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Reconfigurable intelligent surface (RIS) is an emerging technology that isunder investigation for different applications in wireless communications. RISsare often analyzed and optimized by considering simplified electromagneticreradiation models. In this chapter, we aim to study the impact of realisticreradiation models for RISs as a function of the sub-wavelength inter-distancebetween nearby elements of the RIS, the quantization levels of the reflectioncoefficients, the interplay between the amplitude and phase of the reflectioncoefficients, and the presence of electromagnetic interference. We considerboth case studies in which the users may be located in the far-field andnear-field regions of an RIS. Our study shows that, due to design constraints,such as the need of using quantized reflection coefficients or the inherentinterplay between the phase and the amplitude of the reflection coefficients,an RIS may reradiate power towards unwanted directions that depend on theintended and interfering electromagnetic waves. Therefore, it is in generalimportant to optimize an RIS by considering the entire reradiation pattern bydesign to maximize the reradiated power towards the desired directions ofreradiation while keeping the power reradiated towards other unwanteddirections at a low level. Our study shows that a 2-bit digitally controllableRIS with an almost constant reflection amplitude as a function of the appliedphase shift, and whose scattering elements have a size and an inter-distancebetween (1/8)th and (1/4)th of the signal wavelength may be a good tradeoffbetween performance, implementation complexity and cost. However, the presentedresults are preliminary and pave the way for further research into theperformance of RISs based on accurate and realistic electromagnetic reradiationmodels.

Digital Reconfigurable Intelligent Surfaces: On the Impact of Realistic Reradiation Models