Ruoqi Deng, Yutong Zhang, Haobo Zhang, Boya Di, Hongliang Zhang, Lingyang Song
©SHUTTERSTOCK.COM/ARTHEAD
Ultramassive multiple-input, multiple-output (MIMO) is one of the key enablers in the forthcoming 6G networks to provide high-speed data services by exploiting spatial diversity. In this article, we consider a new paradigm, termed holographic radio for ultramassive MIMO, where numerous tiny and inexpensive antenna elements are integrated to realize high directive gain with low hardware cost. We propose a practical way to enable holographic radio by a novel metasurface-based antenna, i.e., reconfigurable holographic surface (RHS). Specifically, RHSs incorporating densely packed tunable metamaterial elements are capable of holographic beamforming. Based on the working principle and hardware design of RHSs, we conducted full-wave analyses of RHSs and built an RHS-aided point-to-point communication platform supporting real-time data transmission. Both simulated and experimental results show that the RHS has great potential to achieve high directive gain with a limited size, thereby substantiating the feasibility of RHS-enabled holographic radio. Moreover, future research directions for RHS-enabled holographic radio are also discussed.
The 6G wireless communications look forward to providing high-speed data services and revolutionary mobile connectivity to handle the explosive growth in the number of mobile devices and applications. By exploiting the spatial diversity, massive MIMO with large-scale phased arrays capable of highly directional beamforming is considered one of the powerful solutions to fulfill the challenging visions of 6G communications [1]. However, considering that phased arrays rely on high-resolution phase shifters, the hardware cost and power consumption will become unaffordable, as massive MIMO evolves into ultramassive MIMO [2].
To overcome the limitations of phased arrays, a new paradigm, termed holographic radio for ultramassive MIMO has been proposed, where high directive gain can be achieved by numerous integrated antenna elements with low hardware cost [3]. To lift the half-wavelength restriction and enable holographic radio in practical systems, RHSs, one of the representative metasurface-based antennas composed of densely packing subwavelength metamaterial elements, have been developed as a promising candidate [4]. Specifically, due to the unique structure and characteristics of the metamaterial elements, the RHS can regulate electromagnetic waves via simple diode-based controllers. This provides a powerful solution to reduce the hardware cost while guaranteeing high directive gain in practice.
Different from another widely used metasurface-based antenna, named reconfigurable intelligent surface (RIS), whose feeds are set outside the metasurface due to the reflection characteristic [5], the feeds of the RHS are attached to the edge of the metasurface. As such, electromagnetic waves generated by the feeds propagate along the RHS elements and excite the RHS elements one by one. This enables the RHS to conveniently serve as an antenna array integrated with the transceiver. Compared with parallel feeding adopted by phased arrays, where each antenna element requires a long feeding line, such series feeding adopted by the RHS also leads to a much simpler wire layout in the implementation of ultramassive MIMO. Based on the holographic interference principle, each RHS element can control the radiation amplitude of the incident electromagnetic waves electrically to generate object beams [6], and such a beamforming technology is also known as holographic beamforming.
Although RHSs provide a promising solution to enable the holographic radio, as nascent metasurfaces, initial research on RHSs has primarily focused on the fundamental hardware component design [7] and holographic beamforming optimization [8], [9]. However, most works have only substantiated the capability of a simple 1D RHS to achieve beamforming or investigated RHS-aided communications, as well as amplitude-controlled beamforming at the theoretical level. The realization of the RHS-enabled holographic radio system has not been studied. In this article, we study the feasibility of utilizing RHSs to enable holographic radio at the system and component levels. More precisely, we contribute to the research on RHS-enabled holographic radio from the following two aspects:
Although RHS-enabled holographic radio provides a powerful solution for the implementation of ultramassive MIMO, several open problems still need to be addressed, which shed light on future research directions, as listed below:
The rest of this article is organized as follows. First, we present the “Working Principle of RHS” section and then the “Hardware Design and Full-Wave Analysis of RHS” section, followed by the “Experimental Prototype of RHS-Enabled Holographic Radio” and “Experimental Results and Discussion” sections. Following the “Future Research Directions of RHS-Enabled Holographic Radio” section, we close the article with the “Conclusions” section.
