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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.

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At least 577 records · Page 32
All sourcesNASA NTRSDOE OSTI
NASA NTRS1989

High rate concatenated coding systems using multidimensional bandwidth-efficient trellis inner codes

A concatenated coding system using two-dimensional trellis-coded MPSK inner codes and Reed-Solomon outer codes for application in high-speed satellite communication systems was proposed previously by the authors (1989). The authors extend their results to systems using symbol-oriented, multidimensional, trellis-coded MPSK inner codes. The concatenated coding systems are divided into two classes according to their achievable effective information rates. The first class uses multidimensional trellis-coded 8-PSK inner codes and achieves effective information rates around 1 b/dimension (spectral efficiency 2 b/s/Hz). The second class employs multidimensional trellis-coded 16-PSK inner codes and provides effective information rates around 1.5 b/dimension (spectral efficiency 3 b/s/Hz). Both classes provide significant coding gains over an uncoded reference system with the same effective information rate as the coded system. The results show that the symbol-oriented nature of multidimensional inner codes can provide an improvement of up to 1 dB in the overall performance of a concatenated coding system when these codes replace bit-oriented two-dimensional codes.

Deng, Robert H.↗
NASA NTRS1989

Single chip fixed frequency bit synchronizer

A single chip, fixed frequency suboptimum bit synchronizer design which was implemented utilizing a programmable logic device is described. The bit synchronizer is modeled after a digital transition tracking loop for symbol estimation and employs a first-order incremental phase modulator for closed-loop symbol synchronization. The BER and tracking performance is modeled and compared to optimum designs. The bit synchronizer was developed for the Space Shuttle.

Butler, Madeline J.↗
NASA NTRS1990

An empirical study of FORTRAN programs for parallelizing compilers

Some results are reported from an empirical study of program characteristics that are important in parallelizing compiler writers, especially in the area of data dependence analysis and program transformations. The state of the art in data dependence analysis and some parallel execution techniques are examined. The major findings are included. Many subscripts contain symbolic terms with unknown values. A few methods of determining their values at compile time are evaluated. Array references with coupled subscripts appear quite frequently; these subscripts must be handled simultaneously in a dependence test, rather than being handled separately as in current test algorithms. Nonzero coefficients of loop indexes in most subscripts are found to be simple: they are either 1 or -1. This allows an exact real-valued test to be as accurate as an exact integer-valued test for one-dimensional or two-dimensional arrays. Dependencies with uncertain distance are found to be rather common, and one of the main reasons is the frequent appearance of symbolic terms with unknown values.

Shen, Zhiyu↗
NASA NTRS1991

Overview of arraying techniques in the deep space network

Four different arraying schemes that can be used by the Deep Space Network are functionally discussed and compared. These include symbol stream combining (SSC), baseband combining (BC), carrier arraying (CA), and full spectrum combining (FSC). In addition, sibeband aiding (SA) is also included and compared even though it is not an arraying scheme, since it uses a single antenna. Moreover, combinations of these schemes are discussed, such as carrier arraying with sideband aiding and baseband combining (CA/SA/BC) or carrier arraying with symbol stream combining (CA/SSC). Complexity versus performance is traded off and the benefits to the reception of existing spacecraft signals are discussed. Recommendations are made as to the best techniques for particular configurations.

Mileant, A.↗
NASA NTRS1991

The advanced receiver 2: Telemetry test results in CTA 21

Telemetry tests with the Advanced Receiver II (ARX II) in Compatibility Test Area 21 are described. The ARX II was operated in parallel with a Block-III Receiver/baseband processor assembly combination (BLK-III/BPA) and a Block III Receiver/subcarrier demodulation assembly/symbol synchronization assembly combination (BLK-III/SDA/SSA). The telemetry simulator assembly provided the test signal for all three configurations, and the symbol signal to noise ratio as well as the symbol error rates were measured and compared. Furthermore, bit error rates were also measured by the system performance test computer for all three systems. Results indicate that the ARX-II telemetry performance is comparable and sometimes superior to the BLK-III/BPA and BLK-III/SDA/SSA combinations.

