Showing papers on "Multipath propagation published in 1991"

The mobile radio propagation channel

Abstract: Fundamentals of VHF and UHF propagation propagation over irregular terrain propagation in built-up areas area coverage and planning tools characterisation of multipath phenomena wideband channel characterisation other mobile radio channels and methods of characterisation sounding sampling and simulation man-made noise and interference multipath mitigation techniques

The land mobile satellite communication channel-recording, statistics, and channel model

Abstract: The communication channel between the MARECS satellite at 26 degrees W and a cruising van was measured and recorded in European areas exhibiting satellite elevations from 13 to 43 degrees . Different environments and mobile antennas were tested. The results of an extensive statistical evaluation include spectra of the fading amplitude; probability density, and distribution of the received signal power; and the percentage of time for fade and nonfade periods. Based on the physical phenomena of multipath fading and signal shadowing, an analog model of the land mobile satellite channel which can readily be used for software and hardware fading simulation is developed. The most important parameter of this model is the time-share of shadowing. The Rice factor which characterizes the channel during unshadowed periods, can vary from 3.9 to 18.1 dB. Block error probability density, error gap distribution, and block error probability are discussed. >

Cdma microcellular telephone system and distributed antenna system therefor

Abstract: A code division multiple access (CDMA) communication system in which cellular techniques are utilized in a wireless Private Branch Exchange (PBX) environment. A microcellular arrangement is defined in which a base station (10) communicates user information signals using CDMA communication signals with subscriber terminals. A distributed antenna system (26) is utilized in the system to provide multipath signals which facilitate signal diversity for enhanced system performance.

Statistical channel impulse response models for factory and open plan building radio communicate system design

Abstract: Statistical radio channel impulse response models are presented for the analysis and design of wireless factory and open plan office communication systems. The models incorporate first- and second-order statistics to characterize the discrete impulse responses of indoor radio channels for both line-of-sight (LOS) and obstructed (OBS) topographies. The effects of large-scale transmitter-receiver separation distance, small-scale receiver movement, and models for the correlation of multipath component amplitudes over 1 m local areas are developed from 1.3 GHz measurements. SIRCIM, a computer simulator based on the models presented, has recreated multipath power delay profiles and CW fading profiles that are representative of measured data. Large-scale models for path loss are implicitly included in this work. >

Adaptive equalization for TDMA digital mobile radio

Abstract: Adaptive equalization for a TDMA (time-division multiple-access) digital cellular system is discussed. A survey of adaptive equalization techniques that includes their performance characteristics and limitations and their implementation complexity is presented. The design of adaptive equalization algorithms for a narrowband TDMA system is considered. It is concluded that, on the basis of implementation complexity and performance in the presence of multipath distortion and signal fading, MLSE (maximum-likelihood sequence estimation) and DFE (decision feedback equalization) are viable equalization methods for mobile radio. >

Signal and interference statistics of a CDMA system with feedback power control

Abstract: A feedback power control approach that allows power commands to be updated at a higher rate than the rate of multipath fading is investigated. The signal and interference statistics as received at the base stations after power control are obtained for a simulated direct-sequence code-division multiple-access (CDMA) system, which includes multiple base stations with diversity receivers and a large number of power-controlled users continuously moving at various speeds. It is shown that often-used analyses based on perfect average power control lead to optimistic capacity results (by 25% to 60%) because interference is underestimated by 1 to 2 dB. >

Characterisation of mobile radio signals: model description

Abstract: Following a brief review of multipath propagation models, the major characteristics of mobile radio signals are determined using an adaptation of the generic three-dimensional model proposed by Aulin. The various multipath component waves, which have equal amplitude and random, uniformly-distributed phase are allowed to have spatial angles of arrival with both vertical and horizontal components. The spatial angle of arrival in the horizontal plane is assumed to be uniformly distributed in the interval (0, 2π) but a new PDF is proposed for the vertical angle of arrival, β. The effective scatterers are assumed to form a cyclinder around the mobile with a radius and height that depends on the maximum value βm of the vertical angle of arrival. Suitable values for βm are estimated from published experimental results. The limitations of two-dimensional scattering models are illustrated by considering not only the power spectral density, but also the cross correlation between antennas spatially separated in the vertical plane.

