Channel-Independent Quantum Mapping-Substitution OFDM-Based Spatial Modulation for Physical-Layer Encryption in VLC Networks

Physical-layer encryption has lately been highlighted as a promising security approach for visible light communications (VLC). Orthogonal frequency-division multiplexing (OFDM) is the main structural entity of multiplecarrier modulation in most modern communication systems, including vehicle ad hoc networks, 5G systems, and the Internet of Things. The integration of OFDM and spatial modulation (SM) can improve the capacity of lowcomplication VLC systems. Two common methods have been proposed for OFDM-based SM schemes: frequencydomain SM (FD-SM) and time-domain SM (TD-SM). TD-SM supports higher spectral efficiency than FD-SM with low complication. In this paper, we propose a physical-layer encryption method, referred to as channelindependent quantum mapping-substitution (CIQMS) TD-SM, for enhancing VLC's confidentiality. The random properties of the input data are used for generating dynamic cypher keys that are updated for each frame using hashing and stream cypher methods. The proposed approach performs four encryption operations: quantum noise stream cypher, substitution of radiated data symbols mapping, substitution of light-emitting-diode information mapping, and polarity encryption of time-domain OFDM subcarriers. The findings show that our CIQMS method provides a considerably higher secrecy rate than channel-dependent methods proposed in the literature. Also, it provides high robustness against both authorized and non-authorized eavesdroppers. In addition to that, the results show that the CIQMS method provides high confidentiality against several attacks, e.g., statistical attacks, chosen-/known-plaintexts, and brute-force attacks.