Showing papers on "Fading published in 1983"

Detection of non-Gaussian signals by frequency domain Kurtosis estimation

Abstract: The detection of signals by power spectrum density (PSD) estimation is a well known and often employed method. The PSD estimate is essentially a second order measure which is not sensitive to the statistical nature of the signals. For non-Gaussian signals, a frequency domain Kurtosis (FDK) estimate supplements the PSD estimate and in some cases of practical importance is superior as a detection statistic. Non-Gaussian signals occur in underwater acoustics due to multipath and frequency modulation effects. Specifically, we have considered sinusoidal and narrowband Gaussian signals which, when propagated through fading or multipath environments, are received as non-Gaussian in terms of a frequency domain Kurtosis estimate. The environment is modeled by introducing amplitude probability density distributions which are due to the fading or multipath conditions. Several distributions were considered previously which included Rayleigh and log-normal, both of which have been experimentally verified to exist in the ocean. The asymptotic probability of detection for a randomly occurring signal is derived for the PSD and FDK. A simulation comparing the probability of detection for the FDK and PSD is also included.

Digital communications over fading radio channels

Abstract: A major contribution to system outage in a terrestrial digital radio channel is deep fading of the frequency transfer characteristic, which in addition to causing a precipitous drop in received signal-to-noise ratio (s/n) also causes signal dispersion that can result in severe intersymbol interference. Because the temporal variation of the channel is slow compared to the signaling rate, the information theoretic channel capacity and the “Efficiency Index” in bits/cycle — a figure-of-merit we use for the communication techniques considered — can be viewed as random processes. Starting from an established mathematical model characterizing fading channels (derived from extensive measurements), we estimate the probability distribution of channel capacity and the distributions of efficiency indices for different communications techniques. The repertoire of communication methods considered involves quadrature amplitude modulation with adaptive linear and decision feedback equalization, and maximum likelihood sequence estimation. For specific outage objectives the maximum number of bits per cycle achievable by each technique is estimated. The sensitivity of the distributions to bit-error-rate objective and unfaded s/n is assessed. For certain desired operating points the efficacy of adaptive equalization is demonstrated. There are some operating points where adaptive equalization alone is not adequate and therefore space diversity should be considered. An estimate of the effect of frequency diversity is also included.

Frequency-Selective Fading Effects in Digital Mobile Radio with Diversity Combining

Abstract: we analyze the effects of frequency-selective fading in a cellular mobile radio system that uses 1) phase-shift keying (PSK) with cosine rolloff pulses, and 2) space diversity with maximal-radio combining. The distorting phenomena with which we deal are multipath fading (which produces the frequency selectivity), shadow fading, and cochannel interference. The relevant quality measure is defined to be the bit error rate averaged over the multipath fading, denoted by (BER). The relevant system performance characteristic is defined to be the probability distribution for (BER), taken over the ensemble of shadow fadings and locations of the desired and interfering mobiles. To obtain numerical results, we use a combination of analysis and Monte Carlo simulation, invoke widely accepted models for the multipath and shadow fadings, and assume a cellular system with seven channel sets and centrally located base stations. The outcome is a set of performance curves that reveal the influences of various system and channel parameters. These include: the number of modulation levels (two or four), the diversity order, the shape of the multipath delay spectrum, and the standard deviation (or delay spread, τ 0 ) of the multipath delay spectrum. Practical factors accounted for in these assessments include fading- and interference-related timing recovery errors and combiner imperfections. Our results highlight the importance of the ratio \tau_{0}/T , where T is the digital symbol period. They show that the delay spectrum shape is of no importance for \tau_{0}/T \leq 0.2 , but can have a profound influence for \tau_{0}/T \geq 0.3 . We also find that using 4-PSK leads to better detection performance, in certain cases, than using 2-PSK.

Frequency-hopped single sideband mobile radio system

Abstract: The present invention relates to a frequency-hopped single sideband (SSB) mobile radio system implemented by hopping the carrier frequency of an input signal (s(t)) every τ seconds. The hopping is controlled by a carrier-frequency-hopped sequence (f i (t)) generated by a carrier-frequency-hopping generator (16,24). When employed in a frequency-hopped SSB transmitter (10), the carrier sequence functions to modulate the input signal, "hopping" it to a different carrier frequency every τ seconds. The carrier-frequency-hopped SSB receiver (20) employs the identical carrier sequence as used by the transmitter to demodulate the transmitted carrier-frequency-hopped SSB signal, thereby recovering the original single sideband signal (s(t)). By frequency hopping the carrier signal of an SSB signal, the present invention mitigates the effects of co-channel interference and frequency selective fading inherent in prior art SSB cellular mobile radio systems.

