Research On Key Technologies Of Time-varying Interference Suppression In Direct Sequence Spread Spectrum Communication Systems

Interference is the important factor that affects the capacity and quality of thecommunication system, direct sequence spread spectrum （DSSS） communicationsystem has been widely employed in military and civilian communications, for its goodanti-interference capability and other characteristics.However, the capability is relatedto the spreading gain, when the spreading gain is restricted, the anti-interferencecapability is limited. How to improve the anti-interference capability of DSSScommunication system is significant for military and civilian communications withoutthe increasing spreading gain.Existing activeinterference suppression methods used in DSSS considered that theinterferences are time-invariant. But in some practical scenarios, the interferences havethe characteristic of time-varying.In this paper, time-varying interferenceis studied，including the suppression of time-varying single tone jamming （STJ）, the suppression oftime-varying multitone jamming （MTJ）, and the suppression of time-varying multipleaccess interference （MAI） in CDMA systems.First, the BER performance of CDMA in the presence of time-varying STJ isanalyzed. The BER expression is derived and the effect of the Doppler shift on theperformance is discussed. The accuracy of the BER estimation evaluated by theexpression is verified by computer simulations. Furthermore, a frequency cancellationmethod is proposed to suppress the time-varying STJ. The BER expression is analyzedto assess the performance of the proposed method. Simulation results demonstrate theeffectiveness of the method in mitigating time-varying STJ and approaching theinterference-free performance limit over practical ranges of STJ power levels andmobility levels. Under the condition of AWGN channel, whenSNR （Signal-to-NoiseRatio） is10dB,the spreading gain is16, Doppler frequency is100Hz, JSR（Jamming-to-Signal power Ratio） is less than30dB, theSTJ can be suppressed by theproposed method.Second, the BER performance of CDMA in the presence of time-varying multi-tonejamming （MTJ） is analyzed. The BER expression is derived and the effect of the Doppler shift on the performance is discussed. The accuracy of the BER estimationevaluated by the expression is verified by computer simulations. Besides, a method offrequency domain interference detection is proposed to suppress the time-varying MTJ.The performance of the proposed method is validated by computer simulations, and thesimulation results show that the effectiveness of the proposed method inmitigatingtime-varying MTJ over practical ranges of MTJ power levels and mobilitylevels.Under the condition of AWGN channel,there are three STJ in the systems, SNR is10dB,the spreading gain is16, Doppler frequency is100Hz, JSR is less than20dB,theMTJ can be suppressed by the proposed method.Finally, a frequency domain interference rejection combining （FD-IRC） algorithm isproposed to suppress the time-varyingMAI for uplink CDMA systems. The theoreticalBER lower bound of the FD-IRC in cyclic prefix CDMA （CP-CDMA） is derived. Theproposed method doesn’t require any priori information about the interfering user andchannel. The simulation results show that the proposed method can improve theperformance of the system compared to frequency domain maximal ratio combining.When there are three interfering users in the system and each of them has the samepower as the desired user, at the BER value of10^-3, andthe spreading gain is16, theSNR of the proposed method is improved by2dB.The research in this thesis explores the problem of time-varyinginterferencesuppression techniques for DSSS systems, which extends the existingresearches and has theoretical and practical values. The research results can be appliedto the3G and next generation mobile communication systems.