Showing papers on "Multipath propagation published in 1993"

Simulation of multipath impulse response for indoor wireless optical channels

Abstract: A recursive method for evaluating the impulse response of an indoor free-space optical channel with Lambertian reflectors is presented. The method, which accounts for multiple reflections of any order, enables accurate analysis of the effects of multipath dispersion on high-speed indoor optical communication systems. A simple algorithm for computer implementation of the technique and computer simulation results for both line-of-sight and diffuse transmitter configurations are also presented. In both cases, it is shown that reflections of multiple order are a significant source of intersymbol interference. Experimental measurements of optical multipath, which help verify the accuracy of the simulations, are discussed. >

Impulse response modeling of indoor radio propagation channels

Abstract: If indoor radio propagation channels are modeled as linear filters, they can be characterized by reporting the parameters of their equivalent impulse response functions. The measurement and modeling of estimates for such functions in two different office buildings are reported. The resulting data base consists of 12000 impulse response estimates of the channel that were obtained by inverse Fourier transforming of the channel's transfer functions. It is shown that the number of multipath components in each impulse response estimate is a normally-distributed random variable with a mean value that increases with increasing antenna separations; a modified Poisson distribution shows a good fit to the arrival time of the multipath components; amplitudes are lognormally distributed over both local and global areas, with a log-mean value that decreases almost linearly with increasing excess delay; for small displacements of the receiving antenna, the amplitude of the multipath components are correlated; the amplitudes of adjacent multipath components of the same impulse response function show negligible correlations; and the RMS delay spread over large areas is normally distributed with mean values that increase with increasing antenna separation. >

Adaptive multichannel combining and equalization for underwater acoustic communications

Abstract: A theoretically optimal multichannel receiver for intersymbol interference communication channels is derived, and its suboptimal versions with linear and decision feedback equalizer are presented. A practical receiver based on any of these structures encounters difficulties in the underwater acoustic channels in which the extended time‐varying multipath is accompanied by phase instabilities. A receiver that overcomes these problems by jointly performing adaptive mean‐squared error diversity combining, multichannel carrier‐phase synchronization and decision feedback equalization is proposed. Its performance is demonstrated on the experimental telemetry data from deep and shallow water long‐range acoustic channels. Presented results indicate superior quality of coherent PSK and QAM reception obtained through joint equalization of very few channels.

Orthogonal multicarrier techniques applied to direct sequence spread spectrum CDMA systems

Abstract: We design a new scheme for DS-CDMA applications. The basic principle consists in applying orthogonal multicarrier transmission to DS spread spectrum systems. Several algorithms for estimating the equalizer coefficients are investigated in order to reduce the interuser interference due to multipath propagation and cross-correlation between the code sequences. The resulting system achieves high robustness against channel impairments and outperforms the conventional single-carrier DS-CDMA system. >

A spread-spectrum multiaccess system with cochannel interference cancellation for multipath fading channels

Abstract: The authors propose and analyze a direct-sequence spread-spectrum multiaccess (DS/SSMA) receiver that employs a cascade of cochannel interference (CCl) cancellers for communication over multipath fading channels. The receiver first coherently demodulates and despreads the received signal to produce correlator outputs and initial data estimates. Based on these estimates, the cancellation scheme essentially creates replicas of the contributions of the CCl embedded in the correlator outputs and removes them for a second improved hard data decision. By repeating this operation over and over, a cascade of CCl cancellers is derived. Through theoretical analysis and simulation, the authors investigate the canceller's bit error rate (BER) performance in both the absence and presence of errors in the amplitude and phase estimates of each user's received signal. Numerical results show the considerably large improvement in performance that can be attained by the cancellation scheme, even under partially degraded estimates. >

Pseudorandom noise ranging receiver which compensates for multipath distortion by dynamically adjusting the time delay spacing between early and late correlators

Abstract: A receiver for pseudorandom noise (PRN) encoded signals consisting of a sampling circuit, multiple carrier and code synchronizing circuits, and multiple digital autocorrelators. The sampling circuit provides digital samples of a received composite signal to each of the several receiver channel circuits. The synchronizing circuits are preferably non-coherent, in the sense that they track any phase shifts in the received signal and adjust the frequency and phase of a locally generated carrier reference signal accordingly, even in the presence of Doppler or ionospheric distortion. The autocorrelators in each channel form a delay lock loop (DLL) which correlates the digital samples of the composite signal with locally generated PRN code values to produce a plurality of (early, late), or (punctual, early-minus-late) correlation signals. Thus, during an initial acquisition mode, the delay spacing is relatively wide, on the order of approximately one PRN code chip time. However, once PRN code synchronism has been achieved, the code delay spacing is narrowed, to a fraction of a PRN code chip time. There are several advantages to this arrangement, especially in environments wherein multipath distortion in the received composite signal is of the same order of magnitude as a PRN code chip time.

