Showing papers on "Fading published in 1985"

Digital communications

Abstract: Spread Spectrum It’s not just for breakfast anymore! Don't blame me, the title is the work of this month's guest columnist, Steve Bible, N7HPR (n7hpr@tapr.org). While cruising the net recently, I noticed a sudden bump in the number of times Spread Spectrum (SS) techniques were mentioned in the amateur digital areas. While QEX has discussed SS in the past, we haven't touched on it in this forum. Steve was a frequent cogent contributor, so I asked him to give us some background. Steve enlisted in the Navy in 1977 and became a Data Systems Technician, a repairman of shipboard computer systems. In 1985 he was accepted into the Navy’s Enlisted Commissioning Program and attended the University of Utah where he studied computer science. Upon graduation in 1988 he was commissioned an Ensign and entered Nuclear Power School. His subsequent assignment was onboard the USS Georgia, a trident submarine stationed in Bangor, Washington. Today Steve is a Lieutenant and he is completing a master’s degree in computer science at the Naval Postgraduate School in Monterey, California. His areas of interest are digital communications, amateur satellites, VHF/UHF contesting, and QRP. His research area closely follows his interest in amateur radio. His thesis topic is Multihop Packet Radio Routing Protocol Using Dynamic Power Control. Steve is also the AMSAT Area Coordinator for the Monterey Bay area. Here's Steve, I'll have some additional comments at the end.

Fading memory and the problem of approximating nonlinear operators with Volterra series

Abstract: Using the notion of fading memory we prove very strong versions of two folk theorems. The first is that any time-invariant (TI) continuous nonlinear operator can be approximated by a Volterra series operator, and the second is that the approximating operator can be realized as a finite-dimensional linear dynamical system with a nonlinear readout map. While previous approximation results are valid over finite time intervals and for signals in compact sets, the approximations presented here hold for all time and for signals in useful (noncompact) sets. The discretetime analog of the second theorem asserts that any TI operator with fading memory can be approximated (in our strong sense) by a nonlinear moving- average operator. Some further discussion of the notion of fading memory is given.

A statistical model for a land mobile satellite link

Abstract: A statistical model for a land mobile satellite link is described. The model assumes that the amplitude of the line-of-sight component under foliage attenuation (shadowing) is lognormally distributed and the received multipath interference has a Rayleigh distribution. Expressions for the level crossing rate and average fade duration are given. Comparison between results calculated from the model and from measurements show reasonably good agreement. The model should be useful for designing communications systems and for simulating propagation effects in the laboratory.

Performance of low-complexity channel coding and diversity for spread spectrum in indoor, wireless communication

Abstract: The application of selection diversity in conjunction with simple channel coding is considered for a multiuser, slowly fading, Spread-Spectrum Multiple Access (SSMA), digital radio system. For the most part, the index of performance for our study is the average bit error probability; we also give some consideration to multipath outage as a performance measure. All subscribers are assumed to communicate to a central station; that is, a star network architecture is assumed. Average power control is also assumed. The average mentioned in this context includes averaging over the channel fading statistics. The modulation is direct-sequence, spread-spectrum, binary phase-shift keying. We assume perfect timing and carrier recovery in our coherent receiver, and a slowly varying, Rayleigh fading, discrete multipath model is used. Previous analyses have found that SSMA can tolerate few simultaneous users for fading radio channels. We find that the combination of spread-spectrum modulation with low-complexity diversity and/or channel coding can restore fading-channel user levels to an acceptable figure. In addition, selection diversity plus channel coding is more effective than either method by itself. Finally, it turns out that SSMA is less sensitive to a change in the value of delay spread of a fading channel than, say, time-division multiple access. The method of moments is used to accurately assess the system error probability. Using this technique, we also assess the accuracy of assuming that the multiuser interference has a Gaussian distribution, which allows it to be analyzed by a simple method. Using this assumption, we compare selection diversity plus channel coding with the maximal-ratio-combining technique for diversity reception. Except for a high order of diversity, the former is more efficient and is always less complex than the latter.

