  Intelligent reflecting surface (IRS) is a promising technology to support
high performance wireless communication. By adaptively configuring the
reflection amplitude and/or phase of each passive reflecting element on it, the
IRS can reshape the electromagnetic environment in favour of signal
transmission. This letter advances the existing research by proposing and
analyzing a double-IRS aided wireless communication system. Under the
reasonable assumption that the reflection channel from IRS 1 to IRS 2 is of
rank 1 (e.g., line-of-sight channel), we propose a joint passive beamforming
design for the two IRSs. Based on this, we show that deploying two cooperative
IRSs with in total K elements can yield a power gain of order O(K^4), which
greatly outperforms the case of deploying one traditional IRS with a power gain
of order O(K^2). Our simulation results validate that the performance of
deploying two cooperative IRSs is significantly better than that of deploying
one IRS given a sufficient total number of IRS elements. We also extend our
line-of-sight channel model to show how different channel models affect the
performance of the double-IRS aided wireless communication system.
