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SCIENCE CHINA Information Sciences, Volume 62 , Issue 8 : 080305(2019) https://doi.org/10.1007/s11432-018-9858-2

Exploring multiamplitude voltage modulation to improve spectrum efficiency in low-complexity visible-light communication

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  • ReceivedJun 21, 2018
  • AcceptedApr 8, 2019
  • PublishedJul 11, 2019

Abstract

Owing to the recent advancements in the Internet of Things (IoT), an increasing number of IoT devices have led to frequency crowding in wireless networks. The low-complexity visible light communication (VLC) system is a promising solution for indoor frequency-crowded wireless networks owing to the ubiquity of light-emitting diodes (LEDs) and readily deployable, low-cost modulation methods. However, due to the inherent limitations of LED materials and growth of data from IoT applications, low-complexity VLC systems can barely boost the data throughput without adding complex modulations and circuits. This study aims to design and implement a spectrum-efficient, low-complexity VLC system to improve data throughput with very little cost to support indoor IoT applications. This system uses multiamplitude voltage to transmit multiple bit streams simultaneously, making it a novel system. However, it is not trivial to achieve this goal as the varying voltage can cause the LEDs to flicker. Therefore, we further propose a voltage-to-current amplifier circuit, which effectively mitigates the effects of changes in the LED's brightness upon human eyes. Finally, we evaluate the system under different speeds, distances, and angles. Extensive experiments demonstrate promising results from the viewpoint of spectrum efficiency, throughput, bit-error rate, and user perception.


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  • Figure 1

    (Color online) Low-complexity visible-light communication (VLC) systems for smart illumination and consumer devices, e.g., smart toys, smart glasses, and other IoT devices.

  • Figure 2

    (Color online) The principles of our multiple-amplitude pulse-position-modulation (MAPPM) scheme, as compared with two readily deployable modulation methods, such as on-off keying (OOK) and 2-PPM, in low-complexity VLC systems. Our MAPPM scheme enables two bit streams to transmit simultaneously by modulating the signal with multiple states (where the blue line represents different states).

  • Figure 3

    (Color online) The system architecture. The second bit stream is embedded in the first by the MAPPM scheme. The synthetic bit stream is transmitted into the V-I circuit so that human eyes cannot perceive the flicker. Then, it passes through the photodetector and amplifier to produce A/D conversion and threshold decision.

  • Figure 4

    (Color online) The architecture of the sequence header. We set the same duration of 600 $\mu~$s for both one bit and sequence header. Then, we set different lengths of time in the sequence header to avoid the flicker and synchronize the receiver. The duration time of the header sequence varies with the duration time of one bit, i.e., the duration time varies with different speeds.

  • Figure 7

    (Color online) The LED is operated by an STM32F407ZGT6 microcontroller and a $V$-$I$ conversion circuit. The light intensity is converted into a voltage signal by the SD5412 photodetector. The voltage signal is sampled by the STM32F103C8 microcontroller. (a) The transmitter module; (b) transmission path; (c) the receiver module.

  • Figure 8

    (Color online) User perception at different viewing angles. We list four different scenarios to observe whether the LED flicker impacts upon human eyes.

  • Figure 9

    (Color online) The BER curve for the MAPPM scheme with Bits-1 (a) and Bits-2 (b) using different speeds and the receiver being located at different distances.

  • Figure 10

    (Color online) The system throughput doubles at different speeds at a distance of 12 cm.

  • Figure 11

    (Color online) The receiving waveform of the photodetector and $V$-$V$ circuit under the interference condition at a distance of 15 cm.

  • Figure 12

    (Color online) The BER curve of Bits-2 under light interference.

  • Figure 13

    (Color online) The receiving waveform when the transmitter has six LEDs at a distance of 80 cm.

  • Figure 14

    (Color online) The receiving waveform when the transmitter has six LEDs at a distance of 80 cm. Meanwhile, the closed-loop gain is $5$.

  • Figure 17

    The curve of the PV cell-response time. We test response time and output voltage under different light intensities. The test data are transmitted at 1 KHz. Note that an enlarged view is given to the rising edge.

  • Table 1   2-PPM modulation scheme$^{\rm~a)}$
    Bit (first symbol, second symbol)
    0 (bright, dark)
    1 (dark, bright)

    a

  • Table 2   Experimental parameters
    System modules
    TX microcontroller STM32F407ZGT6
    TX voltage-to-voltage amplifier LM358A
    TX voltage-to-current amplifier OPA07CPA
    Transistor S8050
    RX voltage-to-voltage amplifier LM358A
    RX microcontroller STM32F103C8
    Power GPS-430C
    LED SSL-LX100133XUWC
    Photodetector SD5412
    TX characteristics
    Average optical power per LED 0.1 W
    LED's FoV $120^\circ~$
    RX characteristics
    Photodetector diameter 2.54 mm
    Receiver's FoV $120^\circ~$
    Photodetector responsivity $R$ 0.87 (A/W)
  • Table 3   User perception about the flicker
    View Speed (kbps) Day flicker (%) Night flicker (%)
    1 100 90
    Direct 30 cm 3.3 0 25
    8 0 0
    1 75 95
    Indirect 10 cm 3.3 5 5
    8 0 0
    1 75 90
    Indirect 35 cm 3.3 0 25
    8 25 0
    1 100 100
    Reflect 20 cm 3.3 0 5
    8 0 0

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