Passive radar for measuring passive sensors: direct signal interference suppression on FPGA using orthogonal matching pursuit and stochastic gradient descent

Abstract

Passive wireless sensors provide an attractive technology in niche applications where battery powered sensors are not applicable. Surface acoustic wave technology provides an optimum implementation of passive wireless transducers acting as cooperative targets to short range radar systems: the signal carrying the information is delayed beyond clutter, with a piezoelectric substrate converting the electromagnetic wave to an acoustic wave whose properties are dependent on the environment. We have tackled the issue of short range radar certification by considering a passive radar approach in which existing non-cooperative radiofrequency sources are used to power the sensor, and the reader is made solely of passive receivers aimed at recording the reference signal generated by the non-cooperative source and the backscattered signal returned by the sensor. A passive radar approach only requires a cross-correlation between the reference and surveillance signals to identify the time delay between the incoming and backscattered signals and hence the recovery of the physical quantity sensed by the acoustic transducer through acoustic velocity modulation. Practical sources do exhibit some short term correlation. Hence, strong copies of the reference signal with delays shorter than the one introduced by the sensor must be canceled by the receiver to allow for the weak sensor response to be extracted. This classical problem is called Direct Signal Interference (DSI) suppression. In a post-processing approach with little computation time limitation, this problem is solved using a least square error optimization approach. In the context of real time sensor measurement using a Field Programmable Gate Array (FPGA) implementation, data recording, frequency transposition and decimation are readily implemented in the gate array matrix. DSI removal appears as the limiting factor for a full FPGA implementation of the short range passive radar reader. We address the challenge by the iterative process of DSI suppression using Orthogonal Matching Pursuit (OMP) algorithm, and additionnally consider the FPGAfriendly Stochastic Gradient Descent (SGD) approach to try to recover DSI coefficients from a pipelined algorithm using streams of measurement data.

Software-Defined Radio Implemented GPS Spoofing and Its Computationally Efficient Detection and Suppression

“…(32) can always act as a reference for resolving the ambiguity of phase difference estimation as observed during various experiments. Besides, we have already addressed in the past the issue of iteratively identifying the LS weight coefficient and had identified Stochastic Gradient Descent (SGD) [22] as a means of avoiding the computing burden of matrix inversion, especially aimed at being implemented in Field Programmable Gate Arrays (FPGA) best suited for iterative processing as each new sample is collected and, due to strong memory constraints, poorly suited to matrix storage and processing. The SGD based LS weight coefficient calculation method is briefly presented as follows.…”

“…The phase differences are used to detect the spoofing phenomenon and to classify the frequency peaks into two groups to distinguish the authentic and spoofing signals for spoofing suppression purpose. At last, with the help of the method proposed by [17] or a stochastic gradient descent (SGD) based iterative least squares (LS) algorithm [22], which is also applicable to strong spoofing and jamming suppression, the steering vector of spoofing signals is formed and the orthogonal projection based beamforming technique [17] is used to suppress spoofing signals and restore authentic signals. The proposed method makes a trade-off between existing pre-correlation and post-correlation methods: it works well in low and high spoofing power cases, the gains of the uncalibrated antennas are assumed to be different for different incoming signal directions, the periodicity of PRN codes is not necessary to be used, and its performance can be improved in low SNR situations by increasing the integration time; it is computationally efficient and can be achieved by a compact, low cost microcontroller before the GPS receiver without running the full acquisition procedure.…”

Capteurs acousto-électriques radiofréquences – Modes d'interrogation

“…Toutes les approches de RADAR à courte portée sont exploitables pour interroger les capteurs [25] : impulsionnel, onde continue balayée ou onde continue émise par paliers, à bruit par la génération d'un code pseudo-aléatoire pour étaler le spectre ou exploitant des sources non-coopératives dans une approche de RADAR passif [26], [27], [28]. Alors que toutes ces méthodes sont théoriquement équivalentes par transformée de Fourier, les conditions de déploiement opérationnel pourront pousser à sélectionner une approche ou l'autre.…”