Showing papers on "Fading published in 1975"
TL;DR: This simulation of the Rayleigh distributed fast fading encountered in mobile radio is presented and should be of interest to all those whose studies involve parameters of a mobile system that interact strongly with the radio environment.
Abstract: A brief description is presented of a computer simulation of the Rayleigh distributed fast fading encountered in mobile radio. This simulation should be of interest to all those whose studies involve parameters of a mobile system that interact strongly with the radio environment.
242 citations
01 Jan 1975
77 citations
TL;DR: This paper presents a survey of diversity techniques as applied to mobile radio, with emphasis on systems designed for analogue communication, and includes a unified approach to systems based on the double-heterodyne phase-stripper principle.
Abstract: After discussing the nature of the electromagnetic field in urban environments, and the use of diversity techniques in fading media, the paper presents a survey of these techniques as applied to mobile radio, with emphasis on systems designed for analogue communication. It includes a unified approach to systems based on the double-heterodyne phase-stripper principle. The discussion is not restricted to present-day receivers only, but includes considerations which may apply in future designs.
58 citations
TL;DR: In this paper, an experiment was performed to determine the fading characteristics of backscattered radar signals from four agricultural targets at 9 GHz, which included two different row crops (corn and soybeans), a continuous canopy (alfalfa) and bare ground.
Abstract: An experiment was performed to determine the fading characteristics of backscattered radar signals from four agricultural targets at 9 GHz. The targets included two different row crops (corn and soybeans), a continuous canopy (alfalfa) and bare ground. After a short review of the statistics of Rayleigh fading backscatter, the data processing method and the results of the experiment are analyzed. Comparison with theory shows adequate agreement with the experimental results provided crop type, soil moisture condition and incidence angle are correctly incorporated in the target model. Because recent studies indicate that a high degree of precision is required for mapping vegetotion types with radar, it is necessary to acquire as much a priori knowledge of target fading characteristics as possible.
47 citations
TL;DR: In this paper, the two-frequency two-position mutual coherence functions are derived under the usual parabolic and Markov approximations and then solved numerically, and it is shown that the mutual coherency functions occur naturally in the study of pulse distortion through a random communication channel and in the investigation of signal correlations.
Abstract: Equations for the two-frequency two-position mutual coherence functions are derived under the usual parabolic and Markov approximations. These equations are then solved numerically. It is shown that the mutual coherence functions occur naturally in the study of pulse distortion through a random communication channel and in the investigation of signal correlations. Contour plots of correlation functions show the possibility of having equal values at two frequency separations for a given spatial separation. This behavior is explainable in terms of overlapping Fresnel zones. Recently it has been observed that radio signals from communication satellites at a frequency as high as several GHz may experience the scintillation phenomenon when received on the ground [Pope and Fritz, 1971; Skinner et al., 1971; Sessions, 1972; _ Craft and Westerlund, 1972; Taur, 1973]. This came as a surprise since scintillation was not anticipated to occur at such a high frequency. After some experimentation it is now believed that the scintillation is caused by the electron density irregularities in the ionosphere. As is well known, for a given electron density fluctuation, th e rms fluctuation in the refractive index of the ionospheric medium is inversely proportional to the square of the signal frequency. Therefore, under conditions when GHz scintillation occurs, signals with a lower frequency will experience even more severe fading. This implies that the effects of multiple scattering on signal statistics are very important under such conditions. To obtain their statistics correctly, it is desirable to develop a scintillation theory that takes into account the multiple scattering effects. Thanks to the recent advances in the theory of wave propagation in random media, it is now possible to develop such a theory [Liu et al., 1974a; Yeh et al., 1975]. In this paper, we shall apply a similar technique to investigate the frequency and spatial correlations of signals passing through the irregularity slab. In satellite communications the ability to transmit wideband data is limited primarily by the inter-symbol interference which is closely
25 citations
Patent•
06 Oct 1975TL;DR: In this paper, an adaptive equalizer is proposed to compensate for the effects of frequency-selective fading on a microwave radio system, which includes two Bode-type bump networks and an AGC amplifier whose slope and gain are respectively controlled by pilot tones added to the message load.
