The researchers have used backscatter radio which uses existing RF signals as the power source which is then used to modify and reflect the signal with encoded data. This is the principle behind passive RFID tags but it has until now been used for communications over a few feet or meters and usually much closer, data rates of the order of kbits per second and small data payloads such as an identity code.
Active radio technologies including Wi-Fi, ZigBee, SigFox , LoRa and LTE-M provide reliable coverage and long ranges but consume power and cost $4 to $6 in volume, according to the research team.
The University of Washington's has now proposed long-range backscatter radio system in a paper a paper to be presented Sept. 13 at UbiComp 2017. The system provides reliable long-range communication at up to 2.8km with sensors that consume 1000 times less power than existing technologies capable of transmitting data over similar distances.
The LoRa Backscatter device prototyped by the research team consumes 9.25 microwatts, operates at 100s of meters and can be powered by printed batteries and button cells. The RF source transmits a single tone, for the purposes of this paper in the 900MHz ISM band, that the backscatter device uses to synthesize chirp spread spectrum (CSS) signals. The challenge that the team has faced is to make sure at the receiver that the backscatter signal is not drowned by noise and does not suffer interference from the RF source.
The University of Washington's contribution to the state of the art was the addition of CSS. Spreading the reflected signals across multiple frequencies allowed the team to achieve much greater sensitivities and decode backscattered signals across greater distances even when it's below the noise.
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