In this section, we present the holographic principle of RHSs, based on which amplitude-controlled holographic beamforming is introduced.
An RHS is a special leaky-wave antenna comprising feeds and metamaterial radiation elements. As shown in Figure 1, the feeds are attached to the edge or the back of the RHS and transform input signals into electromagnetic waves, which are also called reference waves. The RHS adopts series feeding where the incident reference wave propagates along the RHS elements and excites the RHS elements one by one. The reference wave is then transformed into a leaky wave through the slots of RHS elements to emit signals to free space [10]. The radiation amplitude of the leaky wave at each RHS element can be controlled electrically to achieve holographic beamforming. Compared with parallel feeding adopted by phased arrays, where each antenna element requires a long feeding line, the series feeding leads to a much simpler wiring layout in the implementation of ultramassive MIMO.
Figure 1 Illustration of the RHS.
To further interpret the working principle of an RHS, we assume that the RHS has K feeds and N RHS elements. The object wave propagating in the direction ${(}{\theta},{\phi}{)}$ is denoted as ${\Psi}_{\text{obj}}$, while the reference wave generated by feed k is denoted as ${\Psi}_{\text{ref}}$. The RHS can construct a holographic pattern on the metasurface to record the interferogram ${\Psi}_{\text{int}\text{f}}$ between the reference wave and the object wave based on the holographic principle, i.e., ${\Psi}_{\text{int}\text{f}} = \Psi{}_{\text{obj}}{\Psi}_{\text{ref}}^{\ast}$ [11]. When the interferogram ${\Psi}_{{\text{int}}{f}}$ recorded on the RHS is excited by the propagating reference wave, the leaked wave ${\Psi}_{{\text{int}}{f}}{\Psi}_{\text{ref}}$ is proportional to ${\Psi}_{\text{obj}}{\left\vert{\Psi}_{\text{ref}}\right\vert}^{2}$, indicating that its direction is exactly ${(}{\theta},{\phi}{)}$.
Remark 1: The physical structure and operating mechanisms of RHS are different from those of another representative metasurface, called RIS. Specifically, an RHS’s feeds are attached to the edge of the metasurface. Hence, RHSs can be conveniently integrated with transceivers. In contrast, the feeds of an RIS are set outside the metasurface because of the reflection characteristic [5]. An extra link is required between the transmitter (Tx) and the RIS to control the phase shifts of each RIS element. In addition, the RHS utilizes the method of series feeding, while the RIS utilizes the method of parallel feeding, where all RIS elements are excited by the incident signals at the same time. Due to their different physical structure and operating mechanism, RHSs are more likely to serve as transmit/receive antennas directly, while RISs are widely used as relays.
The RHS utilizes an amplitude-controlled method to represent the information contained in the interferogram ${\Psi}_{{\text{int}}{f}}$ by tuning each RHS element based on the phase of the reference wave at each RHS element. Specifically, the phase of the reference wave changes in the process of propagation. At each RHS element, the phase of the reference wave is determined by the product of the propagation vector on the RHS and the distance vector from feed k to the nth RHS element ${\bf{r}}_{n}^{k}$ [11], which is a priori-fixed. If the reference wave is in phase with the object wave at an RHS element, the RHS element will be tuned to radiate much energy of the reference wave into free space. Otherwise, the RHS element will be detuned and not radiate energy into free space.
