Dual-channel near-field holographic MIMO communications based on programmable digital coding metasurface and electromagnetic theory

By using programmable digital coding metasurface, a dual-channel near-field holographic MIMO communication system is realized in low cost and low system complexity.
Dual-channel near-field holographic MIMO communications based on programmable digital coding metasurface and electromagnetic theory
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Holographic multiple-input multiple-output (MIMO) method features spatially continuous apertures and fields, which can surpass the massive MIMO technologies. Holographic MIMO surfaces have huge potentials to enhance the spectral and energy efficiency through dynamic control of electromagnetic (EM) waves. However, the implementation of holographic MIMO faces many engineering problems. For example, reducing the element spacing of traditional antenna arrays will greatly increase the mutual coupling and the number of T/R components, leading to high cost and system complexity.

 Programmable digital coding metasurface (PDCM) can perform real-time and subwavelength- scale modulations on the EM waves at low cost, which makes the holographic MIMO system possible [Cui et al., Light: Science & Applications 3, e218, 2014]. On the other hand, direct digital modulation (DDM) uses tunable devices on the aperture to transmit information and eliminates mixers and T/R components, which greatly reduces the system cost and complexity [Cui et al. Research, 2584509, 2019]. Hence PDCM is an ideal solution for meeting the requirements of the holographic MIMO system.

 In this study, we propose to construct a holographic MIMO communication system by using PDCM (see Fig. 1), in which the transmitter is composed of PDCM and a field programmable gate array (FPGA). Firstly, orthogonal holographic patterns are obtained by Hilbert-Schmidt decomposition of free-space radiation operator under the guidance of electromagnetic theory. Then, FPGA will pre-encode the baseband signals by the orthogonal holographic patterns and generate the control voltages for PDCM, which attaches the information to the carrier and radiates to free space. A similar procedure is adopted at the receiver, and the signal symbols are recovered by calculating the inner product of the received aperture field and orthogonal basis patterns. A prototype is designed and fabricated in the microwave frequency. To verify the multi-channel communication capability of the PDCM-based holographic MIMO system, experiments are carried out in an indoor environment. The measurements indicate that the holographic MIMO system can realize dual-channel signal transmissions under quadrature phase shift keying (QPSK) scheme. Despite the limited performance of hardware, the experimental results still show great potentials of PDCM in revealing the holographic MIMO communications. We hope that our design provides a feasible and practical route to meet the extreme requirements of B5G and 6G communication systems.

Figs. 2a and b illustrate a prototype of the proposed dual-channel holographic MIMO system and the experimental setup. A microwave signal source connected to a linearly-polarized horn antenna serves as the excitation and provides the carrier waves. The desired coding sequences are generated by a control platform (NI PXIe-1082) with a high-speed bus controller, an FPGA module (NI PXIe-7966R), a synchronous clock module (NI PXIe-6674T), and a digital I/O module (NI 6581B), which are then loaded to PDCM through the digital input and output (DIO) lines. On the whole, 48 independent signals are generated to control the 24 columns of 2-bit PDCM elements. An SDR platform (NI USRP-2974) connected with two probes is configured to perform the signal demodulations, including the down-conversion, sampling, and baseband operation. Finally, the transmitted bit streams are recovered, and the real-time constellation results are displayed on the screen, which are straightforward representations of the relation between the baseband signals. Non-line-of-sight (NLOS) experiments are also taken into the consideration. As shown in Fig. 2b, a cardboard with a thickness of about 4 cm is placed in front of the probe array to simulate the NLOS propagation, while all other configurations are the same as those in Fig. 2a. The measured constellation diagrams are shown in Figs. 2c-d. Due to the performance limitations of PDCM and environmental factors, the measured constellations are worse than the simulated results. It is the mutual coupling that causes this gap. The constellation points in one channel vary with the symbols transmitted in another one. Consequently, there is significant coupling between the two channels. However, the constellation points of the two channels show obvious zoning effect in general, and four QPSK symbols can be clearly distinguished. Moreover, by comparing the blue and red dots in Figs. 2c-d, it can be found that the impact of NLOS propagation on our system is negligible. Overall, a PDCM-based dual-channel holographic MIMO system prototype has been built up and measured. The experiment preliminarily verifies the feasibility of constructing orthogonal channels to transmit the information based on the Hilbert-Schmidt decomposition.

Figure 1. Schematic diagram of the proposed holographic MIMO system based on PDCM and EIT. The transmitter is composed of PDCM and FPGA. Firstly, orthogonal holographic patterns are obtained by the Hilbert-Schmidt decomposition of free-space radiation operator and EIT. Then, the FPGA pre-encodes the baseband signals by the orthogonal holographic patterns and generates the control voltages for PDCM, which attaches the information to the carrier and radiates to free space. A similar procedure is adopted at the receiver, and the signal symbols are recovered by calculating the inner product of the received aperture field and the orthogonal basis patterns.

 

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Microwaves, RF Engineering and Optical Communications
Technology and Engineering > Electrical and Electronic Engineering > Microwaves, RF Engineering and Optical Communications
Communications Engineering, Networks
Technology and Engineering > Electrical and Electronic Engineering > Communications Engineering, Networks

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