Engineering topics

Wilson, S. G.

Publications and source records attributed to Wilson, S. G..

16 records

NASA NTRS1986

Rate 5/6 Trellis-coded 8-PSK

Some new short constraint length codes are described for satellite channels which are constant-envelope and which have spectral and energy efficiencies intermediate to other recent coded techniques, namely r = 2/3 coded 8-PSK and r = 3/4 coded 16-PSK. Two intervals of 8-PSK transmission are used to form a modulation base for r = 5/6 Trellis codes. A particularly attractive design is an eight-state code, producing 6.2 dB gain on the Gaussian channel over uncoded 8-PSK, yet sacrificing only 16 percent in spectral efficiency.

Wilson, S. G.↗

NASA NTRS1985

Convolutional coding combined with continuous phase modulation

Background theory and specific coding designs for combined coding/modulation schemes utilizing convolutional codes and continuous-phase modulation (CPM) are presented. In this paper the case of r = 1/2 coding onto a 4-ary CPM is emphasized, with short-constraint length codes presented for continuous-phase FSK, double-raised-cosine, and triple-raised-cosine modulation. Coding buys several decibels of coding gain over the Gaussian channel, with an attendant increase of bandwidth. Performance comparisons in the power-bandwidth tradeoff with other approaches are made.

Pizzi, S. V.↗

NASA NTRS1985

16-QAM and trellis-coded 16-QAM on nonlinear channels

The use of 16-QAM on bandlimited nonlinear satellite channels, in uncoded and trellis-coded form, for bandwidth efficient modulation (ideally 4 bps/Hz) is discussed. To avoid severe degradation due to AM/AM and AM/PM distortion, or to avoid the need for large back-off, predistorting the 16-QAM constellation at the modulator is considered. Performance for varying back-off settings for uncoded and coded 16-QAM is simulated, and it is found that the four-state trellis code has a coding gain of about 8 dB, larger than expected based on linear channel analysis, while the 16-state code gains only marginally beyond this.

Lyons, M. T.↗

NASA NTRS1984

Rate 3/4 convolutional coding of 16-PSK - Code design and performance study

Convolutional coding coupled with 16-PSK modulation is investigated for bandwidth efficient transmission. Rate 3/4, small memory codes are found which are optimized in the free-distance sense on the Gaussian channel. These codes provide up to 4.8 dB of coding gain with 32 states over uncoded 8-PSK, a scheme having the same spectral efficiency as the codes described. The performance is compared with earlier findings of Ungerboeck and some recent results on R = 2/3 coded 8-PSK. In addition, results of a channel transmission study to assess the performance of the four-state code on the band-limited nonlinear channel are presented, and it is found that performance of the coded scheme degrades comparably with uncoded 8-PSK, i.e., coding gain is roughly preserved.

Wilson, S. G.↗

NASA NTRS1984

An improved algorithm for evaluating trellis phase codes

A method is described for evaluating the minimum distance parameters of trellis phase codes, including CPFSK, partial response FM, and more importantly, coded CPM (continuous phase modulation) schemes. The algorithm provides dramatically faster execution times and lesser memory requirements than previous algorithms. Results of sample calculations and timing comparisons are included.

Mulligan, M. G.↗

NASA NTRS1984

Design considerations for a monolithic, GaAs, dual-mode, QPSK/QASK, high-throughput rate transceiver

A monolithic, GaAs, dual mode, quadrature amplitude shift keying and quadrature phase shift keying transceiver with one and two billion bits per second data rate is being considered to achieve a low power, small and ultra high speed communication system for satellite as well as terrestrial purposes. Recent GaAs integrated circuit achievements are surveyed and their constituent device types are evaluated. Design considerations, on an elemental level, of the entire modem are further included for monolithic realization with practical fabrication techniques. Numerous device types, with practical monolithic compatability, are used in the design of functional blocks with sufficient performances for realization of the transceiver.

Kot, R. A.↗

NASA NTRS1984

Carrier recovery methods for a dual-mode modem: A design approach

A dual mode model with selectable QPSK or 16-QASK modulation schemes is discussed. The theoretical reasoning as well as the practical trade-offs made during the development of a modem are presented, with attention given to the carrier recovery method used for coherent demodulation. Particular attention is given to carrier recovery methods that can provide little degradation due to phase error for both QPSK and 16-QASK, while being insensitive to the amplitude characteristic of a 16-QASK modulation scheme. A computer analysis of the degradation is symbol error rate (SER) for QPSK and 16-QASK due to phase error is prresented. Results find that an energy increase of roughly 4 dB is needed to maintain a SER of 1X10(-5) for QPSK with 20 deg of phase error and 16-QASK with 7 deg phase error.

