Directional modulation method to increase the capacity of confidential messages over broadcast channels

This study investigates information and secrecy capacity over broadcast channels through the use of directional modulation. The authors develop and demonstrate the utility of an antenna array control technique that provides communication devices with enhanced information and secrecy capacity by time-varying the array steering vectors using a novel beam control method. They show that an increase in the achievable capacity in a broadcast communication system beyond the currently-accepted Sato bound is possible by using the time domain of array steering vectors as an additional degree of freedom. Furthermore, they increase the overall secrecy capacity of the system by spatially restricting the successful reception of signals. In practical terms, this results in higher-rate, confidential messages for multiple devices for a given power constraint. They present simulation results of the array steering technique showing that direction-dependent modulation can provide broadcast channel capacity and secrecy capacity exceeding theoretical bounds for conventional, static coding techniques. They further describe an experiment and share measured results that corroborate the simulated results at the physical layer and provide insight into the proposed method.