  We study a backscatter communication protocol over a AWGN channel, where a
transmitter illuminates a tag with a directional multi-antenna. The tag
performs load modulation on the signal while hiding its physical presence from
a warden. We show that, if the transmitter-to-tag channel is inaccessible to
the warden, then $\Theta(n)$ reliable and covert bits can be transmitted over
$n$ channel usages. This overcomes the square-root law for covert
communication. This paper provides the first evidence for practical
implementation of covert backscatter communication, with potential applications
in IoT security.
