## 2 Literature Review
### Nomenclature and Literature Terminologies  
IRS is the literature term adopted in this research paper, but the general idea refers to any passive intelligent or software-controlled metasurface with the ability to reconfigure the incident signal toward the user, thus creating a programmable wireless environment [11]. There are many common terminologies that refer to the same idea as an IRS. Some of them are highlighted in a recent survey paper [62]. IRS terminologies including those that are listed in [62] are mentioned below:  
* Reconfigurable intelligent surface (RIS): It is a thin and cheap wallpaper-like surface that can reconfigure the radio propagation with the aid of a software program [63].
* Large intelligent metasurface (LIMS): These are equipped with a large number of low-cost metamaterial antennas with the ability to passively reflect the incident signals by certain phase shifts, without signal processing capability, thus improving the signal at receivers [64].  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Key Concepts** & **Solution** & **References** & **Year** \\ \hline Intelligent Walls & Frequency Selective Surfaces & [48; 49] & 2012 \\ \hline
2-D Meta Materials (MM) & Meta-Surface (MS) & [33] & 2012 \\ \hline Tunable MS & Spatial Microwave Modulators & [50] & 2014 \\ \hline Coding MMs & EM properties & [51] & 2014 \\ \hline Software MMs & Software based & [52] & 2015 \\ \hline Programmable MS & MS equipped with pin diode & [54] & 2016 \\ \hline Phase Reflect Arrays & Smart Reconfiguration & [53] & 2016 \\ \hline Large IS & Transceiver beyond mMIMO & [36; 55; 56] & 2017 \\ \hline Holographic MS & Software control of EM waves & [59; 60] & 2018 \\ \hline IRS & Phase shifting of unit cell & [61] & 2018 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Historical Perspective of IRS.  
Figure 2: IRS timeline.  
* Software defined metasurface (SDMS): It manipulates impinging EM waves in complex ways by altering the direction, power, frequency spectrum, polarity and phase, thus creating a programmable wireless environment [65].
* Large intelligent surface (LIS): It is aided by a large active antenna-array and dedicated RF chains to lower the power consumption of the system [56; 66].
* Passive intelligent surface (PIS): This technology is an alternative to active antenna arrays and passively reflects incident signals, making it an emerging green technology. It can support the high data rate and energy sustainability demands of ubiquitously deployed users in 5G networks [67].
* Smart reflect arrays: These are closely related to an IRS with the capability to solve the problem of signal blockage in mmWave indoor communications by steering the incident signal toward the user destination to establish a robust link between transceivers [68].
* Passive holographic MIMO (HMIMO): It is a low-cost wireless planar surface composed of subwavelength dielectric scattering particles, that alters EM waves according to desired objectives and optimizes the wireless environment while achieving high-throughput, massively connected, and low-latency communications at a reduced power budget [36].  
LIS, RIS, LIMS, SDMS and IRS are one and the same, and all these green technologies are expected to function similarly to improve the EE and SE at a lower cost. These do not perform any signal processing, rather they reconfigure the incident signal to the desired user, thus maximizing efficiency and are mostly based on phase shifting without amplification.