Y. F. Liu

Investigation of 4-ASK modulation with digital filtering to increase 20 times of direct modulation speed of white-light LED visible light communication system

In this demonstration, we propose and experimentally investigate the quaternary-amplitude-shift-keying (4-ASK) modulation with digital filtering to enhance the direct modulation speed of white-light light-emitting-diode (LED) in visible light communication (VLC) system. Here, an ordinary LED commercially available for lighting application with a direct modulation speed of 1 MHz is used. Data rate of 20 Mbit/s can be achieved in a 1 m free space transmission without using optical blue filter. In the previous studies, the transmission rate of LED VLC could only be increased by 2 to 10 times of the direct modulation speed of the white-light LED if using electrical equalization only. Moreover, the adaptive-controlled FIR filter makes the system closer to the matched filtering condition for reducing the inter-symbol-interference (ISI) for the LED VLC. A recorded 20 times enhancement of the direct modulation speed of white-light LED VLC system is demonstrated by using digital filter only and without using optical blue filter.

Mitigation of Optical Background Noise in Light-Emitting Diode (LED) Optical Wireless Communication Systems

One challenge faced by the in-home light-emitting diode (LED) optical wireless communication is the optical noises. Here, we first experimentally characterize the effect of optical background noise to the performance of the LED optical wireless communication channel. We demonstrate using Manchester coding for the LED to mitigate the optical noise. No adaptive monitoring, feedback, or optical filtering is required. The theoretical and numerical analysis of Manchester decoding process to mitigate the optical background noise is provided. Our experimental result shows that Manchester coding can significantly eliminate optical noise generated by the AC-LED operated at <; 500 kHz and fluorescent light.

Demonstration of bi-directional LED visible light communication using TDD traffic with mitigation of reflection interference

In this work, we experimentally demonstrate a bi-directional transmission link using light emitting diode (LED) visible light communication (VLC) for both downlink and uplink paths. Time-division-duplex (TDD) is proposed and demonstrated to significantly eliminate the reflection interference in VLC. A free space bi-directional transmission of 2 m using simple on-off keying (OOK) modulation with bit error rate (BER) of < 10(-5) in both directions are achieved. Moreover, the influence of reflection interference is analyzed, showing the proposed scheme can significantly eliminate the reflection interference.

Background Optical Noises Circumvention in LED Optical Wireless Systems Using OFDM

Background optical noises generated by the conventional fluorescent light sources or AC-power light-emitting diode (LED) can significantly affect the performance of the LED optical wireless communication and produce challenges to implementation. We demonstrate using orthogonal frequency division multiplexing (OFDM) to effectively circumvent the optical background noises. Besides, by using simple equalization at the receiver side, the transmission capacity can be extended from 1 Mb/s to 12 Mb/s. The theory and analysis of the equalization are presented. Experiments at different data rates and different OFDM subcarriers are preformed, and results show that, by adjusting the number of OFDM subcarriers, the influence of the background optical noises can be significantly circumvented.

Equalization and pre-distorted schemes for increasing data rate in in-door visible light communication system

We propose and demonstrate using signal pre-distortion and equalization schemes for increasing the data rate of the white-light LED communication system. Bit-error-rate of &#60;10−10 at 10-Mb/s operation was achieve using 1-MHz bandwidth white-light LED.

Alternating-Signal-Biased System Design and Demonstration for Visible Light Communication

In the literature, previous system designs are mainly focused on the use of direct-current light-emitting diodes (DC-LEDs) with fixed voltage bias. As alternating-current (AC)-LEDs are more energy efficient and becoming more and more popular, a driving system supporting AC-LEDs providing lighting and visible light communication (VLC) functions is highly desirable. This paper investigates the alternating-signal-biased system design for VLC, which combines the AC bias voltage signal and electrical data signal to modulate both DC- and AC-LEDs. In the experiment, a 40-LED array is modulated by our proposed system that makes use of current from AC power source while maintaining nondistorted message signal transmission. On-off-keying and quadrature-phase-shift-keying formats are used, and data rates of 60 and 120 kb/s are demonstrated, respectively. The threshold voltage effect and the system extension to support AC-LED are discussed. The alternating-signal-biased system design supporting both DC- and AC-LEDs is also discussed.

Using dual-mode self-locked semiconductor laser for optical millimeter-wave application

In this study, an optical millimeter-wave (mm-wave) generator is proposed and experimentally demonstrated by using a self-injected Fabry-Perot laser diode (FP-LD), having mode spacing of 1.11 nm, for dual-mode beating in 140 GHz band (terahertz band). The created dual-wavelength also can be also modulated at 1.25, 2.5, and 10 Gb/s with on-off keying (OOK) modulation format by external optical modulator, respectively, in 20 km fiber transmission. Moreover, the dual-mode laser can be selected in difference wave-lengths by tuning the optical filter inside cavity for the future WDM applications.

Optimized OFDM modulation format for white-light LED optical wireless communication with pre-equalization

We demonstrate a simple orthogonal-frequency-division-multiplexing (OFDM) light-emitting-diode (LED) optical wireless communication system using power pre-equalization to enhance the transmission performance. Error free operation using 5Mbps at a distance of 2m is achieved.

Time division duplex for preventing reflection interference in visible light communication

In this work, an experimental time division duplex system is presented to prevent interference in visible light communication. The 40-LED array and a single LED were used for downlink and uplink respectively. The interference of signal reflection on mirrors was discussed and removed using time division duplex. At the distance of 2 m, the data rate achieved is above 1Mbps with bit error rate below 2×10-5 in each of the direction.

QPSK modulation for AC-power-signal-biased visible light communication system

With the integration of light emitting diode (LED), visible light communication (VLC) can provide wireless communication link using the lightning system. Due to the consideration of power efficiency, AC-LED has the design of reducing energy waste with alternating current from the power outlet. In this work, we propose an AC-power-signalbiased system that provides communication on both DC-LED and AC-LED. The bias circuit is designed to combine ACpower signal and the message signal with QPSK format. This driving scheme needs no AC-to-DC converters and it is suitable for driving AC LED. Synchronization is completed to avoid threshold effect of LED.