Showing papers on "Multipath propagation published in 1985"

Analysis and Simulation of a Digital Mobile Channel Using Orthogonal Frequency Division Multiplexing

Abstract: This paper discusses the analysis and simulation of a technique for combating the effects of multipath propagation and cochannel interference on a narrow-band digital mobile channel. This system uses the discrete Fourier transform to orthogonally frequency multiplex many narrow subchannels, each signaling at a very low rate, into one high-rate channel. When this technique is used with pilot-based correction, the effects of flat Rayleigh fading can be reduced significantly. An improvement in signal-to-interference ratio of 6 dB can be obtained over the bursty Rayleigh channel. In addition, with each subchannel signaling at a low rate, this technique can provide added protection against delay spread. To enhance the behavior of the technique in a heavily frequency-selective environment, interpolated pilots are used. A frequency offset reference scheme is employed for the pilots to improve protection against cochannel interference.

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.

Equalizing without altering or detecting data

Abstract: For terrestrial digital radio systems that use Quadrature Amplitude Modulation, the idea of adapting equalizers to multipath distortion, without relying on accurate data estimates, is attractive. Prompt adaptation following a severe fade, when accurate data estimates are unavailable, is useful for reducing outage time. To avoid processing and administrative overhead, the adaptation method should not involve violating the transmitted signal with the insertion of equalizer training signals. We approach this kind of equalization by building on an algorithm of D. Godard (IEEE Transactions on Communications, November 1980)1 that was devised for voiceband polling networks. The method involves a very simple tap update procedure. However, the technique lacks the foundation of the years of analysis and experimentation that underlie least-mean-square adaptation algorithms. The main purpose of this paper is to present new findings, including (1) a proof that the algorithm, thought to require special equalizer initialization, converges regardless of initialization (this offers useful flexibility in digital radio systems, since, after a severe fade, the algorithm could start with any tap misalignment); (2) a preliminary look at convergence speed suggesting the possibility of significant outage reduction; (3) an algorithm that provides phase coherence (the original algorithm requires a follow-on phase-locked loop); and (4) an algorithm for cross-polarization cancellation as well as equalization.

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.

The tone capture properties of CMA-based interference suppressors

Abstract: An earlier paper introduced the constant modulus algorithm (CMA), an adaptive filtering technique for correcting multipath and interference-induced degradations in constant envelope waveforms such as FM and QPSK signals This algorithm exploits the fact that both multipath propagation and additive interference disrupt the constant envelope property of the received signal By sensing the received envelope variations, the adaptive algorithm can reset the coefficients of an FIR digital filter so as to remove the variations and, in the process, suppress the various interference components from the desired signal This paper examines a problem that arises when using CMA to suppress narrow-band interference If both the interferer and the signal have constant envelope and are spectrally nonoverlapping, then it is possible to find two different filter solutions, one which suppresses the interferer and another which "captures" the interferer and suppresses the desired signal This paper examines how "capture, can occur and how it may be prevented This problem is studied by characterizing the algorithm's behavior to an input consisting of only two sinusoids Assuming slow adaptation, the N-dimensional adaptive weight recursion is shown to compress into a two-by-two recursion in the tone output amplitudes This simplified recursion is then analyzed to determine what combinations of input amplitudes (signal-to-interference ratios) and filter initial conditions lead to "lock" and which lead to the capture of the interferer The results are then broadened to include multiple input tones and signals with nonzero bandwidth

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.

Multipath effects on the determination of absolute ionospheric time delay from GPS signals

Abstract: Signals from the Global Positioning System (GPS) satellites can be used to make measurements of absolute ionospheric group delay and relative phase advance. These ionospheric parameters have wide application in providing corrections for military systems requiring knowledge of ionospheric time delay and phase scintillation, such as satellite detection radars. Multipath effects from the local environment of the receiving antenna can cause severe contamination of ionospheric group delay measurements made with GPS pseudorandom noise receivers. Measured variations in the multipath effects in some typical clean and reflective environments are presented and shown to be consistent with signal analysis for pseudorandom noise receivers. Measured single reflector multipath is analyzed briefly and shown to correlate well with calculations. Since antennas must frequently be located in environments that are much less than ideal, data analysis procedures were developed for minimizing the impact of multipath on the group delay measurement. These include low-pass filtering, day-to-day correlation, and calibration to relative differential carrier phase advance measurements by using subsets of the absolute ionospheric group delay data. Results of these processes as applied to the test data from various multipath environments are presented.