Abstract: An adaptive equalizer to compensate for the effects of frequency-selective fading on a microwave radio system. The equalizer includes two Bode-type bump networks and an AGC amplifier whose slope and gain are respectively controlled by pilot tones added to the message load. Importantly, the frequency characteristics of the two Bode networks are non-overlapping, thus ensuring stability.
20 citations
TL;DR: Differential phase-shift keying performance analysis in the presence of multipath is extended to include cases in which the Doppler spread is on the order of the bit rate or greater, with results approaching that of ideal DPSK as expected.
Abstract: Differential phase-shift keying (DPSK) performance analysis in the presence of multipath is extended to include cases in which the Doppler spread is on the order of the bit rate or greater. As the Doppler spread is increased from zero, the performance degrades below that predicted by Jones [1]. However, as the Doppler spread is increased beyond values on the order of the bit rate, performance improves. In the limit as the Doppler spread approaches infinity, with total multipath power held constant, performance approaches that of ideal DPSK as expected. Bit error rate performance in the presence of time-selective Rician fading is presented along with confirming experimental data obtained with a channel simulator.
19 citations
TL;DR: This paper presents a simple formula for the capacity of a coherent digital communication channel operating near threshold with additive noise from a certain class of arbitrary but known probability distributions and the optimun receiver is derived for arbitrary modulations and fading.
Abstract: This paper presents a simple formula for the capacity of a coherent digital communication channel operating near threshold [i.e., with very low signal-to-noise ratio (SNR)] with additive noise from a certain class of arbitrary but known probability distributions. For the noncoherent threshold channel, the optimun receiver is derived for arbitrary modulations and fading. This receiver is shown to specialize to previously derived canonical froms for unmodulated signals.
13 citations
TL;DR: The signal-to-noise (S/N) ratio of digital signals can be improved by using diversity transmission, followed by combining at the receive end, resulting in a lower probability of error P(e) .
Abstract: The signal-to-noise (S/N) ratio of digital signals can be improved by using diversity transmission, followed by combining at the receive end, resulting in a lower probability of error P(e) . Second-order combining schemes for data signals are presented in this paper. The effect of delay difference in the two channels of a diversity system is analyzed, using four-phase coherent, phase-shift keying (PSK) as an example. For comparison with predicted performance, experimental results are included.
7 citations
TL;DR: In this paper, a study of the kinetics of fading for a series of model disperse dyes and direct dyes on cellulose is described, and the results confirm the importance of the filtereffect in determining the rates of fading and a rate equation which allows for the filter effect is given.
Abstract: A study of the kinetics of fading for a series of model disperse dyes on nylon 6.6 and direct dyes on cellulose is described. The results confirm the importance of the filtereffect in determining the rates of fading, and a rate equation which allows for the filter effect is given. It is shown that this equation holds for the change in the rate of fading with concentration of several direct dyes on cellulose.
01 Jun 1975
TL;DR: In this article, a 2.65 km path has been observed for one year over a 1.5 km path and the distribution with time of the depth of fading, duration of fading and the time between fades and information on the weather situations producing rain which gives high attenuation.
Abstract: Observations for one year over a 2.65 km path have yielded distributions with time of the depth of fading, duration of fading and the time between fades, and information on the weather situations producing rain which gives high attenuation. Observed attenuation has also been related to rainfall rate at a point.
05 Aug 1975
TL;DR: In this paper, the effects of ionospheric UHF scintillation on satellite communications are described in terms of relatively simple fading channel models and two methods of countering the effect of SCI induced fading on satellite communication are presented and analyzed.
Abstract: : The effects of ionospheric UHF scintillation on satellite communications are described in terms of relatively simple fading channel models. Two methods of countering the effects of scintillation induced fading on satellite communications are presented and analyzed. First, threshold testing techniques which effectively erase faded data are discussed. These techniques are most easily applied to add scintillation protection to an operating communication system which handles segments or blocks of data. Second, time diversity modulation techniques which can counter scintillation and other channel problems such RFI and excess noise in the same operation are discussed. These time diversity modultaion techniques offer somewhat better performance than the threshold techniques at the cost of a somewhat more complex interface between the basic receiver and the anti-scintillation processor.
