Low complexity OFDM VLC system enabled by spatial summing modulation.

Commercial-off-the-shelf (COTS) devices enabled visible light communication (VLC) for Internet of things (IoT) applications has attracted extensive attentions from both academic and industrial communities, thanks to the pervasive deployments of light emitting diode (LED) lighting infrastructure. However, due to the limitation of frequency response and non-linearity of the commercial illuminating LED light consisting of multiple LED chips, the achievable data rate is far less than that provided by the experimental VLC system with a single LED with specialized devices, e.g., lens. To this end, we propose a power-of-2 arrangement scheme for LED chips to generate spatial summing modulation with low control complexity, and demonstrate its availability in an orthogonal frequency division multiplexing (OFDM) VLC system purely built upon COTS devices. It significantly differs from a conventional OFDM VLC system relying on digital-to-analog converter (DAC) and analog signal chain, which is complex and confined by LED's non-linearity, thanks to we design a novel digital-to-light converter (DLC) which can output 256 light intensities linearly and be directly controlled by the discrete digital signals generated by the OFDM modulator. An experimental demonstration with employing the QAM-OFDM modulation scheme successfully confirms the effectiveness of the proposed spatial summing VLC system, which can achieve low BERs of below the forward error correct (FEC) threshold of 3.8×10-3 for both QAM8 and QAM16 running transmission frequency of 300 kHz under a communication distance of 0.8 m. It demonstrates the promising potential for delivering a data rate at hundred kbps level with this novel spatial summing based OFDM VLC system, which is sufficient for many IoT applications.

Research on the inhibitory effect of Baicalin on the human tongue squamous cell carcinoma cell line SCC15 / 口腔疾病防治

Objective@#To explore the inhibitory effect and possible mechanism of Baicalin on the human tongue squamous cell carcinoma cell line SCC15 and to provide a new idea and experimental basis for the clinical prevention and treatment of tongue squamous cell carcinoma.@*Methods @#SCC15 cells cultured in DMEM alone were used as the control group, and SCC15 cells cultured in 20 mg/mL baicalin solution were used as the baicalin group. Scratch tests and Transwell migration tests were performed to detect changes in cell migration ability, and flow cytometry was used to detect changes in the cell cycle. Western blotting was used to detect differences in the phosphorylation levels of signal transduction and transcription activator 3 (STAT3).@*Results @# Compared with the control group, the scratch test and the Transwell migration test showed that the cell migration ability of cells in the baicalin group was significantly decreased (t=4.927, P=0.008); flow cytometry showed that the number of cells of the baicalin group increased in the G0/G1 phase (t=9.893, P=0.001), decreased in the S phase (t=8.528, P=0.001), and decreased in the G2/M phase (t=3.550, P=0.024); Western blotting results showed that the STAT3 protein of SCC15 cells in the baicalin group decreased (t=3.550, P=0.024), and the phosphorylation level significantly decreased (t=8.262, P=0.001).@*Conclusion @#Baicalin inhibits the human tongue squamous cell carcinoma cell line SCC15, and its mechanism may be related to a decrease in STAT3 pathway phosphorylation activity.

Column generation algorithms for virtual network embedding in flexi-grid optical networks.

Network virtualization provides means for efficient management of network resources by embedding multiple virtual networks (VNs) to share efficiently the same substrate network. Such virtual network embedding (VNE) gives rise to a challenging problem of how to optimize resource allocation to VNs and to guarantee their performance requirements. In this paper, we provide VNE algorithms for efficient management of flexi-grid optical networks. We provide an exact algorithm aiming to minimize the total embedding cost in terms of spectrum cost and computation cost for a single VN request. Then, to achieve scalability, we also develop a heuristic algorithm for the same problem. We apply these two algorithms for a dynamic traffic scenario where many VN requests arrive one-by-one. We first demonstrate by simulations for the case of a six-node network that the heuristic algorithm obtains very close blocking probabilities to exact algorithm (about 0.2% higher). Then, for a network of realistic size (namely, USnet) we demonstrate that the blocking probability of our new heuristic algorithm is about one magnitude lower than a simpler heuristic algorithm, which was a component of an earlier published algorithm.

