Physical-Layer Security Improvement in MIMO OFDM Systems Using Multilevel Chaotic Encryption

Ensuring physical-layer security (PLS) in wireless communication has always been a challenge due to the broadcasting nature of the transmission. In this work, a multilevel chaotic encryption (MCE) system is proposed to improve PLS in Multiple-Input Multiple-Output (MIMO) Orthogonal Frequency Division Multiplexing (OFDM) systems under Rayleigh fading channel. Additionally, the proposed technique improves the transmission performance by reducing the high Peak-to-Average-Power Ratio (PAPR) which is also considered as one of the major drawbacks of Multicarrier Modulations (MCM) like OFDM. In the proposed scheme, multilevel encryption is achieved in two steps. In the first step, a precoding matrix based on a unique chaotic sequence scrambles the modulated symbols. In the second step, phase scrambling is used based on chaotic sequence for selective mapping to provide a second level of encryption for the entire transmission. We evaluated the security performances using randomness of the proposed MCE, achievable key-space, and security analysis under cryptographic attacks from the perspective of Cryptanalysis. We verified the communication performances in terms of computational complexity of the system, PAPR, and error performances considering multipath Rayleigh fading wireless channel. The proposed MCE provides a huge key-space of similar to 10(302) which can resist any cryptographic attacks. Also, the pseudo-random nature of the multilevel chaotic scrambling reduces the PAPR by reducing autocorrelation of the OFDM signal. The proposed low-complexity MCE solution outperforms existing encryption schemes when compared in terms of security, computational complexity, and PAPR. Mathematical analysis and simulation results show that the proposed MCE scheme can enhance physical layer security along with significant reduction in PAPR without compromising the error performances of the system.