Similar articles

Abstract: With the commercialization of fifth generation networks worldwide, research into sixth generation (6G) networks has been launched to meet the demands for high data rates and low latency for future services. A wireless propagation channel is the transmission medium to transfer information between the transmitter and the receiver. Moreover, channel properties determine the ultimate performance limit of wireless communication systems. Thus, conducting channel research is a prerequisite to designing 6G wireless communication systems. In this paper, we first introduce several emerging technologies and applications for 6G, such as terahertz communication, industrial Internet of Things, space-air-ground integrated network, and machine learning, and point out the developing trends of 6G channel models. Then, we give a review of channel measurements and models for the technologies and applications. Finally, the outlook for 6G channel measurements and models is discussed.

面向6G的信道测量与建模：现状与展望

[1]3GPP, 2018. Study on Channel Model for Frequencies from 0.5 to 100 GHz. Technical Report TR 38.901, 3GPP.

[2]3GPP, 2019. Study on Evaluation Methodology of New Vehicle-to-Everything (V2X) Use Cases for LTE and NR. Technical Report TR 37.885, 3GPP.

[3]5G-ACIA, 2018. LS on Channel Model for Indoor Industrial Scenarios. Proposal RP-181521, 5G-ACIA.

[4]Ai Y, Cheffena M, Li Q, 2015. Radio frequency measurements and capacity analysis for industrial indoor environments. Proc 9^th European Conf on Antennas and Propagation, p.1-5.

[5]Ali E, Ismail M, Nordin R, et al., 2017. Beamforming techniques for massive MIMO systems in 5G: overview, classification, and trends for future research. Front Inform Technol Electron Eng, 18(6):753-772.

[6]Almeida JJH, Lopes PB, Akamine C, et al., 2018. An application of neural networks to channel estimation of the ISDB-TB FBMC system. https://arxiv.org/abs/1803.01141

[7]Alpaydin E, 2006. Introduction to Machine Learning. MIT Press, USA.

[8]Al-Hourani A, Kandeepan S, Jamalipour A, 2014. Modeling air-to-ground path loss for low altitude platforms in urban environments. IEEE Global Communications Conf, p.2898-2904.

[9]Al-Saegh AM, Sali A, Mandeep JS, et al., 2017. Channel measurements, characterization, and modeling for land mobile satellite terminals in tropical regions at Ku-band. IEEE Trans Veh Technol, 66(2):897-911.

[10]Arndt D, Ihlow A, Heuberger A, et al., 2011. Antenna diversity for mobile satellite applications: performance evaluation based on measurements. Proc 5^th European Conf on Antennas and Propagation, p.3729-3733.

[11]Baum LE, Petrie T, 1966. Statistical inference for probabilistic functions of finite state Markov chains. Ann Math Stat, 37(6):1554-1563.

[12]Berardinelli G, Mahmood NH, Rodriguez I, et al., 2018. Beyond 5G wireless IRT for Industry 4.0: design principles and spectrum aspects. IEEE Globecom Workshops, p.1-6.

[13]Bishop CM, 2006. Pattern Recognition and Machine Learning. Springer, New York.

[14]Cerwall P, Jonsson P, Möller R, et al., 2015. Ericsson Mobility Report. Telefonaktiebolaget LM Ericsson, Stockholm, Sweden.

[15]Chen JJ, Yin XF, Cai XS, et al., 2017. Measurement-based massive MIMO channel modeling for outdoor LoS and NLoS environments. IEEE Access, 5:2126-2140.

[16]Chen XB, Tian L, Tang P, et al., 2016. Modelling of human body shadowing based on 28 GHz indoor measurement results. IEEE 84^th Vehicular Technology Conf, p.1-5.

[17]Chen XF, Han Z, Zhang HG, et al., 2018. Wireless resource scheduling in virtualized radio access networks using stochastic learning. IEEE Trans Mob Comput, 17(4):961-974.

[18]Cheng CL, Kim S, Zajić A, 2017. Comparison of path loss models for indoor 30 GHz, 140 GHz, and 300 GHz channels. Proc 11^th European Conf on Antennas and Propagation, p.716-720.

