Spatial and wavelength division multiplexing for high-speed VLC systems: An overview
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paper_title: LED Based Wavelength Division Multiplexed 10 Gb/s Visible Light Communications

paper_content:

LED-based visible light communications can provide high data rates to users. This can be further increased by the use of wavelength division multiplexing using the different colours required to generate white light to transmit different data streams. In this paper, a trichromatic approach is described and the influence of colour combination on achievable data rate is analysed. A demonstration of LED-based communications which achieves a data rate of >10 Gb/s by using a rate adaptive orthogonal-frequency-division-multiplexing scheme is also reported.
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paper_title: 125 Mbit/s over 5 m wireless distance by use of OOK-Modulated phosphorescent white LEDs

paper_content:

We report a wireless visible-light link operating for the first time at 125 Mbit/s over 5 m distance indoor by use of on-off-keying (OOK), resulting in bit-error-ratios below 2·10−3.
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paper_title: 5.6 Gbit/s downlink and 1.5 Gbit/s uplink optical wireless transmission at indoor distances (≥ 1.5 m)

paper_content:

We experimentally realized a bi-directional high-speed optical system working at common environment distances and illumination levels. The downlink record aggregate bitrate of 5.6 Gbit/s is achieved exploiting WDM approach, commercial LEDs and DMT modulation.
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paper_title: Cellular architecture and key technologies for 5G wireless communication networks

paper_content:

The fourth generation wireless communication systems have been deployed or are soon to be deployed in many countries. However, with an explosion of wireless mobile devices and services, there are still some challenges that cannot be accommodated even by 4G, such as the spectrum crisis and high energy consumption. Wireless system designers have been facing the continuously increasing demand for high data rates and mobility required by new wireless applications and therefore have started research on fifth generation wireless systems that are expected to be deployed beyond 2020. In this article, we propose a potential cellular architecture that separates indoor and outdoor scenarios, and discuss various promising technologies for 5G wireless communication systems, such as massive MIMO, energy-efficient communications, cognitive radio networks, and visible light communications. Future challenges facing these potential technologies are also discussed.
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paper_title: Improving SINR in indoor cellular visible light communication networks

paper_content:

In this paper, we first investigate the negative impact of light reflections and then introduce an angle diversity receiver (ADR) to mitigate the interference effect resulting from light reflections in an indoor cellular optical wireless communications (OWC) network. As the ADR consists of multiple photodiodes (PDs), a proper signal combining scheme is essential to optimize the system performance. Therefore, four different combining schemes, the select best combining (SBC), the equal gain combining (EGC), the maximum ratio combining (MRC) and the optimum combining (OPC), are investigated. The results indicate that the OPC scheme achieves the best performance among these combination schemes. In particular, when we use a seven-PD ADR as the optical detector, the OPC scheme can significantly suppress the interference signal resulting from light reflections and can achieve a signal to interference plus noise ratio (SINR) performance that is close to the SINR performance of the same configuration that is free of light reflections.
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paper_title: High-Speed Optical Wireless Demonstrators: Conclusions and Future Directions

paper_content:

Two high-speed angle diversity optical wireless systems have recently been implemented, as part of a European Community funded project. One operates at 1.25 Gb/s offering a limited coverage area, and the other at 280 Mb/s, with room scale coverage. In this paper, we summarize the design approach for these systems and their performance. Implications of these results for the design and implementation of future systems are also discussed.
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paper_title: High-Speed Visible Light Communications Using Multiple-Resonant Equalization

paper_content:

White light-emitting diodes (LEDs) are becoming widespread in commercial lighting applications, and there are predictions that they will be in common use in domestic applications in the future. There is also growing interest in using these devices for both illumination and communications. One of the major challenges in visible light communications is the low modulation bandwidth (BW) available from devices, which is typically several megahertz. In this letter, we describe a link that uses 16 LEDs which are modulated using a resonant driving technique, creating an overall BW of 25 MHz. This is used to implement a 40-Mb/s nonreturn-to-zero on-off keying link which operates at low error rates, and also provides illumination at levels sufficient for a standard office environment.
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paper_title: Integrated transceivers for optical wireless communications

paper_content:

Line-of-sight free-space optical links can provide extremely high bandwidth communications, but this usually requires that transmitter and receiver are precisely aligned. In order to allow terminals to be mobile, links must be able to track users within their field of view so that the link is maintained. There are various means to do this, but all require complex subsystems with a number of different optical, optoelectronic, and electrical components. A solid-state tracking architecture is introduced and a seven-channel tracking system demonstration described. The system is designed to operate at 155 Mb/s and is, to the best of our knowledge, the first that uses an integrated approach. Arrays of novel resonant cavity LED (RCLED) emitters that operate at 980 nm are used as sources. These are flip-chip bonded to arrays of CMOS driver circuits and integrated with the necessary transmitter optics. The receiver uses a back-illuminated detector array flip-chip bonded to arrays of custom CMOS receivers. All these components are custom and have performance substantially better than nonoptimized commercially available components. In the paper, the design and fabrication of the optics, optoelectronics, and electronics required for this is described. Successful operation of all the subsystems is detailed, together with results from an initial link demonstration.
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paper_title: Enhanced performance of single-input multiple-output visible light communication system utilizing space diversity technology

paper_content:

Contradictions among receiver optical power, bandwidth of photodiode, and transmission distance are ubiquitous in visible light communication (VLC) systems. In order to moderate such contradictions and improve the performance of VLC systems, we proposed a space diversity receiver for a light-emitting diode (LED)-based VLC system and experimentally demonstrated the highest data rate for an RGB LED with a 1.4 Gbit / s data rate employing two detectors located in different space positions at the same time. The maximal ratio combining algorithm was used in the VLC system for the first time. As a result, the bit error ratios of the receivers can be reduced by 48.08 dB (red LED), 12.04 dB (green LED), and 27.48 dB (blue LED) compared with any separate detector. The performance of the VLC system was significantly improved. It turned out that it was highly desirable to use multiantennas in the VLC system.
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paper_title: Design and analysis of an angular-segmented full-mobility visible light communications receiver

paper_content:

This paper presents the concept, design and analysis for a visible light communications receiver utilising the angular and spatial diversity in order to achieve full mobility and protection from signal blocking in a typical home/office environment. The proposed receiver uses seven sub-receivers, each dedicated to an independent viewing angle and direction. Employing selection combining, the receiver with the strongest signal is used for the communications channel. Full performance evaluation and numerical analysis of the proposed system have been performed in MATLAB and compared to that of a single receiver. The received power, impulse response, corresponding signal-to-noise ratios and root-mean-square delay spread are also investigated. Copyright © 2014 John Wiley & Sons, Ltd.
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paper_title: High-Speed Visible Light Communications Using Multiple-Resonant Equalization

paper_content:

White light-emitting diodes (LEDs) are becoming widespread in commercial lighting applications, and there are predictions that they will be in common use in domestic applications in the future. There is also growing interest in using these devices for both illumination and communications. One of the major challenges in visible light communications is the low modulation bandwidth (BW) available from devices, which is typically several megahertz. In this letter, we describe a link that uses 16 LEDs which are modulated using a resonant driving technique, creating an overall BW of 25 MHz. This is used to implement a 40-Mb/s nonreturn-to-zero on-off keying link which operates at low error rates, and also provides illumination at levels sufficient for a standard office environment.
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paper_title: Practical OFDM signalling for visible light communications using spatial summation

paper_content:

Visible light communication (VLC) channels leverage solid-state illumination devices to provide a dual role as high-speed communication links. These channels are bandwidth-limited and can have high SNRs. Bandwidth-efficient intensity-modulated/direct-detection (IM/DD) orthogonal frequency-division multiplexing (OFDM) has been considered for VLC systems. Key drawbacks of OFDM in IM/DD channels are its high peak-to-average power ratio (PAPR) as well as the difficulty of implementing power efficient, high current, wideband drivers. In this paper, we propose a new form of IM/DD OFDM termed spatial optical OFDM (SO-OFDM) in which multiple narrowband OFDM signals are transmitted simultaneously from different LEDs and allowed to sum in space. In this way, a conventional OFDM receiver can be employed while having a series of lower power transmitters, each with improved PAPR. Simulation results show that in comparison with conventional IM/DD OFDM, SO-OFDM is more robust to amplifier nonlinearities and can be implemented with more efficient line driver topologies.
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paper_title: Superposed pulse amplitude modulation for visible light communication

paper_content:

We propose and experimentally demonstrate a novel modulation scheme called superposed pulse amplitude modulation (SPAM) which is low-cost, insensitive to non-linearity of light emitting diode (LED). Multiple optical pulses transmit parallelly from different spatial position in the LED array and overlap linearly in free space to realize SPAM. With LED arrangement, the experimental results show that using the modulation we proposed the data rate of 120 Mbit/s with BER 1 × 10−3 can be achieved with an optical blue filter and RC post-equalization.
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paper_title: Grouped modulation scheme for led array module in a visible light communication system

paper_content:

Grouped modulation is proposed to generate multiple-level optical signals and enhance the data rate for LED-array-module-based indoor visible light communication systems. By dividing LEDs in an array module into n groups and driving different groups with separate circuits, 2nlevel modulation can be realized with binary modulation for each group. The maximum driven current for respective circuits is reduced compared to that of the same multiple-level PAM for which an array module is driven by a single circuit. The limitation to modulation bandwidth of an LED array module, caused by the product of gain and bandwidth of its driven circuit, is alleviated. The benefit of grouped modulation is that the performance of a visible light communication system can be improved with the medium complexity of driven circuits for different groups. Proof-of-concept experiments demonstrate that the data rate can be enhanced up to 50 Mb/s with three LEDs (LED465E provided by THORLABs) by 2-grouped OOK modulation and the transmission length is 1.0 m with a bit error rate less than 1e-6. Meanwhile, the maximum current of driven circuits for two groups is reduced by a third compared to a circuit driving three LEDs as a whole. The improvements can be further increased greatly if a larger-sized LED array module of hundreds or thousands of LEDs is divided into more groups. Grouped modulation can also be extended to other formats such as pulse position modulation and overlapping pulse position modulation.
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paper_title: Subcarrier Grouping OFDM for Visible-Light Communication Systems

paper_content:

Orthogonal frequency-division multiplexing (OFDM) has been widely studied and adopted in visible-light communication (VLC) systems because of its high spectral efficiency and low-complexity implementation. However, OFDM-based VLC systems suf- fer from a high peak-to-average power ratio (PAPR), along with the nonlinear transfer characteristics of light-emitting diodes (LEDs). In this paper, a subcarrier grouping scheme for an OFDM-based VLC system is proposed. This scheme exploits the property that one LED lamp usually contains multiple independent LED chips. In the proposed scheme, all OFDM signal subcarriers are divided into groups, and each group is trans- mitted by an individual LED chip while the same receiver structure is maintained. The PAPR is reduced significantly because only some of the subcarriers are located on each LED chip. Hence, the achievable input power can be increased, leading to a higher signal-to-noise ratio (SNR) and a lower bit error rate (BER). Simulated and experimental results demonstrate that the BER performance can be improved by over two orders of magnitude. Moreover, an adaptive switching mechanism is also proposed in this paper, in which a subcarrier grouping scheme is employed adaptively when the required input power is high. Simulation results validate that the proposed mechanism can enhance sys- tem robustness significantly.
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paper_title: Optical spatial modulation using colour LEDs

paper_content:

Spatial Modulation (SM) is a combined multiple-input-multiple-output (MIMO) and digital modulation technique. Like in any MIMO scheme, the performance of SM is related to the differentiability, i.e. the correlation, of the multiple channels. In this paper, we analyse the performance of optical SM for a wireless indoor environment. It is shown that the use of colour light-emitting diodes (LEDs) can improve the performance of SM by more than 10 dB. This is due to the fact that the responsivity of photo-diodes is a function of the optical wavelength. As a consequence, using LEDs with different colours induces specific optical power levels at the receiver. These distinctive power levels directly affect the channel characteristic since they reduce the channel correlation. Moreover, we analyse the computational complexity of optical SM and compare it to the complexity of other common MIMO techniques. It is found that SM requires a much lower computational complexity compared to Repetition Coding (RC) and Spatial Multiplexing (SMP).
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paper_title: Experimental proof-of-concept of optical spatial modulation OFDM using micro LEDs

paper_content:

