  Wirelessly powered backscatter communication (WPBC) has been identified as a
promising technology for low-power communication systems, which can reap the
benefits of energy beamforming to improve energy transfer efficiency. However,
existing studies on energy beamforming fail to simultaneously take energy
supply and information transfer in WPBC into account. This paper takes the
first step to fill this gap, by considering the restrictive relationship
between the energy harvesting rate and achievable rate with estimated
backscatter channel state information (BS-CSI). To ensure reliable
communication and user fairness, we formulate the energy beamforming design as
a max-min optimization problem by maximizing the minimum achievable rate for
all backscatter tags subject to the energy constraint. We derive the
closed-form expression of the energy harvesting rate, as well as the lower
bound of the achievable rate for maximum-ratio combining (MRC) and zero-forcing
(ZF) receivers. Our numerical results indicate that our scheme significantly
outperforms state-of-the-art energy beamforming schemes. Additionally, the
achievable rate of our scheme approaches more than $90\%$ of the rate limit
achieved via beamforming with perfect CSI for both receivers.
