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At least 595 records · Page 33

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NASA NTRS1990

On decoding of multi-level MPSK modulation codes

The decoding problem of multi-level block modulation codes is investigated. The hardware design of soft-decision Viterbi decoder for some short length 8-PSK block modulation codes is presented. An effective way to reduce the hardware complexity of the decoder by reducing the branch metric and path metric, using a non-uniform floating-point to integer mapping scheme, is proposed and discussed. The simulation results of the design are presented. The multi-stage decoding (MSD) of multi-level modulation codes is also investigated. The cases of soft-decision and hard-decision MSD are considered and their performance are evaluated for several codes of different lengths and different minimum squared Euclidean distances. It is shown that the soft-decision MSD reduces the decoding complexity drastically and it is suboptimum. The hard-decision MSD further simplifies the decoding while still maintaining a reasonable coding gain over the uncoded system, if the component codes are chosen properly. Finally, some basic 3-level 8-PSK modulation codes using BCH codes as component codes are constructed and their coding gains are found for hard decision multistage decoding.

Lin, Shu↗

NASA NTRS1988

Electrochemical and galvanic corrosion effects in thin-film photovoltaic modules

The electrochemical and galvanic corrosion properties of thin-film photovoltaic (TF-PV) modules and module subcomponents are determined and interpreted in the light of established corrosion science. Results of a detailed study of thin-film aluminum metallization corrosion are presented. Bar-graph corrosion, observed in fielded modules, has been induced experimentally and found to be electrochemical in nature. Corrosion rates and passivation techniques for TF-PV modules are discussed.

Mon, G.↗

NASA NTRS1988

Water-module interaction studies

Mechanisms by which moisture enters photovoltaic modules and techniques for reducing such interactions are reported. Results from a study of the effectiveness of various module sealants are given. Techniques for measuring the rate and quantity of moisture ingress are discussed. It is shown that scribe lines and porous frit bridging conductors provide preferential paths for moisture ingress and that moisture diffusion by surface/interfacial paths is considerably more rapid than diffusion by bulk paths, which implies that thin-film substrate and supersubstrate modules are much more vulnerable to moist environments than are bulk-encapsulated crystalline-silicon modules. Design approaches that reduce moisture entry are discussed.

Mon, G.↗

NASA NTRS1990

Location of a faulty module in a computing system

Considering the interplay between different phases of fault tolerance, a new problem of locating a faulty module in a computing system is formulated and solved. First, the probability of each module being faulty, or faulty probability, is calculated using the likelihood principle from the model parameters for fault detection, diagnostics, error propagation, and error detection. Then, based on the faulty probabilities and a given required diagnostic coverage, the order in which modules are to be diagnosed and the maximum time allotted to diagnose each module are determined by minimizing the average total diagnostic time. An example is presented and analyzed to answer the question of whether or not a system should delay the diagnosis upon detection of an error until more errors are detected.

Lin, Tein-Hsiang↗

NASA NTRS1990

Spread-spectrum code acquisition in the presence of Doppler shift and data modulation

A spread-spectrum code acquisition technique for a direct-sequence (DS) system in the presence of Doppler effect and data modulation is investigated. Both the carrier-frequency offset and code-frequency offset due to severe Doppler effect are considered. The code-chip slipping during the correlation process caused by code-frequency offset can degrade the acquisition performance significantly. However, this issue can be alleviated by compensating code-frequency offset in an appropriate manner. Results are presented for the cases with and without data modulation. Coherent detection is considered when there is no data modulation. If data modulation is present, the authors partition the correlation time into subintervals and the integration results in these subintervals are square-law noncoherently combined for detection. The implementation of this code acquisition technique using the fast Fourier transform (FFT) algorithm is described. The use of theoretical results to estimate the hardware complexity of an actual system is illustrated step by step, showing that implementation is feasible with existing technology. The tradeoff between hardware complexity and acquisition performance is discussed.

Cheng, Unjeng↗

NASA NTRS1989

Photovoltaic module on-orbit assembly for Space Station Freedom

One of the elements of the Space Station Freedom power system is the photovoltaic (PV) module. These modules will be assembled on-orbit during the assembly phase of the program. These modules will be assembled either from the Shuttle Orbiter or from the Mobile Servicing Center (MSC). The different types of assembly operations that will be used to assemble PV modules are described.

