A Novel Continuous Wave Interference Detectable Adaptive Notch Filter for GPS Receivers Ying-Ren Chien 1 , Yi-Cheng Huang 2 , De-Nian Yang 3 , and Hen-Wai Tsao 4 Research Center for Information Technology Innovation, Academia Sinica 1,3 Institute of Information Science, Academia Sinica 3 Graduate Institute of Communication Engineering, National Taiwan University 2,4 E-mail:{curtis 1 ,dnyang 3 }@iis.sinica.edu.tw, kshscoolevan@gmail.com 2 , tsaohw@cc.ee.ntu.edu.tw 4 Abstract—In this paper, we propose an interference detectable adaptive notch filter (ANF) for GPS receivers. The proposed ANF can estimate the existence of continuous wave interference (CWI) and its power, by exploiting the statistic value within an adaptive second-order infinite impulse response (IIR) filter. Moreover, our ANF is modulized, which allows more modules can be included to deal with multiple CWIs. We also design an adaptation algorithm for each ANF module and prove that the ANF module can adaptively notch the strongest CWI at its input, and this merit enables us to notch the primary CWIs with a limited number of ANF modules. Simulation results show that the adaption of the ANF modules converges in four iterations, and the signal to interference plus noise ratio (SINR) improvements can reach 21 dB. I. I NTRODUCTION A Global Positioning System (GPS) receiver inherently has the anti-jamming ability due to its characteristics of spreading spectrum. However, the performance of GPS receivers will severely degrade as the jamming signals are higher than the system’s anti-jamming ability. The existing results [1] indicated that one of the most insidious jamming sources is the continuous wave interference (CWI), which can easily overwhelm a GPS receiver’s analog-to-digital converter (ADC) at the analog front-end part and paralyze the GPS. CWI rejection by adaptive filtering techniques has attracted great attention recently and can be classified into time- domain [2]–[4] and frequency-domain approaches [5]–[7]. Rusch and Poor [2] proposed an enhanced nonlinear method to suppress the interferences. However, this approach requires a nonlinear function tanh, which is more computational inten- sive. Moreover, the performance depends on the correctness of the feedback data. In [4], the authors utilized a notch filter to reduce spurs and combined a feedback filter to cancel inter- symbol-interferences caused by the notch filter. However, the performance may degrade due to the decision errors. Ma et al. [3] proposed an adaptive all-passed based notch filter to reject the CWIs. However, hardware cost may increase because the information of the covariance matrix and its inverse are required during the adaptation process. In addition, Wang et al. [5] designed a partial coefficient updating algorithm to adaptively update N sets of interference rejection blocks in frequency domain. Capozza et al. [6] proposed an N-sigma excision algorithm to null CWIs in frequency domain. Balaei and Dempster [7] devised a statistically hypothesis testing to detect GPS interference in frequency domain. Zhang et al. [8] proposed a wavelet transform based approach to suppress the CWIs. However, the major concerns about the above transferred domain approaches are the hardware complexity, which comes from Fast Fourier transform (FFT), inverse FFT, or wavelet transform blocks in the hardware. Moreover, the windowing blocks are desired to avoid significant spectral leakage issues [6] and hence incurs higher cost. In this paper, therefore, we propose a low-complexity time- domain approach, called the adaptive notch filter (ANF) mod- ule, which is able to detect, estimate, and notch one single-tone CWI. The ANF module is composed of a simple second-order IIR filter with the lattice structure. We show that if the -3 dB bandwidth of the IIR filter is less than about 30 KHz, the non-linear phase response of the IIR filter brings almost zero offset on both the acquisition and tracking loops. Therefore, the proposed ANF does not require complicated FFT blocks. Moreover, to adaptively adjust the notch frequency, we cross- correlate the output and internal adaptive signals of the ANF module. The power of the notched CWI can be simultaneously estimated by using the internal adaptive information embedded within the ANF module as well. Furthermore, we devise a novel jamming signal detection algorithm without using frequency-domain information. When there is no jamming signal, the received signal will bypass the ANF module so that the degradation of signal-to-noise ratio (SNR) caused by the ANF can be avoided. The proposed scheme can deal with multi-tone CWIs by cascading the ANF module, and we prove that each stage can notch the strongest CWI appearing at its input. The rest of this paper is organized as follows. Section II describes the system model and two performance evaluation metrics. We propose the adaptation algorithm of the ANF module and the corresponding CWI detection and estimation algorithms in Section III. Section IV presents simulation results and shows how to determine the -3 dB bandwidth of the ANF modules and the value of CWI detection threshold. The performance of multiple CWIs detection and rejection are demonstrated as well. Finally, we conclude this paper in Section V. 978-1-4244-5637-6/10/$26.00 ©2010 IEEE This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the IEEE Globecom 2010 proceedings.