Search NASA⌕ Search

SEARCH · Search NASA

Results for “SIGNAL FADING”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 253 records · Page 14
All sourcesNASA NTRSDOE OSTI
NASA NTRS2002

Mountain-Top-to-Mountain-Top Optical Link Demonstration

A mountain-top-to-mountain-top optical link was demonstrated between JPL's Table Mountain Facility (TMF), Wrightwood, California, and Strawberry Peak (SP), Lake Arrowhead, California, during the months of June, August, and September of 2000. The bidirectional laser link was nearly horizontal at an altitude of 2 km and spanned a range of 46.8 km. The 780-nm beacon laser transmitted from TMF comprised eight co-propagating mutually incoherent laser beams. The normalized variance or scintillation index (SI) of the individual beacon lasers measured by recording the signal received through 8.50-cm-diameter spotting telescopes on three different nights (June 28-30, 2000) was 1.05 +/- 0.2, 1.76 +/- 0.6, and 0.96 +/- 0.24, respectively. These measurements agreed with values predicted by a heuristic model. The SI of the signal received at SP was found to decrease progressively with an increasing number of beams, and a factor of 3 to 3.5 reduction was achieved for all eight beams. The beam divergence determined by mapping out the point spread function of a few of the individual laser footprints received at SP was 85 to 150 microrad, compared to a design goal of 120 microrad. The 852-nm communications laser beam received at TMF through a 60-cm-diameter telescope on the nights of August 4 and September 14 and 15, 2000, yielded SI values of 0.23 +/- 0.04, 0.32 +/- 0.01, and 0.49 +/- 0.18, respectively, where the reduction was attributed to aperture averaging. The probability distribution functions of the received signal at either end, mitigated by multi-beam averaging in one direction and by aperture averaging in the other direction, displayed lognormal behavior. Consequently, the measured fade statistics showed good agreement with a lognormal model.

Biswas, A.↗
NASA NTRS1987

A prototype 4800 bps voice terminal for the mobile satellite experiment

The authors report the final stage of a research project for the NASA mobile satellite experiment (MSAT-X). The work has resulted in two alternative coding techniques which have achieved good communications quality at 4.8 kb/s while tolerating vehicle noise, fading, and a fairly high-error-rate, bursty channel. The authors have constructed a circuit board based on three AT&T 32-bit floating-point DSP32 digital signal processors (DSP) and providing both excess processing power and considerable flexibility for handling different variations of the algorithms. A Motorola 68HC11 microcomputer chip serves as the board controller and interface handler. Mounted on a wirewrapped card, the prototype's circuit footprint amounts to only 225 cm2 and consumes about 13 W of power. A description is given of the architecture, signal processing and communication functions, circuitry, and firmware of the prototype. Issues such as synchronization, framing, error control, and modem interface are also addressed.

Chan, Wai-Yip↗
NASA NTRS1976

Simulated performance of coded PCM/FM modem on Rician fading channel

This paper describes a simulation approach to characterizing the performance of a coded PCM/FM modem operating on the classical Rician fading channel. In particular, we consider the performance of short constraint length convolutional codes in conjunction with Viterbi maximum likelihood decoding. The receiver consists of a narrowband filter followed by an ideal limiter/discriminator. Primary interest is in the bit error probability performance parameterized by both the fading channel and receiver parameters. Particular attention is given to simulating the impulse noise appearing at the discriminator output. This is the 'click' noise caused by encirclements of the origin by the instantaneous signal-plus-noise phasor. Results include the effects of channel memory and narrowband receiver filtering effects on overall bit error probability.

Modestino, J. W.↗
NASA NTRS1974

Path diversity for reception of satellite signals

The improved system performance resulting from the use of path diversity on earth-space satellite links operating above 10 GHz is discussed. Diversity gain is defined, and it is noted that the optimum diversity gain may be determined quite simply from the knowledge of a simple terminal fade distribution only. This approach may be extended to an arbitrary number of diversity terminals. The available diversity gain data are compared and shown to be consistent. These data, resulting from measurements in Ohio and New Jersey, indicate that diversity gain becomes relatively independent of separation distance for separations greater than approximately 8 km.