[19]Cheng X, Li YR, 2019. A 3-D geometry-based stochastic model for UAV-MIMO wideband nonstationary channels. IEEE Int Things J, 6(2):1654-1662.

[20]Cisco, 2019. Cisco Visual Networking Index: Global Mobile Data Traffic Forecast Update, 2017-2022 White Paper. Cisco Systems, Inc., CA, USA.

[21]CMCC, BUPT, 2018a. New Measurements and Modelling on Fast Fading in IIOT Scenarios. Proposal RP-1904743, 3GPP.

[22]CMCC, BUPT, 2018b. New Measurements and Modelling on Pathloss in IIOT Scenarios. Proposal RP-1904742, 3GPP.

[23]CMRI, 2019. The Outlook and Demand Report for 2030+. China Mobile Resesrch Institute, Bejing (in Chinese). https://cmri.chinamobile.com/news/5985.html [Accessed on Jan. 4, 2020].

[24]Cortes C, Vapnik V, 1995. Support-vector networks. Mach Learn, 20(3):273-297.

[25]Dahlman E, Mildh G, Parkvall S, et al., 2014. 5G wireless access: requirements and realization. IEEE Commun Mag, 52(12):42-47.

[26]Dai L, Zhang H, Zhuang Y, 2018. Propagation-model-free coverage evaluation via machine learning for future 5G networks. IEEE 29^th Annual Int Symp on Personal, Indoor and Mobile Radio Communications, p.1-5.

[27]Darak SJ, Zhang HG, Palicot J, et al., 2017. Decision making policy for RF energy harvesting enabled cognitive radios in decentralized wireless networks. Dig Signal Process, 60:33-45.

[28]Dreyfus SE, 2012. Artificial neural networks, back propagation, and the Kelley-Bryson gradient procedure. J Guid Contr Dynam, 13(5):926-928.

[29]Ericsson, 2019a. Summary of Email Discussion on Additional Modelling Components. Proposal RP-1905197, 3GPP.

[30]Ericsson, 2019b. Views on Additional Modelling Components. Proposal RP-1905203, 3GPP.

[31]Feng QX, McGeehan J, Tameh EK, et al., 2006. Path loss models for air-to-ground radio channels in urban environments. IEEE 63^rd Vehicular Technology Conf, p.2901-2905.

[32]Ferrer-Coll J, Ängskog P, Chilo J, et al., 2012. Characterisation of highly absorbent and highly reflective radio wave propagation environments in industrial applications. IET Commun, 6(15):2404-2412.

[33]Freund Y, Schapire R, Abe N, 1999. A short introduction to boosting. J Jpn Soc Artif Intell, 14(5):771-780.

[34]Gao X, Chen Z, Hu Y, 2013. Analysis of unmanned aerial vehicle MIMO channel capacity based on aircraft attitude. WSEAS Trans Inform Sci Appl, 10(2):58-67.

[35]Giordani M, Polese M, Mezzavilla M, et al., 2019. Towards 6G networks: use cases and technologies. https://arxiv.org/abs/1903.12216

[36]Goddemeier N, Wietfeld C, 2015. Investigation of air-to-air channel characteristics and a UAV specific extension to the rice model. IEEE Globecom Workshops, p.1-5.

[37]Goldhirsh J, Vogel W, 1987. Roadside tree attenuation measurements at UHF for land mobile satellite systems. IEEE Trans Antenn Propag, 35(5):589-596.

[38]Goldsmith A, Jafar SA, Jindal N, et al., 2003. Capacity limits of MIMO channels. IEEE J Sel Areas Commun, 21(5):684-702.

[39]Haas E, 2002. Aeronautical channel modeling. IEEE Trans Veh Technol, 51(2):254-264.

[40]Hanssens B, Kshetri SR, Tanghe E, et al., 2018. Measurement-based analysis of dense multipath components in a large industrial warehouse. 12^th European Conf on Antennas and Propagation, p.1-5.

