Physical Layer Encryption using Chaotic Quadrature Phase Shift Keying Constellation for Secure Orthogonal Frequency Division Multiplexing Data Transmission

A physical-layer encryption scheme using chaotic Quadrature Phase Shift Keying (QPSK) constellation is presented for secure Orthogonal Frequency-Division Multiplexing (OFDM) data transmission. The proposed system encrypted baseband image data using a two-stage chaos-based cypher (bitwise diffusion and substitution via a chaotic S-box) before QPSK modulation and OFDM transmission. The secret chaotic keys effectively randomise the QPSK constellation, ensuring that an eavesdropper observes a noise-like signal while legitimate receivers with the correct keys recover the data. We derive the algorithmic steps of the encryption and decryption process and illustrate the system with a block diagram matching the implemented MATLAB processing pipeline. Simulation results demonstrate that the encrypted transmission achieves security with minimal performance penalty: the bit-error-rate (BER) for an authorised receiver approaches 10⁻⁵ at moderate signal-to-noise ratios, whereas an unauthorised receiver faces a BER around 0.5 (i.e., random guessing). Constellation diagrams confirm that the cipher-text QPSK symbols appear chaotic, and the intended receiver recovers a sample image with high fidelity. The proposed scheme thus provides robust physical-layer security for OFDM systems without requiring complex higher-layer encryption.