Showing papers on "Fading published in 1980"

Frequency-hopped multilevel FSK for mobile Radio

Abstract: A multiple-access modulation technique that uses multilevel frequency shift keying (FSK) to modulate frequency-hopped, spread-spectrum carriers is examined for possible application to digital mobile radiotelephony. This technique, in which all users employ the full system bandwidth simultaneously, would be resistant to the frequency-selective fading so troublesome in mobile radio. We have studied base-to-mobile communication of 32 kb/s per user in the 20-MHz (one-way) bandwidth of the 850-MHz mobile radio band. The number of users that can be served within a given bit error rate criterion depends on the quality of the radio channel. For perfect transmission, where the only degradation is mutual interference, an error rate less than 10−3 can be maintained with up to 209 simultaneous users. Transmission impairments, consisting of white Gaussian noise and frequency-selective Rayleigh fading with an average rf signal-to-noise ratio of 25 dB, reduce the number of simultaneous users to about 170. This capacity is roughly three times that of a phase-shift-keying spread-spectrum system recently proposed for mobile radio. For mobile-to-base transmission of FH-fsk, we have yet to study impairments resulting from delay spread in a synchronous system or, alternatively, the penalty for operating asynchronously. These effects would reduce the number of possible users from the estimates we have given for base-to-mobile transmission.

Analysis of multipath propagation structure in urban mobile radio environments

Abstract: An analysis is made of the physical structure of multipath propagation in an urban area. The incident angle, path length, and field strength are measured for each principal component of the received multipath signal. The results of the analysis reveal that the observed principal waves compare fairly well with those propagating via geometrical optics. The experimental data indicate that a limited number of ray-theoretical wave components play an important role in propagation in urban mobile radio environments.

Spectral parameter estimation of CAT radar echoes in the presence of fading clutter

Abstract: The analysis technique and a part of the results obtained from CAT radar echoes from higher troposphere and lower stratosphere are presented. First, the effect of processing distortion caused by the periodogram method using FFT algorithm on the slowly fading ground clutter echo is discussed. It is shown that an extremely narrow clutter spectrum can spill over the entire frequency range if the data are truncated at a tie sorter than their correlation time affecting largely the estimation of the CAT spectrum contribution, especially when the latter is a few tens of dB weaker than the former. A nonlinear least squares fitting procedure is used to parameterize the observed power spectrum in terms of CAT echo power, Doppler shift, spectral width, and the parameters which specify the shape of the clutter component.

Fast adapting fading channel equalizer

Abstract: A receiver system for processing diversity channel signals to substantially eliminate future and past intersymbol interference (ISI) in received signals which have been transmitted through a time-varying frequency selective transmission medium. The system includes lattice filter means which provide forward error residual signals which are used for generating uncorrelated adaptive weighting signals which eliminate future ISI and decision feedback circuitry which provide uncorrelated adaptive weighting signals which eliminate past ISI.

A polynomial model for multipath fading channel responses

Abstract: We present a channel model useful for analyzing the effects of multipath fading in digital radio systems. The frequency response of a fading channel is represented by the function A 0 — ωB 1 + jωA 1 , where ω (=2πf) is measured from the center of the channel, and A 0 , A 1 ,and B 1 are variable coefficients that change slowly with time. The model consists of this function, the joint probability density function (pdf) for the three coefficients, and the average number of seconds per heavy-fading month for which this response applies. The model is derived from a large base of multipath fading data, obtained on a 26.4-mile path in Georgia in June 1977. It consists of nearly 25,000 recorded measurements of received power vs frequency in a 26.3-MHz bandwidth at 6 GHz. In this paper, we present the methods of data reduction and statistical analysis used to derive the model; describe some assessments of Us validity; and discuss its limitations, virtues and possible uses. By all available measures, the model is highly accurate. It suffers from a potentially important phase ambiguity that can be resolved only via new, coherent measurements. The existing model should prove very useful in the design and planning of such measurements.

