# A New Internet of Things Hybrid VLC/RF System for m-Health in an Underground Mining Industry
## 4 Network Architecture
### VLC Downlink  
With the concept of intelligent environments and the Internet of Things (IoT), we face a high level of activity in indoor positioning systems. The RF-based Global Positioning System (GPS) is one of the most reliable positioning techniques, which has been widely used in outdoor environments. However, GPS-based systems have problems in indoor environments concerning low positioning accuracy. To complement GPS for indoor applications, several wireless positioning technologies occupy RF such as RFID, Wi-Fi, ZigBee,  
Figure 2: Network architecture. and Bluetooth, and these technologies offer low positioning accuracy (PA) due to multipath. Instead, VLC offers several advantages, including high positioning precision; and unlicensed operation; moreover, it is not harmful to human health and does not present radio frequency-based interference [14]. The IEEE 802.15.7 standard for Personal Area Networks in Wireless Optical Communications (OWPAN) relates the intended application with the broadcast topology, and the infrastructure will be able to transmit the cell identifier to the mobile nodes without forming a network with them, in a unidirectional communication and for which the destination address is not required. The standard defines the PHY II physical layer mode used mainly for indoor environments that allows one to use an OOK modulation with Reed Solomon Forward Error Correction (FEC) encoding. The physical layer supports a single data transmission mode that transfers one Physical Protocol Data Unit (PPDU) per frame, the structure of the PPDU frame is formatted as illustrated in Figure 3[13]. The transceiver uses the preamble field to obtain synchronization of the optical clock with an incoming message. The standard defines a Fast Blocking Pattern (FLP) followed by a selection of four Topology-Dependent Patterns (TDPs) to distinguish different PHY topologies. The preamble will be sent at a clock rate chosen by the Transmitter (TX) and supported by the Receiver (RX). The preamble is a sequence in the time domain and has no channel encoding or line encoding. The PSDU field has a variable length and carries the PHY frame data. A tunnel space is considered with dimensions \(a\) m \(\times\)\(b\) m \(\times\)\(c\) m, similar to Figure 4; this space is divided into several hexagonal optical cells. In each of these cells, there is an LED \(i\) that transmits a cell identifier on the air and provides coverage to the region limited by that cell at a specific frequency, which can be reused in a different cell with a minimum distance from the first, thus avoiding adjacent channel interference. To generate the downlink communication link, VLC transmitters using an LED \(i\), which is placed on the tunnel roof at coordinates \((x_{i},y_{i},z_{i}),\forall i\in(1,2,\ldots,i)\) and \(p\) mobile transceivers containing a photodetector and an RF emitter are placed on the safety helmet  
Figure 4: VLC Downlink. Figure 3: VLC Frame. of mining personnel with coordinates \((x_{p},y_{p},z_{p}),\forall p\in(1,2,\ldots,p)\), in addition to \(\Psi\) which is the angle of incidence of light and where \(\phi\) is the angle of irradiation. The optical power received by each photodetector \(p\) from the LED \(i\) is described as:  
\[Pr_{(i,p)}=Pt_{i}Hlos_{(i,p)}, \tag{1}\]  
where, \(Pt_{i}\) is the power transmitted by LED \(i\). The channel response \(Hlos_{(i,p)}\) between the LED \(i\) and the photodetector \(p\) can be expressed as:  
\[Hlos_{(i,p)}= \begin{cases}\frac{A_{t}(m_{1}+1)}{2\pi d_{(i,p)}^{2}}cos^{m_{1}} (\phi)T_{s}(\Psi)g(\Psi)cos(\Psi),&0\leq\Psi\leq\Psi_{c}\\ 0&elsewhere\end{cases} .