| When designing a special type small satellite (STSS) communication system with spread-spectrum technology, the fading and the interference must be considered carefully. The satellite system is doomed to suffer from the multipath fading in the presence of little elevation. Because the speed of the satellite comparing to it of the ground station is fast, the high Doppler shifts may lead to fast fading of the signal. There are many interference sources in the transmission channels of the low-orbit satellite communications, such as the narrow band interference, the barrage interference, the continuous carrier interference, the pulse interference and the multi-access interference. Because the existing of these interferences and fading, the received signal will be characterized by very low signal to noise (SNR) and large amplitude dynamics of the input signal. When it was used in the actual channel, the STSS communication system can't operate properly with slow synchronization. Through analyzing these problems, the author found the reason included the system using the fixed threshold except large amplitude dynamics of the input signal.For efficient operation of the STSS communication system under the environments full of fading and interferences, the adaptive threshold must be adopted. The adaptive decision thresholds are offered based on the studies of the digital matched-filter (DMF) in this paper. Aims at the bad effect of the adaptive decision thresholds under the very low SNR, the author give a new means which combines the adaptive decision threshold with Rake receiver or 2D-Rake receiver. By this way, we can obtain the optimum performance of the STSS communication system. The study of the STSS communication system is as follows:(I) The author fully analyzed the bit error ratio (BER) performance and acquisition performance over non-fading channel and fading channel (including non-frequency selective and frequency selective fading channel) with noise and interference when the system adopts fixed threshold. Using the state transition diagram, the analytic expression of the mean acquisition time of the PN code acquisition by one dwell decision approach is derived. Numerical analysis is also given. It is shown that the system can't operate properly at fixed threshold.(II) Two adaptive algorithms about the decision threshold setting are presented in this paper. In constant false alarm (CFAR), the case of the probability of a false alarm is fixed independently of the SNR. In order to keep constant false alarm, the system needs to estimate the noise power to set an adaptive threshold. After introducing some classic approaches for noise power estimation, a new CFAR detector (OSTMGO) based on ordered statistics (OS) and trimmed mean (TM) is proposed in this paper. The detection performance of OSTMGO is good both in homogeneous background and in non-homogeneous environment caused by strong interfering targets and clutter edges. For efficient operation of the receiver the threshold must be automatically controlled to provide the decision conditions where the sync false alarm probability and sync correct detection probability even in the case of large amplitude dynamics of the input signal. If they are main ingredients to deteriorate the system performance, the influence of the continuous carrier interference and the pulse interference can be eliminated almost totally, by using a median filter as an assistant of the DMF. (III) Because the satellite communication channel is modeled as frequency selective fading, the communication is doomed to suffer from the multipath fading. The performance of the conventional receiver is bad over the frequency selective fading channel because it utilizes only one of multipath components. Therefore, a creative sub-optimum diversity receiver based on the theory of Rake receiver is proposed. The results of emulation indicate that the Rake receiver can largely eliminate the deleterious effects of the multi-path transmission when the received multi-path signal number exceeds...
|
PDF Full Text Request