## 3 Experimental Results and Discussions
### Communication-Performance Evaluations  
In this section, an evaluation of the communication performance of the dual-hop system is given. In order to better understand the influence of the communication performance of the dual-hop system, a comparison is made with the single-hop system. The frequency response and BER performance were experimentally tested. The normalized frequency response of the micro-LED-based VLC system at different links is shown in Figure 6, which illustrates the modulation-bandwidth characteristics under different situations. Without any hardware-equalization technique, it can reach 880 MHz in the single-hop link, which presents this blue micro-LED has great potential as high-speed communication application. Moreover, the values of 3-dB modulation bandwidth decrease to 715 MHz with the relay node. It is easy to see that modulation bandwidth only slightly decreases after the relay nodes and longer distance transmission. Figures 7 and 8 indicate the results of measured BER performance, as well as the obtained insert of it, which represents the corresponding eye diagrams when the data rate is varied. The PRBS pattern length of the OOK signal is set as \(2^{31}-1\). As illustrated in Figure 6, the different OOK modulations of 1.0 Gbps to 2.1 Gbps are applied on Tx1. The maximum data rate is 2.0 Gbps within the forward error correction (FEC) limit (3.8 \(\times\)\(10^{-3}\)), which can fully demonstrate the great potential of the micro-LED used in the high-speed VLC communication system. As a result, the maximum data of the first blue micro-LED can reach 2 Gbps in 2 m free-space transmission at the FEC limit. Meanwhile, through the same 2 m free-space channel and lens, the values of the received optical power are also measured, which remain relatively the same: 67 uW in Figure 7 while 63 uW in Figure 8.  
Then, the maximum modulated data of Tx2 is likewise performed. In the experiment, the different OOK modulations of 500 Mbps to 1.3 Gbps are applied on the second blue micro-LED. The maximum data rate corresponding to the FEC limit is 1.1 Gbps, which is smaller than the single-hop link. After the AF method, the transmitted optical power of the relay node is almost the same. The change of Vpp also confirms this, which is from 137.6 mV to 112.0 mV. Therefore, for single-hop and dual-hop links, 2.0 Gbps and 1.1 Gbps are achievable within the FEC limit, respectively. Figure 6: The comparison of measured normalized frequency response from Hop 1 and Hop 2. In order to be more intuitive, Table 2 summarizes some measured indicators related to communication performance. Under the premise of the extended distance of the dual-hop VLC system which increases from 2 m to 4 m, it is noticeable that some of the communication performance is changed. The value of Vpp goes down from 137.6 mV to 112.0 mV, which is relatively the same due to the AF method and the same bias current. However, the AF method will inevitably introduce synchronous amplification of noise; the noise and long-distance attenuation represent a greater impact in the dual-hop link following an inevitable reduction in the modulation bandwidth and the maximum data rate. There is an admissible decrease in modulation bandwidth between 880 MHz and 715 MHz. Moreover, the data rate stabilizes at the order of Gbps and the relay node in the real-time dual-hop VLC system expands the scope of the limited area, tackling the NLOS problem effectively. Therefore, the cost of the relay node is acceptable. To the best of our knowledge, this is the first experimental demonstration of a real-time dual-hop link based on a micro-LED that could provide a potential solution to tackle the NLOS problem of indoor VLC. In this work, the modulation signal with an OOK format is used to complete the experimental verification of our proposed dual-hop VLC system based on blue micro-LEDs. In future work, higher-order modulation formats can be considered to obtain higher data rates. Figure 8: Measurement of real-time BER with OOK format from Hop 2.