BS-IRS-user reflected link (i.e., $\boldsymbol{V}'$) for each IRS elements group. Then, based on the estimated channels, the BS computes the optimal transmit power allocation (i.e., $\boldsymbol{p}$) and IRS group reflection coefficients (i.e., $\bar{\boldsymbol{\phi}}$) that maximize the achievable rate, and feeds back $\boldsymbol{p}$ and $\bar{\boldsymbol{\phi}}$ to the user and the IRS controller, respectively. Finally, data transmission proceeds in the third phase based on the optimized $\boldsymbol{p}$ and $\bar{\boldsymbol{\phi}}$ and the estimated channels. In the following, we elaborate the three transmission phases in detail.

![Illustration of the proposed protocol with IRS elements grouping.[]{data-label="fig:Preamble"}](Preamble.eps){width="0.9\linewidth"}

Pilot Transmission and Channel Estimation
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Recall that the combined composite BS-IRS and IRS-user channel matrix $\boldsymbol{V}'\in\mathbb{C}^{N\times K}$ comprises all the reflecting channels associated with the respective IRS elements groups, which superpose with each other in the resultant BS-IRS-user link as $\boldsymbol{h}_r=\boldsymbol{V}'\bar{\boldsymbol{\phi}}$. To resolve their individual channels, we perform channel training based on the on/off state control of the IRS reflecting elements, which requires $T_p=K+1$ OFDM symbol durations as shown in Fig. \[fig:Preamble\]. Specifically, let $\boldsymbol{x}_p\in\mathbb{C}^{N\times 1}$ denote the pilot signal sent at every symbol duration in the first phase, and $\boldsymbol{X}_p=\mathrm{diag}(\boldsymbol{x}_p)$. In the first symbol duration, all the IRS reflecting elements are switched off, i.e., $\bar{\boldsymbol{\phi}}={\bf 0}_{K\times 1}$. 