# Revolution or Evolution? Technical Requirements and Considerations towards 6G Mobile Communications
## 5 6G RF Spectrum Technologies
### Visible Light Spectrum  
Using infrared, visible light, and ultraviolet spectra, optical wireless communications (OWC) are seen as a complement to RF-based mobile communications. In OWC, visible light with a spectrum that ranges from 430 to 790 THz (unlicensed spectrum) demonstrates the highest potentials, due to the extensive use of light-emitting diodes (LEDs) and  
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & **5G Technology** & \multicolumn{2}{c}{**6G Technology**} \\ \hline  & **mmWaves** & **THz** & **VLC** \\ \hline Frequency band & 3 GHz–99 GHz & 100 GHz–10 THz & 430 THz–790 THz \\ \hline Supporting data rate & Gigabits/second & Terabits/second & Gigabits/second \\ \hline Propagation loss & Low propagation loss & High propagation loss & High \\  & (compared to THz) & & \\ \hline Underwater communication & No & No & Yes \\ \hline Link & NLOS & NLOS & LOS \\ \hline Spectrum & Licensed & Licensed & Unlicensed \\ \hline Electromagnetic interference & Yes & Yes & No \\ \hline Penetrate through & Yes & Yes & No \\ opaque objects & & & \\ \hline  & & Molecular absorption, & Small coverage, require RF \\  & Circuit design, & circuit design challenges, & uplink, dark objects absorb \\  & High propagation loss & higher propagation loss & light waves, suffer from shot \\  & & (compared to mmWave), high & noise caused by another \\  & & penetration loss & light source \\ \hline Environment communication & Indoor/outdoor & Indoor/outdoor & Mostly indoor \\ \hline  & Wide bandwidth, small & High bandwidth & Low-cost hardware, \\  & antenna size, & (100x mmWave), & low interference, \\  & focused beams, path loss can & small antenna size, & unlicensed spectrum \\  & be compensated, and allowing & focused beams & \\  & spatial multiplexing & & \\ \hline Potential applications & Small cell access, cellular & Autonomous vehicles, cloud, & Li-Fi, visible light ID system, \\  & access, and wireless backhaul & & hospital robots, underwater \\  & & mobile HetNets & communication, and traffic \\ \hline Transmission power & High & High & Low \\ \hline  & & 24-100 & 100-300 \\  & GHz & GHz & GHz \\  & & **Bands** & **Visible Light** \\  & Low and Sub-Bands & High Bands (mmWave) & & **THz Bands** \\ \hline \hline \end{tabular}
\end{table}
Table 2: Comparison of 5G (mmWaves) and 6G (THz and VLC) technologies.  
Figure 4: 4G, 5G, and 6G spectrum bands.  
technological advancements. In the case of LED, unlike former illumination technologies, its light intensity levels can swiftly change, allowing different ways to encode data in emitted light [57]. VLC makes the most of LED for both lighting and high-speed data communication purposes. Unlike the typical RF bands, VLC presents ultra-high bandwidth (THz) and frequency reuse and unrestricted access to spectrum without electromagnetic interference [58]. The use of visible spectrum and VLC technologies (for short-range links) is necessary to achieve optical fiber performance for the 6G mobile network. However, the current VLC indoor technology today is fixed within a range of a few tenths of a megabit per second to 100 Mbps over 5 m. Nevertheless, the launch of emerging new light sources grounded by micro-LED technology (given the absence of beamforming and commercial off-the-shelf light fixtures) can address such restrictions. Using a single-diode LED in the laboratory, such innovation can provide almost 10 Gbps and a bandwidth of over 1 GHz.