By: Bradley Mee
Picture the map room scene from Indiana Jones: Raiders of the Lost Ark – the titular hero armed with the recently liberated headpiece of the Staff of Ra, fixes it to a long stick and places the headpiece within a broad beam of diffuse sunlight penetrating the underground chamber to reveal a map holding the location of the Well of souls: the burnished ruby jewel held within the headpiece acts to focus the sunlight into a narrow beam of electromagnetic energy that Indy steers over the map until the white light shifts to red and reveals the location of the Ark of the Covenant.
Reconfigurable Intelligent Surfaces (RIS) are a 2D metasurface that, as with the Staff of Ra, act to direct and focus electromagnetic (EM) energy by shifting the phase of incident waves. However, rather than visible light, RIS operate at wireless communications frequencies such as mobile phones (1 GHz), Bluetooth (2.4 GHz) or Wi-Fi (5 GHz), and the RIS does not feature an elegant jewel to shift the phase of these signals but in this project, Liquid Crystals (LC) are employed to adjust the phase of radio waves.
Traditional wireless communications systems consist of a transmitter, receiver, and the environment through which the information is sent: while it is possible to design and tailor the transmitter and receiver to meet requirements, the wireless channel has erstwhile not been subject to significant modification. As radio waves propagate between a transmitter and receiver, they take many reflected and scattered paths, which travel slightly different distances, introducing various shifts in phase that causes the overall received energy to combine in a way that denigrates the quality of the received signal – known as destructive interference. The aim of the RIS is to adjust the phase of the reflected signals such that they combine constructively at the receiver to improve the signal quality.
RIS are of particular interest in modern communications systems, with the number of transmitting devices increasing exponentially, as a way to improve energy efficiency and clean up the bustling radio environment. 6G technologies will be characterised by low-latency applications such as control networks of driverless vehicles in urban areas – these will require ubiquitous coverage to unsure no signal dead zones exist: RIS can also be used a means to direct signals around large blocking obstacles to prevent outages.
LC is an intermediary phase of matter between a crystal solid and an isotropic liquid, exhibiting properties of each. The fundamental characteristics of LC can be tuned using small voltages that align the molecular structure in a certain direction, which can cause an EM wave to pass through the material more slowly. This continuous and reversable feature of the LC makes it a candidate to form an integral part of the RIS. By applying a small voltage to the LC, the RIS can adjust the phase of waves arriving at its surface such that the waves are then directed toward a nearby receiver and arrive in-phase.
Even if you have not heard of LC before, you have likely spent hundreds of hours staring at them. Liquid Crystal Display (LCD) TVs operate using thousands of LC cells, or pixels, working independently to alter the phase of visible light which results in a certain colour forming from each pixel to generate the picture. One of the challenges for this project is to apply the theory and techniques from the optical domain to radio waves. Other aspects of the project will focus on developing individual RIS elements to find, through simulations and experiments, the configuration that is best able to apply a full phase-shift to incident signals.
RIS technology will be introduced for future wireless communications standards, such as 6G, and in particular, for high frequency Terahertz communications [1]. They are designed to be a passive technology that do not increase the power of signal and hence reduce energy requirements. Furthermore, an LC-based RIS can exploit solar cell technology to provide the low tuning voltages. They can be situated on the outside of buildings to assist with communications dead spots, prevent eavesdroppers from intercepting signals, or within windows to boost signals that are attenuated by the glass [2]. Although an RIS unlikely to locate the Ark of the Covenant, the overall benefit of these systems is to improve signal quality of wireless communications, without increasing the signal power, and to make use of wasted EM energy that is bouncing around the environment. As the number of wireless communication devices increases to the point where almost all electrical gadgets around the home and office will be connected to the internet of things, RIS will be a vital tool in providing low-power solutions that help to reduced wasted RF energy and clean up the radio environment.
References
[1] C. Liaskos, S. Niw, A. Tsioliaridou, A. Pitsillides, S. Ioannidis, and I. Akyildiz, “A new wireless communication paradigm through software-controlled metasurfaces”, IEEE Comm. Mag., vol. 56, no 9, pp. 162-169, Sep. 2018.
[2] Q. Wu and R. Zhang, “Towards smart and reconfigurable environment: Intelligent reflecting surface aided wireless network”, IEEE Comm. Mag., Jan. 2020.