Antenna Selection for Enhancing Privacy in Radar-Based Vital Sign Monitoring Systems

Radar-based vital sign monitoring (VSM) systems have become valuable for non-contact health monitoring by detecting physiological activities, such as respiration and heartbeat, remotely. However, the conventional phased array used in VSM is vulnerable to privacy breaches, as an eavesdropper can extract sensitive vital sign information by analyzing the reflected radar signals. In this paper, we propose a novel approach to protect privacy in radar-based VSM by modifying the radar transmitter hardware, specifically by strategically selecting the transmit antennas from the available antennas in the transmit array. By dynamically selecting which antennas connect or disconnect to the radio frequency chain, the transmitter introduces additional phase noise to the radar echoes, generating false frequencies in the power spectrum of the extracted phases at the eavesdropper's receiver. The antenna activation pattern is designed to maximize the variance of the phases introduced by antenna selection, which effectively makes the false frequencies dominate the spectrum, obscuring the actual vital sign frequencies. Meanwhile, the authorized receiver, having knowledge of the antenna selection pattern, can compensate for the phase noise and accurately extract the vital signs. Numerical experiments are conducted to validate the effectiveness of the proposed approach in enhancing privacy while maintaining vital sign monitoring.

On the Security of Directional Modulation via Time Modulated Arrays Using OFDM Waveforms

In this paper, we investigate how secure the TMA OFDM system is, by looking at the transmitted signal from an the viewpoint of eavesdropper. First, we propose a novel, low-complexity scheme via which the eavesdropper could defy the scrambling in the received signal and recover the transmitted symbols. We show that the symbols which the eavesdropper sees along the OFDM subcarriers are linear mixtures of the source symbols, where the mixing coefficients are unknown to the eavesdropper. Independent component analysis (ICA) could be used to obtain the mixing matrix but there would be permutation and scaling ambiguities. We show that these ambiguities can be resolved by leveraging the structure of the mixing matrix and the characteristics of the TMA OFDM system. In particular, we construct a k-nearest neighbors (KNN)-based algorithm that exploits jointly the Toeplitz structure of the mixing matrix, knowledge of data constellation, and the rules for designing the TMA ON-OFF pattern to resolve the ambiguities. In general, resolving the ambiguities and recovering the symbols requires long data. Specifically for the case of the constant modulus symbols, we propose a modified ICA approach, namely the constant-modulus ICA (CMICA), that provides a good estimate of the mixing matrix using a small number of received samples. We also propose measures which the TMA could undertake in order to defend the scrambling. Simulation results are presented to demonstrate the effectiveness, efficiency and robustness of our scrambling defying and defending schemes. Complete abstract please see in the paper.

Enhance Security of Time-Modulated Array-Enabled Directional Modulation by Introducing Symbol Ambiguity

In this paper, if the time-modulated array (TMA)-enabled directional modulation (DM) communication system can be cracked is investigated and the answer is YES! We first demonstrate that the scrambling data received at the eavesdropper can be defied by using grid search to successfully find the only and actual mixing matrix generated by TMA. Then, we propose introducing symbol ambiguity to TMA to defend the defying of grid search, and design two principles for the TMA mixing matrix, i.e., rank deficiency and non-uniqueness of the ON-OFF switching pattern, that can be used to construct the symbol ambiguity. Also, we present a feasible mechanism to implement these two principles. Our proposed principles and mechanism not only shed light on how to design a more secure TMA DM system theoretically in the future, but also have been validated to be effective by bit error rate measurements.

How secure is the time-modulated array-enabled ofdm directional modulation?

Time-modulated arrays (TMA) transmitting orthogonal frequency division multiplexing (OFDM) waveforms achieve physical layer security by allowing the signal to reach the legitimate destination undistorted, while making the signal appear scrambled in all other directions. In this paper, we examine how secure the TMA OFDM system is, and show that it is possible for the eavesdropper to defy the scrambling. In particular, we show that, based on the scrambled signal, the eavesdropper can formulate a blind source separation problem and recover data symbols and TMA parameters via independent component analysis (ICA) techniques. We show how the scaling and permutation ambiguities arising in ICA can be resolved by exploiting the Toeplitz structure of the corresponding mixing matrix, and knowledge of data constellation, OFDM specifics, and the rules for choosing TMA parameters. We also introduce a novel TMA implementation to defend the scrambling against the eavesdropper.