# 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)  
PAM and QAM are widely used in VLC systems, due to their ability to transmit higher data rates compared to simpler modulation schemes like OOK. Both PAM and QAM encode multiple bits per symbol, allowing for more efficient use of bandwidth and increasing data throughput [42]. PAM varies the amplitude of each pulse to encode information. For example, in 4-PAM, four distinct amplitude levels are used to represent two bits of information per symbol. In VLC systems, PAM is particularly attractive because it offers a good trade-off between power efficiency and data rate. PAM can achieve higher data rates than OOK by transmitting more bits per symbol, but it comes at the cost of increased complexity and susceptibility to noise, especially in environments with high ambient light interference. This trade-off makes PAM suitable for applications where moderate-to-high data rates are needed but power consumption remains a concern, such as in wearable medical devices or remote health-monitoring systems [43]. In healthcare environments, where devices such as patient monitors or diagnostic equipment may be required to transfer large amounts of data, PAM provides a feasible solution, due to its higher spectral efficiency. However, the implementation of PAM requires more precise control over transmission power and detection sensitivity, as errors due to noise or distortion can degrade performance more than with simpler modulation schemes. Error correction techniques and adaptive modulation schemes can help mitigate these challenges, making PAM a practical solution in environments where data integrity is critical. QAM further improves upon PAM by modulating both the amplitude and phase of the light signal. This dual-modulation approach allows QAM to transmit even more bits per symbol compared to PAM. For example, 16-QAM uses 16 distinct combinations of amplitude and phase to encode four bits per symbol. This results in significantly higher data throughput than PAM or OOK, making QAM a strong candidate for high-bandwidth VLC applications, such as the transmission of medical images, video feeds, or other data-heavy applications in healthcare [44]. QAM is commonly used in VLC systems where high spectral efficiency is required. However, like PAM, QAM is also more susceptible to noise and interference, due to its reliance on amplitude modulation. The complexity of the signal constellation in higher-order QAM schemes (e.g., 64-QAM, 256-QAM) requires sophisticated receivers capable of accurately detecting small changes in both amplitude and phase. This makes QAM better-suited for applications where high-performance hardware is available and the VLC link is relatively free of interference [45]. In the context of healthcare, QAM's higher data rate can support bandwidth-intensive applications, such as real-time monitoring of multiple patients, telemedicine, or the transmission of high-resolution medical imaging data between devices. For instance, in a hospital environment, QAM could be used to transmit large sets of diagnostic data between diagnostic devices and central servers for analysis, or to transmit real-time video feeds for telemedicine consultations. Orthogonal Frequency-Division Multiplexing (OFDM) and Advanced Modulation Techniques for Improved Efficiency  
OFDM is widely used in VLC, due to its ability to handle high data rates and mitigate ISI through the use of multiple subcarriers [46]. It is especially effective in environments where multipath reflection can degrade signal quality, such as hospital rooms with numerous reflective surfaces [47]. In particular, a study utilizing differential carrier offset orthogonal frequency-division multiplexing (DCO-OFDM) demonstrated the effective transmission of data in indoor environments using VLC [48].