Dahlem & McLaughlin, 1969; Suchman, 1966; Wolff, 1966). If dimensional dominance is an example of dimensional saliency, it should be possible to improve performance on S's nonpreferred dimension by reducing the saliency of his preferred dimension and/or by increasing the saliency of his nonpreferred dimension. Once S has been trained to attend to his originally nonpreferred dimension, increasing the physical saliency of cues of his originally dominant and conflicting dimension then should not produce a decrement in performance. This training procedure is a special application of the errorless or fading discrimination procedure. Essentially, it involves converting gradually a simple discriminacion problem into a more difficult problem by "fading in" cues associated with the difficult problem. Fading discrimination procedures have been used to teach form, position, and right-left concepts to normal and retarded children (Bijou, 1963; Moore & Goldiamond, 1964; Stoddud LQ Sidman, 1967). Typically, fewer errors and more solutions are produced by fading procedures than by standard discrimination learning procedure. Adopting the Zeaman and House ( 1963) attentional model as a basis it may be suggested that the fading technique serves as a more efficient teaching pro=Now at The Devereaux Foundadon, Devon, Pennsylvania.

Performance of Digital FMSystems ina Fading Environment

Abstract: Thispaper investigates the effects of fadingand predetection diversity on theerror-rate performance ofdigital FM discriminator systems.Results are presented in graphsshowingthe relative improvement oftheerrorrateforseveral typesofdiversity, andare comparedwiththewell-known FSK systems. Itisalsoshownthat an optimummodulationindexgiving the minimum attainable probability oferrorfornarrowbanddigital FM isnotaffectedby theeffect offading, carrier-to-noise ratio, orthetypesandorderof diversity. Theuseofpredetection diversity combining iswell knownasa technique directed tocombattheeffect of fading onthereceived signals; byeither adding coherently orselecting thelargest ofsignals fromall diversity branches, theoutput signal-to-noise ratio (SNR)isgreatly enhanced [I].Manypapers intheliterature havebeendevoted tothe study oftheeffect ofpredetection diversity techniques on analog FM demodulation [2],[3], butcomparatively few totheeffect onfading digital FM.Inarecent paper, Nelson andSchilling [4]havestudied theeffects ofswitching-type predetection diversity witha truncated Rayleigh distribution andequal-gain diversities forfading digital FM. However, theeffects ofotherimportant predetection diversity techniques, suchasselection combining (SC), maximal-ratio combining (MRC),andequal-gain combining (EGC), forN-order diversity, werenotconsidered. Moveover, there arenotsufficient papers oncomparison ofthe error performance ofdigital FM discriminator systems and other binary FSKsystems (noncoherent orcoherent) ina fading environment andalsoonperformance degradation duetofading. Inthis paper, wetherefore consider theeffects offading andpredetection diversity ontheerror-rate performance of digital FM discriminator systems. Thediversity techniques considered areSC,MRC,andEFC.Their effect ontheerror ratesfora binary digital FM systemundergoing nonselective fading ispresented inmoregeneral form, anda comparative discussion isgiven through numerical results whichshowstheerrorrates asa function ofmean carrier-to-noise ratio (CNR), modulation index, andorder ofdiversity. A comparison ofperformance withnoncoherent andcoherent (ormatched filter) FSKdetectors operating inbothfading andnonfading environments is

A few results of three‐dimensional statistical analysis of ionospheric drift records for an equatorial station

Abstract: Fedor [1967] proposed a general statistical method of analysis based on the concepts of Briggs et al. [1950] and Banerji [1960]. According to this technique, the best-fitting values of the average steady drift (horizontal and vertical), of the random drift, and of the anisotropy parameters and the orientation of the characteristic ellipsoid of the ground diffraction pattern are obtained from an adequate combination of cross correlograms among spaced receiver and spaced frequency fading records. The method offers three advantages over the previous treatments of this problem in that (1) it provides a best fit to the available data in accordance with the principle of least squares, (2) it places no upper limit on the quantity of data that may be fitted, and (3) the method permits a complete description of the average three-dimensional structure of the measured quantity. It has been shown by Fedor that the horizontal drift velocity components are not given correctly by the two-dimensional methods of Briggs et al. [1950] if the vertical movements of anisometric irregularities occur.