  This paper proposes a hybrid-relaying scheme empowered by a self-sustainable
intelligent reflecting surface (IRS) in a wireless powered communication
network (WPCN), to simultaneously improve the performance of downlink energy
transfer (ET) from a hybrid access point (HAP) to multiple users and uplink
information transmission (IT) from users to the HAP. We propose time-switching
(TS) and power-splitting (PS) schemes for the IRS, where the IRS can harvest
energy from the HAP's signals by switching between energy harvesting and signal
reflection in the TS scheme or adjusting its reflection amplitude in the PS
scheme. For both the TS and PS schemes, we formulate the sum-rate maximization
problems by jointly optimizing the IRS's phase shifts for both ET and IT and
network resource allocation. To address each problem's non-convexity, we
propose a two-step algorithm to obtain the near-optimal solution with high
accuracy. To show the structure of resource allocation, we also investigate the
optimal solutions for the schemes with random phase shifts. Through numerical
results, we show that our proposed schemes can achieve significant system
sum-rate gain compared to the baseline scheme without IRS.
