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基于信息超表面的无线中继近年来受到学术界和工业界的广泛关注。有别于在收发机进行设计工作的传统无线通信,信息超表面的引进使得通信系统的设计增加了另一可控的实体——信道。合理且准确的信道模型是无线通信系统研究的理论基础。因此,该节首先介绍了信息超表面无线中继在信道模型建立方面的一系列研究成果,包括信息超表面辅助链路的自由空间路径损耗,信息超表面的反射相移特性以及单元互耦现象。为体现信息超表面在信道调控方面的作用,该节还将介绍现有研究工作如何通过设计信息超表面来改善、定制电磁传播环境。
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尽管信息超表面的设计在电磁领域已进行了广泛的研究,其在通信领域的分析尚处于初始阶段。作为无线中继,信息超表面为发射机和接收机之间的信道额外提供了一条可控的反射链路。鉴于信息超表面将此反射链路分成距离为
$ {d}_{1} $ 的发射机至信息超表面子链路以及距离为$ {d}_{2} $ 的接收机至信息超表面子链路,信息超表面提供的反射链路的自由空间路径损耗与$ {d}_{1} $ 和$ {d}_{2} $ 的关系亟待澄清。当前关于信息超表面自由空间路径损耗的讨论主要在于此损耗是与$ {\left({{d}_{1}+d}_{2}\right)}^{2} $ 成正比,亦或与$ {\left({{d}_{1}d}_{2}\right)}^{2} $ 成正比。在发射机和接收机之间同时存在视距传播路径和信息超表面反射路径场景中,参考文献[36]研究通过设计最优的信息超表面单元反射相位,使得接收机接收到的信号能量最大化。假设地面铺满足够大的信息超表面,该最优反射相位使得自由空间路径损耗与
$ {\left({{d}_{1}+d}_{2}\right)}^{2} $ 成正比。参考文献[37]使用物理光学原理对信息超表面的自由空间路径损耗进行研究分析。理论推导结果表明,当远场源发射的电磁波经由信息超表面反射至同样处于远场的接收机时,即使入射波是一个平面波,反射信号也会呈现一个窄波束的状态,且波束宽度与信息超表面的面积尺寸呈反比。更重要的是,由于发射机与接收机均处于信息超表面的远场,自由空间路径损耗与$ {\left({{d}_{1}d}_{2}\right)}^{2} $ 成正比。因此,参考文献[37]认为自由空间路径损耗与$ {\left({{d}_{1}+d}_{2}\right)}^{2} $ 成正比不具有通用性。若要使该关系成立,须设计信息超表面用于镜像反射电磁波,并使信息超表面的面积趋于无穷,或使之处于发射机和接收机的近场。考虑信息超表面对电磁波的散射效应,参考文献[38]提出了一个简单且严格的路损模型,表明路损与距离乘积成正比。参考文献[38]所提出的模型采用了普适的单元辐射方向图,因此适用于各种不同单元设计的信息超表面。在太赫兹频段,参考文献[39]提供了一个低复杂度的路径损耗模型。不同于已有的研究工作,该模型不仅揭示了路损与距离乘积的关系,还考虑了太赫兹传播媒介特性的影响,例如分子吸收损耗。上述工作研究的对象均为反射式信息超表面,而在实际应用中,透射式信息超表面也具有广泛的应用前景。针对能够工作在反射和透射状态的信息超表面,如图1(a)、(b)所示,参考文献[40]等基于格林定理的矢量推广形式,提出了一个基于物理的自由空间路径损耗解析表征模型。该模型表明,经由信息超表面反射或透射的链路路径损耗可以用一个可计算的积分来表示,该积分取决于传输距离、无线电磁波的极化、信息超表面的大小和所需的表面变换。图 1 具备反射和透射功能的信息超表面路径损耗分析[40]:(a)发射机和接收机在信息超表面的同一侧;(b)发射机和接收机在信息超表面的两侧;(c)信息超表面辅助的无线通信系统[41-42];用于测量不同
$ {d}_{1} $ ,$ {d}_{2} $ ,$ {\theta }_{t} $ ,和$ {\theta }_{r} $ 配置下自由空间路径损耗的测量统[41-42]:(d)原理图;(e)实测照片Figure 1. Pathloss analysis for reflection and transmission RIS[40]: (a) Tx and Rx are on the same side of the RIS; (b) Tx and Rx are on opposite sides of the RIS; (c) RIS-assisted wireless communication system[41-42]; Free-space path loss measurement system for measuring the amount of power reflected from the RIS for different configurations of
$ {d}_{1} $ ,$ {d}_{2} $ ,$ {\theta }_{t} $ and$ {\theta }_{r} $ [41-42]: (d) Diagram; (e) Photograph关于信息超表面路径损耗的研究已引起了众多研究者的关注,然而大部分研究工作局限于理论分析和建模,缺乏实际的信道测量验证。为了获得准确且符合实际通信系统的路径损耗模型,参考文献[41-42]基于信息超表面的物理和电磁特性,提出了信息超表面辅助的自由空间路径损耗通用理论模型,并在两个典型场景中进一步推导特例模型与