Hinedi, S.↗
NASA NTRS1990

Army-NASA aircrew/aircraft integration program (A3I) software detailed design document, phase 3

The capabilities and design approach of the MIDAS (Man-machine Integration Design and Analysis System) computer-aided engineering (CAE) workstation under development by the Army-NASA Aircrew/Aircraft Integration Program is detailed. This workstation uses graphic, symbolic, and numeric prototyping tools and human performance models as part of an integrated design/analysis environment for crewstation human engineering. Developed incrementally, the requirements and design for Phase 3 (Dec. 1987 to Jun. 1989) are described. Software tools/models developed or significantly modified during this phase included: an interactive 3-D graphic cockpit design editor; multiple-perspective graphic views to observe simulation scenarios; symbolic methods to model the mission decomposition, equipment functions, pilot tasking and loading, as well as control the simulation; a 3-D dynamic anthropometric model; an intermachine communications package; and a training assessment component. These components were successfully used during Phase 3 to demonstrate the complex interactions and human engineering findings involved with a proposed cockpit communications design change in a simulated AH-64A Apache helicopter/mission that maps to empirical data from a similar study and AH-1 Cobra flight test.

Banda, Carolyn↗
NASA NTRS1991

On the optimality of code options for a universal noiseless coder

A universal noiseless coding structure was developed that provides efficient performance over an extremely broad range of source entropy. This is accomplished by adaptively selecting the best of several easily implemented variable length coding algorithms. Custom VLSI coder and decoder modules capable of processing over 20 million samples per second are currently under development. The first of the code options used in this module development is shown to be equivalent to a class of Huffman code under the Humblet condition, other options are shown to be equivalent to the Huffman codes of a modified Laplacian symbol set, at specified symbol entropy values. Simulation results are obtained on actual aerial imagery, and they confirm the optimality of the scheme. On sources having Gaussian or Poisson distributions, coder performance is also projected through analysis and simulation.

Yeh, Pen-Shu↗
NASA NTRS1991

Time-domain finite elements in optimal control with application to launch-vehicle guidance

A time-domain finite element method is developed for optimal control problems. The theory derived is general enough to handle a large class of problems including optimal control problems that are continuous in the states and controls, problems with discontinuities in the states and/or system equations, problems with control inequality constraints, problems with state inequality constraints, or problems involving any combination of the above. The theory is developed in such a way that no numerical quadrature is necessary regardless of the degree of nonlinearity in the equations. Also, the same shape functions may be employed for every problem because all strong boundary conditions are transformed into natural or weak boundary conditions. In addition, the resulting nonlinear algebraic equations are very sparse. Use of sparse matrix solvers allows for the rapid and accurate solution of very difficult optimization problems. The formulation is applied to launch-vehicle trajectory optimization problems, and results show that real-time optimal guidance is realizable with this method. Finally, a general problem solving environment is created for solving a large class of optimal control problems. The algorithm uses both FORTRAN and a symbolic computation program to solve problems with a minimum of user interaction. The use of symbolic computation eliminates the need for user-written subroutines which greatly reduces the setup time for solving problems.

Bless, Robert R.↗
NASA NTRS1991

Coding gains and error rates from the Big Viterbi Decoder

A prototype hardware Big Viterbi Decoder (BVD) was completed for an experiment with the Galileo Spacecraft. Searches for new convolutional codes, studies of Viterbi decoder hardware designs and architectures, mathematical formulations, and decompositions of the deBruijn graph into identical and hierarchical subgraphs, and very large scale integration (VLSI) chip design are just a few examples of tasks completed for this project. The BVD bit error rates (BER), measured from hardware and software simulations, are plotted as a function of bit signal to noise ratio E sub b/N sub 0 on the additive white Gaussian noise channel. Using the constraint length 15, rate 1/4, experimental convolutional code for the Galileo mission, the BVD gains 1.5 dB over the NASA standard (7,1/2) Maximum Likelihood Convolution Decoder (MCD) at a BER of 0.005. At this BER, the same gain results when the (255,233) NASA standard Reed-Solomon decoder is used, which yields a word error rate of 2.1 x 10(exp -8) and a BER of 1.4 x 10(exp -9). The (15, 1/6) code to be used by the Cometary Rendezvous Asteroid Flyby (CRAF)/Cassini Missions yields 1.7 dB of coding gain. These gains are measured with respect to symbols input to the BVD and increase with decreasing BER. Also, 8-bit input symbol quantization makes the BVD resistant to demodulated signal-level variations which may cause higher bandwidth than the NASA (7,1/2) code, these gains are offset by about 0.1 dB of expected additional receiver losses. Coding gains of several decibels are possible by compressing all spacecraft data.

