Passive Sensing Technique Using Cyclic Prefix in Power-Constrained Optical Wireless Communication Systems

In the era of 6 G, the merging of wireless communication with sensing technologies has become a key area of research, particularly in addressing limitations in spectrum resources. Optical wireless communication (OWC) is an advanced technology that provides high data rates and broad bandwidth, creating new opportunities for integrated sensing and communication (ISAC). However, current studies have not adequately investigated how to efficiently use communication signals in OWC systems for sensing in scenarios with a transmitted peak power constraint and signal clipping distortions. This work introduces a novel approach using the cyclic prefix (CP) of the signal frame in a clipping-enhanced optical orthogonal frequency division multiplexing (CEO-OFDM) system for integrated passive sensing and communication (IPSAC) using the OFDM signal itself without transmitting extra sensing signals. Employing CEO-OFDM technology mitigates the clipping distortion due to the peak transmitted power restrictions, ensuring optimal communication performance. Additionally, it leverages the inherent cyclic prefix of the OFDM signal for sensing purposes, eliminating the need for extra sensing signals like pilots, thus achieving efficient target detection and time difference of arrival estimation (TDOA). The study also uses the Cramér-Rao lower bound (CRLB) to measure the sensing capabilities of the system. Simulation results demonstrate that the proposed technique surpasses conventional DC-biased optical OFDM (DCO-OFDM) and asymmetrically clipped optical OFDM (ACO-OFDM) systems in sensing performance and bit error rate (BER). From the results, in the multi-frame TDOA estimation process, the TDOA estimation error in the CEO-OFDM system is 4 times less than that of the DCO-OFDM system and 2 times less than that of the ACO-OFDM system. This suggests that the CEO-OFDM system demonstrates notably reduced estimation errors and enhanced precision compared to the other systems.