This paper investigates experimentally the performance of optical spatial modulation (OSM) with orthogonal frequency division multiplexing (OFDM) in a multiple-input and multiple-output (MIMO) based visible light communication (VLC) system. Gallium nitride based micro light emitting diodes (µLEDs) are considered as a transmitter unit. A two transmitter and two optical receiver based MIMO setup is used. The performance of OSM-OFDM system is experimentally analysed by varying the semi-angle at half power, θ½ of the µLEDs. The optical cross-talk in the MIMO channel is quantified and its effect on the achievable data rate and bit error rate (BER) performance is analysed. The experimental results show that OSM-OFDM achieves higher data rates than single-input and single-output (SISO)-OFDM for all the ranges of θ½ of the µLEDs under study. However, the BER performance of OSM-OFDM is worse than in the case of SISO-OFDM. The maximum achievable data rate in OSM-OFDM is 1.34 Gb/s and 1.17 Gb/s in SISO-OFDM for θ½ of 2.27° at a link distance of 1 m using adaptive data loading. The experimental results also show that due to the strongly correlated channels, the system performance is highly dependent on the θ½ of the µLEDs.
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paper_title: Demonstration of the Merit and Limitation of Generalised Space Shift Keying for Indoor Visible Light Communications

paper_content:

This paper demonstrates the merit and de-merit of a low complexity, multiple transmitter generalised space shift keying (GSSK) signalling technique for short range indoor visible light communications. To illustrate the high spectral efficiency credential of GSSK, an array of $N_{{\rm t}}$ white light emitting diodes (LEDs) is used to demonstrate $N_{{\rm t}}$ bits/symbol transmission by exploiting the natural differences in the channel gains of the spatially separated LED array. With two low power white LEDs each having a $\hbox{3}$ dB modulation bandwidth of $\hbox{8}$ MHz, data transmission of up to $\hbox{40}$ Mbits/s ( $\hbox{20}$ Mbaud) is demonstrated. For an average received electrical signal power of ${-12.5}$ dBm and $2\times 10^{4}$ data symbols, the measured symbol error rate at $\hbox{20}$ Mbaud is $7 \times 10^{-2}$ and $5 \times 10^{-5}$ at $\hbox{10}$ Mbaud ( $\hbox{20}$ Mbits/s) without any error control coding. The GSSK technique is experimentally shown to be most suitable for a fixed configuration as its performance is quite sensitive to receiver mobility.
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paper_title: SM/SPPM Aided Multiuser Precoded Visible Light Communication Systems

paper_content:

In this paper, we propose a multiuser precoded visible light communication (VLC) system utilizing optical orthogonal frequency division multiplexing (OFDM), where optical spatial modulation (OSM) and spatial pulse position modulation (SPPM) are invoked for supporting high-rate data transmissions without compromising uniform illumination across multiple light-emitting diodes (LEDs). Both OSM and SPPM exploit the index of data streams for carrying additional information bits, whereas the multiuser interference occurring between different user equipments is mitigated by the MIMO precoder. By evaluating the performances of the proposed VLC systems, we demonstrate that the OSM-aided system is capable of achieving high data rates at sufficiently high signal-to-noise ratios (SNRs). In contrast, the SPPM-aided system has a higher effective bandwidth efficiency at low SNRs, despite the fact that its attainable peak data rate is lower than that of the SM-aided counterpart. Such tradeoffs indicate the potential application scenarios of the proposed schemes for achieving diverse design prerequisites.
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paper_title: High-Speed Visible Light Communications Using Multiple-Resonant Equalization

paper_content:

White light-emitting diodes (LEDs) are becoming widespread in commercial lighting applications, and there are predictions that they will be in common use in domestic applications in the future. There is also growing interest in using these devices for both illumination and communications. One of the major challenges in visible light communications is the low modulation bandwidth (BW) available from devices, which is typically several megahertz. In this letter, we describe a link that uses 16 LEDs which are modulated using a resonant driving technique, creating an overall BW of 25 MHz. This is used to implement a 40-Mb/s nonreturn-to-zero on-off keying link which operates at low error rates, and also provides illumination at levels sufficient for a standard office environment.
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paper_title: Indoor Optical Wireless MIMO System With an Imaging Receiver

paper_content:

This letter reports an experimental demonstration of an indoor optical wireless multiple-input-multiple-output (MIMO) system with an imaging receiver. The system setup consists of a 2 × 2 array of white light-emitting diodes (LEDs) and a 3 × 3 photodetector array, separated by a range of 2 m. The system operates at a bit rate of 2 Mb/s/channel, with error-free operation at certain positions within the system coverage area. An overview of the design specifications, optical design, and experimental setup are reported in this letter, together with results and discussion of how the system might be improved.
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paper_title: Optical Multi-Input Multi-Output systems for short-range free-space data transmission

paper_content:

Radio Frequency Multi-Input Multi-Output (MIMO) systems are becoming ubiquitous, due to the ability of spatial multiplexing to increase capacity where there is limited spectrum available, and the power that can be transmitted is limited by spectral regulation.
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paper_title: High-Speed Visible Light Communications Using Multiple-Resonant Equalization

paper_content:

White light-emitting diodes (LEDs) are becoming widespread in commercial lighting applications, and there are predictions that they will be in common use in domestic applications in the future. There is also growing interest in using these devices for both illumination and communications. One of the major challenges in visible light communications is the low modulation bandwidth (BW) available from devices, which is typically several megahertz. In this letter, we describe a link that uses 16 LEDs which are modulated using a resonant driving technique, creating an overall BW of 25 MHz. This is used to implement a 40-Mb/s nonreturn-to-zero on-off keying link which operates at low error rates, and also provides illumination at levels sufficient for a standard office environment.
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paper_title: Indoor Optical Wireless MIMO System With an Imaging Receiver

paper_content:

This letter reports an experimental demonstration of an indoor optical wireless multiple-input-multiple-output (MIMO) system with an imaging receiver. The system setup consists of a 2 × 2 array of white light-emitting diodes (LEDs) and a 3 × 3 photodetector array, separated by a range of 2 m. The system operates at a bit rate of 2 Mb/s/channel, with error-free operation at certain positions within the system coverage area. An overview of the design specifications, optical design, and experimental setup are reported in this letter, together with results and discussion of how the system might be improved.
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paper_title: Experimental Demonstration of 4 × 4 MIMO Wireless Visible Light Communication Using a Commercial CCD Image Sensor

paper_content:

We report an experimental demonstration of 4 × 4 multiple-input multiple-output (MIMO) wireless visible light communications (VLC) using a charge-coupled device image sensor receiver instead of a photodiode receiver. An image sensor is a kind of digital camera, which is used in most mobile devices such as smart phones or laptop computers and a promising commercial candidate for a VLC receiver. The transmission distance of the experimental result is over 10 m, enough for most indoor communication applications. We expect that the MIMO VLC technique based on image sensor receivers can be widely used with the development of high-speed image sensors.
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paper_title: Imaging diversity receivers for high-speed infrared wireless communication

paper_content:

We discuss two modifications to the design of wireless infrared links that can yield significant performance improvements, albeit at the price of increased complexity. In line-of-sight and non-line-of-sight links, replacement of a single-element receiver by one employing an imaging light concentrator and a segmented photodetector can reduce received ambient light noise and multipath distortion. For a fixed receiver entrance area, such an imaging receiver can reduce transmit power requirements by as much as about 14 dB, depending on the link design and the number of photodetector segments. Imaging receivers also reduce co-channel interference, and may therefore enable infrared wireless networks to employ space-division multiplexing, wherein several transmitters located in close proximity can transmit simultaneously at the same wavelength. In nondirected non-line-of-sight links, replacement of the diffuse transmitter by one that projects multiple narrow beams can reduce the path loss, further reducing the transmit power requirement by several decibels. We describe the design of an experimental 100 Mb/s infrared wireless link employing a multibeam transmitter and a 37-pixel imaging receiver.
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paper_title: On the relation between V-BLAST and the GDFE

paper_content:

This letter shows that the receiver processing of the Vertical Bell Labs Layered Space-Time (V-BLAST) architecture may he viewed as a generalized decision feedback equalizer (GDFE) as applied to a multiple-input-multiple-output (MIMO) channel. It is proven that, depending on the criterion chosen to reduce the interference, the receiver operations of the V-BLAST system are fundamentally equivalent to either a zero-forcing (ZF) or a minimum-mean-square-error (MMSE) GDFE.
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paper_title: Performance Analysis of ZF and MMSE Equalizers for MIMO Systems: An In-Depth Study of the High SNR Regime

paper_content:

This paper presents an in-depth analysis of the zero forcing (ZF) and minimum mean squared error (MMSE) equalizers applied to wireless multiinput multioutput (MIMO) systems with no fewer receive than transmit antennas. In spite of much prior work on this subject, we reveal several new and surprising analytical results in terms of output signal-to-noise ratio (SNR), uncoded error and outage probabilities, diversity-multiplexing (D-M) gain tradeoff and coding gain. Contrary to the common perception that ZF and MMSE are asymptotically equivalent at high SNR, we show that the output SNR of the MMSE equalizer (conditioned on the channel realization) is ρmmse = ρzf+η\ssrsnr, where ρzf is the output SNR of the ZF equalizer and that the gap η\ssrsnr is statistically independent of ρzf and is a nondecreasing function of input SNR. Furthermore, as \ssr snr\ura ∞, η\ssrsnr converges with probability one to a scaled F random variable. It is also shown that at the output of the MMSE equalizer, the interference-to-noise ratio (INR) is tightly upper bounded by [(η\ssrsnr)/(ρzf)]. Using the decomposition of the output SNR of MMSE, we can approximate its uncoded error, as well as outage probabilities through a numerical integral which accurately reflects the respective SNR gains of the MMSE equalizer relative to its ZF counterpart. The e-outage capacities of the two equalizers, however, coincide in the asymptotically high SNR regime. We also provide the solution to a long-standing open problem: applying optimal detection ordering does not improve the D-M tradeoff of the vertical Bell Labs layered Space-Time (V-BLAST) architecture. It is shown that optimal ordering yields a SNR gain of 10log10N dB in the ZF-V-BLAST architecture (where N is the number of transmit antennas) whereas for the MMSE-V-BLAST architecture, the SNR gain due to ordered detection is even better and significantly so.
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paper_title: A Gigabit/s Indoor Wireless Transmission Using MIMO-OFDM Visible-Light Communications

paper_content:

This letter reports an experimental demonstration of indoor wireless visible-light communication transmission at 1 Gb/s. The system consists of a four-channel multiple-input multiple-output link that uses white LED sources, each transmitting signals at 250 Mb/s using orthogonal frequency division multiplexing modulation. A nine-channel imaging diversity receiver is used to detect the signals, and an average bit error rate of 10-3 is achieved at the room illumination level of ~1000 lux at 1-m range.
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paper_title: Fisheye-lens-based space division multiplexing system for visible light communications

paper_content:

This paper presents the high-diversity space division multiplexing (SDM) visible light communications (VLCs) utilizing a fisheye-lens-based imaging receiver. The receiver features ultra-wide field-of-view, good illumination uniformity, ultra-high imaging quality, and compact size; therefore, it can realize omnidirectional receiving and provide high spatial diversity for the SDM VLC system. In the presented system, four data streams are parallelly transmitted by commercial phosphorescent white light-emitting diodes, and discrete multitone signals based on quadrature amplitude modulation and bit loading are employed to achieve high spectral efficiency. The experiment verifies that the interchannel interference is effectively alleviated due to the high spatial diversity provided by the fisheye-lens-based receiver, and an aggregate data rate of 1.3 Gbit/s is achieved with the limited 3-dB bandwidth of 20 MHz. Besides, it is feasible to extend the system to more channels to achieve higher capacity. The experimental results indicate that the fisheye-lens-based imaging receiver is a potential candidate for high-speed VLC applications.
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paper_title: Demonstration of High-Speed 2 × 2 Non-Imaging MIMO Nyquist Single Carrier Visible Light Communication With Frequency Domain Equalization

paper_content:

We experimentally demonstrate a high-speed 2 × 2 non-imaging multiple-input multiple-output nyquist single carrier visible light communication system. Two commercially available blue light emitting diodes with 3 dB electrical bandwidth of 10 MHz and two avalanche photo diodes with 3 dB electrical bandwidth of 100 MHz are used as transmitters and receivers, respectively. A frequency domain equalization method based on two pairs of time-multiplexed training symbols is proposed, which allows de-multiplexing and post-equalization simultaneously processing in one step. Frequency domain averaging and time-domain averaging are also implemented and analyzed in this paper. In this demonstration, the overall data rate is 500 Mb/s. The measured bit error rates for two receivers are both below the 7% pre-forward-error-correction threshold of $ \hbox{3.8} \times 10^{-3}$ after 40 cm free-air transmission.
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paper_title: High-Speed Visible Light Communications Using Multiple-Resonant Equalization

paper_content:

White light-emitting diodes (LEDs) are becoming widespread in commercial lighting applications, and there are predictions that they will be in common use in domestic applications in the future. There is also growing interest in using these devices for both illumination and communications. One of the major challenges in visible light communications is the low modulation bandwidth (BW) available from devices, which is typically several megahertz. In this letter, we describe a link that uses 16 LEDs which are modulated using a resonant driving technique, creating an overall BW of 25 MHz. This is used to implement a 40-Mb/s nonreturn-to-zero on-off keying link which operates at low error rates, and also provides illumination at levels sufficient for a standard office environment.
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paper_title: Indoor gigabit 2 × 2 imaging multiple-input–multiple-output visible light communication

paper_content:

We propose and experimentally demonstrate a 2 × 2 imaging ::: multiple-input-multiple-output (MIMO) Nyquist single carrier visible light ::: communication (VLC) system based on spectral efficient 64/32-ary quadrature ::: amplitude modulation, as well as pre- and post-equalizations. Two commercially ::: available red-green-blue light-emitting diodes (LEDs) with 3 dB bandwidth of 10 MHz ::: and two avalanche photodiodes with 3 dB bandwidth of 100 MHz are employed. Due to ::: the limited experiment condition, three different colors/wavelengths are transmitted ::: separately. The achieved data rates of red, green, and blue LED chips are 1.5, 1.25, ::: and 1.25 Gb/s, respectively. The resulting bit error ratios are below the 7% ::: pre-forward error correction limit of 3.8 × 10-3 after 75 cm indoor transmission. To ::: the best of our knowledge, this is the first experimental investigation of imaging ::: MIMO system, and it is the highest data rate ever achieved in MIMO VLC system.
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paper_title: A MIMO-ANN system for increasing data rates in organic visible light communications systems

paper_content:

This paper presents the first ever experimental demonstration of a multiple-input multiple-output (MIMO) visible light communications system employing four silicon (Si) light emitting diodes (LEDs) and four organic photodetectors (OPDs) as transmitters and receivers, respectively. The proposed link is relatively low cost and it employs the on-off keying (OOK) modulation format offering a data rate of 200 kb/s without the need for equalization, which is a significant increase compared with previous non-equalized systems. In order to speed up date rates further, we implement an artificial neural network (ANN) to classify the signal and correct the error induced by the matrix inversion at the receiver, allowing a gross bit rate of 1.8 Mb/s in the best case.
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paper_title: Performance Comparison of MIMO Techniques for Optical Wireless Communications in Indoor Environments

paper_content:

In this paper, we compare the performance of multiple-input-multiple-output (MIMO) techniques applied to indoor optical wireless communications (OWC) assuming line-ofsight (LOS) channel conditions. Specifically, several 4 × 4 setups with different transmitter spacings and different positions of the receiver array are considered. The following MIMO algorithms are considered: Repetition Coding (RC), Spatial Multiplexing (SMP) and Spatial Modulation (SM). Particularly, we develop a framework to analytically approximate the bit error ratios (BERs) of these schemes and verify the theoretical bounds by simulations. The results show that due to diversity gains, RC is robust to various transmitter-receiver alignments. However, as RC does not provide spatial multiplexing gains, it requires large signal constellation sizes to enable high spectral efficiencies. In contrast, SMP enables high data rates by exploiting multiplexing gains. In order to provide these gains, SMP needs sufficiently low channel correlation. SM is a combined MIMO and digital modulation technique. We show that SM is more robust to high channel correlation compared to SMP, while enabling larger spectral efficiency compared to RC. Moreover, we investigate the effect of induced power imbalance between the multiple transmitters. It is found that power imbalance can substantially improve the performance of both SMP and SM as it reduces channel correlation. In this context, we also show that blocking some of the links is an acceptable method to reduce channel correlation. Even though the blocking diminishes the received energy, it outweighs this degradation by providing improved channel conditions for SMP and SM. For example, blocking 4 of the 16 links of the 4 × 4 setup improves the BER performance of SMP by more than 20 dB, while the effective signal to noise ratio (SNR) is reduced by about 2 dB due to the blocking. Therefore, MIMO techniques can provide gains even under LOS conditions which provide only little channel differences.
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paper_title: Performance of optical spatial modulation and spatial multiplexing with imaging receiver

paper_content:

Spatial modulation (SM) and spatial multiplexing (SMP) are two multiple-input multiple-output (MIMO) techniques for transmitting data over an indoor optical wireless channel. Receivers for SM and SMP can be of the non-imaging type, in which case the channel matrix coefficients can be highly correlated, or of the imaging type, which can reduce the degree of correlation and improve overall system performance. In this work, we propose a new framework to analyze the performance of imaging MIMO systems. This framework is applied to characterize the performance of SM and SMP under both imaging and non-imaging receivers. Results of our analysis indicate that imaging receivers can provide significant signal-to-noise ratio (SNR) improvements up to 45dB under SM and SMP as compared to the use of non-imaging receivers. Finally, the application of the proposed analysis framework indicates specific design principles to optimize imaging receiver parameters.
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paper_title: 1.5 Gbit/s Multi-Channel Visible Light Communications Using CMOS-Controlled GaN-Based LEDs

paper_content:

An on-chip multi-channel visible light communication (VLC) system is realized through a blue (450 nm) GaN-based micron-size light-emitting diode (μLED) array integrated with complementary metal-oxide-semiconductor (CMOS) electronics. When driven by a custom-made CMOS driving board with 16 independent parallel data input ports, this μLED array device is computer controllable via a standard USB interface and is capable of delivering high speed parallel data streams for VLC. A total maximum error-free data transmission rate of 1.5 Gbit/s is achieved over free space by modulating four μLED pixels simultaneously using an on-off key non-return to zero modulation scheme. Electrical and optical crosstalk of the system has also been investigated in detail and the further optimization of CMOS design to minimize the crosstalk is proposed.
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paper_title: High-Speed Integrated Visible Light Communication System: Device Constraints and Design Considerations

paper_content:

Visible light communications (VLC) has the potential to play a major part in future smart home and next generation communication networks. There is significant ongoing work to increase the achievable data rates using VLC, to standardize it and integrate it within existing network infrastructures. The future of VLC systems depends on the ability to fabricate low cost transceiver components and to realize the promise of high data rates. This paper reports the design and fabrication of integrated transmitter and receiver components. The transmitter uses a two dimensional individually addressable array of micro light emitting diodes ( $\mu\text{LEDs}$ )and the receiver uses an integrated photodiode array fabricated in a CMOS technology. A preliminary result of a MIMO system implementation operating at a data rate of 1 Gbps is demonstrated. This paper also highlights the challenges in achieving highly parallel data communication along with the possible bottlenecks in integrated approaches.
---
paper_title: LED Based Wavelength Division Multiplexed 10 Gb/s Visible Light Communications

paper_content:

LED-based visible light communications can provide high data rates to users. This can be further increased by the use of wavelength division multiplexing using the different colours required to generate white light to transmit different data streams. In this paper, a trichromatic approach is described and the influence of colour combination on achievable data rate is analysed. A demonstration of LED-based communications which achieves a data rate of >10 Gb/s by using a rate adaptive orthogonal-frequency-division-multiplexing scheme is also reported.
---
paper_title: Towards a 100 Gb/s visible light wireless access network

paper_content:

Potential visible light communication (VLC) data rates at over 10 Gb/s have been recently demonstrated using light emitting diodes (LEDs). The disadvantage is, LEDs have an inherent trade-off between optical efficiency and bandwidth. Consequently, laser diodes (LDs) can be considered as a very promising alternative for better utilization of the visible light spectrum for communication purposes. This work investigates the communication capabilities of off-the-shelf LDs in a number of scenarios with illumination constraints. The results indicate that optical wireless access data rates in the excess of 100 Gb/s are possible at standard indoor illumination levels.
---
paper_title: Going beyond 4 Gbps data rate by employing RGB laser diodes for visible light communication

paper_content:

With increasing interest in visible light communication, the laser diode (LD) provides an attractive alternative, with higher efficiency, shorter linewidth and larger bandwidth for high-speed visible light communication (VLC). Previously, more than 3 Gbps data rate was demonstrated using LED. By using LDs and spectral-efficient orthogonal frequency division multiplexing encoding scheme, significantly higher data rates has been achieved in this work. Using 16-QAM modulation scheme, in conjunction with red, blue and green LDs, data rates of 4.4 Gbps, 4 Gbps and 4 Gbps, with the corresponding BER/SNR/EVM of 3.3 × 10−3/15.3/17.9, 1.4 × 10−3/16.3/15.4 and 2.8 × 10−3/15.5/16.7were obtained over transmission distance of ~20 cm. We also simultaneously demonstrated white light emission using red, blue and green LDs, after passing through a commercially available diffuser element. Our work highlighted that a tradeoff exists in operating the blue LDs at optimum bias condition while maintaining good color temperature. The best results were obtained when encoding red LDs which gave both the strongest received signal amplitude and white light with CCT value of 5835K.
---
paper_title: 5.6 Gbit/s downlink and 1.5 Gbit/s uplink optical wireless transmission at indoor distances (≥ 1.5 m)

paper_content:

We experimentally realized a bi-directional high-speed optical system working at common environment distances and illumination levels. The downlink record aggregate bitrate of 5.6 Gbit/s is achieved exploiting WDM approach, commercial LEDs and DMT modulation.
---
paper_title: 8-Gb/s RGBY LED-Based WDM VLC System Employing High-Order CAP Modulation and Hybrid Post Equalizer

paper_content:

In this paper, for the first time, we propose the use of a hybrid post equalizer in a high-order carrierless-amplitude-and-phase-modulation-based visible light communication (VLC) system. The hybrid equalizer consists of a linear equalizer, a Volterra-series-based nonlinear equalizer, and a decision-directed least mean squares equalizer to simultaneously mitigate the linear and nonlinear distortions of the VLC system. A commercially available red-blue-green-yellow light-emitting diode (RBGY LED) is utilized for four-wavelength multiplexing. By the hybrid equalizer, an aggregate data rate of 8 Gb/s is experimentally achieved over a 1-m indoor free-space transmission with the bit error rate (BER) below the 7% forward error correction (FEC) limit of 3.8 × 10 ::: -3 ::: . To the best of our knowledge, this is the highest data rate ever reported in high-speed VLC systems.
---
paper_title: Gigabit-class optical wireless communication system at indoor distances (1.5 – 4 m)

paper_content:

In this paper we experimentally realized bidirectional optical wireless communication (OWC) link using four channel visible LED board exploiting wavelength division multiplexing (WDM) for the downlink and infrared LED for uplink. We achieved greater than 5 Gbit/s data rate at common indoor distance (1.5 to 4 m) for downlink and 1.5 Gbit/s for uplink using commercially available LEDs. We achieved these results after a careful choice of the LED emission wavelengths and the optical filter spectra. Moreover, we investigate the optimal LED working current and the optimal modulation depth. The bit error ratios of all the channels were maintained lower than the FEC limit (3.8·10−3).
---
paper_title: 803 Mbit/s visible light WDM link based on DMT modulation of a single RGB LED luminary

paper_content:

We report the first visible light link based on WDM and DMT modulation of a single RGB-type white LED, operating at an aggregate rate of 803 Mbit/s within the FEC 2·10−3 limit.
---
paper_title: Enhanced Performance of a High-Speed WDM CAP64 VLC System Employing Volterra Series-Based Nonlinear Equalizer

paper_content:

The light-emitting diode nonlinearity in visible light communication (VLC) systems is considered to be a major problem that deteriorates system performance. In this paper, we experimentally demonstrate a high-speed WDM CAP64 VLC system employing a Volterra series-based nonlinear equalizer to mitigate the nonlinear effect. A modified cascaded multimodulus algorithm (M-CMMA) is utilized to calculate the error function and update the weights of the nonlinear equalizer without using training symbols. An aggregate data rate of 4.5 Gb/s is successfully achieved over 2-m indoor free-space transmission with a bit error rate (BER) below the 7% forward error correction limit of 3.8 × 10-3. With the Volterra nonlinear equalizer, the Q factor of the VLC system is 1. 6 dB better than that without using the nonlinear equalizer, and the transmission distance is also increased by about 110 cm at the BER of 3.8 × 10-3. To the best of our knowledge, this is the first time that the Volterra nonlinear equalizer is utilized for high-speed carrierless amplitude and phase (CAP) modulation-based VLC systems.
---
paper_title: LED Based Wavelength Division Multiplexed 10 Gb/s Visible Light Communications

paper_content:

LED-based visible light communications can provide high data rates to users. This can be further increased by the use of wavelength division multiplexing using the different colours required to generate white light to transmit different data streams. In this paper, a trichromatic approach is described and the influence of colour combination on achievable data rate is analysed. A demonstration of LED-based communications which achieves a data rate of >10 Gb/s by using a rate adaptive orthogonal-frequency-division-multiplexing scheme is also reported.
---
paper_title: 450-nm GaN laser diode enables high-speed visible light communication with 9-Gbps QAM-OFDM

paper_content:

