Research On Multi-User Detection And Spectrum Coexistence For Space Craft DS-UWB Communication Systems

With the rapid development of space craft technology, people make a claim for higher requirement to the communication and control ability of the space craft and begin to focus on the craft formation flying technology. The key to make the formation flying come true is to build the inter-craft link between each two crafts, and the direct sequence ultra-wideband (DS-UWB) communication system is a right choice based on its high transmission rate, large message capacity, high interference resistance and confidentiality and easy to accomplish multi-access communication. Space craft DS-UWB communication system belongs to the multi-access system which can embrace a certain number of transmitters working simultaneously. Compared to the traditional UWB communication system, there are two main differences. On one hand, the communication power will break through the restraint designated by Federal Communication Committee (FCC). On the other hand, the DS-UWB signal will co-exist with the unified S-band (USB) signal in the condition of formation flying, and due to the wide range of frequency band of DS-UWB system that will overlap the transmission band of USB, there must be some interference between two systems, so it is meaningful and useful to ensure the coexistence between DS-UWB and USB.As for the multi-user detetion technique of DS-UWB system, in this dissertation, we propose a hybrid multi-user detector based on matched filter and error-bit recognizer and regard the multi-user detection as a error-bit recognition and filtering process after the detection of the matched filter. In this detector, we construct a mapping function and map the output codes from matched filter into a one-dimensional feature space to get the amplitude information. After that, set an initial value and classify the codes into right codes and wrong ones by K-means clustering. Finally, make the sign judgment to prevent some right codes being picked out as the wrong codes and correct the wrong codes to reduce the bit error rate (BER) of the system. The simulation results show that the hybrid multi-user detector can improve the BER performance of the system because of high recognition probability of the wrong codes in K-means clustering, especially along with the narrowband interference made by USB. What's more, the proposed hybrid MUD is not sensitive to the user number. It can achieve good BER performance when there are 20 UWB transmitters in the channel, which is fit for the signal detection in the condition of a number of transmitters and enormous communication capacity.As for the spectrum coexistence, in this dissertation, we emphasize on the bit-error performance of USB receiver along with the interference created by the formation craft DS-UWB transmitters. We analyze the impact on the bit error rate of USB receiver with different proportion between pulse repetition frequency (PRF) of DS-UWB signal and the baseband transmission rate of USB signal. The simulation results demonstrate that when PRF of DS-UWB signal is approximately equivalent to the baseband transmission rate of USB signal, the main type of interference is pulse interference which creates a intense immediate impulse to the USB signal; when the PRF of DS-UWB signal is much greater than the baseband transmission rate of USB signal, the output of DS-UWB signal passing through RF filter of USB receiver can be seen as the white Gaussian noise. In order to decrease and eliminate the interference to the USB system, it is a better way to increase the rate of judgment impulse which is can decrease the opportunity that the energy of DS-UWB signals enters the receiver of USB system.