Showing papers on "Multipath propagation published in 1992"

Distributed cochannel interference control in cellular radio systems

Abstract: Distributed power control algorithms that use only the carrier-to-interference ratios (C/I ratios) in those links actually in use are investigated. An algorithm that successfully approximates the behavior of the best known algorithms is proposed. The algorithm involves a novel distributed C/I-balancing scheme. Numerical results show that capacity gains on the order of 3-4 times can be reached also with these distributed schemes. Further, the effects of imperfect C/I estimates due to noise vehicle mobility, and fast multipath fading are considered. Results show that the balancing procedure is very robust to measurement noise, in particular if C/I requirements are low or moderate. However, for required high C/I levels or for a rapidly changing path loss matrix, convergence may be too slow to achieve substantial capacity improvements. >

Theory and Performance of Narrow Correlator Spacing in a GPS Receiver

Abstract: Historically, conventional GPS receivers have used 1.0 chip early-late correlator spacing in the implementation of delay lock loop s (DLLs), However, there are distinct advantages to narrowing this spacing, especially in C/A-code tracking applications. These advantages are the reduction of tracking errors in the presence of both noise and multipath. The primary disadvantage i s that a wider precorrelation bandwidth is required, coupled with higher sample rates and higher digital signal processing rates. However, with current CMOS technology, this is easily achievable and well worth the price. Noise reduction is achieved with narrower spacing because the noise components of the early and late signals are correlated and ten d to cancel, provided that early and late processing are simultaneous (not dithered). Multipath effects are reduced because the DLL discriminator is less distorted by the delayed multipath signal. This paper presents the derivation of these narrow correlator spacing improvements, verified by simulated and tested performance.

Mechanisms governing UHF propagation on single floors in modern office buildings

Abstract: Because of the potential implementation of indoor wireless local area networks (LANs) and personal communication networks (PCNs) it is important to understand propagation of signals in the UHF band inside buildings. The authors explore features of office buildings of modern construction that influence propagation between transmitter and receiver located on the same floor. One feature is the clear space between ceiling and furnishings or floor that results in excess attenuation of the signal. A second feature is reflection and transmission at interior and exterior walls. Diffraction at corners and propagation along the exterior wall are also shown to be a significant means for radiation to reach the receivers. The influences of the first two features are combined into a computer program that evaluates the sector average signal, which is then compared with measurements. >

Method and apparatus for multiplexing communications signals through blind adaptive spatial filtering

Abstract: A method and apparatus for spatial multiplexing of spectrally overlapping communications signals which does not require use of a training signal, computationally intensive direction-finding methods, or antenna calibration is presented. An adaptive antenna array at a base station is used in conjunction with signal processing through self coherence restoral to separate the temporally and spectrally overlapping signals of users that arrive from different specific locations within the locale and to mitigate multipath fading and shadowing at the base station and, by reciprocity, to transmit directively to minimize interfering signals arriving at the mobile (or portable or stationary) units and to mitigate multipath fading and shadowing at the mobile units. The radiation pattern of transmitted signal is matched to the adapted reception pattern of the signal received at the base station. BACKGROUND OF THE INVENTION The Government has rights in this invention pursuant to Grant No. MIP-88-12902 awarded by the National Science Foundation.

Vehicle position estimates by multibeam antennas in multipath environments

Abstract: Vehicle positions in multipath environments are estimated from the angle of arrival of waves received by multibeam antennas installed at two base stations. The estimated root mean square (RMS) position error is about 350 m. This can be reduced to about 200 m by using two kinds of processes: the averaging of time series data and the averaging of two sets of data. The position estimate error is caused by reflection from buildings near the vehicles, and a large error exceeding 500 m is caused by reflections from high buildings located some distance from the vehicle. Precise estimation of vehicle position in multipath environments by measuring angle-of-arrival is difficult because of building reflection. >

A versatile wave propagation model for the VHF/UHF range considering three-dimensional terrain

Abstract: A polarimetric wave propagation model for field strength forecasting and coverage prediction in the VHF/UHF frequency range is presented. The model uses a digital terrain data bank and considers multipath propagation. Based on the uniform geometrical theory of diffraction (UTD) and physical optics an approach is described for calculating the propagation effects in natural 3-D terrain, given by topological and morphographical data. The method for field strength forecasting is described and methods for the analysis of the predicted multipath signal are discussed. It is shown how the complex probability density function (PDF) for the receiver field strength and the field strength delay spectrum can be derived. Methods for further evaluation of the transmitting channel characteristics are discussed. >