Switched diversity with feedback for DPSK mobile radio systems

Abstract: Switched diversity with feedback for differential phase shift keying (DPSK) mobile radio is discussed. The technique uses multiple transmit antennas at the base station but only one receive antenna at the mobile. The base station transmits with one antenna that is switched when the mobile informs the base station that the received signal has fallen below a fixed level. The implementation of switched diversity with feedback in a digital mobile radio system is first described, and then the bit error rate performance of the system is analyzed with fading as a function of several design parameters. Implementation of the system is shown to be relatively simple, yet the system is shown to reduce substantially the required received E b /N 0 for a given error rate at the mobile as compared to a system without diversity. For example, with five transmit antennas the required received E b /N 0 for a 10-3bit error rate is 13 dB less. The system capacity and availability assuming 32 kb/s audio and flat fading is then discussed. It is shown that with three-corner base station diversity and four transmit antennas at each base station, 126 two-way circuits per cell can be used in a fully loaded 40-MHz bandwidth system with a ten-percent probability that the error rate exceeds 10-3.

Reception of PSK Signals Over Fading Channels Via Quadrature Amplitude Estimation

Abstract: We consider here a new approach to PSK signal detection over a slow nonselective Rayleigh fading channel which does not require a carrier recovery loop. The receiver achieves coherent demodulation by making use of estimates of the quadrature amplitudes of the received PSK signals in its likelihood ratio test. The receiver is assumed to have a memory containing information on the past received signals which enables it to generate the estimates. The error rate of the receiver can be evaluated analytically and computer simulation results are presented to verify the predicted performance.

Combined source and channel coding for variable-bit-rate speech transmission

Abstract: Motivated by potential applications to mobile radio, we studied variable-bit-rate speech communication through Gaussian-noise and Rayleigh-fading channels. For convenience we used a constant signaling rate of 32 kb/s and adjusted the source-coding and channel-coding rates in response to changing transmission quality. When the channel quality was good enough, we used all 32 kb/s for speech transmission. When the channel quality was lower, we reduced the source rate to 24 or 16 kb/s and introduced channel coding to control distortion due to transmission errors. We concentrated on specific source and channel codes that could be implemented with hardware of modest complexity. The source code was embedded differential pulse code modulation, which is amenable to variable-bit-rate operation and economical to implement. For error control we introduced punctured convolutional codes and a Viterbi decoder with only 16 states. Although the source/channel codec was simple, it offered good performance. Speech quality was at the level of normal telephony when the channel was good; the error-correcting codes extended by up to 13 dB the range of channel signal-to-noise ratios that support adequate quality. Our performance estimates were based on a new analysis of transmission errors in embedded differential pulse code modulation and on computer simulations of speech transmission through fixed and fading channels.

Two-Dimensional DPCM Image Transmission Over Fading Channels

Abstract: A combined source-channel coding approach is described for the encoding, transmission, and remote reconstruction of image data. The transmission medium considered is that of a fading dispersive communications channel. Both the Rician fading and Rayleigh fading channel models are considered. The image source encoder employs two-dimensional (2-D) differential pulse code modulation (DPCM). This is a relatively efficient encoding scheme in the absence of channel errors. In the presence of fading, however, the performance degrades rapidly. By providing error control protection to those encoded bits which contribute most significantly to image reconstruction, it is possible to minimize this degradation without sacrificing transmission bandwidth. Several modulation techniques are employed in evaluation of system performance including noncoherent multiple frequency shift-keyed (MFSK) modulation. Analytical results are provided for assumed 2-D autoregressive image models, while simulation results are described for real-world images.

Characterization of multipath parameters for line-of-sight microwave propagation

Abstract: Ray equations are derived for microwave multipath propagation in a two-layer atmosphere, and the connection between the different solutions is demonstrated by numerical examples. Analytical expressions are developed to characterize the condition for multiple rays, the variation in angle-of-arrival (AOA), and the delay spread. The results suggest that more than three rays will be rare in practice and that the three-ray model (obtained from the zero-order rays defined in the paper) may provide a good representation of selective fading on microwave links.

