# Intelligent Reflective Surfaces for Wireless Networks:
## 6 Conclusions  
This paper is a comprehensive survey on IRS technology that addresses the main theoretical aspects of the IRSs, the main approached issues in the state-of-the-art, the applications of IRSs for future wireless communication systems, and the open issues to be approached in the future. First, we have introduced the meta-surface concept, its properties, and some specific hardware parameters to be considered in the design of the IRSs. Next, we have addressed the IRS controller by describing its functionalities and some practical implementations. A comprehensive survey on the approached scenarios by considering the IRS technology for wireless communication systems has been performed. The corresponding system models and their particularities such as multi-antenna setups, the radio propagation conditions (NLOS and LOS), and the hardware impairments have been analyzed. The metrics considered to evaluate the system performance and the main results of relevant works in the literature under different communication system scenarios have been summarized. A vast amount of techniques to handle the problems of the IRS design and the channel estimation in these setups have been analyzed, e.g., DL, AO, and PG. Then, some applications where the IRS technology could result beneficial have been described, including the communication blockage --which is common in the mmWave band--the security, the benefits in mobile communications by improving the service quality for cell edge users, the D2D communications where the IRS-assisted systems suit with the required low-power transmission, the IoT systems, where the IRS-aided systems could be employed to alleviate the energy budget issues, the B5G NOMA systems, where the IRS deployment results in benefits to maximize the rate, the CR system where the EE of the secondary transmissions can be improved by considering IRS-aided systems, the MEC systems, where the use of IRSs enhances the EE and the SE, and the UAV systems, which also benefit of the IRS technology and the power transfer systems. Finally, the practical challenges and the identified open problems for the design of the IRS-aided wireless systems such as the joint design of the IRS matrix and the hybrid transceivers in mmWave, the user scheduling in practical systems, and the special considerations under wideband scenarios such as the beam squint effect have been suggested. After reviewing a large number of references in the literature related to the IRS technology, it is clear that this technology represents a very promising strategy to be incorporated in a countless number of wireless communication scenarios. The idea of controlling the propagation environment by using arrays of passive elements with negligible power consumption is extremely attractive to reduce the economic cost and the environmental impact of implementing the future B5G architectures, where the requirements in terms of SE and user accommodation will be extremely challenging. However, the practical deployment of this technology is still in an early stage due to several issues which make it difficult, such as hardware impairments, the difficulty of acquiring accurate channel information, or the need of optimizing the positioning of IRSs depending on the communications scenario. For this reason, it is nowadays complicated to find real implementations of IRS-aided wireless systems in the industry field, and even testbed prototypes that allow evaluating the practical feasibility of this technology. In this regard, it is clear that more research and technical advances are necessary to address multiple open issues with the aim of moving to a true consolidation of IRS-aided communication systems in the future. Hence, though significant performance improvement can be identified in the literature, IRS-aided systems are expected to provide higher performance gains over the current state-of-the-art approaches by addressing a significant number of practical issues regarding mobility, user scheduling, coding, etc.