[41]Hess GC, 1980. Land-mobile satellite excess path loss measurements. IEEE Trans Veh Technol, 29(2):290-297.

[42]Hochreiter S, Schmidhuber J, 1997. Long short-term memory. Neur Comput, 9(7):1735-1780.

[43]Holfeld B, Wieruch D, Raschkowski L, et al., 2016. Radio channel characterization at 5.85 GHz for wireless M2M communication of industrial robots. IEEE Wireless Communications and Networking Conf, p.1-7.

[44]Hu BB, Nuss MC, 1995. Imaging with terahertz waves. Opt Lett, 20(16):1716-1718.

[45]Huang C, Huang KW, Wen Y, et al., 2016. A propose of the ISS space-to-space communication system by multiplexing ground mobile communication frequency resources. 6^th Int Conf on Instrumentation & Measurement, Computer, Communication and Control, p.567-569.

[46]Huang HJ, Yang J, Song Y, et al., 2018. Deep learning for super-resolution channel estimation and DOA estimation based massive MIMO system. IEEE Trans Veh Technol, 67(9):8549-8560.

[47]Huawei, HiSilicon, 2018. Preliminary Channel Measurement on Large-Scale Propagation Loss for Indoor Factory Environment. Proposal RP-1904706, 3GPP.

[48]ITU-R, 2013. Attenuation by Atmospheric Gases. Recommendation P.676-10, ITU-R, Geneva, Switzerland.

[49]ITU-R, 2015. {IMT Traffic Estimates for the Years 2020 to 2030. Report M.2370, ITU-R, Geneva, Switzerland.}

[50]ITU-T, 2019. Architectural Framework for Machine Learning in Future Networks Including IMT-2020. Recommendation Y.3172, ITU-T, Geneva, Switzerland.

[51]Jacob M, Priebe S, Dickhoff R, et al., 2012. Diffraction in mm and sub-mm wave indoor propagation channels. IEEE Trans Microw Theory Technol, 60(3):833-844.

[52]Jansen C, Piesiewicz R, Mittleman D, et al., 2008. The impact of reflections from stratified building materials on the wave propagation in future indoor terahertz communication systems. IEEE Trans Antenn Propag, 56(5):1413-1419.

[53]Jansen C, Priebe S, Moller C, et al., 2011. Diffuse scattering from rough surfaces in THz communication channels. IEEE Trans Terahertz Sci Technol, 1(2):462-472.

[54]Jiang CX, Zhang HJ, Ren Y, et al., 2016. Machine learning paradigms for next-generation wireless networks. IEEE Wirel Commun, 24(2):98-105.

[55]Joo EM, Zhou Y, 2009. Theory and Novel Applications of Machine Learning. IntechOpen, London, UK.

[56]Kalman RE, 1960. A new approach to linear filtering and prediction problems. J Bas Eng, 82(1):35-45.

[57]Karedal J, Wyne S, Almers P, et al., 2007. A measurement-based statistical model for industrial ultra-wideband channels. IEEE Trans Wirel Commun, 6(8):3028-3037.

[58]Khalid N, Akan OB, 2016. Wideband THz communication channel measurements for 5G indoor wireless networks. IEEE Int Conf on Communications.

[59]Khawaja W, Guvenc I, Matolak D, 2016. UWB channel sounding and modeling for UAV air-to-ground propagation channels. IEEE Global Communications Conf, p.1-7.

[60]Khawaja W, Guvenc I, Matolak DW, et al., 2019. A survey of air-to-ground propagation channel modeling for unmanned aerial vehicles. IEEE Commun Surv Tutor, 21(3):2361-2391.

[61]Kim S, Zajić AG, 2015. Statistical characterization of 300-GHz propagation on a desktop. IEEE Trans Veh Technol, 64(8):3330-3338.

[62]Lacoste F, Carvalho F, Fontan FP, et al., 2010. MISO and SIMO measurements of the land mobile satellite propagation channel at S-band. Proc 4^th European Conf on Antennas and Propagation, p.1-5.

