Variable data rate modem for low Earth orbiting satellite (LEOS) communication

There is wide interest in providing mobile satellite services (MSS) via low Earth orbiting satellite (LEOS) instead of by the traditional geosynchronous satellites (GEOS). This development conforms with the trends in personal communication services (PCS) where low cost, low power and high volume users are the goals. The distinct features of LEOS are characterized by its low altitude (4000 km versus 37700 km) and its high flyby speed (6.9 km/sec). Due to shortening of the LOS distance by a factor of 4 that is translated into a 12 dB advantage in power as compared to GEOS terminals, LEOS terminals are more compact. It generally radiates in the neighbourhood of 5 Watts. Currently planned small LEOS systems rely on sending short burst of data packet generally of a duration less than 100 msec. There are plans to optimize the data transmission during the burst. This paper studies the possibility of using a variable data rate modem to increase the total throughput in such a system. Baud rate variations in the range of 1200, 2400, 4800, and 9600 are considered. This will be a new feature at the link layer of the OSI model. It is necessary to consider the available link margin variation in the burst period of a practical small LEOS system. Due to the antenna orientation, nonuniform radiation pattern, and the angular aspect between the terminal and satellite, the available link margin is reduced. The crucial issue is how much dB margin is there. An analysis of the available link margin for a variable rate data modem is presented.