Hodge, D. B.↗
NASA NTRS1990

Trellis-Coded MDPSK System With Doppler Correction

Multiple-differential-phase-shift-keyed (MDPSK) microwave system designed for communications between mobile and/or fixed terrestrial stations via satellite transponders. Stations in system transmit and receive data or digitally-coded voice signals at rates up to 4.8 kb/s in channels only 5 kHz wide. Incorporates advanced encoding, decoding, modulation, and demodulation techniques to minimize effects of error bursts. Uses feedforward techniques for fast recovery from deep fades.

Divsalar, D.↗
NASA NTRS2005

Simulator of Space Communication Networks

Multimission Advanced Communications Hybrid Environment for Test and Evaluation (MACHETE) is a suite of software tools that simulates the behaviors of communication networks to be used in space exploration, and predict the performance of established and emerging space communication protocols and services. MACHETE consists of four general software systems: (1) a system for kinematic modeling of planetary and spacecraft motions; (2) a system for characterizing the engineering impact on the bandwidth and reliability of deep-space and in-situ communication links; (3) a system for generating traffic loads and modeling of protocol behaviors and state machines; and (4) a system of user-interface for performance metric visualizations. The kinematic-modeling system makes it possible to characterize space link connectivity effects, including occultations and signal losses arising from dynamic slant-range changes and antenna radiation patterns. The link-engineering system also accounts for antenna radiation patterns and other phenomena, including modulations, data rates, coding, noise, and multipath fading. The protocol system utilizes information from the kinematic-modeling and link-engineering systems to simulate operational scenarios of space missions and evaluate overall network performance. In addition, a Communications Effect Server (CES) interface for MACHETE has been developed to facilitate hybrid simulation of space communication networks with actual flight/ground software/hardware embedded in the overall system.

Clare, Loren↗
NASA NTRS1987

Laboratory measurements and a theoretical analysis of the TCT fading channel radio system

This paper documents the laboratory and theoretical performance of a pilot-aided digital radio system. The technique considered transmits a midband pilot tone to improve the receiver data detection performance in a multipath fading channel and is referred to as the tone calibrated technique (TCT). The performance of a 860 MHz prototype system carrying 2.4 kbit/s data under Rician fading conditions is reported. Both experimental and analytical results show that the error floor experienced with nonpilot-aided transmission methods is effectively removed by the TCT scheme, resulting in significant performance gains at high signal-to-noise ratio values. The paper also examines the TCT system performance under typical operating conditions and presents a new analysis of the TCT theoretical error probability.

Rafferty, William↗
NASA NTRS1977

Prediction of attenuation of the 28 GHz COMSTAR beacon signal using radar and measured rain drop spectra

Disdrometer measurements and radar reflectivity measurements were injected into a computer program to estimate the path attenuation of the signal. Predicted attenuations when compared with the directly measured ones showed generally good correlation on a case by case basis and very good agreement statistically. The utility of using radar in conjunction with disdrometer measurements for predicting fade events and long term fade distributions associated with earth-satellite telecommunications is demonstrated.

Goldhirsh, J.↗
NASA NTRS2005

Adaptive Thresholding and Parameter Estimation for PPM

A method of adaptive setting of a threshold level for the detection of pulses in a pulse-position modulation (PPM) free-space optical communication system has been developed. In simplified terms, it is desirable to set a threshold value high enough to greatly reduce the probability (PFA as defined below) of erroneously detecting noise as signal pulses but not so high as to greatly reduce the probability (PD as defined below) of detecting any signal pulses that may be present along with noise. In the present method, the threshold level is varied with time, in response to changing conditions in the optical-communication channel, in an effort to maintain a balance between the aforesaid competing requirements. An integral part of this adaptation scheme is a scheme for estimating key parameters of the optical-communication channel in particular, parameters that describe the fading and total attenuation in the channel, and parameters that characterize spreading of pulses by atmospheric and other effects. The method can be implemented by software processing of digitized optoelectronic-detector output, and has been tested by computational simulation. In the first stage of processing by this method, the digitized values of the detector output during noise-only time slots of received PPM symbols are averaged to obtain a background level. This background level is subtracted from the detector output in the hope of reducing or eliminating the noise component in the remaining signal. (This background level should not be confused with the detection threshold, which is computed in the last stage of processing.) Next, the remaining signal - in effect, a vector of pulse samples - is normalized by dividing it by its L1 norm (in general, the L1 norm of a vector is defined as the sum of absolute magnitudes of its orthogonal components).

