论涡旋电磁波轨道角动量传输新维度

doi:10.11959/j.issn.1000-436x.2022087

摘要/Abstract

摘要：

目的：作为未来移动通信中潜在核心关键技术的涡旋电磁波轨道角动量（OAM）传输技术，易与传统基于多天线的多输入多输出（MIMO）传输技术相混淆，从而引发关于涡旋电磁波OAM是否在无线传输中提供新维度的争议。本文明确了只有电磁波量子携带内禀OAM的涡旋电磁波传输系统才可以获得OAM新维度；统计态OAM涡旋波束中的电磁波量子形成的外部OAM与空域维度相耦合，无法构成MIMO传输以外新维度。

方法：本文从电磁波资源利用和发展的历史出发，分析电磁波OAM的物理特征，给出了含有OAM维度的香农信道容量公式，指明了无线传输中OAM新维度的功率复用对容量提升的意义。为了进一步体现OAM新维度特性，依据信道容量的不同，将涡旋电磁波OAM传输系统划分为四个典型信道容量区域。以微波频段为例，指出了只有基于涡旋微波量子的量子态OAM涡旋电磁波传输可以超越传统多天线MIMO容量界，并形成含有OAM维度的新MIMO容量界。

结果：为了说明量子态OAM涡旋电磁波和统计态OAM涡旋电磁波束的应用范围和突出优点，本文对典型OAM传输系统按信道容量由高到低划分为A至D四个区域。其中，区域A属于具有物理新维度的量子态OAM传输系统，采用涡旋微波量子传输信息，其容量界相比于传统多天线MIMO容量界获得提升；区域B、C和D属于统计态OAM涡旋波束，虽然不具备新维度，但在视距直射（LoS）信道中有突出表现。与传统视距MIMO传输相比，区域B为OAM专用天线传输系统，可恢复信道正交性和信道矩阵的秩，获得容量大幅提升，代表着统计态OAM涡旋波束使用的发展趋势；区域C为阵列天线全相位面传输系统，系统复杂度低，且作为早期OAM技术代表，技术成熟度较高；区域D为部分相位传输系统，不需要接收完整相位面，适用于长距离传输。

结论：针对学术界中电磁波OAM是否为无线传输系统提供新维度的争议，本文从电磁波的利用历史出发，分析了电磁波OAM传输机理，并指出：无论是内禀OAM还是外部OAM，都是电磁波可以利用的新物理量，但只有基于内禀OAM的量子态OAM涡旋微波量子传输可以在无线传输中产生新维度；基于外部OAM的统计态OAM涡旋波束则无法构成MIMO传输以外的新维度，只能算作具有螺旋相位面波束赋形能力的多天线MIMO传输系统特例。

关键词: 轨道角动量, 无线通信, 涡旋微波量子, 涡旋电磁波, 内禀轨道角动量, 外部轨道角动量

Abstract:

Purpose:The vortex Electro-Magnetic (EM) wave transmission system with Orbital Angular Momentum (OAM), which is the potential key technology in future mobile communications,is easily confused with the traditional Multiple-Input Multiple-Output (MIMO) transmission system. This leads to the controversy on whether the OAM of vortex EM waves can provide the new dimension for the wireless transmission. This paper points out that only the vortex EM wave transmission system with vortex photons carrying Intrinsic OAM(IOAM)can obtain the new dimension.Furthermore,compared with the MIMO transmission with multiple antennas,Extrinsic OAM(EOAM)formed by the plane microwave photons in the statistical OAM beam is coupled with the space domain and cannot provide additional new dimension.

Method:This paper analyzes the physical characteristics of the EM wave with OAM and traces back to the history of the EM wave resource utilization and development. Moreover, the formula of Shannon channel capacity containing OAM dimension is given, and the significance of power multiplexing of new dimension for capacity enhancement is specified. In order to show the insight of the new dimensional characteristics of vortex EM waves, the typical vortex EM wave OAM transmission systems are classified into four different regions according to the channel capacity. Taking the microwave band as an example,it is pointed out that only the quantum OAM vortex EM wave transmission based on vortex microwave photons can surpass the traditional multi-antenna MIMO capacity bound and form a new MIMO capacity bound containing the OAM dimension.

Consequence:In order to illustrate the application scope and the outstanding advantages utilized by quantum OAM electromagnetic waves and statistical OAM Beams, this paper classifies the typical OAM transmission systems into four regions based on channel capacity.From the high channel capacity to the low channel capacity, Region A belongs to the quantum OAM transmission system with the new dimension of OAM, using vortex microwave photon to convey information, and the corresponding capacity bound can be enhanced to surpass the traditional multi-antenna MIMO capacity bound;Regions B,C and D belong to the statistical OAM vortex beams,which do not own new dimensions in MIMO transmission but have outstanding performance in the Line-of-Sight (LoS) channel. Compared with the traditional LoS MIMO transmission, Region B refers to the OAM dedicated antenna transmission system, which can recover the channel orthogonality and the rank of the channel matrix,so that a significant capacity enhancement is obtained,which represents the development trend of statistical OAM vortex beam.Region C refers to the array antenna full-phase plane transmission system.It enjoys low system complexity and high technical maturity as a representative of early OAM technology. Region D refers to the partial phase transmission system, which does not need to receive the complete phase plane, and is suitable for long-distance transmission.

Conclusion:In response to the controversy on whether OAM electromagnetic waves can provide the new dimension for the wireless transmission, this paper analyzes the physical insight of the EM wave transmission with OAM in history.It can be concluded that:both the intrinsic OAM and the extrinsic OAM can be utilized by EM waves,but only quantum OAM vortex microwave photons transmission based on intrinsic OAM can generate new dimensions in the wireless transmission besides the MIMO transmission. The statistical OAM vortex beams based on extrinsic OAM cannot constitute new dimensions beyond MIMO transmission and just belongs to the special case of multi-antenna MIMO systems which can generate the beams with helical phase front.

Key words: orbital angular momentum, wireless communication, vortex microwave photon, vortex electro-magnetic wave, intrinsic OAM, extrinsic OAM

中图分类号:

TN92

张超, 王元赫. 论涡旋电磁波轨道角动量传输新维度[J]. 通信学报, 2022, 43(6): 211-222.

Chao ZHANG, Yuanhe WANG. New dimension in vortex electro-magnetic wave transmission with orbital angular momentum[J]. Journal on Communications, 2022, 43(6): 211-222.

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容量界	信道容量/(bit·(s·Hz)^-1)
开环MIMO传输容量界（区域C与D分界线）	14.7
闭环MIMO传输容量界（区域B与C分界线）	17.6
传统多天线MIMO传输容量界（区域A与B分界线）	27.6
含OAM维度的新多天线MIMO容量界（2模态复用）	41.3
含OAM维度的新多天线MIMO容量界（2模态复用和2模态键控）	49.3