Fig. 4: Dense leaky-wave backscatter nodes.

a The measured half-power spectral bandwidth of a leaky-wave backscatter as a function of the angle of the tag’s angular location. The bandwidth is directly measured from the frequency response of the received time domain signal. We observe that the bandwidth monotonically decreases with larger incident angles. The half-power bandwidth of the backscattered signal can be tens of GHz to a few GHz, depending on the impinging angle. b In a network consisting of multiple leaky-wave backscatters, we can establish several concurrent interference-free links by separating the backscattered signal in the spatial and spectral domains. Since the spectral band at each tag depends on its angular location (i.e., the angle of incident on the tag), to ensure orthogonal multi-tag operation, the emitted signals from the interrogator should impinge on the tags at slightly different angles. Given two backscatter tags (a primary and a secondary), we plot the minimum required angular separation between the two tags as a function of the incident angle on the primary user. We observe that the minimum required angular separation is fairly uniform across space, independent of the location or orientation of the primary user. This angular separation is a difference between the incident angles of free-space waves on the two users. Hence, radial movement of the secondary leaky-wave backscatter away from the primary tag or rotating it both result in increasing the angular separation in this context. This is shown in the top and bottom insets, respectively.