Recently, intelligent reflecting surface (IRS) has become a research focus for its capability of controlling the radio propagation environments. Compared to the conventional terrestrial IRS, aerial IRS (AIRS) exploiting unmanned aerial vehicle (UAV)/high-altitude platform (HAP) can provide better deployment flexibility. To this end, a three-dimensional (3D) one-cylinder model is first developed for AIRS-assisted multiple-input multiple-output (MIMO) narrowband channels. In order to change the wireless channel with AIRS and create a favorable propagation environment, we propose a novel method of designing the phase-shifts for the IRS elements. Based on the model, channel impulse response (CIR), space-time correlation function, and channel capacity are derived and thoroughly investigated. A key observation in this paper is that multipath and Doppler effects in radio propagation environments can be effectively mitigated via adjusting the phase-shifts of IRS. More specifically, for the special propagation environments in the absence of any scatterers, it is found that the effects of multipath fading can be completely eliminated by IRSs. While for the general propagation environments with multiple scatterers, a small number of IRS elements can also significantly reduce the Doppler spread and the deep fades of the channels. Based on the numerical investigation of channel correlations, it is shown that channel non-stationarity is not introduced into the time domain when the phase shift of IRS is linear related to the time. Moreover, the channel capacity can also be improved by the proposed methods. Finally, the model with non-ideal IRSs is considered and it is found that using non-ideal IRSs results in poor performances compared with using ideal IRSs. These conclusions will provide a fundamental support for developing intelligent and controllable propagation environments of the future sixth-generation (6G) wireless networks.

中文翻译：

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空中智能反射面MIMO多径信道建模与分析


近年来，智能反射面（IRS）因其控制无线电传播环境的能力而成为研究热点。与传统的地面IRS相比，利用无人机（UAV）/高空平台（HAP）的空中IRS（AIRS）可以提供更好的部署灵活性。为此，首先为AIRS辅助的多输入多输出（MIMO）窄带信道开发了三维（3D）单圆柱模型。为了用 AIRS 改变无线信道并创造有利的传播环境，我们提出了一种设计 IRS 元件相移的新方法。基于该模型，导出并深入研究了信道脉冲响应（CIR）、空时相关函数和信道容量。本文的一个关键观察结果是，通过调整 IRS 的相移可以有效减轻无线电传播环境中的多径和多普勒效应。更具体地说，对于没有任何散射体的特殊传播环境，发现IRS可以完全消除多径衰落的影响。而对于具有多个散射体的一般传播环境，少量的IRS元件也可以显着减少多普勒扩展和信道的深度衰落。基于信道相关性的数值研究表明，当IRS的相移与时间呈线性关系时，信道非平稳性并未引入时域。此外，所提出的方法还可以提高信道容量。最后，考虑非理想 IRS 的模型，发现与使用理想 IRS 相比，使用非理想 IRS 会导致性能较差。 这些结论将为开发未来第六代（6G）无线网络的智能可控传播环境提供基础支撑。