## 8 VLC-IoT: Challenges, Solutions, and Future Directions
### Challenges and Solutions  
In this section, we discuss the specific challenges that are met when implementing light waves for IoT applications due to the unique characteristics of light waves and provide possible solutions. The challenges imposed by VLC on VLC-IoT are presented in Table 2 and include:  
1. Limited modulation bandwidth of off-the-shelf LEDs  
One of the major challenges of implementing VLC for IoT is the limited modulation bandwidth of off-the-shelf LEDs. In the current infrastructure system, the wavelength of existing off-the-shelf LEDs deployed for lighting purposes and employed for data transmission in the VLC technology is too short to encode information in the signals phase using the current technology. This is because of the limited linear operation region of LEDs for electro-optical conversion which requires that modulating signal falls within strict amplitude constraints in order to mitigate nonlinear effects [63]. \begin{table}
\begin{tabular}{c c c} \hline \hline
**S/N** & **Challenges** & **Solutions** \\ \hline \multirow{2}{*}{1} & Limited modulation bandwidth of & 1. The use of multiple-input multiple-output (MIMO) advanced \\  & off-the-shelf LEDs. & modulation and multiple access schemes; 2. the use of smart LEDs. \\ \hline \multirow{2}{*}{2} & Extended link distance in outdoor & 1. Using relays to extend the transmission range of VLC-IoT \\  & VLC-IoT applications. & applications; 2. deploying LEDs-to-camera VLC for extended range \\  & & of transmission; 3. development of hybrid OFDM and CDMA \\  & & resource allocation algorithms. \\ \hline \multirow{3}{*}{3} & Line-of-sight (LOS) signal loss and & 1. Using high spatial diversity to guaranteed connection; 2. use \\  & shadowing effects. & multiple MIMO antennas to reduce the risk of LOS signal loss due to \\ \cline{1-1}  & & interrupted signals; 3. using transmitters with ultra-wide FOVs to \\ \cline{1-1}  & & eliminate loss of signals. \\ \hline \multirow{2}{*}{4} & Lack of recognized channel models for & 1. Study characteristics and theoretical details of indoor and outdoor \\ \cline{1-1}  & VLC-IoT communications. & VLC; 2. Validate identified channel model in different transmission \\ \cline{1-1}  & & medium for possible standardization \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Challenges and solutions of VLC-IoT concept. 2. Suggested possible solutions To mitigate this challenge, we suggest (1) the use of multiple-input multiple-output (MIMO) advanced modulation and multiple access schemes including those proposed in some parts of this survey, (2) the use of smart LEDs, which in the long run saves cost because it last longer and can have different degrees of brightness and different colors which mitigate the problem associated with modulation when using \"dumb\" LEDs
3. Extended link distance in outdoor VLC-IoT applications In outdoor VLC-IoT, e.g., in satellite and deep-space applications, unlike most indoor VLC applications, the distance between the transmitter and the receiver is relatively longer. Moreover, the short-wavelength of OWC limits the transmission range of VLC to a few meters. Therefore, the extended distances in outdoor VLC-IoT applications limit the number of areas in which VLC high data rate can be applied for IoT since increasing the distance by a few meters decrease the data rates [64]. Other challenges imposed by the extended transmission range of outdoor VLC-IoT applications include ISI, inter-cell interference (ICI), and co-channel interference (CCI). 4.