An Application of Digital Filters to Spread Spectrum Communications

1.本文提出了一种分析复系数自适应无限冲激响应（IIR）陷波器稳态性能的方法，该陷波器可用于四相相移键控（QPSK）扩频通信系统中的多窄带干扰抑制。得到了信噪比改善因子，由此可以得出陷波滤波器使用的有效性.研究了直接序列码分多址（DS CDMA）通信系统中自适应IIR陷波器抑制窄带干扰的问题，提出了一种基于二阶格型的自适应IIR陷波器，并给出了归一化算法。通过收敛性分析发现，对于较大的NBI功率或干扰信号功率比（ISR），归一化算法的收敛速度与DS CDMA系统的活跃用户数和NBI功率无关，这是一个理想的结果。我们分析了所提出的二阶格型所带来的DS CDMA系统的性能增益，的自适应IIR陷波算法，以判决变量的信噪比改善因子和误码率（BER）为优值。研究发现，信噪比改善因子随窄带干扰功率的增大而增大，随DS-CDMA系统活跃用户数的增大而减小.提出了一种基于多小波变换的感兴趣区域（ROI）图像编码方法。该方法避免了JPEG-2000在较低码率约束下对非感兴趣区域造成的大量信息损失，从而提高了恢复图像整体的主观图像质量，保持了感兴趣区域优于非感兴趣区域的质量。

1. We proposed a method to analyze the steady-state performance of a complex coefficient adaptive infinite impulse response (IIR) notch filter which is useful for the rejection of multiple narrow-band interference in quadrature phase shift keying (QPSK) spread spectrum communication systems. The signal-to-noise ratio (SNR) improvement factor has been obtained from which the validity of the use of the notch filter can be concluded.2. We have investigated the problem of narrow-band interference (NBI) suppression in a direct-sequence code-division multiple-access (DS CDMA) communication system using adaptive IIR notch filter algorithms.We first proposed a second-order lattice-based adaptive IIR notch filter with a normalized algorithm. We found out from the convergence analysis that for large NBI power or interference-to-signal power ratio (ISR), the convergence speed of the normalized algorithm is independent of the number of DS CDMA system active users and power of the NBI which is a desirable result.Secondly, we analyzed the performance gain of the DS CDMA system afforded by the proposed second-order lattice-based adaptive IIR notch filter algorithms using SNR improvement factor of the decision variable and bit error rate (BER) as figures of merit. We found out that the SNR improvement factor increases with the power of the NBI and decreases with the number of DS CDMA system active users.3. We proposed a region-of-interest (ROI) image coding based on multiple wavelet transforms. The proposed method can avoid much information loss for non ROI at lower constrained bit rate caused in JPEG-2000, therefore the subjective image quality for whole of restored image will be improved, maintaining the quality of ROI higher than that of non-ROI.