Leo Satellite Cdma Short Data Mobile Communications System Design And Analysis

Land mobile satellite systems seems to be one important part for realizing global personal communications network, they can provide a wide range services (e.g., position finding, radio paging, interconnecting to the PSTN, voice communication, and data communication) to numerous types of terminal, for the vast demand of message transmission in domestic civil and military markets, we proposed a CDMA mobile satellite system, which provides short message transmission for mobile and fixed users and relies on micro-satellite constellation in low earth orbit, we call that little LEO system in order to differ from the big LEO systems, which provides, voice communication mainly and relies on big or medium satellite constellation in low earth orbit. In this dissertation, both theoretical studies and computer simulations are carried out based on this CDMA little LEO systems. Firstly the propagation characteristics of a mobile satellite radio channel is analyzed, a statistical propagation model and a software channel simulator are developed for our CDMA little LEO system. Secondly, the performance of CDMA little LEO system, in terms of bit error probability and throughput when system employing CDMA-S-ALOHA scheme is investigated, the environment are proposed, the influence of propagation effects on the performance is given, the effects of diversity technique and FEC coding are also considered. In a direct-sequence code-division multiple-access system, accurate~ adaptive power control, especially in reverse link, is critical for a good aggregate system performance. Because the conflict of long round trip delay and short data burst duration between the mobile and base-station, closed-loop APC is typical not feasible in our CDMA little LEO system, so an open-loop APC scheme, which relies on channel state information obtained on the opposite link is proposed, an analytical model for the open-loop power on the mobile channel is developed, and performance of control error is illustrated through computer simulation and numerical computation. Results indicated that open-loop power is perfect for unshadowed users, but not properly for deep shadowed users. Rapid initial code acquisition is crucial in spread-spectrum communications, especially to packet burst transmission; the length of synchronization preamble decided the efficiency of frame transmission. In this dissertation, we proposed two code acquisition schemes, hybrid acquisition scheme and parallel acquisition scheme, respectively to forward link and reverse link, circular-state diagram technique is employed to described the code acquisition process, the mean acquisition time, the acquisition probability and lost probability on mobile satellite channel is derived, Acquisition performance is demonstrated through numerical result. Finally, we give a briefly discussion of the CDMA little LEO system, ways to solve the design problem and improve system performance are explored.