[2] [2] GIORDANI M,ZORZI M. Non-terrestrial networks in the 6G era: challenges and opportunities[J]. IEEE Network, 2021,35(2): 244-251. doi:10.1109/MNET.011.2000493.

[3] [3] CUI Tiejun, QI Meiqing, WAN Xiang, et al. Coding metamaterials, digital metamaterials and programmable metamaterials[J]. Light:Science & Applications, 2014(3):218. doi:10.1038/lsa.2014.99.

[4] [4] TEKBIYIK K,KURT G K,EKTI A R,et al. Reconfigurable intelligent surfaces empowered THz communication in LEO satellite networks[J]. IEEE Access, 2022(10):121957-121969. doi:10.1109/ACCESS.2022.3223086.

[5] [5] ROTSHILD D,ABRAMOVICH A. Wideband reconfigurable entire Ku-band metasurface beam-steerable reflector for satellite communications[J]. IET Microwaves Antennas & Propagation, 2019,13(3):334-339. doi:10.1049/iet-map.2018.5417.

[6] [6] MATTHIESEN B,BJORNSON E,DE-CARVALHO E, et al. Intelligent reflecting surface operation under predictable receiver mobility: a continuous time propagation model[J]. IEEE Wireless Communications Letters, 2020, 10(2): 216-220. doi: 10.1109/LWC.2020.3024781.

[7] [7] KHAN W U, LAGUNAS E, MAHMOOD A, et al. Energy-efficient RIS-enabled NOMA communication for 6G LEO satellite networks[C]// 2023 IEEE the 97th Vehicular Technology Conference(VTC2023-Spring). Florence,Italy:IEEE, 2023: 1-6. doi: 10.1109/VTC2023-Spring57618.2023.10200793.

[8] [8] WU Qingqing,ZHANG Rui. Intelligent reflecting surface enhanced wireless network via joint active and passive beamforming[J]. IEEE Transactions on Wireless Communications, 2019,18(11):5394-5409. doi:10.1109/TWC.2019.2936025.

[9] [9] HUANG Chong,CHEN Gaojie,ZHOU Yitong,et al. Deep learning empowered secure RIS-Assisted non-terrestrial relay networks[C]// 2022 IEEE the 96th Vehicular Technology Conference(VTC2022-Fall). London,United Kingdom:IEEE, 2022:1-5. doi:10.1109/VTC2022-Fall57202.2022.10012808.

[10] [10] XU Sai,LIU Jiajia,CAO Yurui,et al. Intelligent reflecting surface enabled secure cooperative transmission for satellite-terrestrial integrated networks[J]. IEEE Transactions on Vehicular Technology, 2021,70(2):2007-2011. doi:10.1109/TVT.2021.3055965.

[11] [11] TIAN Xiaowen, GONZALEZ-PRELCIC N, SHIMIZU T. Enabling NLoS LEO satellite communications with reconfigurable intelligent surfaces[DB/OL]. (2022-05-31)[2023-12-21]. https://arxiv.org/abs/2205.15528.

[12] [12] DONG Hao, HUA Cunqing, LIU Lingya, et al. Intelligent reflecting surface-aided integrated terrestrial-satellite networks[J]. IEEE Transactions on Wireless Communications, 2023,22(4):2507-2522. doi:10.1109/TWC.2022.3212049.

[13] [13] ZHENG Ziyuan,JING Wenpeng,LU Zhaoming,et al. RIS-enhanced LEO satellite communication:joint passive beamforming and orientation optimization[C]// 2022 IEEE Globecom Workshops(GC Wkshps). Rio de Janeiro,Brazil:IEEE, 2022: 874-879. doi: 10.1109/GCWkshps56602.2022.10008708.

[14] [14] ZHENG Beixiong,LIN Shaoe,ZHANG Rui. Intelligent reflecting surface-aided LEO satellite communication:cooperative passive beamforming and distributed channel estimation[J]. IEEE Journal on Selected Areas in Communications, 2022, 40(10): 3057-3070. doi:10.1109/JSAC.2022.3196119.

[15] [15] TEKBIYIK K, KURT G K, EKTI A R, et al. Graph attention networks for channel estimation in RIS-assisted satellite IoT communications[DB/OL]. (2021-04-01)[2023-12-21]. https://arxiv.org/abs/2104.00735.

[16] [16] ZHANG Zijian,DAI Linglong,CHEN Xibi,et al. Active RIS vs. passive RIS:which will prevail in 6G?[J]. IEEE Transactions on Communications, 2023,71(3):1707-1725. doi:10.1109/TCOMM.2022.3231893.