Arabshahi, Payman↗
NASA NTRS1993

Propagation measurements for satellite radio reception inside buildings

Swept CW signals (from 700 to 1800 MHz) were received inside six buildings of brick, corrugated sheet-metal, wood-frame, mobile-home, and reinforced concrete-wall construction. A transmitter antenna was mounted outdoors on top of an 18 m tower to simulate a satellite, and a linearly scanned directional receiver antenna was used to probe the spatial, spectral, and temporal variability of the signal indoors. Levels were found to have much structure in the spatial and frequency domain, but were relatively stable in time. Typically, people moving nearby produced variations of less than 0.5 dB, whereas a person blocking the transmission path produced fades of 6 to 10 dB. Severe losses (17.5 dB) were observed in the concrete-wall building, which also exhibited the longest multipath delays (over 100 ns). Losses inside a mobile home were even larger (over 20 dB) and were independent of antenna orientation. The power-frequency distortion increased with the logarithm of the bandwidth, but could be reduced by moving to a position of higher power. Only the losses showed a clear frequency dependence, but they could be mitigated by moving the antenna.

Vogel, Wolfhard J.↗
NASA NTRS1997

Multidimensional Trellis Coded Phase Modulation Using a Multilevel Concatenation Approach: Code Design - Part 1

The first part of this paper presents a simple and systematic technique for constructing multidimensional M-ary phase shift keying (MMK) trellis coded modulation (TCM) codes. The construction is based on a multilevel concatenation approach in which binary convolutional codes with good free branch distances are used as the outer codes and block MPSK modulation codes are used as the inner codes (or the signal spaces). Conditions on phase invariance of these codes are derived and a multistage decoding scheme for these codes is proposed. The proposed technique can be used to construct good codes for both the additive white Gaussian noise (AWGN) and fading channels as is shown in the second part of this paper.

Rajpal, Sandeep↗
NASA NTRS1991

Satellite sound broadcast propagation measurements

Power transmitted from atop a 17.9 m tower in simulation of a satellite signal, emitted by a tone generator sweeping from 700 to 1800 MHz, was received using a 90 deg beamwidth linearly scanning antenna at many locations inside six buildings of solid brick, corrugated sheet-metal, wood-frame, mobile home, and concrete wall construction. The signal levels are found to have much structure in the spatial and frequency domain but were relatively stable in time. Typically, people moving nearby produced less than 0.5 dB variations, whereas a person blocking the transmission path produces 6 to 10 dB fades. Losses, which at an average position in a room increased from 6 to 12 dB over 750 to 1750 MHz, could be mitigated to 2 to 6 dB by moving the antenna typically less than 30 cm. Severe losses (17.5 dB, mitigated to 12.5 dB) were observed in a concrete wall building, which also exhibited the longest multipath delays (greater than 100 ns). Losses inside a mobile home were even larger (greater than 20 dB) and independent of antenna orientation. The losses showed a clear frequency dependence.