A fading reduction method for maritime satellite communications

Abstract: A maritime satellite communication system has been established mainly for large vessels. In the near future it will be expected to service not only large vessels but also small ones. Studies on reducing the fading of the received signals have become very important because a compact antenna designed to be mounted on small vessels has a broad beamwidth which can be greatly influenced by reflected waves from the sea surface. A simple method and antenna equipment which can reduce the fading are proposed and, further, the effects by experiments are confirmed.

Optimum Reception in Non-Gaussian Electromagnetic Interference Environments: II. Optimum and Suboptimum Threshold Signal Detection in Class A and B Noise

Abstract: In this second part of an ongoing study, the general problem of optimum and suboptimum detection of threshold (ie weak) signals in highly nongaussian interference environments is further, developed from earlier work ([la],[lb];[34]) Both signal processing algorithms and performance measures are obtained canonically, and specifically when the electromagnetic interference environment (EMI) is either Class A or Class B noise Two types of results are derived: (1), canonical analytic threshold algorithms and performance measures, chiefly error probabilities and probabilities of detection; and (2), various typical numerical results which illustrate the quantitative character of performance Suboptimum systems are also treated, among them simple cross– and auto–correlators (which are optimum in gaussian interference), and clipper–correlators which employ hard limiters (and are consequently optimum in "Laplace noise") The various modes of reception considered explicitly here include:(i), coherent and incoherent reception; (ii), "composite" or mixed reception (when there is a nonvanishing coherent component in the received signal; (iii), "on–off" and binary, signals, as well as varieties of fading and doppler spread Both local optimality (LO) and asymptotic optimality (AO) are demonstrated, along with the critical influence of the proper bias in the optimum algorithms, which maintain their LO and AO character as sample size is increased, without having to add additional terms in the original threshold expansion (and thus produce insurmountable system complexity for the very large samples required for effective detection of weak signals) It is shown that for AO, as well as LO, two conditions may be needed to establish the largest magnitude of the minimum detectable input signal which can be permitted and still maintain the optimal character of the algorithm In addition to the more general Bayes risk and probabilistic measures of performance, Asymptotic Relative Efficiencies (ARE1s) are also included and their limitations discussed A number of numerical examples which illustrate the determination of performance and performance comparisons are provided, with an extensive set of Appendices containing many of the analytic details developed and presented here for future use, as well

A Simplified Method for Prediction of Multipath Fading Outage of Digital Radio

Abstract: We propose a simplified method based on the correlated Rayleigh probability distribution of in-band radio signals. It can be efficiently applied to the calculation of the outage for various radio path length, propagation terrain, radio frequency, bandwidth, and digital modulation techniques. Predicted results are compared to various propagation data.

Fading properties of a NaF:F+2* color center laser

Abstract: An investigation of the temporal stability of a color center laser based on F+2* centers in NaF is presented. It is found that the fading of the output power strongly depends on pump power, intracavity power, pump wavelength, and temperature. Optimum conditions for operation were thus derived to improve the temporal stability of this tunable laser.

Co-channel interference measurement system

Abstract: A co-channel interference measurement system for radio communications using angular modulation comprises an envelope detector (12) for detecting a desired signal (D) and an undesired signal (U) by envelope detection. A low-frequency spectrum component detector (23) detects a spectrum similar to that of a fading spectrum, the low frequency component relating to the sum of the desired signal and the undesired signal. A high-frequency spectrum component detector (27,28,29,30) detects a spectrum higher than a fading spectrum, the high frequency component relating to the product of the desired signal and the undesired signal. A divider (25) provides the ratio of the low frequency spectrum component to the high frequency spectrum component, and a level meter (26) indicates the value of the interference ratio (D/U) of the desired signal to the undesired signal according to the ratio of the low frequency spectrum component to the high frequency spectrum component.

The 'overflow' theory of ascites formation: a fading concept?

Abstract: (1983). The ‘Overflow’ Theory of Ascites Formation: A Fading Concept? Scandinavian Journal of Gastroenterology: Vol. 18, No. 7, pp. 833-837.