A digital DS spread-spectrum receiver with joint channel and Doppler shift estimation

Abstract: A digital spread-spectrum receiver design is presented for communication over multipath channels with severe Doppler shifts. The characteristics of the underwater channel relevant to spread-spectrum system design are discussed, and a channel model for short-range communications (less than 10 km) is defined. The receiver considered uses a digital coherent RAKE combiner, coupled with an extended Kalman filter (EKF)-based estimator for channel parameters and pseudonoise code delay. Receiver performance is evaluated by computing average bit-error rate (BER) versus iterations of the EKF joint estimator, using both fixed and time-varying channels. It is shown that the BER obtained using the EKF joint estimator closely tracks the optimum BER obtained when the channel, delay, and Doppler parameters are known exactly. Finally, the Cramer-Rao lower bound for time-invariant joint channel, delay, and Doppler estimation is derived, and compared with the ensemble averaged mean-squared error of the EKF estimator. >

Broadband CDMA for personal communications systems

Abstract: Multipath propagation in a broadband CDMA environment is described. A propagation model for broadband spread-spectrum signals is presented. Experimental results relating to the sharing of the band by fixed service microwave users and mobile personal communications network (PCN) users are discussed. Field tests indicate that PCN systems can provide high-quality communications when sharing the spectrum with fixed-service microwave systems in suburban and urban areas. >

MLSE equalization and decoding for multipath-fading channels

Abstract: The performance of a receiver using a combined MLSE (maximum likelihood sequence estimation) equalizer/decoder and D-diversity reception is analyzed for multipath Rayleigh fading channels. An upper bound on the (decoded) bit error probability is derived. Comparisons to simulation results show that this upper bound is quite tight when the system has a high signal-to-noise ratio or when diversity reception is used. The upper bound involves an infinite series that must be truncated at a point where the remainder can be safely assumed to be small. An algorithm based on a one-directional stack algorithm is proposed for this calculation because it makes efficient use of computer memory. >

Parabolic equation modelling of the effects of multipath and ducting on radar systems

Abstract: The parabolic equation approach to the modelling of clear-air propagation, and in particular to the assessment of radar performance in the presence of multipath and ducting, is described. The derivation and solution of the parabolic equation by means of the split-step algorithm are described. A discussion of the radar equation is given to provide a connection with conventional assessment methods. The model is applied to the problem of anomalous propagation and the influence of the ground. One of the main advantages of the parabolic equation approach over alternative ray and mode methods is its ability to make quantitative predictions for two-dimensional refractive-index structures without the need for a priori identification of the structures. New applications, such as wideband effects and the influence of refractive-index fluctuations and measurement errors on prediction accuracy, are discussed. Clutter modelling and other extensions to the model are briefly mentioned.< >

Radio propagation and anti-multipath techniques in the WIN environment

Abstract: The unique characteristics of wireless in-building network (WIN) systems based on microwave radio technology are addressed, focusing on the multipath propagation problem which most distinguishes radio from wired LAN media. This is followed by a discussion of the three principal techniques that have been developed within the radio communication industry to deal with this multipath problem: adaptive equalization, spread spectrum, and antenna diversity. A review of the strengths and weaknesses of each of these techniques reveals that antenna diversity is clearly the preferred choice for use in WIN systems operating in the recently allocated spectrum near 18 GHz. An example of such an antenna system is examined. It is shown how these multiple, directional antenna systems function in three types of environment. >

An experimental TDMA indoor radio communications system using slow frequency hopping and coding

Abstract: The general principles, implementation, and performance of a 1-Mb/s time-division-multiple-access (TDMA), slow frequency hopping and coding, 1.5-GHz radio communications system for a medium-sized office building are discussed. TDMA is provided for service flexibility and slow frequency hopping and coding for immunity against multipath fading and interference. Measurements show that, with a hallway-mounted distributed antenna system installed on one floor of the building and with the mobile unit transmitting only 1 mW of peak RF power from anywhere on that floor, only a single 384-b frame out of a total of 200000 transmitted frames suffered an unrecoverable error. This kind of performance is comparable to that of wired data modems. Similar performance was obtained from a central antenna covering the same floor. However, the required transmitted power in this case was 100 mW. Implementation issues, such as spectrum allocation, cellular subdivisions, and spectrum efficiency, are also discussed. >

