6G无线通信物理层关键技术

doi:10.11959/j.issn.1000-0801.2023250

摘要/Abstract

摘要：

业界预计在2030年前后商用6G技术。与5G技术相比，6G将在流量密度、连接数密度和能量效率等多个性能指标方面大幅度提升。为满足 2030 年及以后的垂直行业的个性化、多样化需求，6G 将采用诸多新技术，如新型网络架构、新物理层无线传输技术以及新的安全可持续性技术。从6G物理层潜在关键技术入手，重点介绍了太赫兹（THz）、超大规模多输入多输出（MIMO）、智能反射面（IRS）及物理层安全等技术。此外，还概括地介绍了6G潜在关键技术挑战，希望为未来6G物理层相关研究提供一定的参考。

关键词: 6G, 太赫兹, 超大规模MIMO, 智能反射面, 挑战

Abstract:

6G technologies are expected to be deployed commercially around 2030.It is expected that 6G will provide performance superior to 5G in the key performance indicators, such as traffic density, connection density, and energy efficiency.In order to meet the personalized and diversified requirements of vertical industries in 2030 and beyond, 6G will adopt many new technologies, such as new network architecture, new physical layer wireless transmission technologies, and new security sustainability technologies.Starting from the potential key technologies in physical layer of 6G, terahertz (THz), ultra-large-scale multiple-input multiple-output (MIMO), intelligent reflecting surface (IRS), and physical security were introduced mainly.In addition, the challenges of potential key technologies of 6G were briefly introduced, which were expected to provide some reference for future 6G physical layer-related research.

Key words: 6G, THz, ultra-large-scale MIMO, IRS, challenge

中图分类号:

TN929.5

倪善金, 沈亮, 宁珊, 万辛. 6G无线通信物理层关键技术[J]. 电信科学, 2023, 39(12): 1-18.

Shanjin NI, Liang SHEN, Shan NING, Xin WAN. Key technologies in physical layer of 6G wireless communications[J]. Telecommunications Science, 2023, 39(12): 1-18.

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当前物理层安全技术的主要研究方向及已发表论文[65,66,67,68,69,70,71,72,73]提出的解决方案"

研究方向	当前主要解决方案
无密钥物理层	文献[65]在莱斯信道基础上提出一种线性预编码器设计方案来进行数据和人工噪声传输，以增强大规模 MIMO 无人驾驶飞行器通信系统的安全性
安全传输技术	文献[66]探究了如何利用 THz 通信固有的多路径特性来降低消息窃听概率，同时提出在通信实体间共享当前可用的多个 THz 传播路径上的数据传输，虽然以牺牲部分链路容量为代价，但这种方式即使在多个窃听者协作窃听时仍然可以有效降低消息窃听概率
	文献[67]面向人工噪声辅助的安全MIMO-IRS无线通信系统，提出在发射功率限制和IRS相移单位模数的约束下，联合优化基站处的发射预编码矩阵、人工噪声的协方差矩阵和IRS处的相移，以最大化保密速率
	文献[68]利用射频指纹的唯一性与第三方不可复制性，提出一种基于射频指纹的接收信号来源安全识别方案，从而实现了信息保护和设备的快速认证
基于信道的物理层安全密钥生成技术	文献[69]对6G无线通信内生安全的理念与构想进行了总结与分析，提出逼近香农“一次一密”的通信安全一体化概念，指出当前基于信道的物理层安全生成技术主要包括：一是利用无线信道的独一无二性、不可复制性等内在安全属性，从无线信道中生成安全密钥，从而保证通信的可靠性^[70]；二是将无线信道天然传播特性与6G新兴使能技术（如THz、超大规模MIMO、IRS等）结合，通过挖掘新兴使能技术带来的信道新特征以及新的传播特性，从数据传输速率及无线密钥生成速率两个方面入手，逼近实现“一次一密”效果^[71-72]；三是将传播信号与无线密钥设计结合，实现面向通信安全一体化的高速密钥生成及数据安全高效传输机制^[73]

