  The performance of a device-to-device (D2D) underlay communication system is
limited by the co-channel interference between cellular users (CUs) and D2D
devices. To address this challenge, an intelligent reflecting surface (IRS)
aided D2D underlay system is studied in this paper. A two-timescale
optimization scheme is proposed to reduce the required channel training and
feedback overhead, where transmit beamforming at the base station (BS) and
power control at the D2D transmitter are adapted to instantaneous effective
channel state information (CSI); and the IRS phase shifts are adapted to
slow-varying channel mean. Based on the two-timescale optimization scheme, we
aim to maximize the D2D ergodic rate subject to a given outage probability
constrained signal-to-interference-plus-noise ratio (SINR) target for the CU.
The two-timescale problem is decoupled into two sub-problems, and the two
sub-problems are solved iteratively with closed-form expressions. Numerical
results verify that the two-timescale based optimization performs better than
several baselines, and also demonstrate a favorable trade-off between system
performance and CSI overhead.
