Low-probability of Intercept/Detect (LPI/LPD) Secure Communications Using Antenna Arrays Employing Rapid Sidelobe Time Modulation

Abstract

We present an electronically-reconfigurable antenna array offering low probability of intercept/detect (LPI/LPD) and secure communications capabilities simultaneously at the physical layer. This antenna array is designed to provide rapidly time-varying sidelobes and a stationary main lobe. By performing rapid sidelobe time modulation (SLTM), the signal transmitted in the undesired directions (i.e., through sidelobes) undergoes spread-spectrum distortion making it more difficult to be detected, intercepted, and deciphered while the signal transmitted in the desired direction (i.e., through the main lobe) is unaffected. Therefore, the intended receiver would not need additional modifications (i.e. encryption keys) to detect and recover the signal. We describe the operating principles of this SLTM array and validate its spread-spectrum SLTM sequence generation in undesired directions through theory, simulations, and experiments. Using a fabricated SLTM prototype operating at X band, we conducted system-level measurements to demonstrate its LPI/LPD, secure communications, and jamming resilience capabilities. The presented method is a physical layer technique, which can bring LPI/LPD capabilities to existing communications systems by simply replacing their antennas with SLTM arrays. This technique can be used independently or in combination with additional coding and signal-processing techniques to achieve further enhancements in LPI/LPD and secure communications.

Figures (20)

• Figure 1: (a) Eight operational modes of an eight-element SLTM array. (b) Array factor of SLTM elements, array factor of non-SLTM elements, and their total array factor in 0$^{\circ}$ in Mode 1. (c) Array factor of SLTM elements, array factor of non-SLTM elements, and their total array factor in 30$^{\circ}$ in Mode 1. (d) Amplitude of array factor in 8 different modes in 0$^\circ$. (e) Phase of array factor in 8 different modes in 0$^\circ$. (All the figures are generated when $d$ is a half wavelength)
• Figure 2: Graphical representation of the power spectral density of the (a) SLTM sequence, (b) transmitted signal in main lobe and sidelobe directions and (c) zoom-in view of the transmitted signal. $f_b$=$\frac{1}{T_{b}}$.
• Figure 3: Graphical representation of the time domain $Re[x(t)]$ and the real part of the TM signal $r(t)$.
• Figure 4: Topology of the slotted patch antenna with a 1-bit electronically reconfigurable phase shift. (a) 3D perspective view. (b) Top view. (c) Bottom view.
• Figure 5: Equivalent circuit models of the PIN diodes, capacitors and inductors employed in the design of the 1-bit, phase-reconfigurable patch antenna.