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.

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.

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.

Adaptive multipath delay estimation

Abstract: Two algorithms are proposed for the continuous tracking of multipath delay. The first is an efficient method for adaptively tracking the secondary peak in the autocorrelation function caused by multipath. In principle, it is equivalent to the correlation method commonly used in practice. The second is an adaptive inverse filter which can exactly remove the multipath transfer function. The methods use an adaptive delay line interpolated by a first-order filter. The interpolation coefficient is explicitly estimated using the recursive Gauss-Newton (RGN) algorithm often used for recursive system identification.

Decision Feedback Equalization of Dispersive Radio Channels

Abstract: The performance of adaptive decision feedback equalization applied to high bit rate digital radio systems in the presence of multipath propagation is analytically investigated. Minimum phase (MP) and nonminimum phase (NMP) type fades as well as the transition periods between these two states are considered. Insight is given into the IF recovered timing epoch and its consequences on the decision feedback equalizer (DFE) behavior, especially during transition periods. Required conditions on the DFE structure for ensuring its maximum efficiency are derived and a modified updating algorithm is presented. Finally, dynamic simulation results are reported and compared to theoretical results. They show that the proposed structure is capable of coping with most propagation conditions.

UHF wideband statistical model and simulation of mobile radio multipath propagation effects

Abstract: A local wideband statistical model is presented for UHF mobile multipath propagation. The complex echo-envelope statistics have been derived from 10 MHz wideband pulse soundings at 436 MHz over a large area in suburban and urban localities. In the proposed model, the effects of Doppler-induced RF phase shifts have been considered. Echo paths are shown to fit a Poisson distribution for the excess time delays, while the echo envelopes conform to Rayleigh fading statistics. Correlated scattering, evident from echo path clusters in time-delay and echo-amplitude correlation coefficients in neighbouring path delay bins, is shown to be significant for excess path time delays less than 0.3 μs. Disagreements observed for local and nonlocal scattering in different localities are discussed. The implementation of a wideband multipath simulator based on the statistical model is described. Digital computer simulation of DPSK is used as an illustrative example to demonstrate its usefulness.

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.

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.

Diversity techniques in communications receivers

Abstract: This chapter discusses the radiowaves that arrive in communication receivers. In all except very simple transmission conditions, radio communication links are subjected to conditions in which energy can travel from the transmitter to the receiver via more than one path. This "multipath" situation arises in different ways depending upon the application, for example in MF and HF skywave transmissions it arises because of reflections from two or more ionospheric layers, or from two-hop propagation. In mobile radio, it arises because of reflection and scattering from buildings, trees and other obstacles along the path.

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.

Erratum: Large area characterisation of urban UHF multipath propagation and its relevance to the performance bounds of mobile radio systems

Abstract: A large number of small-area statistical channel descriptors in the form of average-power/time-delay profiles have been obtained in urban and suburban areas. The average excess delay and the delay spread are meaningful parameters which adequately describe the dispersiveness of the channel, and the variability of these parameters over a large area has been used as part of a two-stage model for characterisation of a UHF mobile radio channel. Suburban and urban areas exhibit quite different characteristics, reflecting the different nature of the environment. The variability of the statistical parameters is also a useful estimator of the performance bounds of radio systems. Using the measured data, together with relationships available in the literature for the performance of a given system in the presence of multipath, consideration is given to several system, such as DPSK data, SSB with pilot, FM voice and a phase ranging automatic vehicle location system. It is concluded that system techniques such as diversity reception can often lead to useful improvements in performance.

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.

