Showing papers on "Multipath propagation published in 1988"

An improved spatial smoothing technique for bearing estimation in a multipath environment

Abstract: It is well known that signal subspace algorithms perform poorly when coherent or highly correlated signals are present. Recently, the so-called spatial smoothing technique was devised to preprocess the array covariance matrix so that signal subspace algorithms can be applied irrespective of the signal correlation. Unfortunately, the application of this technique reduces the effective aperture of the array. A modified spatial smoothing technique that is capable of increasing the effective aperture of the array over that of conventional spatial smoothing methods is explored. It is shown that under certain conditions, the modified algorithm may fail to yield the desired increase in array aperture, and some simulation results concerning the sensitivity of the modified spatial smoothing algorithm to these conditions are provided. >

Propagation characteristics on microcellular urban mobile radio channels at 910 MHz

Abstract: To mitigate fading and reduce time dispersion on urban mobile radio channels, it has been proposed that future systems be configured with many small cells having low-powered base stations with street-lamp-level antennas. The results of measurements made to determine propagation characteristics on urban mobile radio channels with low-base-station antennas and line-of-sight between the base and mobile units are reported. Cumulative distribution functions for envelope fading as well as delay spread and frequency correlation statistics are presented. Comparisons are made with similar statistics for conventional channels. Results show that multipath propagation conditions would be significantly less severe if small-celled systems were implemented. Root mean square delay spread averages are reduced by a factor of approximately four in comparison with those typical in conventional systems. In addition, microcellular-type channels have Rician, rather than Rayleigh, envelope fading characteristics, and correspondingly different frequency correlation statistics, which offers further advantages. >

On carrier signal multipath effects in relative GPS positioning

Abstract: Discusses errors in GPS carrier phase observations resulting from multipath interference. A mathematical model for multipath induced carrier phase errors is derived. It is shown that multipath can be monitored over short baselines by analyzing dual frequency measurements. Two related experiments are reported. The first example demonstrates that extreme multipath interference can render useless GPS observations for precise positioning applications. The 2nd example shows the danger of occurrence of long periodic multipath errors caused by conducting material in the immediate vicinity of the receiver antenna.

High bit rate communication system for overcoming multipath

Abstract: A communication system for relatively high data bit rate RF communication overcomes multipath interference by employing relatively narrow beam antenna sectors and by selecting the best communication path established between two terminals, at least one of which has narrow beam antenna sectors. The communication path selection process includes determining the signal integrity of data communicated between the terminals. One implementation includes the narrow beam antenna sectors at each terminal.

Phase coherent TDMA quadrature receiver for multipath fading channels

Abstract: A method and apparatus for phase-coherently demodulating a multipath-impaired time division multiple access QPSK data timeslot is disclosed. A quadrature separator (105, 107, 111) generates multipath-impaired intermediate signals which, during a predetermined synchronizing sequence for the timeslot, are applied to a pair of synchronizing correlators (303,305) to generate quadrature multipath profiles. These profiles then are used to modify subsequently received QPSK timeslot signals at the separator outputs to coherently construct multipath compensated I and Q channel data.

Analysis of the time variant structure of microwave line-of-sight multipath phenomena

Abstract: The authors report a novel technique, the SVDP (singular value decomposition prony) algorithm, which has demonstrated its superiority in identifying the multipath rays in line-of-sight microwave links. The technique has been applied to field data and it has proved itself to be a very useful tool in analyzing the time-variant structure of the actual physical mechanism, i.e. the evolution of the delay and amplitude of the multipath rays. The authors present the results of analysis of field measurements made at 11 GHz over a wide bandwidth of 400 MHz on the French CNET at Lannion, using a 50 km link. The major applications of this technique are: the modelling of a multipath channel using the least possible number of rays with the objective of minimizing the RMS error; the analysis of the time-variant structure of a channel and hence the design of adaptive equalizers; and the generation of the statistics of the time-domain multipath parameters, particularly the delay and amplitude distributions. >