A decision feedback equalizer with time-reversal structure

Abstract: This work describes the use of a receiver with a time-reversal structure for low-complexity decision feedback equalization of slowly fading dispersive indoor radio channels. Time-reversal is done by storing each block of received signal samples in a buffer and reversing the sequential order of the signal samples in time prior to equalization. As a result, the equivalent channel impulse response as seen by the equalizer is a time-reverse of the actual channel impulse response. Selective time-reversal operation, therefore, allows a decision feedback equalizer (DFE) with a small number of forward filter taps to perform equally well for both minimum-phase and maximum-phase channel characteristics. The author evaluates the theoretical performance bounds for such a receiver and quantifies the possible performance improvement for discrete multipath channels with Rayleigh fading statistics. Two extreme cases of DFE examples are considered: an infinite-length DFE; and a DFE with a single forward filter tap. Optimum burst and symbol timing recovery is addressed and several practical schemes are suggested. Simulation results are presented. The combined use of equalization and diversity reception is considered. >

Delay spread measurements for the digital cellular channel in Toronto

Abstract: This paper describes a set of measurements performed in five existing cells in the Toronto area, in order to assess the impact of multipath propagation on the performance of the CTIA IS-54 digital standard. A sweeping correlator apparatus with a 0.1 mu s resolution and 910 MHz carrier frequency was used. Generally, the measurements exhibit multipath propagation with considerably smaller excess delays than some reported recently but are consistent with earlier results obtained in the U.S. and Europe. In view of the results, it appears that the long delay (high selectivity) problem has been somewhat over-emphasized in the past, at least for areas like Toronto. Problems associated with short delays (flat fading), on the contrary, appear to have been underestimated, although these would be typical in any dense urban environment. >

Equalization for interference cancellation in spread spectrum multiple access systems

Abstract: The performance of a fractionally spaced decision feedback equalizer (DFE) for spread-spectrum multiple access (SSMA) systems, which does not make explicit use of the interferers' spreading sequences, is evaluated. It is found that the system performance is acceptable even for small spreading gains in a slow-fading multipath environment. Good performance has been achieved without the knowledge of the desired user's spreading sequence. This could lead to less complex receivers and higher capacity of users for SSMA than that of other access techniques. >

Multipath effects on GPS code phase measurements

Abstract: An analysis of multipath errors in GPS code phase measurements is presented, assuming the code phase is measured with the widely used noncoherent delay lock loop. Both specular and diffuse reflections are considered. A rough estimate for the fading bandwidth is given for mobile and stationary users, showing that averaging is far more effective for mobile than for stationary users. In general, however, a certain mean range error is present for both stationary and mobile users, depending on the power-delay profile of the received signal. Measurements near buildings confirm the asymmetry of multipath errors, which causes the nonzero mean, and show that range errors can take values of more than 100 m.

Radio frequency broadcasting systems and methods using two low-cost geosynchronous satellites

Abstract: High quality audio broadcasts at radio frequencies to mobile receivers at or near the earth's surface are provided by substantially simultaneous transmission of the same signal from two geosynchronous, spatially-separated satellites (#1 and #2) on the geosynchronous orbit which virtually eliminates multipath fading and foliage attenuation and thereby permits the use of a low-cost space segment.

Digitally implemented fast frequency estimator/demodulator for low bit rate maritime and mobile data communications without the use of an acquisition preamble

Abstract: A low-bit-rate, low-cost, all-digital preambleless demodulator for maritime and mobile data communications operates under severe high noise conditions, fast Doppler frequency shifts, large frequency offsets, and multipath fading. Sophisticated algorithms, including an FFT-based burst acquisition system, a cycle-slip resistant carrier phase tracker, an innovative Doppler tracker, and a fast acquisition symbol synchronizer, provide reliable burst reception. The compact DSP-based demodulator includes an input buffer receiving a complex sampled baseband input signal and providing a baseband output to a coarse frequency estimator fast Fourier transform (FFT) or discrete Fourier transform (DFT) module which produces a first estimation of the carrier frequency. A fine frequency estimator FFT or DFT module receives the first estimation and provides a second estimation of the carrier frequency. An extra coarse frequency estimator FFT or DFT module may be provided between the buffer and the coarse frequency estimator.

