# Comprehensive Survey on VLC in E-Healthcare: Channel Coding Schemes and Modulation Techniques
## 3 Modulation Techniques for VLC in E-Healthcare Systems
### Modulation Techniques in IEEE 802.15.7-2018 Standard and Their Adaptation to E-Health Environments  
The IEEE 802.15.7-2018 standard for VLC outlines several modulation schemes, including OOK, OFDM, variable pulse position modulation (VPPM) and color shift keying (CSK) [61; 62]. These modulation techniques are designed to optimize communication for various applications, balancing data rate, power efficiency, and robustness against interference--factors critical for VLC systems in healthcare environments.  
In this sense, VPPM combines the benefits of pulse position modulation (PPM) with the ability to control the intensity of light, making it a highly flexible modulation technique [63]. In VPPM, the position of the pulse is varied to encode the data, while the intensity of the light can be adjusted for dimming control. This dual capability allows VLC systems to perform data transmission and lighting functions simultaneously, which is particularly valuable in healthcare settings where the intensity of the light must be carefully managed for patient comfort and medical procedures.  
For example, in environments such as operating rooms or ICUs, VPPM can ensure that lighting conditions are optimal for medical personnel while maintaining continuous data transmission for patient monitoring devices. VPPM also offers better noise and interference resistance compared to OOK, making it more suitable for healthcare environments where lighting conditions and ambient light sources may vary. This modulation technique can be used in systems where low-to-moderate data rates are sufficient, such as transmitting patient vitals (e.g., heart rate, temperature, and oxygen levels) to monitoring stations without interrupting the visual experience of the room lighting [64].  
On the other hand, CSK is a more advanced modulation technique specified in the IEEE 802.15.7-2018 standard that encodes data by varying the color of the transmitted light [65]. By using different wavelengths of light, typically red, green, and blue (RGB) LEDs, CSK can transmit multiple bits of information simultaneously by altering the color combinations. This modulation scheme is highly spectrally efficient, as it leverages the color properties of LEDs without requiring significant bandwidth. The basic transmission scheme using CSK modulation can be seen in Figure 1:  
Figure 1: Transmission scheme using CSK modulation.  
In e-health applications, CSK could be particularly useful for transmitting larger datasets or more complex medical information, such as medical images or real-time video for telemedicine. Because CSK uses multiple wavelengths, it can achieve data rates higher than those of OOK or VPPM, making it suitable for bandwidth-intensive applications. For example, in a telemedicine scenario where high-resolution video and diagnostic data need to be transmitted with minimal latency, CSK can provide the necessary data throughput while still allowing the lighting system to function at its regular capacity [2; 61].  
Furthermore, CSK's ability to modulate data through color changes rather than intensity adjustments makes it less sensitive to ambient light interference, ensuring a more stable communication link in environments like hospital wards or emergency rooms. This capability allows CSK to be applied in scenarios where both lighting and communication are critical but must not interfere with each other.