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参考文献 101

[1]	倪善金, 赵军辉 . 5G无线通信网络物理层关键技术[J]. 电信科学, 2015,31(12): 40-45.
	NI S J , ZHAO J H . Key technologies in physical layer of 5G wireless communications network[J]. Telecommunications Science, 2015,31(12): 40-45.
[2]	大唐电信. 6G愿景与技术趋势白皮书[R]. 2021.
	Datang Mobile Communications Equipment Co.,Ltd.. 6G vision and technology trends[R]. 2021.
[3]	张辛欣我国正式发放5G商用牌照[EB]. 2019.
	ZHANG X X . China officially issues 5G commercial license[EB]. 2019.
[4]	YOU X H , WANG C X , HUANG J ,et al. Towards 6G wireless communication networks:vision,enabling technologies,and new paradigm shifts[J]. Science China Information Sciences, 2020,64(1): 1-74.
[5]	中国移动通信有限公司研究院. 2030+愿景与需求白皮书[R]. 2020.
	Research Institution of China Mobile. 2030+ vision and requirements[R]. 2020.
[6]	王晴天, 刘洋, 刘海涛 ,等. 面向 6G 的网络智能化研究[J]. 电信科学, 2022,38(9): 151-160.
	WANG Q T , LIU Y , LIU H T ,et al. Research on network intelligence for 6G[J]. Telecommunications Science, 2022,38(9): 151-160.
[7]	TARIQ F , KHANDAKER M R A , WONG K K ,et al. A speculative study on 6G[J]. IEEE Wireless Communications, 2020,27(4): 118-125.
[8]	GIORDANI M , POLESE M , MEZZAVILLA M ,et al. Toward 6G networks:use cases and technologies[J]. IEEE Communications Magazine, 2020,58(3): 55-61.
[9]	ZONG B Q , FAN C , WANG X Y ,et al. 6G technologies:key drivers,core requirements,system architectures,and enabling technologies[J]. IEEE Vehicular Technology Magazine, 2019,14(3): 18-27.
[10]	IMT-2030 (6G) 推进组. 6G总体愿景与潜在关键技术白皮书[R]. 2021.
	IMT-2030 (6G) Promotion Group. 6G vision and candidate technologies[R]. 2021.
[11]	蒋瑞红, 冯一哲, 孙耀华 ,等. 面向低轨卫星网络的组网关键技术综述[J]. 电信科学, 2023,39(2): 37-47.
	JIANG R H , FENG Y Z , SUN Y H ,et al. A survey on networking key technologies for LEO satellite network[J]. Telecommunications Science, 2023,39(2): 37-47.
[12]	李晖, 魏克军, 闻立群欧洲6G启动，我国正与全球同步推进[EB]. 2021.
	LI H , WEI K J , WEN L Q ,et al. European 6G is launched,and China is advancing in step with the world[EB]. 2021.
[13]	University of Oulu. White paper on 6G networking[EB]. 2021.
[14]	6th Generation Innovation Centre. 6G wireless:a new strategic vision[R]. 2020.
[15]	德国弗劳恩霍夫应用研究促进协会. 6G，下一代移动通信技术[EB]. 2021.
	Fraunhofer-Gesellschaft. 6G,the next generation mobile communication technology[EB]. 2021.
[16]	白云飞, 韩国力推6G核心技术自主化[EB]. 2021.
	BAI Y F . Republic of Korea pushes for independence of 6G core technology[EB]. 2021.
[17]	姚建铨, 迟楠, 杨鹏飞 ,等. 太赫兹通信技术的研究与展望[J]. 中国激光, 2009,36(9): 2213-2233.
	YAO J Q , CHI N , YANG P F ,et al. Study and outlook of terahertz communication technology[J]. Chinese Journal of Lasers, 2009,36(9): 2213-2233.
[18]	谢莎, 李浩然, 李玲香 ,等. 太赫兹通信技术综述[J]. 通信学报, 2020,41(5): 168-186.
	XIE S , LI H R , LI L X ,et al. Survey of terahertz communication technology[J]. Journal on Communications, 2020,41(5): 168-186.
[19]	ITU. Attenuation by atmospheric gases,ITU-R recommendation:ITU-R P.676-9[S]. 2012.
[20]	MA J J , SHRESTHA R , MOELLER L ,et al. Invited article:channel performance for indoor and outdoor terahertz wireless links[J]. APL Photonics, 2018,3(5).
[21]	王敏, 王俊峰, 吴秋宇 ,等. 340 GHz太赫兹室内信道测量及仿真研究[J]. 物理学报, 2014,63(15): 154101.
	WANG M , WANG J F , WU Q Y ,et al. Research on indo or channel measurement and simulation at 340 GHz[J]. Acta Physica Sinica, 2014,63(15): 154101.
[22]	YANG Y H , YAMAGAMI Y , YU X B ,et al. Terahertz topological photonics for on-chip communication[J]. Nature Photonics, 2020,14(7): 446-451.
[23]	邓若琪, 张雨童, 张浩波 ,等. 全息无线电:全息超表面赋能的超大规模 MIMO 新范式[J]. 电子学报, 2022,50(12): 2984-2995.
	DENG R Q , ZHANG Y T , ZHANG H B ,et al. Holographic radio:a new paradigm for ultra-massive MIMO enabled by reconfigurable holographic surfaces[J]. Acta Electronica Sinica, 2022,50(12): 2984-2995.
[24]	WU Q Q , ZHANG R . Towards smart and reconfigurable environment:intelligent reflecting surface aided wireless network[J]. IEEE Communications Magazine, 2020,58(1): 106-112.
[25]	BJ?RNSON E , ?ZDOGAN ? , LARSSON E G . Reconfigurable intelligent surfaces:three myths and two critical questions[J]. IEEE Communications Magazine, 2020,58(12): 90-96.
[26]	WU Q Q , ZHANG S W , ZHENG B X ,et al. Intelligent reflecting surface-aided wireless communications:a tutorial[J]. IEEE Transactions on Communications, 2021,69(5): 3313-3351.
[27]	鲁磊 . 全球首颗6G试验卫星“电子科技大学号”成功发射[EB]. 2020.
	LU L . The world’s first 6G test satellite “University of Electronic Science and Technology of China” was successfully launched[EB]. 2020.
[28]	宫碧莹 . 我国5G基站超300万个！今年前7个月通信业运行稳健[EB]. 2023.
	GONG B Y . There are more than 3 million 5G base stations in China! The communications industry performed steadily in the first seven months of this year[EB]. 2023.
[29]	HAN C , BICEN A , AKYILDIZ I F . Multi-ray channel modeling and wideband characterization for wireless communications in the terahertz band[J]. IEEE Transactions on Wireless Communications, 2015,14(5): 2402-2412.
