In recent gears, we have witnessed the rapid adoption of the Department of Defense's Global Positioning System (GPS) for navigation in a number of military and civilian applications. Unfortunately, the low-power GPS signal is susceptible to interference. This paper presents a novel VLSI architecture that removes narrow-band signals from the wide-bandwidth GPS spectrum. The interference suppression technique employed is frequency-domain excision. The single-chip frequency-domain excisor transforms the received signal (GPS signal+noise+interference) to the frequency domain, computes signal statistics to determine an excision threshold, removes all spectral energy exceeding that threshold, and restores the remaining signal (GPS signal+noise) to the temporal domain. The heart of this VLSI implementation is an on-chip 256-point fast Fourier transform processor that operates at 40 million complex samples per second. It processes 12-bit (for each I and Q) sampled complex data. The 1.57 million-transistor chip was fabricated in 0.5-/spl mu/m CMOS triple metal technology and is fully functional.

A single-chip narrow-band frequency-domain excisor for a Global Positioning System (GPS) receiver

A coherent demodulation device, for the demodulation of a digital signal of the type constituted by digital elements distributed in the time-frequency space and transmitted in the form of symbols constituted by a multiplex of N orthogonal carrier frequencies modulated by a set of the digital elements and broadcast simultaneously, the digital signal comprising reference elements, having a value and a position, in the time-frequency space, that are known to the demodulation device, comprising means for the estimation, by Fourier transform, of the frequency response of the transmission channel at any instant, carrying out the transformation of the received samples, corresponding to reference elements, from the frequency domain to the temporal domain, the multiplication in the temporal domain of the transformed samples by a rectangular temporal window (f n) and the reverse transformation, after the multiplication, of the obtained samples from the temporal domain into the frequency domain, the estimation means comprising means for the thresholding of the samples in the temporal domain, providing for the systematic elimination of the samples below a certain threshold.

Device for the coherent demodulation of time-frequency interlaced digital data, with estimation of the frequency response of the transmission channel and threshold, and corresponding transmitter

A wireless communication apparatus employing spread spectrum modulation which detects and eliminates interference waves and temporarily stops and restarts communication for maintaining fine communication status. If the interference wave frequency band is known, a reception signal of the frequency band is eliminated. Communication is temporarily halted and restarted corresponding to interference occurrence period and/or radiowave propagation condition, and communication status is notified to a user. Further, if the interference wave occurrence period is unknown, the frequency of the interference wave is detected and a signal of the detected frequency is eliminated. In a case where communication is performed in packet form, the number of packets which can be transmitted sequentially without receiving reception confirmation is controlled based on the radiowave propagation condition. Moreover, a reception signal can be divided into a plurality of frequency band signals and interference wave detection can be performed in band signal units.

Spread spectrum modulation communication apparatus for narrow band interference elimination

Jammers characterized by their instantaneous frequencies can be effectively mitigated in direct sequence spread spectrum communications by using open-loop adaptive excision filters. The primary requirement for these filters is that they must possess a notch in tune with the jammer instantaneous frequency (IF) to annihilate the interference power at every time sample. The interference time-varying frequency can be obtained using existing IF estimators, including quadratic time-frequency distribution methods. Without focusing on any specific estimator, we develop expressions for the receiver performance using a general class of multiple-zero FIR excision filters and show the dependence of the bit error rate (BER) on the filter order and its group delay. The effect of inaccuracies in the jammer instantaneous frequency information on the receiver performance is considered and evaluated as a function of the filter notch bandwidth. The latter is defined by the filter zero multiplicity, which is shown to be an important factor in the analysis of the correlator signal-to-noise ratio (SNR).

Performance analysis of instantaneous frequency-based interference excision techniques in spread spectrum communications

The capacity of direct-sequence spread-spectrum modulation to reject narrow-band interference can be significantly improved by eliminating narrow-band energy at the receiver in a process called frequency excision. This paper considers several algorithms that operate on the real-time discrete Fourier transform (DFT) of the received signal to perform frequency excision. The case in which only the signal and additive white Gaussian noise (AWGN) are present at the receiver is considered as a means of comparing the relative performance of different algorithms that operate without knowledge of the power spectral density of the interference. An approach for analysis, using the postcorrelation signal-to-noise ratio (SNR) as the figure of merit, is presented that is valid for a broad class of spreading modulations. First, the algorithm that sets a fixed fraction of the frequency domain record to zero is examined using rank-order statistics as an analytical tool. This result is then generalized to confirm previous estimates of SNR degradation for the algorithm that sets all values that exceed a threshold to zero. These results are again generalized to apply to the algorithm that sets a fixed fraction of the band to a fixed amplitude while retaining phase information in an algorithm called fraction clip. The relative performances of several clip algorithm options are derived as special cases. Finally, a performance measure of the algorithms in the presence of multiple narrow-band interference is provided and illustrated with an example.

Analysis of DFT-based frequency excision algorithms for direct-sequence spread-spectrum communications

Analyses and performance predictions of an FFT-based narrowband interference suppression filter for use in PN (pseudonoise) spread-spectrum communications systems are presented. The received baseband signal is processed in fixed-length blocks, transformed to the frequency domain with an FFT, filtered there by using an appropriate weighting, and then transformed back to the time domain. A general expression for the symbol matched filter signal-to-noise ratio (SNR) is given in terms of signal parameters and filter weighting coefficients. An optimal (maximum SNR) filter scheme and several suboptimal thresholding algorithms are discussed. Performance predictions, based on a realistic high-frequency interference model, are presented for these algorithms and FFT-based architecture. >

An FFT based technique for suppressing narrow-band interference in PN spread spectrum communications systems

A method for determining the number of sources impinging on a uniform linear array, which is applicable even in the extreme case of fully correlated sources, is presented. This technique uses modified rank sequences, a modification of the construction implicit in the matrix decomposition methods of A. Di (1985). The authors prove that if a particular rank sequence stabilizes to a value strictly less than the common row size of the defining block matrices, then this value equals the number of sources provided that the number has not exceeded a Bressler-Macovski (1986) type bound. Using the above characterization of stability, they formulate an algorithm that either determines the number of sources or indicates that the resolution capability of the algorithm has been exceeded. Rank determinations are based on an additive perturbation model. A threshold that approximates the largest singular value of the error matrix is determined and used to separate the large and small singular values of the matrices that induce the rank sequence. >

R.C. DiPietro

Papers

An FFT based technique for suppressing narrow-band interference in PN spread spectrum communications systems

Enumeration of fully correlated signals by modified rank sequences