[63]Lacoste F, Lemorton J, Casadebaig L, et al., 2012. Measurements of the land mobile and nomadic satellite channels at 2.2 GHz and 3.8 GHz. 6^th European Conf on Antennas and Propagation, p.2422-2426.

[64]Lei MY, Zhang JH, Lei T, et al., 2015. 28-GHz indoor channel measurements and analysis of propagation characteristics. IEEE 25^th Annual Int Symp on Personal, Indoor, and Mobile Radio Communication.

[65]Li HH, Li YZ, Zhou SD, et al., 2017. Wireless channel feature extraction via GMM and CNN in the tomographic channel model. J Commun Inform Netw, 2(1):41-51.

[66]Li JZ, Ai B, He RS, et al., 2017. Indoor massive multiple-input multiple-output channel characterization and performance evaluation. Front Inform Technol Electron Eng, 18(6):773-787.

[67]Li W, Zhang JH, Ma XC, et al., 2019. The way to apply machine learning to IoT driven wireless network from channel perspective. China Commun, 16(1):148-164.

[68]Li WZ, Law CL, Dubey VK, et al., 2001. Ka-band land mobile satellite channel model incorporating weather effects. IEEE Commun Lett, 5(5):194-196.

[69]Li Y, Zhao L, Wang H, 2012. A novel mobility model for clustered MANET. 8^th Int Conf on Wireless Communications, Networking and Mobile Computing, p.1-4.

[70]Li YP, Zhang JH, Ma ZY, et al., 2018. Clustering analysis in the wireless propagation channel with a variational Gaussian mixture model. IEEE Trans Big Data, online.

[71]Lin L, Zhu M, 2018. Efficient tracking of moving target based on an improved fast differential evolution algorithm. IEEE Access, 6:6820-6828.

[72]Liu GY, Hou XY, Wang F, et al., 2016. Achieving 3D-MIMO with massive antennas from theory to practice with evaluation and field trial results. IEEE Syst J, 11(1):62-71.

[73]Liu JJ, Shi YP, Fadlullah ZM, et al., 2018. Space-air-ground integrated network: a survey. IEEE Commun Surv Tutor, 20(4):2714-2741.

[74]Liu L, Zhang K, Tao C, et al., 2018. Channel measurements and characterizations for automobile factory environments. 20^th Int Conf on Advanced Communication Technology, p.234-238.

[75]Liu XQ, Chen HH, Chen SY, et al., 2017. Symbol cyclic-shift equalization algorithm—a CP-free OFDM/OFDMA system design. IEEE Trans Veh Technol, 66(1):282-294.

[76]Loo C, 1996. Statistical models for land mobile and fixed satellite communications at Ka band. Proc Vehicular Technology Conf, p.1023-1027.

[77]Lu B, Wang CX, Jie H, et al., 2018. Predicting wireless mmwave massive MIMO channel characteristics using machine learning algorithms. Wirel Commun Mob Comput, 2018:9783863.

[78]Luan FY, Zhang Y, Xiao LM, et al., 2013. Fading characteristics of wireless channel on high-speed railway in hilly terrain scenario. Int J Antenn Propag, 2013:378407.

[79]Luo SP, Polu N, Chen ZX, et al., 2011. RF channel modeling of a WSN testbed for industrial environment. IEEE Radio and Wireless Symp, p.375-378.

[80]Lutz E, Cygan D, Dippold M, et al., 1991. The land mobile satellite communication channel-recording, statistics, and channel model. IEEE Trans Veh Technol, 40(2):375-386.

[81]Ma XC, Zhang JH, Zhang YX, et al., 2017. A PCA-based modeling method for wireless MIMO channel. IEEE Conf on Computer Communications Workshops, p.874-879.

[82]Martínez ‘AO, de Carvalho E, Nielsen JØ, 2014. Towards very large aperture massive MIMO: a measurement based study. IEEE Globecom Workshops, p.281-286.

[83]Marzetta TL, 2010. Noncooperative cellular wireless with unlimited numbers of base station antennas. IEEE Trans Wirel Commun, 9(11):3590-3600.

