Multiple Access Methods for Low Earth-Orbital Satellite Communication Systems

研究了低地球轨道卫星（LEOS）多点波束通信系统的多址接入方法。为了讨论TDMA系统中切换失败和呼叫阻塞的缓解问题，考虑了两层信道配置，其中保留一部分卫星信道，并在需求分配的基础上分配给满载的小区。此外，还考虑了相邻单元的故意重叠，并发现具有良好的切换性能。在对TDMA进行讨论的基础上，对CDMA方案的几个特点进行了研究。针对LEOS系统载波频移较大的特点，研究了多普勒频移对CDMA信号PN码采集的影响，阐明了星间干扰和频偏的影响。在CDMA系统中，多址干扰限制了系统的最大容量。因此，本文还讨论了基于减法的非线性消去器和基于自适应阵列天线的空间消去器的消去技术。虽然对消去方案的讨论是针对地面系统进行的，但将其应用于低轨道卫星系统是很直接的。

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.