To map the phase difference between the object wave and the reference wave to the radiation amplitude of the RHS elements, the real part of the interference ${\text{Re}}{\left[{\Psi}_{{\text{int}}{f}}\right]}$ (i.e., the cosine value of the phase difference) is considered. Since the value of ${\text{Re}}\left[{{\Psi}_{{\text{int}}{f}}}\right]$ decreases as the phase difference grows, satisfying the amplitude control requirements, ${\text{Re}}{\left[{\Psi}_{{\text{int}}{f}}\right]}$ is a direct representation of the radiation amplitude distribution on the RHS. Hence, the holographic pattern m, i.e., the radiation amplitude of each RHS element that can generate the object wave with the direction of ${(}{\theta},{\phi}{)}$, is parameterized mathematically by: \[{m}{(}{\bf{r}}_{n}^{k},{\theta},{\phi}{)} = \frac{{\text{Re}}{\left[{\Psi}_{{\text{int}}{f}}{\left({\bf{r}}_{n}^{k},{\theta},{\phi}\right)}\right]} + {1}}{2} \tag{1} \] where ${\text{Re}}{\left[{\Psi}_{{\text{int}}{f}}\right]}$ is normalized to [0,1] to avoid negative values. This formula represents the basic principle for amplitude-controlled holographic beamforming. The effectiveness of this formula to achieve holographic beamforming will also be verified in the “Experimental Results and Discussion” section.
In this section, we present the hardware design of the RHS element. Full-wave analyses of RHSs are then introduced.
As shown in Figure 2, the kernel of the designed RHS element with controllable radiation amplitude is a complementary electric-LC (cELC) resonator connected with p-i-n diodes [7]. (The RHS element with controllable radiation amplitudes can also be fabricated by loading varactor diodes and liquid crystals [6].) For symmetry, two p-i-n diodes are connected across the gaps separating the central metal patch from the microstrip line. By controlling the biased voltages applied to the p-i-n diodes, the cELC resonator’s mutual inductance, together with the radiation amplitude of the RHS element, can be changed. Specifically, when two p-i-n diodes are in the OFF states, the RHS element radiates the energy of the reference wave into free space. When the p-i-n diodes are in ON states, the reference wave’s energy is hardly radiated. The design requirement of the radiation efficiency of an RHS element is related to the size of the RHS to guarantee that most of the input energy from the feed can be radiated into free space. For example, for a 1D RHS with 16 RHS elements, the radiation efficiency of an RHS element with p-i-n diodes in the OFF and ON states is required to be 30%–40% and lower than 15%, respectively [7].
Figure 2 Hardware design of the RHS element.
The size of the RHS element is ${0.82}\,{\times}\,{1.7}\,{\times}\,{0.11}{\text{cm}}^{3}$. The aimed working frequency is set as 12 GHz for satellite communications. To cover the working frequency, we chose MACOM MADP-000907–14020 with low insertion loss for p-i-n diodes [12]. The whole RHS element is composed of five layers, given as follows:
Moreover, the radiation characteristics of an RHS element, such as its radiation efficiency and resonant frequency, are determined by its geometric parameters. Considering that the geometric parameters of the RHS element are coupled with each other and cannot be adjusted independently through a single dimension, a parameter optimization procedure based on full-wave analyses is also applied to achieve the desired radiation characteristics.
Based on the simulation of the RHS element, we conducted full-wave analyses of RHSs with different sizes utilizing the CST Microwave Studio.
To begin with, we considered a 1D RHS with 16 RHS elements. (The experimental prototype of the considered 1D RHS will be presented in the “Experimental Prototype of RHS-Enabled Holographic Radio” section.) The element spacing of the RHS is 0.82 cm, which is approximately one-third of the wavelength at 12 GHz. The RHS is fed from the port at the left end, where the electromagnetic wave generated by port 1 continuously radiates energy from each RHS element in the propagation process. The residual energy of the electromagnetic wave will be absorbed by the port at the right end.
To determine the ON/OFF state of the p-i-n diodes of each RHS element, for a given object beam, we calculated the theoretical radiation amplitude of each RHS element by (1). If the radiation amplitude is larger than a predefined threshold, the p-i-n diodes will be in the OFF state. In contrast, if the radiation amplitude is less than the threshold, the p-i-n diodes will be in the ON state [13]. The full-wave analysis shows the main lobe’s direction of the beam pattern of the RHS matches the object beam’s direction and the simulated gain is about 4 dBi, which is obtained directly from the far-field results in the CST Microwave Studio.
Considering that a 2D RHS can achieve 3D holographic beamforming, i.e., the elevation angle ${\theta}$ and the azimuth angle ${\phi}$ of the generated beam can be controlled, we evaluated the beam-steering capability of the RHS in both a horizontal plane and vertical plane.
