## 5 Micro-LED Array-Based VLC
The violet \(\upmu\)-LED can achieve bit allocation until 650 MHz, however, it still suffers from optical power loss of about 11 % because of the dichroic mirror. In addition, it is possible to technically incorporate the two dual-color micro-LED arrays into the same package to provide a three-color output that will open a way in terms of WDM VLC connections. In another study related to \(\upmu\)-LED array for VLC application, [PERSON] et al. [79] report fabrication and characterization of blue-only nitride \(\upmu\)-LED arrays with series-biasing and demonstrate their application in VLC. The \(\upmu\)-LED array reported in this experiment contains flip-chip configured \(\upmu\)-LED components arranged in the form of a \(3\times 3\)\(\upmu\)-LED matrix where every component has an emissive area of 1256 \(\upmu\)m\({}^{2}\) and a diameter of 40 \(\upmu\)m. The \(\upmu\)-LED array presented in this report has the benefit of the fact that light-emitting devices working at high driving current densities have a shorter differential carrier lifetime and consequently an enhanced value of modulation bandwidth. The 6-dB electrical bandwidth of the demonstrated \(\upmu\)-LED array comes out to be more than 280 MHz when driven at a current density value of 3200 A/cm\({}^{2}\), as shown in Figure 9a. It is noted that the collective optical power of an \(\upmu\)-LED array with nine components is a bit less than nine times the optical power given out by an individual \(\upmu\)-LED component of the same size and operating under the same condition [75]. The literature suggests that absorption of light among adjacent components and thermal effects are responsible for this phenomenon [80]. The joint modulation bandwidth of the \(\upmu\)-LED array, on the other hand, is more than the expected value suggested by the bandwidth of a single component driven at the similar conditions. This happens because series biasing of \(\upmu\)-LED components leads to a lower capacitance, as reported in [81]. The details of this experimental set-up can be found  
Figure 8: For the blue-green \(\upmu\)-LED array: signal-to-noise ratio and allocated bits plots in both WDM schemes attained by (**a**) the blue \(\upmu\)-LED and (**b**) the green \(\upmu\)-LED; for the blue-violet \(\upmu\)-LED array: signal-to-noise ratio and allocated bits plots in both WDM schemes attained by (**c**) the blue \(\upmu\)-LED and (**d**) the violet \(\upmu\)-LED. [77] Figures reproduced with permission from Optical Society of America. in [79]. The free-space VLC implementation for this series-biased \(\upmu\)-LED array has been demonstrated with On-off keying (OOK), Pulse Amplitude Modulation (PAM), and Orthogonal Frequency-Division Multiplexing (OFDM) modulation formats. A comparison between eye diagrams of the transmitted and received signals is presented in Figure 9b,c where the OOK modulation scheme is adopted at a bit rate of 700 Mbps and the reported \(\upmu\)-LED array is used as a transmitter. It can be observed that the amplitude and temporal distortions created by the channel are easily observable at a comparatively low bit rate used in the experiment. These distortions lead to undesirably high BERs by introducing inter symbol interference. To address this issue, a finite impulse response filter is used as a feedforward equalizer. The equalizer used in the experiment minimizes non-uniformity of the received symbols before this equalizer, which is a consequence of Intersymbol Interference (ISI) as reported in [82]. The equalizer used in the experiment keeps received symbols below a specific decision threshold, which keeps the spread of the received symbols relatively narrower and ultimately lowers the BER. The SNR for the \(\upmu\)-LED for OFDM modulation format is shown in Figure 10a as a function of bandwidth. High SNR ensures a high probability of the transmitted signal being distinguished by the decoder.