Performance of power-controlled wideband terrestrial digital communication

Abstract: Performance of a wideband multipath-fading terrestrial digital coded communication system is treated. The analysis has applications to a cellular system using direct-sequence spread-spectrum code-division multiaccess (CDMA) with M-ary orthogonal modulation on the many-to-one reverse (user-to-base station) link. For these links, power control of each multiple-access user by the cell base station is a critically important feature. This feature is implemented by measuring the power received at the base station for each user and sending a command to either raise or lower reverse link transmitter power by a fixed amount. Assuming perfect interleaving, the effect of the power control accuracy on the system performance is assessed. >

Passive high accuracy geolocation system and method

Abstract: A system for accurately locating a mobile cellular radio includes plural radio receivers, the position of each being precisely known, a satellite-based time source for providing the actual time of arrival of a radio signal at the plural radio receivers, and a device for removing multipath and cochannel interference from the received signals so that actual time of arrival of the radio signal may be determined accurately. The Global Positioning System (GPS) may be used to provide the time signal and to provide the precise location of the plural radio receivers. The system may be used to passively determine the location of the mobile radio and is accurate enough to use with a computer generated map system.

Modulated spread spectrum in RF identification systems method

Abstract: A method for RF communication between transceivers in a radio frequency identification system that improves range, decreases multipath errors and reduces the effect of outside RF source interference by employing spread spectrum techniques. By pulse amplitude modulating a spread spectrum carrier before transmission, the receiver can be designed for simple AM detection, suppressing the spread spectrum carrier and recovering the original data pulse code waveform. The data pulse code waveform has been further encrypted by a direct sequence pseudo-random pulse code. This additional conditioning prevents the original carrier frequency components from appearing in the broadcast power spectra and provides the basis for the clock and transmit carrier of the transceiver aboard an RFID tag. Other advantages include high resolution ranging, hiding transmissions from eavesdroppers, and selective addressing.

Advantages of CDMA and spread spectrum techniques over FDMA and TDMA in cellular mobile radio applications

Abstract: A unified theoretical method for the calculation of the radio capacity of multiple-access schemes such as FDMA (frequency-division multiple access), TDMA (time-division multiple access), CDMA (code-division multiple access) and SSMA (spread-spectrum multiple access) in noncellular and cellular mobile radio systems is presented for AWGN (additive white Gaussian noise) channels. The theoretical equivalence of all the considered multiple-access schemes is found. In a fading multipath environment, which is typical for mobile radio applications, there are significant differences between these multiple-access schemes. These differences are discussed in an illustrative manner revealing several advantages of CDMA and SSMA over FDMA and TDMA. Novel transmission and reception schemes called coherent multiple transmission and coherent multiple reception are briefly presented. >

Linear unbiased data estimation in mobile radio systems applying CDMA

Abstract: Data estimation in the uplink of a synchronous mobile radio system applying code-division multiple access (CDMA) is considered. In mobile radio systems applying CDMA, multipath propagation leads to intersymbol interference (ISI) and together with time variance, to cross interference between the signals of different users regardless of whether the user codes are chosen orthogonal or not. A linear unbiased data estimation algorithm is presented that eliminates both ISI and cross interference perfectly by jointly detecting the different user signals, leading to unbiased estimates of the transmitted data symbols. By theoretical analysis and simulation, the performance of the linear unbiased data estimation algorithm is examined under the assumption that the radio channel impulse responses are known at the receiver. The price to be paid for the interference elimination are SNR degradations, which are calculated for typical mobile radio situations in urban areas. The resulting average uncoded bit error probabilities lead to the conclusion that systems applying the linear unbiased data estimation algorithm are well suited for mobile radio applications. >

