THz Bistatic Backscatter Side-Channel Sensing at a Distance

This article presents the sensing and detection of backscattered terahertz (THz) side channels unintentionally created by field-programmable gate array (FPGA) activity using a bistatic arrangement. At first, a single frequency is modulated onto a THz carrier due to the switching activity inside the FPGA and this modulated frequency is received at a distance. The effects of polarization and the receiver distance on the backscattered signal are studied and it is found that deliberately introducing a polarization mismatch between the transmitter and the receiver can improve the signal-to-noise ratio (SNR) by more than 10 dB. This allows the signal to be received at distances greater than 45 cm with SNR above 54 dB, making detection feasible at several meters away. Through the use of a near-field focuser, the properties of the side-channel signal are measured over the surface of the FPGA board with a resolution of 0.5 mm. Next, a backscatter signal at four distinct frequencies is created and detected through splitting the FPGA into four distinct modules. The relative strength of the frequencies is compared, and conclusions about the physical location and the strength of these signals originating from distinct modules are analyzed. It is found that focusing the backscatter system on certain locations on the FPGA can preferentially receive the signal from one module while filtering out the other modules. This helps isolating the signals created by various modules in an FPGA and significantly improving the effectiveness of side-channel detection techniques.