[30]	HAN C , CHEN Y . Propagation modeling for wireless communications in the terahertz band[J]. IEEE Communications Magazine, 2018,56(6): 96-101.
[31]	LIN C , LI G Y . Distance-aware multi-carrier indoor terahertz communications with antenna array selection[C]// Proceedings of 2014 IEEE 25th Annual International Symposium on Personal,Indoor,and Mobile Radio Communication (PIMRC). Piscataway:IEEE Press, 2015: 522-526.
[32]	LIN C , LI G Y . Adaptive beamforming with resource allocation for distance-aware multi-user indoor terahertz communications[J]. IEEE Transactions on Communications, 2015,63(8): 2985-2995.
[33]	KüRNER T , PRIEBE S . Towards THz communications - status in research,standardization and regulation[J]. Journal of Infrared,Millimeter,and Terahertz Waves, 2014,35(1): 53-62.
[34]	PENG B L , JIAO Q , KüRNER T . Angle of arrival estimation in dynamic indoor THz channels with Bayesian filter and reinforcement learning[C]// Proceedings of 2016 24th European Signal Processing Conference (EUSIPCO). Piscataway:IEEE Press, 2016: 1975-1979.
[35]	GAO X Y , DAI L L , ZHANG Y ,et al. Fast channel tracking for terahertz beamspace massive MIMO systems[J]. IEEE Transactions on Vehicular Technology, 2017,66(7): 5689-5696.
[36]	DI B Y , ZHANG H L , LI L L ,et al. Practical hybrid beamforming with finite-resolution phase shifters for reconfigurable intelligent surface based multi-user communications[J]. IEEE Transactions on Vehicular Technology, 2020,69(4): 4565-4570.
[37]	REN H , LI L X , XU W J ,et al. Machine learning-based hybrid precoding with robust error for UAV mmWave massive MIMO[C]// Proceedings of ICC 2019 - 2019 IEEE International Conference on Communications (ICC). Piscataway:IEEE Press, 2019: 1-6.
[38]	CHEN Z , MA X Y , ZHANG B ,et al. A survey on terahertz communications[J]. China Communications, 2019,16(2): 1-35.
[39]	NIE S , JORNET J M , AKYILDIZ I F . Intelligent environments based on ultra-massive mimo platforms for wireless communication in millimeter wave and terahertz bands[C]// Proceedings of ICASSP 2019 - 2019 IEEE International Conference on Acoustics,Speech and Signal Processing (ICASSP). Piscataway:IEEE Press, 2019: 7849-7853.
[40]	HAN C , JORNET J M , AKYILDIZ I . Ultra-massive MIMO channel modeling for graphene-enabled terahertz-band communications[C]// Proceedings of 2018 IEEE 87th Vehicular Technology Conference (VTC Spring). Piscataway:IEEE Press, 2018: 1-5.
[41]	WEN C K , SHIH W T , JIN S . Deep learning for massive MIMO CSI feedback[J]. IEEE Wireless Communications Letters, 2018,7(5): 748-751.
[42]	NING B , TIAN Z , MEI W ,et al. Beamforming technologies for ultra-massive MIMO in terahertz communications[J]. arXiv preprint, 2021,arXiv:2107.03032.
[43]	冯瑞, 王承祥, 郑一 ,等. 6G 超大规模天线信道波束域传播特性分析[J]. 电波科学学报, 2023,38(1): 35-43.
	FENG R , WANG C X , ZHENG Y ,et al. Beam domain propagation characteristics analysis of 6G ultra-massive MIMO channels[J]. Chinese Journal of Radio Science, 2023,38(1): 35-43.
[44]	丁瑞, 钱晓涵, 刘道华 ,等. 超大规模 MIMO 信道测量建模研究综述[J]. 电讯技术, 2022,62(7): 1014-1022.
	DING R , QIAN X H , LIU D H ,et al. Survey of channel measurement and modeling for extra-large scale massive MIMO[J]. Telecommunication Engineering, 2022,62(7): 1014-1022.
[45]	孙蕊蕊, 韩瑜, 金石 ,等. 低复杂度超大规模 MIMO 无线传输设计研究[J]. 电信科学, 2023,39(9): 87-96.
	SUN R R , HAN Y , JIN S ,et al. Research on low-complexity XL-MIMO wireless transmission design[J]. Telecommunications Science, 2023,39(9): 87-96.
[46]	廖勇, 杨植景, 李雪 . 人工智能在 6G空口物理层的潜在应用[J]. 北京邮电大学学报, 2022,45(6): 21-30.
	LIAO Y , YANG Z J , LI X . Research progress of potential applications of AI in 6 G air interface physical layer[J]. Journal of Beijing University of Posts and Telecommunications, 2022,45(6): 21-30.
[47]	YANG W N , LI M , LIU Q . A practical channel estimation strategy for XL-MIMO communication systems[J]. IEEE Communications Letters, 2023,27(6): 1580-1583.
[48]	DARDARI D . Communicating with large intelligent surfaces:fundamental limits and models[J]. IEEE Journal on Selected Areas in Communications, 2020,38(11): 2526-2537.
[49]	ZHANG X Q , SHAO R W , XU H ,et al. Electromagnetic effective degree of freedom of MIMO system in nonfree space[C]// Proceedings of 2022 International Applied Computational Electromagnetics Society Symposium (ACES-China). Piscataway:IEEE Press, 2023: 1-3.
[50]	WEI L , HUANG C W , ALEXANDROPOULOS G C ,et al. Multi-user holographic MIMO surfaces:channel modeling and spectral efficiency analysis[J]. IEEE Journal of Selected Topics in Signal Processing, 2022,16(5): 1112-1124.
[51]	NING B Y , TIAN Z B , MEI W D ,et al. Beamforming technologies for ultra-massive MIMO in terahertz communications[J]. IEEE Open Journal of the Communications Society, 2023(4): 614-658.
[52]	BASAR E , YILDIRIM I . Reconfigurable intelligent surfaces for future wireless networks:a channel modeling perspective[J]. IEEE Wireless Communications, 2021,28(3): 108-114.
[53]	?ZDOGAN ? , BJ?RNSON E , LARSSON E G . Intelligent reflecting surfaces:physics,propagation,and pathloss modeling[J]. IEEE Wireless Communications Letters, 2020,9(5): 581-585.
