Spherical Modes Driven Directional Modulation With a Compact MIMO Antenna

Abstract

In this letter, a compact beamsteering multiple-input-multiple-output (MIMO) antenna is proposed for directional modulation applications. A generalization of the beamsteering principle is presented and by exploiting the antenna's unidirectional beamsteering capability within the entire azimuth plane, the system realizes secure steerable transmissions in the direction of the legitimate receiver, with a low bit error rate of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\boldsymbol{10^{-5}}$</tex-math></inline-formula> , while a high error rate of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\boldsymbol{10^{-1}}$</tex-math></inline-formula> is seen outside the desired regions. Numerical and experimental verifications are carried out to validate the proposed concept and comparisons with circular arrays are conducted, revealing that the system achieves comparable performance but with up to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\boldsymbol{35\%}$</tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\boldsymbol{71\%}$</tex-math></inline-formula> reduction in antenna diameter and profile, respectively. The method is proposed as a good candidate to enhance the security of small Internet of Things (IoT) devices.