# Visible Light Communication System Technology Review: Devices, Architectures, and Applications
## 1 Introduction
However, when the blue photon hits yellow phosphor [8; 9; 10; 11], the transition causes a relatively reduced penetration power and frequency modulation bandwidth, thereby limiting the signal-to-noise ratio (SNR) and transmission capacity of Li-Fi systems. In 2020, [PERSON] et al. modulated the limited bandwidth of an LED with Discrete  
Figure 1: A schematic of a VLC System (Tx: Transmitter; Rx: Receiver; DAC/ADC: Digital/Analog converter; TIA: Transimpedance amplifier). Multitone (DMT) format, achieving 513 Mbit/s of data rate [12]. In 2012, [PERSON] et al. reached a data rate of 1 Gbit/s with a phosphorescent white LED by the utilization of a DMT modulation scheme [13]. To improve spectrum utilization, Li-Fi systems mostly use carrierless amplitude and phase (CAP) and M-ary quadrature-amplitude-modulation (M-QAM) as the modulation format. In 2013, [PERSON] et al. used CAP and QAM-orthogonal frequency-division multiplexing (OFDM) to modulate an RGB-LED, reaching a data rate of 2.93 and 3.22 Gbit/s, respectively [14]. However, the coherence, noise performance, and direct modulation bandwidth of the traditional LEDs were all hampered by the spontaneously radiating photons of the active region stimulated by the photons. To overcome the problems mentioned above, resonant cavity LEDs or micro-LED arrays (\(\upmu\)-LEDs) have been proposed due to their high internal quantum efficiency (IQE), light extraction rate, and variable modulation bandwidth. Since the RC constant of the device cannot limit the bandwidth of the \(\upmu\)-LED [15] and the GaN \(\upmu\)-LED is a much smaller LED (\(<\)100 \(\upmu\)m), it has been proven to provide better white light than traditional LEDs, with a higher output intensity and wider modulation bandwidth [16; 17]. Laser diodes (LDs) are another light source often employed in VLC systems. They have stronger coherence, narrower spectral linewidth, and larger modulation bandwidth, making them more suited for data rates exceeding Gbit/s [18; 19]. Unlike LEDs, which can only provide up to a few hundred MHz of usable bandwidth, a commercial transistor-outline can (TO-can)-packaged GaN blue LD was reported in 2013 carrying 2.5 Gbit/s of error-free on-off keying (OOK) transmission, which can provide a VLC system with 1.4 GHz of bandwidth [20]. In 2013, [PERSON] et al. demonstrated a bidirectional VLC that can provide 2.5 Gbit/s 16-QAM OFDM, using a red laser pointer [21]. To increase the data rate, in 2013, [PERSON] et al. achieved 9 Gbit/s VLC in a 5 m point-to-point (PtP) free space link by utilizing 64-QAM OFDM to modulate a GaN blue LD [22]. The lighting function of an LD is similar to an LED, which is realized by covering the output end of blue LD with yellow phosphor or using the collimated beam of mixed R/G/B LD. In 2015, [PERSON] et al. demonstrated a non-return-to-zero OOK (NRZ-OOK) data stream that transmitted at a data rate of 2 Gbit/s using a blue LD covered with a yellow phosphor [23]. In the same year, [PERSON] et al. achieved a data rate of 4 Gbit/s by embedding the yellow fluorescence of the blue LD into remote color conversion [24]. In 2015, [PERSON] et al. also illustrated a blue LD-based white light converted by a phosphorous diffuser, with a correlated color temperature (CCT) of 5217K, which can transmit 16-QAM OFDM data at a data rate of 5.2 Gbit/s in a 0.6 m free space channel [25]. Additionally, [PERSON] et al. expected to achieve a data rate exceeding 100 Gbit/s by using an R/G/B hybrid LD light source for white light VLC carrying a QAM-OFDM signal [26]. In 2017, [PERSON] et al. achieved an 8 Gbit/s data rate with 16-QAM OFDM over 0.5 m free space by using a commercial R/G/B TO-can-packaged LD [27]. These illustrations suggested that visible light LDs have greater data transmission potential than LEDs and that this potential will grow in the future. Mainstream white light VLC light sources will be achieved by using laser and color mixing technology, and a brief comparison of light source used in VLC systems is presented in Table 1.