Test equipment for simulating multipath interference

Abstract: The invention relates to a device for simulating multipath interference in order that the correction stages of a radio receiver can be tested. The device includes parallel paths, simulating the multipath environment, and a pilot tone generator for introducing pilot tone into the paths. A feedback loop continuously adjusts the phase and amplitude of segments in the paths so as to cancel pilot tone from the output whereby the loop maintains the adjustment need for maximum attenuation at the frequency of the pilot tone.

Statistical model for amplitude and delay of selective fading

Abstract: The transmission performance of digital radio systems is controlled by spectral distortion caused by multipath fading. To evaluate this performance for digital systems with high-order modulation schemes, a statistical model for frequency-selective fading is needed. New propagation data obtained in Gainesville, Florida, were used to generalize Rummler's model to include group delay response. The introduction of the delay response data into the model of the fading channel enabled the classification of the fades as minimum phase and nonminimum phase. We found that 24 percent of all fades have significant delay distortion, and can be characterized as being minimum phase or non-minimum phase. In the range of practical interest, there are as many minimum phase as nonminimum phase fades. The results of this work will facilitate a better understanding of the fading channel, which will be beneficial in the engineering of radio routes and digital radio design. The results also demonstrate the need for a description of the geographical occurrence of dispersion, which will differ from that for multipath fading at a single frequency. This is based on the observation, presented in this paper, that the relative amount of dispersive fading is significantly greater in Gainesville, Florida, than in Palmetto, Georgia. The availability of a dispersive fading map will facilitate the accurate engineering of digital radio routes.

Effect of sound‐speed profile on differential time‐delay estimation

Abstract: The sensitivity of correlograms to various sound‐speed profiles is being studied in order to estimate the impact of refracted multipath acoustic energy on differential time‐delay estimation (TDE). To do this, we utilize the generic sonar model to compute the crosscorrelation function for an array of two separated receivers in realistic ocean environments. The effects of the obtained correlogram peaks that show the differential time delays were compared with those predicted when assuming isovelocity profiles. The influence of a signal source’s range and bearing, as well as the vertical separation of the receivers on the correlograms, is demonstrated for each environment considered. Our results indicate that TDE may be significantly influenced by the processing of highly refracted rays of many different types due to the selected sound‐speed profile. Also, the Cramer–Rao lower bounds to the standard deviation of time‐delay errors at various signal‐to‐noise ratios are compared with the results that were derived using the generic sonar model and found to be in close agreement.

Adaptive sidelobe cancellation of wide-band multipath interference

Abstract: The steady-state performance of a narrow-band sidelobe canceller using a single auxiliary element with tapped delay line is analyzed for a simple two-path interference model, and an impulse response model is developed to provide insight as to the cancellation performance that might be expected. Computer solutions are then presented for various combinations of signal and multipath parameters. It is shown that auxiliary delay taps are required for effective cancellation of wide-band multipath interference, even for fractional bandwidth-delay products. Good cancellation is generally achieved provided that 1) the delay line taps are spaced much closer than the Nyquist sampling rate, and 2) the total length of the delay line considerably exceeds the longest multipath delay, depending on multipath component strengths.

A comparison of multipath distortion characteristics among digital modulation techniques

Abstract: In a digital mobile radio transmission, long-delayed multi-path propagation causes frequency selective fading, resulting in signal waveform distortion and hence bit error rate (BER) degradation. Comparison of multipath distortion among various digital modulation techniques (binary, quaternary, offset quaternary, and minimum phase shift keying (BPSK, QPSK, OQPSK and MSK)) is made using a two-ray propagation model, which is a fundamental model to evaluate multipath tolerant capability. The analysis clarified the multipath conditions causing severe distortions and also the mechanisms of eye pattern degradation, showing the superiority of BPSK in multipath tolerance.