$ {d}_{1} $ 和$ {d}_{2} $ 的关系,同时进行相应的数值仿真和实验测量验证。参考文献[41-42]考虑了如图1(c)所示的信息超表面辅助的无线通信系统,假设发射机与接收机之间的直达链路被障碍物完全阻挡,收发机仅能依靠信息超表面提供的反射路径建立通信链路。参考文献[41-42]将信息超表面置于直角坐标系的x-o-y平面上,并使信息超表面的几何中心与直角坐标系的原点重合。图1(c)所示的信息超表面由N行M列规则排布的电磁响应可重构的单元组成。每个电磁单元沿y轴方向的长度为
$ {d}_{y} $ ,沿x轴方向的长度为$ {d}_{x} $ ,其大小一般为亚波长尺度,即二分之一波长至十分之一波长。发射机到直角坐标原点的距离,接收机到直角坐标原点的距离,直角坐标原点到发射机的仰角和方位角,直角坐标原点到接收机的仰角和方位角分别用$ {d}_{1} $ ,$ {d}_{2} $ ,$ {\theta }_{t} $ ,$ {\phi }_{t} $ ,$ {\theta }_{r} $ 和$ {\phi }_{r} $ 来表示。参考文献[41-42]的研究结果显示,当位于$ \left({{x}_{t},y}_{t}{,z}_{t}\right) $ 的发射机发射波长为λ的信号经信息超表面的反射被位于$ \left({{x}_{t},y}_{t}{,z}_{t}\right) $ 的接收机所接收,自由空间路径损耗通用模型可表示为:$$ P{L_{general}} = \dfrac{{16{\pi ^2}}}{{{G_t}{G_r}{{\left( {{d_x}{d_y}} \right)}^2}{{\left| {\displaystyle\sum\limits_{m = 1 - \frac{M}{2}}^{\frac{M}{2}} {\displaystyle\sum\limits_{n = 1 - \frac{N}{2}}^{\frac{N}{2}} {\dfrac{{\sqrt {F_{n,m}^{combine}} {\varGamma _{n,m}}}}{{r_{n,m}^tr_{n,m}^r}}{{\rm e}^{\dfrac{{ - j2\pi \left( {r_{n,m}^t + r_{n,m}^r} \right)}}{\lambda }}}} } } \right|}^2}}} $$ (1) 式中:
$ {F}_{n,m}^{combine} $ 为发射天线、信息超表面、接收天线的联合归一化功率辐射方向图,为角度相关的因子。通过自由空间路径损耗通用模型,参考文献[41-42]揭示了信息超表面反射路径的自由空间路径损耗与发射/接收天线的增益以及电磁单元大小的平方成反比。此外,路径损耗还与发射/接收天线以及电磁单元的联合归一化功率方向图、电磁单元的数量、电磁单元的反射系数设计以及发射机/接收机与各电磁单元之间的距离有关。为更清晰地展示路损模型与传播距离的关系,参考文献[41-42]针对两个信息超表面典型应用场景进行讨论,以便得到典型场景下自由空间路径损耗的直观见解。参考文献[41-42]讨论的两个典型信息超表面应用场景为:辅助波束成形以及辅助信号广播。在信息超表面辅助波束成形场景中,将配置信息超表面单元的反射系数,使得反射信号在特定用户处相干叠加,以获得最大的接收功率。而在信息超表面辅助信号广播场景中,将配置信息超表面单元的反射系数,使得反射信号均匀覆盖特定区域内的所有用户,以获得公平的服务质量。
在远场辅助波束成形场景下,参考文献[41]给出的自由空间路径损耗模型为:
$$ PL_{far{\text{ }}field}^{beam} = \frac{{16{\pi ^2}{{\left( {{d_1}{d_2}} \right)}^2}}}{{{G_t}{G_r}{{\left( {MN{d_x}{d_y}} \right)}^2}F\left( {{\theta _t},{\varphi _t}} \right)F\left( {{\theta _r},{\varphi _r}} \right){A^2}}} $$ (2) 与通用模型相比,信息超表面远场辅助波束成形场景下的路损更加直观,揭示了此场景中路径损耗与
$ {\left({{d}_{1}d}_{2}\right)}^{2} $ 成正比。在近场辅助信号广播场景下,参考文献[41-42]给出的自由空间路径损耗模型为:
$$ PL_{near{\text{ }}field}^{broadcast} = \frac{{16{\pi ^2}{{\left( {{d_1} + {d_2}} \right)}^2}}}{{{G_t}{G_r}{\lambda ^2}{A^2}}} $$ (3) 与通用模型相比,信息超表面近场辅助信号广播场景下的路损模型揭示了该场景路径损耗与
$ {\left({{d}_{1}+d}_{2}\right)}^{2} $ 成正比。通过以上两个特定场景的讨论,参考文献[41-42]揭示了信息超表面所提供的辅助链路的路径损耗与其具体应用场景有关。根据不同的应用场景,该路损可与
$ {\left({{d}_{1}d}_{2}\right)}^{2} $ 成正比,也可与$ {\left({{d}_{1}+d}_{2}\right)}^{2} $ 成正比。为验证这一结论,参考文献[41-42]进一步开展了实际路损测量工作,搭建了如图1(d)、(e)的测量系统。通过改变系统中的$ {d}_{1} $ ,$ {d}_{2} $ ,$ {\theta }_{t} $ ,和$ {\theta }_{r} $ 对路损进行测量,不同频段和不同场景下的实测数据验证了信息超表面远场波束成形场景下,路径损耗与两段传输距离乘积平方成正比,在信息超表面近场广播场景下,路径损耗与两段距离和的平方成正比。 -
