  Reconfigurable intelligent surface (RIS) assisted radio is considered as an
enabling technology with great potential for the sixth-generation (6G) wireless
communications standard. The achievable secrecy rate (ASR) is one of the most
fundamental metrics to evaluate the capability of facilitating secure
communication for RIS-assisted systems. However, the definition of ASR is based
on Shannon's information theory, which generally requires long codewords and
thus fails to quantify the secrecy of emerging delay-critical services.
Motivated by this, in this paper we investigate the problem of maximizing the
secrecy rate under a delay-limited quality-of-service (QoS) constraint, termed
as the effective secrecy rate (ESR), for an RIS-assisted multiple-input
single-output (MISO) wiretap channel subject to a transmit power constraint. We
propose an iterative method to find a stationary solution to the formulated
non-convex optimization problem using a block coordinate ascent method (BCAM),
where both the beamforming vector at the transmitter as well as the phase
shifts at the RIS are obtained in closed forms in each iteration. We also
present a convergence proof, an efficient implementation, and the associated
complexity analysis for the proposed method. Our numerical results demonstrate
that the proposed optimization algorithm converges significantly faster that an
existing solution. The simulation results also confirm that the secrecy rate
performance of the system with stringent delay requirements reduce
significantly compared to the system without any delay constraints, and that
this reduction can be significantly mitigated by an appropriately placed
large-size RIS.
