The goal of this clinical trial aims to investigate whether a non-invasive implantation window prediction model, constructed based on characteristic changes in peripheral blood miRNAs during the peri-implantation period, can improve pregnancy outcomes in patients with unexplained recurrent implantation failure undergoing frozen embryo transfer. Researchers will compare the clinical pregnancy outcomes between individualized embryo transfer guided by implantation window results and conventional embryo transfer, to observe whether implantation window-guided individualized embryo transfer can improve the clinical efficacy for patients with recurrent implantation failure (RIF). Participants will complete implantation window testing as required by the study protocol, undergo randomization and blinding, and proceed with embryo transfer according to their assigned group.