by the IRS and the remaining non-reflected paths directly from the transmitter. Finally, the achievable rate for the user is determined by both the IRS reflection coefficients and the transmit power allocation over OFDM SCs, which are intricately coupled and thus need to be jointly optimized. In general, the size of the joint optimization problem increases with the number of IRS elements and/or OFDM SCs. In this paper, we tackle the aforementioned challenges via novel protocol and algorithm designs. The main contributions of this paper are summarized as follows:

-   First, we propose a novel design by dividing the IRS elements into groups, where each group consists of adjacent elements that are assumed to share a common reflection coefficient. Hence, only the combined channel of each group needs to be estimated. Based on this grouping method, a practical transmission protocol with pilot training is further proposed, where the combined channel associated with each group is estimated sequentially by turning on only the IRS elements of the corresponding IRS group while the other elements are set to be off. Note that compared to the straightforward approach of estimating the channel of each of the IRS’s elements and jointly designing their reflection 