[54]	WEI X H , SHEN D C , DAI L L . Channel estimation for RIS assisted wireless communications—part II:an improved solution based on double-structured sparsity[J]. IEEE Communications Letters, 2021,25(5): 1403-1407.
[55]	LIU C , LIU X M , NG D W K ,et al. Deep residual learning for channel estimation in intelligent reflecting surface-assisted multi-user communications[J]. IEEE Transactions on Wireless Communications, 2022,21(2): 898-912.
[56]	GUO H Y , LIANG Y C , CHEN J ,et al. Weighted sum-rate maximization for reconfigurable intelligent surface aided wireless networks[J]. IEEE Transactions on Wireless Communications, 2020,19(5): 3064-3076.
[57]	ZHANG S W , ZHANG R . Capacity characterization for intelligent reflecting surface aided MIMO communication[J]. IEEE Journal on Selected Areas in Communications, 2020,38(8): 1823-1838.
[58]	ZHANG Z J , DAI L L . A joint precoding framework for wideband reconfigurable intelligent surface-aided cell-free network[J]. IEEE Transactions on Signal Processing, 2021,69: 4085-4101.
[59]	NING B Y , CHEN Z , CHEN W R ,et al. Terahertz multi-user massive MIMO with intelligent reflecting surface:beam training and hybrid beamforming[J]. IEEE Transactions on Vehicular Technology, 2021,70(2): 1376-1393.
[60]	SIRIWARDHANA Y , PORAMBAGE P , LIYANAGE M ,et al. AI and 6G security:opportunities and challenges[C]// Proceedings of 2021 Joint European Conference on Networks and Communications ＆ 6G Summit (EuCNC/6G Summit). Piscataway:IEEE Press, 2021: 616-621.
[61]	YANG N , WANG L F , GERACI G ,et al. Safeguarding 5G wireless communication networks using physical layer security[J]. IEEE Communications Magazine, 2015,53(4): 20-27.
[62]	刘杨, 彭木根 . 6G 内生安全:体系结构与关键技术[J]. 电信科学, 2020,36(1): 11-20.
	LIU Y , PENG M G . 6G endogenous security:architecture and key technologies[J]. Telecommunications Science, 2020,36(1): 11-20.
[63]	NGUYEN V L , LIN P C , CHENG B C ,et al. Security and privacy for 6G:a survey on prospective technologies and challenges[J]. IEEE Communications Surveys ＆ Tutorials, 2021,23(4): 2384-2428.
[64]	黄开枝, 金梁, 钟州 . 5G物理层安全技术:以通信促安全[J]. 中兴通讯技术, 2019,25(4): 43-49.
	HUANG K Z , JIN L , ZHONG Z . 5G physical layer security technology:enhancing security by communication[J]. ZTE Technology Journal, 2019,25(4): 43-49.
[65]	MAENG S J , YAPICI Y , GüVEN? ? , ,et al. Precoder design for physical-layer security and authentication in massive MIMO UAV communications[J]. IEEE Transactions on Vehicular Technology, 2022,71(3): 2949-2964.
[66]	PETROV V , MOLTCHANOV D , JORNET J M ,et al. Exploiting multipath terahertz communications for physical layer security in beyond 5G networks[C]// Proceedings of IEEE INFOCOM 2019 - IEEE Conference on Computer Communications Workshops (INFOCOM WKSHPS). Piscataway:IEEE Press, 2019: 865-872.
[67]	HONG S , PAN C H , REN H ,et al. Artificial-noise-aided secure MIMO wireless communications via intelligent reflecting surface[J]. IEEE Transactions on Communications, 2020,68(12): 7851-7866.
[68]	YU J B , HU A Q , LI G Y ,et al. A robust RF fingerprinting approach using multisampling convolutional neural network[J]. IEEE Internet of Things Journal, 2019,6(4): 6786-6799.
[69]	金梁, 楼洋明, 孙小丽 ,等. 6G无线内生安全理念与构想[J]. 中国科学(信息科学), 2023,53(2): 344-364.
	JIN L , LOU Y M , SUN X L ,et al. The concept and conception of 6G wireless endogenous security[J]. Scientia Sinica (Informationis), 2023,53(2): 344-364.
[70]	ZHANG J Q , RAJENDRAN S , SUN Z ,et al. Physical layer security for the Internet of things:authentication and key generation[J]. IEEE Wireless Communications, 2019,26(5): 92-98.
[71]	HU X Y , JIN L , HUANG K Z ,et al. Intelligent reflecting surface-assisted secret key generation with discrete phase shifts in static environment[J]. IEEE Wireless Communications Letters, 2021,10(9): 1867-1870.
[72]	厉东明, 杨旋 . 6G物理层安全技术综述[J]. 移动通信, 2022,46(6): 60-63.
	LI D M , YANG X . Surveys on physical-layer security for 6G[J]. Mobile Communications, 2022,46(6): 60-63.
[73]	MITEV M , CHORTI A , POOR H V ,et al. What physical layer security can do for 6G security[J]. IEEE Open Journal of Vehicular Technology, 2023(4): 375-388.
[74]	ZHANG Z , ZHANG C S , JIANG C J ,et al. Improving physical layer security for reconfigurable intelligent surface aided NOMA 6G networks[J]. IEEE Transactions on Vehicular Technology, 2021,70(5): 4451-4463.
[75]	LI Y N , WANG W J , WANG J H ,et al. Fast-convolution multicarrier based frequency division multiple access[J]. Science China Information Sciences, 2019,62(8): 1-17.
[76]	李华, 郝诗雅, 巩彩红 ,等. 面向 6G 的新型多址与波形技术[J]. 电信科学, 2022,38(10): 36-45.
	LI H , HAO S Y , GONG C H ,et al. New multiple access and waveform technology for 6G[J]. Telecommunications Science, 2022,38(10): 36-45.
[77]	PING L , LIU L H , WU K Y ,et al. Interleave division multiple-access[J]. IEEE Transactions on Wireless Communications, 2006,5(4): 938-947.
