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Christopher Long
Christopher Long

Visible Light Communications: Modulation And Si...



High-speed visible light communication (VLC), as a cutting-edge supplementary solution in 6G to traditional radio-frequency communication, is expected to address the tension between continuously increased demand of capacity and currently limited supply of radio-frequency spectrum resource. The main driver behind the high-speed VLC is the presence of light emitting diode (LED) which not only offers energy-efficient lighting, but also provides a cost-efficient alternative to the VLC transmitter with superior modulation potential. Particularly, the InGaN/GaN LED grown on Si substrate is a promising VLC transmitter to simultaneously realize effective communication and illumination by virtue of beyond 10-Gbps communication capacity and Watt-level output optical power. In previous parameter optimization of Si-substrate LED, the superlattice interlayer (SL), especially its period number, is reported to be the key factor to improve the lighting performance by enhancing the wall-plug efficiency, but few efforts were made to investigate the influence of SLs on VLC performance. Therefore, to optimize the VLC performance of Si-substrate LEDs, we for the first time investigated the impact of the SL period number on VLC system through experiments and theoretical derivation. The results show that more SL period number is related to higher signal-to-noise ratio (SNR) via improving the wall-plug efficiency. In addition, by using Levin-Campello bit and power loading technology, we achieved a record-breaking data rate of 3.37 Gbps over 1.2-m free-space VLC link under given optimal SL period number, which, to the best of our knowledge, is the highest data rate for a Si-substrate LED-based VLC system.




Visible Light Communications: Modulation and Si...


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The principle of BPL-DMT modulation is shown in the upper flow diagram of Fig. 2a). First, system SNR should be measured to guide the optimal bit allocation. To maximize the measured SNR, the optical power of the light onto the PIN-PD needs to approach the threshold optical power of PIN-PD to get the maximum PD response, and thus the attenuation value of the ND filter needs adaptive adjustment. The training signal for SNR measurement is a DMT-modulated quadrature phase shift keying (QPSK) signal generated according to the standard DMT modulation process [25]. After it is transmitted over the VLC channel, the received signal from OSC is immediately demodulated into a QPSK signal referred to standard DMT demodulation process, which is subsequently equalized by classical zero-forcing equalization and intra-symbol frequency-domain averaging (ISFA). Next, the SNR could be estimated by calculating the error vector magnitude between the transmitted QPSK and the received QPSK signal [26]. The forward error code (FEC) threshold uses the 7% overhead FEC threshold with the bit error ratio (BER) of 3.8e-3. The ISFA technology is applied to smooth the estimated SNR response and suppress noise.


Abstract:In the nearest decades, the rapidly increasing demand of wireless connectivity has resulted in the ubiquitous deployment of wireless systems as well as heavily congested wireless spectrum. Owing to the various inherent advantages, such as spectral and bandwidth relief, no healthy concern, high security, low cost, and low interference with Radio Frequency (RF) waves, visible light communication (VLC) has been an emerging optical wireless data transmission approach that can act as a good complement to and substitute for Radio Frequency. How to achieve a high-speed data transmission is a key problem to be solved in the VLC system. This review mainly focuses on the enabling technologies for high-speed VLC systems, including novel transmitter architectures, blue filters and advanced modulation, and equalization technologies. And the inherent advantages, potential applications, and some issues of VLC that need further study are presented as well. Finally, a comprehensive survey on the recent developments and the key contributions by research groups involved in the field of high-speed VLC is provided.Keywords: visible light communications; high-speed; LED; blue filter; modulation; equalization


The broad transparency spectra of most ENZ materials, from ultraviolet (UV) to far-infrared (FIR), have been experimentally proven, which makes them promising for applications like visible light communications (VLC) and IR telecommunication windows of 1.3, 1.55, and 2 μm17,18,19,20.


As mentioned before, it is difficult to further improve the modulation speed in electro-optical modulators due to the functional response time limitations of the electrical section55,56,57. Nevertheless, all-optical modulation can overcome the modulation rate limitations by using one light beam to control the transmission/reflection of another light beam30,58. Several papers have reported all-optical modulation schemes with high speeds of 200 GHz based on graphene devices, in which a graphene sheet covers each structure. As a result of the required low insertion loss, these schemes have relatively low modulation depth or modulation efficiency59,60,61.