Multipath noise reduction for spread spectrum signals

Abstract: The invention includes the concepts of early-prompt delay tracking, multipath correction of early-prompt delay tracking from correlation shape and carrier phase multipath correction. In early-prompt delay tracking, since multipath is always delayed with respect to the direct signals, the system derives phase and pseudorange observables from earlier correlation lags. In multipath correction of early-prompt delay tracking from correlation shape, the system looks for relative variations of amplitude across the code correlation function that do not match the predicted multipath-free code cross-correlation shape. The system then uses deviations from the multipath-free shape to infer the magnitude of multipath, and to generate corrections pseudorange observables. In carrier phase multipath correction, the system looks for variations of phase among plural early and prompt lags. The system uses the measured phase variations, along with the general principle that the multipath errors are larger for later lags, to infer the presence of multipath, and to generate corrections for carrier-phase observables.

Spread-spectrum code and carrier synchronization errors caused by multipath and interference

Abstract: The influence of multipath propagation and spread-spectrum interference on code and carrier synchronization is investigated. Both coherent and noncoherent delay lock loops (DLLs) are considered, with arbitrary early-late spacings up to one chip time. The coherent DLL is shown to have a major advantage; for a relatively high fading bandwidth, it has negligible tracking errors, while a noncoherent DLL always has a certain bias error. The results are particularly interesting for spread-spectrum positioning systems like Global Positioning System (GPS) and GLONASS. >

Analysis of a direct-sequence spread-spectrum cellular radio system

Abstract: The uplink and downlink performance of a digital cellular radio system that uses direct sequence code division multiple access is evaluated. Approximate expressions are derived for the area averaged bit error probability while accounting for the effects of path loss, log-normal shadowing, multipath-fading, multiple-access interference, and background noise. Three differentially coherent receivers are considered: a multipath rejection receiver, a RAKE receiver with predetection selective diversity combining, and a RAKE receiver with postdetection equal gain combining. The RAKE receivers are shown to improve the performance significantly, except when the channel consists of a single faded path. Error correction coding is also shown to substantially improve the performance, except for slowly fading channels. >

Two signaling schemes for improving the error performance of frequency-division-duplex (FDD) transmission systems using transmitter antenna diversity

Abstract: Two signaling schemes that exploit the availability of multiple (N) antennas at the transmitter to provide diversity benefit to the receiver are proposed. In the first scheme, a channel code of length N and minimum Hamming distance d/sub min/ /spl les/ N is used to encode a group of K information bits. The channel code symbol c/sub i/ is transmitted with the i/sup th/ antenna. At the receiver, a maximum likelihood decoder for the channel code provides a diversity of d/sub min/, as long as each transmitted code symbol is subjected to independent fading. The second scheme introduces deliberate resolvable multipath distortion by transmitting the data bearing signal with antenna 1, and N - 1 delayed versions of it with antennas 2 through N. The delays are unique to each antenna and are chosen to be multiples of the symbol interval. At the receiver, a maximum likelihood sequence estimator resolves the multipath in an optimal manner to realize a diversity benefit of N.

Effect of fading correlation on adaptive arrays in digital wireless communications

Abstract: The effect of correlations among the fading signals at the antenna elements of an adaptive array in a digital wireless communication system is investigated. With an adaptive array, the signals received by multiple antennas are optimally weighted and combined to suppress interference and combat desired signal fading. A model of local scattering where the received multipath signals arrive within a given bandwidth is presented, and a closed-form expression for the correlation as a function of antenna spacing is derived. Results show that the degradation in performance with correlation in an adaptive array that combats fading and suppresses interference is only slightly larger than that for combating fading alone, i.e., with maximal ratio combining. This degradation is small, even with correlation as high as 0.5. >

A novel approach to modeling and efficient simulation of frequency-selective fading radio channels

Abstract: A multipath radio channel concept based on channel orthogonalization techniques in two inner product spaces is presented and compared to the conventional approach of modeling each individual Rayleigh or Rice fading multipath ray individually. The proposed simulator can be applied to linear amplitude/phase modulation and linear fading channels including Nyquist filtering. It is shown to be a good approximation to the conventional model in the case of tight rolloff factors. Channels having quasi-or truly continuous delay profiles can now be properly represented at significantly reduced computational complexity. The proposed simulation concept leads to the lowest level of complexity being achieved for the prevailing channel and noise conditions on a particular channel. >

