Engineering topics

Motamedi, Masoud

Publications and source records attributed to Motamedi, Masoud.

7 records

NASA NTRS1992

A direct broadcast satellite-audio experiment

System studies have been carried out over the past three years at the Jet Propulsion Laboratory (JPL) on digital audio broadcasting (DAB) via satellite. The thrust of the work to date has been on designing power and bandwidth efficient systems capable of providing reliable service to fixed, mobile, and portable radios. It is very difficult to predict performance in an environment which produces random periods of signal blockage, such as encountered in mobile reception where a vehicle can quickly move from one type of terrain to another. For this reason, some signal blockage mitigation techniques were built into an experimental DAB system and a satellite experiment was conducted to obtain both qualitative and quantitative measures of performance in a range of reception environments. This paper presents results from the experiment and some conclusions on the effectiveness of these blockage mitigation techniques.

Vaisnys, Arvydas↗

NASA NTRS1990

Multiple Access Trade Study

The Personal Access Satellite System (PASS) strawman design uses a hybrid Time Division Multiple Access (TDMA)/Frequency Division Multiple Access (FDMA) implementation. TDMA is used for the forward direction (from Suppliers to Users), and FDMA for the return direction (from Users to Suppliers). An alternative architecture is proposed that will require minimal real time coordination and yet provide a fast access method by using random access Code Division Multiple Access (CDMA). The CDMA system issues are addressed such as connecting suppliers and users, both of whom may be located anywhere in the CONUS, when the user terminals are constrained in size and weight; and providing efficient traffic routing under highly variable traffic requirements. It is assumed that bandwidth efficiency is not of paramount importance. CDMA or Spread Spectrum Multiple Access (SSMA) communication is a method in which a group of carriers operate at the same nominal center frequency but are separable from each other by the low cross correlation of the spreading codes used. Interference and multipath rejection capability, ease of selective addressing and message screening, low density power spectra for signal hiding and security, and high resolution ranging are among the benefits of spread spectrum communications.

Motamedi, Masoud↗

NASA NTRS1990

Antenna Beam Coverage Concepts

The strawman Personal Access Satellite System (PASS) design calls for the use of a CONUS beam for transmission between the supplier and the satellite and for fixed beams for transmission between the basic personal terminal and the satellite. The satellite uses a 3 m main reflector for transmission at 20 GHz and a 2 m main reflector for reception at 30 GHz. There are several types of spot beams under consideration for the PASS system besides fixed beams. The beam pattern of a CONUS coverage switched beam is shown along with that of a scanning beam. A switched beam refers to one in which the signal from the satellite is connected alternatively to various feed horns. Scanning beams are taken to mean beams whose footprints are moved between contiguous regions in the beam's coverage area. The advantages and disadvantages of switched and/or scanning beams relative to fixed beams. The consequences of using switched/scanning in lieu of fixed beams in the PASS design and attempts are made to evaluate the listed advantages and disadvantages. Two uses of switched/scanning beams are examined. To illustrate the implications of switched beams use on PASS system design, operation at two beam scan rates is explored.

Estabrook, Polly↗

NASA NTRS1990

Impact of Non-geostationary Orbits on PASS

The use of satellites in non-geostationary orbits (NGO) for PASS (Personal Access Satellite System) is discussed. The following subject areas are covered: (1) orbit parameters (circular and elliptical orbit characteristics); (2) link characteristics (Doppler shift, propagation loss, CONUS coverage antenna, multibeam antenna gain, and impact on link equations); (3) number of satellites required for continuous CONUS coverage (circular and elliptical orbit results); and (4) advantages and disadvantages of NGOs.

Estabrook, Polly↗

NASA NTRS1990

A CDMA architecture for a Ka-band Personal Access Satellite System

A Code Division Multiple Access (CDMA) architecture is currently being studied for use in a Ka-band Personal Access Satellite System (PASS). The complete architecture consisting of block diagrams of the user terminal, the supplier station, the network management center, and the satellite is described along with the access methods and frequency/time plans. The complexity of developing this system using the CDMA architecture is compared to that of a Frequency Division Multiple Access (FDMA) architecture. The inherent advantages and disadvantages of the two architectures are compared and their respective capacities are discussed.

Motamedi, Masoud↗

NASA NTRS1990

Use of elliptical orbits for a Ka-band personal access satellite system

The use of satellites in elliptical orbits for a Ka-band personal communications system application designed to provide voice and data service within the continental U.S. is examined. The impact of these orbits on system parameters such as signal carrier-to-noise ratio, roundtrip delay, Doppler shift, and satellite antenna size is quantized for satellites in two elliptical orbits, the Molniya and the ACE orbits. The number of satellites necessary for continuous CONUS coverage has been determined for the satellites in these orbits. The increased system complexity brought about by the use of satellites at such altitudes is discussed.

Motamedi, Masoud↗

NASA NTRS1990

Multiple access capacity trade-offs for a Ka-band personal access satellite system

System capability is critical to the economic viability of a personal satellite communication system. Ka band has significant potential to support a high capacity multiple access system because of the availability of bandwidth. System design tradeoffs are performed and multiple access schemes are compared with the design goal of achieving the highest capacity and efficiency. Conclusions regarding the efficiency of the different schemes and the achievable capacities are given.

Dessouky, Khaled↗