Apparatus for improved underwater acoustic telemetry utilizing phase coherent communications

Abstract: An underwater acoustic communications system utilizes phase coherent modulation and demodulation in which high data rates are achieved through the use of rapid Doppler removal, a specialized sample timing control technique and decision feedback equalization including feedforward and feedback equalizers. The combined use of these techniques drammatically increases data rates by one and sometimes two orders of magnitude over traditional FSK systems by sucessfully combating fading and multipath problems associated with a rapidly changing underwater acoustic channel that produce intersymbol interference and make timing optimization for the sampling of incomming data impossible.

Analysis of a multiple-cell direct-sequence CDMA cellular mobile radio system

Abstract: The performance of a multiple-cell direct-sequence code division multiple-access cellular radio system is evaluated. Approximate expressions are obtained for the area-averaged bit error probability and the area-averaged outage probability for both the uplink and downlink channels. The analysis accounts for the effects of path loss, multipath fading, multiple-access interference, and background noise. Two types of differentially coherent receivers are considered: a multipath rejection receiver and a RAKE receiver with predetection selective combining. Macroscopic base station diversity techniques and uplink and downlink power control are also topics of discussion. >

Superresolution of multipath delay profiles measured by PN correlation method

Abstract: Time resolution of multipath delay profiles measured by using the autocorrelation of a pseudonoise (PN) code sequence is generally limited by the chip interval of the PN code sequence. A superresolution PN correlation method (SPM) is proposed which improves the time resolution of delay profiles measured by the conventional PN correlation method. The SPM is based on a decomposition of the eigenvector space of the correlation matrix of the delay profile data vector and gives the number of paths and their delay times with higher resolution. It is verified by computer simulations and experiments using coaxial delay lines that the SPM can resolve two paths with a delay difference of a few tenths of the chip interval. The applicability of the SPM to the analysis of an indoor multipath environment in which many delayed waves arrive with short delay differences is demonstrated by an indoor radio propagation experiment at 2.3 GHz. >

Analysis of a High Performance C/A Code GPS Receiver in Kinematic Mode

Abstract: The results of a land semikinematic test, conducted under normal operational conditions to assess the performance of a multipath resistant, lo-cm C/A code accuracy, C/A code receiver technology introduced in 1991 , are presented. A semikinematic or stop and go test, where the vehicle stops at selected control points, was conducted to provide an accurate assessement of both the carrier phase and code measurements under static and kinematic conditions. The data was analysed with kinematic software packages developed at The University of Calgary, and used previously to assess other geodetic receivers. The control points calculated with the carrier phase measurements are in agreement at the cm-level with the values determined previously through a conventional static GPS survey. The double difference code noise was found to behave in a quasi-random manner, with an estimated standard deviation of 50 cm, with or without the use of an antenna ground plane to reduce multipath. The effective multipath rejection characteristics of this new receiver type is attributed to the use of a narrow correlator spacing in the code tracking loops. Kinematic positioning tests based on code and carrier phase-smoothed code measurements using the between-receiver single difference technique, which is well suited for real- time applications, resulted in one metre and 50 cm accuracies, respectively, in terms of rms in each of the three coordinates. These accurate results will permit the effective implementation of rapid carrier phase ambiguity resolution techniques for both the static and kinematic case.

Estimation of the parameters of a multipath channel using set-theoretic deconvolution

Abstract: A method for the estimation of the parameters of multipath communication channels is developed. The method makes use of matched filtering, set-theoretic deconvolution and autoregressive modeling. Pulse compression waveforms are used as channel probing signals. The results of a feasibility study in which the proposed method is applied to specular multipath channels to estimate the multipath parameters at a number of different signal-to-noise ratios (SNR) are presented. >

Hybrid DS/SFH-SSMA with predetection diversity and coding over indoor radio multipath Rician-fading channels