Measurement and modeling of land mobile satellite propagation at UHF and L-band

Abstract: A propagation experiment is described in which a stratospheric balloon served as a transmitter platform at 870 and 1502 MHz in simulation of a land mobile satellite. A vehicle followed the drifting balloon along roads of western Texas and New Mexico, collecting at L-band amplitude and phase, and at UHF amplitude information only for elevation angles between 25 degrees and 45 degrees . The data obtained has been analyzed and is presented along with results from modeling of multipath scattering and roadside tree attenuation. The signal, with variations caused by multipath propagation and tree shadowing, was reduced by 3 dB at L-band and 2 dB at UHF for one percent of all locations. A median ratio of 3.9 was found between peak-to-peak phase (degrees) and power (dB) fluctuations. The ratio between L-band and UHF dB attenuation averages varied from 1.3 to 1.0 at fade levels from 6 to 23 dB. Optical sky brightness was measured and used to predict fade distribution with great accuracy. A single-scatterer multipath model is introduced. It is used to duplicate some of the measured data and to show the dependence of power variations on satellite elevation angle. Using Fresnel diffraction theory, the attenuation caused by a model tree was calculated to be near 10 dB and the maximum fade was found to increase by the logarithm of the number of branches. >

Multipath reduction system

Abstract: The effects of multipath and other interference signals in communication receivers are reduced by using adaptive signal processing. The feedback of the adaptive signal processor (104) is changed so that an external reference signal is not required. The invention addresses a signal environment in which the directions of arrival and the times of arrival of the signal of interest and the unwanted multipath or interference signals are unknown. The system uses the strongest received signal as the signal of interest.

Performance of differentially coherent digital communications over frequency-selective fading channels

Abstract: The performance of binary differential phase-shift keyed (DPSK) communications over frequency-selective wide-sense-stationary uncorrelated-scattering fading channels is considered. A technique is described for obtaining bounds on the average error probability for DPSK in terms of one or two parameters obtained from multipath spread or frequency-correlation-function channel measurements. A method of approximating the performance of systems using various data-pulse shapes and systems where it is difficult to obtain a complete channel characterization is developed. Numerical examples and comparisons of the most interesting cases are given. >

High-resolution multipath time delay estimation for broad-band random signals

Abstract: The resolution limits to multipath time delay estimation for broadband random signals are examined. First, appropriate Cramer-Rao lower bounds (CRLBs) are derived to establish baseline performance for unbiased estimation. The bounds are then compared to computer simulation results. For the two-path case, a maximum-likelihood estimator is implemented, while for the three-path case, the modified forward-backward linear prediction algorithm developed for high-resolution frequency estimation is used. It is shown that both of these techniques can achieve performance close to the appropriate CRLB; in particular, reliable multipath estimation can be achieved even when the multipath time delays are much closer than could be resolved by conventional autocorrelation processing. A criterion is developed for predicting when the bound is no longer reachable. It is shown that even relatively poor a priori knowledge of the attenuation can significantly help the estimate of time delay for small delays. >

Accuracy of source localization using multipath delays

Abstract: The depth and range of underwater source can be estimated from measurements of propagation delay differences along multiple propagation paths. The accuracy of depth and range estimation using the Cramer-Rao lower bound is studied. The formulas derived can be used in conjunction with a propagation medium (nonconstant velocity profile). Explicit formulas for the bounds are derived for the case of homogeneous medium (constant velocity profile). Numerical examples are presented to illustrate the behavior of these bounds. >

Fade measurements at L-band and UHF in mountainous terrain for land mobile satellite systems

Abstract: Fading results related to land mobile satellite communications at L-band (1502 MHz) and UHF (870 MHz) are described. These results were derived from an experiment performed in a series of canyon passes in the Boulder, Colorado region of the USA. The experimental configuration involved a helicopter as the source platform, which maintained a relatively fixed geometry with a mobile van containing the receiver and data-acquisition system. An unobstructed line of sight between the radiating sources and the receiving van was, for the most part, also maintained. In this configuration, the dominant mechanism causing signal fading (or enhancement) is a result of multipath. The resulting fade distributions demonstrated that at the 1% and 5% levels, 5.5 and 2.6 dB fades were on the average exceeded at L-band and 4.8 and 2.4 dB at UHF, respectively, for a path elevation angle of 45 degrees . The canyon results as compared with previous roadside-tree-shadowing results demonstrate that the deciding factor dictating fade margin for future land mobile satellite systems is tree shadowing rather than fades caused by multipath. >

