Secrecy enhancement in MIMO-OFDM-IM systems with limited RF chains

Abstract

A physical layer security scheme is proposed to resist eavesdropping of a multiple-input multiple-output (MIMO) system using orthogonal frequency-division multiplexing with index modulation (OFDM-IM) in the presence of a multiple-antenna passive eavesdropper, where limited number of radio frequency (RF) chains is required and additional bits are conveyed through the indices of activated transmit antennas. The secure transmission against eavesdropping is achieved by injecting artificial noise (AN) in the frequency and spatial domains. When 2 RF chains are used, the structure of quadrature spatial modulation (QSM) is exploited. The generated OFDM-IM symbols are split into two orthogonal parts, and precoded ANs are injected into each part to produce combined symbols for transmission. The secure scheme is extended to general scenarios when other number of RF chains are available based on the idea of generalized spatial modulation (GSM) and AN. The secrecy rates of the proposed schemes are analyzed, and a closed-form approximation for the ergodic secrecy rate is derived to reduce the computation complexity. The power allocation ratio and design parameters are discussed through numerical simulations. Compared to the existing secure OFDM-IM scheme (modified for MIMO systems), the proposed secure MIMO OFDM-IM shows better performance against eavesdropping when the eavesdropping and legitimate links have the same channel quality and the eavesdropping-legitimate channel correlation is considered. When 2 RF chains are available, the secure scheme based on QSM is recommended since it outperforms the secure scheme based on GSM with 2 RF chains.