Double-IRS Auxiliary Communications: Models and Performance Prediction

In urban scenario, the direct link between transmitter (Tx) and receiver (Rx) may be unavailable due to severe blockage, and two cooperative intelligent reflecting surfaces (IRSs) are used to boost system performance. Considering the effective coverage area of the IRS, the activation conditions of the two cooperative IRSs are obtained, which are determined by the positions and directions of the Tx, IRSs, and Rx. Based on this, the received signal power is investigated for the double reflection link, and the optimal power gain is proportional to the fourth power of the total geometric area of the two IRSs, rather than the total number of the unit cells of the two IRSs. Our numerical results show that when the two cooperative IRSs are both activated, the double reflection link can provide good performance gains for the complex urban environment. However, the performance gain of the double reflection link is lower than that of the single reflection link under the condition of the existence of single reflection link and a reasonable size of the IRS. Eventually, the proposed channel model is equivalent to a virtual Rician fading channel, and the upper bound of the ergodic sum capacity is investigated, including the case of high signal-to-noise ratio (SNR). The tight upper bound was successfully acquired and also proved by Monte-Carlo simulation results.