The 2D RHSs have multiple feeds attached to the edge of the RHS. Note that the radiation amplitude of each RHS element is related to the distance between the RHS element and the feed according to (1). For a multifeed 2D RHS, the radiation amplitude of each RHS element can be calculated as a summation of the radiation amplitude distribution corresponding to each feed, i.e.: \[{m}_{n}{(}{\theta},{\phi}{)} = \mathop{\sum}\limits_{{k} = {1}}\limits^{K}{\frac{{\text{Re}}\left[{{\Psi}_{{\text{int}}{f}}\left({{\bf{r}}_{n}^{k},{\theta},{\phi}}\right)}\right] + {1}}{2{K}}}{.} \tag{2} \]
Figure 3(a) and (b) show the normalized far-field beam pattern of the 2D RHS with 64 elements in the horizontal plane with the desired direction scanned from −60° to 60° and that in the vertical plane with the desired direction scanned from −30° to 30°, respectively. (The sidelobe can be cancelled by superposing an auxiliary control pattern to reduce the holographic pattern corresponding to the sidelobe on the original holographic pattern [11].) This validates the capability of the 2D RHS to achieve 3D holographic beamforming through the amplitude-controlled method. Moreover, the simulated gain of the RHS with 32 and 64 RHS elements is about 7 dBi and 10 dBi, respectively. The simulated results show that the gain of the RHS will increase 3 dB when the number of RHS elements doubles, indicating that the RHS has the same gain-boosted capability as traditional antennas [14]. Hence, the conclusion in “Remark 2” stands.
Figure 3 Normalized far-field beam patterns of the RHS in the horizontal plane and the vertical plane. (a) Horizontal plane. (b) Vertical plane.
Remark 2: Capable of the same gain boosting as traditional antennas, the RHS has great potential to achieve a high directive gain through numerous RHS elements with low hardware cost. Hence, the RHS provides a practical way to enable holographic radio.
Moreover, since the RHS adopts series feeding, the input signals are only required to be connected with the feeds rather than all RHS elements, leading to a simple wiring layout when implementing ultramassive MIMO.
In this section we introduce the implementation of the RHS prototype. The beam pattern measurement procedures of the prototype and the RHS-aided point-to-point communication platform are then presented.
As shown in Figure 4, the designed 1D RHS prototype consists of 16 RHS elements and the size of the RHS is ${15.2}\,{\times}\,{1.7}\,{\times}\,{0.11}{\text{cm}}^{3}$. The radiation amplitude of each RHS element can be controlled based on a field-programmable gate array (FPGA). Specifically, the bias voltage applied to each p-i-n diode can be changed by controlling the FPGA, and thus the ON/OFF state of the p-i-n diode can be controlled. In addition, we utilize SubMiniature version A connectors attached to the edge of the RHS to feed signals into the RHS and introduce a trapezoidal evolutionary structure to the edge of the RHS for impedance matching.
Figure 4 Beam pattern measurement of the RHS.
As shown in Figure 4, a vector network analyzer (VNA) is utilized to measure the beam pattern of the RHS. Specifically, port 1 of the VNA is connected to the RHS, while port 2 of the VNA is connected to a standard horn antenna. The RHS is placed on an antenna rotating platform. To measure the transmit beam pattern of the RHS, the relative signal strength (i.e., ${S}_{21}$ parameter) received at the horn antenna corresponding to different angles is measured and normalized. To validate the transceiver reciprocity of the RHS, the receive beam pattern is obtained by measuring and normalizing the value of ${S}_{21}$ corresponding to different angles.
The hardware modules of the RHS-aided point-to-point communication platform are shown in Figure 5 and their functions are introduced as follows:
Figure 5 RHS-aided point-to-point communication platform.
In this section, we present the experimental setup and experimental results obtained with the implemented RHS prototype.