Vogel, Wolfhard J.↗
NASA NTRS2010

Notes on SAW Tag Interrogation Techniques

We consider the problem of interrogating a single SAW RFID tag with a known ID and known range in the presence of multiple interfering tags under the following assumptions: (1) The RF propagation environment is well approximated as a simple delay channel with geometric power-decay constant alpha >/= 2. (2) The interfering tag IDs are unknown but well approximated as independent, identically distributed random samples from a probability distribution of tag ID waveforms with known second-order properties, and the tag of interest is drawn independently from the same distribution. (3) The ranges of the interfering tags are unknown but well approximated as independent, identically distributed realizations of a random variable rho with a known probability distribution f(sub rho) , and the tag ranges are independent of the tag ID waveforms. In particular, we model the tag waveforms as random impulse responses from a wide-sense-stationary, uncorrelated-scattering (WSSUS) fading channel with known bandwidth and scattering function. A brief discussion of the properties of such channels and the notation used to describe them in this document is given in the Appendix. Under these assumptions, we derive the expression for the output signal-to-noise ratio (SNR) for an arbitrary combination of transmitted interrogation signal and linear receiver filter. Based on this expression, we derive the optimal interrogator configuration (i.e., transmitted signal/receiver filter combination) in the two extreme noise/interference regimes, i.e., noise-limited and interference-limited, under the additional assumption that the coherence bandwidth of the tags is much smaller than the total tag bandwidth. Finally, we evaluate the performance of both optimal interrogators over a broad range of operating scenarios using both numerical simulation based on the assumed model and Monte Carlo simulation based on a small sample of measured tag waveforms. The performance evaluation results not only provide guidelines for proper interrogator design, but also provide some insight on the validity of the assumed signal model. It should be noted that the assumption that the impulse response of the tag of interest is known precisely implies that the temperature and range of the tag are also known precisely, which is generally not the case in practice. However, analyzing interrogator performance under this simplifying assumption is much more straightforward and still provides a great deal of insight into the nature of the problem.

Barton, Richard J.↗
NASA NTRS1986

Coherent versus noncoherent signaling for satellite-aided mobile communications

The use of coherent versus noncoherent communications is an unresolved issue for the mobile satellite community. Should one select the more robust but less efficient noncoherent strategy for communications over satellite-aided mobile channels, or does the introduction of a space platform in the mobile link improve signal stability (both amplitude and phase) such that conventional coherent schemes become attractive? This publication tries to answer some of the questions by discussing the results from experiments using a coherent QPSK receiver. The issues discussed include items such as the measured performance in Rician fading, the link error floor in a fading environment, etc. The results are compared and contrasted with that of a noncoherent limiter/discriminator FM receiver.

Davarian, F.↗
NASA NTRS2000

Seven Years of ACTS Technology Verification Experiments Reviewed

The Advanced Communications Technology Satellite (ACTS) was designed to achieve a 99.5-percent system availability rate and signals with less than one error in 10(exp 7) bits throughout the continental United States. To accomplish such a high rate of system availability, ACTS uses multiple narrow hopping beams and very small aperture terminal (VSAT) technology. In addition, ACTS uses an adaptive rain fade compensation protocol to reduce the negative effects of propagation on the system. To enhance knowledge on how propagation and system variances affect system availability, researchers at the NASA Glenn Research Center at Lewis Field performed technology verification experiments over a 7-yr period (from September 1993 to the present). These experiments include T1VSAT System Availability, Statistical Rain Fade Compensation Characterization, Statistical Characterization of Ka-Band Propagation Effects on Communication Link Performance, and Multibeam Antenna Performance.

Acosta, Roberto J.↗
NASA NTRS1986

Model verifies design of mobile data modem

It has been proposed to use differential minimum shift keying (DMSK) modems in spacecraft-based mobile communications systems. For an employment of these modems, it is necessary that the transmitted carrier frequency be known prior to signal detection. In addition, the time needed by the receiver to lock onto the carrier frequency must be minimized. The present article is concerned with a DMSK modem developed for the Mobile Satellite Service. This device demonstrated fast acquisition time and good performance in the presence of fading. However, certain problems arose in initial attempts to study the acquisition behavior of the AFC loop through breadboard techniques. The development of a software model of the AFC loop is discussed, taking into account two cases which were plotted using the model. Attention is given to a demonstration of the viability of the modem by an approach involving modeling and analysis of the frequency synchronizer.

Davarian, F.↗
NASA NTRS1990

An implementation of a reference symbol approach to generic modulation in fading channels

As mobile satellite communications systems evolve over the next decade, they will have to adapt to a changing tradeoff between bandwidth and power. This paper presents a flexible approach to digital modulation and coding that will accommodate both wideband and narrowband schemes. This architecture could be the basis for a family of modems, each satisfying a specific power and bandwidth constraint, yet all having a large number of common signal processing blocks. The implementation of this generic approach, with general purpose digital processors for transmission of 4.8 kilobits per sec. digitally encoded speech, is described.

Young, R. J.↗