Onyszchuk, I. M.↗
NASA NTRS1991

A carrier-arraying demonstration at Goldstone for receiving Pioneer 11 signals

A carrier arraying technique was demonstrated at Goldstone. The Block III receivers of two 34-m antennas, Deep Space Station 12 and 15, were arrayed together to receive S band (2.3 GHz) signals from the Pioneer 11 spacecraft. Carrier phases in the two receivers were synchronized by the analog phase lock loops, and carrier signals were added at an intermediate frequency to enhance tracking performance. The receiver at DSS 15, which has been unable to lock up and track the Pioneer 11 signal by itself due to a wider tracking loop bandwidth and a higher system temperature, was now able to track the carrier and produce usable baseband signals. The receiver at DSS 12 achieved a reduction of the rms phase error, increasing the telemetry symbol SNR by an average of 0.35 dB. The baseband signals from both antennas were then synchronized and combined using the existing Baseband Assembly, thereby achieving a total symbol SNR increase of 2.5 + or - 0.7 dB relative to DSS 12 alone. A more comprehensive model is presented to permit evaluation of both thermal and phase noise effects. The analysis agrees well with observed data.

Pham, T. T.↗
NASA NTRS1991

Multi-dimensional modulation codes for fading channel

Some new codes are presented which have good performance on Rician fading channel with small decoding complexities. A new M-way partition chain is proposed for the L x MPSK (L less than or equal to M) signal set which maximizes the intra-set distance of each subset at each partition level. Based on this partition chain, a class of asymptotical optimum codes was found. For M = 4, these codes have both large symbol distances and product distances. Multi-level coding scheme allows to construct a code by hand such that the code meets some desired parameters, e.g., symbol distance, product distance, etc. In design of a multi-level code, all factors were considered which affect the performance and complexity of the code, such as, the decoding scheme, decoding complexity, and performance under the decoding scheme, e.g., if the multi-stage decoding scheme is used, the performance degradation due to the suboptimum decoding is taken into consideration. The performance for most of the presented codes was simulated on Rayleigh fading channel, and the results show that these codes have good performance with small decoding complexities.

Lin, Shu↗
NASA NTRS1990

Toward modeling a dynamic biological neural network

The organization of a mammalian macular endorgan is interpreted on physical and engineering principles. Having accomplished this task, it is possible to mathematically and symbolically model information processing by the macular neural network. A complex symbolic model is produced through use of mathematical notations that describe the functioning system. The model, consisting of six tiers, is constructed to mimic the neural system. On the basis of initial simulations, it is concluded that the network functions best when some of the detecting elements (i.e., type I hair cells) are weakly inhibitory. The simulations also illustrate the importance of disinhibition of receptors located in the third tier in shaping nerve discharge patterns at the sixth tier in the model system.

Ross, M. D.↗
NASA NTRS1992

Open-Loop Acquisition Of Frequency In BPSK

Proposed open-loop analog/digital signal-processing system would be Costas-loop error detector functioning in closed-loop manner overall. Detector estimates difference between frequency of input signal and internal reference oscillator. Estimate used to close frequency-control loop. Precise symbol timing not necessary. Performance better than systems that effect open-loop acquisition using integrators instead of low-pass filters in arms of Costas loops and in which performance varies with symbol timing.

