Nonstationary Interference Excision in Time-Frequency Domain Using Adaptive Hierarchical Lapped Orthogonal Transform for
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Nonstationary Interference Excision in Time-Frequency Domain Using Adaptive Hierarchical Lapped Orthogonal Transform for Direct Sequence Spread Spectrum Communications Li-ping Zhu Department of Electronics Engineering, Shanghai Jiao Tong University, Shanghai 200030, China College of Information Engineering, Dalian Maritime University, Dalian, Liaoning 116026, China Email: [email protected]
Guang-rui Hu Department of Electronics Engineering, Shanghai Jiao Tong University, Shanghai 200030, China Email: [email protected]
Yi-Sheng Zhu College of Information Engineering, Dalian Maritime University, Dalian, Liaoning 116026, China Email: [email protected] Received 22 November 2002 and in revised form 15 June 2003 An adaptive hierarchical lapped orthogonal transform (HLOT) exciser is proposed for tracking, localizing, and rejecting the nonstationary interference in direct sequence spread spectrum (DSSS) communications. The method is based on HLOT. It utilizes a fast dynamic programming algorithm to search for the best basis, which matches the interference structure best, in a library of lapped orthogonal bases. The adaptive HLOT differs from conventional block transform and the more advanced modulated lapped transform (MLT) in that the former produces arbitrary time-frequency tiling, which can be adapted to the signal structure, while the latter yields fixed tilings. The time-frequency tiling of the adaptive HLOT can be time varying, so it is also able to track the variations of the signal time-frequency structure. Simulation results show that the proposed exciser brings significant performance improvement in the presence of nonstationary time-localized interference with or without instantaneous frequency (IF) information compared with the existing block transform domain excisers. Also, the proposed exciser is effective in suppressing narrowband interference and combined narrowband and time-localized impulsive interference. Keywords and phrases: nonstationary interference excision, adaptive hierarchical lapped orthogonal transform, hierarchical binary tree, best basis selection, dynamic programming algorithm.
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INTRODUCTION
Over the past several years, interference excision techniques based on time-frequency representations of the jammed signal have received significant attentions in direct sequence spread spectrum (DSSS) communications [1, 2, 3, 4]. The attraction of the time-frequency domain interference excision techniques is that they have the capability of analyzing the time-varying characteristics of the interference spectrum, while the existing time domain and transform domain techniques do not. The time-frequency representation of a signal refers to
expanding the signal in orthogonal basis functions which give orthogonal tilings of the time-frequency plane. Herley et al. [5] use time-frequency tile of a particular basis function to designate the region in the time-frequency plane which contains most of that function’s energy. The timefrequency tiles of the spread spectrum signal and the channel additive white Ga
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