Active Intelligent Surfaces for Next Generation Radio Systems: An Overview on Large Intelligent Surfaces and Radio Stripes

Massive multiple-input-multiple-output (mMIMO) represents one of the most important breakthroughs in modern telecommunication theory. While actual smart antenna systems developed in the fifth generation (5G) era allow very large peak spectral efficiencies (SEs) in areas close to the base station, beyond-5G (B5G) systems will demand consistent, high SE over larger areas, so that the users can experience a ubiquitous service. For this purpose, the B5G mMIMO system's research tendency is based on the increase of antenna density in the communication structures. Following this approach, large intelligent surfaces (LIS) and radio stripes (RS) systems have been extensively studied in recent years. However, since LIS and RS systems take the mMIMO concept to an extreme level, it brings some important challenges to the table. Its practical implementation requires simplified hardware and signal processing techniques in order to maintain the entire system at an affordable complexity. This includes the development of efficient transmission/detection and resource allocation techniques. This work explores the state-of-the-art designs of the LIS and RS systems while mentioning the upcoming communication challenges that will be present in future B5G wireless communication systems that incorporate an antenna ecosystem formed by several LIS and RS.