Showing papers on "Fading distribution published in 1979"

Reception Through Nakagami Fading Multipath Channels with Random Delays

Abstract: The Nakagami fading distribution is shown to fit empirical results more generally than other distributions. A statistical model for a noisy, Nakagami fading multipath channel is given, following Turin's delay-line model. Optimal receivers are derived for two states of knowledge of the channel-known path delays and random path delays. Upper bounds on the probability of error are computed, for binary, equal-energy, equiprobable signals, which are uniformly orthogonal and have equal, triangular, autocorrelation moduli. Results are graphically displayed and show the dependence of error probability on number of paths, amount of fading and spread of path delays.

Analysis of Direct-Sequence Spread-Spectrum Multiple-Access Communication Over Rician Fading Channels

Abstract: This paper is concerned with the performance of biphase direct-sequence spread-spectrum multiple-access communication for a general class of fading channels. The channels considered are those for which the channel output consists of a strong stable specular signal plus a faded version of this signal. Such channels, which are referred to as Rician fading (or Rice fading) or specular-plus-Rayleigh fading, are the result of a transmission medium which gives rise to a major stable communication path and a number of additional weaker communication paths. The fading channel is modeled as a general wide-sense-stationary uncorrelatedscattering (WSSUS) channel-a model which is general enough to exhibit both time and frequency selectivity and to impose no restrictions on the fading rate. For the general WSSUS model, results are obtained on the average signal-to-noise ratio at the receiver output in terms of the spread-spectrum signature sequences and the covariance function for the fading process. These results are then specialized to each of two important classes of WSSUS channels: time-selective fading channels and frequency-selective fading channels. Numerical evaluations are presented for specific examples of each of these two types of channels. Analytical expressions are derived for a spread-spectrum multiple-access system with random signature sequences, and the use of these expressions in preliminary system design is discussed.

Performance and Optimization of Switched Diversity Systems for the Detection of Signals with Rayleigh Fading

Abstract: The performance and optimization of switched diversity systems are considered. First, the one-dimensional distribution and probability density functions of the envelope of the received signal are obtained for three different switching strategies. This information is used to obtain the average probability of bit error for the case of non-coherent detection of binary FSK signals with Rayleigh fading envelopes and additive white Gaussian noise. The optimization of two of these switching strategies is then considered, and it is shown that by proper selection of switching thresholds, the average probability of bit error during detection can be minimized. It is also shown that these optimized switching strategies yield a significant improvement in performance over non-diversity systems and can approach the performance of more complex receivers such as maximal ratio combining. Computer simulations of switched diversity systems using a practical field model are used to verify the analysis.

Digital Radio outage due to selective fading — observation vs prediction from laboratory simulation

Abstract: A statistical model (introduced in a companion paper) of fading on a radio path is used with laboratory measurements on a digital radio system to estimate the outage due to multipath fading, where outage is the time that the bit error rate (ber) exceeds a threshold. Over the range of ber of interest (10−6 to 10−3), the calculated outage agrees favorably with the outage observed during the period for which the fading model was developed. It is further shown that the calculated outage, when scaled to a heavy fading month on the basis of single-frequency, time-faded statistics, agrees equally well with the outage observed on the same path during a heavy fading month The agreement between measured and predicted outage substantiates the selective fading model. The prescribed laboratory measurements characterize the sensitivity of the radio system to selective fading. Thus, the methodology provides a useful basis for comparing the outage of alternative realizations of digital radio systems.

Fading Effects on the Performance of a Spread Spectrum Multiple Access Communication System

Abstract: The effects of fading on the performance of a spread spectrum multiple access communication system using direct sequence modulation are investigated. Expressions for the system signal-signal-to-noise ratio and for the mean and variance of the desired and interfering users are derived in terms of the spreading code and fading correlation functions. The system error probability is also evaluated for the special case of slow Rayleigh fading. The results suggest that under conditions of fading and interference, long code sequences may be required to maintain low error rates.

Transmitter diversity for a digital FM paging system

Abstract: Multipath fading is one of the most serious problems for the signal transmission in mobile radio. To combat this problem, the application of transmitter diversity to a digital FM paging system is described. The method is to transmit two digital FM paging signals with different modulation indices, each of which is modulated by an identical binary signal, from separate antennas. It is shown that the same action obtained with the well-known selection diversity can be achieved when the difference between the modulation indices is equal to two. The diversity effect on the bit-error-rate (BER) performance is theoretically analyzed. The diversity effect on the BER performance in a 600-bit/s Manchester-coded digital FM system is verified by the laboratory simulation tests using a Rayleigh fading simulator.