Abstract: The authors investigate the bit-error-rate (BER) performance of hybrid direct-sequence/slow-frequency-hopped spread-spectrum multiple-access (DS/SFH-SSMA) systems operating over a multipath Rician-fading channel (which models indoor radio propagation in factories). They consider both phase-shift-keying (PSK) modulation with coherent demodulation and differential phase-shift-keying (DPSK) modulation with noncoherent demodulation. Predetection multipath diversity (maximal ratio combining for coherent reception and equal gain combining for noncoherent reception) and simple interleaved channel coding are employed for improving the BER performance. The BER of both coherent and noncoherent hybrid systems is obtained using a Gaussian interference approximation. >

Timing and Automatic Frequency Control of Digital Receiver Using the Cyclic Properties of a Non-Linear Operation

Abstract: A technique for correcting the sampling time and carrier frequency error in a receiver for digitally modulated signals. Discrete-time, complex-valued samples of the incoming signal are fed to a pair of non-linear operators such as correlators. The first correlator provides a signal having a first phase component related to the symbol timing error and a second phase component related to the carrier frequency error. The second correlator provides a signal having a first phase component related to the symbol timing error and a second phase component related to the negative of the carrier frequency error. The phase components are then separated and detected to extract an estimate of the symbol timing error and the carrier frequency error. In the preferred embodiment, the complex-valued samples are frequency-shifted, before being fed to the correlators, so that the phase components of interest appear at zero frequency. The invention satisfactorily demodulates transmitted digital signals in applications, such as cellular time-division multiple access (TDMA), where they are susceptible to multipath fading.

Least‐squares time‐delay estimation for transient signals in a multipath environment

Abstract: The problem of estimating the arrival times of overlapping ocean‐acoustic signals from a noisy received waveform that consists of scaled and delayed replicas of a deterministic transient signal is considered. It is assumed that the transmitted signal and the number of paths in the multipath environment are known, and an algorithm is developed that gives least‐squares estimates of the amplitude and time delay of each path. A method is given to ensure that the global minimum of the error surface is found in spite of the existence of numerous local minima. The algorithm is then extended to the case in which the transmitted signal is not known precisely, but is assumed to belong to a parametric class of signals. The extended algorithm additionally obtains the parameters that characterize the transmitted signal. The algorithm is demonstrated on the class of signals consisting of gated sinusoids, using both simulated and experimental data.

ID microwave holographic sensor

Abstract: A method and apparatus for measuring both physical and electrical properties of a dielectric object using sequential spatial and spectral microwave data. The apparatus is a step CW microwave transmitter and receiver with a spatially modulated antenna positioned linearly in the Poynting vector direction. The method removes the antenna dispersion, removes the antenna image, transfers the image plane to the surface, removes diffraction and then removes the nuisance multipath by spatially integrating along the Poynting vector. The result is a clean complex surface reflection coefficient that contains only the subsurface or object plane information. This clean complex surface reflection coefficient may be compared to reflection coefficients of known objects in order to ascertain physical characteristics of the object.

Coherent and differentially coherent multiuser detectors for asynchronous CDMA frequency-selective channels

Abstract: The authors consider an asynchronous CDMA (code-division multiple-access) frequency-selective Rayleigh fading channel in which a central receiver has knowledge of the signature waveforms of all users. Low-complexity coherent and differentially coherent multiuser detectors are analyzed for this scenario. The receivers consist of a bank of matched filters followed by a multipath decorrelating detector, appropriate multipath diversity combining, and demodulation. The performance of the derived detectors is expressed by the single-user error rate. The conventional single-user RAKE receiver suffers from near-far effects in the multiuser scenario, due to the fading nature of the channel. This results in an error probability floor for high signal-to-noise ratios and limits the multiple-access capability of the system. In contrast, both multiuser detectors alleviate the near-far problem, providing significant performance improvement in the CDMA frequency-selective fading channel. >

Apparatus for multipath DSSS communications

Abstract: Circuit for recovery of a plurality of multipath direct sequence spread spectrum signal components received over a time span, wherein the multipath signal components are components of a signal generated from digital data comprised of data bits at a data bit rate modulated by a pseudo-noise code, consisting of a sequence of chips having a chip rate greater than the data bit rate, wherein the sequence of chips is a code repetition period equal or greater than the received time span of the multipath signal components. A plurality of correlators is provided. Each correlator has a first input for coupling to the multipath signal components, a second input, and an output. A pseudo-noise code generator generates a duplicate of the pseudo-noise code. A time delayer is coupled between the second inputs of a mulitiplicity of the correlators and the pseudo-noise code generator for delaying the duplicate of the pseudo-noise code by a multiple of a time delay equal to the time of a chip duration or less. A signal combiner is coupled to the plurality of phase correcting devices.

