  Due to extremely low power consumption, backscatter has become the
transmission mechanism of choice for battery-free devices that operate on
harvested energy. However, a limitation of recent backscatter systems is that
the communication range scales with the strength of the ambient carrier
signal(ACS). This means that to achieve a long range, a backscatter tag needs
to reflect a strong ACS, which in practice means that it needs to be close to
an ACS emitter. We present TunnelScatter, a mechanism that overcomes this
limitation. TunnelScatter uses a tunnel diode-based radio frequency oscillator
to enable transmissions when there is no ACS, and the same oscillator as a
reflection amplifier to support backscatter transmissions when the ACS is weak.
Our results show that even without an ACS, TunnelScatter is able to transmit
through several walls covering a distance of 18 meter while consuming a peak
biasing power of 57 microwatts. Based on TunnelScatter, we design battery-free
sensor tags, called TunnelTags, that can sense physical phenomena and transmit
them using the TunnelScatter mechanism.
