# An Overview of Indoor Localization System for Human Activity Recognition (HAR) in Healthcare
## 6 Systems for HAR
### Intelligent Metasurfaces  
In this subsection, an attractive, innovative technology is reported: intelligent metasurfaces, representing an emerging research topic that involves various disciplines, with digitization, programmability, and intelligence capabilities. Intelligence refers to the extraordinary property of making decisions, self-programming, and performing tasks without human intervention. It is an attractive technology for its potential to configure the propagation environment, making it more controlled. In wireless transmissions, a penalizing phenomenon for radio waves is represented by the presence of the adverse effects produced by the propagation of incident EM waves; walls, ceilings, floors, and objects act as scatters, creating multipath paths. Controlling the propagation of radio waves can mitigate these adverse effects. Research in recent years in wireless technology has led to the creation of reconfigurable intelligent surfaces (RISs). These metasurfaces can counteract the adverse effects caused by path loss, signal absorption, and multipath fading, effectively controlling the EM response in reflected, refracted, and diffracted waves. A metasurface is an artificial planar structure that contains repeated conductive elements, i.e., meta-atoms, on a dielectric substrate. The meta-atom is integrated with tunable functional materials and is designed to be reprogrammable. There are two different topologies of these surfaces: a programmable thin wallpaper and a programmable thin glass. Both have the following characteristics:  
* Do not emit new radio waves;
* No power amplification;
* Low power consumption for operation;
* Low processing capacity for surface configuration. It is a technology that is applied in different sectors. Metasurfaces can be installed in buildings to improve coverage, increase spectral efficiency and reduce exposure to EM radiation [73]. They can also be used on cars, trains, and planes to improve communication between infrastructure and carriers while reducing passenger exposure to the EM field. In smart homes, RISs can be used to improve device connectivity. In smart clothing, the meta-atom is integrated with tunable functional materials and is designed for reprogrammable clothing. RISs are used to make wearable body networks for monitoring people's health. In the field of communications, this enables the creation of wireless networks that can be reconfigurable to adapt to changes in the surrounding environment. In wireless transmissions, RISs can be used as antennas to replace conventional antenna arrays because they do not need to process incoming signals to send them to the intended receiver. Instead, they reflect the signal transmitted directly from the base station to the receiver. They are dynamic but passive elements that function as reflectors and can be controlled individually, whose phase response can be adjusted and optimized for beam orientation and focus. This solution is advantageous as it allows efficient communication, reducing the power consumption and hardware cost of conventional phased arrays [74]. [PERSON] et al. [75] studied intelligent metasurfaces tracing their future development as an intelligent platform for data mining, communication, energy harvesting, and sensing by directly processing illuminated information-carrying waves. They explored the applications of intelligent metasurfaces in developing new wireless architectures and intelligent sensing. Artificial intelligence is the core of intelligent surfaces, and the combination with deep learning techniques has made it possible to design intelligent devices and systems. [PERSON] et al. [76] newly proposed coding and programmable metamaterials, where each meta-atom has a finite number of quantized physical states and can be used to encode digital information. The intelligent metasurface integrates the reprogrammable coding metasurface with deep learning algorithms by acquiring the intelligence of self-programming and decision making to adapt to changing surroundings without human supervision.