Optimal Closed-Form Designs for Directional Modulation with Practical Hardware Limitations.

In this paper, directional modulation (DM) is investigated to enhance physical layer security. Practical transmitter designs are exploited under imperfect channel state information (CSI) and hardware limitations, such as finite-resolution phase shifters (PSs) at transmitter. Considering the most common scenario where eavesdroppers (Eves)' information is completely unknown, we exploit the DM design to optimize legitimate users (LUs)' receiving performance while randomizing the Eves' received signal. Explicitly, tailored for the practical hardware limitations and imperfect CSI in realizing DM, we design optimal precoders in closed-form with Lagrangian and Karush-Kuhn-Tucker conditions, enhancing the practicality of the symbol-level based DM. Simulation demonstrates that the algorithm achieves lower symbol error rate (SER) at the LUs while deteriorating the Eves' SER, leading to an improved secrecy rate over the benchmarks.