Pick up nearly any publication and you’ll see at least one story about how Fifth Generation (5G) networks will extend the scope of mobile communication services from interpersonal communication to smart inter-connections between billions of devices – 27 billion by the end of this year, according to Statistica.
Fewer people are discussing the new challenges being posed to underlying cellular network resources by this massive number of interconnected devices and the diversified nature of the services that they support. To address these challenges, the 3GPP has recently completed the global standard for 5G radio interfaces in its Release 15, which focuses on fulfilling key performance indicators that include greater than 10 Gbps peak downlink rates for enhanced Mobile Broadband (eMBB), at least one million connected devices per square kilometer for the Internet of Things (IoT) / massive Machine Type Communications (mMTC) and less than one millisecond latency for Ultra Reliable Low Latency Communications (URLLC). The 5G standard has been a result of various technologies that include massive multiple-input multiple-output (MIMO), millimeter wave (mmWave) communication and network densification.
However, these technologies face two key practical limitations. First, they consume a lot of power, which is a critical issue for practical implementation. Second, they struggle to provide the users with uninterrupted connectivity and quality of service guarantee in harsh propagation environments, due to the lack of control over the wireless propagation channel – posing serious challenges to emerging applications like augmented / virtual reality, autonomous driving and drones. For example: the network’s total energy consumption scales linearly as more base stations (BS)s are added to densify the network or more antennas are added to the massive MIMO arrays. Moreover, massive MIMO performance is known to suffer when the propagation environment exhibits poor scattering conditions, whereas communication at mmWave frequencies suffers from high path and penetration losses. These two limitations result in the need for green and sustainable cellular networks where the network operator has control over the propagation environment.
Making the Physical Environment Software-Controlled
One concept that is emerging to address these needs is that of a smart propagation environment, where the wireless propagation environment is turned into an intelligent reconfigurable space that plays an active role in transferring radio signals from the BS to the users. This contrasts with the current wireless networks, which view the propagation environment as fixed and as an adversary to the communication process – the effects of which need to be counteracted at the transmitter and receiver.
Smart propagation environments largely expand the concept of software-controlled networks from the logical domain to the physical domain. The propagation environment itself is viewed as a software entity, which can be remotely programmed, configured, and optimized. This concept is realizable through different emerging technologies, which include deploying smart reflect arrays in the environment or coating the environmental objects with reconfigurable meta-surfaces that apply wave transformations on the impinging electromagnetic waves to shape them in desirable ways. These surfaces, broadly referred to as intelligent reflecting surfaces (IRSs) in the literature, are passive in the sense that they enable wireless network operators to enhance coverage and performance without generating new radio waves but by recycling and reconfiguring those that already exist in the environment. This also allows battery-constrained tiny devices, like wearable medical sensors, to backscatter the radio waves generated by cellular base stations in order to report their sensed data to mobile phones ‘for free.’ In fact, unlike BSs and relays, IRSs do not really require active power sources for their operation. The circuitry required to make these surfaces reconfigurable can be powered with energy harvesting modules, rendering this technology truly energy-neutral.
Making IRSs a Reality
Smart propagation environments enabled by IRSs is a recent but feasible technology with current research activities focusing on fabricating new metasurfaces and reflect arrays to make them re-configurable using software-defined protocols and implementing testbeds. The communication-theoretic foundation of the system model for smart propagation environments is still unknown. My research focuses on understanding how the availability of IRSs will allow wireless communication researchers to redesign common and well-known network communication paradigms. My particular emphasis is on 1) incorporating IRSs into 5G mmWave MIMO communication systems, 2) enabling the resulting systems by developing accurate channel models and low-complexity channel estimation protocols and 3) designing transmit and reflect beamforming schemes for the BS and IRSs respectively to yield unprecedented massive MIMO-like gains but with a reduced number of active antennas at the BS – thereby enabling cheaper implementation and lower energy consumption.
In one of our projects on this subject, we showed that when using a smart surface with N reflecting elements in a single-user MIMO system, the received signal strength can scale as N2 for a fixed transmit power at the BS. This also implies that the required power at the BS can be scaled down by a factor of 1/N2 without compromising the received signal strength. This squared gain is because the IRS not only achieves a beamforming gain of N in the IRS-to-user link (similar to massive MIMO), but also yields an inherent aperture gain of N by collecting all the radiated power in the BS-to-IRS link. The latter cannot be achieved by increasing the number of antennas in massive MIMO antenna arrays.
While contributing on the theoretical front, I am also collaborating with researchers from the Advanced Materials and Manufacturing research division at UBC to carry out proof-of-concept experiments with the developed hardware prototypes for IRSs. These experiments aim to demonstrate the commercial feasibility of this technology and its efficiency in overcoming signal blockages and enhancing coverage, especially at mmWave frequencies. We hope to report some measurement results by the year-end.
The Impact
The societal and economic impact of smart propagation environments can be radical and deep. The idea of turning the environmental objects, which usually act as unintentional adversaries to the wireless communication process, into programmable entities constitutes a transformative wireless future, where communication and information transfer can be made cheaper, more reliable and more energy efficient.
For example, smart propagation environments can capitalize on the programmed reflections of waves to make the received signals stronger, which not only enables mmWave communication in dense urban scenarios with high-rise buildings, but also has major benefits in virtual reality (VR) applications. For example, these surfaces can enable multi-Gbps wireless communication between VR headsets and PCs/smart phones/gaming consoles etc. Smart surfaces can also help us reduce the transmit power in sensitive environments, such as airplanes, where the walls and ceilings of cabins can be coated with IRSs to enable strong Wifi connections with reduced emission of radio waves from the devices. This concept will also enable us to think of smart cities, where “smart” encompasses the physical environment as well.
