  In this work, we consider a backscatter communication system wherein multiple
asynchronous sources (tags) exploit the reverberation generated by a nearby
radar transmitter as an ambient carrier to deliver a message to a common
destination (reader) through a number of available subchannels. We propose a
new encoding strategy wherein each tag transmits both pilot and data symbols on
each subchannel and repeats some of the data symbols on multiple subchannels.
We then exploit this signal structure to derive two semi-blind iterative
algorithms for joint estimation of the data symbols and the subchannel
responses that are also able to handle some missing measurements. The proposed
encoding/decoding strategies are scalable with the number of tags and their
payload and can achieve different tradeoffs in terms of transmission and error
rates. Some numerical examples are provided to illustrate the merits of the
proposed solutions.