Tdma radio system employing bpsk synchronisation for qpsk signals subject to random phase variation and multipath fading

Abstract: A time division multiple access (TDMA) radio system conveying a quadrature phase shift keying (QPSK) modulated data signal is disclosed. In order to overcome multipath distortion and phase variation introduced by the radio channel, a synchronizing poriton of a TDMA timeslot is transmitted in only one vector of the QPSK modulation. The system utilizes the one vector synchronizing portion to develop a channel profile estimate which is applied to the received message portion of the QPSK data signal to compensate for the multipath distortion and phase variation during one TDMA timeslot.

Joint Spatial And Temporal Equalization In A Decision-directed Adaptive Antenna System

Abstract: This paper describes two methods for jointly optimizing the weights of a single-weight per channel adaptive diversity combiner followed by a fractionally-spaced equalizer. It is shown that such a system provides an efficient and effective means of mitigating multipath distortion and cochannel interference for a wide variety of digital communication applications. Both "blind" and decision-directed weight-update algorithms are given.

Delay-Doppler resolution performance of large time-bandwidth-product linear FM signals in a multipath ocean environment

Abstract: Active sonar systems that transmit large time‐bandwidth (TW) product linear frequency‐modulated (LFM) waveforms and receive echoes from targets of unknown speed can suffer considerable correlation losses that cannot be predicted from conventional (narrow‐band) ambiguity function theory. As is well known, the theory can be modified to include the effects of Doppler distortion on large TW‐product signals by correlating the received signal against a reference that is a time‐compressed version of the transmitted signal. In this paper, the effects of multipath (or target highlight structure) and Doppler on the correlation process for rectangular weighted large TW‐product LFM waveforms are examined. When the received echo contains no multipath, the correlator peak output is a maximum for the reference channel that is closest in Doppler to the target. However, in a multipath environment, the correlator output peak does not generally occur at the correct Doppler reference channel. The spreading of the cross‐ambiguity function as a result of Doppler mismatch and multipath is calculated in terms of the delay‐Doppler coordinates and magnitude of the extrema. It is shown that, under multipath conditions, correct estimates of target parameters can be recovered through integration of the correlator output.

Diversity protections for digital radio-summary of ten-year experiments and studies

Abstract: A summary is presented of a series of critical experiments that led to recent discoveries of large improvement factors for digital radio performance by antenna pattern diversity, antenna angle diversity, frequency diversity, and space diversity. The measured diversity improvement factors for digital radio against multipath dispersive fading are larger than those predicted by the existing analog radio model by at least one order of magnitude. Applications of these findings will lead to substantial savings in the cost for diversity protections for digital radio routes. These discoveries stimulated the development of new models of diversity improvement factors for digital radio and the development of the DRDIV computer program for engineering digital radio routes. Background information is given on multipath fading and diversity concepts, and a glossary of terms is included. >

Method of demodulation in digital communication systems with multipath propagation

Abstract: Method of demodulation in digital communication systems with multipath propagation in a channel between transmitter and receiver, based on the use of algorithms of the Viterbi, Fano, Stack and similar types, with decoding of the soft decision type, where there is included a calculation of reference values in the receiver. On account of the complexity, possibly current consumption, the receiver is adapted to process only sequences each having a limited number (nu) of consecutive data symbols, here denoted ''known'' data symbols, the channel and the transmitter together containing more than the ''known'' data symbols, and these other symbols being denoted ''unknown'' data symbols. The method is based on the concept that the receiver by means of a channel estimator (23) forms an estimate of the instantaneous impulse response (h(t)) of the multipath propagation channel (possibly convolved with the ambiguity function ($(1,1)$(t)) of the channel estimated). The calculation of reference values comprises utilization of the impulse response (h(t)) of the channel and knowledge of the signal transmitted by the transmitter for a given sequence with known and unknown data symbols, and this transmitted signal is averaged over at least a part of the unknown data symbols.