Maximum-power and amplitude-equalizing algorithms for phase control in space diversity combining

Abstract: Space diversity receivers equipped with continuous combiners have, in recent years, found wide use in terrestrial microwave radio systems as a means of mitigating the effects of multipath fading. An analysis of two phase-control algorithms for space diversity combining, comparing the performance capabilities of a maximum-power and an amplitude-equalizing algorithm, is presented in this paper. The extended capabilities of the equalizing algorithm in mitigating linear and quadratic channel distortions are investigated through computer simulations. A reduction in digital radio outage time afforded by the equalizing algorithm (relative to that of the maximum-power algorithm) is approximated for over-water-paths where it is assumed that fading can be described in terms of a two-ray fading model. For the model chosen, we show that the additional outage reduction owing to the equalizing phase-control algorithm is highly dependent on the statistics of the multipath delay parameter, τ, and can vary from unity to a factor of five for different probability-density functions of τ.

Fading on the S+, the S-, both, or neither.

Abstract: Mentally retarded adults were presented with size and number discriminations using four stimulus conditions. In Condition 1, the S+ circles faded from large (Size 10) to smaller (Size 6) while the S- circles remained at Size 5 and number of circles varied randomly. In Condition 2, the S- squares faded in number from 1 to 5 while the S+ squares remained at Number 6, and size varied randomly. In Condition 3, simultaneous fading of the S+ and S- triangles occurred on size for 6 subjects and on number for the other 6. In Condition 4, fading did not occur on the S+ or S- hexagons. Each of the fading procedures (Conditions 1, 2, and 3) produced fewer errors on all 80 trials than did the no-fading control condition but did not differ significantly among themselves. These same results generally occurred for fading and test trials. Although fading has been used successfully to teach a variety of skills, systematic analyses of fading procedures clearly delineating component stimulus operations have been neglected.

Characterization of the Underwater Channel Application to Communication

Abstract: This paper is a synthesis of the CEPHAG’s last years studies on the submarine acoustic channel characterization and its application to optimal communication. The methodology used for the characterization is recalled and some experimental results are given. More recently we have used some of these models to construct and value an optimal binary communication scheme. The more advanced results concern the fading multipath case and the frequency dispersive channel. Other results, concerning the estimation of data transmitted in a multipath channel, are also mentioned.

An experimental study of atmospheric optical transmission

Abstract: This paper reports measurements made on a 23-mile, experimental, atmospheric, optical-transmission link for possible use as a standby substitute for microwave radio when the radio suffers severe multipath or obstruction fading. To allow comparison of transmission on a microwave and on an optical path, we used two parallel systems. One, a microwave system at 11 GHz, allowed frequency-selective fading to be measured, and the other, an optical system at 6328A, allowed amplitude changes of the received optical signal to be obtained. The measured clear-air loss on the optical path is 27 dB. This measurement is made up of 17 dB of atmospheric scattering and 10 dB due to the receiving antennas intercepting only 10 percent of the beam at the receiver. The signal-to-noise ratio, calculated using measured background sky-noise and measured received power, is about 60 dB for a 100-MHz band. The beam diameter was measured to be 32 feet where the signal is down 20 dB. On the single occasion when frequency-selective microwave fading was observed, there was no fading of the optical signal. We find that it is necessary to control the transmitter elevation angle with a servo error signal from the receiver; the azimuth angle needs only occasional manual correction. The optical beam can be automatically reacquired after severe atmospheric attenuation, and that scintillation is usually several decibels, and occasionally as much as 10 dB.

Method of providing smooth transition from one to another of two sequences of digitally coded sound signals (fading)

Abstract: Sudden changes in loudness of the sound signal between successive television scenes are avoided by substituting faded transitions between the sound signals of successive scenes instead of simple abutment of sound signal sequences one behind the other. This is produced, in a system in which the sound signals are digitally encoded, by distributing digital sound signal samples individually or by pairs to two recording channels in alternation and providing the transitions from the sound signals of one scene to the sound signal of the next scene earlier in one channel than in the other, by about 10 milliseconds. During that time span, one signal is faded out and the other is faded in by digital filtering or multiplication. It is convenient to provide such a reciprocal fading operation for the sound signals between each picture field or at least between each picture frame. During the reciprocal fading intervals, the sound signal samples of each sequence may be filled in with interpolated sample values inserted at the same time that a sample value of the other sequence is provided in the other channel. The provision of continuous forming of interpolated values can be used for substitution of interpolated values for sample values of a signal that has dropped out or been subjected to errors by disturbances.

