Research On Narrowband Interference Suppression In DSSS TT & C System

As tracking telemetry & command (TT & C) system is widely used in aeronautics and astronautics, it is extended from a common communication system to an advanced one comprising functions of remote control, telemetry and tracking. At the same time, direct spread sequence spectrum (DSSS) technology has become the most popular communication mechanism in modern TT & C system because of its low power spectrum density, well-concealed signals, high range resolution, strong capacity of multi-address communication, as well as remarkable anti-jamming and anti-noise abilities. A system applying DSSS has inherent spreading gain that may help resisting intentional or unintentional interference in channels. When such a system suffers high-power narrowband interference(NBI) or when central frequency of interference signals gets close to central frequency of DSSS signals, however, the simple way of enlarging spreading gain cannot resist all interferences due to limited frequency resource. Therefore, it is practically necessary to study how to promote the capacity of a TT & C system to resist NBI in atrocious weather.Current ways of resisting interference consist of interference cancellation and interference avoidance. Implementation of interference cancellation is based on the increase in the amount of calculation or the expense of signal energy, while avoiding interference with the increase in system complexity is to achieve interference suppression without reducing communication quality.The thesis focuses on interference avoidance in TT & C system. Because former technologies focuses on time-domain,space-domain and frequency-domain without using sequences characteristics,the thesis puts forward NBI avoidance technologies on the basis of code characteristics for the first time,which avoids NBI through designing spread sequences with different spectrum structures and applying different communication modes. First, the thesis studies m-Walsh composite sequence(m-W CS) and makes out the changing rule of various m-W CS signal spectrums. Based on the research on binary spread sequence spectrum signals, the thesis works over spectrum structures of m-W CS by referring to generation process and correlation features of composite sequence and characteristics of m sequence and Walsh sequence, and makes a conclusion that it is feasible to control spectrums of m-W CS. This preliminary conclusion acts as a theoretical basis on avoiding NBI in a DSSS TT & C system by using code characteristics.Second, the thesis provides a method to avoid weak NBI in Gaussian channels by using various m-W CSs. Specific implementation steps of the method are positioning NBI in frequency-domain, designing m-W CS as spread sequence spectrum, and minimizing energy of the signals suffering NBI. In this way NBI can be avoided. Simulation experiment result shows this avoidance technology that makes use of m-W CS can mitigate the effect on signals due to interference.Third, the thesis puts forward an adoptive, code-hopping, DSSS-based anti-jamming approach to cope with time-varying or multi-frequency interference occurred in Gaussian channels. The approach combines m-W CS with code-hopping DSSS technology to offer DS-FH distinctions with DSSS signals, and enables a DSSS system avoiding NBI on a self-adaptation basis through link quality analysis and sequence shift algorithm. Simulation experiment result shows an adaptive, code-hopping DSSS system has strong anti-jamming ability under spot narrowband jamming and time-varying or multi-frequency interference.Fourth, the thesis gives a frequency diversity means to suppress spot NBI in Gaussian channels based on the characteristic of code-domain. This means integrates m-W CS with orthogonal multicode spread spectrum technology, and uses diversified m-W CSs over identical information to realize multi-channel parallel spread spectrum in consideration of the point that different m-W CSs have different spectrum structures in a specific frequency domain. Thus, frequency diversity is realized and diversity combination technologies are used to strengthen the ability of system to resistance against NBI. Simulation experiment result shows this methodology effectively protects the system from interference by incorporating diversity multi-channel information when spot NBI disturbs some frequencies.Fifth, the thesis suggests an m-W composite-sequence-based M-ary orthogonal spread spectrum solution in a high-velocity, limited-band TT & C system in which NBI in channels is strong and the amount of m-W CS used to avoid interference is low. Based on typical M-ary orthogonal spread spectrum, the thesis creates a new M-ary orthogonal spread spectrum mode by amalgamating m-W CS with M-ary orthogonal spread spectrum, and studies structures, transmission efficiency, bit error rate(BER) in Gaussian channels and anti-jamming capacity of the new mode. In accordance with theoretical analysis and simulation experiment result, the amalgamation of CS and M-ary orthogonal spread spectrum overmatches typical M-ary spread spectrum technology with regard to transmission efficiency or system complexity. Furthermore, the convergence of this new mode and the avoidance technology by using m-W CS remarkably enhances the ability of an M-ary spread spectrum system to resist NBI.