Coherent Detector for Pseudo-FSK Backscatter Under Ambient Constant Envelope Illumination

This work studies ambient backscatter, where the tag utilizes a frequency-shifted form of on-off keying (OOK), which may be coined as pseudo-frequency shift keying (pseudo-FSK). Such scheme, offers the possibility of simple, frequency-domain multiple access (due to FSK), by appropriate selection of the switching frequencies among (possibly receiverless) tags, while reserving bandwidth (due to OOK). A constant envelope-modulated ambient signal is assumed to illuminate the tag, resembling signals from (analog) FM radio, (digital) minimum-shift keying (MSK), or phase-shift keying (PSK) broadcasting stations. Fully coherent, maximum likelihood (ML) detection is derived for tag information, without estimating or detecting the ambient signal; instead, the law of large numbers is exploited, in conjunction with channel estimation techniques, even though the ambient unknown signal changes between successive tag bits. Closed-form expression for the probability of error is also given and simulations verify theoretical results. Ambient is a special case of bistatic backscatter; thus, useful design principles for ambient systems can stem from the bistatic backscatter literature.