Packet Radio Performance Over Slow Rayleigh Fading Channels

Abstract: Expressions for the throughput and average packet delay for a Pure-ALOHA single-hop packet radio system operating in slow Rayleigh fading are derived. For noncoherent frequency-shift-keying (NCFSK), an exact closed form expression is presented. For coherent phase-shift-keying (CPSK) an excellent approximation for large packet sizes is derived. This approximation technique is valid in general for other modulation schemes and for other fading channel statistical characterizations. The packet length which maximizes the useful data throughput in slow Rayleigh fading is found. The results of this investigation indicate that a packet radio system can be designed with a modest link margin for fading and achieve identical throughput performance over a nonfading channel and a fading channel with only a small increase in average packet delay for the fading channel.

Fading channel communications

Abstract: Adaptive processing can reduce the effects of fading on beyond-the-horizon digital radio links. Two radio propagation channels for beyond-thehorizon communications, troposcatter, and HF are currently being reexamined. In the past, transmission over these radio channels had been considered unreliable due to fading effects. Recently, conversion from analog to digital transmission and the use of new adaptive signal processing techniques have offered promise of acceptable network communication quality. In addition, over-the-horizon radio provides economic and/or secu-· rity advantages relative to satellite, cable, or line-of-sight terrestrial microwave links. There is renewed interest in . the use of high frequency (HF) and troposcatter communications for networks carrying digital traffic.

Computer simulation of multipath fading in the land mobile radio environment

Abstract: This paper describes a radically new approach to the simulation of multipath fading in the civil land mobile radio environment. The simulation package described takes into account vehicle movement through a particular environment, signal shadowing and coherent waves. Statistical results obtained with the simulation are compared with those obtained by v.h.f. field trials for three areas within the City of Bath which exhibit differing fading characteristics, and are shown to be in excellent agreement.

Frequency-hopping, multiple-access, phase-shift-keying system performance in a Rayleigh fading environment

Abstract: Recently, various Frequency-Hopping, Multiple-Access (FHMA) schemes have been proposed as alternatives to frequency-division (FDMA) techniques for guarding against interference from other users and multipath fading in transmitting digitized speech in mobile radio. While the advantage of frequency diversity against fading is well known, the system degradation of frame-asynchronous FHMA-PSK due to interference from other users and fading has not been studied, except by modeling the interference as additive white Gaussian noise (AWGN). In this paper, we analyze the performance of two FHMA-PSK systems in a Rayleigh fading environment. We assume that the interferers' addresses are assigned at random and derive expressions for the union upper bound on the bit error rate (P b ) for U users, each assigned N frequencies, i.e., N chips. As an example, we consider transmitting digitized speech (R = 31.25 kb/s) over the mobile radio channel (bandwidth W = 20 MHz), using orthogonal coding of rate r = 5/32 (N = 32). For the differentially coherent FHMA-PSK case, the number of users in a two-way radio system is limited to 26 by the performance of the mobile-to-base link, which corresponds to an equivalent bandwidth of 770 kHz per user.

A UHF channel simulator for digital mobile radio

Abstract: A device to simulate the channel propagation characteristics of the ground mobile environment at UHF has been designed, built, and characterized. This device provides a flexible, easily changed set of simulated channel characteristics, which allows the performance of a mobile radio unit to be evaluated in the laboratory under controlled conditions. The channel simulator was designed to simulate mobile platform speeds up to 675 mi/h and multipath components having differential delays approaching 10 µs. The channel simulator can provide up to four easily selectable, independently fading, multipath components, having calculated time delay spreads of up to 3.5 µs. The adjustable fading bandwidths and the exceptional long delays were implemented using the relative new signal processing technologies of charged-coupled devices and surface wave devices. Envelope statistics such as fading distributions and level crossing rates produced by the channel simulator show excellent agreement with theoretical prediction and documented experimental data.

Phase and amplitude dispersion for Earth-satellite propagation in the 20- to 30-GHz frequency range

Abstract: Amplitude and phase dispersion have been measured for over a year on a 19- and 28-GHz earth-space propagation path. In the experiment amplitude and phase differences were compared for a 28-GHz carrier with \pm264 -MHz sidebands and a 19-GHz carrier, all transmitted from a COMSTAR satellite. No dispersion (frequency selective fading) was found of the type caused by multipath propagation with a large spread in time delay or by resonances in the propagation medium. The only frequency dependences evident were due to the bulk properties of water in rain. The conclusion from this investigation is that amplitude and phase dispersion should not pose a problem for wideband (on the order of 1 GHz) satellite communication systems operating at frequencies greater than 10 GHz with elevation angles from the earth terminals of greater than 15\deg .

