  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.
