  A long-standing vision of backscatter communications is to provide long-range
connectivity and high-speed transmissions for batteryless Internet-of-Things
(IoT). Recent years have seen major innovations in designing backscatters
toward this goal. Yet, they either operate at a very short range, or experience
extremely low throughput. This paper takes one step further toward breaking
this stalemate, by presenting PolarScatter that exploits channel polarization
in long-range backscatter links. We transform backscatter channels into nearly
noiseless virtual channels through channel polarization, and convey bits with
extremely low error probability. Specifically, we propose a new polar code
scheme that automatically adapts itself to different channel quality, and
design a low-cost encoder to accommodate polar codes on resource-constrained
backscatter tags. We build a prototype PCB tag and test it in various outdoor
and indoor environments. Our experiments show that our prototype achieves up to
10$\times$ throughput gain, or extends the range limit by 1.8$\times$ compared
with the state-of-the-art long-range backscatter solution. We also simulate an
IC design in TSMC 65 nm LP CMOS process. Compared with traditional encoders,
our encoder reduces storage overhead by three orders of magnitude, and lowers
the power consumption to tens of microwatts.
