Li-Fi Technology Continues to Break Through the Progress of Visible Lighting Communication to Commercialization

In the past ten years, Light-Fidelity (Li-Fi) technology for free space lighting and communication, and process technology and mass production of gallium nitride (GaN) blue light and laser diode components have been proposed by Professor Herald Haas and others in the UK. Under the vigorous development of commodities, it can be seen that in a few short years, lighting communication has become a key development topic for the enthusiastic discussion of the industry, academia and academic circles. In the near future, Li-Fi will inevitably become an indispensable and revolutionary technology in smart home life because of its dual use of lighting and communication integration.

Expand the scope of application Li-Fi/Wi-Fi complement each other

Li-Fi is a technology that can complement each other with wireless communication (Wi-Fi) with the highest degree of spatial freedom and optical fiber communication network with the highest bit rate capacity. In the future, it is a special space for indoor or mobile vehicles or underwater. Terrain or electromagnetic shielding environment can have its development prospects. In addition, according to the latest research on intelligent service technology, the market value of GaN blue light-emitting diodes applied to the automatic vehicle light source will reach more than one billion US dollars, and it is expected that the future light will be the annual growth of the rear light module before the automatic vehicle will be More than 10%, and light-emitting diode car lighting in the next few years with the gradual increase in penetration will create considerable profits.

However, these assessments are based solely on the automotive lighting segment and their estimates do not include the potential production value of visible light communication for vehicles. In today's transportation systems, traffic signals such as traffic lights are limited to giving the driver a visual message and thereby achieving traffic flow control. However, such systems are not enough today to give drivers enough information such as navigation, traffic and safety messages. Especially in the future development and mature promotion of unmanned automatic vehicles, it is very important to prevent collisions and ensure the safety of the surrounding environment parameters to quickly monitor the operation of the system.

Visible light communication blessing traffic safety add force

The faster the vehicle speed, the shorter the tolerance time and distance sensed by the system. Today's safe communication sensor technologies for automatic vehicles include ultrasonic, microwave short-range radar and video recognition technologies. In order to escape the traditional framework, the concept of the Vehicle Information and Communication System (VICS) was proposed in 1996. The main concept is to use the infrared light emitted by the optical signal source set beside the road to detect Automatic vehicles traveling on the road and real-time traffic information in order to control the traffic flow in the first time. However, the reason why VICS has not been put into practical use till now is that a large number of optical beacons will generate huge amounts of public transportation system construction costs.

In order to realize a more practical and low-cost sub-era intelligent transportation system, it has been pointed out that the use of light-emitting diodes instead of traffic lights can simultaneously establish the feasibility of visual communication and visible light communication. The system uses a green light-emitting diode to carry Binary Phase Shift Keying (BPSK) direct-tuning data for the transmission source, achieving a 1 Mbps transmission rate and an angular deviation tolerance range of 5o and a bit error rate of 10-6. .

Subsequently, in 2009, a new road-to-vehicle (R2V) visible light communication system was proposed, which transmitted Quadrature Phase Shift Keying (QPSK) and reached 60 meters 1Mbps and 40 meters. 2 meters of free space visible light transmission.

In order to break through the limitation of one-way information transmission, in 2008 there was a study that proposed the concept of Inter-Vehicle Communication (IVC), using current network architectures such as Wi-Fi and Bluetooth as vehicles for inter-vehicle communication. Meet the needs of a large number of communication between automatic vehicles such as shop positioning, flow control, navigation information and driving safety.

Related Research To achieve automatic vehicle-to-vehicle IVC, headlamps have been used in combination with positioners to demonstrate a 100 Mbps visible light transmission system. In 2013, more research teams used LED headlamps of automatic vehicles to carry Pulse Position Modulation (PMM) data format to achieve 10 kbps and 20 meters of visible light transmission. In addition, in 2014, it was confirmed that when the LED headlights were offset within 0.2 to 0.4 meters, the constructed C2C visible light transmission system still had a transmission distance of 20 meters and transmission. The performance is 2Mbps.

In order to achieve an intelligent transportation system, a combination of vehicle information communication and automatic vehicle-to-carriage communication is a potential solution. It achieves multi-party message transmission and exchange by using traffic signals near the road and headlights and positioning lights of automatic vehicles. This provides applications such as workshop positioning, flow control, navigation information and driving safety. However, under such concept, the speed of the automatic carrier must affect the time that the receiving end can capture data. For example, the high-speed movement of the automatic carrier will cause the receiving angle to rapidly shift and cause the receiving end to fail to completely receive the information.

