Precise Frequency Response of COTS LED for VLC Using Internal Quantum Efficiency Metric

Visible light communications (VLC) utilizing LEDs for transmissions have been widely considered a revolutionary solution for next-generation networks such as 6G. Frequency response (FR) is of great importance for LEDs, but the inherent internal quantum efficiency (IQE) of LEDs is often disregarded, resulting in imprecise FR models. Although it is widely known that IQE varies with the injected direct current (DC), the impact of IQE on the frequency response has not been thoroughly analyzed. To address this issue, we have developed a precise electro-optical (E-O) FR model by incorporating the ABC model of IQE, named IQE-FR. To validate the accuracy of IQE-FR, we conducted the experiments using commercial off-the-shelf (COTS) LEDs including a tri-color LED and three monochromatic LEDs. The IQE-FR model aligns well with the experimental findings in terms of the maximum FR value, while state-of-the-art FR models that ignore the IQE effects do not yield consistent results. Specifically, we discovered that the red LEDs exhibit a stable DC gain when the biasing current rises to 300 mA, but the highest DC gain for green and blue LEDs can only be achieved under 100 mA. For green and blue LEDs, the biasing currents are much lower than their typical working currents (350 mA and 500 mA). To address the trade-off between luminance and DC gain, we propose cascading multiple LEDs operating at a low current corresponding to the maximum response. Experimental results demonstrate that four cascaded monochromatic LEDs exhibit similar bandwidth and response gain larger than 11 dB compared to a single LED at the same biasing current while delivering four times the emission power, which is very close to the theoretical value of 12 dB. Furthermore, we find that the DC gain improvement by cascading multiple LEDs is nonlinear and bounded by an upper limit as the number of LEDs increases, and as a cost-effective solution, a few tens of LEDs is recommended. To the best of our knowledge, this is the first attempt to apply the IQE model to study the LEDs’ precise FR performances for VLC. We believe that the precise IQE-FR model opens new avenues for optimizing VLC system performance by considering the effects of IQE.