[84]Matolak DW, 2015. Channel characterization for unmanned aircraft systems. 9^th European Conf on Antennas and Propagation, p.1-5.

[85]Matolak DW, Sun RY, 2014. Antenna and frequency diversity in the unmanned aircraft systems bands for the over-sea setting. IEEE/AIAA 33$^rm rd$ Digital Avionics Systems Conf, p.1-10.

[86]Matolak DW, Sun RY, 2017a. Air-ground channel characterization for unmanned aircraft systems. Part I: methods, measurements, and models for over-water settings. IEEE Trans Veh Technol, 66(1):26-44.

[87]Matolak DW, Sun RY, 2017b. Air-ground channel characterization for unmanned aircraft systems. Part III: the suburban and near-urban environments. IEEE Trans Veh Technol, 66(8):6607-6618.

[88]Matolak DW, Sen I, Xiong WH, et al., 2005. 5 GHz wireless channel characterization for vehicle to vehicle communications. IEEE Military Communications Conf, p.3016-3022.

[89]Meredith J, 2016. Study on Channel Model for Frequency Spectrum above 6 GHz. Technical Report TR 38900, 3GPP.

[90]Miaoudakis A, Lekkas A, Kalivas G, et al., 2005. Radio channel characterization in industrial environments and spread spectrum modem performance. IEEE Conf on Emerging Technologies and Factory Automation, p.87-93.

[91]Molisch AF, 2012. Wireless Communications. John Wiley & Sons, New York.

[92]Moral PD, 1996. Non-linear filtering: interacting particle resolution. Markov Process Rel Field, 2(4):555-581.

[93]Nachmani E, Marciano E, Burshtein D, et al., 2017. RNN decoding of linear block codes. https://arxiv.org/abs/1702.07560

[94]Nachmani E, Marciano E, Lugosch L, et al., 2018. Deep learning methods for improved decoding of linear codes. IEEE J Sel Top Signal Process, 12(1):119-131.

[95]Navabi S, Wang CW, Bursalioglu OY, et al., 2018. Predicting wireless channel features using neural networks. IEEE Int Conf on Communications, p.1-6.

[96]Newhall WG, Mostafa R, Dietrich C, et al., 2003. Wideband air-to-ground radio channel measurements using an antenna array at 2 GHz for low-altitude operations. IEEE Military Communications Conf, p.1422-1427.

[97]Nikolaidis V, Moraitis N, Kanatas AG, 2016. Dual polarized MIMO LMS channel measurements and characterization in a pedestrian environment. 10^th European Conf on Antennas and Propagation, p.1-5.

[98]Ono F, Takizawa K, Tsuji H, et al., 2015. S-band radio propagation characteristics in urban environment for unmanned aircraft systems. Int Symp on Antennas and Propagation, p.1-4.

[99]O’Shea TJ, Hoydis J, 2017. An introduction to machine learning communications systems. https://arxiv.org/abs/1702.00832v1

[100]Petropoulou P, Michailidis ET, Panagopoulos AD, et al., 2014. Radio propagation channel measurements for multi-antenna satellite communication systems: a survey. IEEE Antenn Propag Mag, 56(6):102-122.

[101]Piesiewicz R, Kleine-Ostmann T, Krumbholz N, et al., 2005. Terahertz characterisation of building materials. Electron Lett, 41(18):1002-1004.

[102]Piesiewicz R, Jacob M, Koch M, et al., 2008. Performance analysis of future multigigabit wireless communication systems at THz frequencies with highly directive antennas in realistic indoor environments. IEEE J Sel Top Quant Electron, 14(2):421-430.

[103]Pometcu L, D’Errico R, 2018. Large scale and clusters characteristics in indoor sub-THz channels. Proc 29^th Annual Int Symp Personal Indoor and Mobile Radio Communications, p.1405-1409.

[104]Priebe S, Kuerner T, 2013. Stochastic modeling of THz indoor radio channels. IEEE Trans Wirel Commun, 12(9):4445-4455.

[105]Priebe S, Jastrow C, Jacob M, et al., 2011. Channel and propagation measurements at 300 GHz. IEEE Trans Antenn Propag, 59(5):1688-1698.

