Light emitting diodes (LEDs) based visible light communication (VLC) has been considered as a promising technology for indoor high-speed wireless access, due to its unique advantages, such as low cost, license free and high security. To achieve high-speed VLC transmission, carrierless amplitude and phase (CAP) modulation has been utilized for its lower complexity and high spectral efficiency. Moreover, to compensate the linear and nonlinear distortions such as frequency attenuation, sampling time offset, LED nonlinearity etc., series of pre- and post-equalization schemes should be employed in high-speed VLC systems. In this paper, we make an investigation on several advanced pre- and postequalization schemes for high-order CAP modulation based VLC systems. We propose to use a weighted preequalization technique to compensate the LED frequency attenuation. In post-equalization, a hybrid post equalizer is proposed, which consists of a linear equalizer, a Volterra series based nonlinear equalizer, and a decision-directed least mean square (DD-LMS) equalizer. Modified cascaded multi-modulus algorithm (M-CMMA) is employed to update the weights of the linear and the nonlinear equalizer, while DD-LMS can further improve the performance after the preconvergence. Based on high-order CAP modulation and these equalization schemes, we have experimentally demonstrated a 1.35-Gb/s, a 4.5-Gb/s and a 8-Gb/s high-speed indoor VLC transmission systems. The results show the benefit and feasibility of the proposed equalization schemes for high-speed VLC systems.
Visible light communication (VLC) has no doubt become a promising candidate for future wireless communications due to the increasing trends in the usage of light-emitting diodes (LEDs). In addition to indoor high-speed wireless access and positioning applications, VLC usage in outdoor scenarios, such as vehicle networks and intelligent transportation systems, are also attracting significant interest. However, the complex outdoor environment and ambient noise are the key challenges for long-range high-speed VLC outdoor applications. To improve system performance and transmission distance, we propose to use receiver diversity technology in an outdoor VLC system. Maximal ratio combining–based receiver diversity technology is utilized in two receivers to achieve the maximal signal-to-noise ratio. A 400-Mb/s VLC transmission using a phosphor-based white LED and a 1-Gb/s wavelength division multiplexing VLC transmission using a red–green–blue LED are both successfully achieved over a 100-m outdoor distance with the bit error rate below the 7% forward error correction limit of 3.8×10−3. To the best of our knowledge, this is the highest data rate at 100-m outdoor VLC transmission ever achieved. The experimental results clearly prove the benefit and feasibility of receiver diversity technology for long-range high-speed outdoor VLC systems.
We propose and experimentally demonstrate a full-duplex high-speed visible light communication (VLC) access network based on star topology architecture to offer high-speed optical wireless access for a large number of users. Optical fiber is used as the backbone of the VLC network and directly connected to the light-emitting diode lamps. Frequency division multiplexing (FDM) is utilized for both the downlink and uplink. The bidirectional transmission of 32 quadrature amplitude modulation orthogonal FDM signals at an overall throughput of 4 Gb/s is successfully achieved to support four users access, and each user is offered 500 Mb/s downstream and 500 Mb/s upstream. The measured bit error rates of the downlink and uplink for all four users are <7% pre-forward error correction limit of 3.8×10−3 after a 25 km standard single-mode fiber and 65 cm free space, which clearly validates the promising potential of the proposed VLC network architecture to offer more than 10 Gb/s wireless access.
KEYWORDS: Optical filters, Polarization, Phase shift keying, Digital signal processing, Electronic filtering, Signal to noise ratio, Signal processing, Receivers, Signal detection, Tolerancing
Quadrature duobinary (QDB) spectrum shaping polarization division multiplexed-quadrature phase shift keying (PDM-QPSK) signal with Nyquist and super-Nyquist spectrum efficiency will be a promising candidate for future ultrahigh speed ultrahigh spectrum efficiency coherent optical fiber transmissions systems. Several equalization algorithms including constant modulus algorithm (CMA), CMA plus postfilter, and cascaded multimodulus algorithm (CMMA) have been proposed as effective solutions for QDB-PDM-QPSK signal. For the first time as far as we know, the application conditions and performances for these three algorithms are analyzed and compared. System performances for a 112-Gb/s QDB-PDM-QPSK signal as a function of the optical filtering bandwidth and the optical SNR have been theoretically investigated. The results show that CMA would be the best choice in terms of convergence rate for general filtering. However, CMMA can outperform the other two schemes with a good receiver sensitivity and high-dynamic range of optical signal to noise ratio giving a strong filtering effect of super-Nyquist signaling.
Visible Light Communication (VLC) has received more and more attention in recent years. In this paper, we propose a
novel configuration of unidirectional subcarrier multiplexing (SCM) -wavelength division multiplexing (WDM) -VLC
system adopting quadrature amplitude modulation (QAM) orthogonal frequency division multiplexing (OFDM)
modulation. Based on this configuration, we successfully demonstrate a 3x1 multiple-input single-output (MISO)
OFDM-VLC transmission experiment using three blue phosphor LEDs. The transmission distance can be as long as
140cm. We also demonstrate an 800-Mb/s MISO SCM-WDM VLC transmission experiment based on RGB-LED with
66-cm transmission distance. In the two experiments, pre-equalization and post-equalization both are implemented to
compensate for the poor frequency response of LEDs. And the properties of EVM and BER in all channels are measured
and analysed.
A simple and cost-effective technique to generate coherent optical multi-carriers by using cascaded phase modulators is
proposed and investigated. A detailed theoretical analysis is presented. The phase and amplitude relation of RF signals on
two concatenated phase modulators are investigated. Simulation results are in good agreement with the theoretical analysis,
and the experimental performance of 22 subcarriers with amplitude difference of 5dB shows that this scheme is a
promising technique for the coming Tb/s optical communication.
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