Richards, C. W.↗

NASA NTRS1984

Four-dimensional modulation and coding - An alternate to frequency-reuse

Four dimensional modulation as a means of improving communication efficiency on the band-limited Gaussian channel, with the four dimensions of signal space constituted by phase orthogonal carriers (cos omega sub c t and sin omega sub c t) simultaneously on space orthogonal electromagnetic waves are discussed. 'Frequency reuse' techniques use such polarization orthogonality to reuse the same frequency slot, but the modulation is not treated as four dimensional, rather a product of two-D modulations, e.g., QPSK. It is well known that, higher dimensionality signalling affords possible improvements in the power bandwidth sense. Four-D modulations based upon subsets of lattice-packings in four-D, which afford simplification of encoding and decoding are described. Sets of up to 1024 signals are constructed in four-D, providing a (Nyquist) spectral efficiency of up to 10 bps/Hz. Energy gains over the reuse technique are in the one to three dB range t equal bandwidth.

Wilson, S. G.↗

NASA NTRS1983

Four-dimensional modulation and coding: An alternate to frequency-reuse

Four dimensional modulation as a means of improving communication efficiency on the band-limited Gaussian channel, with the four dimensions of signal space constituted by phase orthogonal carriers (cos omega sub c t and sin omega sub c t) simultaneously on space orthogonal electromagnetic waves are discussed. "Frequency reuse' techniques use such polarization orthogonality to reuse the same frequency slot, but the modulation is not treated as four dimensional, rather a product of two-d modulations, e.g., QPSK. It is well known that, higher dimensionality signalling affords possible improvements in the power bandwidth sense. Four-D modulations based upon subsets of lattice-packings in four-D, which afford simplification of encoding and decoding are described. Sets of up to 1024 signals are constructed in four-D, providing a (Nyquist) spectral efficiency of up to 10 bps/Hz. Energy gains over the reuse technique are in the one to three dB range t equal bandwidth.

Wilson, S. G.↗

NASA NTRS1983

Advanced digital modulation: Communication techniques and monolithic GaAs technology

Communications theory and practice are merged with state-of-the-art technology in IC fabrication, especially monolithic GaAs technology, to examine the general feasibility of a number of advanced technology digital transmission systems. Satellite-channel models with (1) superior throughput, perhaps 2 Gbps; (2) attractive weight and cost; and (3) high RF power and spectrum efficiency are discussed. Transmission techniques possessing reasonably simple architectures capable of monolithic fabrication at high speeds were surveyed. This included a review of amplitude/phase shift keying (APSK) techniques and the continuous-phase-modulation (CPM) methods, of which MSK represents the simplest case.

Wilson, S. G.↗

NASA NTRS1982

An improved algorithm for evaluating trellis phase codes

Mulligan, M. G.↗

NASA NTRS1982

Error bounds for multi-h phase codes

Upper and lower bounds for the probability of a decoding error event and for symbol error probability are developed for the class of time-varying phase codes known as multi-h codes. The effect of finite decoder memory is also treated. The analysis is illustrated with numerical examples, and simulation results are compared.

Wilson, S. G.↗

NASA NTRS1982

Rate 3/4 16-PSK phase codes

An investigation is carried out of convolutional-coding coupled with 16-PSK modulation for bandwidth efficient transmission. Small memory codes (rate three-fourths) are found that are optimum in the free-distance sense on the Gaussian channel. These codes furnish up to 4.8 dB of coding gain with 32 states over uncoded 8-PSK, a scheme having the same spectral efficiency as the codes described. The performance is compared with the earlier findings of Ungerboeck (1976, 1982) and certain recent results on R = 2/3 coded 8-PSK. It is shown that with 4 decoder states the coding gain is 3.5 dB and with 32 states it is 4.8 dB. What is more, the codes compare favorably with coded 16-QASK when envelope variations and peak energy constraints are considered.

Wilson, S. G.↗

NASA NTRS1981

Trellis phase codes for power-bandwith efficient satellite communications

Support work on improved power and spectrum utilization on digital satellite channels was performed. Specific attention is given to the class of signalling schemes known as continuous phase modulation (CPM). The specific work described in this report addresses: analytical bounds on error probability for multi-h phase codes, power and bandwidth characterization of 4-ary multi-h codes, and initial results of channel simulation to assess the impact of band limiting filters and nonlinear amplifiers on CPM performance.

Wilson, S. G.↗

NASA NTRS1981

Power spectra of multi-h phase codes

A procedure is presented for calculating the power spectrum of the class of digital continuous-phase signals known as multi-h phase codes. These signals have been shown to have attractive power advantages over other uncoded digital schemes such as QPSK, and represent a bandwidth-efficient alternative to other coding techniques. The method is general, handling M-ary signaling, various frequency pulse shapes, and arbitrary sets of modulation indices. Numerical results are shown for several specific codes. In addition, the issue of spectral lines is resolved, and simple spectral approximations are developed.

Wilson, S. G.↗

NASA NTRS1980

Joint map data/phase sequence estimation for trellis phase codes

A unified estimation procedure is formulated for the coherent detection of trellis phase codes, including partial response FM codes and multi-h codes. The unknown carrier phase is imbedded in the system state vector, and by simple augmentation of the decoder's state space, joint data and carrier estimation may be accomplished. The procedure is general to all trellis phase codes analyzed thus far. The effect of a static phase error on detection performance is also treated.

Wilson, S. G.↗