## 4 Multiplexing
### Multiple-Input Multiple-Output  
[PERSON] et al. [106] deploy polar coded MIMO FSO communication systems. Polar codes are error-correcting codes to achieve the capacity of binary-input memoryless symmetric channels. In simpler words, the sender can transmit data at the fastest data rate. Moreover, encoding and decoding are accomplished at a low complexity rate [107]. Photodetectors in this project neglect ambient light. Simulation results confirms that the polar-coded multiple photodiodes technique reduces turbulence-induced fading beneficially. Moreover, polar codes perform better than Low-density parity-check (LDPC) codes. On top of that, MIMO is a technology that utilizes multiple antennas at transceivers to sustain numerous paths for data to reach the receiver. Nevertheless, [PERSON] et al. [108] propose a decode-and-forward (DF) relay that executes an end-to-end hybrid RF/FSO multi-hop utilizing multiple pulse position modulation (MPPM) coding with a MIMO scheme as shown in Figure 2, which illustrates a one-way communication (represented by unidirectional arrows). The DF relay decodes the received signal from the source to reach the destination successfully [109]. The connection between the source and the DF relay is done in RF communication. Contrarily, both DF relay and destination are using a MIMO FSO communication. Results confirm that the complexity of serial amplify-and-forward (AF) multi-hop is the main problem and requires focused studies and analysis to enhance its performance. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Type** & **Power (W)** & **Coverage Radius (km)** & **Users** & **Location** \\ \hline Picocell & 0.1–0.2 & 0.01–0.05 & 4–16 & Indoor public areas \\ Femtocell & 0.25 & 0.2 & 32–100 & High-capacity enterprise areas \\  & & & & Utilized outdoor to increase \\ Microcell & 2–10 & 2 & 200 & signal coverage in locations that \\  & & & & macro BSs do not or only \\ Macrocell & 40–100 & 10–40 & 1000+ & Outdoor \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Comparison of small cells in 5G networks [103; 104]. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **SISO** & **SIMO** & **MISO** & **MIMO** \\ \hline Definition & Single-Input & Single-Input & Multiple-Input & Multiple-Input \\ Number of & Single-Output & Multiple-Output & Single-Output & Multiple-Output \\ Transmitter Antennas & One & One & Multiple & Multiple \\ Number of & & & & \\ Receiver Antennas & One & Multiple & One & Multiple \\ Channel Capacity & Low & Better than SISO & Better than SIMO & High \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Difference Between SISO, SIMO, MISO, and MIMO. However, [PERSON] et al. [110] propose a mixed RF and hybrid RF/FSO half-duplex relay network model, where authors assume that each user has only one antenna in their device for sending and receiving packets, and a DF relay requires more antennas than the number of users X as illustrated in Figure 3. If any error occurs during the relay's transmission, the destination will recover the lost signal from the combined signal from the source and relay. Or else; the extra users will be blocked until an active user becomes idle or look for another relay to serve him/her. On the relay-destination link, the primary link is the FSO with the technology of heterodyne detection (HD) as it has low noise, and the RF is the backup link that uses the same frequency as the user-relay link. Moreover, the system utilizes the N2 \(\times\) N3 MIMO system, and the N3 \"number of antennas at destination in relay-destination link\" must be greater than the N2 \"number of antennas at relay in relay-destination link\". The significance of this project is that there is a buffer with a maximum size of L, where L is the number of packets stored. The buffer is in the physical layer of the relay node for storing data of users temporarily until the condition of the channel becomes suitable for data transmission in the relay-destination link.