Digital Radio Receiver Responses for Combating Frequency-Selective Fading

Abstract: Frequency-selective fading in a radio channel, caused by multipath propagation or any other phenomenon, can seriously degrade the effectiveness of digital transmission. Receiver processing that adapts in an appropriate way to the prevailing channel response can strongly reduce the harmful effects of such fading. We examine here the theoretical possibilities of adaptive processing by deriving and analyzing receiver responses for three different criteria. The criteria used permit an analytical approach that is simple and exact. The performance characteristics derived for the three receivers bracket the "best" performance that is possible in practice and reveal possible tradeoffs between performance and practicality. The analysis applies generally to the broad class of quadrature amplitude modulation (QAM) signals, which are assumed to be filtered in the transmitter to enforce spectral emission requirements. For each receiver response we derive a formula for the "power penalty", defined as the increase in transmitter power (over some theoretical minimum) needed to compensate for transmit filtering and channel fading. The power penalty formulas are evaluated for a two-ray multipath fading channel and for each of two common forms of modulation/transmit filtering. Graphical results are given for numerous combinations of the multipath parameters and the transmitter bandwidth-to-symbol rate ratio. The results of this study can be used to predict the probability (time fraction) of multipath outage, provided that the statistics of the channel parameters are known. Some cursory comparisons are made between the new results and those of a previous analysis for fixed receivers. The outage reductions made possible by using appropriate receiver responses are found to be quite large, possibly two or more orders of magnitude.

Signal-to-Noise Ratio Statistics for Nondispersive Fading in Radio Channels with Cross Polarization Interference Cancellation

Abstract: The signal-to-noise ratio (SNR) statistics of a crosspolarized interference cancellation scheme is derived in terms of the statistics of nondispersive fading of the signals in the radio channels. The results are applied to certain digital radio channels subjected to nondispersive fading. It is found that error rates not exceeding 10-6could be maintained during 99 percent of the duration of a 30 dB fade in a 26.4 mi path. The method and data presented support the feasibility of cross polarization interference cancellation. They are applicable to the determination of SNR budget in the transmission system.

Multipath propagation in line-of-sight links

Abstract: Using a classical model, and adding two complementary propositions, it is possible to obtain a general method for calculating the fading distribution laws on line-of-sight links of indifferent lengths undergoing multipath propagation. In particular, it is shown that the conventional empirical d3.5 power law gives a rough approximation in a very limited domain, but is not a general law.

Error Performance of Direct Sequence Spread Spectrum Systems on Non-Selective Fading Channels

Abstract: The effects of fading on the performance of a spread spectrum multiple-access communication system using direct sequence modulation are investigated. General series expansions are developed for the average probability of error and specific results are given for the irreducible error rate associated with propagation over independent Rician fading channels or correlated Rayleigh fading channels. Simple single-term approximations for the irreducible error rate, which are proportional to the second moment of the cross-correlation between the spreading codes employed by the users, are derived for these cases.

Probability of error performance of the spread-spectrum mobile communications receiver in a non-Rayleigh fading environment

Abstract: In previous work to evaluate the probability of error performance of the spread-spectrum mobile communications receiver the usual assumptions about the 900 MHz urban mobile channel were made [1], [2], [3], [4], [5]. These "usual assumptions" are that the highly frequency-selective fading has Rayleigh statistics with a slowly-changing, non-frequency-selective median, the median has log-normal characteristics [6]. This paper studies the effects of the fading distribution on the performance of the Spread-Spectrum receiver. The Spread-Spectrum systems uses transmitter power control to counter the effects of shadow fading in the mobile to base station channel. This study is thus confined to the effects of deviations from the Rayleigh fading assumption. The effects of the distribution of the median are not considered at this time. The design of the signal set used in the spread-spectrum system incorporates inherent diversity into the operation of the receiver. Loose bounds on the probability of error can be set by considering the cases of flat fading over the entire spread-spectrum signal, and independent fading of each component of the spread-spect rural signal. The results of Monte-Carlo computer simulations show that fading distributions that are the result of dominant specular reflections cause deleterious probability of error performance. The result of Jao and Elbaum [7] for single pulse detection in non-Rayteigh fading channels provides the motivation and the genesis for the work presented here.

Fading of shortwave radio signals in the tropical zone

Abstract: The paper describes the nature, type and occurrence of fading of shortwave signals with special reference to ob servations in the Tropical Zone. The probable causes for the occurrence of different types of fading are indicated- Theoretical analysis of the fading curres, both with regard to distribution of amplitude and the variation with time is discussed at length. Experimental observations by various workers indicate Rayleigh, normal. log-normal and m type amplitude probability distribution and these have been explained from theoretical considerations. Time variation analysis shows that the fading rate varies, in general, between 0.07 and 0.3 Hz. Propagation in the equatorial belt is highly influenced by q-type sporadic-E during day-time and results in enhanced fading rates of the order of 02 to 5 Hz. Very rapid fading rate commonly called ‘flutter’ upto 15 Hz is noticed in the evening hours around ± 20° of geomagnetic equator. The cause of flutter fading is attributed to spread-F Seasonally, it is more...