当前针对基于信息超表面的无线中继系统研究大多将信息超表面在通信系统中的作用建模成幅度不变或者幅相独立可调的复对角矩阵。然而在实际设计过程中,硬件不理想因素使信息超表面单元的电磁反射幅相特性呈现复杂的特性。
从信息超表面单元的等效传输线模型出发,参考文献[43]提出了一种幅相依赖的反射系数模型。常见印刷电路板构成的信息超表面单元包括底层的金属底板,中间介质层,顶层金属贴片及可调半导体器件,可由图2(a)所示等效传输线模型表示。发射电磁信号的接入点作为源,通过被视为一段传输线的自由空间与信息超表面单元构成完整的回路。在等效传输线模型中,信息超表面单元可建模成并联的谐振电路。
如图2(a)所示,第n个信息超表面单元对应的并联谐振电路的阻抗可表示为:
$$ {Z_n}\left( {{C_n},{R_n}} \right) = \frac{{j\omega {L_1}\left( {j\omega {L_2} + \dfrac{1}{{j\omega {C_n}}} + {R_n}} \right)}}{{j\omega {L_1} + \left( {j\omega {L_2} + \dfrac{1}{{j\omega {C_n}}} + {R_n}} \right)}} $$ (4) 式中:
$ {L}_{1} $ ,$ {L}_{2} $ ,$ {C}_{n} $ ,$ {R}_{n} $ 以及$ \mathrm{\omega } $ 分别表示信息超表面单元底层电感,顶层电感,可调半导体器件电容,可调半导体器件电阻以及入射信号的角频率。因此,在传输线模型下,信息超表面单元的反射系数可以表示为:$$ {v_n} = \frac{{{Z_n}\left( {{C_n},{R_n}} \right) - {Z_0}}}{{{Z_n}\left( {{C_n},{R_n}} \right) + {Z_0}}} $$ (5) 式中:
$ {Z}_{0} $ 为自由空间阻抗。由此可见,信息超表面单元的反射系数受控于可调半导体器件的电容和电阻。因此,该反射系数的幅度和相位是互相影响,如无特殊设计,则无法实现幅度不变或者幅相独立可调。尽管参考文献[43]等提出的反射系数模型揭示了信息超表面单元反射系数受控于可调半导体器件的电容和电阻,且幅度和相位具有相互依赖的关系,但该模型忽视了实际通信场景中电磁波传播特性对反射系数的影响。作为具有二维平面结构的信息超表面,其对电磁波的反射特性很大程度上取决于单元几何结构的设计。当电磁波从不同角度入射至信息超表面时,信息超表面单元在电磁波垂直面上的几何结构投影将发生变化,从而影响反射特性。考虑到实际通信场景中,入射至信息超表面的电磁波将来自四面八方,参考文献[44]中提出了一种角度依赖的相移器模型。如图2(b)所示,该模型中,信息超表面单元几何结构在传输线模型所体现的等效电容、电感以及电阻,均为电磁波入射角
$ \theta $ 的函数。在实际通信场景中,采用角度依赖相移器模型能够更好地反映信息超表面对电磁波的操纵能力,同时也对基于信息超表面无线中继系统的反射系数优化提出一个切实而有具有挑战的难题。 -
硬件不理想特性不仅使信息超表面单元呈现复杂的电磁响应,更引发了单元与单元之间的互耦效应。在信息超表面单元互耦研究工作中,参考文献[45]提出了一个物理和电磁兼容的通信模型,用于分析和优化信息超表面无线中继系统。图3为参考文献[45]提出的模型的原理图,信息超表面由
$ {N}_{\text{ris}}=M\times N $ 个单元组成,发射机和接收机分别配备$ {N}_{t} $ 和$ {N}_{r} $ 根天线单元。基于辐射元件(收发机天线,信息超表面单元)之间的互阻抗,参考文献[45]中建立的信息超表面无线中继信道模型具有四个主要的特点:(1)端到端:该模型表述的是收发机之间的等效信道,体现了多输入多输出(Multiple-Input-Multiple-Output, MIMO)通信系统中发射机天线端口馈电电压$ {V}_{\text{T}i} $ ($i=\mathrm{1,2},\dots ,{N}_{t}$ )与接收机天线端口测量电压$ {V}_{\text{L}j} $ ($j=\mathrm{1,2},\dots ,{N}_{r}$ )的一一对应关系;(2)电磁兼容:该模型解释了电磁场在收发机以及信息超表面上的产生与传播过程;(3)互耦感知:该模型解释了信息超表面亚波长尺寸单元之间的电磁耦合现象;(4)单元感知:该模型解释了信息超表面单元对电磁波的反射幅度和反射相位之间的纠缠现象。基于参考文献[45]中提出的包含互耦效应的信道模型,参考文献[46]研究了单输入单输出(Single-Input-Single-Output, SISO)通信系统的信息超表面可调阻抗优化问题,旨在最大化收发机的端到端信噪比。针对无互耦效应的信息超表面参考文献[46]得出了信息超表面最优可调阻抗的闭式表达,并内在地解释了信息超表面幅度响应和相位响应的相互作用。参考文献[46]通过数值仿真发现,当信息超表面单元间距小于半波长,互耦效应对端到端信噪比的影响尤为明显。这种情况下,有针对性地根据信息超表面单元互耦效应优化可调阻抗,能够提高系统端到端的信噪比。
