Space-Time Line Code Hopping for Physical-Layer Secure Communications

This study explores the security of the existing space-time line code (STLC) scheme. A novel STLC-hopping scheme is proposed to ensure physical-layer secure communication of the STLC transceiver, overcoming the limitations of the conventional STLC systems posed by correlated legitimate and eavesdropping channels. The transmitter randomly selects an STLC encoding matrix for every two symbols, and the legitimate receiver decodes them based on the transmitter's chosen matrix under the assumption that the transceiver shares a pseudorandom sequence generator for the STLC-hopping pattern. The proposed STLC-hopping system is scalable for the number of transmit antennas and compatible with a conventional STLC scheme. Even with highly correlated eavesdropping channels, the proposed STLC-hopping method prevents an eavesdropper from decoding STLC signals. Numerical results confirm the effectiveness of the proposed STLC-hopping method by showing it can almost sustain the secrecy rate lower bound, regardless of the eavesdropping channel correlations, and offer enhanced physical-layer security for STLC systems. Further, bit-error-rate performance results verify that hopping between merely two STLCs can provide sufficient secure communications.