CDMA for Mobile LEO Satellite Communications

Abstract

Satellite orbiting the earth would appear to be the ideal way to obtain worldwide coverage to mobile users. During WARC’92¹several frequency bands were established for that purpose [14 ]. Since then there have been numerous proposals for implementing such systems. There are proposals for using geosynchronous orbit satellites (GEOS) at an altitude 35,784 km, medium earth orbit satellites (MEOS) at 5,000 – 10,000 km and Low Earth Orbit Satellites (LEOS) at 150 – 1,500 km. Of these, the LEOS have attracted the most attention because of the novelty of having many satellites, handoffs and a cellular-like configuration. The advantages would be small propagation loss so that handsets could be used for direct communication from a mobile user and small propagation delay for better performance in voice, data and other interactive services. A disadvantage is that more satellites are required -- offset by cheaper launch costs -- and increased probability of shadowing. The design challenges include worldwide coverage, system user capacity, margins of fading, interaction delay, call handoffs, spectrum sharing, handheld battery life and user health hazards (transmit power). The frequency bands to be used for LEOS is 1616.5–1626.5 MHz for the uplink and 2483.5–2600 MHz for the downlink if frequency division duplex (FDD) is used and just the former band if time division duplex (TDD) is used. Of five major proposals for “big” LEOS, four have proposed CDMA in one form or another, and only one has proposed FDMA/TDMA. Because of this, and the general interest in CDMA in LEOS we have focused our effort in this paper on LEOS CDMA and, in particular, how it differs from terrestrial cellular CDMA which has already demonstrated its virtues and has become one of several standards in the United States. The network architecture of a typical CDMA based LEOS is shown in Figure 1 where the satellites act as “dumb transponders” or “bent pipes”. Thus, while communications can take place between users within the footprint of the satellites, the signals are decoded at an earth-based gateway. Communications from users in different footprints access their respective satellites directly but communications between them involves both gateways to/from the satellites and terrestrial lines between gateways.

World Administrative Radio Conference, Torremolinos, Spain, 1992.