Fast adaptive equalizers for narrow-band TDMA mobile radio

Abstract: It is pointed out that the future European cellular digital mobile radio system in the 900 MHz band adopts a narrowband time division multiple access (TDMA) scheme with Gaussian minimum-shift keying (GMSK) modulation and burst type transmission. Consequently, very fast adaptation methods are necessary to cope with the time- and frequency-selective distortions produced by Rayleigh and multipath fading. The authors examine a few theoretical aspects of the application of recursive least squares (RLS) adaptation algorithms to the narrowband TDMA mobile radio system and give the relevant performance results for the fast Kalman algorithm, which turns out to be suitable for the considered application. In particular, signature curves, bit error rate, speed of convergence, steady-state behavior, numerical stability, required accuracy, and hardware complexity are discussed. Linear transversal and nonlinear decision-feedback equalizers are considered. >

Use of a highly deterministic multipath signal model in low-angle tracking

Abstract: A maximum-likelihood estimation technique based on deterministic modelling is presented. The algorithm is designed to be used specifically for radar tracking of low-angle targets or radar direction-finding in a naval environment. In this algorithm, a highly deterministic multipath signal model is used for formulating the likelihood function, which is maximised with respect to the angles of arrival. In addition, the Cramer-Rao bounds that apply specifically to this model are derived. It is shown, by using both simulated data and real data, that this technique provides excellent performance in resolving multiple signals. The measured data that are used in this study were recorded using a 32-element sampled aperture antenna on an 'overwater' path.

Statistical modelling and computer simulation of indoor radio channel

Abstract: Multipath profiles obtained from radio propagation measurements at 910 MHz are used to analyse the statistical characteristics of the indoor radio channel. The data base is divided into two classes: manufacturing floors and college offices. In the manufacturing floors, there is plenty of open space without any presence of walls, as a result of which most of the received power is concentrated in the initial paths. The college office areas have a wider spread of power in delay, because of less open space and the frequent obstruction of the signal between the transmitter and the receiver by one or more walls. The statistical parameters required for computer simulation of multipath profiles in each environment are determined. The arrival of the paths is shown to fit a modified Poisson process and the amplitude of the paths follow a log-normal distribution. The mean and the standard deviation of the lognormal distribution are shown to decay exponentially with delay. To evaluate the performance of this simulation model, the distribution of the RMS delay spreads from the simulated profiles is compared with that obtained from the measured profiles.

A statistical model for the factory radio channel

Abstract: A statistical model, based on experimental data, is presented to characterize the multipath propagation medium in typical manufacturing environments. The statistical behavior of the medium is described by a time-varying impulse response which includes the distribution of the parameters of the channel such as the gain, the interarrival time, and the number of paths. The resolvable components in a number of experimentally measured pulse responses of indoor factory radio channels were used to estimate the channel statistics. The interarrival times of the signals were modeled by the Weibull distribution. A model for the distribution of signals was presented using the modified beta distribution. The path gain coefficients were represented using the Rayleigh, Rician, or log-normal distributions. The model for the factory channel is flexible enough to fit the data for other indoor sites by changing the parameters of the factory channel appropriately relative to the size, topography, and other structural features of the site. >

Performance of an RCPC-coded OFDM-based digital audio broadcasting (DAB) system

Abstract: A system proposal for DAB is investigated. The kernel is orthogonal frequency division multiplexing (OFDM) with 4-DPSK (differential phase shift keying) modulation, rectangular pulse-shaping, and a guard interval to reject multipath distortions. Emphasis is on the use of source-adapted channel coding with rate-compatible punctured convolutional (RCPC) codes. Based on analytical and simulated BER (bit error rate) curves for several propagation conditions and on preliminary source significance information (SSI), a static code design with unequal error protection (UEP) is presented that also takes auxiliary data services into account. The gain due to UEP is on the order of 8dB in signal power or 25% in bandwidth. >

Wireless propagation measurements in indoor multipath environments at 1.7 GHz and 60 GHz for small cell systems