Shah, Biren N.↗
NASA NTRS1991

Programmable digital modem

The design of the Programmable Digital Modem (PDM) is outlined. The PDM will be capable of operating with numerous modulation techniques including: 2-, 4-, 8- and 16-ary phase shift keying (PSK), minimum shift keying (MSK), and 16-ary quadrature amplitude modulation (QAM), with spectral occupancy from 1.2x to 2x the data symbol rate. It will also be programmable for transmission rates ranging from 2.34 to 300 Mbit/s, where the maximum symbol rate is 75 Msymbol/s. Furthermore, these parameters will be executable in independent burst, dependent burst, or continuous mode. In dependent burst mode the carrier and clock oscillator sources are common from burst to burst. To achieve as broad a set of requirements as these, it is clear that the essential signal processing must be digital. In addition, to avoid hardware changes when the operational parameters are changed, a fixed interface to an analog intermediate frequency (IF) is necessary for transmission; and, common system level architectures are necessary for the modulator and demodulator. Lastly, to minimize size and power, as much of the design as possible will be implemented with application specific integrated circuit (ASIC) chips.

Poklemba, John J.↗
NASA NTRS1992

An 8-PSK TDMA uplink modulation and coding system

The combination of 8-phase shift keying (8PSK) modulation and greater than 2 bits/sec/Hz drove the design of the Nyquist filter to one specified to have a rolloff factor of 0.2. This filter when built and tested was found to produce too much intersymbol interference and was abandoned for a design with a rolloff factor of 0.4. The preamble is limited to 100 bit periods of the uncoded bit period of 5 ns for a maximum preamble length of 500 ns or 40 8PSK symbol times at 12.5 ns per symbol. For 8PSK modulation, the required maximum degradation of 1 dB in -20 dB cochannel interference (CCI) drove the requirement for forward error correction coding. In this contract, the funding was not sufficient to develop the proposed codec so the codec was limited to a paper design during the preliminary design phase. The mechanization of the demodulator is digital, starting from the output of the analog to digital converters which quantize the outputs of the quadrature phase detectors. This approach is amenable to an application specific integrated circuit (ASIC) replacement in the next phase of development.

Ames, S. A.↗
NASA NTRS1990

Combined trellis coding and feedforward processing for MSS applications

The idea of using a multiple (more than two) symbol observation interval to improve error probability performance is applied to differential detection of trellis coded MPSK over a mobile satellite (fading) channel. Results are obtained via computer simulation. It is shown that only a slight increase (e.g., one symbol) in the length of the observation interval will provide a significant improvement in bit error probability performance both in AWGN and fading environments.

Divsalar, D.↗
NASA NTRS1992

Bandwidth efficient CCSDS coding standard proposals

The basic concatenated coding system for the space telemetry channel consists of a Reed-Solomon (RS) outer code, a symbol interleaver/deinterleaver, and a bandwidth efficient trellis inner code. A block diagram of this configuration is shown. The system may operate with or without the outer code and interleaver. In this recommendation, the outer code remains the (255,223) RS code over GF(2 exp 8) with an error correcting capability of t = 16 eight bit symbols. This code's excellent performance and the existence of fast, cost effective, decoders justify its continued use. The purpose of the interleaver/deinterleaver is to distribute burst errors out of the inner decoder over multiple codewords of the outer code. This utilizes the error correcting capability of the outer code more efficiently and reduces the probability of an RS decoder failure. Since the space telemetry channel is not considered bursty, the required interleaving depth is primarily a function of the inner decoding method. A diagram of an interleaver with depth 4 that is compatible with the (255,223) RS code is shown. Specific interleaver requirements are discussed after the inner code recommendations.

Costello, Daniel J., Jr.↗
NASA NTRS1992

Rotative quadrature phase-shift keying

A rotative quadrature phase-shift keying (RQPSK) modulation scheme is proposed. By rotating the QPSK signal constellation by pi/2, either clockwise or anticlockwise, during a symbol duration, the conventional QPSK scheme can be modified to transmit 3 bits per symbol to achieve both power and bandwidth efficiency.

Liu, J.↗