| Research Abstract |
In this research, multiple access methods for low earth-orbital satellite (LEOS) communication systems with multiple spot beams are investigated. In order to discuss the mitigation of handover failure and call blockings in TDMA systems, two-layered channel configuration is considered, where a part of satellite channels are reserved and assigned to the cells of fully loaded on demand assignment basis. Also, intentional overlap of adjacent cells are considered and found to have good handover performance. In addition to the discussion on TDMA, several features of CDMA scheme are studied. Since one of the most peculiar feature of LEOS systems is large Doppler shift in carrier frequency, PN code acquisition of a CDMA signal under Doppler shift is examined and the influence of the inter-satellite interference and the frequency offset is clarified. In CDMA systems, multiple access interference limits the maximum capacity of the systems. Thus interference cancellation techniques with a subtraction based nonlinear canceller and with a spatial cancellation by adaptive array antenna are also discussed. Though the discussion on the cancellation schemes are done for terrestrial systems, it is straight forward to apply them in LEOS systems.
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