A Spectrum-Efficient Cross-Layer RF Distance Bounding Scheme

Figure 5

The scenario of data communication on SFCW signals. On the downlink, the varying directions of the signal frequency (i.e., the signal varies from high frequency to low frequency or in reverse) represent challenge bits; on the uplink, the SFCW signals which have been encoded with challenge bits are further encoded with response bits in ASK modulation by employing backscatter communication.