Causes of burst errors in multipath fading channel (mobile radio)

Abstract: The so-called irreducible error due to frequency-selective fading is known to have a serious effect on mobile radio communication systems. Thus, the analysis of such errors is a prerequisite for making high-speed digital signal transmission over a fading channel feasible. The authors attempted to elucidate the physical mechanisms causing such errors using laboratory measurements of microscopic bit error rate. The results clarified some sources of burst errors in a multipath fading channel. In particular, large and rapid timing fluctuations of eye pattern due to severe delay distortion of the multipath channel was found to be the most dominant source of error. >

An error protected 16 kbit/s voice transmission for land mobile radio channel

Abstract: Robust speech coding and an efficient error correction coding is indispensable for toll-quality voice transmission in mobile radio because of degradation due to multipath fading. A decision procedure for the bit-selective forward-error-correction (BS-FEC) scheme is proposed as a promising solution to this problem, and backward-type prediction speech coding is shown to be more robust against transmission errors than forward-type prediction. Combinations of BS-FEC (channel coding) and backward-type prediction speech coding are designed, and improvements in the SNRs of transmitted voice signals are examined. Simulation results show that BS-FEC can provide good speech quality even at a bit error rate (BER) of 10/sup -2/ in Rayleigh fading environments, at the cost of a slight degradation in the SNR (signal-to-noise ratio) at low BERs. >

Multipath interconnection: a technique for reducing congestion within fast packet switching fabrics

Abstract: The banyan interconnection is prone to internal link congestion, resulting in a blocking switch architecture. Several solutions that have been implemented to reduce the severity of link congestion offer packets a multiplicity of paths, which tend to increase packet delay variability and allow delivery of out-of-sequence packets. This, in turn, can lead to an increase in end-to-end protocol complexity, particularly in the case of real-time services. A solution called multipath interconnection is proposed to overcome this difficulty. Multiple (i.e., alternate) paths are provided and one is selected at call-setup time. Subsequent packets belonging to the call are constrained to follow the selected path. A number of path selection strategies are presented. >

A simple digital fading simulator for mobile radio

Abstract: An instrument which can be used to simulate the multipath fading associated with VHF/UHF mobile radio channels is described. The implementation is considered simple, flexible, and economical and allows the fading waveforms to be repeated exactly. Level crossing rate and probability distribution function measurements over a wide range of Doppler frequencies indicate good agreement with theoretical results. >

Wide-band packet radio for multipath environments

Abstract: A direct-sequence spread-spectrum packet radio is described that has versatile signal-processing and local-control capabilities designed to support the functions required of a robust mobile communications network. Noteworthy capabilities include eleven selectable data rates with accurate range measurements in a fading multipath channel. The radio uses a hybrid analog/digital signal processor and nonrepeating spreading codes for suppression of intersymbol interference and jamming. It incorporates two sets of monolithic surface-acoustic-wave convolvers as programmable matched filters with time-bandwidth products of 64 and 2000. The analog matched filters are coupled with binary postprocessing for the functions of detection, RAKE demodulations and ranging measurements over a wide multipath spread. The data rate can be varied in response to channel conditions from 1.45 Mb/s down to 44 b/s with an almost ideal tradeoff in signal-processing gain from 18 dB up to 61 dB prior to multipath combining. >

Phase reference navigation system and method

Abstract: A phase reference navigation system and method employs a plurality of fixed narrow beamwidth antennas arranged to cover a predetermined angular sector and such that the patterns of adjacent antennas crossover one another at their half-power points. Each antenna radiates an RF carrier which is amplitude modulated by a reference phase signal and by one of two different frequency subcarriers. Adjacent antennas radiate different frequency subcarriers. Each subcarrier is frequency modulated by a variable phase signal which has a phase relative to the reference signal which corresponds to the direction of orientation of the antenna from which that subcarrier radiates. A receiver simultaneously receives the RF energy radiated by two adjacent antennas. Upon demodulation, the two variable phase signals are recovered and combined to produce a resultant signal which has a phase that varies as a function of angle between the two antennas. The invention utilizes FM capture effect principles to substantially reduce multipath effects and is useful for omni-directional, or bi-directional or uni-directional systems.

