  Integrated Sensing And Communication (ISAC ) systems are expected to perform
accurate radar sensing while having minimal impact on communication. Ideally,
sensing should only reuse communication resources, especially for spectrum
which is contended by many applications. However, this poses a great challenge
in that communication systems often operate on narrow subbands with low sensing
resolution. Combining contiguous subbands has shown significant resolution gain
in active localization. However, multiband ISAC remains unexplored due to
communication subbands being highly sparse (non-contiguous) and affected by
phase offsets that prevent their aggregation (incoherent). To tackle these
problems, we design HiSAC, the first multiband ISAC system that combines
diverse subbands across a wide frequency range to achieve super-resolved
passive ranging. To solve the non-contiguity and incoherence of subbands, HiSAC
combines them progressively, exploiting an anchor propagation path between
transmitter and receiver in an optimization problem to achieve phase coherence.
HiSAC fully reuses pilot signals in communication systems, it applies to
different frequencies and can combine diverse technologies, e.g., 5G-NR and
WiGig. We implement HiSAC on an experimental platform in the millimeter-wave
unlicensed band and test it on objects and humans. Our results show it enhances
the sensing resolution by up to 20 times compared to single-band processing
while occupying the same spectrum.
