# A Review of Hybrid VLC/RF Networks: Features, Applications, and Future Directions
## 2 Hybrid VLC/RF Networks in Indoor Scenarios under Ideal Conditions
### Hybrid Network Topologies  
Network topologies are primarily used to describe the physical or logical structure of a network, i.e., the topology defines how and where the various devices in the network are connected. The correct design of the topology of a hybrid network is very important because it allows for optimization of the location and interconnection between the different access points of the network [77]. The main topologies designed for hybrid VLC/RF networks are separated into six major groups: symmetric with interference, symmetric without interference, asymmetric, dual transmission, separate transmission, and hybrid transmission, as shown in Figure 6.  
Symmetrical without interference is a topology in which both networks are independent. Both the VLC network and the RF network can be used for downlink and uplink connection, as shown in Figure 7A. There is no interference between the networks [56; 63; 65; 67; 69; 75; 76; 77; 78; 79; 80; 81; 82; 83; 84; 85; 86].  
In symmetric with interference topologies, the VLC and RF networks are linked in the uplink; therefore, they are not independent and interference occurs between them. Figure 7B shows a schematic of a symmetrical topology with interference. In the downlink, both networks can be used simultaneously without interference, while in the uplink only the  
Figure 6: Types of hybrid topologies in the literature.  
RF network is used to transmit information from both networks, which causes interference between RF users and VLC users transmitting over the RF network [87; 88; 89; 90; 91].  
Figure 7C shows an asymmetric topology scheme. The downlink and uplink connections are routed in different directions for the two networks. The VLC network is only used for the downlink connection, while the RF network is only used for the uplink [52; 77; 92; 93; 94; 95; 96; 97; 98; 99; 100; 101; 102; 103; 104].  
In dual transmission topologies only one of the two networks can be connected. The VLC network is used for the connection between the VLC access points and the RF access point via a relay, while the RF network is used for the connection between the RF access point and the users. Both networks transmit on the downlink and uplink, as shown in Figure 7D [18; 105; 106; 107; 108; 109; 110].  
In separate transmission, the two networks are separated into independent connections. As shown in Figure 7E, users can choose between one of the two networks depending on the location and movement of each user as well as the SNR of the links. While this topology has great flexibility for users, it is difficult to control and is not optimal [49; 111; 112; 113; 114; 115; 116; 117; 118; 119; 120; 121; 122; 123].  
Figure 7F shows a hybrid transmission topology, in which both networks are interconnected by a control unit. Depending on the location of the users and the network conditions, the control unit allocates network resources to each user, ensuring fairness. The connection can be made by both networks in both the downstream and upstream directions [50; 68; 124; 125; 126; 127; 128; 129; 130; 131; 132; 133; 134; 135].