# Intelligent Reflective Surfaces for Wireless Networks:
## 4 IRS Applications  
By considering the tremendous amount of benefits offered by the IRSs, several application scenarios can be examined by considering this technology. Figure 5 shows several applications which could benefit from the use of IRS technology. The first illustrated application is related to scenarios where some user is totally blocked and not available to communicate with a BS, (e.g., [75]). In this case, the IRS could be employed to create a feasible connection, i.e., a virtual link between both ends. This is particularly useful for problems of coverage extension in mmWave and THz communications systems due to the unfavourable free-space omnidirectional path loss in these frequency bands. The second presented application in Figure 5 constitutes a scenario with legitimate users and an eavesdropper device. In this case, the IRS could be employed to mitigate the effect of the eavesdropper by properly cancelling its signal and thus increasing the communication system security. In [80] the authors employ the IRS technology to increase the communication system security by developing a strategy for jamming interference mitigation. In the third scenario illustrated in Figure 5, as it is considered in [39; 67], a cellular network with a cell edge user--which can suffer high signal attenuation from the BS and co-channel interference from near BSs--is considered. In this scenario, the IRS deployment could be helpful to increase the coverage area in the cellular network. The fourth application focuses on device-to-device (D2D) networks [90], where the IRS could be used to cancel the interference, support the required low-power transmission in these systems, and enhance individual data links in these communication systems. The fifth illustrated application in Figure 5 is the use of IRSs for the mmWave band, where a high sensitivity to blockage is present in the propagation [91]. The IRSs are useful in these scenarios to increase the received power, the channel rank, and thus the spatial diversity needed for outdoor systems. The sixth application shown in Figure 5 is the use of IRSs for Internet of Things (IoT) systems (e.g., [61]), where a multi-IRS scenario seems to be useful to compensate the power loss over long distances and alleviate the energy budget issue in energy-constrained IoT networks via the passive IRS beamforming. The use of the IRS is also interesting for B5G-NOMA systems (e.g., [66; 67]). In these scenarios, the IRS could be considered to increase the number of served users and enhance the rate of communication, which constitutes the major requirement to be accomplished in these systems. Another interesting application illustrated in Figure 5 is the use of the IRS technology for CR networks (see [77]), where the IRS could be used to increase the degrees of freedom to further improve the CR network performance. Specifically, the IRS could be employed to improve the efficiency of the secondary transmissions. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Reference** & **System Model** & **Performance Metric** & **Main Results** \\ \hline \multirow{3}{*}{[86]} & MIMO and & & Small gap \\  & MISO setups, & & between \(\infty\) bit and \\  & IRS with & & low resolution & low resolution IRSs \\ \hline \multirow{3}{*}{[87]} & Discrete/continuous & & High RE for \\  & IRS phase shifts. & & low resolution \\  & Partial CSI & & phase shifts \\ \hline \multirow{3}{*}{[89]} & MEC LOS, & Completed & Close match between \\  & DL & task-input bits & the BCD performance \\ \cline{1-1}  & & & and the low complexity \\ \cline{1-1}  & & & algorithm performance \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Multiuser MIMO approaches. The use of IRSs is beneficial for MEC systems by enhancing both the EE and the SE. For example, in [71] the transmit power of the mobile users is minimized by considering an infrastructure to perform ML tasks in the MEC server.