参考文献[46]针对互耦效应进行的信息超表面可变阻抗优化设计仅适用于单天线收发机与单个信息超表面组成的无线中继系统,缺乏普适性。对于更具一般性的多信息超表面辅助的多用户MIMO通信系统,参考文献[47]以最大化用户和速率为目标,提出了一个可证明收敛的优化算法来设计每个信息超表面的可调阻抗。
参考文献[48]也对信息超表面单元间互耦进行了研究。参考文献[48]利用严格的散射参数网络分析建立的通信模型与参考文献[45]所提的模型具有四个不同之处:(1)与参考文献[45]中基于阻抗参数提出的模型相比,参考文献[48]发现用反射系数和散射参数来解释信息超表面的反射机理更为自然;(2)参考文献[48]所建立的通信模型包含了收发机和信息超表面的阻抗不匹配因素以及互耦效应,较参考文献[45]所提模型更为通用;(3)参考文献[48]等解释了信息超表面相移以及恒模约束的物理意义;(4)参考文献[48]不仅研究了常见单连接信息超表面,更进一步提出了全连接结构以及组连接结构。
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自香农定义了通信信道的容量以来,通信行业一直致力于改进通信系统以期不断提高、不断逼近香农极限。以往对通信系统的设计主要体现在收发机上,而收发机之间的传播信道通常被视为无法主动改变仅能被动适应的不可控外生实体。根据固有传播信道设计的通信系统,往往处于被动的困境,无法发挥其最大潜力。例如在强视距传播的毫米波通信系统中,不论天线个数如何增长,处于各自远场的收发机之间的信道矩阵都将存在秩亏的现象,无法获得多天线通信系统的空间复用增益。信息超表面的出现,有望打破信道不可控这一铁律。
在基于信息超表面的无线中继系统中,信息超表面作为收发机之间传播信道中一个可控的电磁实体,通过改变其对电磁波的响应,能够实现对信道的操纵与改善。已有研究工作[49-50]显示,通过在信道中部署多个信息超表面,信道的秩能够得到一定程度的提升。参考文献[51]使用基于梯度的优化方法设计信息超表面的反射系数,以最大化包含信息超表面的信道的有效秩及其最小奇异值。在实验应用方面,参考文献[52]将包含65个单元的信息超表面部署于如图4(a)所示的
$ 1.45\;\mathrm{m}\times 1\;\mathrm{m}\times 0.75\;\mathrm{m} $ 混沌铝腔中,用于提升两个最多配备8根天线的收发机的传输速率。为了提升$ N\times N $ MIMO信道矩阵的有效秩,参考文献[52]采用顺序迭代优化算法设计65个信息超表面单元的反射系数。在每次迭代优化后,测量信道矩阵并计算信道矩阵的有效秩,若获得更高的信道有效秩,则更新信息超表面的反射系数。图4(b)的实验测量结果表明对于不同的天线数$ N $ ,优化过的信息超表面都能使系统获得满秩的信道,降低了不同子信道的串扰。参考文献[49-52]虽然从仿真、优化以及实测的角度分别证明了信息超表面能够用于改善信道的秩,但从理论上揭示信息超表面反射系数与信道秩的关系的工作仍然比较少。参考文献[53]中提出了一种信道定制的方法。如图4(c)所示,在强视距传播的毫米波混合天线架构系统中,收发机大规模天线阵列所产生的强方向性波束和具有稀疏特性的毫米波信道将使得收发机之间的信道仅存在一条传播链路。若无信息超表面的引入,信道的秩将骤降至1,无法实现多数据流传输。为解决信道秩亏的问题,该研究工作引入了分布式信息超表面,通过信息超表面的位置部署以及反射系数设计,实现信道秩的任意定制。该研究工作得出了信息超表面反射系数与信道奇异值的闭式表达,因此能够通过调整信息超表面的反射系数定制截断条件数最小,即各个子信道均匀的传播环境。
图 4 (a)信息超表面覆盖的铝腔实验装置,用于增强LoS
$ N\times N $ MIMO无线通信链路在2.47 GHz WiFi频率下的多径散射[52];(b) LoS$ N\times N $ MIMO(红线表示$ N=2 $ ,蓝线表示$ N=4 $ ,橙线表示$ N=6 $ )的有效秩随着算法迭代步骤的演进。图中包含瑞利衰落信道和完美正交信道作为比较基准[52];(c)分布式信息超表面辅助的强视距毫米波通信系统[53]Figure 4. (a) The aluminium cavity experimental setup incorporating RIS coated on the wall in order to enrich multipath scattering in a LoS
$ N\times N $ MIMO wireless communication link at the WiFi frequency 2.47 GHz[52]; (b) The evolution of the effective rank$ {R}_{\text{eff}} $ of the LoS$ N\times N $ channel matrix (red for$ N=2 $ , blue for$ N=4 $ , and orange for$ N=6 $ ) over the number of algorithmic steps. Benchmarks for Rayleigh fading and perfect channel orthogonality are also included[52]; (c) Distributed RISs-assisted strong LoS mmWave communication systems[53]
