Array Phase Center Dynamics Using Spatial Amplitude Modulation for High-Efficiency Secure Wireless Communication

We present an approach to secure wireless communication based on dynamically changing the apparent phase center (PC) of a two-element antenna array using spatial amplitude dynamics. We implement PC dynamics by modulating the relative amplitudes of the signals fed to two antenna elements. By moving the PC symmetrically with respect to the geometric center of the array, the far-field amplitude pattern remains mostly constant while the phase pattern changes, imparting modulation onto the transmitted or received signals that is a function of the angle. The resulting directional modulation (DM) effectively scrambles the information in the radiated waveform at angles outside of the information beam, where the modulation impacts are negligible. The result is a narrow region where information is recoverable, and since the radiated power does not change, the efficiency remains high. We present the design of a 2.5-GHz two-element array with asymmetric amplitude modulation and characterize the PC location as a function of the amplitude ratio between the elements. We demonstrate secure wireless communication experimentally in a high signal-to-noise ratio (SNR) environment, demonstrating the reduction in throughput due solely to the PC dynamics. Finally, we analyze the information beamwidth (IB) and present a design procedure relating the amplitude ratio, SNR, element spacing, and QAM modulation format to the IB.