Secrecy performance analysis of CIOD–OFDM systems over wireless fading channels

Space–time block codes from coordinate interleaved orthogonal designs (CIODs) have been attracted attention due to their capability for full-diversity and full-rate transmission along with their needs for only a single-symbol decoding. When the number of transmit antennas is more than two, the code rate of orthogonal space–time block coding (OSTBC) is mostly lower than one, whereas, the CIOD offers rate-one transmission for two, three and four transmit antennas, yielding a higher transmission rate compared to that of the OSTBC. In addition, physical layer (PHY) security performance of CIOD scheme is an important issue but has not been evaluated yet. In this paper, the PHY security performance of a combined coordinate interleaved orthogonal design and orthogonal frequency-division multiplexing (CIOD–OFDM) is evaluated and studied. Closed-form expressions for two performance evaluation metrics of PHY security such as the average secrecy capacity and the secrecy outage probability are derived and presented for the considered CIOD–OFDM system and compared with the secrecy performance of the OSTBC–OFDM system. Mathematical expressions are derived for the general case, where multiple antennas are used at the transmitter and receiver sides. It is shown that the CIOD–OFDM system can provide a better security performance than the OSTBC–OFDM system. Furthermore, union bound expressions for the symbol error rate performance of CIOD–OFDM system is derived. Then, an efficient method for enhancing the PHY security of the signal transmission in CIOD–OFDM is presented. The Monte-Carlo simulation results are also presented for verifying the results obtained from mathematical and theoretical expressions.