# Software-Defined Visible Light Communication for Internet of Things: A Low-Complexity Approach
## 2 Architecture of SD-VLC System for IoT Applications  
In VLC systems, modulation methods used in coherent wireless optical communication, such as optical phase modulation, cannot be directly applied because the transmitting source primarily uses LED lighting devices, whose emitted light is incoherent. Coherent modulation relies on a stable phase of the light wave, whereas the light emitted by LEDs is broadband, extended, and lacks a fixed phase relationship, making it impossible to directly apply modulation methods based on coherent detection (e.g., QAM, PSK) in the optical domain of VLC. Additionally, the receiver typically uses photodiodes for direct detection, while coherent detection usually requires a local oscillating light source and interferometry. Therefore, VLC primarily adopts IM/DD to accommodate the non-coherent characteristics of LEDs and to meet the illumination requirements. Moreover, the modulation signal in VLC must avoid flicker, and its modulation frequency typically falls within the kHz to MHz range, which is much higher than the critical flicker frequency of the human eye (approximately 100 Hz), ensuring that the emitted signal adheres to the real-valued, unipolar, and non-negative constraints. Since VLC uses IM/DD for optical signal transmission, its modulation mainly relies on the variation of the luminous intensity of LEDs. However, the modulation bandwidth of LEDs is limited by factors such as carrier recombination time, the resistive-capacitive (RC) effects arising from the LED's internal resistance and junction capacitance, and packaging materials, with a typical bandwidth usually in the range of 100 MHz, which is much lower than the GHz bandwidth of RF communication [16, 17]. Therefore, to achieve high data rate transmission within the limited bandwidth, modulation techniques with high spectral efficiency, such as orthogonal frequency division multiplexing (OFDM) [18], need to be adopted in the electrical domain. These techniques optimize the spectral efficiency of the signal, enabling VLC to carry information more efficiently and meeting the needs of diverse applications, such as smart lighting, IoT device interconnection, and indoor positioning. Building on this, SD-VLC enables the system to adaptively select modulation methods, optimize resource allocation, and dynamically adjust signal processing strategies according to different IoT application scenarios through a flexible software control architecture to enhance communication performance. Figure 1 illustrates the application of SD-VLC in IoT scenarios, including key areas such as smart lighting, vehicle-to-vehicle, and industrial automation. The SD-VLC architecture depicted in Figure 1 integrates software-controlled signal processing with programmable hardware for flexible and efficient VLC-enabled IoT applications. This system is implemented on the ZYNQ platform, leveraging the PYNQ framework, where the processing system (PS), running ARM/Linux, supports network applications, while the programmable logic (PL), based on a high-speed FPGA, ensures flexibility and real-time communication adaptability. A critical component of this system is the analog front-end (AFE), which enables both illumination and communication and is analyzed in detail with respect to its design and impact on VLC performance [19]. Meanwhile, the digital front-end (DFE) module serves as a bridge between the digital and analog domains. However, it is important to note that the digital down converter (DDC) and digital up converter (DUC) in this work do not perform traditional spectral shifting to intermediate frequency (IF) or radio frequency (RF), as seen in conventional software-defined radio (SDR) systems. Instead, their primary function is to ensure that the transmitted signal satisfies the real-valued, unipolar, and non-negative constraints required by VLC.