## 7 Hybrid Communication Systems: VLC with Other Communication Technologies  
### VLC with Acoustic Communication Systems  
Hybrid environments could be a setting where VLC is combined with acoustic communication, proving that this would be a promising solution for extending communication range and improving robustness. VLC operates within the 380-780 nm spectral range and offers high-speed data transmission without RF interference issues. On the other hand, acoustic communication has great capabilities regarding extended-range data transfer and penetration through obstacles, a complementary aspect to VLC. It will be of huge potential in various applications such as underwater communication, vehicular systems, and smart cities since it exploits the strengths of both modalities. VLC provides the advantages of high data rates and interference-free communication that is favorable in a localized environment. At the same time, acoustic systems increase robustness for scenarios where line-of-sight communication is not practical [65]. Applications of VLC-acoustic integration span a variety of domains. In underwater communication, VLC's high-speed functionalities respond to the bandwidth constraints of acoustic signals, thus facilitating efficient data transfer. In vehicular and smart city systems, VLC's localized high-speed communication finds a complement in the long-range functionality of acoustic systems for robust network coverage. Integrating VLC's millimeter-level positioning precision with the non-line-of-sight capabilities of acoustic systems enhances positioning accuracy in complex environments [65]. These examples give an idea of how the hybrid system could assure high-speed, interference-free communications while surmounting range and obstacle-related problems. Advanced modulation and synchronization strategies are key enablers of the functionality of hybrid VLC-acoustic systems. Adaptive modulation techniques adapt to environmental conditions dynamically, hence providing a reliable communication system. Ref. [66] Highlighted the contribution of adaptive schemes in VLC for maintaining signal quality under varying conditions; this consolidates the robustness of the acoustic signals. Synchronization mechanisms are equally important, with the work in [67] proposing cross-layer coordination techniques, like OFDM, for aligning VLC and acoustic signal timing so as to minimize errors, hence enhancing system reliability. Hybrid signal processing plays an important role in effective synchronization. Energy efficiency indeed faces great challenges in hybrid VLC-acoustic systems. The dual-purpose illumination and communication using LEDs in VLC bring several complications while trying to balance power consumption and data transmission. Indeed, higher data rates in VLC can mostly be achieved with higher power input, affecting energy efficiency [68]. For achieving energy-efficient designs targeting applications, it is essential to optimize components such as LED drivers and acoustic transducers. In [69], the need for energy-efficient LED drivers was pointed out, while synchronization between VLC and acoustic signals increases the computational load, which requires low-power processing algorithms. Such challenges call for novel approaches in energy-efficient hybrid system design. Hybrid VLC-acoustic networks also pose some unique security challenges and opportunities. Spatial confinement of VLC signals provides resistance to RF interference and eavesdropping a strong basis for secure communication [70]. On the other hand, the non-line-of-sight nature of acoustic communications makes them more vulnerable to being intercepted; hence, robust encryption and authentication mechanisms are required. Apart from the above-mentioned issues, synchronization protection is another important concern: a secure feedback path in adaptive modulation schemes helps to avoid timing disruptions and system security collapses [66].