The value of Doppler information for multiple access and power control in LEO satellite systems

A key feature of LEO satellite systems is the fast changing geometry between the satellite and the ground based mobile terminals. This feature manifests itself in the significant and smooth "S" shaped variation of the Doppler time curve of the forward channel (from satellite to mobile terminal) frequency. We provide an overview of the processing of this Doppler information and its value to communication functions such as multiple access and power control at the terminal. The Doppler curve has a simple and accurate analytic approximation for circular orbit LEO satellites. Also, the Doppler curve parameters can be estimated at the mobile terminals from two Doppler and Doppler-rate measurements. In Doppler based multiple access (DBMA), traffic on the inbound channel (from mobile terminal to satellite) can be controlled by specifying a region of eligibility (ROE) in terms of the Doppler curve parameters. Only terminals located in the ROE are permitted to transmit. For one scheme of power control, termed Doppler based transmit power control (DBTPowC), terminals estimate their distance from the satellite using the Doppler-time curve and modify their transmitted power to compensate for the path loss differential to the satellite. In the second scheme, termed Doppler based transmit permission control (DBTPermC), the earth-station permits only those terminals to transmit whose distance from the satellite is within a specified range.