Sours, T.↗

NASA NTRS1990

Analysis of energy states in modulation doped multiquantum well heterostructures

A precise and effective numerical procedure to model the band diagram of modulation doped multiquantum well heterostructures is presented. This method is based on a self-consistent iterative solution of the Schroedinger equation and the Poisson equation. It can be used rather easily in any arbitrary modulation-doped structure. In addition to confined energy subbands, the unconfined states can be calculated as well. Examples on realistic device structures are given to demonstrate capabilities of this procedure. The numerical results are in good agreement with experiments. With the aid of this method the transitions involving both the confined and unconfined conduction subbands in a modulation doped AlGaAs/GaAs superlattice, and in a strained layer InGaAs/GaAs superlattice are identified. These results represent the first observation of unconfined transitions in modulation doped multiquantum well structures.

Ji, G.↗

NASA NTRS1990

A concatenated coded modulation scheme for error control

A concatenated coded modulation scheme for error control in data communications is presented. The scheme is achieved by concatenating a Reed-Solomon outer code and a bandwidth efficient block inner code for M-ary PSK modulation. Error performance of the scheme is analyzed for an AWGN channel. It is shown that extremely high reliability can be attained by using a simple M-ary PSK modulation inner code and relatively powerful Reed-Solomon outer code. Furthermore, if an inner code of high effective rate is used, the bandwidth expansion required by the scheme due to coding will be greatly reduced. The proposed scheme is particularly effective for high speed satellite communications for large file transfer where high reliability is required. Also presented is a simple method for constructing block codes for M-ary PSK modulation. Some short M-ary PSK codes with good minimum squared Euclidean distance are constructed. These codes have trellis structure and hence can be decoded with a soft-decision Viterbi decoding algorithm.

Kasami, Tadao↗

NASA NTRS1991

Spatial Light Modulator Would Serve As Electronic Iris

In proposed technique for controlling brightness of image formed by lens, spatial light modulator serves as segmented, electronically variable aperture. Offers several advantages: spatial light modulator controlled remotely and responds faster than motorized iris or other remotely controlled mechanical iris. Unlike iris, modulator also configured so as not to vary depth of field appreciably. Unlike lead lanthanum zirconate titanate crystal, spatial light modulator does not require high voltage.

Gutow, David A.↗

NASA NTRS1991

Combinatorial FSK modulation for power-efficient high-rate communications

Deep-space and satellite communications systems must be capable of conveying high-rate data accurately with low transmitter power, often through dispersive channels. A class of noncoherent Combinatorial Frequency Shift Keying (CFSK) modulation schemes is investigated which address these needs. The bit error rate performance of this class of modulation formats is analyzed and compared to the more traditional modulation types. Candidate modulator, demodulator, and digital signal processing (DSP) hardware structures are examined in detail. System-level issues are also discussed.

Wagner, Paul K.↗

NASA NTRS1991

Multi-stage decoding for multi-level block modulation codes

Various types of multistage decoding for multilevel block modulation codes, in which the decoding of a component code at each stage can be either soft decision or hard decision, maximum likelihood or bounded distance are discussed. Error performance for codes is analyzed for a memoryless additive channel based on various types of multi-stage decoding, and upper bounds on the probability of an incorrect decoding are derived. It was found that, if component codes of a multi-level modulation code and types of decoding at various stages are chosen properly, high spectral efficiency and large coding gain can be achieved with reduced decoding complexity. It was found that the difference in performance between the suboptimum multi-stage soft decision maximum likelihood decoding of a modulation code and the single stage optimum decoding of the overall code is very small, only a fraction of dB loss in SNR at the probability of an incorrect decoding for a block of 10(exp -6). Multi-stage decoding of multi-level modulation codes really offers a way to achieve the best of three worlds, bandwidth efficiency, coding gain, and decoding complexity.

