  A general optimization framework is proposed for simultaneously transmitting
and reflecting reconfigurable surfaces (STAR-RISs) with coupled phase shifts,
which converges to the Karush-Kuhn-Tucker (KKT) optimal solution under some
mild conditions. More particularly, the amplitude and phase-shift coefficients
of STAR-RISs are optimized alternatively in closed form. To demonstrate the
effectiveness of the proposed optimization framework, the throughput
maximization problem is considered in a case study. It is rigorously proved
that the KKT optimal solution is obtained. Numerical results confirm the
effectiveness of the proposed optimization framework compared to baseline
schemes.
