## 7 Improvement of Modulation Bandwidth  
Even though various \(\mu\)-LEDs for VLC applications exhibit high modulation bandwidths for short distances, some challenges still remain when implemented for long distance transmission. The modulation bandwidth for the commercial white LEDs based on Y\({}_{3}\)Al\({}_{5}\)O\({}_{12}\):Ce\({}^{3+}\) (YAG:Ce) phosphor is quite low due to the long carrier lifetime resulting in a limited transmission rate [90]. Hence, increasing the modulation bandwidth of the source of white light becomes a key issue. Researchers have therefore made significant efforts to design high-speed white emitting GaN-based LEDs and make further improvement in the existing bandwidth. To date, several approaches have been made to increase the modulation bandwidth in order to minimize the RC time constant by decreasing the active layer thickness, using the barrier dop-ed MQs, and by decreasing the effective active region. However, once the size of the LEDs exceeds its limit, the RC time cannot be further decreased and the bandwidth is constrained by the recombination time. Hence, several studies reported various ways of improving the modulation bandwidth, such as optimizing the LED chips structure, using blue-filter and using equalization circuit. Since high capacitance can be attributed to large chip size, it is important to optimize the structure of LED chips, as their layer structure specifies the bandwidth of a high-power LED. Optimization of LEDs can be achieved in various ways, which are intended to reduce the RC time constant and carrier recombination time, thus increasing the modulation bandwidth of LED chips. In addition, blue filter is another crucial component for VLC phosphor-based white LEDs that could effectively eliminate unwanted signals and increase the data rate, but it cannot influence the modulation bandwidth of the system. Some studies also indicated that an equalization circuit consisting of a resistor, inductor, and capacitor could reduce the signal distortions and increase the modulation bandwidth. [PERSON] et al. demonstrated a phosphor-LED VLC and found that using pre-equalization technology, the modulation bandwidth of the phosphor-LED increases from 1 MHz to 12 MHz without utilizing  
Figure 13: Eye diagrams at data rates of (**a**) 400 Mb/s and (**b**) 700 Mb/s using a p-i-n (PIN) photodetector; eye diagrams with received powers of (**c**) 80 \(\mu\)W and (**d**) 0.1 \(\mu\)W at 100 Mb/s using the Avalanche Photodiode (APD) photodetector. [89] Figures reproduced with permission from Optical Society of America. the blue filter [91]. Figure 14a illustrates the pre-equalization Resistance-Inductance-Capacitance (RLC) circuit for compensation for LED impedance matching. The pre-equalization technique will certainly increase the modulation bandwidth by reducing signal distortion. However, the values of resistance and capacitance need to be adjusted to obtain the ideal pre-equalization characteristic. Moreover, the LED lighting must have a decent modulation bandwidth to accomplish a real-time VLC framework. Figure 14b-f display the modulation bandwidth for the phosphor-LED with and without the pre-equalization technique. It is clear that without pre-equalization, the bandwidth of the LED is 1.2 MHz, but when the pre-equalization technique is implemented, the measured bandwidth lies between 11.6 MHz and 12.1 MHz. Therefore, the modulation bandwidth of LEDs for VLC applications can be significantly increased using the pre-equalization technique. Keeping in mind the various approaches stated above, [PERSON] et al., as part of the study to improve modulation bandwidth, showed that the modulation bandwidth could be improved by incorporating quantum dots (QDs) with LED [92]. Since QDs have the potential to confine the excitons resulting in a short fluorescent lifetime that is much less than commercial phosphorus, QD-LED could enhance not only the color quality but also the white LED bandwidth to achieve better performance for VLC [93].