Principal Investigator: Professor Chen Xudong, ECE

Wireless communication systems have been significantly advanced in the last several decades and will continue to evolve in years ahead to support the ever-growing number of wireless devices and meet their more demanding communication requirements in terms of data rate, reliability, and latency, as driven by the emerging new applications such as AR/VR, autonomous vehicles, edge computing, industry IoT, etc. To achieve this end, most of existing wireless systems (e.g., 5G, WiFi) have been designed following the same approach of adding increasingly more active helping nodes (e.g., base station (BS), access point, active relay) with more antennas into the wireless network (a.k.a. ultra-dense network (UDN), massive MIMO, etc.) in order to increase the system area spectral efficiency in bits per second per Hertz per unit area. However, this approach not only aggravates the network interference issue, but also incurs increasingly higher costs in hardware, deployment and energy consumption, especially for systems that are to operate in higher frequency bands (e.g. mmWave, THz). Thus, it is still imperative to research into new and more efficient approaches for designing future wireless network (e.g., 6G) to sustain its capacity growth cost-effectively. On the other hand, existing physical-layer wireless transmission techniques (such as beamforming, power control, adaptive modulation, etc.) were devised to cater to the time-varying wireless channels
due to users’ mobility. However, the wireless channel or propagation environment itself has been traditionally regarded as random and uncontrollable. Given the knowledge of time-varying channels, existing techniques can already approach closely the fundamental capacity limit of wireless systems (known as the Shannon capacity, defined as the maximum mutual information, I, between the channel input, X, and output, Y, conditioned on the given channel, H, i.e., I(X;Y|H)). However, it still remains open what will be the new and higher capacity limit of future wireless systems if the channel H is no more treated as random, but instead can be arbitrarily controlled/programmed to further enhance I(X;Y|H) with favorable H’s.

To address the two important issues above, this project proposes a fundamentally new approach to design wireless networks in the future that will be different from all existing ones. Specifically, unlike traditional wireless systems that are composed of active nodes/antennas only, the proposed new network will consist of not only the active components, but also a new type of passive devices called reconfigurable intelligent surface (RIS). RIS is a planar array consisting of a large number (say, hundreds or even thousands) of reconfigurable passive elements (e.g., low-cost printed dipoles), which are able to independently induce a certain amplitude variation and/or phase shift (controlled by a smart controller attached to them) on the incident electromagnetic (EM) wave, without the need of any active RF circuit like A/D converter or power amplifier (thus, featuring very low hardware and energy costs). By jointly designing the reflection coefficients of its passive elements, each RIS is capable of smartly altering its local signal propagation and a network of densely deployed RISs will enable a smart and reconfigurable propagation environment for wireless communication, which can potentially lead to a quantum leap in its capacity limit, yet at much reduced cost as compared to traditional active beamforming/relaying.