Secure and private NOMA VLC using OFDM with two-level chaotic encryption.

In this paper, we propose and demonstrate a secure and private non-orthogonal multiple access (NOMA) based visible light communication (VLC) system. Orthogonal frequency division multiplexing (OFDM) modulation is applied in the system and a two-level chaotic encryption scheme is further implemented, which can guarantee both the security of legitimate users against eavesdroppers and the privacy among all the legitimate users. An experimental demonstration with two legitimate users and one eavesdropper successfully verifies the feasibility of the proposed secure and private NOMA VLC system. To the best of our knowledge, it is the first time that simultaneous security and privacy improvement is considered for NOMA VLC systems.

Improving performance of channel equalization in RSOA-based WDM-PON by QR decomposition.

In reflective semiconductor optical amplifier (RSOA)-based wavelength division multiplexed passive optical network (WDM-PON), the bit rate is limited by low modulation bandwidth of RSOAs. To overcome the limitation, we apply QR decomposition in channel equalizer (QR-CE) to achieve successive interference cancellation (SIC) for discrete Fourier transform spreading orthogonal frequency division multiplexing (DFT-S OFDM) signal. Using an RSOA with a 3-dB modulation bandwidth of only ~800 MHz, we experimentally demonstrate a 15.5-Gb/s over 20-km SSMF DFT-S OFDM transmission with QR-CE. The experimental results show that DFTS-OFDM with QR-CE attains much better BER performance than DFTS-OFDM and OFDM with conventional channel equalizers. The impacts of several parameters on QR-CE are investigated. It is found that 2 sub-bands in one OFDM symbol and 1 pilot in each sub-band are sufficient to achieve optimal performance and maintain the high spectral efficiency.

Investigation of PMD in direct-detection optical OFDM with zero padding.

We investigate the polarization-mode dispersion (PMD) effect of zero padding OFDM (ZP-OFDM) in direct-detection optical orthogonal frequency division multiplexing (DDO-OFDM) systems. We first study the conventional equalization method for ZP-OFDM. Then an equalization method based on sorted QR decomposition is proposed to further improve the performance. It is found that the performance improvement of ZP-OFDM is due to the frequency domain oversampling (FDO) induced inter-carrier interference (ICI). Numerical simulation results show that compared with cyclic prefix OFDM (CP-OFDM), ZP-OFDM has a significantly higher tolerance to PMD in DDO-OFDM systems when the channel spectral nulls occur at certain differential group delay (DGD) values.

Performance of dimming control scheme in visible light communication system.

We investigate the performance of visible light communication (VLC) system with a pulse width modulation (PWM) dimming control scheme. Under this scheme, the communication quality in terms of number of transmitted bits and bit error rate (BER) of less than 10(-3) should be guaranteed. However, for on-off-keying (OOK) signal, the required data rate becomes 10 times as high as the original data rate when the duty cycle of dimming control signal is 0.1. To make the dimming control scheme easy to be implemented in VLC system, we propose the variable M-QAM OFDM VLC system, where M is adjusted according to the brightness of LED light in terms of duty cycle. The results show that with different duty cycles the required data rates are not higher than the original value and less LED lamp power is required to guarantee the communication quality, which makes the dimming control system that satisfies both communication and illumination requirements easy to be implemented and power-saving.

Performance improvement of on-off-keying free­space optical transmission systems by a co­propagating reference continuous wave light.

In this paper, we analytically investigate an optical signal detection scheme to mitigate the scintillation effect with the assistance of a co-propagating reference continuous wave (CW) light. Using the correlation coefficient between the intensities of the data light and the reference CW light, we mathematically derive their joint intensity distributions under two widely used atmospheric turbulence channel models, namely log-normal distributed channel model and Gamma-Gamma distributed channel model, respectively. We also carry out the Monte-Carlo (MC) simulation and show that theoretical results agree with simulation results well. Our analytical results reveal that when the correlation coefficient is 0.99, the power reductions to achieve BER of 10⁻³ are 12.3 dB and 20.4 dB under moderate and strong atmospheric turbulence conditions (i.e., Rytov variances of 1.0 and 4.0), respectively. In addition, the feasibility of the scheme applied to wavelength-division-multiplexed (WDM) free-space-optical (FSO) transmission systems is also investigated, where only a single reference CW light could be used to mitigate the scintillation effects on all WDM channels.