Abstract: Given the current interest being shown in the utilization of the frequency band around 1.7/1.8 GHz and 60 GHz, work has been undertaken to determine channel characteristics in these two ranges. The authors describe wideband measurements which have been conducted in a variety of indoor locations. RMS (root mean square) delay spreads were calculated and used to compare levels of multipath activity at the two frequencies. The levels of multipath measured at the two frequencies can be related to the size of the rooms involved and the measurement techniques. Any differences due to atmospheric absorption at 60 GHz were found to be negligible. >

Fiber-reflection-induced impairments in lightwave AM-VSB CATV systems

Abstract: Impairments due to multiple fiber reflections are evaluated for amplitude-modulated (AM) vestigial-side-band (VSB) lightwave cable TV (CATV) systems using directly modulated distributed feedback (DFB) lasers or externally modulated diode-pumped YAG laser transmitters. For DFB lasers, the spectral broadening caused by laser chirp results in a Gaussian optical spectrum with a width near 5 GHz. Square-law detection of multipath signals from two -30-dB reflections results in an effective intensity noise near -150 dB/Hz. For externally modulated YAG lasers, the same multiple reflections result in interference that cannot be described by an effective RIN. Although the total interference power is comparable to that of the DFB lasers, the signal and interference are highly correlated. As a result of this correlation, multipath interference for YAG-based systems cannot be measured using the standard test procedure with unmodulated carriers. >

Comparative performance study of several blind equalization algorithms

Abstract: This paper examines the transient and steady-state characteristics of several Bussgang-type blind equalization algorithms. A combination of computer simulations and analysis is used to assess the relative performance of the various algorithms. The computer simulations involve channel characteristics typical of those found in an urban multipath environment, and they include the effects of frequency offset. The equalizer structures considered in this paper are comprised of a T/2 fractionally-spaced linear finite impulse response filter. The analysis of misadjustment is based on an approximate Gaussian model of the data.© (1991) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Channel-estimation-based adaptive DFE for fading multipath radio channels

Abstract: The performance of an adaptive decision feedback equaliser (DFE) over an HF channel when its taps are derived from an estimate of the sampled channel impulse response is examined. The conventional approach to adaptation is to adjust the equaliser taps directly to minimise a least squares error cost function. It is shown that the channel estimate approach not only yields superior performance, but also involves fewer computations than the conventional square-root-Kalman approach. The problem of channel estimation is mathematically analysed for both the steepest descent and recursive least squares algorithms, and theoretical results are compared with simulation. It is also shown how simple theoretical predictions of the performance of the DFE can be made when using this alternative method of implementation.

Cramer-Rao bound for location systems in multipath environments

Abstract: The Cramer-Rao lower bound for source localization is studied in the context of multipath stochastic sources, multipath propagation, and observations, in an array of sensors. A general expression is derived and then specialized to simpler configurations and related to results previously reported in the literature. The special case of a single stochastic source in a multipath environment is treated. The relative importance for source localization of the temporal (multipath) and spatial (array baseline) structures of the incoming wavefield is assessed. It is shown that for an array of K sensors the multipath contribution to the Fisher information matrix can be interpreted as the contribution of K independent arrays whose size depends on the number of spatially resolved replicas. The degradation due to unknown source spectra is analyzed. When the source spectrum is completely arbitrary, source location is not possible with a single sensor. If a parametric form of the source spectrum is available, the multipath structure can be used to locate the acoustic source. >

Channel modelling of short-range radio links at 60 GHz for mobile intervehicle communication

Abstract: Within the European PROMETHEUS research program, the DLR has the task of investigating the 60 GHz link and developing a channel model. Some propagation effects for the intervehicle radio link, such as power law with distance, oxygen and rain absorption, edge diffraction, and frequency- and time-selectivity, are theoretically examined. The simple two-path propagation model is analyzed for this application and compared with measurement results. The model is then expanded to multipath propagation and compared with a measurement. The minima of the two-path model are filled up by the multipath signal, but the fade durations are still on the order of the suggested packet length. >

Performance of a CDMA radio communications system with feed-back power control and multipath dispersion

Abstract: An investigation is made of the performance of a CDMA (code-division multiple access) radio system with feedback power control. Feedback power control delay, power delay profile and channel fading rate are used as parameters to characterize the channel performance. Tradeoffs between the use of single and multiple correlators per antenna for the multipath dispersive channel are shown. It is found that in some delay-spread environments, it may be better to turn off one of the correlators or lock all the correlators onto the strongest path. >

