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The experimental setup of a 2×2 MIMO-OFDM VLC system is shown in Fig.2. At the transmitter, the generated signals are uploaded into an arbitrary function generator. DC offffset is also supplied by signal generator to ensure the positivity of the transmitted signals. Then, mixed signals are transmitted by two LEDs. At the receiver, the optical signals are converted into electrical signals by two photodetectors (PDs), which are recorded by a high-speed digital oscilloscope. The parameters used in the experiment are shown in Table 1.
System parameters Values Modulation order 8 QAM、16 QAM、64 QAM Bandwidth/MHz 12.5 FFT length 1024 Subcarrier number 256 CP length 16 Transmission distance/m 0.5 Distance between TXs/cm 7 Distance between RXs/cm 10 Table 1. Experimental parameters
Figure 3 shows the BER curves of BPSK, QPSK, 8 QAM, 16 QAM and 64 QAM modulation respectively. The BER of the five modulation methods is gradually decreasing with the increase of SNR; SNR required for PSK modulation is smaller at the same BER level compared with QAM modulation; the BER of the system is also increasing with the increase of QAM modulation order; when BER is at the 7% pre-forward error correction (pre-FEC) threshold of 3.8×10−3, the SNR required for BPSK, QPSK, 8 QAM, 16 QAM and 64 QAM modulation are 3.8 dB, 7 dB, 9.6 dB, 13 dB and 20.7 dB; according to estimate of the SNR on the receiver, the modulation mode of the transmitter can be adaptively selected through the feedback channel, BPSK, QPSK or 8 QAM modulation is adopted if the SNR is small, otherwise 16 QAM or 64 QAM modulation. Under the same BER, it can greatly improve the utilization of frequency band adaptive modulation method compared with a certain fixed modulation method is used.
Figure 4 shows the curves of the SNR with the subcarrier index in the traditional MIMO-OFDM system, STBC coding, OCT precoding and STBC-OCT precoding. In the traditional MIMO-OFDM, the SNR is reduced from 22 dB to 8 dB, and the higher-frequency fading is obvious, which is mainly caused by the nonlinear characteristics of the LED; the SNR of a single STBC code is reduced from 27 dB to 12 dB, and the average SNR is about 4 dB higher than that of traditional MIMO-OFDM. Therefore, it is beneficial for higher-frequency fading. However, the system using a single STBC code is still limited by bandwidth, and its SNR curve drops monotonously; a single OCT precoding has a SNR curve of about 14 dB within the subcarrier index range; STBC-OCT precoding still maintains the relatively flat SNR advantage of a single OCT precoding, and the SNR is increased by about 3 dB compared with OCT precoding, so it can effectively overcome the problem of bandwidth limitation in MIMO-OFDM VLC system.
Figure 5 shows the simulation results of the CCDF curves of the traditional MIMO-OFDM system, STBC coding, OCT precoding and STBC-OCT precoding schemes. The PAPR performance is similar to the MIMO-OFDM system, STBC coding and OCT precoding, while the PAPR of the STBC-OCT precoding system is much lower. At the CCDF=10−4, the STBC-OCT precoding scheme was used to obtain a gain of 2 dB over the PAPR of the other three schemes.
BER is an important indicator to measure system performance, so the relationship between BER performance of MIMO-OFDM VLC system and driving peak voltage (Vpp) and direct current offset (DC) is further analyzed.
Figure 6 shows the BER of the traditional MIMO-OFDM, OCT precoding and STBC-OCT precoding schemes under different DC bias conditions. Using Rebel Star 01 LED, the LED’s starting light-emitting voltage is 2.2 V through experimental tests, and the LED has a good linearity in the range of 2.5 V to 2.9 V[14-15]. In the experiment, Vpp is set to 2.8 V, and the DC bias is 2.4-3.1 V, the BER of each curve shows a trend of first decreasing and then increasing with the DC bias increases. When the DC is 2.7 V, this value is exactly in the middle of the linear voltage region of the LED, so the error performance of the three curves is the best, and the BER are 5.45×10−1, 2.3×10−1, 8.8×104. The STBC-OCT precoding has the best BER performance and is always lower than the 7% pre-FEC threshold of 3.8×10−3, which is mainly due to the flatness of the SNR, which reduces the higer-frequency attenuation error, thereby improving the BER performance of the system.
In addition, in the experiment, the DC offset is set to 2.7 V, and the Vpp-value is from 2.1 V to 3.3 V, the BER performance of different schemes is shown in Fig.7. The BER of each curve also shows a trend of first decreasing and then increasing with the increase of Vpp, When Vpp= 2.7 V, the BER of STBC-OCT precoding is 1×10−3, Vpp=3.1 V, the BER of MIMO-OFDM and OCT precoding are 2.6×10−3, 2.5×10−3, the STBC-OCT precoding performance is the best and it is lower than the 7% pre-FEC threshold of 3.8×10−3. Experimental results show that the BER decreases first with the increase of Vpp, because increasing Vpp can improve the SNR, Then the BER increases again, and when Vpp continues to increase, nonlinear distortion plays a leading role.
Research of adaptive modulation STBC-OCT precoding in MIMO-OFDM VLC system
doi: 10.3788/IRLA20210652
- Received Date: 2021-09-09
- Rev Recd Date: 2021-12-07
- Accepted Date: 2021-12-24
- Publish Date: 2022-07-05
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Key words:
- visible light communication /
- multiple input-multiple output-orthogonal frequency division multiplexing /
- space-time block code /
- orthogonal circulant matrix transform /
- adaptive modulation
Abstract: An efficient precoding scheme combing adaptive modulation, space-time block code (STBC) and orthogonal circulant matrix transform (OCT) is proposed to improve the performance of multiple input-multiple output-orthogonal frequency division multiplexing (MIMO-OFDM) based visible light communication (VLC) systems in this work. A 2×2 MIMO-OFDM VLC system is designed and established to evaluate the performance of the proposed adaptive modulation STBC-OCT precoding scheme. The performances of bit error rate (BER), peak-to-average power ratio (PAPR) are investigated experimentally for different modulation and coding schemes. The performances of BER are studied in different direct current bias (DC) and driving peak-to-peak voltages (VPP) offsets. The experimental results show that the system using proposed adaptive modulated STBC-OCT precoding obtains a relatively flat and higher signal-to-noise ratio (SNR) values, lower PAPR compared with other precoding methods. The proposed scheme can obtain the best BER performance, and it is always lower than the 7% pre-forward error correction (pre-FEC) threshold of 3.8×10−3with the transmission distance is 0.5 m, DC is set to 2.7 V and VPP is 2.7-2.8 V, it can effectively overcome the bandwidth limitation of MIMO-OFDM VLC system and provide the best reliability.