Obviously, how the visible light communication system between mobile carriers improves the information transmission capacity per unit time is a great challenge at the present stage. The simultaneous development of visible light illumination communication sources that can tolerate high-speed movement without sacrificing the data transmission system rate will become One step research hotspot. Therefore, laser ranging and communication sensing technologies have also recently been evaluated and applied, especially because of the high image resolution depth that can be achieved by using high-modulus variable bandwidth GaN blue laser diodes, and their future and GaN. The single crystallizing process compatibility of the blue phototransistor will allow the imaging sensing speed to be improved by an order of magnitude or more over the same module using a silicon transistor driver.

Therefore, the gallium nitride blue laser diode is used as a light source to develop the communication and sensing technology of the next-day Vehicle-to-Vehicle (V2V), and its high-speed transmission performance is optimized to avoid collisions and blind spots. Inspection systems may be quite feasible with products that optimize imaging speed and image depth. If the required information is assigned to the multi-task and multi-task distribution, a visible light communication intelligent transportation system can also be established to control the movement of the automatic vehicle in real time and retrieve information such as speed, location, direction of travel, and driving conditions, thereby reaching the next era. The vision of driving an automatic vehicle makes the traffic environment safer and more convenient. The concept is shown in Figure 1.

Li-Fi Technology Continues to Break Through the Progress of Visible Lighting Communication to Commercialization

Figure 1 Li-Fi system with D-wave multi-tasking PON to construct an intelligent transportation system. Data source: SMF: Single-mode fiber. Sweet Home 3D, Copyright(c) 2005-2015 Emmanuel PUYBA RET/eTeks

Based on the above-mentioned requirements for application technologies such as intelligent residential lighting communication or automotive lighting communication sensing, the demand for research and development of visible light illumination and communication Li-Fi light sources and related signal processing modules is unstoppable. In theory, the key to building a Li-Fi system is to seek a source of visible light that provides both long-term lighting and high-speed transmission. The current mainstream product of white light illumination source is LED, which is widely used in public construction and information products as a standard light source due to its high brightness, low power consumption and long life. Therefore, the use of light-emitting diodes to build Li-Fi systems has long been considered a universal solution with creative and long-term development value.

In order to realize white light-emitting diodes that can simultaneously provide illumination and data transmission, the current research mostly uses red, green and blue light-emitting diodes to form a white light source or a yellow phosphor (Yellow Phosphor) to convert the blue light-emitting diode output into a white light source. Basically, the white light illumination light source generated by the GaN blue light emitting diode plus the color conversion phosphor can reduce the light source complexity and the system cost more than the white light generation technology using the red, blue, and green three-color light emitting diodes. However, the conventional quantum light-emitting diodes used in the two have limited internal quantum efficiency and light extraction rate due to the internal reflection effect of the component interface, so that some photons excited by the active layer are confined to the inside of the component, resulting in limited output optical power.

To overcome this limitation, an array of micro-LEDs (μLED) with high internal quantum efficiency, light extraction rate, and modulation bandwidth was proposed.

Li-Fi light source potential shares miniature LED array debut

Since 2017, there have been new developments in the technology of light-emitting diodes and laser diode (LD) components. Since the size of light-emitting diodes has been reduced to micro-light-emitting diodes, the response speed has become faster, the modulation bandwidth has become larger, and LD has grown differently. Coherence performance optimization of the component pointing to the substrate results in narrowing of the laser output line width and widening of the direct modulation bandwidth.

All of the above developments will contribute to the improvement of the overall transmittable bit rate capacity of visible light communication. In the aspect of hardware technology, the bandwidth of the light source and the fluorescence conversion time of the fluorescent powder should be shortened while taking into account the requirements of the lighting performance. Xianyin is the key to white lighting Li-Fi having sufficient downlink bit rate for initial commercialization and popularization. However, it is unavoidable that the required energy transition of the carrier during the use of the fluorescent color conversion technology will still relatively reduce the modulation bandwidth of the blue light emitting diode due to a certain relaxation time, thereby limiting the transmission of the Li-Fi system. capacity.

