A General Physics-Based IRS Auxiliary Channel Model for Wireless Communications

A general physics-based intelligent reflecting surface (IRS) auxiliary channel model considering the projected structure (PS) of the IRS unit is proposed in this letter. Based on the proposed model, we investigate the effect of IRS placement on the performance. Using the Cardano formula, the optimum IRS placement, which depends on the distance between the transmitter (Tx) and the receiver (Rx) and the heights of the Tx and the IRS, is derived. An interesting result is observed: the optimal IRS position is affected by the proportional relationship between the height of the IRS and that of the Tx. When the height of the IRS is less than half of that of the Tx, the optimal IRS position is located near the Rx, and vice versa. Also, when the height of the IRS is half of that of the Tx, there are two optimal IRS positions, one located near the Tx and the other located near the Rx. Using the derived IRS positions, the closed-form expression of the optimum received signal power is derived. The simulation results show that the IRS PS should be considered in IRS-assisted communication systems, otherwise it will lead to the overestimation of the performance gains. In addition, the simulation results show that the IRS can combat the large path loss experienced in millimeter wave communications.