Rate adaptive OFDM (RA-OFDM) spread spectrum system for LEO satellite communications

Wireless communications systems in a tactical environment must be able to function effectively in a channel characterized by multipath fading and interference. Additionally, current and future communications requirements will necessitate the transmission of many different types of information with a wide range of data rates over these wireless links. Rate adaptive signaling is a powerful way to design a robust system across such broad range of channel conditions because the system will not have to be designed for the worst case scenario and system resources can be utilized more efficiently. In this paper, we apply our previously proposed rate adaptive OFDM Spread Spectrum System concept to a LEO satellite communications application. We demonstrate the feasibility of rate adaptation by analyzing the link margin of a LEOS link characterized by free space loss, multipath fading and interference. A Doppler based rate adaptation algorithm is proposed to constantly measure and predict the mobile terminal received signal strength and to control the system processing gain accordingly. Desired system performance can be maintained across a large dynamic range of signal strength by the proper control of the processing gain.