Performance of MRC Diversity Systems for the Detection of Signals with Nakagami Fading

Abstract: Bit error rate (BER) is analyzed theoretically for diversity reception in Nakagami fading environment using an M -branch maximal ratio combiner (MRC). Coherent and incoherent reception of frequency shift keying (FSK) are considered, using the multiple branch diversity system for both identical and different diversity branch fading parameters. The effect of correlation is also considered for the dual diversity case. The results are extended to include coherent phase shift keying (CPSK) and differential phase shift keying (DPSK).

Performance of Noncoherent Direct-Sequence Spread-Spectrum Communications Over Specular Multipath Fading Channels

Abstract: The performance of coherent direct-sequence spread-spectrum communications over specular multipath fading channels is investigated. The average probability of error of the correlation receiver is derived for an arbitrary number of paths with deterministic or random gain coefficients. The gain coefficients, delays, and phase angles of any two distinct paths are modeled as mutually independent random variables. Numerical results for several values of the system and channel parameters are presented.

Performance of nondiversity receivers for spread spectrum in indoor wireless communications

Abstract: In this work we have considered direct-sequence spread-spectrum transmission for indoor wireless communications. We have modeled the indoor communications medium, which is a multipath fading channel, by a discrete set of Rayleigh faded paths. We have proposed new analytical techniques to evaluate the probability of error for the receiver terminals studied in this work. Numerical results reveal that, for the nondiversity receivers considered here, Rayleigh fading is very hostile to this form of modulation/multipleaccess technique. The results also indicate that either some form of operation to prevent Rayleigh fading or diversity operation to counteract Rayleigh fading is required.

Capacity and Cutoff Rate of Noncoherent FSK with Nonselective Rician Fading

Abstract: The capacity and cutoff rate of frequency-shift keying (FSK) modulation and noncoherent reception when the signal is subject to Rician fading are calculated. Both hard and soft decisions with maximum likelihood combining are considered, as well as soft decisions with square-law combining. Optimal code rates are found that minimize the required signal-to-noise ratio for reliable communication.

An Approximate Statistical Analysis of the Widrow LMS Algorithm with Application to Narrow-Band Interference Rejection

Abstract: The Widrow LMS algorithm is considered for the implementation of an adaptive prewhitening filter in a direct-sequence (DS) spread-spectrum receiver. Exact expressions for the steady-state tapweight covariance matrix and resulting average excess mean square error are developed for the real LMS algorithm when the input contains a random binary sequence (used to model a pseudonoise spreading sequence). It is shown here that the output samples of the adaptive filter possess approximately Gaussian statistics under the conditions of slow convergence and a large number of filter taps. Using this approximation, expressions for the resulting bit error rate (BER) when the adaptive algorithm is used to suppress a fading gone jammer are developed, and numerical results obtained from these expressions are compared to simulation results for the DS receiver.

Binary Code-Division Multiple-Access Systems Operating in Multipath Fading, Noisy Channels

Abstract: Binary code-division multiple-access (CDMA) systems operating in multipath fading, noisy channels (MFNC) are studied. Interference from undesired users and multipath are analyzed; these are modeled as equivalent noise. The error probability is given as a function of channel parameters and system parameters. The maximum possible number of users is estimated as a function of the tolerable error probability.

Binary error probabilities on Earth-space links subject to scintillation fading

Abstract: Theoretical estimates are derived of the average probability 〈Pb〉 of bit error rate that would occur for a binary CPSK signal on an Earth-space link in the presence of scintillation fading and additive white Gaussian noise. Strong scintillations are shown to produce a significant degradation in 〈Pb〉. Experimental results of the signal amplitude distribution obtained on a 7.1° elevation satellite downlink lead to values of 〈Pb〉 against 〈Eb/No〉 which agree well with the results obtained using the theoretical Moulsley-Vilar distribution (1982).