[78]	Qualcomm Incorporated. May:R1- 164688[R]. 2016.
[79]	NIKOPOUR H , BALIGH H . Sparse code multiple access[C]// Proceedings of 2013 IEEE 24th Annual International Symposium on Personal,Indoor,and Mobile Radio Communications (PIMRC). Piscataway:IEEE Press, 2013: 332-336.
[80]	DAI X M , ZHANG Z Y , BAI B M ,et al. Pattern division multiple access:a new multiple access technology for 5G[J]. IEEE Wireless Communications, 2018,25(2): 54-60.
[81]	王钢, 许尧, 周若飞 ,等. 无线网络中的功率域非正交多址接入技术[J]. 无线电通信技术, 2019,45(4): 329-336.
	WANG G , XU Y , ZHOU R F ,et al. Power domain non-orthogonal multiple access technology for wireless communication networks[J]. Radio Communications Technology, 2019,45(4): 329-336.
[82]	OZYURT S , KUCUR O . Quadrature NOMA:a low-complexity multiple access technique with coordinate interleaving[J]. IEEE Wireless Communications Letters, 2020,9(9): 1452-1456.
[83]	周天航, 杨闯, 刘子乐 ,等. 太赫兹无线组网:原理、现状与挑战[J]. 电信科学, 2021,37(6): 33-44.
	ZHOU T H , YANG C , LIU Z L ,et al. Terahertz wireless networking:principles,status and challenges[J]. Telecommunications Science, 2021,37(6): 33-44.
[84]	SIEGEL P H . Terahertz technology[J]. IEEE Transactions on Microwave Theory and Techniques, 2002,50(3): 910-928.
[85]	中国移动通信有限公司研究院. 2030+技术趋势白皮书[R]. 2020.
	Research Institution of China Mobile. 2030+ technology trend[R]. 2020.
[86]	张振宇, 邹润民, 惠峥 ,等. 面向6G超大规模MIMO系统分布式基带处理信号检测算法[J]. 移动通信, 2021,45(4): 16-20.
	ZHANG Z Y , ZOU R M , HUI Z ,et al. Signal detection algorithm for decentralized baseband processing in 6G ultra-massive MIMO systems[J]. Mobile Communications, 2021,45(4): 16-20.
[87]	AMIRI A , REZAIE S , MANCHON C N ,et al. Distributed receiver processing for extra-large MIMO arrays:a message passing approach[J]. IEEE Transactions on Wireless Communications, 2022,21(4): 2654-2667.
[88]	SABBAGH R , PAN C H , WANG J Z . Pilot allocation and sum- rate analysis in cell-free massive MIMO systems[C]// Proceedings of 2018 IEEE International Conference on Communications (ICC). Piscataway:IEEE Press, 2018: 1-6.
[89]	LIU P , LUO K , CHEN D ,et al. Spectral efficiency analysis of cell-free massive MIMO systems with zero-forcing detector[J]. IEEE Transactions on Wireless Communications, 2020,19(2): 795-807.
[90]	BASHAR M , CUMANAN K , BURR A G ,et al. Cell-free massive MIMO with limited backhaul[C]// Proceedings of 2018 IEEE International Conference on Communications (ICC). Piscataway:IEEE Press, 2018: 1-7.
[91]	安建成 . 面向可重构智能表面的信道估计与被动波束成形技术研究[D]. 成都:电子科技大学, 2021.
	AN J C . Research on channel estimation and passive beamforming for reconfigurable intelligent surface[D]. Chengdu:University of Electronic Science and Technology of China, 2021.
[92]	TANG W K , CHEN M Z , CHEN X Y ,et al. Wireless communications with reconfigurable intelligent surface:path loss modeling and experimental measurement[J]. IEEE Transactions on Wireless Communications, 2021,20(1): 421-439.
[93]	DING Z G , SCHOBER R , POOR H V . On the impact of phase shifting designs on IRS-NOMA[J]. IEEE Wireless Communications Letters, 2020,9(10): 1596-1600.
[94]	DING Z G , VINCENT POOR H . A simple design of IRS-NOMA transmission[J]. IEEE Communications Letters, 2020,24(5): 1119-1123.
[95]	MISHRA D , JOHANSSON H . Channel estimation and low-complexity beamforming design for passive intelligent surface assisted MISO wireless energy transfer[C]// Proceedings of ICASSP 2019-2019 IEEE International Conference on Acoustics,Speech and Signal Processing (ICASSP). Piscataway:IEEE Press, 2019: 4659-4663.
[96]	WEI X H , SHEN D C , DAI L L . Channel estimation for RIS assisted wireless communications—part I:fundamentals,solutions,and future opportunities[J]. IEEE Communications Letters, 2021,25(5): 1398-1402.
[97]	YU X H , XU D F , SCHOBER R . Optimal beamforming for MISO communications via intelligent reflecting surfaces[C]// Proceedings of 2020 IEEE 21st International Workshop on Signal Processing Advances in Wireless Communications (SPAWC). Piscataway:IEEE Press, 2020: 1-5.
[98]	WU Q Q , ZHANG R . Intelligent reflecting surface enhanced wireless network via joint active and passive beamforming[J]. IEEE Transactions on Wireless Communications, 2019,18(11): 5394-5409.
[99]	HUANG C W , ZAPPONE A , ALEXANDROPOULOS G C ,et al. Reconfigurable intelligent surfaces for energy efficiency in wireless communication[J]. IEEE Transactions on Wireless Communications, 2019,18(8): 4157-4170.
[100]	陈健锋, 崔苗, 张广驰 ,等. 双智能反射平面辅助无线携能通信系统的安全通信优化[J]. 电信科学, 2022,38(1): 47-60.
	CHEN J F , CUI M , ZHANG G C ,et al. Secure communication optimization for double-IRS assisted SWIPT system[J]. Telecommunications Science, 2022,38(1): 47-60.
[101]	景小荣, 宋振远, 罗悦 ,等. IRS与人工噪声辅助的MIMO通信系统物理层安全方案设计[J]. 系统工程与电子技术, 2022,44(10): 3266-3274.
	JING X R , SONG Z Y , LUO Y ,et al. Design of physical layer safety scheme aided with IRS and artificial noise for MIMO communication systems[J]. Systems Engineering and Electronics, 2022,44(10): 3266-3274.