Variable pulse position modulation (VPPM) is a new modulation scheme that supports simultaneously illumination with dimming control and communication. In this paper, the transmitter, optical wireless channel, and receiver structure of VPPM were modeled mathematically, and their error performance for examining the communication performance according to dimming level and data rate was analyzed. The results highlight the need for careful consideration of the tradeoff between the dimming flexibility and communication performance according to the channel condition in VPPM.


In the IEEE standard, the variable pulse position modulation (VPPM) scheme was proposed and adopted as one of the modulation schemes for VLC. To support illumination with dimming control and communication simultaneously, this scheme uses binary PPM for communication and the pulse width for dimming control. Figure 1 shows an operating example of the VPPM. In Figure 1, the dimming and communication functions of the VPPM appear to operate independently. Although the VPPM concept is well described in the standard, no attempt has been made to model the structure of VPPM and analyze its ability to examine the performance tradeoff between the dimming and communication functions according to the channel condition. More efficient communication and lighting parameters for VPPM can be realized if its communication performance can analyzed according to the dimming level and channel environment.


I'm an associate professor in NTUEE. My research focuses on high speed VCSELs for optical interconnects, GaN-on-Si power and rf electronics, 2D material field-effect transistors, Si photonics, transistor lasers for optical logic gates, and GaN LED for visible light communications.


Li-Fi is a light communication system that is capable of transmitting data at high speeds over the visible light, ultraviolet, and infrared spectrums. In its present state, only LED lamps can be used for the transmission of data in visible light.[2]


Li-Fi can potentially be useful in electromagnetic sensitive areas without causing electromagnetic interference.[10][14][11] Both Wi-Fi and Li-Fi transmit data over the electromagnetic spectrum, but whereas Wi-Fi utilizes radio waves, Li-Fi uses visible, ultraviolet, and infrared light. [15] Researchers have reached data rates of over 224 Gbit/s,[16] which was much faster than typical fast broadband in 2013.[17][18] Li-Fi is expected to be ten times cheaper than Wi-Fi.[19] The first commercially available Li-Fi system was presented at the 2014 Mobile World Congress in Barcelona.


Although Li-Fi LEDs would have to be kept on to transmit data, they could be dimmed to below human visibility while still emitting enough light to carry data.[19] This is also a major bottleneck of the technology when based on the visible spectrum, as it is restricted to the illumination purpose and not ideally adjusted to a mobile communication purpose, given that other sources of light, for example the sun, will interfere with the signal.[20]


The general term "visible light communication" (VLC), whose history dates back to the 1880s, includes any use of the visible light portion of the electromagnetic spectrum to transmit information. The D-Light project at Edinburgh's Institute for Digital Communications was funded from January 2010 to January 2012.[23] Haas helped start a company to market it.[24]


The modulation formats recognized for PHY I and PHY II are on-off keying (OOK) and variable pulse-position modulation (VPPM). The Manchester coding used for the PHY I and PHY II layers includes the clock inside the transmitted data by representing a logic 0 with an OOK symbol "01" and a logic 1 with an OOK symbol "10", all with a DC component. The DC component avoids light extinction in case of an extended run of logic 0's.[citation needed]


Effective dynamic modulation of visible light properties has been significantly desired for advanced imaging and sensing technologies. In particular, phase-change materials have attracted much attention as active material platforms owing to their broadband tunability of optical dielectric functions induced by the temperature-dependent phase-changes. However, their uses for visible light modulators are still limited to meet multi-objective high performance owing to the low material quality factor and active tunability in the visible regime. Here, a design strategy of phase-change metafilm absorber is demonstrated by making the use of the material drawbacks and extending design degree of freedom. By engineering tunability of effective anisotropic permittivity tensor of VO2-Ag metafilm around near-unity absorption conditions, strong dynamic modulation of reflection wave is achieved with near-unity modulation depth at desired wavelength regions without sacrificing bandwidth and efficiency. By leveraging effective medium theory of metamaterial and coupled mode theory, the intuitive design rules and theoretical backgrounds are suggested. It is also noteworthy that the dynamic optical applications of intensity modulation, coloring, and polarization rotation are enabled in a single device. By virtue of ultrathin flat configuration of a metafilm absorber, design extensibility of reflection spectrum is also verified. It is envisioned that our simple and powerful strategy would play a robust role in development of miniaturized light modulating pixels and a variety of photonic and optoelectronic applications. 041b061a72


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