Cross-polarization cancellation and equalization in digital transmission over dually polarized multipath fading channels

Abstract: A theory for data-aided equalization and cancellation in digital data transmission over dually polarized fading radio channels is presented. The present theory generalizes and extends previous work by admitting decision feedback structures with finite-tap transversal filter implementations. Subject to the assumption that some past and/or future data symbols are correctly detected, formulas and algorithms for evaluating the least mean-square error for different structures are presented. In a sequence of curves we evaluate and compare the performance of various structures for a particular propagation model and several fading events. We find improvement in performance for decision feedback over linear equalization. More importantly, we discovered that in this application, as in the single-channel transmission case, decision feedback/ canceler structures are much less sensitive to timing phase than linear equalizers.

Comparison of DFE and MLSE Receiver Performance on HF Channels

Abstract: Data communication at rates near or above 2 kbits/s on 3 kHz-baadwidth HF radio channels is subject to impairment from severe linear dispersion, rapid channel time variation, and severe fading. In this investigation, recorded 2.4 kbit/s QPSK signals received from HF channels were processed to extract a time-varying estimate of the channel impulse response. From the estimated channel impulse responses, performance-related parameters were computed for ideal matched filter reception, maximum-likelihood sequence-estimation (MLSE), and decision feedback equalization (DFE). The results indicated that the simpler DFE receiver suffered only a small theoretical performance degradation relative to the more complex MLSE receiver. Other HF channel impulse response statistics were also obtained to shed light on equalization and filter adaptation techniques.

Data Directed Estimation Techniques for Single-Tone HF Modems

Abstract: This paper is concerned with data directed estimation (DDE) techniques for use in single-tone HF modems. The proposed scheme uses least-squares adaptive techniques for dealing with fading and multipath radio HF channels. The DDE is based on direct estimation of channel parameters and data blocks. Training data is imbedded into the transmission in the form of blocks of training bits alternated with blocks of source data at a fixed (though selectable) fraction of the transmission. Two DDE techniques, i.e., linear and nonlinear, are discussed in this paper. In the linear DDE, the source data block is estimated by a Levinson recursive algorithm. In the nonlinear DDE, only the end symbols of the data block are kept, quantized, and subtracted from the simultaneous equations. The symbols left in the block are estimated by a Levinson algorithm again, and so forth. The procedure described here is used recursively until all the known symbols are obtained. In both linear and nonlinear DDE techniques, the channel is estimated by a steepest-descent algorithm. As with a linear equalizer, the linear DDE lacks the ability to cope with the fading dispersive HF radio channels. However, the nonlinear DDE works well under the same channel conditions. The analysis shows that the nonlinear DDE can achieve performance similar to the decision-feedback equalization (DFE) technique with known channel conditions. The channel estimator used in the DDE techniques will converge faster than the DFE (see Section VII) if a similar adaptive algorithm is utilized.

Analysis of Four Decision-Feedback Carrier Recovery Loops in the Presence of Intersymbol Interference

Abstract: The performance of four decision-feedback carrier recovery techniques is evaluated in the presence of additive noise and intersymbol interference (ISI). For QAM signal constellations, a closed-form expression is given for the phase jitter variance (PJV) of each loop, and the loop tracking performance is examined. The analytic results are then computed in the case of a 16 QAM digital radio system subjected to multipath fading. Two cases are considered: in the first case no countermeasure techniques are used against selective fading, while in the second case a three-coefficient decision-feedback equalizer (DFE) is used. Computer simulations using a pseudorandom sequence to estimate loop performance are also reported which support the theoretical results.

Synchronization of multichannel receiver based on higher quality channels

Abstract: A synchronization of a multichannel receiver based on higher-quality channels capable of establishing and maintaining timing synchronism stably even when the transmission channels deteriorate due to fading. A plurality of power detectors measure the quality of the baseband signals of a plurality of channel lines and generate a quality signal. A plurality of equalizers generate timing phase deviation signals for the channel lines. A selecting circuit selects at least one channel line using the quality signal. A frequency controlling circuit generates a frequency control signal in accordance with at least one timing phase deviation signal from the selected channel line and controls a sampling frequency of a sampling circuit which converts a transmitted signal into digital form. Thus, the sampling frequency for all channel lines is determined by using the timing phase deviation signal obtained from a high quality channel line. As a result, the timing phase control can be established with stability.