[106]Priebe S, Kannicht M, Jacob M, et al., 2013. Ultra broadband indoor channel measurements and calibrated ray tracing propagation modeling at THz frequencies. J Commun Netw, 15(6):547-558.

[107]Priebe S, Jacob M, Kuerner T, 2014. Angular and RMS delay spread modeling in view of THz indoor communication systems. Radio Sci, 49(3):242-251.

[108]Quinlan JR, 1986. Induction of decision trees. Mach Learn, 1(1):81-106.

[109]Raimundo X, Salous S, Cheema A, 2018. Indoor dual polarised radio channel characterisation in the 54 and 70 GHz bands. IET Microw Antenn Propag, 12(8):1287-1292.

[110]Rappaport TS, McGillem CD, 1987. Characterising the UHF factory radio channel. Electron Lett, 23(19):1015-1017.

[111]Rappaport TS, Xing YC, MacCartney GR, et al., 2017. Overview of millimeter wave communications for fifth-generation (5G) wireless networks: with a focus on propagation models. IEEE Trans Antenn Propag, 65(12):6213-6230.

[112]Rasmussen CE, 2003. Gaussian processes in machine learning. In: Bousquet O, Luxburg U, Rätsch G (Eds.), Advanced Lectures on Machine Learning. Springer Berlin Heidelberg, p.63-71.

[113]Rey S, Eckhardt JM, Peng B, et al., 2017. Channel sounding techniques for applications in THz communications: a first correlation based channel sounder for ultra-wideband dynamic channel measurements at 300 GHz. Proc 9^th Int Congress on Ultra Modern Telecommunications and Control Systems and Workshops, p.449-453.

[114]Richter F, Fehske AJ, Fettweis GP, 2009. Energy efficiency aspects of base station deployment strategies for cellular networks. IEEE 70^th Vehicular Technology Conf, p.1-5.

[115]Rieche M, Ihlow A, Arndt D, et al., 2015. Modeling of the land mobile satellite channel considering the terminal’s driving direction. Int J Antenn Propag, 2015:372124.

[116]Samimi MK, Rappaport TS, MacCartney GR, 2015. Probabilistic omnidirectional path loss models for millimeter-wave outdoor communications. IEEE Wirel Commun Lett, 4(4):357-360.

[117]Series M, 2015. {IMT Vision-Framework and Overall Objectives of the Future Development of IMT for 2020 and Beyond. Report M.2083-0, ITU-R, Geneva, mboxSwitzerland.}

[118]Sexton D, Mahony M, Lapinski M, et al., 2005. Radio channel quality in industrial wireless sensor networks. Sensors for Industry Conf, p.88-94.

[119]Shafin R, Liu LJ, Chandrasekhar V, et al., 2019. Artificial intelligence-enabled cellular networks: a critical path to beyond-5G and 6G. https://arxiv.org/abs/1907.07862

[120]Simunek M, Pechac P, Fontan FP, 2011. Excess loss model for low elevation links in urban areas for UAVs. Radioengineering, 20(3):561-568.

[121]Solomitckii D, Orsino A, Andreev S, et al., 2018. Characterization of mmWave channel properties at 28 and 60 GHz in factory automation deployments. IEEE Wireless Communications and Networking Conf, p.1-6.

[122]Strinati EC, Barbarossa S, Gonzalez-Jimenez JL, et al., 2019. 6G: the next frontier. https://arxiv.org/abs/1901.03239

[123]Sun RY, Matolak DW, 2017. Air-ground channel characterization for unmanned aircraft systems. Part II: hilly and mountainous settings. IEEE Trans Veh Technol, 66(3):1913-1925.

[124]Tang P, Zhang J, Molisch AF, et al., 2018. Estimation of the $K$-factor for temporal fading from single-snapshot wideband measurements. IEEE Trans Veh Technol, 68(1):49-63.

[125]Tanghe E, Joseph W, Verloock L, et al., 2008. The industrial indoor channel: large-scale and temporal fading at 900, 2400, and 5200 MHz. IEEE Trans Wirel Commun, 7(7):2740-2751.

