Composed of artificially engineered structures, metamaterials have tailorable properties that are not available in naturally occurring materials. During the past two decades, metamaterials have received great attention due to their powerful capabilities to control electromagnetic (EM) waves. As the two-dimensional versions of metamaterials, metasurfaces have also attracted great research interests owing to their simple fabrication, negligible electrical thickness and lower insertion losses. As a key branch, time-varying and spatiotemporally-modulated metamaterials and metasurfaces have currently attracted strong interests and brought many exotic applications by exploring the temporal dimension of their constitutive parameters.
As the digital version of metasurfaces, the concept of digital coding and programmable metasurfaces was originally put forward by our group in 2014 [Cui et al., Light: Science & Applications 3, e218, 2014]. By designing two distinct coding elements with opposite reflection phases (e.g., 0 and 180°) as the digital bits “0” and “1”, one can manipulate the EM waves by changing the coding pattern. Besides providing a versatile and effective technological platform for wave manipulations, the digital coding metasurface has built a bridge between the physical and digital worlds, making it possible to revisit metamaterials from the perspective of information science. In 2018, we further expanded this paradigm by introducing the concept of “space-time-coding digital metasurfaces” [Zhang et al., Nature Communications 9, 4334, 2018], which allows to attain simultaneous manipulations of EM waves in both space and frequency domains, i.e., to control the propagation direction and harmonic frequency distribution simultaneously. More importantly, we can encode the digital messages into the spatial beams and frequency spectra simultaneously, significantly increasing the information encoding capabilities.
In the area of wireless communications, multiplexing techniques can successfully establish multiple independent channels between the transmitters and receivers, improving the capacity of the network. Several multiplexing techniques have emerged in the past decades, such as the frequency-division multiplexing (FDM), time-division multiplexing (TDM), code-division multiplexing (CDM), and space-division multiplexing (SDM). FDM always requires high-performance filters and mixers to divide the frequency range, and SDM is typically requires many antennas, each with a radio frequency (RF) chain, to form a phased array, which leads to the communications systems with high cost and high complexity.
Specifically, the space-time-coding digital metasurfaces have the characteristics of low cost, simple structures, and easy implementations in practice, which can be used to control the spatial and spectral characteristics of the EM waves simultaneously. This important property motivates our study here, which is essentially suitable for implementing SDM and FDM together in the wireless communications without using the traditional radiofrequency components such as antenna arrays, filters and mixers. Hence, we leverage the physical property of the space-time-coding digital metasurface to propose a new wireless communication scheme with both space- and frequency-division multiplexing. In our method, the original information is directly modulated in the spatial and spectral characteristics of the carrier wave reflected by the space-time-coding digital metasurface. By encoding space-time-coding matrices through multiple channels, the digital messages can be directly transmitted to different users at different locations simultaneously and independently (see Fig. 1), without the need for digital-analog conversions and mixing processes in the traditional technologies. Each designated user has its own independent receiving channel via a specific harmonic frequency, while the undesired users located at other directions cannot recover the correct information.
The space-time-coding digital metasurface plays the role of information modulation and energy radiation at the same time. With the aid of space-time-coding strategy, the information encoding and processing can be performed not only in the time domain but also in the space domain. To illustrate the direct data transmission, we build a dual-channel wireless communication system based on a 2-bit space-time-coding digital metasurface and use it to transmit two different pictures to two users simultaneously in real time (see Fig. 2). The dual-channel system has low interference between different user channels and can achieve a transmission rate of 2.5 Mbps. The corresponding experimental results with good performance validate the information encoding scheme. Overall, our approach offers a low-cost and low-complexity solution for implementing SDM and FDM in a wireless communications network, and also has the characteristics of directional modulation and secure information transmission, which will find potential applications in the next-generation wireless communication and radar systems.
For more details, please read our recent publication in Nature Electronics: "A wireless communication scheme based on space- and frequency-division multiplexing using digital metasurfaces".