Fortunately, in recent years, the academic community has also made important progress in the development of transfer wavelength fluorescent materials. Fluorescent materials that can be mass-produced and have a short-lived period of several nanoseconds have been available. These high-speed components and materials mentioned above will have an organic increase in the transmission speed of white light Li-Fi. If a blue light diode is used instead of a white light source formed by a light emitting diode and phosphor, there is no problem in terms of bandwidth, but it is necessary to consider how to improve the lumen efficiency, control the color rendering, and control the residual blue light so as not to harm the human eye. If ultraviolet wavelength laser diodes are used with phosphor technology, the amount of residual light in the blue-sensitive region of the human eye can be avoided to reduce the blue light damage. Secondly, the bandwidth of the yellow-green region after wavelength conversion can be adjusted to improve the color rendering. help.

In addition to the existing GaN blue laser diodes, Sumitomo Corporation has even developed a green-light side-emitting laser diode for this future market value. The wavelet multitasking technology is also considered as an effective solution for constructing high-speed visible light communication transmission systems. In the current research, the multi-tasking multi-tasking visible light transmission system mostly uses the hybrid form of red, blue, and green light sources as the transmission light source, which can not only achieve the three-carrier sub-wave multi-task transmission but also provide white light illumination. In order to realize the split-wave multi-task visible light transmission system, research work was first carried out in 2011 to construct a split-wave multi-task visible light transmission system with red, blue and green light-emitting diode arrays of wavelengths of 700, 530 and 470 nm [8], and to use discrete multi-carrier modulation. (Discrete Multitone, DMT) as a modulation format, the transmission error rate is 2×10-3, and its value is less than the error rate of 3.8×10-3 as defined by Forward Error Correction (FEC). .

In order to reduce the construction cost of sub-wave multi-tasking visible light transmission systems to increase the possibility of commercialization, in 2012, some research teams proposed the use of commercially available 671-nm red and 532-nm green laser pens to construct dual-wavelength subwave multi-tasking visible light transmission. System [9] and use the NRZ-OOK data format. At the receiving end, with the help of the preamplifier and adaptability filter, a free space transmission of 10 meters was successfully performed, and the bit error rate was less than 10-9 at each wavelength reaching a transmission rate of 500 Mbps.

In addition, current commercial systems cannot increase their bit rates within the limited bandwidth of the same light emitting diode or laser diode due to the use of conventional digital signal formats with lower spectrum utilization. One of the important breakthroughs in the future is the need to apply the high-bandwidth bit format signal transceiving circuit and module technology that is currently in the development of the laboratory and widely used in wireless networks to the Li-Fi commercial module. Promote Li-Fi to complement and complement the current 4G or even the next phase of 5G wireless network technology.

At present, in the white light illumination Li-Fi system, the usage rate of the adjustable variable spectrum is increased to increase the total communication bit rate capacity, and the carrierless amplitude phase and the multi-ary quadrature amplitude modulation-orthogonal frequency division multi-task (QAM- OFDM) as a modulation format. With the above cross utilization of various hardware and software technologies, Tsonev and his research team used the blue micro light-emitting diodes for the first time in 2014 to conduct visible-light transmission of OFDM at a bit rate of 3Gbps, which can be achieved when the free-space transmission distance is 5 cm. Bit rate < 2 x 10-3 transmission performance. In order to further increase the transmission capacity and distance of white light illumination Li-Fi systems, the use of blue laser diodes to replace blue light emitting diodes has a freely usable high direct modulation bandwidth (~GHz), is immune to electromagnetic waves, and has a low transmission loss in the atmosphere The advantage is that the blue-ray laser to white light lighting Li-Fi has a very high potential to become the main axis of visible light wireless communication in the second era.

Therefore, Watson and his research team used non-return-to-zero (NRZ) On-Off Keying (OOK) with wavelengths of 422 and 450 nm to achieve 2.5 Gbps in 2013. Free space transmission, in addition, Chi and his research team built a 16-QAM OFDM data link with a transmission capacity of 9Gbps ​​and a low transmission error rate with a GaN blue laser diode with a center wavelength of 450nm in 2015. Free space transmission at 3.6×10-3 and distance up to 9 meters.

These studies demonstrate the feasibility of using GaN blue LEDs/laser diodes in Li-Fi systems to simultaneously achieve white light illumination and visible light wireless communication. In order to further provide white light illumination, research colleagues first used the commercially available red, blue and green white light-emitting diodes to construct a split-wave multi-task visible light transmission system in 2013, and adopted the OFDM and CAP modulation formats. In addition, they also introduced pre-compensation. And discriminating feedback equalizer (Decision Feedback

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