Wireless communications based on information metasurfaces (Invited)
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摘要: 信息超表面由于其强大的处理空间电磁波的能力,成为国内外物理和信息领域的研究热点之一。文中主要介绍信息超表面在无线通信领域的一系列研究进展。信息超表面能实时操控电磁波及直接处理数字编码信息,并进一步对信息进行感知、理解,甚至记忆、学习和认知,这使其在无线通信领域展现出巨大潜能。文中首先介绍信息超表面在承担无线中继职能时所涉及的信道建模研究进展、以及其对信道的改善作用;其次介绍信息超表面在新体制发射机中的应用,通过对入射到信息超表面上的载波进行幅度或相位调制,实现了多种简化的发射机架构。此外,文中还介绍了利用信息超表面近场、远场以及散射场等信息,实现了多种新型无线通信系统。最后,文章对基于信息超表面的无线通信进行了总结和展望。Abstract: Information metasurfaces have become one of the research hotspots in the field of physics and information, because of the ability of manipulating electromagnetic waves. A series of research progress in the field of wireless communications based on information metasurfaces was introduced. Information metasurface can manipulate electromagnetic waves in real time and directly process digital coding information, and can further perceive, understand, even memorize, learn and recognize information, which makes it show great potential in the field of wireless communications. Firstly, the research progress of channel modeling was introduced and the channel improvement that information metasurfaces could achieve when they worked as a wireless relay. Secondly, the application of information metasurface in the new transmitter system was also introduced, which modulated the amplitude or phase of the carrier waves. Thus several simplified transmitter architectures could be realized. Thirdly, the realization of several new wireless communication systems using the information of the near field, far field and scattering field of the information metasurface was introduced. Finally, the future wireless communication based on information metasurface was summarized and prospected.
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Key words:
- metamaterial /
- information metasurfaces /
- wireless communications
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图 1 具备反射和透射功能的信息超表面路径损耗分析[40]:(a)发射机和接收机在信息超表面的同一侧;(b)发射机和接收机在信息超表面的两侧;(c)信息超表面辅助的无线通信系统[41-42];用于测量不同
$ {d}_{1} $ ,$ {d}_{2} $ ,$ {\theta }_{t} $ ,和$ {\theta }_{r} $ 配置下自由空间路径损耗的测量统[41-42]:(d)原理图;(e)实测照片Figure 1. Pathloss analysis for reflection and transmission RIS[40]: (a) Tx and Rx are on the same side of the RIS; (b) Tx and Rx are on opposite sides of the RIS; (c) RIS-assisted wireless communication system[41-42]; Free-space path loss measurement system for measuring the amount of power reflected from the RIS for different configurations of
