On 5 February 2019, the research group headed by Professor Tiejun Cui from Southeast University, China has proposed a completely new digital wireless communication architecture based on the digital coding metasurface. The research article entitled “Topologically Protected Edge State in Two-Dimensional Su–Schrieffer–Heeger Circuit” has been published in Research (Research 2019, Article ID: 8609875 DOI: 10.1155/2019/8609875) https://spj.sciencemag.org/research/2019/8609875/.
Digital wireless communication system has experienced rapid development in the past thirty years, and has become indispensable in daily life. In the currently used wireless communication systems, the information to be sent is firstly digitized, converted to analog, modulated, and finally transmitted to terminals through a series of modules including digital-analog (D/A) convertor, modulator, demodulator, mixer, digital up convertor (DUC) and digital down convertor (DDC), and expensive RF components. One of the most important processes in the wireless communication systems is the signal modulation, where the digital bit stream is conveyed into electromagnetic waves that can be physically transmitted. Some widely used modulation techniques include amplitude-shift keying (ASK), frequency-shift keying (FSK), phase-shift keying (PSK), and quadrature amplitude modulation (QAM), all of which are designed to modulate the digital signal through variations in the amplitude, frequency, and phase of the carrier wave (see Figure 1a). The modulated signal will be then up-shifted to radio frequency, amplified by a series of power amplifiers, and finally radiated to free space using antennas.
Recently, a new kind of metasurface, digital coding and programmable metasurface, has been proposed, which can manipulate the electromagnetic waves in real time by changing the digital coding sequences applied on the metasurface. Since the radiation pattern provides the information of the digital coding sequences on the programmable metasurface, it is possible to utilize the variation of far-field radiation patterns as a new modulation approach for the wireless communication system, as illustrated in the last row in Figure 1a. Based on this idea, researchers from Southeast University, China, proposed a completely new digital wireless communication architecture based on the digital coding metasurface. As the signal is directly modulated by the dynamically changing radiation patterns and does not require the modules used in the traditional digital wireless communication systems (e.g. the mixers and D/A convertors), the system is named as directly digital modulation (DDM). Owing to the simplified system architecture of the DDM system in both hardware and software, the digitized signal can be directly applied on the digital coding metasurface, and is sent to free space under the illumination of a feeding antenna (see Figure 1b). The information, being modulated in all possible radiation patterns of the metasurface, can be correctly received by multiple receivers distributed in different locations in the far field region.
Figure 1: (a) The popular modulation techniques ASK, FSK, and PSK for traditional wireless communication systems (the first three rows) and the radiation-pattern-based modulation for the proposed DDM system (the last row). (b) Schematic illustration of the working mechanism of the proposed DDM system. (c) Prototype of the DDM wireless communication system.
A prototype was fabricated to validate the new wireless communication system, as shown in Figure 1c. The authors tested the performance of the DDM system by transmitting a binary image (Figure 2a) under different circumstances. The image was correctly received with two receivers in the 2-bit-symbol transmission mode at a distance of 50cm (Figure 2b). They also developed a channel optimization algorithm to improve the transmission rate as well as system robustness against channel disturbance, allowing the DDM system to work properly with the presence of obstacles in the wireless channel. Figure 2c shows that severe transmission error occurs as soon as a metal plate is inserted between the transmitter and receivers, but a correct image can still be received under the help of the channel optimization algorithm by dynamically updating the best digital state for optimum transmission (Figure 2d). Most importantly, the new modulation technique can protect the information from being intercepted from either a single or multiple positions, making it very promising for secrete communication. It is expected that the revolutionary digital-metasurface-based DDM system will find promising applications in situations where the information security is highly demanded, for example, satellite to earth communication and military wireless communication.
Figure 2: (a) The original image to be transmitted. (b) The received image under the 2-bit-symbol transmission mode. (c) The received image when a metal plate is placed between the programmable metasurface and two receiving antennas. (d) The correctly received image after turning on the channel auto-scan program.
Tag: Emerging materials research