An Evanescent-Propagating Wave Conversion Method for Expanding the DoF in Holographic MIMO

doi:10.12142/ZTECOM.202602010

ZTE Communications ›› 2026, Vol. 24 ›› Issue (2): 83-92.DOI: 10.12142/ZTECOM.202602010

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An Evanescent-Propagating Wave Conversion Method for Expanding the DoF in Holographic MIMO

Liu Guohao¹, Fang Min²^,³, Peng Lin²^,³, Luo Jun²^,³, Sun Zhi¹()

^1.Department of Electronic Engineering, Tsinghua University, Beijing 100084, China
^2.State Key Laboratory of Mobile Network and Mobile Multimedia Technology, Shenzhen 518055, China
^3.ZTE Corporation, Shenzhen 518057, China

Received:2025-02-03 Online:2026-06-16 Published:2026-06-16
About author:Liu Guohao received his BS degree from the Department of Electronic Engineering, Tsinghua University, China in 2024, where he is currently pursuing his PhD degree. His current research interests include electromagnetics-based communication theory and communication systems based on novel antenna designs.
Fang Min received her PhD degree from the Department of Electronic Engineering, Tsinghua University, China in 1999. She joined ZTE Corporation in 2004 and has been engaged in innovation in new technologies and standardization for next-generation wireless communications. She led ZTE’s standard team to accomplish the 3GPP LTE Release 8 standardization and ZTE’s first UMTS FDD work item. She also made a significant contribution to the ZTE’s 5G R&D on the Pre5G massive MIMO technology, which won both the Best Mobile Technology Breakthrough and the CTO Choice Awards in MWC 2016. She is currently coordinating the wireless prototyping activities on new traffic models, novel architectures, and key technologies such as ultra-massive MIMO, mmWave coverage, and agent-based communication for 6G and beyond.
Luo Jun received his BS degree in electronic science and technology from the University of Electronic Science and Technology of China in 2016, and MS degree in electromagnetic fields and microwave technology from Southeast University, China in 2019. He is currently a senior engineer with ZTE Corporation, and his research interests include RIS, near-field communication, novel MIMO antenna systems, and millimeter-wave and terahertz technologies.
Sun Zhi (zhisun@ieee.org) received his BS degree in telecommunication engineering from Beijing University of Posts and Telecommunications, China in 2004, MS degree in electronic engineering from Tsinghua University, China in 2007, and PhD degree in electrical and computer engineering from Georgia Institute of Technology, USA in 2011. Since 2021, he has been a tenured associate professor with Tsinghua University, China. Prior to that, he was a tenured associate professor at the University at Buffalo, The State University of New York, USA, which he joined as an assistant professor in 2012. His research interests include AI-enabled 6G wireless communication systems, underground and underwater wireless communications and networking, and communication systems based on novel antenna designs. He received the NSF CAREER Award in 2017, the Best Demo Award at IEEE INFOCOM 2017, and the Best Paper Award at IEEE GLOBECOM 2010. He has served as an editor for IEEE Transactions on Mobile Computing, IEEE Transactions on Wireless Communications, and Computer Networks.
Supported by:
ZTE Industry?University?Institute Cooperation Funds(HC?CN?20200923014)

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Abstract

Abstract:

Holographic multiple-input multiple-output (HMIMO) systems deploy ultra-dense antennas in confined spaces, yet spatial degrees of freedom (DoF) fail to scale with element count. Only by harnessing evanescent waves can the potential gains of ultra-dense arrays be unlocked. However, in practical scenarios, antenna apertures are typically too small relative to communication distances to generate significant near-field effects for capturing these evanescent waves. This paper proposes a method to alter the dispersion relation via locally resonant metamaterials (LRM), thereby enabling the radiation of near-field evanescent wave components into the far field. This approach leverages the DoF gains offered by ultra-dense elements within a confined aperture. Full-wave simulations validate the effectiveness of the evanescent-propagating wave conversion method, demonstrating an increase in the spatial DoF radiated into the far field by the HMIMO system even with a limited aperture.

Key words: spatial DoF, evanescent-propagating wave conversion, holographic MIMO

Liu Guohao, Fang Min, Peng Lin, Luo Jun, Sun Zhi. An Evanescent-Propagating Wave Conversion Method for Expanding the DoF in Holographic MIMO[J]. ZTE Communications, 2026, 24(2): 83-92.

Figures/Tables 9

Figure 1 Current distribution on densely packed antennas and interpolated current distribution on a continuous aperture

Figure 2 System model: (a) xm is modulated onto the orthogonal basis of mode currents ?ms, where m=1,2,…,?M, and is realized through a discretized dense dipole antenna array; (b) an LRM structure is employed to achieve the evanescent-propagating waves conversion; (c) the signal radiates and propagates forward into the far field; (d) the signal, received by an ideal aperture, undergoes time-domain reversal and is backpropagated; (e) the time-reversed field excites currents on the HMIMO transmitter; (f) the current distribution on the aperture is recovered through interpolation

Figure 3 Comparison of a 1 000-element discrete MIMO array at different element spacings with an HMIMO array of the same aperture size, where the upper limit of the DoF of discrete MIMO is given by Eqs. (10) and (11), and the upper limit of the DoF of HMIMO is given by Eq. (8)

Figure 4 Spatial spectrum of each mode under element spacings: (a) dx=λ/20, (b) dx=λ/10, (c) dx=λ/4, and (d) dx=λ/2, where each color represents the spatial spectrum components of a current distribution mode and the red dashed box indicates the visible region

Table 1 Classification of patterns for a 1×5 dipole array

Element Spacing	$λ / 20$	$λ / 10$	$λ / 4$	$λ / 2$
Radiating mode	/	Mode 1	Modes 1–2	Modes 1–5
Evanescent mode	Modes 1–5	Modes 2–5	Modes 3–5	/
DoF	<1	1	2	5

Table 1 Classification of patterns for a 1×5 dipole array

Element Spacing	$λ / 20$	$λ / 10$	$λ / 4$	$λ / 2$
Radiating mode	/	Mode 1	Modes 1–2	Modes 1–5
Evanescent mode	Modes 1–5	Modes 2–5	Modes 3–5	/
DoF	<1	1	2	5

Figure 5 Locally resonant metamaterials: (a) dispersion relations for TE and TEM modes and (b) electromagnetic field distribution

Figure 6 Simulation model and results: (a) simulation scenario, (b) normalized E-field magnitude with resonance frequencies and (c) visible region on the dispersion curve; in both scenarios (with and without LRM), the transverse wave number associated with the highest resonance frequency defines the range of the visible region

Figure 7 Normalized current magnitude distribution of time-reversal signals and transmitted signals

Figure 8 Relationship between effective rank(solid lines) and antenna aperture (deshed lines) as the number of elements varies

References 14

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An Evanescent-Propagating Wave Conversion Method for Expanding the DoF in Holographic MIMO

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