Bandwidth Enhancement in White LED Visible Light Communication Systems Using Blue Optical Filtering and Multi-Stage Equalization

Visible Light Communication (VLC) enables short-range high-speed data transmission using white light-emitting diodes (LEDs) for both illumination and communication. However, phosphor-converted white LEDs suffer from limited modulation bandwidth due to slow phosphor relaxation dynamics, resulting in signal attenuation, inter-symbol interference (ISI), and degradation of signal-to-noise ratio (SNR) at higher frequencies. In this work, performance enhancement using blue optical filtering combined with electrical equalization was experimentally investigated for phosphor-converted white LEDs transmitting On-Off Keying Non-Return-to-Zero (OOK-NRZ) signals. Measurements were conducted over a 40 cm indoor line-of-sight link with data rates varied from 0.5 to 20 Mbps. Two equalizer configurations were evaluated: a single-stage RC network (EQ1) and a three-stage cascaded RC network (EQ2). Frequency sweep measurements (representing OOK symbol rate variation) were performed to evaluate received signal strength, SNR, Bit Error Rate (BER), cumulative signal improvement, and normalized performance metrics. Results show that EQ2 provides significantly superior performance compared to EQ1, achieving a peak SNR of 34 dB at 10 Mbps with a corresponding BER reduction to 10⁻⁹, representing approximately 8 dB SNR improvement over EQ1 at the optimal frequency. The optimal enhancement region for EQ2 lies between 0.5 and 12 MHz, where LED channel attenuation is most dominant. Polynomial fitting was used as an empirical approximation, with fifth-order models providing the highest regression accuracy for EQ2 (R² = 0.90). The results demonstrate that the combination of blue optical filtering and multi-stage equalization effectively mitigates LED bandwidth limitations, significantly improving VLC performance for indoor smart lighting and short-range wireless applications.