Cx-1 "leo Satellite Ground Handheld Terminal Spread Spectrum Software Demodulator Architecture And Key Algorithms

Short-data transferring via LEO micro-satellite communication systems see a good future in development of national economic and defense, because of many advantages it takes, e.g. small-size terminal, comparatively low cost and so on. Portable terminal is an important part of CX-1 LEO micro-satellite communications systems, in which the low-power, rapid signal-acquisition and adaptive interference- canceling are the must.With the background of R&D of CX-1 micro-satellite's portable terminal, this paper investigated the architecture and key algorithms of direct-sequence spread-spectrum (DSSS) software demodulator's design deeply. The research targets of this paper are as follows: 1. how to implement the portable terminal's challenging specs, i.e. fast DSSS signal acquisition with the large Doppler-shift and adaptive interference-canceling of CX-1 downlink DSSS signal when received in the bad EM conditions. 2. how to reduce the DSSS software demodulator's computation load, in order to reduce the portable terminal's power, without sacrificing the performance of demodulator.First this paper introduced the DSSS demodulator' system model and deduced the theoretical BER's value of DSSS demodulator in the CX-1 micro-satellite's portable terminal. Then studied the DSSS software demodulator's hardware and software design, at the same time conclude that DSSS software demodulating is feasible from the point of view of algorithms' complexity and computation load.Second, this paper studied the digital down-conversion (DDC) algorithm and the algorithm of fast acquisition of DSSS signal. After derived the relationship between the DDC's frequency- synthesis look-up table's length and the spur-free dynamic range, it proposed a modified DDC method with the large Doppler-shift. It also proposed a correlator or in frequency domain (CFD), which can decrease the computation load from O(N2) to O(Nlog2(N))-Finally, this paper compared two methods of estimating Doppler residual frequency offset and proposed a minimum-mean-square-error(MMSE) interpolator's algorithm.