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In this paper, we propose employing a 682 nm vertical-cavity surface-emitting laser (VCSEL) with 1 GHz bandwidth for high-speed and power-sharing wireless visible light communication (VLC) in the different transmission distances of 2 to 5 m. In the measurement, the data rate of 0.52 to 11.86 Gbit/s (0.44 to 10.8 Gbit/s in a net data rate) can be achieved by using spectral-efficient orthogonal frequency division multiplexing (OFDM) modulation with bit-loading algorithm. Therefore, 4- to 256-quadrature amplitude modulations (QAMs) are employed simultaneously in the modulation bandwidth for VCSEL-based VLC. The proposed power-sharing VLC system can be divided to four end-users, when three beam splitters (BSs) are used simultaneously. Moreover, all of the measured bit error rates (BERs) are below the forward error correction (FEC) threshold (BER = 3.8 × 10−3).
© 2016 Optical Society of America
1. Introduction
Recently, the phosphor light-emitting-diode (LED) based visible light communication (VLC) has been developed to provide indoor lighting and communication simultaneously [1,2]. However, due to its limited modulation bandwidth (few to tens MHz), it is hard to support > Gbps VLC traffic rate [3,4]. Moreover, to enhance the modulation rate of LED, using high spectral-efficient orthogonal frequency division multiplexing (OFDM) modulation have been proposed and discussed in wireless VLC [5,6].
Utilization of visible laser for VLC transmission has been matured due to its remarkable optical characteristics [7,8], such as no electromagnetic interference (EMI), license-free, and wide modulation bandwidth of > GHz. Moreover, using laser-based VLC architecture will be a potential candidate for the end-user access applications [9,10]. Compared with the LEDs, the visible laser diode (LD) shows the higher pumping efficiency and higher direct modulation speed for constructing the alternative VLC systems. More recently, there were several related researches of visible LD-based VLC. For instance, Hanson et al. proposed using 532 nm LD with 1 Gbit/s data rate for under water VLC in 2008 [11]. Janjua et al. utilized the red-green-blue (RGB) LDs to achieve 4 Gbit/s 16-QAM OFDM traffic rate in 2015 [12]. And Chi et al. employed a Gallium Nitride (GaN)-based blue LD for 9 Gbit/s 64-QAM OFDM data rate in 2015 [13]. Besides, to extend the modulation bandwidth to 40 GHz, Tsai et al. proposed multi-stage external-injected vertical-cavity surface-emitting lasers (VCSELs) to achieve a 40 Gbit/s on-off keying (OOK) transmission rate [14]. However, using injection-locking method required additional lasers to enhance the bandwidth and also increased the deployment cost. As mentioned above, the traffic capacity of LD-based VLC is still limited by the inefficient modulation format and bandwidth.
In this paper, we first propose using a 682 nm VCSEL with 1 GHz bandwidth for power-sharing wireless VLC under various transmission distances of 2 to 5 m. Here, utilizing OFDM modulation with bit-loading algorithm can achieve the data rates of 0.52 to 11.86 Gbit/s (0.44 to 10.8 Gbit/s in a net data rate), respectively, when 1 to 3 beam splitters (BSs) are used in the power-sharing VLC system for supporting one to four end-users. In this measurement, 4- to 256-quadrature amplitude modulations (QAMs) are applied simultaneously on the VCSEL laser for adaptively VLC transmission. The entire measured bit error rates (BERs) can be below the forward error correction (FEC) threshold (BER = 3.8 × 10−3) [15]. In addition, the data rate and performance of proposed VLC system are also analyzed and discussed under the different free spacing transmission distances. We believe that the proposed power-sharing VLC system using VCSEL with low bias current (3.5 mA) can be a promising alternative for >10 Gbit/s VLC transmission.
2. Experiment and results
Figure 1(a) depicts the experiment setup of the proposed power-sharing VCSEL-based VLC system. In the experiment, a 682 nm VCSEL with 1 GHz bandwidth was directly modulated by OFDM signals with a driving voltage (Vpp) of 1 V. After free-space transmission distances of 2 to 5 m with and without BSs, optical OFDM signals were directly detected using a 2 GHz silicon-based PIN photodiode (PD) with an active area diameter of 100 μm, as indicated in Fig. 1. It should be noted that each BS increases an end-user but leads to power loss of 3 dB in the VLC system. In this paper, we demonstrate power-sharing VLC system using 3 BSs at most, and the interval among each BS was set to 5 cm. Figure 1(b) depicts the L-I characteristics of the 682 nm VCSEL at 25 °C. The threshold and operated currents of VCSEL are 0.8 mA and 3.5 mA at room temperature, respectively. And the rising/falling time of VCSEL is ~100 ps. Here, the emitted light from the VCSEL is launched into the first convex lens for transmitting in the free space. Then, the parallel light launched into the second convex lens and focused on the PD. Here, the 1st convex lens is employed to transform the divergent light beam into a parallel beam. The 2nd convex lens is used to focus the parallel light into a point. Moreover, to obtain a better coupling efficiency into the PD, we use the travel linear stages in the PD side for alignment to ensure the optimal SNR output.
