Countering RF Side-Channel Sniffing in Optical Wireless Communication

Optical Wireless Communication (OWC) has been emerging as a promising complementary technology to traditional Radio Frequency (RF) communication for 6G and beyond. Due to the inherent low penetration property of light, the optical signal can be easily blocked by non-transparent objects, such as walls, rendering OWC highly secure at the physical layer. However, recent studies unveil that OWC systems can be eavesdropped even outside the wall. This is because while communicating via light, OWC transmitters also emit RF signals carrying the same information as light signals, thus leading to potential security vulnerabilities. Though Commercial Off-The-Shelf (COTS) OWC products are required to follow Electromagnetic Compatibility (EMC) regulations with minimum RF signal leakage, some OWC systems still struggle to meet the requirements when deployed in practical scenarios. To bridge this gap, we are the first to propose and implement a physical layer countermeasure protocol, Dynamic Frequency and Duration (DFD), to counter RF side-channel eavesdropping in OWC systems. The key idea is to transmit the data by dynamically changing the carrier frequency and transmission duration of the modulated signals that are fed into the OWC transmitters. We first validate the RF side-channel eavesdropping phenomenon and examine the effects of the different eavesdropping antennas, Modulation Coding Schemes (MCSs), signal gains, and Light-Emitting Diode (LED) transmitters on the leaked RF side-channel signal power, based on the developed programmable software-defined OWC and eavesdropping testbed. Then, we implement and evaluate DFD with extensive experiments, where the results demonstrate that DFD is highly effective in mitigating the security risks posed by the RF side-channel leakage without significantly affecting the performance of OWC system.