# Comprehensive Survey on VLC in E-Healthcare: Channel Coding Schemes and Modulation Techniques
## 3 Modulation Techniques for VLC in E-Healthcare Systems
### Pulse Amplitude Modulation (PAM) and Quadrature Amplitude Modulation (QAM)
OFDM's advantage lies in its ability to divide the data stream into multiple lower-rate streams transmitted simultaneously, making it suitable for applications that require reliable, high-speed data transmission, such as the transmission of high-resolution medical images or video feeds in real time. Variations of OFDM, including ACO-OFDM, DCO-OFDM, and PAM-DMT, have demonstrated high power efficiency and low complexity, making them practical for VLC systems in healthcare environments where both data integrity and low power consumption are critical [49; 50]. ACO-OFDM reduces the peak-to-average power ratio (PAPR) by clipping the negative portion of the signal, making it ideal for low-power medical devices, such as wearable health monitors, where continuous real-time data transmission is required. This modulation scheme simplifies hardware implementation while maintaining sufficient data integrity, making it a good fit for real-time monitoring applications. DCO-OFDM, which adds a DC bias to make the signal positive, is suitable for higher-data-rate applications, like medical imaging or telemedicine, though it requires more power, due to the added bias. PAM-DMT, combining pulse amplitude modulation with discrete multi-tone transmission, offers a balanced solution by enhancing power efficiency and spectral efficiency. It is particularly useful for longer-range VLC communication in hospitals, where data such as patient vitals need to be reliably transmitted across larger distances [50]. These OFDM variations ensure robust communication in healthcare settings, where signal integrity and low interference are essential for the reliable operation of multiple medical devices. By offering a range of trade-offs between data rate and power efficiency, ACO-OFDM, DCO-OFDM, and PAM-DMT are well-suited for diverse healthcare applications, from wearable technology to high-bandwidth hospital communication networks. In this sense, several advanced modulation techniques, including HACO-OFDM (hybrid asymmetrically clipped optical OFDM), LACO-OFDM (layered asymmetrically clipped optical OFDM), AAO-OFDM (asymmetrically amplified optical OFDM), and ALACO-OFDM (adaptive layered asymmetrically clipped optical OFDM), offer significant improvements in spectral and power efficiency compared to traditional OFDM schemes [51; 52; 53; 54]. These techniques enhance data throughput by efficiently utilizing available bandwidth while maintaining low power consumption, which is essential in environments like healthcare, where both energy conservation and high data rates are vital. HACO-OFDM and LACO-OFDM specifically improve spectral efficiency by clipping parts of the signal while maintaining a low peak-to-average power ratio (PAPR), allowing for effective transmission without the need for complex receivers. These advanced modulation schemes are particularly well-suited for high-performance healthcare applications, where low latency and high spectral efficiency are critical. In real-time patient monitoring systems, for example, the ability to transmit high volumes of data--such as vital signs or imaging data--quickly and efficiently is crucial for timely medical interventions. Additionally, in telemedicine, where high-quality video and data streams must be transmitted with minimal delay, the power efficiency of AAO-OFDM and ALACO-OFDM ensures that these high-bandwidth services can be delivered reliably without overloading the system or requiring excessive power consumption. These advanced schemes allow for robust, high-speed communication in healthcare settings, making them ideal for applications that demand both performance and efficiency [51; 52; 53; 54]. On the other hand, recent research has explored the use of VLC for long-range transmission and imaging applications. For example, a study used low-resolution cameras and intensity modulation/direct-detection orthogonal frequency-division multiplexing (IM/DD-OFDM) to estimate distances between vehicles, showing high-speed data transfer even in challenging underwater environments [55].