Conventional beamforming methods for intelligent reflecting surfaces (IRSs) or reconfigurable intelligent surfaces (RISs) typically entail the full channel state information (CSI). However, the computational cost of channel acquisition soars exponentially with the number of IRSs. To bypass this difficulty, we propose a novel strategy called blind beamforming that coordinates multiple IRSs by means of statistics without knowing CSI. Blind beamforming only requires measuring the received signal power at the user terminal for a sequence of randomly generated phase shifts across all IRSs. The main idea is to extract the key statistical quantity for beamforming by exploring only a small portion of the whole solution space of phase shifts. We show that blind beamforming guarantees a signal-to-noise ratio (SNR) boost of Theta(N^{2L}) under certain conditions, where L is the number of IRSs and N is the number of reflecting elements per IRS. The proposed conditions for achieving the optimal SNR boost of Theta(N^{4}) in a double-IRS system are much easier to satisfy than the existing ones in the literature. Most importantly, the proposed conditions can be extended to a fully general L-IRS system. The above result significantly improves upon the state of the art in the area of multi-IRS-assisted communication. Moreover, blind beamforming is justified via field tests and simulations. In particular, as shown in our field tests at 2.6 GHz, our method yields up to 17 dB SNR boost; to the best of our knowledge, this is the first time that the use of multiple IRSs gets verified in the real world.

翻译：传统针对智能反射面（IRS）或可重构智能表面（RIS）的波束赋形方法通常需要完整的信道状态信息（CSI）。然而，随着IRS数量的增加，信道获取的计算成本呈指数级增长。为克服这一困难，我们提出一种名为"盲波束赋形"的新策略，该方法无需CSI即可通过统计手段协调多个IRS。盲波束赋形仅需在用户终端测量所有IRS上随机生成的相位偏移序列对应的接收信号功率。其核心思想是通过仅探索相位偏移完整解空间的一小部分，提取用于波束赋形的关键统计量。我们证明，在特定条件下，盲波束赋形可保证信噪比（SNR）提升达到Θ(N^{2L})，其中L为IRS数量，N为每个IRS的反射单元数量。在双IRS系统中，实现最优Θ(N^{4})信噪比提升所需的条件比现有文献中的条件更易满足。更重要的是，所提条件可推广至完全通用的L-IRS系统。上述结果显著优于多IRS辅助通信领域现有技术水平。此外，通过现场测试和仿真验证了盲波束赋形的有效性。特别是在2.6 GHz频段的现场测试中，我们的方法实现了高达17 dB的信噪比提升；据我们所知，这是首次在真实环境中验证多IRS的实际应用效果。