# IRS, LIS, and Radio Stripes-Aided Wireless Communications: A Tutorial
### Massive MIMO, Ultra Massive MIMO, and Cell-Free Massive MIMO  
MIMO is a technique of spatial multiplexing that makes use of several antennas at both the transmitter and the receiver. Because the receiver side sees several versions of the identical signal that has been sent, it can extract information more effectively, preventing the amount of fading that occurs on a smaller scale and consequently improving communication dependability. The number of wireless devices in use along with the amount of data utilized on each device is rapidly increasing, resulting in an exponential increase in data traffic demand. Unfortunately, present MIMO systems are incapable of meeting such needs due to the restrictions of only a few antennas at base stations (BS) [10]. Due to interference, the capacity to serve multiple consumers at a given time-frequency resource is limited, restricting the multiplexing gain. Massive MIMO, a new type of multi-userMIMO, has recently become a promising way to improve wireless communication in the future [11; 12].  
Massive MIMO BS have so many antennas that fading on a small scale, interference, and noise have almost no effect due to the array gain. A mMIMO BS can provide service to tens of users at once for a given time-frequency resource, resulting in huge multiplexing benefits and significant improvements in terms of spectral efficiency (SE), measured in bits/s/Hz, and energy efficiency (EE), measured in bits/J [10; 13]. Massive MIMO systems deliver faster data rates without requiring additional bandwidth or the installation of extra BS. The primary advantages of huge MIMO systems are better scalability, less power used to send signals, better use of the spectrum, and easier signal processing [14]. The signal-to-noise ratio varies dramatically between cell-center and cell-edge UEs, which is a major issue for mMIMO. These issues restrict the deployment of mMIMO beyond 5 G (B5G) networks, which is addressed by the cell-free mMIMO network. The term \"CF mMIMO systems\" refers to a better way to use decentralized mMIMO systems in the real communication world [15]. In CF mMIMO systems, all APs are linked to a CPU. The CPU manages each AP as a mMIMO network with no cell borders so that every user can receive and send data at the same time. By using spatial multiplexing techniques, the CPU makes it possible for all APs to serve every user using a similar time-frequency range of resources. Cell-free massive MIMO makes use of the advantageous propagation and channel hardening properties when the number of APs is sufficient to multiplex a lot of users on the same time-frequency resource with low inter-user interference [16]. As a result, it may provide high spectral efficiency with minimal signal processing. Additionally, a cell-free massive MIMO system places the service antennas near the users, resulting in a high level of macro-diversity and reduced path losses. As a result, numerous users can be serviced at the same time with consistently high-quality service. Furthermore, cell-free massive MIMO outperforms standard tiny-cell technology, in which every user is serviced through a single AP. Cell-free massive MIMO is a promising approach for next-generation wireless communication technologies [16].  
Transmission distance in the THz band shows a restriction due to power limitations and high propagation losses. Ultra-massive MIMO (UM-MIMO) communications, which might incorporate a huge number of nano-antennas in small dimensions to boost communication area and data rates at THz frequencies, are suggested as a solution. Consequently, system capacity is increased. In comparison to massive MIMO communication systems, the number of antennas and the dimension of the antenna array are enhanced in ultra-massive MIMO communication systems, resulting in unique channel characteristics [17; 18].