On beamforming in presence of multipath

Abstract: We consider a source radiated signal arriving at an array as a group of wavefronts, each having a different angle of arrival and with arbitrary amplitude, phase and inter wavefront correlation. Several such sources may be present and the measurement data is assumed to be corrupted by sensor to sensor uncorrelated noise. The task of the beamformer is to make optimal estimates of each source signal of interest by using the information in all the wavefronts generated by the source. The proposed processor begins with no apriori information about the environment and constructs the optimal beamformer by a bootstrapping approach which uses a two tier eigenstructure analysis of the array covariance. We show that this new beamformer has substantial advantages over the usual optimal beamformers and present results of computer simulation carried out to verify its performance.

Millimeter–Wave Urban and Suburban Propagation Measurements Using Narrow and Wide Bandwidth Channel Probes

Abstract: Measurements reported are part of a study of propagation characteristics for millimeter–wave communication links operating in an urban#8211;suburban environment. Absorption data were collected for signals propagated through some common building materials at 9.6 (reference frequency), 28.8, and 57.6 GHz. At these same channel frequencies, paths at street level obstructed by office buildings and residential properties were examined in terms of received signal levels resulting from direct penetration and/or propagation by diffraction modes with terminal separation of from 0.1 to 1.2 km. Signal fading was measured for line#8211;of#8211;sight paths up to 0.9 km as the transmitter terminal, operating at 11.4, 28.8, and 30.3 GHz, traveled down an urban street. Narrow#8211; and wide#8211;beam antenna patterns and both modes of linear antenna polarization were used to compare multipath fading characteristics. A unique 30.3 GHz wide bandwidth channel impulse probe was used to record multipath signal amplitudes and delay times, relative to the direct path, for the same antenna parameters indicated above. Impulse response measurements were recorded at about 10 m intervals as the transmitter traveled along the urban street.

A comparison of prediction methods for 800 MHz mobile radio propagation

Abstract: The design and installation of a mobile radio system involves the evaluation or prediction of the propagation characteristics associated with the geographical area of interest. A wide variety of prediction techniques has been developed for different mobile environments, so that the task of selecting an appropriate propagation prediction method can be difficult and complex. This is especially true for systems operating in the upper-UHF band of 800-950 MHz, where mobile radio services are rapidly growing. A practical comparison is given of current prediction techniques available in the literature (for slow lognormal fading rather than multipath Rayleigh fading), with an emphasis on three essential criteria to consider in the selection process: the type of terrain or geography covered by each method, the form of prediction provided (e.g., transmission loss estimates, field strength contour maps, etc.), and the implementation difficulty and degree of sophistication.

A simulation study of space diversity and adaptive equalization in microwave digital radio

Abstract: In this paper we analyze the performance of M-level quadrature amplitude modulation digital radio systems subjected to microwave multipath fading. We consider two kinds of adaptive receiver techniques, either singly or in combination: dual space diversity and adaptive equalization. The space diversity is assumed to be of either the selection type or the continuous-combining type, and the equalization is assumed to be ideal. We describe a specific form of combining which is optimal when no post-combiner equalization is used. A primary aim of the study is to quantify the performance of this combining approach and to compare it with alternate strategies. The study uses Monte Carlo simulations of the dual-channel fading response functions based on a recently published statistical model. For each response pair generated, a receiver detection measure is derived analytically in terms of the system parameters and receiver approach. Probability distributions of this measure, obtained by simulating several thousand response pairs, are then computed. They can be interpreted as displaying the link outage probability as a function of the number of modulation levels (M). We find that the appropriate combining scheme can serve in some cases to avoid the need for adaptive equalization. Also, where post-combiner equalization is used, the same scheme, while no longer optimal, can sharply reduce the dispersion seen by the equalizer input.

Meteorological features leading to multipath propagation, observed in the PACEM 1 experiment

Abstract: Because multipath propagation constitutes a major source of impairments for line of sight radio links, the investigation of meteorological situations associated with it is of some importance. This paper describes the meteorological measurements carried out during thePacem Iexperiment and discusses their accuracy. The usefulness of acoustic soundings and the use of a ray-tracing program are specially detailed points. The great variety of the features encountered calls for a long time derivation of statistical properties, and for further experimental data taking into account the non horizontal homogeneity of the atmospheric boundary layer along the radio link.