  For large-scale Internet of Things (IoT), backscatter communication is a
promising technology to reduce power consumption and simplify deployment.
However, backscatter communication lacks stability, along with limited
communication range within a few meters. Due to the limited computation ability
of backscatter tags, it is burdensome to effectively adapt the bitrate for the
time-varying channel. Thus, backscatter tags are failed to fully utilize the
optimal transmission rate. In this paper, we design a system named COOK with
self-reliant bitrate adaptation in backscatter communication. Channel symmetry
allows backscatter tags to adjust bitrate depending on the received signal
strength of the excitation source (ES) without feedback. In addition, the chirp
spreading signal is exploited as the ES signal to enable backscatter tags to
work under noise floor. Our modulation approach is denoted as Chirp-OOK since
the tags reflect the chirp signal by employing the on-off keying modulation. It
allows that receiver can decode under the noise floor and the bitrate varies
flexibly as the communication range changes. We have implemented the prototype
system based on the universal software radio peripheral (USRP) platform.
Extensive experiment results demonstrate the effectiveness of the proposed
system. Our system provides valid communication distance up to $27m$, which is
7 times as compared with normal backscatter system. The system significantly
increases the backscatter communication stability, by supporting bitrate
adaptation ranges from 0.33kbps to 1.2Mbps, and guaranteeing the bit error rate
(BER) is below 1%.