$ {d}_{1} $ ,$ {d}_{2} $ ,$ {\theta }_{t} $ and$ {\theta }_{r} $ [41-42]: (d) Diagram; (e) Photograph图 4 (a)信息超表面覆盖的铝腔实验装置,用于增强LoS
$ N\times N $ MIMO无线通信链路在2.47 GHz WiFi频率下的多径散射[52];(b) LoS$ N\times N $ MIMO(红线表示$ N=2 $ ,蓝线表示$ N=4 $ ,橙线表示$ N=6 $ )的有效秩随着算法迭代步骤的演进。图中包含瑞利衰落信道和完美正交信道作为比较基准[52];(c)分布式信息超表面辅助的强视距毫米波通信系统[53]Figure 4. (a) The aluminium cavity experimental setup incorporating RIS coated on the wall in order to enrich multipath scattering in a LoS
$ N\times N $ MIMO wireless communication link at the WiFi frequency 2.47 GHz[52]; (b) The evolution of the effective rank$ {R}_{\text{eff}} $ of the LoS$ N\times N $ channel matrix (red for$ N=2 $ , blue for$ N=4 $ , and orange for$ N=6 $ ) over the number of algorithmic steps. Benchmarks for Rayleigh fading and perfect channel orthogonality are also included[52]; (c) Distributed RISs-assisted strong LoS mmWave communication systems[53]图 5 不同编码图案的信息超表面及其远场模式[63]。(a)对称编码、(b)随机编码和(c)不同编码图案的几何和物理熵之间的关系;(d)信息超表面携带的信息与超表面辐射图信息之间的关系示意图[64]
Figure 5. Non-periodic coding metasurfaces and their far-field patterns[63]. (a) Symmetric coding; (b) random coding, and (c) the relationship between the geometrical and physical entropies; (d) Schematic of information relation between the metasurface and its radiation pattern[64]
图 6 (a)基于信息超表面的BFSK调制方案的简化无线通信系统及发射机细节(右侧)[54];(b)BFSK调制方案下接收到的图片数据;(c)接收到的实时信号的频谱;(d)该系统在不同通信距离、夹角下信息传输误比特率与馈源天线发射功率关系图
Figure 6. (a) Experimental scenario of the BFSK wireless communication system based on information metasurface with transmission process described on the right[54]; (b) The received messages by the BFSK wireless communication system; (c) The instantaneous experimental results for the receiving spectrum.; (d) The relation diagram between the bit error rate and the transmitting power of the feed antenna at different communication distances and incident angles
图 7 (a)基于信息超表面的QPSK调制方案无线通信系统的示意图[55];(b) QPSK调制方案下不同数据速率下接收的星座图;(c)基于信息超表面的8 PSK调制方案无线通信系统的实验场景图[56];(d) 8PSK发射机和传统的基于SDR的发射机的接收端信噪比(SNR)和相应的误码率(BER)
