# Hybrid RF/VLC Network Architecture for the Internet of Things
## 1 Introduction  
Nowadays there are many remote monitoring techniques based on technologies framed within the Internet of Things (IoT) ecosystem. Nevertheless, there are still areas such as rural, maritime, or even ports, without high-speed radio coverage (GPRS, WiFi, etcetera). Although the possibility of monitoring environmental parameters is interesting and necessary in these areas, connecting them to the rest of the network is not easy. The inclusion of novel low-speed long-range communication technologies would enable the connection of these areas with an Internet access point, allowing the data produced by these sensors to be consumed on the network at low cost. Other types of wireless sensor communication technologies such as ZigBee allow for the automatic implementation of a wireless network within an area, interconnecting nodes over moderate distances with reduced power consumption [1; 2]. Moreover, in many of the aforementioned scenarios, there is a possibility of finding a solar-powered lighting point. Therefore, a cell can be established at those locations, providing communication capabilities within its coverage area, as well as the interconnection of its elements with Internet or with a central node. Under these circumstances, it would also be possible to remotely operate actuators such as irrigation control, light dimming, and color temperature. The main objective of this work was to develop a network architecture to cover these types of models (rural, maritime, or port environments) using low-power and long-range devices at moderate transmission speeds [3, 4]. The main addressed challenge is the coordination of a hybrid network with several involved technologies. The system must allow inter-operability between different communication systems or standards in a transparent way to the user and the rest of the network. In this case, LoRa, ZigBee, and visible light communication (VLC) are explored as candidates to develop the described system. A cellular-type network structure has been developed. The backbone is performed by a low-power and long-range technology that uses long range (LoRa) modulation techniques. Figure 1 shows the cell network model. Each cell comprising the structure makes use of different technologies to provide node interconnection (both sensors and actuators). The considered technologies are low-range and energy-efficient:  
* **ZigBee**. This technology is in charge of connecting the central node of each cell with the secondary nodes. It provides communications for actuators, as well as location information. Since ZigBee is low-power, the devices can be battery-powered and present a small form factor. * **Visible Light Communications (VLC)**. VLC technology is based on the transmission of data by switching the lamp [5, 6], in a way that is not visible to the human eye, and, in turn, allows two capabilities. One is the downlink communication between the lamp and the secondary nodes or between the lamp and an end user equipped with any mobile device (employing optical camera communications techniques) [7, 8, 9, 10]. This technology also allows for the positioning of a mobile node [11, 12, 13] within its coverage. This latter capability could be especially useful in remote indoor environments such as greenhouses. Furthermore, a CMOS sensor could be added to implement the uplink of the VLC system. In summary, the general objectives of this work can be broken down into the following:  
1. offering low speed data coverage to remote areas for monitoring;
2. implementing a prototype that allows inter-operability of different technologies;
3. enabling location within the coverage areas of the cell;
4. allowing the network to transport and store digital contents (maps, images, audio, etc.) to be stored in the central nodes of each cell;
5. developing a star network topology that serves as a backbone to the cellular hybrid network. Figure 1: Cell communication between nodes using different technologies.