Because of its ability to flexibly control the electromagnetic characteristics in the channel environment, the Reconfigurable Intelligent Surface (RIS) has developed rapidly in academic research and industrial promotion and is considered as one of the key candidate technologies for 5G Advanced and 6G networks. RIS has the opportunity to build an intelligent wireless electromagnetic environment through its ability to flexibly and abnormally regulate radio wave transmission. The introduction of RIS may build a new network paradigm, which brings new possibilities to the future network, but also leads to many new challenges in technology and engineering applications.
5G technology is currently in a period of rapid commercial use, while 6G technology is also in intensive research and development. The massive data transmission between people and people, people and things, and things and things brought by the interconnection of everything poses new challenges to the reliability, real-time, transmission rate, network capacity, traffic density and other aspects of the existing network. Among many emerging technologies, intelligent hypersurface (RIS) technology with intelligent perception and control capabilities has gradually attracted the attention of scholars in the field of global communications and has become one of the key enabling technologies for 6G wireless communications.
Supersurface is an artificial electromagnetic structure formed by sub wavelength scale units (typically 1/10~1/3 wavelength) arranged in a specific space. It has the characteristics of lightweight, low profile, easy integration, easy conformability, etc. After the design and manufacture of the traditional super surface, its electromagnetic wave response and electromagnetic function are solidified, and can no longer be changed according to the actual needs. However, in order to meet the needs of complex electromagnetic systems, the electromagnetic characteristics of hypersurfaces often need to be flexibly adjusted, so the concept of programmable hypersurfaces came into being.
By loading specific regulatory devices, such as PIN tube (P-I-N diode), varactor and MEMS switch, RIS can conduct real-time programmable regulation on the amplitude, phase, frequency, polarization and other characteristics of electromagnetic waves, thus controlling the propagation behavior of electromagnetic waves in free space, breaking through the limitations of traditional wireless channels that cannot be actively regulated, and building a new paradigm of intelligent programmable wireless environment. At the same time, the hardware architecture of wireless communication transceivers based on RIS has the characteristics of simple architecture, low power consumption, low cost, etc. Only using RIS and baseband modules can complete information modulation and transmission, eliminating signal mixing, up-conversion, amplification and other processes, providing a new solution for the next generation of wireless communication.
When there are insurmountable obstacles between the base station and the terminal, there is a nonline of sight channel between them. If the signal propagation environment is single and there is no reflection path, the signal received by the terminal is very weak. With RIS, you can control the reflected beam, aim at the terminal in the blind area and track dynamically, which is equivalent to creating a virtual LOS path and expanding the coverage of the cell.
When the network detects eavesdroppers or illegal users, the phase of the reflected signal of RIS can be adjusted to offset the direct signal when receiving, so as to reduce information leakage.
When the signal transmission environment is relatively simple, there is often a lack of independent multipath, and it is difficult to achieve sufficient multi-stream transmission. Through the reflection of RIS, the signal propagation path can be artificially increased to better achieve multi-stream transmission and improve the throughput of hot users.
When the terminal is located at the edge of the cell, RIS is used to dynamically control the reflected signals of the serving cell and the adjacent cell, so that the signals of the serving cell are in phase superimposed and enhanced, and the signals from the adjacent cell are inversely superimposed and canceled, thus effectively eliminating the interference of the adjacent cell.
RIS can perform interference suppression through intelligent reflection of multi-channel signals, and simultaneously conduct low-power transmission, which is conducive to large-scale D2D communication.
Transmission of wireless power and information in the Internet of Things. Synchronous wireless information and energy transmission technology can simultaneously transmit signal and energy, providing energy for wireless devices while exchanging information with wireless devices. RIS can play a similar role as a relay. It compensates for the huge energy consumption caused by long-distance transmission through passive beams and helps the charging area improve the wireless transmission power.
To solve indoor coverage, outdoor base station signals can penetrate the building's exterior walls or windows, or professional room subsystems can be deployed. Both of these methods can be used by RIS. For the way of outdoor penetrating indoor, transparent RIS boards can be deployed on the glass surface of building windows to control the signal to enter the room and achieve a certain gain.
In addition to reshaping the wireless channel, RIS can also realize the function of signal transmitters or receivers. How is this achieved?
Since RIS can be encoded in real time, we import the baseband signal into the RIS controller in the form of encoding, and then transmit the RF carrier of the target frequency band to the RIS, so that the baseband signal can be modulated onto the carrier through reflection. The transmitter with this architecture can save complex and inefficient RF links, high energy consumption mixers, power amplifiers and other devices, thus significantly reducing the cost and power consumption of the transmitter.
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