Instantaneous frequency based nonlinear adaptive filter for interference suppression in spread spectrum systems
This paper presents an approach based on jammer instantaneous frequency estimation for suppression of frequency modulated (FM) jammers in spread spectrum systems. The FM jammers are instantaneous narrowband and have clear time-frequency signatures that are distinct from spread spectrum code. In the proposed approach, an augmented state-space representation of the received signal (spread spectrum signal+noise+interference signal) is developed using a three-coefficient FIR model for the FM interference. Based on the state-space representation developed, a Kalman-type nonlinear adaptive filter namely, augmented state approximate conditional mean filter (ASACMF) is formulated to estimate the unknown jammer instantaneous frequency and the FM interference from the received signal. The efficacy of the proposed filter is corroborated with simulation examples for FM interference suppression in spread spectrum systems. The performance improvement achieved with the proposed filter is quantified in comparison with the three coefficient FIR excision filter. Simulation results show that the proposed filter is effective in suppressing the FM interference in spread spectrum systems with good receiver output SNRs.