A TO-38-can packaged Gallium nitride (GaN) blue laser diode (LD) based free-space visible light communication (VLC) with 64-quadrature amplitude modulation (QAM) and 32-subcarrier orthogonal frequency division multiplexing (OFDM) transmission at 9 Gbps is preliminarily demonstrated over a 5-m free-space link. The 3-dB analog modulation bandwidth of the TO-38-can packaged GaN blue LD biased at 65 mA and controlled at 25°C is only 900 MHz, which can be extended to 1.5 GHz for OFDM encoding after throughput intensity optimization. When delivering the 4-Gbps 16-QAM OFDM data within 1-GHz bandwidth, the error vector magnitude (EVM), signal-to-noise ratio (SNR) and bit-error-rate (BER) of the received data are observed as 8.4%, 22.4 dB and 3.5 × 10−8, respectively. By increasing the encoded bandwidth to 1.5 GHz, the TO-38-can packaged GaN blue LD enlarges its transmission capacity to 6 Gbps but degrades its transmitted BER to 1.7 × 10−3. The same transmission capacity of 6 Gbps can also be achieved with a BER of 1 × 10−6 by encoding 64-QAM OFDM data within 1-GHz bandwidth. Using the 1.5-GHz full bandwidth of the TO-38-can packaged GaN blue LD provides the 64-QAM OFDM transmission up to 9 Gbps, which successfully delivers data with an EVM of 5.1%, an SNR of 22 dB and a BER of 3.6 × 10−3 passed the forward error correction (FEC) criterion.
---
paper_title: Going beyond 4 Gbps data rate by employing RGB laser diodes for visible light communication

paper_content:

With increasing interest in visible light communication, the laser diode (LD) provides an attractive alternative, with higher efficiency, shorter linewidth and larger bandwidth for high-speed visible light communication (VLC). Previously, more than 3 Gbps data rate was demonstrated using LED. By using LDs and spectral-efficient orthogonal frequency division multiplexing encoding scheme, significantly higher data rates has been achieved in this work. Using 16-QAM modulation scheme, in conjunction with red, blue and green LDs, data rates of 4.4 Gbps, 4 Gbps and 4 Gbps, with the corresponding BER/SNR/EVM of 3.3 × 10−3/15.3/17.9, 1.4 × 10−3/16.3/15.4 and 2.8 × 10−3/15.5/16.7were obtained over transmission distance of ~20 cm. We also simultaneously demonstrated white light emission using red, blue and green LDs, after passing through a commercially available diffuser element. Our work highlighted that a tradeoff exists in operating the blue LDs at optimum bias condition while maintaining good color temperature. The best results were obtained when encoding red LDs which gave both the strongest received signal amplitude and white light with CCT value of 5835K.
---
paper_title: High-Speed Visible Light Communications Using Individual Pixels in a Micro Light-Emitting Diode Array

paper_content:

The high-frequency modulation of individual pixels in III-nitride-based micro-pixel light-emitting diode arrays, where each array consists of 16 × 16 individually addressable 72-μm-diameter pixels, are reported. The devices investigated have peak emission wavelengths at 370, 405, and 450 nm, respectively. The optical -3-dB modulation bandwidth of a typical pixel from the 450-nm-emitting device was found to be approximately 245 MHz. Data transmission at rates of up to 1 Gb/s is demonstrated from a single pixel emitting at 450 nm, using on-off keying nonreturn-to-zero modulation, with a bit-error ratio of less than 1 × 10-10. Such devices have potential for free-space or fiber-coupled visible light communications.
---
paper_title: Visible light communication using laser diode based remote phosphor technique

paper_content:

This paper reports a demonstration of high speed visible light communications using a blue laser diode combined with a remote phosphor. This method both generates white light and supports multi Gb/s communications. Data rates of up to 5.62 Gb/s and 6.52 Gb/s were achieved by using OFDM with fixed-rate and adaptive loading approaches, respectively. A total data rate of 10 Gb/s was found to be achievable by employing a two channel imaging system. These results demonstrate the considerable promise of the laser diode based remote phosphor approach in visible light communications.
---
paper_title: A 3-Gb/s Single-LED OFDM-Based Wireless VLC Link Using a Gallium Nitride $\mu{\rm LED}$

paper_content:

This letter presents a visible light communication (VLC) system based on a single 50- μm gallium nitride light emitting diode (LED). A device of this size exhibits a 3-dB modulation bandwidth of at least 60 MHz - significantly higher than commercially available white lighting LEDs. Orthogonal frequency division multiplexing is employed as a modulation scheme. This enables the limited modulation bandwidth of the device to be fully used. Pre- and postequalization techniques, as well as adaptive data loading, are successfully applied to achieve a demonstration of wireless communication at speeds exceeding 3 Gb/s. To date, this is the fastest wireless VLC system using a single LED.
---
paper_title: 8-Gb/s RGBY LED-Based WDM VLC System Employing High-Order CAP Modulation and Hybrid Post Equalizer

paper_content:

In this paper, for the first time, we propose the use of a hybrid post equalizer in a high-order carrierless-amplitude-and-phase-modulation-based visible light communication (VLC) system. The hybrid equalizer consists of a linear equalizer, a Volterra-series-based nonlinear equalizer, and a decision-directed least mean squares equalizer to simultaneously mitigate the linear and nonlinear distortions of the VLC system. A commercially available red-blue-green-yellow light-emitting diode (RBGY LED) is utilized for four-wavelength multiplexing. By the hybrid equalizer, an aggregate data rate of 8 Gb/s is experimentally achieved over a 1-m indoor free-space transmission with the bit error rate (BER) below the 7% forward error correction (FEC) limit of 3.8 × 10 ::: -3 ::: . To the best of our knowledge, this is the highest data rate ever reported in high-speed VLC systems.
---
paper_title: Multiband Carrierless Amplitude Phase Modulation for High Capacity Optical Data Links

paper_content:

