Multiple Access Methods for Low Earth-Orbital Satellite Communication Systems

研究了多点波束低轨卫星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，考虑了两层信道配置，其中一部分卫星信道被预留并被分配给按需全载分配的小区。此外，考虑了相邻小区的有意重叠，并发现具有良好的切换性能。除了对时分多址的讨论外，还研究了码分多址方案的几个特点。针对低轨卫星系统的特点之一是载波频移较大的特点，研究了在多普勒频移下对码分多址信号的伪码捕获，分析了星间干扰和频偏对捕获性能的影响。在码分多址系统中，多址干扰限制了系统的最大容量。因此，还讨论了基于相减的非线性对消器和自适应阵列天线的空间对消干扰对消技术。虽然对抵消方案的讨论是针对地面系统的，但将其应用于LEOS系统是直截了当的。

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.