The Research Of Interference Suppression Technologies In DSSS Communication Systems

Direct sequence spread spectrum (DSSS) communication systems have many advantages, including low probability of intercept (LPI) transmission, multiple accessing, multipath suppression and narrowband interference suppression. It has been widely used in personal communication network (PCN), wireless LAN (WLAN), 3G mobile communication and military tactical communication. As is well known, the inherent processing gain of spread spectrum systems will, in many cases, provide the system with interference rejection capability. But if the level of interference is too high, the interference immunity can be improved significantly by using signal processing techniques that complement the spread spectrum modulation.The research of this dissertation is based on the demands of DSSS narrowband interference suppression intermediate frequency digitalized receiver. A simplified signal model of digitalized receiver and the signal model of interference, such as tone interference, autoregressive interference and digital interference, are provided. And it is analyzed in detail the capability of DSSS communication systems in suppressing broadband Gaussian interference, single tone interference and partial band interference.Different time domain interference suppression techniques can be established for different interference models. Firstly, the performances of linear and nonlinear algorithms based on interference estimator/subtracter are compared. Secondly, a novel interference suppression algorithm based on subspace tracking is proposed for the tone interference model. Finally, the stability performance of the transform domain normalized least mean square (TDNLMS) adaptively implementing structure of minimum bit error ratio (MBER) receiver is analyzed.Filter bank is the multirate signal processing structure, which decomposes a digital signal into a bank of frequency bands. The discrete Fourier transform (DFT), lapped orthogonal transform (LOT) and cosine modulated filter banks (CMFB) are all special cases of this general structure. Firstly, the signal-to-noise ratio degradation of windowed discrete Fourier transform (WDFT) is studied and the improved methods, namely overlap-add and overlap-select, are proposed. In the case that only the direct sequence spread spectrum signal and additive Gaussian noise are present at the receiver, the square of amplitude of the received signal after WDFT is assumed to be exponential distributed. An improved narrowband interference detection and suppression algorithm is proposed based on this assumption. Cosine-modulated filter banks have been studied extensively due to their virtues of easily design and efficient implementation. The design of prototype filter satisfying perfect reconstruction property has been formulated as a quadratic-constrained least-squares (QCLS) optimization problem, whose constrained matrices are symmetric and positive definite. A near perfect reconstruction prototype filter design technique is employed in this dissertation. And a novel algorithm for narrowband interference suppression based on subchannel power estimation and modified K-bins is introduced.The interference characterized by its instantaneous frequency can be effectively mitigated in DSSS communications using open-loop adaptive excision filters. On condition of the accurate estimation of instantaneous frequency of the interference, the closed-form expression of signal to interference and noise ratio (SINR) improvement of Kwan-Martin IIR filter is derived. The IIR filter introduces less distortion to DSSS signal compared with the five-coefficients FIR filter and the improvement of SINR is independent of the instantaneous frequency of interference and the spreading gain.