TL;DR: In this article, the problem of discrete-time demodulation of angle-modulated analog signals transmitted over fading channels, with emphasis on Rayleigh and Rician channels, was addressed.
01 Jan 1975
TL;DR: The ``fading memory'' filter of Sorenson and the ``finite memory''filter of Jazwinski are rigorously tied together and the resultant linear filter is shown to be sequential embodying the salient advantages of the fading and finite policies.
Abstract: The ``fading memory'' filter of Sorenson and the ``finite memory'' filter of Jazwinski are rigorously tied together. The resultant linear filter is shown to be sequential embodying the salient advantages of the fading and finite policies.
TL;DR: It is found that the advantages of the technique in the pulsed/digital system are similar to those obtained in the cw/analog system.
Abstract: Part 1 of this paper [Appl. Opt. 14, 666 (1975)] dealt with the cw radar and analog communications uses of three-frequency nonlinear heterodyne detection. In this paper, we evaluate the technique for a number of specific pulsed radar and digital communications applications. Both the vacuum channel and the lognormal turbulent atmospheric channel are considered. It is found that the advantages of the technique in the pulsed/digital system are similar to those obtained in the cw/analog system. Computer generated error probability curves as a function of the input signal-to-noise ratio are presented for a variety of binary receiver parameters and configurations and for various levels of atmospheric turbulence. Orthogonal and nonorthogonal signaling schemes, as well as dependent and independent fading, are considered. When Doppler information is poor, performance is generally superior to that of the conventional heterodyne system.
TL;DR: In this article, the theory of an effective means for combining the signals from a twin antennae space diversity arrangement using phase sweeping in opposite directions for the two antennae is given.
Abstract: Fading due to multi-path propagation is a serious problem in mobile radio communications. Consideration is given to the theory of an effective means for combining the signals from a twin antennae space diversity arrangement using phase sweeping in opposite directions for the two antennae. An analysis is presented for various modulation methods. The resulting system should be compatible with existing receivers. Some experimental results for acommercial v.h.f. a.m. system are included.
TL;DR: In this paper, a new technique is proposed to improve the performance of a troposcatter system, where the transmitting or receiving antenna, or both, are continuously scanning the space to find the scattering volume that produces the optimal signal strength.
Abstract: This is a new technique proposed to improve the performance of a troposcatter system. In this technique, the transmitting or receiving antenna, or both, are continuously scanning the space to find the scattering volume that produces the optimal signal strength. The overall effect is to reduce both short-term and long-term fadings. It is equivalent to a combination of using selection angle diversity for short-term fast fading and increasing system margin for long-term slow fading, by picking up the best signal. An extensive experimental study is suggested.
01 Nov 1975
Patent•
24 Apr 1975
TL;DR: In this article, the signals are applied to the two inputs over two paths, one with a linear, the other with a nonlinear characteristic, the fading circuit output delivers the corrected t.v. signal.
Abstract: The signals are applied to the two inputs over two paths, one with a linear, the other with a nonlinear characteristic, the fading circuit output delivers the corrected t.v. signal. The nonlinear path contains a low-pass filter passing only the lower range of the t.v. signal frequency band, while the linear signal path contains a delay section with a time constant which ensures that the t.v. signals at the fading circuit output are in-phase; they are provided in at least one channel of a colour t.v. camera, in the part of the luminance signal path following a matrix which carries out the signal correction necessary for a certain colour reproduction.
TL;DR: Using a multiple-scatter propagation theory, the characteristics of the transionospheric satellite-earth communication links are related to the parameters of the ionosphere and general equations for the frequency correlation functions are derived.
