Engineering topics

Sayegh, Soheil

Publications and source records attributed to Sayegh, Soheil.

3 records

NASA NTRS1994

Modulation and synchronization technique for MF-TDMA system

This report addresses modulation and synchronization techniques for a multi-frequency time division multiple access (MF-TDMA) system with onboard baseband processing. The types of synchronization techniques analyzed are asynchronous (conventional) TDMA, preambleless asynchronous TDMA, bit synchronous timing with a preamble, and preambleless bit synchronous timing. Among these alternatives, preambleless bit synchronous timing simplifies onboard multicarrier demultiplexer/demodulator designs (about 2:1 reduction in mass and power), requires smaller onboard buffers (10:1 to approximately 3:1 reduction in size), and provides better frame efficiency as well as lower onboard processing delay. Analysis and computer simulation illustrate that this technique can support a bit rate of up to 10 Mbit/s (or higher) with proper selection of design parameters. High bit rate transmission may require Doppler compensation and multiple phase error measurements. The recommended modulation technique for bit synchronous timing is coherent QPSK with differential encoding for the uplink and coherent QPSK for the downlink.

Faris, Faris↗

NASA NTRS1988

A flexible on-board demultiplexer/demodulator

Optimization of satellite communications system performance from the point of view of reduced overall system cost per unit of service delivered, requires that economies be exercised at all levels. Introduction of onboard regeneration which necessarily involves on-board demultiplexing and demodulation is an essential ingredient in meeting this goal. To achieve maximum benefit from this approach, the uplink transmission data rate and bandwidth from each earth terminal and consequently uplink EIRP needs to be minimized. This necessarily introduces multiple carriers of different bandwidths on the uplinks and creates an attending need onboard for simultaneous demodulation of many carriers. This paper addresses a digitally implemented bulk demultiplexer/demodulator that processes a large number of carriers having differing carrier bandwidths, bit rates and center rrequencies. The device is programmable from the ground to accommodate changes in the complexion of the carriers.

Campanella, S. Joseph↗

NASA NTRS1987

Onboard multichannel demultiplexer/demodulator

An investigation performed for NASA LeRC by COMSAT Labs, of a digitally implemented on-board demultiplexer/demodulator able to process a mix of uplink carriers of differing bandwidths and center frequencies and programmable in orbit to accommodate variations in traffic flow is reported. The processor accepts high speed samples of the signal carried in a wideband satellite transponder channel, processes these as a composite to determine the signal spectrum, filters the result into individual channels that carry modulated carriers and demodulate these to recover their digital baseband content. The processor is implemented by using forward and inverse pipeline Fast Fourier Transformation techniques. The recovered carriers are then demodulated using a single digitally implemented demodulator that processes all of the modulated carriers. The effort has determined the feasibility of the concept with multiple TDMA carriers, identified critical path technologies, and assessed the potential of developing these technologies to a level capable of supporting a practical, cost effective on-board implementation. The result is a flexible, high speed, digitally implemented Fast Fourier Transform (FFT) bulk demultiplexer/demodulator.

Campanella, S. Joseph↗