Lin, Shu↗

NASA NTRS1990

The heliospheric current sheet and modulation of Galactic cosmic rays

The posssible effect of the heliospheric current sheet on the modulation of cosmic rays is examined by examining the number and the nature of coronal mass ejections on the basis of data collected on an abrupt onset of cosmic ray modulation observed in May 1987 on earth and in September 1987 by Pioneer 10 and 11 and in a previous study of modulation for the years 1976-1986. The measure used to examine the gradient-drift theory (according to which modulation is associated with solar cycle changes in the current sheet) is the value of the difference in the maximum latitudinal extents for the current sheet in the northern and the southern solar hemispheres. These were obtained from contours of the current sheet produced by extrapolating photospheric magnetic field measurements to a solar wind source surface. The observations are found to be consistent with the predictions of the gradient drift model.

Smith, Edward J.↗

NASA NTRS1990

Cross-over component code construction for multi-level block modulation codes

This paper investigates the multilevel technique for combining block coding and modulation. Several specific methods for constructing multilevel block modulation codes with interdependency among component codes are presented. Given a multilevel block modulation code C with no interdependency among the binary component codes, the proposed methods give a multilevel block modulation code C-prime which has the same rate as C, a minimum squared Euclidean distance not less than that of C, a trellis diagram with the same number of states as that of C, and a smaller number of nearest neighbor codewords than that of C.

Kasami, Tadao↗

NASA NTRS1990

Optical properties of KD*P modulators

Longitudinal KD*P modulators are used in ground-based solar magnetographs to eliminate seeing effects. Although the modulators can be used as variable retarders, the optical properties when zero voltage is applied influences the performance on instruments requiring very accurate polarization measurements. Measurements at the Marshall Space Flight Center are discussed in terms of the optical properties of KD*P modulators when zero voltage is applied. The measurements can be used to predict the modulation characteristics of the devices and to determine the polarization accuracy that can be expected from the vector magnetograph.

West, E. A.↗

NASA NTRS1991

Combinatorial FSK modulation for power-efficient high-rate communications

Wagner, Paul K.↗

NASA NTRS1991

Theory of the resonance line shape in velocity-modulation spectroscopy

Velocity-modulation spectroscopy is often used to obtain absorption spectra of ions in discharges. However, there have been few studies of the line shape. The resonance line shape in velocity-modulation spectroscopy is derived in the present work. Both sinusoidal modulation and square-wave modulation are treated, and the differences in the resulting line shapes are emphasized. The results are relevant to measurements of ion mobilities, resolved with respect to the internal state of the ion.

Farley, John W.↗

NASA NTRS1992

Modulating Concentration Of Hydrazine Vapor To Aid Detection

Three techniques, flow modulation, filament modulation, and reactive-filter modulation developed to obtain selective response to hydrazine or to methyl hydrazines from photoionization detector. Implemented in portable selective hydrazine detectors, useful where continuous or rapid, repetitive quantitative measurements of concentrations of vapors of these toxic substances required. Measure concentration of gaseous hydrazines in presence of other gases like cyclohexane, chlorofluorocarbons, and ammonia. Produce modulated signal, amplitude and phase indicating amount of hydrazine, monomethyl hydrazine, or 1,1-dimethyl hydrazine present in mixture. Used with such other detectors as electrochemical cells.

Stetter, Joseph R.↗

NASA NTRS1991

Observations of x ray pulsars from the Kvant module

The Roentgen international x ray observatory on the Kvant module of the Mir space station has been successfully operating since the beginning of June 1987. Many x ray sources were observed and among them were several x ray pulsars. Four telescopes mounted on board the Kvant module cover a wide energy range with good timing resolution. Timing analysis of the Kvant module data suffers from the presence of only short continuous intervals of source observations, separated by 90 min gaps (90 min is the orbital period of the Mir space station around the Earth). The presence of 90 min gaps leads to the appearance of beat frequencies v=v sub 0 + or - n/90 min (n = 1, 2, 3). Special analysis was applied to avoid this difficulty. Results are presented of the pulsation period measurements of the x ray pulsars Her X-1, Cen X-3, SMC X-1, Vela X-1, A0535 + 26 by the instruments on board the Kvant module in 1987 to 1989. The values of the periods are reduced to the solar system barycenter and to the binary system barycenter (excluding A0535 + 26).

Gilfanov, M.↗