  Intelligent reflecting surface (IRS) is a promising technology to improve the
performance of backscatter communication systems by smartly reconfiguring the
multi-reflection channel. To fully exploit the passive beamforming gain of IRS
in backscatter communication, channel state information (CSI) is indispensable
but more practically challenging to acquire than conventional IRS-assisted
systems, since IRS passively reflects signals over both the forward and
backward (backscattering) links between the reader and tag. To address this
issue, we propose in this letter a new and efficient channel estimation scheme
for the IRS-assisted backscatter communication system. To minimize the
mean-square error (MSE) of channel estimation, we formulate and solve an
optimization problem by designing the IRS training reflection matrix for
channel estimation under the constraints of unit-modulus elements and full
rank. Simulation results verify the effectiveness of the proposed channel
estimation scheme as compared to other baseline schemes.
