  Some important indoor localization applications, such as localizing a lost
kid in a shopping mall, call for a new peer-to-peer localization technique that
can localize an individual's smartphone or wearables by directly using
another's on-body devices in unknown indoor environments. However, current
localization solutions either require pre-deployed infrastructures or multiple
antennas in both transceivers, impending their wide-scale application. In this
paper, we present P2PLocate, a peer-to-peer localization system that enables a
single-antenna device co-located with a batteryless backscatter tag to localize
another single-antenna device with decimeter-level accuracy. P2PLocate
leverages the multipath variations intentionally created by an on-body
backscatter tag, coupled with spatial information offered by user movements, to
accomplish this objective without relying on any pre-deployed infrastructures
or pre-training. P2PLocate incorporates novel algorithms to address two major
challenges: (i) interference with strong direct-path signal while extracting
multipath variations, and (ii) lack of direction information while using
single-antenna transceivers. We implement P2PLocate on commercial off-the-shelf
Google Nexus 6p, Intel 5300 WiFi card, and Raspberry Pi B4. Real-world
experiments reveal that P2PLocate can localize both static and mobile targets
with a median accuracy of 0.88 m.