Fig. 1 (a) Experiment setup of proposed power-sharing VCSEL-based wireless VLC system. (b) L-I characteristics of 682 nm VCSEL.
Here, the OFDM modulation with bit-loading algorithm was utilized to obtain and maximize the bandwidth efficiency. In this work, we used off-line Matlab programs to generate OFDM signals with the parameters set as follows: inverse fast Fourier transform (IFFT) size of 256, cyclic prefix (CP) of 3.03%, and the subcarrier number of 80. Figure 2(a) depicts the flowchart of OFDM signal generation. First, serial binary sequences were converted to parallel ones using serial-to-parallel (S/P) conversion and subsequently mapped to QAM symbols. It should be noted that the QAM orders (4-QAM to 256-QAM) were determined using bit-loading algorithm. Second, the QAM symbols were then transformed into time-domain discrete signals using IFFT. After inserting CP, the parallel discrete signals were converted to serial ones using parallel-to-serial (P/S). Then, the OFDM signal was generated using an arbitrary waveform generator (AWG, Tektronix AWG7122) at a sampling rate of 6 GSample/sec corresponding to a modulation bandwidth of 1.875 GHz, and the digital-to-analog (D/A) conversion resolution and the output peak-to-peak voltage (Vpp) were 8 bit and 1 V, respectively. In the measurement, 80 subcarriers are utilized in the bandwidth of 0.047 to 1.895 GHz corresponding to the effective modulation bandwidth of VCSEL. In this experiment, the highest signal frequency of 1.895 GHz is mainly restrained by the VCSEL with an available bandwidth of 1 GHz. It should be noted that using bit-loading algorithm, the OFDM signal can adaptively change the signal bandwidth in accordance with the entire channel response. Then the OFDM signal was applied on the 682 nm VCSEL through the 2.5 GHz bias-tee (BT). After a free-space link, the VLC signal was detected and converted to electrical signal using a 2 GHz PIN PD at the receiver side. Then, the electrical OFDM signal was amplified using a wideband RF amplifier and subsequently retrieved by a real-time oscilloscope (Tektronix CSA 7404) at a 10 GSample/sec sampling rate and 8 bit analog-to-digital (A/D) conversion resolution for OFDM signal demodulation. The demodulation was implemented using off-line Matlab programs with fast Fourier transform (FFT) size of 256. Figure 2(b) presents the flowchart of OFDM signal generation. After performing synchronization, S/P conversion, and CP removal, the received OFDM signal was transformed into frequency-domain discrete symbols using FFT. Then, training symbols were separated, and the one-tap equalization was performed using these training symbols. Subsequently, the signal to noise ratio (SNR) was measured and the bit error rate (BER) was counted through bit-by-bit comparison.
In the proposed power-sharing VLC system, OFDM modulation with bit-loading algorithm was applied to the VCSEL. Because the BER is a function of the SNR, the SNR of each subcarrier can be manipulated by controlling the power level and thus achieve a target BER. Here, the relation between SNR and BER of a square M-ary QAM can be written as:
where BERtarget denotes the target BER we attempt to achieve. By considering Eq. (1), we can yield the BER of a square M-ary QAM signal with the SNR lower than the target BER. Therefore, the total data throughput can be expressed as:where b denotes bits per symbol, n the subcarrier index, N the total number of subcarriers, and Pn the power for each subcarrier. By the use of Eq. (2), therefore, the data rate of OFDM signal with bit-loading algorithm can be determined.Figure 3(a) presents the SNR of each subcarrier in the frequency range of 0.047 to 1.895 GHz at a transmission distance of 2 m with and without BSs. As the VLC system are without and with 1, 2 and 3 BSs, SNRs ranges between 8.80~28.97 dB, 8.80~28.61 dB, 7.77~25.31 dB, and 8.01~19.58 dB, respectively. As mentioned in 2nd section, paragraph 1, each BS would results in 3 dB insertion power loss and therefore degrades the SNR. In addition, SNR degrades with increases in frequency. This could limit the spectral efficiency of OFDM signals and eventually restrain the data rate. In order to maximize the spectral efficiency of OFDM signals, we applied bit-loading algorithm to OFDM modulation. As mentioned in 2nd section, paragraph 3, the QAM-order was adjusted based on the SNR value of each subcarrier. Figure 3(b) illustrates the corresponding determined bit number of each subcarrier at a transmission distance of 2 m without and with 1, 2 and 3 BSs. In the experiment, 4-QAM to 256-QAM OFDM signals are applied to OFDM signals, as shown in Fig. 3(b). As can be seen, determined bit numbers ranges between 2~8 bit/sec/Hz without using BS. By the use of three BSs, determined bit numbers ranges between 2~5 bit/sec/Hz.
Fig. 3 (a) Measured SNR and (b) determined bit number versus frequency, when the proposed power-sharing VLC is without and with 1, 2 and 3 BSs respectively, in a transmission distance of 2 m.