An implementation of a CMA adaptive array for high speed GMSK transmission in mobile communications

Abstract: The hardware implementation of an adaptive array as a technique for compensating multipath fading in mobile communications is described. The number of the antenna elements is four. The target communication system is modulated by 256 kbps Gaussian-filtered minimum shift keying (MSK) and has a time-division multiplexing (TDM) architecture with 24 time slots. Based on the digital beamforming concept, all of the signals and the array weights are digital-signal processed. The constant modulus algorithm (CMA) is employed for weight optimizing. In an additive white Gaussian noise channel, this system has 5.6-dB gain in an energy-per-bit-to-noise-density ratio at a bit error rate (BER) of 1.0*10/sup -3/, compared with a single antenna system. The result of the basic field test shows that the gain at a BER of 1.0*10/sup -3/ reaches 22.3 dB in a nonselective, slow Rayleigh fading channel given a 5 Hz maximum Doppler shift. >

On the use of multicarrier direct sequence spread spectrum systems

Abstract: The authors propose a multicarrier direct sequence spread spectrum system. By using this system, it is possible to get not only robustness against multipath fading, but to also achieve narrowband interference suppression; these advantages are obtained without the use of either an explicit RAKE structure or an interference suppression filter. The authors evaluate the performance of this system over both a frequency selective Rayleigh fading channel, and an additive white Gaussian noise channel in the presence of single tone interference. >

Apparatus for estimating a line of sight signal propagation time using a reduced-multipath correlation function

Abstract: A method and device for processing a signal are described, wherein an estimate of a multipath-induced contribution to a demodulated navigation signal is calculated and subtracted from said demodulated navigation signal to obtain an estimated line of sight contribution to said demodulated navigation signal, and a propagation time τ 0 is calculated from the thus calculated line of sight contribution to said demodulated navigation signal, such that a very accurate propagation time τ 0 of said navigation signal can be calculated.

Bit error simulation for pi /4 DQPSK mobile radio communications using two-ray and measurement-based impulse response models

Abstract: An accurate software/hardware bit-by-bit error simulator for mobile radio communications is described. Simulation results in indoor and outdoor channels are compared with theoretical results. Bit error rate (BER) results in simulated frequency-selective fading channels generated by several channel models such as two-ray, constant amplitude, and simulated indoor radio channel impulse models (SIRCIMs) are presented. It is shown that BER is not only dependent on the RMS delay spread, but also on the distribution of temporal and spatial multipath components in local areas. An important result is that a two-ray Rayleigh fading model is a poor fit for indoor wireless channels and, if used, can underestimate the BER by orders of magnitude. A real-time bit error simulation of video transmission using the bit-by-bit error simulator is described. The simulator, called BERSIM, is shown to be a useful tool for evaluating emerging data transmission products for digital mobile communications. >

Impulse noise mitigation strategies for multicarrier modulation

Abstract: Short time-duration and high magnitude noise, taking on the form of time-domain impulse noise over copper twisted-pairs or multipath fading in wireless communication, can potentially be the limiting impairment on performance in many high-speed data transmission applications. Coding techniques are presented, designed specifically for multicarrier modulated systems to mitigate the effects of such impairment. Several coding combinations, involving soft error protection, in addition to hard forward error correction, are considered. The techniques can be applied with minor variation to a number of possible applications employing multicarrier modulation, including wireless digital audio broadcasting (DAB) systems and high-speed data/video transmission over coaxial cables. >

Closely spaced monopoles for mobile communications

Abstract: The spatial cross-correlation function for antenna signals in multipath environments depends on the mutual coupling between the antennas. In general, this function is different from the spatial autocorrelation coefficient function of the multipath, fading field. This difference influences the design criteria for the element spacing of compact (that is, closely spaced elements) antennas. The relationship between the spatial autocorrelation function and the cross-correlation function is presented and used for practical, compact diversity antenna design

CDMA communication system

Abstract: A CDMA communication system using a spreading code has a plurality of mobile stations and at least one base station. The mobile stations and the base station are provided with respective receiving sections, each of which has a path diversity function for reducing multipath fading. The base station includes an array antenna having a plurality of integrated antenna elements spaced by a distance substantially equal to one to several wavelengths from each other so as to produce a space diversity effect. A plurality of delay devices are coupled with the respective antenna elements, for applying time delays to signals which pass through the respective antenna elements. The time delay are different from each other by a one symbol period of the spreading code or more.