Abstract: Using a multiple-scatter propagation theory, the characteristics of the transionospheric satellite-earth communication links are related to the parameters of the ionosphere. General equations for the frequency correlation functions are derived. Some approximate solutions are obtained and numerical examples are given. Also discussed are the degree of coherence and potential applications of the procedure to characterize the intense fading channel.
TL;DR: Quasi-optimum digital baseband demodulation algorithms using various nonlinear estimation techniques are derived and digital demodulator structures are simulated on a digital computer for an FM system with first order message spectrum for various values of the parameters for Rayleigh and Rician channels.
TL;DR: In this article, the effects of snow on the antennas and in the propagation path of a propagation experiment in the Mendlesham area, Suffolk, England, were analyzed and the advantage of using switched-path diversity was shown.
Abstract: The severe fading caused by the snow of 1st March 1974 on microwave links of a propagation experiment being conducted in the Mendlesham area, Suffolk, England, is analysed. The figures derived from this event include the relative effects of snow on the antennas and in the propagation path, and the advantage offered by using switched-path diversity.
TL;DR: In the United Kingdom, the National Radiological Protection Board operates a four-weekly issue Fast Neutron Personal Monitoring Service without unacceptable fading problems as mentioned in this paper, which can be found in the British National Archives.
TL;DR: The results of field trials show the usefulness of a single-receiver diversity system in reducing the effects of fading in builtup areas.
Abstract: A single-receiver diversity system for use in the lower u.h.f. mobile-radio band has been built. The results of field trials show the usefulness of such a system in reducing the effects of fading in builtup areas.
01 Jan 1975
TL;DR: A simple sub-optimal threshold strategy, which is only marginally inferior to the optimum one, can reduce the effect of feedback error by several orders of magnitude.
Abstract: Two rate or intermittent data transmission with perfect channel state feedback has been shown to be extremely effective at combatting the effects of fading due to multipath effects. However, such a scheme usually requires a two state decision to be sent back to the transmitter over a similar fading channel. Possible feedback errors result in incorrect transmitter decisions which greatly increase the number of errors in the main data stream. However, these decisions can be weighted by a function of the feedback channel signal-to-noise ratio so as to minimise the effect of unreliable decisions. In fact, a simple sub-optimal threshold strategy, which is only marginally inferior to the optimum one, can reduce the effect of feedback error by several orders of magnitude. This simple strategy is also used to assure the receiver follows the transmitter decisions to allow negligible performance deterioration due to feedback or feedforward error.
Patent•
05 Dec 1975
TL;DR: In this article, a fading device for a motion picture camera includes a pair of sectors rotated with respect to each other to vary the angle of a light passing section formed thereby to obtain a fade-in or fade-out effect.
Abstract: A fading device for a motion picture camera includes a pair of sectors rotated with respect to each other to vary the angle of a light passing section formed thereby to obtain a fade-in or fade-out effect. The sectors are rotated relatively to each other by means of a drive mechanism operated by a motor by way of a change-over mechanism which is operable to change the direction of the relative rotation of the sectors. An indicator is connected with the change-over mechanism to indicate whether the fade-in or fade-out is possible in accordance with the situation of the sectors.
Journal Article•
TL;DR: In this paper, the performance of convolutional codes in conjunction with non-coherent MFSK modulation and Viterbi maximum likelihood decoding on a Rician fading channel is examined in detail.
Abstract: The performance of convolutional codes in conjunction with noncoherent multiple frequency shift-keyed (MFSK) modulation and Viterbi maximum likelihood decoding on a Rician fading channel is examined in detail. While the primary motivation underlying this work has been concerned with system performance on the planetary entry channel, it is expected that the results are of considerably wider interest. Particular attention is given to modeling the channel in terms of a few meaningful parameters which can be correlated closely with the results of theoretical propagation studies. Fairly general upper bounds on bit error probability performance in the presence of fading are derived and compared with simulation results using both unquantized and quantized receiver outputs. The effects of receiver quantization and channel memory are investigated and it is concluded that the coded noncoherent MFSK system offers an attractive alternative to coherent BPSK in providing reliable low data rate communications in fading channels typical of planetary entry missions.