I'm reading the paper Model-Driven Deep Learning for Joint MIMO Channel Estimation and Signal Detection by Hengtao He, Chao-Kai Wen, Shi Jin, and Geoffrey Ye Li on Orthogonal Approximate Message Passing (OAMP) detectors.
There is an step that it is getting me crazy, in Equation $(42)$ in Appendix B. It is the following:
At first, I thought that it was a consequence of a Hypothesis, but then, to derive the next expression in Equation $(44)$, it is used again this formula.
Is there some kind of property which I might be missing?