Performance of a novel LED lamp arrangement to reduce SNR fluctuation for multi-user visible light communication systems.

This paper investigates the performance of our recently proposed LED lamp arrangement to reduce the SNR fluctuation from different locations in the room for multi-user visible light communications. The LED lamp arrangement consists of 4 LED lamps positioned in the corners and 12 LED lamps spread evenly on a circle. Our studies show that the SNR fluctuation under such a LED lamp arrangement is reduced from 14.5 dB to 0.9 dB, which guarantees that users can obtain almost identical communication quality, regardless of their locations. After time domain zero-forcing (ZF) equalization, the BER performances and channel capacities of 100-Mbit/s and 200-Mbit/s bipolar on-off-keying (OOK) signal with most significant inter-symbol interference (ISI) are very close to that of the channel without any ISI caused by this LED lamp arrangement.

An efficient energy-saving scheme incorporating dozing and sleep modes for CLS-based WDM-PONs with colorless ONUs.

We propose and demonstrate an efficient energy-saving scheme incorporating dozing and sleep modes for WDM-PONs with centralized light sources (CLS). The novel scheme is based on simple power detection and local transmission request. Two logic control units are designed to switch the operation modes of the respective ONUs and their associated transceivers in the OLT. The scheme feasibility is experimentally verified with 10 Gbit/s downstream and 1.25 Gbit/s upstream transmissions. The simulation results reveal that the energy-saving of the ONUs in the online state mainly arises from the dozing mode, not from the sleep mode, while the energy-saving of the associated transceivers in the OLT is contributed mainly from the situation where ONUs are in the offline state.

Performance improvement by tilting receiver plane in M-QAM OFDM visible light communications.

We propose a scheme to improve the SNR distribution as well as the spectral efficiency of M-QAM OFDM signal for indoor visible light communication by tilting the receiver plane. Newton method is employed for the photo-detector to receive maximum power by finding the optimal tilting angle. This method is a fast algorithm that only three searching steps are needed. The simulation results show that in the case of one LED source, the maximum spectral efficiency improvement is 0.44bit/s/Hz when the launching power of LED source is 12W; while in the case of four LED sources, the maximum spectral efficiency improvement is 0.21bit/s/Hz when the total launching power of the four LED sources is 12W.

Multipoint free-space optics system for short-range communications between flight platforms.

The problems associated with using typical free-space optics (FSO) transceivers for multipoint communications on flight platforms, such as satellites and unmanned aerial vehicles, in a cluster are discussed and the transceivers are found to be unsuitable for concurrent multipoint communications in those platforms. To overcome the problems, a multipoint FSO system design is proposed. The design of the multipoint system is described and the performance of the multipoint receiver is evaluated in terms of receive gain under different conditions. The proposed multipoint system design offers a promising way to implement concurrent multipoint communications on flight platforms with power, weight, and size advantages compared to the use of multiple transceivers.

Beam divergence changing mechanism for short-range inter-unmanned aerial vehicle optical communications.

The problems associated with using a single fixed beam divergence for short-range inter-unmanned aerial vehicle free-space optical communications are discussed. To overcome the problems, a beam divergence changing mechanism is proposed. Four different methods are then proposed to implement the beam divergence changing mechanism. The performance of these methods is evaluated in terms of transmission distance under adverse weather conditions. The results show that the performance is greatly improved when the beam divergence changing mechanism is used.

Fiber-optic parametric amplifier and oscillator based on intracavity parametric pump technique.