Figures 4(a) and 4(b) depicts SNR and determined bit number versus frequency at a free-space link of 5 m, respectively. Additionally, 1 to 3 BSs were used to a power-sharing VLC system. It is known that the received optical power of VCSEL could decrease when the transmission distance was extended to 5 m. Compared with the results at a transmission distance of 2 m, the SNRs and corresponding determined bit numbers decline, as depicted in Figs. 4(a) and 4(b). Without using BS, the signal bandwidth is approximately 1.73 GHz (0.047~1.781 GHz) and SNRs ranges between 7.79 and 19.65 dB. As three BSs are employed simultaneously, the signal bandwidth narrowed down to approximately 0.28 GHz (0.047~0.328 GHz) and SNRs ranges between 7.80 and 12.46 dB. Besides, as shown in Fig. 4(b), all of determined bit numbers distribute between 2 to 5 bit/sec/Hz at a transmission distance of 5 m. In short, increases in BS number and transmission distance leads to decreases in signal bandwidths, SNRs, and determined bit numbers.
Fig. 4 (a) Measured SNR and (b) obtained bit number versus frequency, when the proposed power-sharing VLC is without and with 1, 2 and 3 BSs respectively, in a transmission distance of 5 m.
Figures 5(a) and 5(b) present the corresponding constellations of OFDM signal with bit-loading algorithm in proposed power-sharing VLC system without and with three BSs under the transmission distance of 2 m, respectively. It should be noted that, in this work, the rectangular 8-QAM is applied rather than the circular 8-QAM, owing to the simpler implementation of decision regions. Figures 5(c) and 5(d) display the corresponding constellations of OFDM signal with bit-loading algorithm in proposed power-sharing VLC system without and with three BSs under the transmission distance of 5 m, respectively. As can be seen in Fig. 5, these constellations are clear among all cases.
Fig. 5 Measured corresponding constellations of OFDM signal under a 2 m transmission distance (a) without and (b) with three BSs. Measured corresponding constellations of OFDM signal under a 5 m transmission distance (c) without and (d) with three BSs.
Figures 6(a) and 6(b) depicts data rates and corresponding BERs at transmission distances of 2, 3, 4 and 5 m, respectively, without and with BSs of 1 to 3. It should be noted that, those data rates presented in Fig. 6 are gross data rates with 15% overheads. As shown in Fig. 6(a), the data rates of 11.86, 10.64, 9.02 and 6.82 Gbit/s are achieved at a free-space link of 2, 3, 4 and 5 m without using BS, respectively. When 1, 2, and 3 BSs are added in the proposed VLC system to support 2, 3 and 4 end-users, respectively, the traffics will drop to 10.52, 8.39, 6.00 and 3.84 Gbit/s; and 8.20, 4.90, 3.21 and 1.57 Gbit/s; and 5.04, 2.53, 1.45 and 0.52 Gbit/s, respectively, at various free space distances of 2, 3, 4 and 5 m. Therefore, with the increases in the number of BSs and the transmission distance, data rates drop due to the decreases in SNRs. Figure 6(b) presents counted BERs of the proposed power-sharing VLC system with the same operation condition as above. The red dash line of Fig. 6(b) is FEC threshold. All BERs are lower than FEC threshold (BER = 3.8 × 10−3), as illustrated in Fig. 6(b). Furthermore, the outstanding challenges of practical implementation for the VLC system were the enhancement of data rate and alignment measurement [11,14,16]. As a result, using a VCSEL with a bandwidth of only 1 GHz, we can achieve 11.86 Gb/s VLC transmission over 2 m free-space link using OFDM modulation with bit-loading algorithm.
Fig. 6 Corresponding (a) data rate and (b) BER of proposed VCSEL-based VLC system using OFDM modulation with bit-loading algorithm.
3. Conclusion
In summary, we first proposed employing a 1 GHz red-light VCSEL laser for high-speed wireless power-sharing VLC transmission under the free space distances of 2 to 5 m. In general, the previous studies were focusing in a point to point (P2P) connection [13,14,17]. Here, we proposed a power-sharing VLC to achieve a point to multi-point (P2M) transmission for increasing the client side number. The proposed VLC transmission also could be used for time-division-multiplexing (TDM) upstream access. In the measurement, utilizing OFDM modulation with bit-loading algorithm could achieve the data rates of 0.52 to 11.86 Gbit/s (0.44 to 10.8 Gbit/s in a net data rate), respectively, when 1 to 3 beam splitters (BSs) were used in the power-sharing VLC system to support one to four end-users. Here, 4- to 256-QAMs was applied simultaneously on the VCSEL laser for adaptively VLC transmission. All of the measured BERs can be below the FEC threshold. In addition, the data rate and performance of proposed VLC system are also analyzed and discussed under the different free spacing transmission distances.
Funding
This work was supported by the Ministry of Science and Technology, Taiwan, MOST-103-2218-E-035-011-MY3.
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