Radio-Frequency (RF) backscatter has emerged as a low-power communication technique. Backscatter systems either rely on active signal generators (spectrum efficient, but dedicated infrastructure) or existing ambient wireless transmissions (existing infrastructure, but spectrum inefficient). In this paper, we aim to make RF backscatter spectrum efficient and at the same time work with existing infrastructure. We propose to leverage the deployment of LiFi networks built upon LED bulbs for pervasive RF backscatter. We experimentally demonstrate that LiFi, which passively leaks RF signals, can be exploited as a radio carrier generator for low-power RF backscatter. We further design LeakageScatter, the first backscatter system operating in the ISM band and exploiting LiFi-leaked RF signals, without the need to actively generate the carrier wave. We customize the design of the loop at the LiFi transmitter, as well as the coil antennas at the tag and RF backscatter receiver, to optimize the system performance. We propose to opportunistically enable the oscillator of the backscatter tag in the software that could reduce the energy consumption on backscattering by up to 75%. Experimental results show that LeakageScatter achieves a backscattering distance up to 10 m and 18 m in indoor and outdoor scenarios, respectively, without using a dedicated RF carrier generator.

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Visible Light Communication (VLC) is widely considered a promising technology for the coming 6G networks. Recent studies show that a VLC transmitter not only emits visible light signals but also leaks RF signals during the transmission. In this work, we devote effort to harvesting the free leaked RF energy from VLC transmissions. We observe that the surrounding objects could help a coil antenna harvest significantly more RF energy. Based on this observation, we propose our system Bracelet+, which involves the human body in the harvesting system to increase the harvested power. After careful analysis of the influence of the human body on the harvested power, we prototype the coil antenna as a bracelet that achieves both high harvested power and convenience for wearing. The average power of the RF energy harvested by our design is 10 larger than that of the conventional coil antenna, without causing any interference to the communication of VLC systems. The harvested power can reach up to micro-watts in our tested scenarios. Such a micro-watt level of harvested energy has the potential to power up ultra-low-power sensors such as temperature sensors and glucose sensors.@en

Visible Light Communication (VLC) is a promising technology for future wireless communications. By modulating the visible light - -that has about 10,000x larger frequency band than that of radios - -to transmit data, VLC has the potential to provide ultra-high-speed wireless connectivities. However, it also has limitations such as i) surrounding objects can easily block VLC links, and ii) intense ambient light can saturate the photodiodes of VLC receivers. In this work, from a different angle compared with state-of-the-art solutions, we utilize the side channel of VLC - -a Radio Frequency (RF) channel created unintentionally during the transmission process of VLC - -to break the above-mentioned VLC limitations. The key enabler is that the side RF channel also contains the data information transmitted in the VLC link. When the VLC link is blocked or saturated, we can utilize the side channel, capable of penetrating through blockages and not affected by ambient light, to assist VLC transmissions. Thus a user service relying on VLC transmissions will not be interrupted. Besides the simple Single-Input Single-Output (SISO) case, we consider challenging scenarios where multiple VLC chains are synchronized to form Multiple-Input Multiple/Single-Output (MIMO/MISO) transmission strategies. To make our system practical, we address several challenges spanning from hardware to software. Compared to state-of-the-art design, we reduce the size of the receiving coil by nearly 90%. Experimental evaluations show that our system can decode overlapped RF signals created by a 3X3 MIMO VLC network five meters away, with various blockages in between. Our system also works under intense ambient light conditions (> 100,000 lux).

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