To avoid environmental scatterings, the RHS prototype is deployed in a microwave anechoic chamber. To measure the far-field beam pattern of the RHS, the horn antenna acting as the receive/transmit antenna is placed 2 m away from the RHS. (The Rayleigh distance of the RHS ${d}_{\text{Ray}}$ is ${2}{D}^{2} / {\lambda}$, where D is the maximum dimension of the RHS, i.e., 15.2 cm, and ${\lambda}$ is the wavelength. Since the working frequency of the RHS is 12 GHz, ${d}_{\text{Ray}}$ is 1.8 m.) In the deployed RHS-aided point-to-point communication platform, the transmit power of the Tx USRP is 2 dBm, and the gain of the horn antenna is 20 dBi.
Figure 6 shows the normalized far-field beam pattern of the RHS. For convenience, we set the holographic pattern of the RHS generating the beam with the direction of 0° according to (1). Specifically, when the theoretical radiation amplitude of an RHS element is less than 0.5, the p-i-n diodes of the RHS element will be set in ON states and vice versa. It can be seen that the measured main lobe’s direction of the beam pattern matches the object direction, indicating that holographic beamforming can be achieved based on (1). We also observe that the transmit beam pattern and the receive beam pattern are almost the same, validating the transceiver reciprocity of the RHS. Moreover, the power consumption of the p-i-n diode in the implemented RHS is 0.01 W, which is far smaller than that of phase-shifting circuits. Hence, compared with traditional phased arrays, the RHS also provides a powerful solution to reduce power consumption in practice. (More discussion and comparison about the performance of RHSs and traditional phased arrays can be found in [8] and [13], to which the interested readers are referred for further information.)
Figure 6 Normalized far-field beam pattern of the RHS.
Figure 7 depicts the graphical interface of the Tx and Rx signals on the host computer when transmitting a real-time data stream. Specifically, the data stream generated by the vector source of the Tx USRP is a vector beginning with a 1 followed by a sequence of 0. We observed that the received signal demodulated by the Rx USRP is the same as the transmit signal. This validates that our RHS-aided point-to-point communication platform supports real-time data transmission. Moreover, from the received spectrum at the Rx, it can be seen that the signal-to-noise ratio of the RHS-aided point-to-point communication is about 20 dB under the experimental environment.
Figure 7 Experimental results of RHS-aided point-to-point communications.
In this section, we present future research directions for RHS-enabled holographic radio.
Low-Earth-orbit (LEO) satellite communication networks are being developed with the promise of providing high-capacity backhaul or data-relay services for terrestrial networks. However, the high-mobility of LEO satellites and the severe path loss put stringent requirements on antenna technologies in terms of accurate beam steering and high antenna gain. Traditional antennas integrated with UTs, such as dish antennas and phased arrays, either require heavy mechanics or costly phase shifters, making their implementation in practical systems prohibitive.
Since RHSs are ultrathin and lightweight antennas and can achieve beamforming with low hardware cost and low power consumption, RHS-enabled holographic radio can be utilized in integrated terrestrial–satellite networks to overcome the shortfalls in traditional antennas. Since existing algorithms optimizing traditional complex-valued analog beamformers do not work well for real-valued holographic beamformers, a holographic beamforming scheme generating multiple directional beams toward the satellites needs to be developed for sum rate maximization. In addition, considering that the high-mobility of LEO satellites leads to time-varying beam patterns, the RHS-aided multisatellite communication protocol design is also worth exploring.
ISAC, where the radar and communication systems share the common spectrum, is one of the most promising candidates to mitigate the spectrum congestion issue [15]. However, the high-power consumption and hardware cost of traditional phased arrays restrict the implementation of ultramassive MIMO, leading to an insufficient ISAC performance.
Benefitting from the capability of achieving high directive gain with low hardware cost, RHS-enabled holographic radio can be applied in ISAC systems to enhance sensing and communication performance. In ISAC systems, the amplitude-controlled holographic beamforming design and the estimation method for target parameters are coupled with each other, which should be simultaneously optimized in the sensing scheme. Moreover, to effectively detect targets and serve communication users, a holographic beamforming optimization algorithm needs to be carefully designed while considering the tradeoff between the sensing and communication functionalities.