Figure 7. (a) Photograph of the QPSK wireless communication system based on the information metasurface[55]; (b) The dependence of the measured constellation diagram of the QPSK wireless communication system on the message transmission rate at receiving terminal; (c) Photograph of the 8 PSK wireless communication system based on the information metasurface[56]; (d) The signal-to-noise ratio (SNR) and the corresponding BER at the receiver of the 8 PSK transmitter and the conventional SDR-based transmitter
图 8 (a)测量的QPSK、8 PSK和16 QAM的星座图[57];(b)实验中搭建的基于信息超表面的256 QAM调制毫米波无线通信系统[58]。不同载波下测得的256 QAM的星座图载频:(c) 27 GHz;(d) 28 GHz;(e) 29 GHz
Figure 8. (a) The measured constellation diagrams for QPSK, 8 PSK and 16 QAM[57]; (b) Photograph of the mmWave 256 QAM wireless communication system based on the information metasurface[58]. The 256 QAM constellation diagrams with a frequency interval of 78.125 kHz at (c) fc=27 GHz, (d) 28 GHz, and (e) 29 GHz, respectively
图 9 (a)利用信息超表面的远场变化直接传输数字信息的示意图[59];(b)测试原型系统的性能而发送的原始图像;(c)~(e)三种测试场景下接收机接收的图像:(c)无障碍物,(d)有障碍物,(e)有障碍物并应用信道自适应算法
Figure 9. (a) The schematic diagram of transmitting digital message directly via the farfield change of the information metasurface[59]; (b) The original image to be transmitted for testing the performance of the prototype system; (c)-(e) The received image: (c) without barrier in the channel, (d) with barrier in the channel; (e) The received image after running the self-adaption program with barrier in the channel
图 10 (a)三通道直接传输的8种编码图案和(b)对应的近场振幅分布[60];(c)基于超表面的散射无线通信概念说明[61];(d)超表面辅助大规模后向散射无线通信中8种编码模式的幅度分布;(e)分别实现单通道、双通道和三通道BPSK调制方案的星座图;(f)从三个用户接收到的三张单色图像及合成的彩色图像
Figure 10. (a) The direct transmitted 8 phase codes in three channels and (b) corresponding near-field patterns[60]; (c) Schematic of the experimental setup of the metasurface-assisted massive backscatter wireless communication[61]; (d) Amplitude distributions of the 8 coding patterns in the metasurface-assisted massive backscatter wireless communication; (e) BPSK constellation results of single-channel, double-channel and three-channel; (f) Individual monochrome and synthesized full-color images transmitted from Alice to Bobs
图 11 (a)基于信息超表面的双通道无线通信系统实验照片[62];(b) 4种编码图案在双通道直接信息传输系统中的辐射方向图测量结果
Figure 11. (a) Experimental scenario of the dual-channel wireless communication system based on information metasurface[62]; (b) Measured radiation patterns of the four coding patterns for dual-channel direct information transmissions
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