Alan R. Lindsey

Optimum interference excision in spread spectrum communications using open-loop adaptive filters

A generalized approach for interference suppression in PN spread spectrum communications using open-loop adaptive excision filtering is introduced. The excision filter coefficients under this technique depend on the jammer power and its instantaneous frequency (IF) information, and both values can be gained in the time-frequency domain. The dependency of the excision filter characteristics on the interference power, which was absent in past contributions in this area, is of significant importance as it allows optimum tradeoff between interference removal and the amount of self-noise generated from the induced correlation across the PN chip sequence, due to filtering. This tradeoff is bounded by the two extreme cases of no self-noise, which implies preprocessing disabled, and full interference excision, which the case previously considered. In this paper, we derive the FIR excision filters that maximize the receiver signal-to-noise ratio for narrowband interference and discuss the generalization to nonstationary jamming environment.

Subspace array processing for the suppression of FM jamming in GPS receivers

The mitigation of FM interference in GPS receivers is considered. In difference to commonly assumed wideband and narrowband interferers, the FM interferers are wideband, but instantaneously narrowband, and as such, have clear time-frequency (TF) signatures that are distinct from the GPS coarse acquisition (C/A) spread spectrum code. In the proposed technique, the estimate of the FM interference instantaneous frequency (IF) and the interference spatial signature are used to construct the spatiotemporal interference subspace. The IF estimates can be provided using existing effective linear or bilinear TF methods. The undesired signal arrival is suppressed by projecting the input data on the interference orthogonal subspace. With a multisensor receiver, the distinctions in both the spatial and TF signatures of signal arrivals allow effective interference suppressions. The deterministic nature of the signal model is considered and the known underlying structure of the GPS C/A code is utilized. We derive the receiver signal-to-interference-plus-noise ratio (SINR) under exact and perturbed IF values. The effect of IF estimation errors on both pseudorange measurements and navigation data recovery is analyzed. Simulation results comparing the receiver performances under IF errors in single and multiantenna GPS receivers are provided.

Broadband interference excision for software-radio spread-spectrum communications using time-frequency distribution synthesis

A new method is introduced for interference excision in spread-spectrum communications that is conducive to software-radio applications. Spare processing capacity in the receiver permits the use of time-frequency techniques to synthesize a nonstationary interference from the time-frequency domain using least squares methods. The synthesized signal is then subtracted from the incoming data in the time domain, leading to jammer removal and increased signal-to-interference-and-noise ratio at the input of the correlator. The paper focuses on jammers with constant modulus that are uniquely described by their instantaneous frequency characteristics. With this a priori knowledge, the jammer signal amplitude is restored by projecting each sample of the synthesized signal to a circle representing its constant modulus. With the phase matching provided by the least squares synthesis method and amplitude matching underlying the projection operation, a significant improvement in receiver performance/bit-error rates is achieved over the case where no projection is performed. Software-radio aspects including computational complexity and processing modes are also discussed.

Anti-jamming GPS receivers based on bilinear signal distributions

In this paper, we propose a nonstationary jammer suppression technique for GPS receivers. This technique is based on using the time-frequency distributions (TFDs) to define the jammer time-frequency (t-f) signature. A mask can be constructed and applied such that the masked t-f region captures the jammer energy, but leaves out most of the GPS signals. The jammer signals are synthesized from the masked TFDs and removed from the received signal by orthogonal projection. A method for determining an appropriate mask is discussed. We extend the proposed jammer excision method to multi-antenna GPS receivers.

Broadband nonstationary interference excision for spread spectrum communications using time-frequency synthesis

A new method is introduced for interference excision in spread spectrum communications. Time-frequency synthesis techniques are used to synthesize the nonstationary jammer from the time-frequency domain using least-squares methods. The synthesized jammer is then subtracted from the incoming data in the time domain, leading to increased signal to interference ratio at the input of the correlator. The paper focuses on jammers with constant modulus where the jamming signal is a polynomial phase. With this a priori knowledge, the jammer signal amplitude is restored by projecting each sample of the synthesized signal to a circle representing its constant modulus. With the phase matching provided by the least-squares synthesis method and amplitude matching underlying the projection operation, the paper shows a significant improvement in receiver performance/bit error rates over the case where no projection is performed.

Subspace projection techniques for anti-FM jamming GPS receivers

This paper applies subspace projection techniques as a pre-correlation signal processing method for the FM interference suppressions in GPS receivers. The FM jammers are instantaneous narrowband and have clear time-frequency (t-f) signatures that are distinct from the GPS C/A spread spectrum code. In the proposed technique, the instantaneous frequency (IF) of the jammer is estimated and used to construct a rotated signal space in which the jammer occupies one dimension. The anti-jamming system is implemented by projecting the received sequence onto the jammer-free subspace. This paper focuses on the characteristics of the GPS C/A code and derives the signal to interference plus noise ratio (SINR) of the GPS receivers implementing the subspace projection techniques.

A wavelet packet based model for time-varying wireless communication channels

Wavelet packets have found many applications in communications. Promising results have been obtained for a number of modulation schemes based on wavelet packets. However, effects of multipath and time-varying wireless communication channels on these wavelet packet signature waveforms have not been seriously dealt with. We present a method to model complex time-varying wireless communication channels using wavelet packets as bases. This model has the advantages of flexibility, efficiency, and accuracy. Test results using field measurement data show the effectiveness of our model.

Wavelet packet waveforms for multicarrier CDMA communications

Wavelet packets have been found a promising candidate for user signature waveforms in code division multiple access communication systems due to their good orthogonality property. In this paper we investigate the use of wavelet packets in multicarrier CDMA. In conventional multicarrier CDMA systems, the multi channel signals are modulated on sub carriers of sinusoid waveforms, using FFT to achieve fast computation. To combat multipath channel effect, a guard interval is inserted between consecutive data symbols. On the other hand, wavelet packet waveforms have the property of localization in both time and frequency domains. This property enables the time domain diversity of the multi carrier signal so that the need for guard intervals is eliminated by time domain diversity combining. Simulations are shown to compare the performance of the wavelet packet receiver design with the DFT based multi carrier CDMA system.

Spatial Time-Frequency Distributions: Theory and Applications

This chapter presents a comprehensive treatment of the hybrid area of time-frequency distributions (TFDs) and array signal processing. The application of quadratic ‘l’F’Ds to sensor signal processing has recently become of interest, and it was necessitated by the need to address important problems related to processing nonstationary signals incident on multiantenna receivers. Over the past few years, major contributions have been made to improve direction finding and blind source separation using time-frequency signatures. This improvement has cast quadratic TFDs as a key tool for source localization and signal recovery, and put bilinear transforms at equal footing with second-order and higher-order statistics as bases for effective spatial-temporal signal processing. This chapter discusses the advances made through time-frequency analysis in direction-of-arrival estimation, signal synthesis, and near-field source characterization.

On the application of time-frequency distributions in the excision of pulse jamming in spread spectrum communication systems

Direct sequence spread spectrum (DSSS) systems are widely used in in a variety of applications including the suppression of a strong interfering signal due to jamming or multipath propagation. Time-frequency distributions (TFD) are applied for interference excision in spread spectrum communication systems. The focus is on jammers consisting of pulses of constant envelope frequency modulated interference. The time-support and the instantaneous frequency (IF) information provided by the TFD are used to reduce the jammer effect on the receiver performance. This is achieved by applying an excision notch filter with a null placed at the interference IF. The filter is turned on and off in synchronisation with the interference duty cycle. The bit error rates at different frequencies are given and compared with those obtained using multiresolution analysis.