Wireless SLAM, which estimates the position of the user and builds up the map of an unknown environment, is a promising technique to empower location-based services. It utilizes antennas to estimate the time of arrival and the angle of arrival based on the amplitudes of received multipath components (MPCs). Hence, the accuracy of the SLAM system is determined by the directive gain of the antennas.
Considering that the RHS is composed of numerous RHS elements, leading to a superior beam-steering capability and high directive gain, the RHS can be utilized in wireless SLAM systems. By adjusting the amplitude of each RHS element properly, the amplitudes of MPCs can be enhanced, such that the accuracy of the wireless SLAM system can be improved.
In this article, we have investigated RHS-enabled holographic radio for 6G communications. The working principle of RHSs including holographic beamforming has been introduced. The hardware design of the RHS element and full-wave analyses of both single-feed 1D RHS and multifeed 2D RHS have then been presented. Notably, we have proposed a prototype of RHS-enabled holographic radio where an RHS-aided point-to-point communication platform supporting real-time data transmission has been built. It has been proved that due to diode-based controllers and series feeding, the RHS has great potential to achieve high directive gain with low hardware cost, low power consumption, and a simple wiring layout, thereby providing a practical way toward ultramassive MIMO. Future research directions for RHS-enabled holographic radio, including satellite communications, ISAC, and SLAM, have also been discussed.
This work was supported in part by the National Key R&D Project of China under Grant 2022YFB2902800, the National Science Foundation under Grants 62271012 and 6194110, and the Beijing Natural Science Foundation under Grants L212027 and 4222005.
Ruoqi Deng (ruoqi.deng@pku.edu.cn) is a Ph.D. student at the School of Electronics, Peking University, Beijing 100871, China. She received her B.S. degree in electronic engineering from Peking University, Beijing, China in 2019. Her current research interests include reconfigurable holographic surface, integrated aerial access, and satellite networks.
Yutong Zhang (yutongzhang@pku.edu.cn) is a Ph.D. student at the School of Electronics, Peking University, Beijing 100871, China. She received her M.E. degree at the School of Software and Microelectronics, Peking University, Beijing, China, in 2020. Prior to that, she received her B.S. degree in Electronics Information Science and Technology from Yunnan University, Kunming, China, in 2017. Her current research interest includes intelligent surface, wireless communications, and edge computing.
Haobo Zhang (haobo.zhang@pku.edu.cn) is a Ph.D. student at the School of Electronics, Peking University, Beijing 100871, China. He received his B.S. degree at School of Electrical Engineering and Computer Science in Peking University, Beijing, China in 2019. His research interests include metasurface, wireless networks, and optimization theory.
Boya Di (boya.di@pku.edu.cn) is an assistant professor at the School of Electronics, Peking University, Beijing 100871, China. She obtained her Ph.D. degree from the Department of Electronics, Peking University, Beijing, China in 2019. Her current research interests include reconfigurable intelligent surfaces, edge computing, vehicular networks, and aerial access networks. She received the best doctoral thesis award from China Education Society of Electronics in 2019 and is the recipient of 2021 IEEE ComSoc Asia-Pacific Outstanding Paper Award.
Hongliang Zhang (hongliang.zhang@pku.edu.cn) is an assistant professor at the School of Electronics, Peking University, Beijing 100871, China. He received his Ph.D. degree at the School of Electrical Engineering and Computer Science at Peking University, Beijing, China in 2019. His current research interest includes reconfigurable intelligent surfaces, aerial access networks, and game theory. He is the recipient of 2021 IEEE Comsoc Heinrich Hertz Award for Best Communications Letters.
Lingyang Song (lingyang.song@pku.edu.cn) is a Boya Distinguished Professor at the School of Electronics, Peking University, Beijing 100871, China. He earned his Ph.D. degree from the University of York, York, U.K., in 2007, where he received the K.M. Stott Prize for excellent research. He has worked as a research fellow at the University of Oslo, Norway, and at Philips Research U.K., Farnborough, U.K.
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Digital Object Identifier 10.1109/MVT.2022.3233157