[126]Tu HD, Shimamoto S, 2009. A proposal of wide-band air-to-ground communication at airports employing 5-GHz band. IEEE Wireless Communications and Networking Conf, p.1-6.

[127]Vogel WJ, Goldhirsh J, 1986. Tree attenuation at 869 MHz derived from remotely piloted aircraft measurements. IEEE Trans Antenn Propag, 34(12):1460-1464.

[128]Vogel WJ, Goldhirsh J, 1988. Fade measurements at L-band and UHF in mountainous terrain for land mobile satellite systems. IEEE Trans Antenn Propag, 36(1):104-113.

[129]Vogel WJ, Goldhirsh J, 1993. Earth-satellite tree attenuation at 20 GHz: foliage effects. Electron Lett, 29(18):1640-1641.

[130]Wang CX, Bian J, Sun J, et al., 2018. A survey of 5G channel measurements and models. IEEE Commun Surv Tutor, 20(4):3142-3168.

[131]Wang Z, Li L, Xu Y, et al., 2018. Handover control in wireless systems via asynchronous multiuser deep reinforcement learning. IEEE Int Thing J, 5(6):4296-4307.

[132]Wang ZY, Shen C, 2017. Small cell transmit power assignment based on correlated bandit learning. IEEE J Sel Area Commun, 35(5):1030-1045.

[133]Watkins C, 1989. Learning from Delayed Rewards. PhD Thesis, University of Cambridge, Cambridge, UK.

[134]Wentz M, Stojanovic M, 2015. A MIMO radio channel model for low-altitude air-to-ground communication systems. IEEE 82^nd Vehicular Technology Conf, p.1-6.

[135]Willink TJ, Squires CC, Colman GW, et al., 2015. Measurement and characterization of low-altitude air-to-ground MIMO channels. IEEE Trans Veh Technol, 65(4):2637-2648.

[136]WP5D I, 2017. {Guidelines for Evaluation of Radio Interface Technologies for IMT-2020. Report M.2412, ITU-R, Geneva, Switzerland.}

[137]Xie YJ, Fang YG, 2000. A general statistical channel model for mobile satellite systems. IEEE Trans Veh Technol, 49(3):744-752.

[138]Yang GS, Zhang Y, He ZW, et al., 2019. Machine-learning-based prediction methods for path loss and delay spread in air-to-ground millimetre-wave channels. IET Microw Antenn Propag, 13(8):1113-1121.

[139]Yanmaz E, Kuschnig R, Bettstetter C, 2011. Channel measurements over 802.11a-based UAV-to-ground links. IEEE GLOBECOM Workshops, p.1280-1284.

[140]Zhang C, Hui YN, 2011. Broadband air-to-ground communications with adaptive MIMO datalinks. IEEE/AIAA 30^th Digital Avionics Systems Conf, p.4D4-1.

[141]Zhang J, 2016. The interdisciplinary research of big data and wireless channel: a cluster-nuclei based channel model. China Commun, 13(S2):14-26.

[142]Zhang J, Pan C, Pei F, et al., 2014. Three-dimensional fading channel models: a survey of elevation angle research. IEEE Commun Mag, 52(6):218-226.

[143]Zhang JH, Tang P, Tian L, et al., 2017a. 6–100 GHz research progress and challenges for fifth generation (5G) and future wireless communication from channel perspective. Sci China Inform Sci, 60(8):080301.

[144]Zhang JH, Zhang YX, Yu YW, et al., 2017b. 3D MIMO: how much does it meet our expectations observed from channel measurements? IEEE J Sel Areas Commun, 35(8):1887-1903.

[145]Zhang JH, Zheng Z, Zhang YX, et al., 2018. 3D MIMO for 5G NR: several observations from 32 to massive 256 antennas based on channel measurement. IEEE Commun Mag, 56(3):62-70.

[146]Zhang P, Niu K, Tian H, et al., 2019. The outlook for 6G mobile communication technologies. J Commun, 4(1):145-152.