  Wireless power transfer (WPT) using energy beamforming is a promising
solution for low power Internet of Things (IoT) devices. In this work, we
consider WPT from an energy transmitter (ET) employing retrodirective WPT using
a large phased antenna array to an energy receiver (ER) capable of ambient
backscatter. The advantage of retrodirective WPT is that no explicit channel
estimation is needed at the ET and the use of ambient backscattering eliminates
the need for active transmission at the ER. We propose a training sequence
design, i.e., pattern of varying the reflection coefficient at the ER, to
eliminate the direct-link interference from the ambient source. We show that
when the ambient symbol duration is known, the ambient interference is fully
cancelled by the proposed design. We analytically model the system and find the
average harvested power at the ER considering Nakagami-$m$ fading channels and
non-linear energy harvesting model. Our results clearly show that the proposed
solution is robust to a small timing offset mismatch at the correlator. When
interference from undesired neighbouring sources in the ambient environment is
not significant, the ER can successfully harvest tens to hundreds of $\mu$W of
power, which is an important improvement for low-power IoT devices.
