# Securing Smart Grid Enabled Home Area Networks with Retro-Reflective Visible Light Communication
## 1 Introduction
Although more advanced 802.1X authentication [8] provides a stronger key protection, it requires a Remote Authentication Dial-In User Service (RADIUS) server and possibly also an Active Directory server, which will be costly for residential and small business settings. In this paper, we propose a heterogeneous radio frequency (RF) and Retro-VLC system (Figure 1) to secure wireless communication between smart appliances and IoT sensors in a HAN and the HAN gateway interfaced with the advanced metering infrastructure (AMI) of a smart grid. As shown in Figure 1, each smart appliance or IoT sensor in a HAN connects to the HAN gateway through two different wireless channels--a duplex Retro-VLC link for data exchange of sensitive tasks, e.g., key exchange, mutual authentication, association process; and a duplex RF link for transmitting and receiving data encrypted by the key shared through the Retro-VLC link. The study in [9] validates the feasibility of eavesdropping on a VLC signal, while intercepting both the VLC downlink and the Retro-VLC uplink is very difficult to achieve. The Retro-VLC link is established by interfacing retro-reflective tags with smart appliances and IoT sensors in a HAN. Optical modulator, e.g., liquid crystal shutters, are mounted on top of the retro-reflective tags to modulate the uplink data transmission from the tags to the lighting infrastructure. The Retro-VLC solution is adopted due to its extremely narrow interception range. As shown in Figure 2, different from a mirror reflection, retro-reflection reflects the incident light beams back to its source with minimal scattering. Replacing the low-cost retro-reflective film [10] with an accurately calibrated corner-cube retro-reflector [11] array can further reduce the scattering effect of the retro-reflection such that the possible sniffing range will be narrowed down to the line-of-sight between the tag and the reader. To study the feasibility of the proposed heterogeneous Retro-VLC + WLAN system, a multi-pixel Retro-VLC prototype is designed and implemented. As the optical intensities from the individual pixels constructively add up, the effective retro-reflected intensity is generated by the sum of all intensities, such that the effective signal is constructed over the optical uplink channel but not in the electrical domain. The multi-pixel mode supports more spectral-efficient modulation schemes, e.g., orthogonal frequency division multiplexing (OFDM), than on-off keying (OOK) in the single-pixel mode. Due to the digital formation process, the generated sinusoidal signal through the multi-pixel design is imperfect. We investigate the non-idealities using a quadrature amplitude modulation (QAM) signal and its corresponding I-Q constellation diagram. The distortion of a QAM signal is quantified based on the relationship between the number of pixels and the QAM signal's amplitude and phase offsets in the I-Q plane. The bit error rate (BER) performance of the Retro-VLC link is studied in simulations and experiments, and integrated into a heterogeneous network configuration for the evaluation of working range, sniffing range, and key exchange latency. The results demonstrate the suitability of duplex Retro-VLC link with respect to securing low-volume sensitive data exchange. The manuscript is organized as follows: in Section 2, we derive a close-form expression of BER for multi-pixel Retro-VLC and build on the architecture of a heterogeneous Retro-VLC + WLAN system and the testbed setup. Simulation and experimental results are presented in Section 3 to evaluate the BER performance, key exchange latency, and working and sniffing range. We discuss the findings, implications and future research challenges in Section 4.