  We present the first wireless protocol that scales to hundreds of concurrent
transmissions from backscatter devices. Our key innovation is a distributed
coding mechanism that works below the noise floor, operates on backscatter
devices and can decode all the concurrent transmissions at the receiver using a
single FFT operation. Our design addresses practical issues such as timing and
frequency synchronization as well as the near-far problem. We deploy our design
using a testbed of backscatter hardware and show that our protocol scales to
concurrent transmissions from 256 devices using a bandwidth of only 500 kHz.
Our results show throughput and latency improvements of 14--62x and 15--67x
over existing approaches and 1--2 orders of magnitude higher transmission
concurrency.