Short range optical data links are experiencing bandwidth limitations making it very challenging to cope with the growing data transmission capacity demands. Parallel optics appears as a valid short-term solution. It is, however, not a viable solution in the long-term because of its complex optical packaging. Therefore, increasing effort is now put into the possibility of exploiting higher order modulation formats with increased spectral efficiency and reduced optical transceiver complexity. As these type of links are based on intensity modulation and direct detection, modulation formats relying on optical coherent detection can not be straight forwardly employed. As an alternative and more viable solution, this paper proposes the use of carrierless amplitude phase (CAP) in a novel multiband approach (MultiCAP) that achieves record spectral efficiency, increases tolerance towards dispersion and bandwidth limitations, and reduces the complexity of the transceiver. We report on numerical simulations and experimental demonstrations with capacity beyond 100 Gb/s transmission using a single externally modulated laser. In addition, an extensive comparison with conventional CAP is also provided. The reported experiment uses MultiCAP to achieve 102.4 Gb/s transmission, corresponding to a data payload of 95.2 Gb/s error free transmission by using a 7% forward error correction code. The signal is successfully recovered after 15 km of standard single mode fiber in a system limited by a 3 dB bandwidth of 14 GHz.
---
paper_title: High-Spatial-Diversity Imaging Receiver Using Fisheye Lens for Indoor MIMO VLCs

paper_content:

This letter proposes an imaging receiver scheme for an indoor multiple-input-multiple-output (MIMO) visible light communication (VLC), in which a fisheye lens with ultrawide field-of-view is used for high-quality imaging, so it can realize omnidirectional receiving and provide high-spatial diversity for decoding of the MIMO signals. In addition, the fisheye lens projects planar and small-sized images, which means that the integration with compact planar receiver array is feasible. Using the polynomial projection model, the optical intensity on the receiving plane is obtained, which is in accordance with the experimental result and shows that the images of the light-emitting diodes are clearly separated. The simulation results indicate that low correlations of the channel matrix are achieved, so high spectral efficiency is realized with various receiver positions under indoor circumstance. Consequently, this fisheye-lens-based imaging receiver is a potential candidate for high-performance indoor MIMO VLC applications.
---
paper_title: Enhanced Performance of a High-Speed WDM CAP64 VLC System Employing Volterra Series-Based Nonlinear Equalizer

paper_content:

The light-emitting diode nonlinearity in visible light communication (VLC) systems is considered to be a major problem that deteriorates system performance. In this paper, we experimentally demonstrate a high-speed WDM CAP64 VLC system employing a Volterra series-based nonlinear equalizer to mitigate the nonlinear effect. A modified cascaded multimodulus algorithm (M-CMMA) is utilized to calculate the error function and update the weights of the nonlinear equalizer without using training symbols. An aggregate data rate of 4.5 Gb/s is successfully achieved over 2-m indoor free-space transmission with a bit error rate (BER) below the 7% forward error correction limit of 3.8 × 10-3. With the Volterra nonlinear equalizer, the Q factor of the VLC system is 1. 6 dB better than that without using the nonlinear equalizer, and the transmission distance is also increased by about 110 cm at the BER of 3.8 × 10-3. To the best of our knowledge, this is the first time that the Volterra nonlinear equalizer is utilized for high-speed carrierless amplitude and phase (CAP) modulation-based VLC systems.
---
paper_title: High gain, wide field of view concentrator for optical communications.

paper_content:

The field of view and gain of optical concentrators used within free space optical communications systems are constrained by conservation of etendue. In this Letter, consideration of the processes in a fluorescent concentrator leads to a simple design strategy for these concentrators for this application. Significantly, because fluorescent concentrators do not conserve etendue, this can lead to concentrators with wider fields of view and higher gains. A model of a fluorescent concentrator containing a quantum dot material suggests that it could have a gain 50 times higher than an etendue conserving concentrator with the same field of view.
---
paper_title: Compensation of a VLC Phosphorescent White LED Nonlinearity by Means of Volterra DFE

paper_content:

In this letter, a visible light communication link is analyzed that employs a white phosphorescent light emitting diode (LED) as transmitter and a p-i-n photodiode combined with optical filter as receiver. Four- and eight-level pulse amplitude modulation is used for data transmission. The link is not only limited by a rather small bandwidth of ~20 MHz of the LED, but also its nonlinearity becomes a problem when the modulation index exceeds ~60%. It is shown that a receiver with a decision feedback equalizer (DFE) with nonlinear Volterra feed-forward section up to the second order can efficiently compensate effects of nonlinearity of the transmitting LED and performs up to 5 dB better in terms of optical power than using a standard DFE.
---
paper_title: High-Speed Integrated Visible Light Communication System: Device Constraints and Design Considerations

paper_content:

Visible light communications (VLC) has the potential to play a major part in future smart home and next generation communication networks. There is significant ongoing work to increase the achievable data rates using VLC, to standardize it and integrate it within existing network infrastructures. The future of VLC systems depends on the ability to fabricate low cost transceiver components and to realize the promise of high data rates. This paper reports the design and fabrication of integrated transmitter and receiver components. The transmitter uses a two dimensional individually addressable array of micro light emitting diodes ( $\mu\text{LEDs}$ )and the receiver uses an integrated photodiode array fabricated in a CMOS technology. A preliminary result of a MIMO system implementation operating at a data rate of 1 Gbps is demonstrated. This paper also highlights the challenges in achieving highly parallel data communication along with the possible bottlenecks in integrated approaches.
---

<form>
Title: Spatial and wavelength division multiplexing for high-speed VLC systems: An overview

Section 1: INTRODUCTION
Description 1: Introduce the motivation, importance, and recent developments in the field of Visible Light Communication (VLC) systems, particularly focusing on spatial and wavelength division multiplexing.

Section 2: SPATIAL MULTIPLEXING/MODULATION
Description 2: Provide an overview of spatial multiplexing techniques in VLC systems, exploring approaches such as ganging, MIMO, and modulation techniques to improve data rates and system performance.

Section 3: Ganging Approach
Description 3: Discuss the ganging approach in VLC systems where multiple LEDs transmit identical information to enhance signal-to-noise ratio (SNR) and coverage.

Section 4: MIMO Approach
Description 4: Describe the MIMO approach in VLC systems, focusing on spatially superposed modulation, space shift keying, spatial multiplexing, and associated optics and decoding algorithms.

Section 5: WAVELENGTH DIVISION MULTIPLEXING (WDM)
Description 5: Examine the WDM technique for VLC, explaining how different optical wavelengths are used to carry independent data streams and its impact on system capacity.

Section 6: CURRENT STATUS AND FUTURE OUTLOOK
Description 6: Summarize the current status of VLC systems, highlighting key advancements, challenges, and potential future directions for achieving higher data rates and more reliable communication.

</form>