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At least 145 records · Page 8
All sourcesNASA NTRSDOE OSTI
NASA NTRS1972

A high speed sequential decoder

The performance and theory of operation for the High Speed Hard Decision Sequential Decoder are delineated. The decoder is a forward error correction system which is capable of accepting data from binary-phase-shift-keyed and quadriphase-shift-keyed modems at input data rates up to 30 megabits per second. Test results show that the decoder is capable of maintaining a composite error rate of 0.00001 at an input E sub b/N sub o of 5.6 db. This performance has been obtained with minimum circuit complexity.

Lum, H., Jr.↗
NASA NTRS1972

On the detection of differentially encoded polyphase signals.

Consideration of the transmission and detection of differentially encoded multiple phase-shift-keyed (MPSK) signals, paying particular attention to the ambiguity resolution problem resulting from suppression of the transmitted carrier. A study is made of the coherent detection of differentially encoded MPSK signals, and the performance of a differentially encoded MPSK system is compared with that of a system which transmits absolutely encoded polyphase signals and performs perfect ambiguity resolution. Both the perfect and noisy reference signal cases are treated. Also, the performance of coherent detection of differentially encoded MPSK signals is compared with that of differentially coherent reception of the same signal set.

Lindsey, W. C.↗
NASA NTRS1972

On the technology of aerospace communication in multipath.

The mechanism, effects, and modelling of multipath propagation, caused by rough earth reflection, are examined for aerospace communication. Emphasis is on binary digital signalling for aircraft and hybrid vehicles, such as Shuttle. The cases of direct Air-Ground and satellite relay (Aerosat) are treated. The recursive, adaptive, coherent Bayes detector for binary phase-shift-keying in nonselective multipath is presented. The derivation for the frequency-shift-keying detector is indicated.

Painter, J. H.↗
NASA NTRS1973

Balloon-aircraft ranging, data, and voice experiment.

The test facilities used in the experiment consisted of a ground station, a balloon platform, radar tracking stations, and a test aircraft. As a direct result of the experiment, several modifications have been incorporated into the equipment. The two most important modifications were the introduction of a 10-sec delay into the search mode and the use of differentially coded phase shift keying for the data channel.

Wishna, S.↗
NASA NTRS1973

A review of multiple amplitude-phase digital signals

This paper reviews the data rate, error rate, and signal-to-noise ratio relationship for various uncoded M-ary digital amplitude modulation (AM), phase modulation (PM), and combined AM-PM systems. These signal systems have the common virtue that expanding the number of possible signals to be transmitted increases the data rate but not the bandwidth. A general treatment of the error rate of M-ary digital AM-PM permits development of a simple yet accurate expression which approximates the increase in average signal-to-noise ratio (over that of binary phase shift keying) required for constant error performance. This equation provides insight into why arrays differ in their signal-to-noise ratio requirements.

Smith, J. G.↗
NASA NTRS1974

Signal simulator model A-111

A signal simulator designed to function as a signal source for Doppler tracking high data rate receivers is described. The simulator produces modulated signals whose carrier frequency can be varied between 200 and 900 MHz at rates greater than 20 MHz/sec. The modulation is phase shift keying with data rate up to 300 megabits per second.

Flattau, T.↗
NASA NTRS1974

Wideband infrared receiver backend

The design, operation, and configuration of the receiver backend for a wideband infrared tracking receiver are described. A squaring loop is incorporated into the receiver design to enable it to track the Doppler shifted carrier which is Phase Shift Keyed modulated. The receiver has a 400 MHz instantaneous bandwidth and tracks signals whose carrier frequency at the backend input is between 200 and 900 MHz with frequency variation rates greater than 20 MHz/second. The output data is compatible with MECL 3 logic.

Flattau, T.↗
NASA NTRS1974

Performance of convolutional codes on fading channels typical of planetary entry missions

The performance of convolutional codes in fading channels typical of the planetary entry channel is examined in detail. The signal fading is due primarily to turbulent atmospheric scattering of the RF signal transmitted from an entry probe through a planetary atmosphere. Short constraint length convolutional codes are considered in conjunction with binary phase-shift keyed modulation and Viterbi maximum likelihood decoding, and for longer constraint length codes sequential decoding utilizing both the Fano and Zigangirov-Jelinek (ZJ) algorithms are considered. Careful consideration is given to the modeling of the channel in terms of a few meaningful parameters which can be correlated closely with theoretical propagation studies. For short constraint length codes the bit error probability performance was investigated as a function of E sub b/N sub o parameterized by the fading channel parameters. For longer constraint length codes the effect was examined of the fading channel parameters on the computational requirements of both the Fano and ZJ algorithms. The effects of simple block interleaving in combatting the memory of the channel is explored, using the analytic approach or digital computer simulation.

Modestino, J. W.↗
NASA NTRS1974

Decision-directed automatic gain control for MAPSK systems

An automatic gain control (AGC) loop is presented for use with M-ary amplitude and phase shift keying (MAPSK) systems. The gain control amplifier is regulated by an error signal formed by the difference between the estimated amplitude level and the received amplitude level. The AGC performance is thus independent of the short-term average received signal energy. AGC loop analysis and simulation is presented for M-ary amplitude shift keying and quadrature amplitude shift keying. The AGC is shown to have a negligible degradation on the symbol probability of error for most practical cases. A generalized AGC for an arbitrary MAPSK system is presented.

