Imperceptible Flicker Noise Reduction Using Pseudo-Flicker Weight Functionalized Derivative Equalization in Light-Fidelity Transmission Link.

A new technique to reduce flicker noise generated in light-fidelity (Li-Fi) transmission links based on the white light-emitting diode (LED) is proposed. Here, flicker noise with a frequency of 120 Hz, which is twice the frequency of AC power (60 Hz), is generated. The proposed technique is implemented in the receiver of the Li-Fi link. It can reduce flicker noise regardless of various digital modulation formats. In addition, there is no need to change the structure of the electrical circuit driving the LED to reduce the flicker noise. As a result, the non-return to-zero-on-off-keying (NRZ-OOK) signal waveform is tilted according to the flicker noise waveform. We implement the derivative equalization with a pseudo-flicker weight function to reduce the flicker noise. The derivative value of the NRZ-OOK signal mixed with flicker noise becomes larger than that without the flicker noise. In the proposed technique, the derivative value between adjacent sampling points is suppressed below the preset thresholds when it is greater than the preset threshold. Furthermore, a pseudo-flicker weight function is applied to accelerate the flicker noise reduction. As a result, using the proposed technique, a 2 dB signal-to-noise ratio (SNR) gain is obtained based on the bit error rate (BER) threshold (3.5 × 10-5) corresponding to 10% flicker modulation, which is known to have no serious effect on human health. This means that it is possible to implement a Li-Fi transmission link based on an illumination environment with a flicker modulation reduced from 10% to 7%.

Computational Large Field-of-View RGB-D Integral Imaging System.

The integral imaging system has received considerable research attention because it can be applied to real-time three-dimensional image displays with a continuous view angle without supplementary devices. Most previous approaches place a physical micro-lens array in front of the image, where each lens looks different depending on the viewing angle. A computational integral imaging system with a virtual micro-lens arrays has been proposed in order to provide flexibility for users to change micro-lens arrays and focal length while reducing distortions due to physical mismatches with the lens arrays. However, computational integral imaging methods only represent part of the whole image because the size of virtual lens arrays is much smaller than the given large-scale images when dealing with large-scale images. As a result, the previous approaches produce sub-aperture images with a small field of view and need additional devices for depth information to apply to integral imaging pickup systems. In this paper, we present a single image-based computational RGB-D integral imaging pickup system for a large field of view in real time. The proposed system comprises three steps: deep learning-based automatic depth map estimation from an RGB input image without the help of an additional device, a hierarchical integral imaging system for a large field of view in real time, and post-processing for optimized visualization of the failed pickup area using an inpainting method. Quantitative and qualitative experimental results verify the proposed approach's robustness.

Ambient LED Light Noise Reduction Using Adaptive Differential Equalization in Li-Fi Wireless Link.

For Li-Fi wireless links based on a white light emitting diode, an adaptive differential equalization (ADE) technique that reduces various noises such as interference noise and shot one generated from ambient light sources is pro-posed. The ADE technique reduces noise by taking advantage of the fact that the derivative between adjacent sampling points of signal with digital waveform is very different from that of noise with the random analog waveform. Furthermore, a weighting function that reflects the Poisson characteristics of shot noise is applied to the ADE technique in order to maximize the reduction efficiency of ambient noise. The signal-to-noise ratio of input non-return-to-zero-on-off keying (NRZ-OOK) signal is improved by 7.5 dB at the first-generation forward error correction (FEC) threshold (the bit error rate (BER) of 8 × 10-5) using the optical wireless experimental link. In addition, it is confirmed that it is possible to maintain the transmission performance corresponding to the BER of 1 × 10-5 by using the proposed ADE technique, even when the intensity of the ambient light source increases by 6 dB.

Compressive sensing-based channel bandwidth improvement in optical wireless orthogonal frequency division multiplexing link using visible light emitting diode.

A new technique, which can compensate for the lack of channel bandwidth in an optical wireless orthogonal frequency division multiplexing (OFDM) link based on a light emitting diode (LED), is proposed. It uses an adaptive sampling and an inverse discrete cosine transform in order to convert an OFDM signal into a sparse waveform so that not only is the important data obtained efficiently but the redundancy one is removed. In compressive sensing (CS), a sparse signal that is sampled below the Nyquist/Shannon limit can be reconstructed successively with enough measurement. This means that the CS technique can increase the data rate of visible light communication (VLC) systems based on LEDs. It is observed that the data rate of the proposed CS-based VLC-OFDM link can be made 1.7 times greater than a conventional VLC-OFDM link (from 30.72 Mb/s to 51.2 Mb/s). We see that the error vector magnitude (EVM) of the quadrature phase shift keying (QPSK) symbol is 31% (FEC limit: EVM of 32%) at a compression ratio of 40%.

Colorless WDM-PON based on a Fabry-Pérot laser diode and reflective semiconductor optical amplifiers for simultaneous transmission of bidirectional gigabit baseband signals and broadcasting signal.

A novel WDM-PON system delivering bidirectional baseband data and broadcasting data is proposed and demonstrated. A subcarrier multiplexing signal is broadcasted to all users by modulating a broadband optical source based on a Fabry-Pérot laser diode. Reflective semiconductor optical amplifiers are used as colorless modulators for the baseband data at both optical line terminal and remote optical network units. Transmission performance including bit error rate of bidirectional gigabit data and error vector magnitude of broadcasting data of many optical channels is investigated. Additionally, the data rate for the broadcasting signal was improved by using an external modulator.

Gigabit radio-over-fiber link for converged baseband and millimeter-wave band signal transmission using cascaded injection-locked Fabry-Pérot laser diodes.

A novel scheme, for both baseband and millimeter-wave band gigabit data transmission in radio-over-fiber system, is proposed and experimentally demonstrated by using cascaded injection-locked Fabry- Pérot laser diodes. It was able to improve suppression ratio of carrier suppressed signal using the cascaded injection-locking. The suppression ratio improvement of the optical carrier suppressed signal of 20 dB was verified. Applying this mechanism, 60-GHz millimeter-wave carrier of enhanced signal quality could be accomplished. Its peak power and phase noise were obtained as -40 dBm and -103.5 dBm/Hz respectively, which was suitable for 60-GHz data transmission. In addition, a successful bidirectional transmission of 1.25-Gbps wired and wireless data was achieved by adopting remodulation technique using a gain-saturated reflective semiconductor optical amplifier for uplink.