LAUSR

This letter investigates the limit covert rate of ambient backscatter communications over additive white Gaussian noise channel, where Alice expects to reliably transmit information to Bob via reflecting the incident signals from an ambient radio frequency (RF) source without being detected by Willie. The RF source is not dedicatedly deployed for covert communication and has its own data stream to send. Our analyses show that the limit covert rate is subject to the well-known square root law (i.e., $\mathcal {O}(\sqrt {n})$ bits in ${n}$ channel uses). Particularly, this limit can be achieved even if Willie can recover the information from the RF source without error, and two properly chosen reflection coefficients are sufficient for achieving this limit.