## 2 Brief Overview of the 5G, 6G, and IoT Requirements  
5G will provide an order of magnitude improvement in some key characteristics compared to 4G to efficiently support the increasing heterogeneous multimedia applications with a diverse set of requirements [11]. The 5G requirements are already specified, and the 5G system is expected to be fully deployed in 2020. The 5G key requirements are summarized as follows:  
_High traffic volume_: The mobile data volume per area will be 1000 times compared to that of the 4G wireless networks, and the number of connected wireless devices will be 100 times higher. _Massive connectivity_: 5G will provide massive connectivity. Ten to 100 times more devices will be connected compared to the 4G communication system [11]. _High user data rate link_: The 5G networks must have the capability to support a very high user data rate; the user will achieve up to 10 Gbps data rate, which will be 10 to 100 times higher compared to 4G. _Low-energy consumption_: Low-energy consumption is an important requirement in the 5G communication system. It will reduce energy consumption by more than 90% (i.e., 10 times lower compared to 4G networks) [11]. _Extremely low latency_: The end-to-end latency will be within only a sub-millisecond level to a few milliseconds [11]. Researchers are focusing on standardizing the requirements of 6G networks [12; 13; 14; 15; 31; 32; 33; 34]. The ultra-high bit rates per device (e.g., 10s of Gbps to Tbps) are expected to be one of the key requirements of 6G [12; 31]. Moreover, 6G is expected to be characterized by 1000 times higher \begin{table}
\begin{tabular}{l l} \hline \hline
**Acronym** & **Definition** \\ \hline
5G & Fifth generation \\
6G & Sixth generation \\
5 GB & 5G-and-beyond \\ BLE & Bluetooth Low Energy \\ DSRC & Dedicated short-range communications \\ eHealth & Electronic health \\ EH & Energy harvesting \\ FSO & Free-space optics \\ HBC & Human bond communication \\ HMD & Head-mounted displays \\ IoT & Internet of Things \\ IR & Infrared \\ LD & Laser diode \\ LED & Light emitting diode \\ LIFI & Light fidelity \\ LOS & Line-of-sight \\ MBS & Macrocellular base station \\ NR & New radio \\ OCC & Optical camera communication \\ OWC & Optical wireless communication \\ P2P & Point-to-point \\ P2 mP & Point-to-multipoint \\ PD & Photodetector \\ NLOS & Non-line-of-sight \\ QoE & Quality of experience \\ QoS & Quality of service \\ RF & Radio frequency \\ UV & Ultraviolet \\ V2V & Vehicle-to-vehicle \\ V2X & Vehicle-to-everything \\ VL & Visible light \\ VLC & Visible light communication \\ WiFi & Wireless fidelity \\ \hline \hline \end{tabular}
\end{table} 
Table 1: List of acronyms. simultaneous wireless connectivity than that of 5G. An ultra-long-range communication with ultra-low-power consumption and ultra-low latency of less than 1 ms is expected for the user experience [13]. The other key expected characteristics of 6G are featured spatial multiplexing, higher spectral efficiency (100 bps/Hz), ultra-high wireless security, ultra-reliability, ultra-low-power consumptions, and massively connected complex networks. The networks will have some special type of characteristics to support the demands of 5 GB wireless communication systems. The key characteristics of future 5G and 6G networks are summarized as follows:  
_Ultra-high-dense network_: To provide uniform QoE, massive connectivity, and high capacity demands, 5 GB network deployments are expected to be much denser and comprise ultra-dense heterogeneous networks compared to 4G networks. _Small-cell networks_: The concept of high-dense small-cell networks has been pointed out as a core characteristic for the 5 GB communication systems. _Higher spectral efficiency_: 5 GB systems are also expected to guarantee an efficient use of the frequency spectrum by using multiple-input and multiple-output, advanced coding and modulation schemes, and a new waveform design. The spectral efficiency on 5G should be at least three times higher than that on the 4G networks.