System and method for joint demodulation of CDMA signals

Abstract: Methods and systems for jointly demodulating CDMA signals are presented The disclosed methods and systems provide, among other advantages, for accurate demodulation of such signals in multipath, time-dispersion environments Single signal or multiple signal demodulation, with or without taking intersymbol interference into account can be accomplished according to the present invention

Tomographic measurement of temperature change in phantoms of the human body by chirp radar-type microwave computed tomography

Abstract: The chirp radar-type microwave computed tomograph (CT) measures the temperature change in a human body noninvasively. The paper examines its feasibility. A chirp pulse signal between 1 and 2 GHz is radiated from the transmitting antenna to the phantom. The transmitted waves are detected by the receiving antenna, which is place on the opposite side of the object, and the beat signal between the incident wave and the transmitted wave is produced by the mixer. By spectral analysis of the beat signal, only those signals transmitted on the straight line between the transmitting antenna and the receiving antenna are discriminated from multipath signals. The microwave tomogram can therefore be reconstructed easily using the conventional algorithms for an X-ray CT image. The microwave CT can use the chirp signal to remove the influence of multipath signals caused by diffraction and reflection. The imaging of dielectric materials with complicated structures is thus possible. The experimental results using phantoms show that the spatial resolution of this microwave CT is about 10 mm and that a two-dimensional distribution of temperature change can be measured.

The dependence of indoor radio channel multipath characteristics of transmit/receiver ranges

Abstract: The authors report experimental and modeling studies that investigate the dependence of indoor radio channel multipath characteristics on a transmit/receive range. Specifically, a simple model for estimation of the complex baseband equivalent impulse response for indoor channels is explained. Using this model, the relationship between RMS delay spread and range on static indoor channels is estimated. Results show that this relationship is nonmonotonic and has a maximum at a range that depends on the building dimensions and the electrical properties of reflecting surfaces. The model is used in infinite, as well as finite, resolution modes to supplement the measurements of impulse response characteristics in different buildings using a limited resolution channel sounder. Experimental and modeling results are combined to derive conclusions that confirm the anticipated nonmonotonic relationship exists in empty buildings. Measurement results demonstrate that furniture has the effect of destroying this relationship, making RMS delay spread almost independent of range. >

Method and apparatus for measuring the impulse response of a radio channel

Abstract: A system for measuring a multipath radio channel impulse response by cross-correlating digital samples of a received pseudo-random signal with a digital copy of the transmitted signal. A transmitter within the system repeatedly transmits the pseudo-random signal across a multipath radio channel. A receiver within the system receives the transmitted signal, as well as delayed versions of the original signal, and digitally samples the received signals. A computer then cross-correlates the samples of the received signal with the digital stored version of the transmitted signal to produce a measure of the impulse response of the multipath radio channel.

Multipath torus switching apparatus

Abstract: Disclosed is a new torus switch with low latency performance. The present invention improves the torus network connection time by providing the capability to try multipaths in one single high speed operation. This multipath approach can be directed at establishing a connection between two specific nodes over various alternate routes simultaneously. The invention is such that if only one route is available, the multipath approach will find that path instantanteously and establish the desired connection with minimal latency. If several links are available, the multipath method establishes the desired connection over only one of the available links and leaves the other options free to be used by other connections. In addition, routing at intermediate torus network stages will be a vast improvement of the wormhole approach.

Pre-RAKE diversity combination for direct sequence spread spectrum communications systems

Abstract: A novel method of multipath diversity signal combination is proposed for mobile direct sequence spread spectrum (DS-SS) communications. Multiple transmissions are made for each spread signal. Each transmission is independently delayed and amplified in the complex domain according to the delay profile and the estimated path strength of the channel. This is done to facilitate reception of a signal at the mobile unit which is already a RAKE combination of the multipath signals. This method is called a pre-RAKE combination system because the RAKE combination function is performed pre-transmission. Using this method the size and complexity of the mobile unit can be kept to a minimum. The pre-RAKE method retains the advantages of diversity reception in a multi-path fading environment. An examination of the SNR for the traditional RAKE and the pre-RAKE systems and computer simulations show that the performance of the pre-RAKE system is equivalent to that of the RAKE system. >