# VLC Network Design for High Mobility Users in Urban Tunnels
## 3 Overview of IEEE 802.15.7
### IEEE 802.15.7 Physical Layer  
A VLC system consists of a transmitter based on lighting LED sources, a propagation channel ranging from 380 to 780 nm in the electromagnetic spectrum, and light-receiving elements such as photodiode detectors or image sensors. Next, we summarize the relevant aspects of the IEEE 802.15.7 PHY layer for the optical wireless communication standard. First, a LED-based VLC system cannot encode information in the phase or amplitude of the light signal. Instead, it uses Intensity Modulation/Direct Detection (IM/DD) schemes to transmit data. But the presence of other light sources in the same medium plays a major role due to the carrier limitation. Despite this, some recent efforts on multiple access-based modulation schemes show the potential to alleviate this restriction. An example of such schemes includes Color-Shift Keying (CSK) modulation, which allows multiple access through the variation of the color emitted by the red, green, and blue light-emitting diodes (RGB LEDs). These schemes are, however, limited to only three different bands (one per color). Other popular modulations schemes such as QAM, OFDM, or other variants as DCO-OFDM, have also been adapted to deal with these types of restrictions [53]. The first release of the IEEE 802.15.7 standard defined three PHY layer types (I, II, and III) which were grouped by data rate. More recently, amendments to this standard added three more PHY modes (IV, V, and VI) for low-rate photodiode communication and optical camera communications (OCC). While PHY I is intended for outdoor applications with data rates from 11.67 kbps to 266.6 kbps, PHY II is designed for indoor usage with  
Figure 3: The transmission algorithm during CAP in standard IEEE 802.15.7.  
moderate data rate applications from 1.25 Mbps to 96 Mbps, and PHY III for applications using multiple optical sources with data rates from 12 Mbps to 96 Mbps. The modulation schemes used in PHY I and II modes are on-off keying (OOK) and variable pulse position modulation (VPPM), and for PHY III is color-shift keying (CSK) that allows multiple light sources and detectors. On the other hand, PHY types IV, V, and VI incorporate multiple modulation schemes that target a broad range of OCC applications. A detailed description of the OWC PHY specification modes can be found within the IEEE 802.15.7 m standard [22]. Additionally, the standard also includes two codifications stages. The first is Run-Length Limited(RLL) which uniforms the illumination level. The second is Reed-Salomon codification which decreases the errors on the transmission. Another relevant point is the channel effects in the communication. On the one hand, the received signal power decreases quickly as the distance grows. At the same time, the scenario reflections create multi-path propagation effects that can limit the maximum data rate. In general, VLC channel models are composed of line-of-sight (LOS) and non-line-of-sight (NLOS) contributions. While LOS can be calculated with a closed formula, NLOS depends on the scenario layout. In this way, the Modified Monte-Carlo simulations (MMC) method with ray-tracing [54] is an alternative tool to obtain the impulse response of the VLC channel. MMC considers the light radiation patterns to generate random rays and emulates the light propagation in the scenario. Each rebound has a deterministic LoS contribution to reducing the number of rays necessary to get a good approximation of the CIR. The channel information provides the power and bandwidth restrictions at determined moments and positions. At the same time, the signal propagating in the channel affects the Signal-to-Noise Ratio (SNR) and increases the bit error rate (BER). Particularly, the power SNR at the output of a PIN photodiode can be formulated as  
\[SNR=\frac{\left(P_{tx}H(0)R_{\lambda}\right)^{2}}{\sigma_{th}^{2}+2q(P_{tx}H(0 )R_{\lambda}+i_{d}+i_{b})B} \tag{4}\]  
The three main noise sources are included in this equation.