Theoretical studies of a simulcast digital radio paging system using a carrier frequency offset strategy

Abstract: In a multistation simulcast digital radio paging system, each base station transmits the same RF signal simultaneously with the resulting efficient frequency utilization and simplified receiver design. In this system a paging receiver in the overlapping area receives several RF signals transmitted from different base stations. When frequency-shift keying (FSK) is used as a modulation method, experimental test results have already shown that the timing of each RF signal should be synchronized as closely as possible, but that the carrier frequency of each transmitter should be set following a certain offset assignment. The signal transmission performance in a multipath fading environment can then be markedly improved. The cause of this improvement effect is theoretically analyzed. It is clarified that the improvement effect is caused by transforming the probability distribution of time-averaged signal power.

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.

Extraction of multipath parameters from swept measurements on a line-of-sight path

Abstract: A computerized method of obtaining the multipath parameters from swept measurements is described, and some results are discussed. The accuracy of the multipath parameters determined is about \pm0.3 ns in delay and \pm0.1 in amplitude. The time variation of the propagation medium during the 331-ms sweep time is included in the muitipath model. This has a significant impact on the analysis and on the results obtained.

Distribution of burst error lengths in Rayleigh fading radio channels

Abstract: A method for measuring burst errors which occur in v.h.f. or u.h.f. digital land mobile radio channels is described. Then it is noted that the cumulative distribution of burst error lengths in a Rayleigh fading channel is approximately governed by the log-normal distribution.

Impairment mechanisms for SSB mobile communications at UHF with pilot-based doppler/fading correction

Abstract: Several impairments are associated with the pilot-based correction of a single sideband (SSB) voice signal for the effects of vehicle motion in a Rayleigh multipath field. These include the distortion caused by a limit on the available correction gain, the distortion resulting from frequency selective fading, the degradation of the signal-to-interference (S/I) [noise (S/N)] ratio, and the distortion induced because of interference to the pilot from a cochannel pilot or from noise. We have investigated these impairments for a specific feedforward, phase- and gain-correcting, ideal receiver. For this receiver, the S/I ratio is degraded at baseband because of the time-varying nature of the gain correction signal and the assumption that the correction signal is correlated with the desired voice signal fading envelope but is independent of the interferer fading envelope. A limit on correction gain appears necessary to limit the magnitude of several of the impairments, and has been considered as a parameter in the analysis of each impairment type. The results are here extended to characterize an “equal-gain” space diversity receiver. Diversity combining is applied after phase correction, but before gain correction to maintain proper voice correction while retaining the benefits of the improved envelope statistics. This paper deals only with the SSB idealized channel, not with a system employing SSB modulation; the authors recognize that many complicated systems problems (eg., antenna combining, channel generation, and cost reduction) must be solved before a working system using the channel analyzed herein can become a reality.

Digital radio transmission system for transmitting a plurality of information signals by a network of transmitters having substantially the same carrier frequencies

Abstract: In this system in which the information signals to be transmitted are converted into the digital form, transmission is done via a frequency-division multiplexer which so transmit the bits of the information signals in parallel that the duration of the bits transmitted through the channels of the multiplexer is longer than double the difference in propagation time between two carriers obtained from the two transmitters which are nearest in the receiving zone, where the said carriers have levels which are near to one another. This solves the problem of overlap between the information signals. To control the problem of fading, three types of transmitters which have three carriers whose deviation is very small compared with the bandwith of a channel are used in the transmitter network.

Enhanced learning of new discriminations after stimulus fading

Abstract: Humans learned to name three sets of braille patterns presented visually. After learning a practice set by traditional discrimination training, half the subjects learned a new set by fading, and the remaining subjects learned them by traditional discrimination training. Finally, all subjects learned a third set of new patterns by traditional discrimination training. Subjects who learned the second set by fading learned the third set faster than did subjects who learned the second set by traditional means. This effect was explained in terms of the differential strengthening of observing behavior by fading and by traditional discrimination training.