Performance of an FH Multilevel FSK for Mobile Radio in an Interference Environment

Abstract: This paper considers a frequency-hopped multilevel frequency shift keying (FH-MFSK) spread-spectrum communication system applied to cellular mobile radiotelephony. We present a mobile-to-base transmission model that allows us to study system impairments, such as interference from nonsynchronous users and adjacent frequency channels in the presence of matched tuned receiver filters. For interceli interference, the usual Gaussian approximation is used, but the variance calculation takes into account shadow fading. Power control in the mobiles, a mean path loss exponent of -3.5 and fast Rayleigh and slow Iognormal fading have been assumed. We have obtained results with mobile-to-base communication of 32 kbits/s per user in a 20 MHz (one-way) bandwidth. In an isolated cell system a bit error probability less than 10-3can be maintained with up to 110 simultaneous users for practical average SNR ratio of 25 dB. The presence of intercell interference degrades the bit error probability enormously, and clustering of cells is required for controlling interference.

The AR6A Single-Sideband Microwave Radio System: Equalization for multipath fading

Abstract: Multipath fading can introduce severe amplitude distortion and level changes in a radio channel. These must be dynamically equalized to meet the toll transmission requirements of AR6A. This article describes equalizer circuitry at intermediate frequencies, which continuously senses the level as a function of frequency in the transmission band and dynamically corrects the effects of multipath fading.

Digital radio outage prediction with space diversity

Abstract: The letter presents a new outage prediction technique for digital radio systems when baseband bit combining is employed. The method utilises the concept of a `normalised system signature?, an approach which enables comparisons of modulation schemes and equaliser implementations.

Detecting circuit for identical frequency interference

Abstract: PURPOSE:To an interference detecting signal corresponding to the electric power ratio of an interference wave to a desired wave under no influence of variation in reception level, by detecting the beat signal of the composite wave of the desired wave and interference wave. CONSTITUTION:In a receiver used for digital communication which employs an angle modulation system, a received signal applied to a terminal 10 is converted by a square-law detector 11 into a signal 17 showing momentary received electric power to obtain a signal showing the amplitude of the received signal, which is applied to an HPF12 and an LPF13. The HPF12 outputs a signal 18 showing momentary received electric power while the DC component of received electric power and the variation component due to fading are removed, and the LPF13 outputs a signal 19 showing mean received electric power which contains the variation component due to the fading to apply them to a divider 14 respectively. The output of the divider 14 is converted by an envelope detector 15 into an envelope voltage, so that an identical frequency interference detecting signal voltage corresponding to the electric power ratio of an interference wave to a desired wave is developed at a terminal 16.

Diversity Improvement in Frequency-Hopping Multilevel FSK Systems Under the Influence of Rayleigh Fading and Log-Normal Shadowing

Abstract: Recently, various frequency-hopping schemes have been proposed for possible application to digital mobile radio telephony. While the advantage of the inherent frequency diversity against fading is well known, the improvement in the system performance due to the use of space diversity has not been studied. In this paper we analyze the performance of FH-MFSK under the influence of Rayleigh fading and log-normal shadowing due to the combined use of frequency and space diversities. It is shown by analysis that the system capacity can be increased from the corresponding nondiversity figure by 26.3 percent relative to the full system capacity (i.e., 209 users) with the use of two diversity branches at channel condition of 10 dB normalized area mean and worst case of shadowing \sigma = 12 dB. Also, graphical results are given for different channel environments, and required signal-to-noise ratio.

Quality of service and bandwidth efficiency of cellular mobile radio with variable-bit-rate speech transmission

Abstract: We have investigated the performance of an adjustable source/channel codec in a cellular mobile-radio environment. The speech transmission rate and the amount of forward error correction change in response to changing channel conditions. The channel rate is constant at 32 kb/s and when the channel is good all of these bits are used for speech transmission. In intermediate and poor channels the speech rate is 24 or 16 kb/s and the remaining channel symbols are used for forward error correction. Relative to conventional transmission this approach offers an improved grade of service. For example, the outage rate (the proportion of "poor or worse" communications) goes from 9% with fixed-rate to 3% with variable-rate transmission. Alternatively, this improved grade of service can be exchanged for higher bandwidth efficiency. The fixed-rate system (with 9% outage) has 23 users per cell. With 52 users per cell the outage of the variable-rate system is only 6%.

Anomalous Union Bound Behavior for MFSK Signaling on Inverse Linear Channels

Abstract: Application of the union bound to MFSK signaling on certain channels where probability of bit error varies in an inverse linear fashion with E_{b}/N_{0} can lead to the erroneous conclusion that performance degrades with increasing alphabet size. The anomaly is shown for the worst-case partial band Gaussian interference channel and the Rayleigh fading channel.