A technique for improving radio system performance during fading

Abstract: A technique for reducing the average fade duration of signals in a radio system by simulating motion of a plurality of transmitting antennas is disclosed. This simulation is accomplished by slowly and continuously varying a characteristic of the modulated carrier signal supplied to the antennas. The altered characteristic can either be the power of the modulated carrier signal supplied to each of the antennas or the phase between modulated carrier signals coupled to the antennas. This technique can also be applied at a receiver to reduce the average fade duration of signals arriving thereat by simulating motion of a plurality of receiving antennas.

Fade margin calculation for channels impaired by Rician fading

Abstract: Excess path loss due to multipath severely restricts the performance of power limited mobile networks such as those using satellite-aided links. To reduce multipath related losses, the higher elevation angle of the spacecraft can be exploited by utilizing mobile antennas which reduce the strength of the multipath reflections in favor of the line-of-sight signal. The presence of a strong and stable path in a fading link will change the envelope statistics of the received waveform from Rayleigh to a more favorable Rician distribution. It is determined that the excess path loss, or fade margin, of a Rician channel when coherent detection of binary phase shift keying (BPSK) or quaternary phase shift keying (QPSK) signals is considered. The results are presented parametrically such that they can be applied to a wide range of propagation characteristics from heavy fading to nonfading situations. Furthermore, similar results are also given for the case where only limited coverage is provided.

Characteristics of sea reflection fading in maritime satellite communications

Abstract: The Japanese domestic experimental mobile satellite system (EMSS) has been studied to establish high quality channels for small vessels with compact and low-gain antennas. The present international maritime satellite communication system (INMARSAT) has also been studied for the next generation to extend services for small ships, for which the present ship terminals including the antenna systems are too large and too heavy. By using a low-gain antenna, however, multipath fading caused by sea reflections will become a serious problem in designing the new system. To take effective countermeasures to the fading effect, it is very important to get a better knowledge of the fading phenomena caused by sea reflections. From a standpoint of maritime satellite communications, experiments measuring the amount of fading were performed under various sea conditions and in two frequency bands. The wave height, which is considered the most important experimental parameter, was measured objectively by a wave rider buoy. The experimental results show that the amount of fading depends on a wave height and radio frequency and, as the most interesting result, as a wave increases above a certain level, the amount of fading decreases. These results are also discussed theoretically.

Outage probability in a mobile radio system subject to fading and shadowing

Abstract: Outage probability calculations are presented for the case when the transmission from a wanted base station is received at a mobile in the presence of an interfering cochannel signal, and reception is subject to fading and shadowing. These calculations consider the joint requirement that for satisfactory reception both an adequate signal and an acceptable signal/interference ratio are required simultaneously.

Modelling the transfer function in medium bandwidth radio channels during multipath propagation

Abstract: A method to compute the effects of a multipath propagation channel on a line-of-sight link consists in establishing a statistical model of the channel transfer function. It is then possible to compute the outage time previsions for various communication systems, or to compare these systems by means of their signatures. To carry out this method, it is first necessary to choose a representation model of the transfer function, then to establish on an experimental basis the joint statistics of the model parameters. The authors discuss the various steps in the construction and validation of such a model and compare several proposed models from the point of view of their applications. These points are illustrated by means of results from thepacem Iexperiment.

Comparison of High-Level Modulation Schemes for High-Capacity Digital Radio Systems

Abstract: This paper presents a comparative study of four 2n-state quadrature amplitude modulation techniques, namely, 16, 32, 64, and 128 QAM, in a digital radio system environment. The effects of filtering, interference, amplifier nonlinearities, and selective fading are investigated using computational and simulation methods. Increase of the carrier-to-noise ratio (CNR) at a fixed symbol error probability (10^{-3} ) is taken as a robustness criterion.

