Engineering topics

Kwatra, S. C.

Publications and source records attributed to Kwatra, S. C..

26 records · Page 2

NASA NTRS1990

Performance evaluation of land mobile satellite system under vegetative shadowing using differential multiple TCM and QPSK

A comparative analysis by computer simulation of the land mobile satellite system (LMSS) is presented for the uncoded quadrature phase shift keying (QPSK), rate 2/3 8-PSK trellis coded modulation (TCM), and rate 4/6 multiplicity 2 8-PSK TCM schemes. An analytical model is used for simulating the fading channel. The simulation results show that under Rayleigh fades, the TCM designed for the fading channel is superior to uncoded QPSK and to the conventional TCM optimized for the additive white Gaussian noise (AWGN) channel. The performance of differentially detected TCM under AWGN and fading is examined based on the preliminary results. Results of the uncoded QPSK with fading channel modeled with the empirical fade data are given.

Kim, Junghwan↗

NASA NTRS1990

Study on the spectral efficiency of SFH-GMSK in land mobile telephone communication by direct simulation

A spread spectrum system, slow frequency hopping with GMSK (Gaussian minimum shift keying) modulation (SFH-GMSK), is proposed for mobile telephone communications. The system performance is evaluated using computer simulation and is compared with an unspread system. Results show that under multipath fading conditions, when the signal-to-noise ratio (SNR) is greater than 15 dB, slow frequency hopping gives some bit error rate improvement over the unspread system. Theoretical predictions indicate that a system efficiency of 20-65 users per cell can be achieved in the cellular configuration. Joint use of SFH-GMSK and FM is also investigated. It is shown that FM interference can cause serious degradation to the SFH-GMSK performance.

Bao, H.↗

NASA NTRS1986

Simulated performance of the NASA 30/20 GHz test transponder using multi-H phase coded modulation

The performance of a proposed NASA 30/20 GHz satellite communications system is studied for multi-h phase coded modulation (MHPM) schemes. The techniques used to model and simulate a satellite communications channel including transmitter, receiver, filters, nonlinearities, and interferers are presented. The performance of various MHPM schemes is compared for several different channel configurations. As a measure of performance, the probability of bit error vs Eb/NO is computed using a Monte Carlo simulation technique. It is found that, regardless of the channel configuration, MHPM schemes can provide power efficiency over serial minimum shift keying modulation.

Meader, C. B.↗

NASA NTRS1986

Image data compression with vector quantization in the transform domain

In this paper, an algorithm is presented for image data compression based upon vector quantization of the two-dimensional discrete cosine transformed coefficients. The ac energies of the transformed blocks are used to classify them into eight different ac classes. The ac coefficients of the transformed blocks of class one are set to zero, while those of classes two through eight are transmitted by seven different code books. The dc coefficients of all eight classes are scalar quantized by an adaptive uniform quantizer. As a result, only 4.5 bits instead of eight bits are required to transmit the dc coefficient with negligible additional degradation. Overall, this algorithm requires approximately 0.75 bits per pixel and gives an average reconstruction error of 7.1.

Abdelwahab, A. A.↗

NASA NTRS1984

Modeling of NASA's 30/20 GHz satellite communications system

NASA is in the process of developing technology for a 30/20 GHz satellite communications link. Currently hardware is being assembled for a test transponder. A simulation package is being developed to study the link performance in the presence of interference and noise. This requires developing models for the components of the system. This paper describes techniques used to model the components for which data is available. Results of experiments performed using these models are described. A brief overview of NASA's 30/20 GHz communications satellite program is also included.

Kwatra, S. C.↗

NASA NTRS1983

NTSC composite video at 1.6 bits/pel

An adaptive algorithm for intraframe compression of NTSC composite color video signal is described. With an average of 80 percent compression (1.6 bits/pixel) and a bit error rate of 0.0001, excellent broadcast quality pictures were obtained. This is confirmed by a subjective evaluation of processed pictures by five viewers. The algorithm and the results of the subjective evaluation are given in this paper.

Kwatra, S. C.↗

NASA NTRS1983

An allpass filter design for luminance and chrominance separation of NTSC signals

Abstract- Digital techniques for separation of luminance and chrominance components from composite NTSC broadcast television signals are explored. First, comb filters are introduced. Secondly, filters based on linear regression are developed. Finally, a third filtering form, the allpass filter is considered. Techniques are compared based on mean squared error criteria and subjective testing. It is demonstrated that the allpass filtering technique is superior to both the comb filtering as well as the regression filtering.

Border, D. A.↗

NASA NTRS1981

A new architecture for adaptive transform compression of NTSC composite video signal

In orthogonal transform coding of NTSC video signal, the data is processed in blocks. Processing a block of data that is maximally correlated leads to larger compression. This paper describes a new architecture for arranging the pixels in a block that yields higher correlation than the adjacent pixel blocks. The data is processed using adaptive, intrafield, discrete cosine transform, variable bit allocation coding. In subjective tests, the picture quality was good for an average of about 2 bits/pel for intrafield processing.

Ekambaram, C.↗