| Small satellites communications will play important role in global personal and military communications in the 21st century. Direct-sequence code-division multiple access (DS-CDMA) is frequently selected as the access scheme because of its noteworthy low-power flux density emission, and robustness again inter- and intra-system interference, soft beam and satellite hand-off, path diversity exploitation, and so on. But there are a lot of questions in DS-CDMA satellite communication system, such as the fading of received signal, the multiple access interference (MAI), large Doppler frequency offset, the CW and narrow band interferences in channel and so on. So the key technologies to be studied are: ⑴ Rapid code acquisition and tracking technologies: Generally, the orbit period of small satellite varies between ninety minutes and hours, the maximum time during which a satellite in LEO orbit is above the local horizon for an observer on the earth is up to 20 minutes (generally, a few minutes). In order to establish successfully between the satellite and user on earth, the system must achieve synchronization. The approaches suited for small satellite communication system are matched filter acquisition, large step delay locked loop acquisition. ⑵ Effective anti-interference techniques: There are lots of interferences in the channel such as CW and narrow band interferences, MAI, and noises. So we must find the effective anti-interference technologies to copy with these interferences. The anti-interference technologies are multiuser detection (to suppress MAI), smart antenna, adaptive filtering, smart-AGC, CW interference canceling. ⑶ Doppler compensation techniques: The large Doppler frequency offset can case the performance degraded in PN and carrier synchronizing. The coherent/incoherent correlation method, matched filters, sequential detection and FFT technique can use to combat the effects of Doppler in PN acquisition. The QPSK and Dual-Channel modulator/demodulator have robustness to Doppler shift. ⑷ The satellite-switched and the signal processing techniques: The satellite will not only as a repeater but also as a switching center in space. The switching and processing onboard is very important to small satellites. ⑸ Diversity techniques: Because of multipath propagation, the different replicas of the same user signal are received with differential delay greater than one chip. The RAKE receiver is also applied in DS-CDMA small satellite system. In order to design and implement the DS-CDMA small satellite communication system, this paper will focus on key techniques in DS-CDAM satellite communication. The main study works on the subjection as follows: ⑴ Based on development of DS-CDMA technology and the requirement of user, this paper achieves to design the DS-CDMA small satellite communication system, both on board and on earth. ⑵ This paper systematically investigates the small satellite communication channel models and the simulation methods. The BER performance of BPSK and DCPSK based on DS-CDMA small satellite communication channel are analyzed. ⑶ In this paper, multiuser detection and RAKE receiver techniques are studied in detail, and a new method to reduce the MAI by wavelet denoising is proposed. ⑷ Studies on the PN acquisition schemes, introduces a new method of PN acquisition suited to small satellite communication, and proposes a new approach to estimate the initial phase of received PN by wavelet analysis in DSSS. ⑸ Introduces the anti-interference techniques and algorithms based on adaptive filtering. ⑹ Design and implement a switching system on board, including the hardware architecture and software programming.
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