Directional Modulation Design under a Given Symbol-Independent Magnitude Constraint for Secure IoT Networks

Directional modulation (DM) is an important technology for physical layer security in wireless communications. Recently, a symbol-independent magnitude constraint for all antennas was proposed in DM design to reduce the design complexity of its analogue implementation. However, a limitation of the method is that it can only set the magnitude to a certain value, and all the antenna coefficients have the same magnitude. In this article, a more flexible solution is provided and the challenge of the design is the nonconvex constraint enforcing an arbitrary symbol-independent magnitude for all coefficients. To solve the problem, a convex iterative method is proposed, based on which the magnitudes of weight coefficients for all antennas can be chosen by designers in advance according to the specific requirements, allowing more freedom in the design process, which is the major difference between the previously proposed design and the newly proposed one. Two design examples are provided to demonstrate the effectiveness of the proposed design. One is is a general example, where coefficient magnitudes for different symbols are the same for the same antenna, but different for different antennas; the other one is a special case where magnitudes for all antennas are the same.