Tropospheric microwave propagation--An X band diagnostic system

Abstract: A microwave system for direct measurement of angles of arrival and multipath delay times is described. Based on the utilization of interferometer and very wide frequency sweeping (1- GHz range) techniques, high accuracy measurements on these parameters are possible. Some experimental results are presented which suggest that movements in the angle of arrival of a single ray path (as opposed to multipath) may be responsible for much of the fading experienced on light-of-sight microwave links in southwestern Ontario.

Postdetection selection diversity effects in a digital fm land mobile radio with discriminator and differential detections

Abstract: In the digital FM land mobile radio communication with fast fading whose maximum Doppler frequency reaches several ten Hz, the error occurs due to the signal amplitude variation and random FM noise caused by the phase variation. This paper discusses the performance improvement by the postdetection selection diversity for the digital FM radio communication with discriminator and differential detections under a fast Rayleigh fading environment. The conditional error ratio is derived for the instantaneous detector input amplitude at fast Rayleigh fading and the average error rate at the diversity reception is obtained as a general solution with the effect of random FM noise taken into account. Further, the improvement effect in the presence of co-channel interference is analyzed. The comparison is made with the postdetection synthesis diversity.

Effects of Intersymbol Interference on a Rayleigh Fast-Fading Channel

Abstract: It is shown in this paper that the performance of certain Rayleigh fast-fading channels can be accurately and efficiently evaluated using the elementary trapezoidal integration rule. The effects of intersymbol interference on such channels are analyzed.

Phase Detector Response to Noisy and Noisy Fading Signals

Abstract: Transfer characteristics of sinusoidal, triangular, and sawtooth-type phase detectors in response to noisy and noisy fading signals have been studied in detail. A new analytical model of the swatooth-type phase detector has been suggested. Detailed experimental results have been supplemented in support of the theoretical findings. Both the theoretical findings and the experimental results clearly indicate that the superiority of a sawtooth-type phase detector over the other two varieties is completely lost in noisy and noisy fading environments.

Time diversity by pitch-synchronized interpolation

Abstract: In mobile communications such as automobile telephone systems, an instantaneous interruption is caused by rapid fading, and speech quality is markedly deteriorated. A pitch-synchronized interpolation method is proposed to improve this quality. This method utilizes the fact that most speech signal regions have a pitch period. In this method, when an instantaneous interruption is detected at the receiver, a speech signal, which includes instantaneous interruption noise, is deleted and interpolated by repetition of the speech signal received one pitch interval wave before the instantaneous interruption. This method can be applied to receivers used in analog and digital transmission systems. A receiver using this method has been constructed, and it was shown that a 10-dB carrier-to-noise ratio (CNR) gain can be obtained by this receiver.

Useful Bounds on the Performance of a Spread-Spectrum Mobile Communication System in Various Fading Environments

Abstract: Previous results on the spread-spectrum system for mobile communication proposed by Cooper and Nettleton have been obtained by computer simulation. In this paper, closed-form bounds on the bit error rate are derived for flat, Rayleigh, and Rician fading conditions. For more general types of fading, the saddle point method is used. Since the bounds are easily computed, they are more useful than computer simulations for analyzing system performance.

Multiple terminal 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 Doppler (relative velocity) and Doppler variation (relative acceleration) problems inherent in a scenario with mobile endpoints A Figure-of-Merit (FOM) 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 considerable endpoint relative motion

C.P.S.K. receiver performances in presence of carrier-frequency offset

Abstract: Error probability in a binary c.p.s.k. receiver with a carrier-frequency offset is studied. The error probability has been evaluated as a function of the p.l.l. bandwidth. The optimum value of the p.l.l. bandwidth for a given frequency offset is determined with and without the slow Rice fading.

Saddle-point approximation for M-ary phase-shift keying with adjacent satellite interference

Abstract: As the geosynchronous orbit for satellite communication becomes increasingly crowded, the effect of adjacent satellite interference is of primary concern. We describe here the saddle-point approximation technique for evaluating the error probability of M-ary PSK systems with adjacent satellite interference. In comparison with previous methods, this technique has the advantages of both computational simplicity and accuracy. Results include a sample calculation to study the effect of fading on the uplink of a 12/14-GHz quadraphase-shift-keying system with 3-degree satellite spacing and 3-m earth stations.