Multiplication of cellular radio capacity by blind adaptive spatial filtering

Abstract: A scheme for cellular radio which greatly increases the allowable number of simultaneous users in each cell is proposed. The scheme employs space-division multiplexing by means of an adaptive antenna array at the base station (1) to separate the temporally and spectrally overlapping signals of users that arrive from different locations within the cell and to mitigate multipath fading and shadowing at the base station and (2) by reciprocity to transmit directively to minimize interfering signals arriving at the mobile units and to mitigate multipath fading and shadowing at the mobile units. A novel aspect of the proposed scheme is that no training signals are required for adaptation, nor are computationally intensive direction-finding methods required, nor is any calibration data required for the array. Results from computer simulations on the performance of the new scheme show that the proposed scheme can accommodate up to five times as many users as a proposed CDMA scheme using the same system bandwidth. >

Statistical modeling of the indoor radio propagation channel. II

Abstract: For pt.I see ibid. (1992). Statistical analyses of indoor radio propagation data are reported. The database includes 12000 impulse response profiles of the channel collected at two office buildings. Statistical properties of the number of multipath components for each profile are described. A Poisson fit to the arrival time data proved to be unsatisfactory. A modified Poisson model showed good fit to the data. Amplitudes were found to be lognormally distributed over both local and global areas, with a log-mean value that decreases almost linearly with increasing the excess-delay. There is a high degree of spatial correlations on path variables which has been quantified for the amplitude sequence. >

Passive Ranging in Multipath Dominant Environments: Part II-Unknown multipath parameters

Abstract: Passive ranging accuracy using arrays deteriorates at long ranges because of the vanishing wavefront curvature observable by an array The idea of including a priori knowledge of propagation conditions in ranging procedures for the purpose of improving the range estimate has received considerable attention in recent years The interest is based on an observation that a large propagation delay measured between two coherent arrivals at a single receiver is equivalent geometrically to a delay generated by a single arrival in propagating to two widely spaced receivers If the equivalent sensor spacing is on the order of or larger than the dimensions of an actual array, multipath ranging appears to be an attractive alternative to conventional curvature measurement approaches This paper considers the potential performance improvements attainable in the process of exploiting a second path in long range ranging The performance of line arrays is pursued in detail for a problem setting in which all of the parameters describing the signal and channel parameters are known a priori >

Space and frequency diversity measurements of the 1.7 GHz indoor radio channel using a four-branch receiver

Abstract: The practical space and frequency diversity performance achievable inside a building at 1.75 GHz under fading conditions due to the motion of a portable terminal and due to the movement of people are investigated. Data are collected using a four-branch dual-frequency envelope receiver positioned throughout one floor of a university building of common construction type. The measurement environment is characterized for large-scale path loss and wall transmission loss. Envelope cross correlations are calculated, and performance of diversity is measured for various frequency separations and antenna spacings which would be applicable given the physical size of portable telephones and data terminals. Two-branch space diversity is directly compared to two-branch frequency diversity and to four-branch hybrid diversity based on simultaneous measurements of each using selection combining. The distributions of correlations and diversity gain at different locations are also investigated. Results indicate that two- and four-branch diversity can be a very effective way to combat signal fading for portable terminals in an indoor radio environment. >

Internetworking wireless terminals to local area networks via radio bridges

Abstract: The author presents a novel distributed architecture for wireless local area networks (WLANs), where each wireless terminal internetworks with a backbone local area network (LAN) via several radio bridges. The site diversity affords some protections against slow fading and shadowing in the indoor environment. Multiaccess interference from multiple sites sharing the same radio channel is minimized by exploiting the capture effect of spread-spectrum signalling, which also gives additional protections against interference from multipath signals. The internetworking protocol employs a self-learning routing algorithm, which automatically adapts to changes in terminal locations, and prevents multiple copies of each data frame from being forwarded over the backbone LAN. The distributed WLAN architecture eases the management of network topological changes and terminal mobility relative to centralized cellular architectures. >