To achieve the more significant passive beamforming gain in the double-intelligent reflecting surface (IRS) aided system over the conventional single-IRS counterpart, channel state information (CSI) is indispensable in practice but also more challenging to acquire, due to the presence of not only the single- but also double-reflection links that are intricately coupled and also entail more channel coefficients for estimation. In this paper, we propose a new and efficient channel estimation scheme for the double-IRS aided multi-user multiple-input multiple-output (MIMO) communication system to resolve the cascaded CSI of both its single- and double-reflection links. First, for the single-user case, the single- and double-reflection channels are efficiently estimated at the multi-antenna base station (BS) with both the IRSs turned ON (for maximal signal reflection), by exploiting the fact that their cascaded channel coefficients are scaled versions of their superimposed lower-dimensional CSI. Then, the proposed channel estimation scheme is extended to the multi-user case, where given an arbitrary user's cascaded channel (estimated as in the single-user case), the other users' cascaded channels can also be expressed as lower-dimensional scaled versions of it and thus efficiently estimated at the BS. Simulation results verify the effectiveness of the proposed channel estimation scheme and joint training reflection design for double IRSs, as compared to various benchmark schemes.

中文翻译：

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双 IRS 辅助多用户 MIMO 系统的高效信道估计


为了在双智能反射表面（IRS）辅助系统中实现比传统单IRS对应系统更显着的无源波束形成增益，信道状态信息（CSI）在实践中是必不可少的，但由于存在信道状态信息（CSI），因此获取起来也更具挑战性。不仅是单反射链路，而且双反射链路也是复杂耦合的，并且还需要更多的信道系数进行估计。在本文中，我们为双IRS辅助的多用户多输入多输出（MIMO）通信系统提出了一种新的高效信道估计方案，以解决其单反射和双反射链路的级联CSI。首先，对于单用户情况，在 IRS 都打开（以获得最大信号反射）的情况下，通过利用它们级联的事实，在多天线基站 (BS) 处有效地估计单反射和双反射信道。信道系数是其叠加的低维 CSI 的缩放版本。然后，将所提出的信道估计方案扩展到多用户情况，其中给定任意用户的级联信道（在单用户情况下估计），其他用户的级联信道也可以表示为低维缩放版本并因此在 BS 处进行有效估计。与各种基准方案相比，仿真结果验证了所提出的双 IRS 信道估计方案和联合训练反射设计的有效性。