A practical slotted quasi-synchronous CDMA access system for LEO micro-satellite short message transmission

A system to provide non-realtime short message transmission service to a wide-area of users via Low Earth Orbit (LEO) micro-satellite is developed. To increase the system capacity, a novel and efficient slotted quasi-synchronous CDMA access protocol is specially designed to deal with the serious Doppler shift and propagation delay variation. Before each message burst transmission, the user will randomly choose an access slot to send a short request to satellite and listen to the echo from the satellite. The channel delay and Doppler shift are then estimated from the echo and compensated in the immediate following message transmission slot, thus to achieve quasisynchronization between all user message bursts. The short request also acts as a reservation indicator, which prevents other users to use the immediate following message slot as access slots. The system throughput is analyzed by exploring the capture effects of the quasi-synchronous message bursts and the asynchronous request bursts, respectively. Transceiver requirements and implementation considerations are discussed, and the prototype is also described. Keywords-Quasi-synchronous CDMA; LEO satellite;Access System