Engineering topics

Sumida, J.

Publications and source records attributed to Sumida, J..

7 records

NASA NTRS2000

Preparation and Fluorescence Anisotropy Study of a Ribonuclease-Lucifer Yellow Conjugate

We have prepared a chemical derivative of ribonuclease A (RNase) with lucifer yellow (LY). The rotational dynamics of the LY-RNase conjugate were characterized by steady state and time resolved fluorescence techniques. Steady state anisotropy measurements were performed at varying viscosities at 10C and 20C, and the rotational correlation time of both RNase and the covalently linked LY probe were determined by time resolved frequency domain measurements. Our data suggest that the fluorophore is rigidly bound at 10C.

Malone, C. C.↗

NASA NTRS1986

Digital transmitter - Simple, yet precise

A transmitter developed for operation at high digital transmission speeds is described. It is a complex system that produces a clean waveform with data rates as high as 1 Mb/s; and its design makes it possible to vary all possible modulation schemes by reprogramming a chip, to vary data-transmission rates with the clock frequency, and to shape pulses without distortion. The design, furthermore, can be adapted to very-large-scale-integration (VLSI) fabrication and mass production. A prototype QPSK residual carrier modulation transmitter was fabricated and tested. The purpose was to create a deep null in the data spectrum to transmit a tone or residual carrier along with the modulation.

Davarian, F.↗

NASA NTRS1986

Coherent versus noncoherent signaling for satellite-aided mobile communications

The use of coherent versus noncoherent communications is an unresolved issue for the mobile satellite community. Should one select the more robust but less efficient noncoherent strategy for communications over satellite-aided mobile channels, or does the introduction of a space platform in the mobile link improve signal stability (both amplitude and phase) such that conventional coherent schemes become attractive? This publication tries to answer some of the questions by discussing the results from experiments using a coherent QPSK receiver. The issues discussed include items such as the measured performance in Rician fading, the link error floor in a fading environment, etc. The results are compared and contrasted with that of a noncoherent limiter/discriminator FM receiver.

Davarian, F.↗

NASA NTRS1986

Model verifies design of mobile data modem

It has been proposed to use differential minimum shift keying (DMSK) modems in spacecraft-based mobile communications systems. For an employment of these modems, it is necessary that the transmitted carrier frequency be known prior to signal detection. In addition, the time needed by the receiver to lock onto the carrier frequency must be minimized. The present article is concerned with a DMSK modem developed for the Mobile Satellite Service. This device demonstrated fast acquisition time and good performance in the presence of fading. However, certain problems arose in initial attempts to study the acquisition behavior of the AFC loop through breadboard techniques. The development of a software model of the AFC loop is discussed, taking into account two cases which were plotted using the model. Attention is given to a demonstration of the viability of the modem by an approach involving modeling and analysis of the frequency synchronizer.

Davarian, F.↗

NASA NTRS1985

2400 bit/s DMSK modem for mobile satellite service

The letter describes the implementation of a differentially coherent receiver suitable for bursty signal transmission over fading channels. To meet the severe system phase response requirement, this implementation is conducted at baseband. Means of frequency correction (AFC) is also shown. This receiver is simple and can easily be fabricated using VLSI technology.

Davarian, J.↗

NASA NTRS1985

DMSK: A practical 2400-bps receiver for the mobile satellite service: An MSAT-X Report

The partical aspects of a 2400-bps differential detection minimum-shift-keying (DMSK) receiver are investigated. Fundamental issues relating to hardware precision, Doppler shift, fading, and frequency offset are examined, and it is concluded that the receiver's implementation at baseband is more advantageous both in cost and simplicity than its IF implementation. The DMSK receiver has been fabricated and tested under simulated mobile satellite environment conditions. The measured receiver performance in the presence of anomalies pertinent to the link is presented in this report. Furthermore, the receiver behavior in a band-limited channel (GMSK) is also investigated. The DMSK receiver performs substantially better than a coherent minimum-shift-keying (MSK) receiver in a heavily fading environment. The DMSK radio is simple and robust, and results in a lower error floor than its coherent counterpart. Moreover, this receiver is suitable for burst-type signals, and its recovery from deep fades is fast.

Davarian, F.↗

NASA NTRS1985

Evaluate interference in digital channels

Any future mobile satellite service (MSS) which is to provide simultaneous mobile communications for a large number of users will have to make very efficient use of the spectrum. As the spectrum available for an MSS is limited, the system's channels should be packed as closely together as possible, with minimum-width guard bands. In addition the employment of frequency reuse schemes is an important factor. Difficulties regarding these solutions are related to the introduction of interference in the link. A balance must be achieved between the competing aims of spectrum conservation and low interference. While the interference phenomenon in narrowband FM voice channels is reasonably well understood, very little effort, however, has been devoted to the problem in digital radios. Attention is given to work, which illuminates the effects of cochannel and adjacent channel interference on digital FM (FSK) radios.

Davarian, F.↗