Performance of some Rake modems over the non-disturbed wide band HF channel

Abstract: The author presents performance evaluations for four Rake modems used to communicate over the wideband (e.g. 1 MHz) HF channel. Attention is confined to performance for the nondisturbed HF channel. Consideration is given to the impact of mode Doppler shift and multipath spread due to dispersion. The effects of additive noise, self-noise, and imperfect channel measurements are included. Results are presented for decision-directed, parallel probe, serial probe, and DPSK Rake modems. Performance to within 1-3 dB of the additive white Gaussian noise channel is predicted, depending upon the modem and the channel dispersion, if the Doppler shift of the modem can be tracked to within 0.1 Hz. >

Apparatus in a mobile telephone receiver for reducing bit error

Abstract: An apparatus in coherent mobile telephone receivers included in a digital mobile telephone system (TDMA) for reducing the bit error occuring as a result of whether single path or multipath propagation is present for the transmitted channels. A phase locked loop (10, 11, 12) is included in the apparatus as well as an equalizer (3) and a decision circuit (7). In addition, there is included a correlator (2) which from a known bit pattern (SW) calculates the channel impulse response for each time slot. The phase locked loop is connected when the impulse response denotes multipath propagation. The impulse response can also be calculated by the parameter calculation unit (5) of the equalizer (3).

Delay spread and time delay jitter for the UHF factory multipath channel

Abstract: The results of wideband delay spread and time-delay jitter measurements collected from five factory buildings are presented. Carrier frequency was 1282 MHz. The data show that excess mean delay, RMS delay spread, and time-delay jitter values in single-storey factory buildings are similar to values observed in and around multistorey office buildings. Differential time-delay jitter measurements reveal that the excess mean delay can vary by as much as 180 ns for 5.5-cm shifts in receiver location. Results from 950 multipath power delay profiles collected in 50 local areas are summarized. >

Adjustable frequency diversity acoustic communications system

Abstract: An underwater acoustic communication system provides an operator with adjable frequency diversity so that data rates may be tailored to specific multipath environments. Transmitted messages are sent with precursor transmission/reception synchronization data and transmission parameter data so that the receiving communication end will recognize when message data starts, tone length as-well-as the frequency diversity of the transmitted message. Timing is extracted from the data to compensate for Doppler shift.

A simple prediction method for L-band multipath fading in rough sea conditions

Abstract: Quantitative analysis of the coherent and incoherent power of sea-reflected waves at 1.5 GHz was performed using multipath-fading data obtained by field experiments. The experiments were carried out for shore-to-shore, satellite-to-shore, and satellite-to-ship paths, and antennas with gains from 13 dBi to 21 dBi were used for signal reception. Results indicate that in rough sea conditions where the incoherent component is dominant, the power of the incoherent component can reach the maximum power of the coherent component (generated in calm sea) in most cases, except for a few cases such as when measurements using narrow-beam antennas or elevation angles below 7 degrees are made. On the results obtained, a simple prediction method for multipath fading caused by sea reflection is presented. >

Air traffic control radar beacon system multipath reduction apparatus and method

Abstract: A system and method for detecting and removing specular multipath reports from terminal ATCRB systems for aircraft targets having Mode 3/A transponder reporting capabilities. Reports are delayed for a first time, or azimuth, interval to determine whether the report is multipath or to identify future occurrences of multipath reports. After the first azimuth interval, the report is released for display and a fragment of the report is retained for an additional azimuth interval. Of two reports with matching Mode 3/A code, the one with the greater range is considered to be a multipath reply. If the multipath report range falls within a multipath range interval and has not been released for display, it is discarded by the system as an erroneous report.

Broadband Matched Field Processing

Abstract: The analysis here will illustrate the use of full wavefield (matched field) processing techniques in a multipath environment. Experimental data consists of the arrivals at a, hydrophone array from an explosive source in shallow water. A coherent recombination of the multipath arrivals is performed in order to provide enhanced signal-to-noise ratio for source detection as well as range/depth localization.