性能指标	6G	5G
峰值速率/（Gbit·s^-1）	＞100	10
用户体验速率/（Gbit·s^-1）	＞10	1
流量密度/（Tbit·(s·km²) ^-1）	＞100	10
连接数密度/（百万·km^-2）	＞10	1
时延	＜1 ms	毫秒级
移动性/（km·h^-1）	＞1 000	350
频谱效率	大于5G的3倍	4G的3～5倍
能量效率	大于5G的10倍	4G的1 000倍
覆盖率	＞99%	70%左右
可靠性	＞99.999%	99.9%左右
定位精度	厘米级	米级
接收机敏感度/dBm	＜-130	-120左右

研究方向	当前主要解决方案
信道建模与信道测量	当前 THz 信道模型主要分为两类：确定性信道模型^[29-30]及统计信道模型^[31-32]。文献[29-30]利用射线追踪技术探究了在视距与非视距路径下的THz信道，并对其进行了简单描述。文献[31-32]根据实验数据，并结合MIMO技术，给出了经典的Saleh-Valenzuela模型
	文献[33]利用射线追踪方法对具体场景的THz信道进行了测量
信道估计	文献[34]通过建立三维坐标对到达角（angle of arrival，AoA）特性进行表征，并利用贝叶斯滤波器对移动用户的AoA进行估计
	文献[35]针对THz移动通信场景，基于THz信道稀疏特性以及移动用户的物理空间特征，利用波束空间技术，对用户的波束空间信道进行信道估计与追踪
覆盖增强	THz信号存在较大的路径损耗以及较差的穿透能力，文献[36]提出利用IRS辅助THz进行通信以扩大THz频段的有效通信范围
	文献[37]提出利用无人驾驶飞行器（unmanned aerial vehicle，UAV）作为空中基站/中继，建立与地面用户的视距连接，从而实现THz通信系统的超可靠低时延通信
调制解调	THz调制解调技术可以助力实现数据在THz频段的远距离传输。THz无线通信的调制方式主要分为直接调制^[38]、太赫兹混频调制和太赫兹光电调制^[17]