Space and frequency correlation characteristics of L‐band multipath fading due to sea surface reflection

Abstract: When maritime satellite communication is conducted at low elevation angles, fading due to sea surface reflection becomes a problem. For suppression of the fading, space diversity and frequency diversity may be employed. This paper studies the correlation characteristics of multipath fading due to sea reflection in the spatial and frequency domains in L-band operation. This is important in investigating the diversity effect. First, from the theoretical model of sea surface reflection, the correlation scattering cross sections of the coherent and incoherent components of the reflected wave are obtained. Next, from numerical calculations, the correlation characteristics are obtained with the antenna elevation angle and the height over the sea as the parameters. It is found that in space diversity good diversity effects can be expected from two antennas- separated in the vertical direction by about 40 cm. It is also seen that fading reduction cannot be expected with frequency diversity in the case of narrowband communications such as in maritime satellite communications. The effects of various fading reduction measures in the up and down links are considered.

An Analysis of the Sensitivity of Digital Modulation Techniques to Frequency-Selective Fading

Abstract: This Paper investigates the sensitivity to frequency-selective fading of different modulation techniques commonly used in digital radio systems. The modulation methods considered are phase-shift-keying ( M ary PSK), nonoffset and offset quadrature amplitude modulation ( M -ary QAM), and quadrature partial response signaling (QPRS). The performances of these methods during multipath fading are compared on the basis of the "signatures" calculated for idealized systems. Results are presented from which the relative outage probability was determined, assuming a propagation channel with frequency-selective Rician fading.

Path loss characteristics of 60 GHz transmissions

Abstract: Propagation measurements of 60 GHz transmissions between a hand-held roving transmitter and a receiver located at street-lamp elevations were made in urban and rural locations. The propagation distance exponent was approximately 2.2, and fades 20 dB below the mean envelope level occurred 1% of the time.

On Optimal Minimax Jamming and Detection of Radar Signals

Abstract: A game between an intelligent jammer J and decision maker DM is considered. DM seeks to detect a coherent slowly fading narrowband signal under a Neyman-Pearson criterion. His observations are corrupted with additive narrowband noise, the source of which is J's jamming with a power constraint, but otherwise almost arbitrary statistics. DM knows J's action but the converse is not true. When the number of samples increases asymptotically, a minimax solution for the game exists where the jamming is Gaussian, independent of the desired signal amplitude level and probability distribution. The same result also holds for detection of a nonrandom baseband signal.

Propagation at 500 MHz for mobile radio

Abstract: The basic characteristics of radio paths relevant to mobile radio communication at UHF are reviewed. Data gathered in a comprehensive survey of urban and suburban Melbourne were analysed to test for consistency with several simple models: Rayleigh envelope, a power spectrum model, log-normal distribution of mean, and the effects of distance and slope of terrain. Factors limiting the validity of some of the models are discussed. Other factors investigated were repeatability under varying weather conditions, the averaging required for reliable representation of local field strengths, and distributions associated with slow and fast fading patterns. A study of the applicability of space diversity showed that a saving of at least 75% of the transmitter power, for a given grade of service, could be made if diversity were used. Because of the number and complexity of the factors influencing the propagation to a vehicle, and because weather and time of day were not found to be significant influences, it was concluded that experimental assessment is a cost-effective method for determining accurately a service area. Examples of suitable data gathering phase are given.

Generating rice processes with given spectral properties

Abstract: The autocorrelation function (acf) of a stochastic Rician fading process is numerically evaluated, and useful approximations are given. The spectral properties of the fading are related to the component processes constituting the fading process. The result is that for Rayleigh fading and first-order Butterworth characteristic of the component processes the fading bandwidth is double the component bandwidth. The fading bandwidth decreases to the component bandwidth, however, when the Gaussian channel is approached or when high-order faltered components are assumed.