A cost-effective fiber optical parametric amplifier (FOPA) based on the laser intracavity pump technique has been proposed and demonstrated experimentally. The parametric process is realized by inserting a 1 km highly nonlinear dispersion-shifted fiber (HNL-DSF) into a fiber ring-laser cavity that consists of a high-power erbium-doped fiber (EDF) amplifier and two highly reflective fiber Bragg gratings. Compared with the conventional parametric pump schemes, the proposed pumping technique is free from a tunable semiconductor laser as the pump source and also the pump phase modulation. When the oscillating power of 530 mW in the EDF laser cavity is achieved to pump the HNL-DSF, a peak parametric gain of 27.5 dB and a net gain over 45 nm are obtained. Moreover, a widely tunable fiber-optic parametric oscillator is further developed using the FOPA as a gain medium.

Stable and spacing-adjustable multiwavelength Raman fiber laser based on mixed-cascaded phosphosilicate fiber Raman linear cavity.

A novel multiwavelength Raman fiber laser based on the mixed-cascaded Stokes effects of phosphosilicate fiber is proposed and demonstrated experimentally. By using stimulated Raman scattering of both P(2)O(5) and SiO(2) along 1 km phosphosilicate fiber pumped with a 1064 nm double-clad fiber laser, the mixed-cascaded Raman linear cavity is formed by a pair of fiber Bragg gratings at 1239 nm, a polarization-maintaining fiber (PMF) Sagnac loop filter, and a conventional optical loop mirror. Up to 15-wavelength stable oscillations around 1320 nm are obtained with a wavelength spacing of 0.44 nm and power nonuniformity of less than 4 dB. By changing the length of the PMF in the Sagnac loop filter from 10 to 5.5 m, the wavelength spacing is adjustable from 0.44 to 0.8 nm. The extinction ratio of the laser is more than 30 dB. Excellent stability is also observed with a peak power fluctuation of less than 0.8 dB in 1 h.

New transmitter configuration for subcarrier multiplexed DPSK systems and its applications to chromatic dispersion monitoring.

We present a new transmitter configuration using a single dual-electrode Mach-Zehnder Modulator (DE-MZM) for generating DPSK signal and subcarrier simultaneously. We then apply this new configuration to monitoring chromatic dispersion (CD) in DPSK systems by detecting the RF power at both of the clock and subcarrier frequencies. Our investigations show that the method can greatly improve CD monitoring capability. The impact of subcarrier on the DPSK performance is also investigated. Results show that by simply selecting the subcarrier close to half of the DPSK data rate frequency, we can significantly suppress the subcarrier induced power penalty to DPSK signal.

High-speed WDM-PON using CW injection-locked Fabry-Pérot laser diodes.

We propose a new WDM-PON architecture using Fabry-Pérot laser diodes (FP-LDs) that are injection-locked by continuous wave (CW) seed light. The modulation characteristics of the CW light injection-locked FP-LD are first investigated. Both uplink and downlink transmissions at 10 Gb/s are experimentally demonstrated using the proposed CW injection-locked FP-LDs. It is shown that up to 16 laser cavity modes can be selectively injection-locked with side mode suppression ratio larger than 30dB. The effects of the location of FP-LD cavity modes, transmission distance, and injection wavelength detuning on the overall transmission performance are investigated. The possibility of eliminating polarization dependence of the proposed CW injection scheme is also experimentally demonstrated by properly configuring a depolarizer. The deployment cost for the proposed WDM PON is potentially low from the fact that the CW laser sources located at the central office can be shared by many WDM-PONs and low-cost FP-LDs are used as light sources for data rates as high as 10 Gb/s.

New bit-error-rate monitoring technique based on histograms and curve fitting.

We propose a new method for estimating the bit error rate (BER) of optical channels suffering from crosstalk and waveform distortion. We demonstrate experimentally that the new method can provide a good BER estimation with an error range less than one order of magnitude.

High-birefringence linearly chirped grating based optical device for PMD compensation.

We propose a novel delay element for polarization mode dispersion (PMD) compensation by employing a tunable high-birefringence linearly chirped grating. The device can adjust differential group delay in a linearly continuous way and its performance is demonstrated by compensating 10-Gb/s signal with the first-order PMD. The tradeoff between PMD compensation capability of the device and the power penalty caused by the chromatic dispersion of the grating has also been studied.