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We propose quasi-linear preequalization to be used for high speed visible light communication (VLC) system. Compared with zero-forcing preequalization, this kind of preequalization method is more suitable for a real VLC experimental system with peak power limitation. We carry out simulations and experiments to test the performance of quasi-linear preequalization. With this equalizer, the 3 dB bandwidth of the system can be extended from 17 MHz to 450 MHz. We implement 2.32 Gbit/s phosphorescent white light-emitting diode (LED) VLC transmission over 1 m distance with quasi-linear equalizer, bit and power loading OFDM, differential amplification positive intrinsic negative diode (PIN) receivers, and maximum ratio combining algorithm. To our knowledge, this is the highest transmission data rate based on a phosphorescent white LED VLC system.

With the advantage of combining lighting and communication, VLC has become more and more attractive. Compared with other wireless communication technologies, VLC is preferred due to its low cost, high accuracy, high security, immunity to electromagnetic interference, and long life [

Recent research results applying preequalization.

LED type | Modulation format | Data rate | Distance | Data source |
---|---|---|---|---|

White | OOK-NRZ | 80 Mbit/s | 10 cm | ECOC 2008 [ |

RGB red | OOK | 614 Mbit/s | 40 cm | ECOC 2012 [ |

RGB red | OOK-NRZ | 477 Mbit/s | 40 cm | OFC 2013 [ |

RGB blue | OOK-NRZ | 662 Mbit/s | 15 cm | ECOC 2014 [ |

White | OOK-NRZ | 200 Mbit/s | 110 cm | COL 2014 [ |

White | OOK-NRZ | 550 Mbit/s | 60 cm | OE 2014 [ |

White | OFDM | 225 Mbit/s | 66 cm | OE 2013 [ |

White | DMT | 1 Gbit/s | 10 cm | PTL 2015 [ |

White | 16QAM-OFDM | 1.6 Gbit/s | 1 m | OE 2015 [ |

White | BPL OFDM | 2 Gbit/s | 1.5 m | PJ 2015 [ |

In this paper, we carry out simulations and implement a high speed VLC system based on a phosphorescent white LED utilizing quasi-linear software preequalization. Instead of hardware equalization circuits reported in most recent high speed experiments based on phosphorescent white LED, we employ software equalization to provide more accurate and complex preequalization. Zero-forcing preequalization will bring the most flat received spectrum theoretically; however it is not the most suitable software preequalization method in a practical VLC experimental system considering the constrained peak and total power. We select three kinds of linear preequalization lines to be the software equalizer. We test the system performance using the lines through simulation and experiment demonstration. Besides, we also apply maximum ratio combining (MRC) algorithm, differential amplification PIN receivers, and adaptive bit and power loading orthogonal frequency division multiplexing (OFDM) for better system performance. We successfully carry out the experimental demonstration of 2.32 Gbit/s VLC transmission utilizing a phosphorescent white LED over 1 m free space transmission distance with bit error rate (BER) lower than 3.8 × 10^{−3}. To our knowledge, this is the highest transmission data rate based on a phosphorescent white LED VLC system.

Preequalization aims at attenuating low frequency components and amplifying high frequency components which can flatten the whole receiving spectrum for better system performance. Zero-forcing preequalization is the most traditional way. The principle of applying zero-forcing equalization in VLC system is shown in Figure

Zero-forcing preequalization.

SNR of each subcarrier is estimated using BPSK-OFDM to implement adaptive bit and power loading in VLC system. SNR is computed with estimation of error vector magnitude (EVM) using [

^{−3}, this subcarrier’s allocated bit number can be computed using (

The total data rate is calculated using (

The frequency of a phosphorescent white LED is modeled using (^{6} rads/s:

The thermal noise variance of PIN receivers is computed using (

We propose three kinds of quasi-linear software equalization lines shown in Figure

Quasi-linear software equalizer lines.

To test the system performance employing different software preequalization lines, we carry out simulations. The simulation setup is shown in Figure

Simulation setup.

We use linear kind with different slope

Average bit numbers using different

Then, we simulate the system using two-staged oblique line equalizer to find the best combination of

Average bit numbers using different

We select

Average bit numbers using linear kind, concave kind, and convex kind equalization.

The block diagram with quasi-linear equalizer and experimental setup of VLC system are shown in Figures

Block diagram with quasi-linear equalizer.

Experimental setup of VLC system.

Quasi-linear equalization is implemented by a Matlab program. Then the signal is loaded into AWG (Tektronix AWG710). After being amplified by EA (Electrical Amplifier, 25 dB gain, 50 Ω input impedance, and 50 Ω output impedance), the signal is coupled with direct current by Bias Tee. Then, it is applied to a phosphorescent white LED (OSRAM, LCWCRDP.EC-KULQ-5L7N-1, luminous flux about 120 lm at 350 mA). The 3 dB bandwidth of this LED is about 17 MHz [

Measured spectra (spectrum analyzer, HP8562A) are shown in Figure

Measured electrical spectra before VLC system (a) without preequalization, (b) with zero-forcing preequalization, (d) with linear preequalization, (f) with convex preequalization, and (h) with concave preequalization and receiving electrical spectra after VLC system (c) with zero-forcing equalization, (e) with linear preequalization, (g) with convex preequalization, and (i) with concave preequalization.

Figure ^{−3}. To our knowledge, this is the highest transmission data rate based on a phosphorescent white LED VLC system. The results also verify that zero-forcing preequalization is not suitable for experimental VLC systems because of peak power limitation caused by PIN receivers and quasi-linear preequalization can improve system performance greatly. The corresponding ^{−3} under the limitation of forward error correction.

Comparison of experimental data rate using linear kind, concave kind, and convex kind equalization.

The bit allocation using three kinds of preequalization is shown in Figure

Bit allocation for 512 subcarriers applying linear kind, concave kind, and convex kind equalization.

In this work, we propose quasi-linear preequalization for high speed visible light communication system. Compared with zero-forcing preequalization, this kind of preequalization method can improve system performance significantly because of the peak power limitation caused by the PIN receivers in VLC system. Adaptive bit and power loading OFDM, differential amplification receivers, and MRC algorithm are also applied in this system for better performance. We carry out both simulation and experimental demonstration to test and verify the performance of different kind of quasi-linear preequalization. According to the results, the performance of the concave kind is worse than the linear and the convex kind. When ^{−3} is experimentally achieved for the first time. To our knowledge, this is the highest transmission data rate based on a phosphorescent white LED VLC system.

The authors declare that they have no competing interests.

This work was supported by the ZTE project (2015ZTE01-02-03) and the Provincial Science and Technology Program of Guangdong (2014B010119003).