Weber, W. J., III↗
NASA NTRS1974

Transmitter efficiency considerations in MAPSK signal selection

A simple model of the efficiency-power relationship for a microwave transmitter permits comparison of M-ary amplitude and phase shift keyed (MAPSK) signal array performance on the basis of average DC power in, rather than average RF signal power out. This is of interest in comparing MAPSK arrays with comparable polyphase, since the polyphase transmitter is operated at a single power level, while MAPSK varies over a range of power levels, each of differing efficiency. A constant efficiency-power curve permits comparison of signal arrays in terms of array average power, while a linear curve requires comparison on the basis of array peak power. Since typical efficiency curves will lie between these extremes, the array peak and average power levels provide bounds on array requirements for average DC input power. Examination of the performance of several MAPSK sets reveals a distinct power advantage of quadrature amplitude shift keying (QASK) over most viable signal sets.

Smith, J. G.↗
NASA NTRS1974

Convolutional code performance in fading channels

The performance of convolutional codes in fading channels typical of the planetary entry channel is examined in detail. Short constraint length codes are considered in conjunction with binary phase-shift-keyed (BPSK) modulation and Viterbi maximum likelihood decoding while for longer constraint length codes we consider sequential decoding utilizing both the Fano and Zigangirov-Jelinek (ZJ) algorithms. For short constraint length codes we are primarily interested in the bit error probability performance parameterized by the fading channel parameters. For longer constraint length codes interest will center on the effect of the fading channel parameters on the computational requirements of both the Fano and ZJ algorithms. In either case the effects of simple block interleaving in combatting the memory of the channel is thoroughly explored. The approach is analytic where possible otherwise resort is made to digital computer simulation.

Modestino, J. W.↗
NASA NTRS1974

Odd-bit symmetric QASK

Multiple-amplitude and phase-shift-keyed (MAPSK) signal set selection is influenced by factors such as average and/or peak signal-to-noise ratio for a given error probability, dynamic range of signal amplitudes, simplicity of generation and detection, and number of bit errors per adjacent symbol error. This paper compares two possible quadrature-amplitude-shift-keyed (QASK) signal sets for the case where the number of bits per symbol is odd (for the even-bit case, the square array is the only viable QASK choice). The symmetric QASK version out-performs the rectangular QASK set at a very modest implementation penalty. This permits symmetric QASK to be considered in future odd-bit system studies.

Smith, J. G.↗
NASA NTRS1977

A linear phase demodulator

Circuit operates on feedback principle to reduce modulation index of received signal, and thus reinserts carrier component in suppressed carrier signal. Device can demodulate phase-shift-keyed signals which do not have carrier component, and it has linear region of + or - 150 deg.

Rippy, R. R.↗
NASA NTRS1976

Carrier and symbol synchronization system performance study

Results pertinent to predicting the performance of convolutionally encoded binary phase-shift keyed communication links were presented. The details of the development are provided in four sections. These sections are concerned with developing the bit error probability performance degradations due to PN despreading by a time-shared delay locked loop, the Costas demodulation process, symbol synchronization effects and cycle slipping phenomena in the Costas loop. In addition, Costas cycle slipping probabilities are studied as functions of Doppler count time and signal-to-noise conditions. The effect of cycle slipping in the symbol synchronizer is also studied as a function of channel Doppler and other frequency uncertainties.

Lindsey, W. C.↗
NASA NTRS1975

Long frame sync words for binary PSK telemetry

Correlation criteria have previously been established for identifying whether a given binary sequence would be a good frame sync word for phase-shift keyed telemetry. In the past, the search for a good K-bit sync word has involved the application of these criteria to the entire set of 2 exponent K binary K-tuples. It is shown that restricting this search to a much smaller subset consisting of K-bit prefixes of pseudonoise sequences results in sync words of comparable quality, with greatly reduced computer search times for larger values of K. As an example, this procedure is used to find good sync words of length 16-63; from a storage viewpoint, each of these sequences can be generated by a 5- or 6-bit linear feedback shift register.

Levitt, B. K.↗
NASA NTRS1976

Convolutional code performance in the Rician fading channel

The performance of short constraint length convolutional codes in conjunction with binary phase-shift keyed (BPSK) modulation and Viterbi maximum likelihood decoding on the classical Rician fading channel is examined in detail. Primary interest is in the bit error probability performance as a function of E sub b/N sub 0 parameterized by the fading channel parameters. Fairly general upper bounds on bit error probability performance in the presence of fading are obtained and compared with simulation results in the two extremes of zero channel memory and infinite channel memory. The efficacy of simple block interleaving in combating the memory of the channel is thoroughly explored. Results include the effects of fading on tracking loop performance and the subsequent impact on overall coded system performance. The approach is analytical where possible; otherwise resort is made to digital computer simulation.

Modestino, J. W.↗