  In this paper, a novel concept called simultaneously transmitting and
reflecting RIS (STAR-RIS) is introduced into the wireless-powered mobile edge
computing (MEC) systems to improve the efficiency of energy transfer and task
offloading. Compared with traditional reflecting-only RIS, STAR-RIS extends the
half-space coverage to full-space coverage by simultaneously transmitting and
reflecting incident signals, and also provides new degrees-of-freedom (DoFs)
for manipulating signal propagation. We aim to maximize the total computation
rate of all users, where the energy transfer time, transmit power and CPU
frequencies of users, and the configuration design of STAR-RIS are jointly
optimized. Considering the characteristics of STAR-RIS, three operating
protocols, namely energy splitting (ES), mode switching (MS), and time
splitting (TS) are studied, respectively. For the ES protocol, based on the
penalty method, successive convex approximation (SCA), and the linear search
method, an iterative algorithm is proposed to solve the formulated non-convex
problem. Then, the proposed algorithm for ES protocol is extended to solve the
MS and TS problems. Simulation results illustrate that the STAR-RIS outperforms
traditional reflecting/transmitting-only RIS. More importantly, the TS protocol
can achieve the largest computation rate among the three operating protocols of
STAR-RIS.