[17] [17] LIU Yuanwei, MU Xidong, XU Jiaqi, et al. STAR: simultaneous transmission and reflection for 360° coverage by intelligent surfaces[J]. IEEE Wireless Communications, 2021,28(6):102-109. doi:10.1109/MWC.001.2100191.

[18] [18] TANG Wankai,CHEN Mingzheng,CHEN Xiangyu, 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. doi: 10.1109/TWC.2020.3024887.

[21] [21] LEE J, SHIN W, LEE J. Performance analysis of IRS-assisted LEO satellite communication systems[C]// 2021 International Conference on Information and Communication Technology Convergence(ICTC). Jeju Island,Korea:IEEE, 2021: 323-325. doi: 10.1109/ICTC52510.2021.9621010.

[22] [22] XIONG Baiping, ZHANG Zaichen,JIANG Hao, et al. A statistical MIMO channel model for reconfigurable intelligent surface assisted wireless communications[J]. IEEE Transactions on Communications, 2022, 70(2): 1360-1375. doi: 10.1109/TCOMM. 2021.3129926.

[23] [23] AZIZ M A,RAHMAN M H,SEJAN M A S,et al. Spectral efficiency improvement using Bi-Deep learning model for IRS-Assisted MU-MISO communication system[J]. Sensors, 2023,23(18):7793. doi:10.3390/s23187793.

[24] [24] TEKBIYIK K,KURT G K,YANIKOMEROGLU H. Energy-efficient RIS-assisted satellites for IoT networks[J]. IEEE Internet of Things Journal, 2022,9(16):14891-14899. doi:10.1109/JIOT.2021.3112881.

[25] [25] ALEXANDROPOULOS G C,VLACHOS E. A hardware architecture for reconfigurable intelligent surfaces with minimal active elements for explicit channel estimation[C]// 2020 IEEE International Conference on Acoustics,Speech and Signal Processing(ICASSP). Barcelona,Spain:IEEE, 2020:9175-9179. doi:10.1109/ICASSP40776.2020.9053976.

[26] [26] MISHRA D, JOHANSSON H. Channel estimation and low-complexity beamforming design for passive intelligent surface assisted MISO wireless energy transfer[C]// 2019 IEEE International Conference on Acoustics, Speech and Signal Processing (ICASSP). Brighton,UK:IEEE, 2019:4659-4663. doi:10.1109/ICASSP.2019.8683663.

[27] [27] ZHENG Beixiong,ZHANG Rui. Intelligent reflecting surface-enhanced OFDM: channel estimation and reflection optimization[J]. IEEE Wireless Communications Letters, 2020,9(4):518-522. doi:10.1109/LWC.2019.2961357.

[28] [28] WANG Peilan, FANG Jun, DUAN Huiping, et al. Compressed channel estimation for intelligent reflecting surface-assisted millimeter wave systems[J]. IEEE Signal Processing Letters, 2020(27):905-909. doi:10.1109/LSP.2020.2998357.

[29] [29] HU Chen,DAI Linglong,HAN Shuangfeng,et al. Two-timescale channel estimation for reconfigurable intelligent surface aided wireless communications[J]. IEEE Transactions on Communications, 2021,69(11):7736-7747. doi:10.1109/TCOMM.2021. 3072729.

[30] [30] SHEN Decai, DAI Linglong. Dimension reduced channel feedback for reconfigurable intelligent surface aided wireless communications[J]. IEEE Transactions on Communications, 2021,69(11):7748-7760. doi:10.1109/TCOMM.2021.3100428.

[31] [31] YANG Yifei, ZHENG Beixiong, ZHANG Shuowen, et al. Intelligent reflecting surface meets OFDM: protocol design and rate maximization[J]. IEEE Transactions on Communications, 2020,68(7):4522-4535. doi:10.1109/TCOMM.2020.2981458.

[32] [32] TAHA A,ALRABEIAH M,ALKHATEEB A. Enabling large intelligent surfaces with compressive sensing and deep learning[J]. IEEE Access, 2021(9):44304-44321. doi:10.1109/ACCESS.2021.3064073.

[33] [33] HUANG Chongwen,MO Ronghong,YUEN C. Reconfigurable intelligent surface assisted multiuser MISO systems exploiting deep reinforcement learning[J]. IEEE Journal on Selected Areas in Communications, 2020, 38(8): 1839-1850. doi: 10.1109/JSAC. 2020.3000835.

[36] [36] BJORNSON E, WYMEERSCH H, MATTHIESEN B, et al. Reconfigurable intelligent surfaces: a signal processing perspective with wireless applications[J]. IEEE Signal Processing Magazine, 2022,39(2):135-158. doi:10.1109/MSP.2021.3130549.