研究方向	当前主要解决方案
信道建模	文献[43]对典型城市环境下的超大规模 MIMO 无线信道在波束域的传播特性进行了信道测量实验与数据分析工作，重点分析了超大规模MIMO在波束域的描述以及波束内与波束间的新信道特性
	文献[44-45]对当前文献中与超大规模 MIMO 相关的信道测量、特征分析与建模方法进行综述，并对超大规模MIMO因天线维度增加所表现出来的信道新特性及新模型架构进行了详细阐述
信道估计	文献[46]给出了人工智能技术在6G空口物理层方面的应用，并总结了机器学习技术使能的6G空口物理层大规模MIMO信道估计算法
	文献[47]研究了一种考虑混合场（即远场与近场都考虑）信道模型的实用策略，以实现超大规模MIMO通信系统的有效信道估计，该策略首先提供了一个准则来确定混合场比例，然后根据比例获得相应的信道估计结果
性能分析	自由度（degrees of freedom，DoF）和有效自由度（effective degrees of freedom，EDoF）是测量超大规模MIMO系统性能的重要指标，文献[48]基于二维采样理论给出DoF的近似表达式，文献[49]则给出了EDoF的近似表达式
波束成形/预编码	对于超大规模 MIMO 而言，设计轻量级信号处理方案来降低超大维度信号计算复杂度至关重要。文献[50]提出了一种低计算量的破零预编码方案，该方案运用诺依曼级数展开代替矩阵求逆，为轻量级信号处理设计提供了重要启示
	文献[51]对THz超大规模MIMO系统的相关波束成形技术进行了总结，并提出了基于波束训练的THz频段超大规模MIMO波束成形策略