Computer-based bit error simulation for digital wireless communications

Abstract: A communications stimulation system allows a user to perform a quantitative or subjective test of digital baseband devices over wireless channels using actual measured or modeled propagation data. The digital wireless communication simulation system is capable of simulating the transient nature of channels and radio hardware so that loss of synchronization can be included in the simulation. The simulator is a combination of computer software and hardware that computes a convolution, in the time domain, of a sequence of binary digits or data symbols (i.e., the data stream) with a computer model of a radio transmitter, a propagation channel or channels and a receiver. The transmitter typically comprises a coder, a pulse shaper, a modulator, and a spreader. The propagation channel or channels may include impulsive and average noise levels, co-channel interference and adjacent interference levels, fading and multipath propagation events, and non-linear channel and radio system effects. The receiver system typically comprises at least a filter bank, a demodulator, a despreader, a synchronizer, a detector, and a decoder. The data stream may either be random or applied by the user. The software computes the bit-by-bit sequence for replay at a later time. Once stored, the bit-by-bit error sequence can be clocked through a hardware data port and compared with an applied data stream in real time. The output of the hardware data port is a real time sequence of bits that has errors due to the bit-by-bit simulation computed earlier by the software.

Direction-of-arrival estimation in applications with multipath and few snapshots

Abstract: This paper analyzes two direction-of-arrival (DOA) estimation algorithms used in the presence of multipath propagation and with very few snapshots. The conditional maximum likelihood (CML) algorithm and the method of direction estimation (MODE) are discussed. The estimates provided by these algorithms are shown to coincide for large number of snapshots or large signal-to-noise ratio. Necessary and sufficient conditions are derived for the algorithms to yield unique estimates. It is shown that their uniqueness conditions coincide with the minimal uniqueness condition on the array, that is independent of the algorithm used (if the array does not satisfy this minimal condition, no DOA estimation method can give unique estimates). Numerical examples are presented to demonstrate the theoretical results.

Space-time processing, environmental-acoustic effects

Abstract: The authors examine the subject of space-time processing and review fundamental environmental effects and their influence on arrays in the deep ocean sound channel. Space-time transforms are reviewed to demonstrate the analogy between spatial and temporal properties to stress the importance of convolution and matched field processing. A criterion is presented by which the resolution of such measurement systems could be calculated. The static source-receiver case is shown to be influenced by the randomness in signal phase due to scattering. Calculations and data are used to show the importance of multipath effects on the relative gain of line array measurement systems and the difficulties encountered for the determination of coherence lengths. Single path coherence lengths were found to be large and predictable using an environmental parameter and the Beran-McCoy mutual coherence functional form. However, multipath effects appeared to be dominant. The temporal fluctuation problem is briefly introduced to stress the fact that for relative source-receiver speeds of 1.5 m/s (3 knots) or greater, the fluctuations are dominated by the changes in the multipath arrivals. >

Pulse ranging radiolocation technique and its application to channel assignment in digital cellular radio

Abstract: A sophisticated simulation package for automatic vehicle monitoring (AVM) systems using a pulse ranging technique has been developed. Using a multipath propagation database, location-error statistics have been obtained for mobile units operating in areas with various degrees of urbanization. The effect of the number and relative position of sensors on location-error statistics has been studied in detail. The performance of various linear and nonlinear optimization techniques has been tested by computer simulation. Application of the package to locating vehicles and portables in digital cellular radiotelephone systems for the purpose of channel assignment and handoff supervision has been studied. It is shown that such techniques are quite feasible even in the worst multipath propagation conditions. >

Cancellation of specular and diffuse jammer multipath using a hybrid adaptive array

Abstract: It has been demonstrated that specular or diffuse jammer multipath can be canceled to a desired level by using an adaptive array that combines bandwidth partitioning with tapped delay lines. Such hybrid systems are studied. In particular, the author studies an ideal two-element array that uses bandwidth partitioning in both the main and auxiliary channels, with an Mth-order adaptive finite impulse response filter in each subband of the auxiliary. The ability of this system to cancel specular moderately diffuse and diffuse multipath is studied. The combinations of bandwidth partitioning and filter order that can achieve a specified jammer cancellation level are discussed. >