## 7 Improvement of Modulation Bandwidth
To analyze the impact of QDs on modulation bandwidth, LEDs based on YAG:Ce phosphor, red QDs, green QDs and a mixture of red and green QD solutions have been developed. Figure 15a,b display the frequency response and modulation bandwidth comparisons for various LEDs, including fabricated ones and blue LED chips that do not contain phosphor or QDs. The measured 3 dB bandwidth for the  
Figure 14: (**a**) Resistance-Inductance-Capacitance (RLC) circuit design for pre-equalization technology; (**b**) bandwidth of phosphor-LED without pre-equalization. The 3 dB bandwidth of phosphor-LED with pre-equalization under received power of (**c**) 1100; (**d**) 900; (**e**) 700; (**f**) 500 Lux, respectively. [91] Figures reproduced with permission from Optical Society of America. LEDs based on phosphor, red QDs, green QDs, red and green QDs mixture, and GaN LED chip are, respectively, 1.55 MHz, 2.70 MHz, 2.55 MHz, 2.60 MHz and 3.65 MHz. It is evident from the figure that there is an increase in the modulation bandwidth of 74.19% to phosphor-based LEDs with the implementation of red QDs, while the green QD-LED and QD-WLED show a reasonable bandwidth with improvements of 64.52% and 67.75%. However, of all these LEDs, the GaN LED chip has the best performance in terms of both frequency response and modulation bandwidth reaching up to 3.65 MHz. In the research of [PERSON] et al., luminescent microspheres (LMS) were developed to enhance the optical efficiency of QDs and sustain large luminescent properties of QDs in white LED. LMS is fabricated by incorporating CdSe/ZnS QDs and the silica microsphere. LED-based LMS were designed to study the influence of fluorescent microspheres in the modulation bandwidth. Figure 16a,b display the LEDs based on YAG:Ce phosphor and the red LMS layer, respectively, along with their emission light. The phosphor-based LED exhibits a luminous efficacy of 119.23 lm/W corresponding to applied current of 54 mA and voltage of 2.84 V, while the LED based on LMS shows a luminous efficacy of 23.761 lm/W, corresponding to 1.98 V applied voltage and a current of 90 mA. Figure 16c,d display comparisons of frequency response and modulation bandwidth between phosphor-based LEDs and LMS, and it is found that the LED with LMS exhibits a higher 3 dB bandwidth of 1.90 MHz than that of a 1.55 MHz phosphor-based LED. It can be realized that the bandwidth for LEDs can be improved by 22.58% using LMS as the light conversion material compared to phosphor. However, the LMS-based LED bandwidth is not as high as the QD solution-based bandwidth because there are some defects forming in the synthesis phase that result in lower light attenuation. Although having a lower bandwidth, LMS-based LEDs have long-term operating performance compared to QD-based solutions, and, thus, LMS still has great potential for QD-LED-based VLC. Figure 15: Comparisons of (**a**) frequency responses and (**b**) modulation bandwidths for different LEDs with phosphor, red, green, a mixture QDs and without phosphor or QDs. [92] Figures reproduced with permission from Optical Society of America. As mentioned in the above section, the modulation bandwidth is limited by the size of the chips which will degrade the performance of the VLC application. Several research reports have been made based on the fact that because of QW- Surface Plasmon (SP) coupling, SPs can establish a new energy transition channel between electron and holes and thus increase the rate of the radiative recombination considerably [94; 95]. SPs have higher state density that leads to a faster QW-SP coupling rate, thereby increasing the recombination rate and reducing the carrier lifetime. Hence, [PERSON] et al. demonstrated Surface Plasmon (SP) coupled with GaN-based nanorod LEDs and Ag nanoparticles, showing that SPs would improve the modulation bandwidth [39]. Figure 17 demonstrates the schematic flow of GaN-LED fabrication based on nanorod and Ag nanoparticles (NPs).