研究方向	当前主要解决方案
信道建模	文献[52]重点介绍了开源软件SimRIS信道仿真器v2.0版本在模拟阵列响应、元件增益、实际路径损耗和阴影模型等方面的优势，并阐明了IRS在未来无线系统中的潜在用例
	文献[53]利用物理光学技术推导出IRS远场路径损耗，并解释了IRS元件单独充当漫散射体时可以以一定的波束宽度在所需方向上联合波束形成信号的原因
信道估计	文献[54]基于基站-IRS-用户间级联信道的稀疏特性，提出了一种双结构正交匹配追踪算法，从而实现基站-IRS-用户级联信道准确估计
	文献[55]将IRS信道估计问题建模为去噪问题，并采用深度残差学习方法隐式学习残差噪声，以便从基于噪声的接收信号中恢复信道系数
性能分析	文献[56]提出在IRS辅助多用户多输入单输出下行链路通信系统中，通过联合设计基站波束成形与IRS相移矩阵实现所有用户权重和速率最大化
	文献[57]通过联合优化IRS反射系数和MIMO传输协方差矩阵来表征IRS辅助点对点MIMO通信系统的基本容量限制
波束成形/预编码	文献[58]提出在宽带IRS辅助无线通信网络中对基站与IRS的预编码进行联合设计以实现最大化网络容量，并采用改进型交替优化算法来解决预编码联合设计存在的非凸性和高复杂性问题
	文献[59]探究了THz通信场景中大规模MIMO与IRS联合部署下的低成本模拟域数字域混合波束成形方案

研究方向	当前主要解决方案
功率分配的NOMA	文献[81]梳理了功率域非正交多址技术的基本原理，并对当前功率域NOMA的相关研究方向及实际应用中面临的难题进行了归纳讨论
基于交织的NOMA	文献[77]对交织分割多址进行了梳理，并基于密度演化技术提出了一种半解析技术来估计系统的误码率，其可以快速且相对准确地预测交织分割多址方案的性能
	文献[82]提出一种基于坐标交织的低复杂度NOMA方法，该方法通过正确利用两个正交载波，可以同时实现性能增强和连续干扰消除操作数量的减少
基于扰码的NOMA	文献[78]提出了基于扰码的资源扩展多址接入技术，该技术通过为用户分配不同的签名，在同一时间同一频率上发送，以此分离用户
基于扩频的NOMA	文献[79]提出了低复杂度稀疏码多址技术，该技术将比特到正交振幅调制（quadrature amplitude modulation， QAM）符号映射过程和扩频过程组合在一起，并将输入比特直接映射为稀疏码多址码本集的多维码字
图样分割多址接入	文献[80]对基于稀疏扩频的图样分割多址接入进行了详细介绍，通过对发射机和接收机的联合设计，可以实现基于低复杂度的连续干扰消除的多用户检测，有效提高系统性能