Two Practical Random-Subcarrier-Selection Methods for Secure Precise Wireless Transmissions

Abstract

In directional modulation networks, two practical random-subcarrier-selection methods are proposed to transmit confidential message, and by constructing random subcarrier set and performing a randomization procedure, only a single receive power peak is formed in the desired user position per orthogonal frequency division multiplexing (OFDM) symbol. This scheme completely addresses the crucial problem facing secure precise wireless transmission (SPWT); how to achieve the SPWT per OFDM symbol while the traditional SPWT holds only in the statistically average sense. Several necessary conditions for SPWT per OFDM are derived and proposed: random, distributed, and distinct subcarrier spacing pattern mapped onto transmit antenna array. Two random subcarrier index sets, quadratic subcarrier set (QSS), and prime subcarrier set (PSS) are constructed. Subsequently, a random metric is defined, and a randomization procedure (RP) is proposed. Its detailed process include the following steps: integer mod, ordering, blocking, and block interleaving (BI) where BI is repeated until the random metric is greater than the predefined value. This yields a single high energy main peak at the desired position while at other positions, i.e., outside the main peak, only weak received energy peak seriously contaminated by the artificial noise are harvested. Simulation results confirm that our proposed two methods, QSS plus RP and PSS plus RP, perform significantly better than conventional methods such as linear subcarrier set plus RP. They actually form a single high energy main peak at desired position, and generate only extremely small leakage outside the main peak.