2.3. Fundamental Building Block of Intelligent Reflecting Surfaces

The metamaterial concept also plays a fundamental role in building reflection based surfaces, particularly in the context of intelligent reflecting surfaces (IRS) [10,11,44]. An IRS consisting of ideal unit-cells is preferred (and often assumed) to have a uniform gain radiation mask within a coverage area along polar and azimuth (i.e., ϕ) directions [45,46,47]. This is not the case when practical unit-cells like the one shown in Figure 5 are used to build a reflective metasurface used as an IRS. To explain this, let us look at a simulation setup given in Figure 7 in which a pathloss system model is implied to record received signal profile when it is served by a non-ideal (practical) IRS. We considered a metasurface having a length of lx=15λ along the x-axis and ly=15λ along the y-axis. The total path loss at the receiver when it’s distance from the metasurface is dr and the distance between the transmitter and the metasurface is dt can be written as:

(a) Pathloss system model with non-ideal reflective metasurface developed using an array of unit-cells, used as an IRS, (b) Total path loss incurred by received signal at the receiver device when it is moved along polar angle (θ).

When Gt and Gr are the transmitter and receiver antenna gains [47,48], Gm(θ) is the metasurface reflective gain pattern along the polar angle θ, while θi and θr are the transmitter and receiver positions relative to the metasurface’s broadside direction. For the sake of simplicity in analysis, consider uniform gains of the transmitter and receiver antennas Gt=Gr=3 dBi, and uniform distance between transmitter and metasurface (dt), and a uniform distance between metasurface and the receiver (dr), i.e., dt=dr=30 m. We simulated a receiver moving from θ=0° to θ=90° as shown in Figure 7, while the metasurface was separately configured to serve the receiver at intervals 15° apart within the polar angle range of θ=0° to θ=90°. The simulated results are presented in Figure 7. It is clear that the total pathloss profile is close to ideal (and also predictable) when the receiver is located at the IRS broadside, i.e., 0°, but as the receiver moves away from the broadside along the polar direction, the pathloss profile deviates from ideal, and becomes almost unpredictable after a specific polar angle (in the given example, after 60°). Thus, contrary to popular belief, the results in Figure 7 show that the service quality of an IRS-aided communication system relies significantly on equalized and consistent radiating fields from the reflective metasurfaces. This makes designing and testing of non-ideal unit-cells a research priority if we wish to achieve efficient IRS design, which can facilitate high data rate communication.