In recent years, with the development of white light-emitting diode (LED) technology, Visible Light Communication (VLC) has become one of the research hotspots of next-generation wireless communication technology. VLC is also called LiFi (Light Fidelity). In 2011, German physicist Hardal Hass from the University of Edinburgh gave a speech on LiFi technology at the TED conference. For the first time, "VLC" was called "LiFi".
LiFi is an emerging wireless communication technology based on light (rather than radio waves) that combines the lighting and data communication functions of light, as shown in Figure 1. LiFi modulates the signal on the LED light source without affecting the LED illumination, and transmits the data by quickly switching a high-frequency flicker signal that the human eye cannot perceive. 
Advantages of LiFi <br> Compared with the current mainstream WiFi communication technology, LiFi has the following advantages:
(1) In terms of capacity, the spectrum of radio waves is very crowded, and the spectrum width of visible light (about 400 THz) is 10,000 times more than that of radio waves;
(2) In terms of efficiency, the efficiency of the radio wave base station is only 5%, most of the energy is only consumed by the cooling of the base station, and the data of the LiFi can be transmitted in parallel while improving the efficiency;
(3) In terms of practicality, radio waves are only acquired in the base station, and WiFi cannot be used on the aircraft, in the operating room or at the gas station, and each lamp in the world can be easily connected to the LiFi hotspot;
(4) In terms of safety, radio waves are easily invaded, and visible light cannot pass through walls or even curtains, providing privacy and security for the network.
As a new technology that combines lighting and communication, LiFi does not affect the quality and requirements of lighting, especially in the development of light sources, while pursuing high transmission rates. The light source of LiFi must have the advantages of good modulation performance, high transmission power and high response sensitivity of the communication light source, and it also needs to meet the characteristics of high brightness, low power consumption and wide radiation range of the illumination source.
LiFi light source selection
1, LED
At present, most of the light sources used in LiFi technology are white LEDs, and a large part of the reason is due to the rapid development of LED technology. The realization of white LEDs mainly includes: blue LED chip excitation yellow-green phosphor converted into white light (PC-LED), ultraviolet light or ultraviolet LED excitation three primary color phosphors to produce white light and red, green and blue three kinds of LED chips are packaged in Mix together to produce white light (RGB-LED). The commercial white LED products at this stage are mainly divided into two categories according to the different spectral components: PC-LED and RGB-LED. The spectrum of the two types of white LEDs is shown in Figure 2.
The modulation bandwidth of the LED determines the channel capacity and transmission rate of the communication system. Studying the modulation characteristics of the LED device is one of the key issues to improve the performance of the new LiFi system. The LED modulation bandwidth is defined as the frequency at which the AC light output from the LED drops to 50% of a reference frequency value (-3 dB). Since the photoelectric response of the spectral portion of the yellow phosphor of the PC-LED is relatively lagging, as shown in FIG. 3, the modulation bandwidth of the LiFi light source is limited to within a few megahertz, thereby limiting the communication rate of the entire system, even at the receiving end. The blue filter also failed to significantly improve the defects of the light source.
Therefore, more and more LiFi research turns the light source to RGB LED, which can provide higher modulation bandwidth, improve channel capacity by wavelength division multiplexing on three colors of light waves, and modulate different data in parallel transmission. At the receiving end, three colors are respectively received by the filters of the respective colors, thereby effectively improving the transmission efficiency. However, LEDs of different colors in RGB-LED have different operating temperature dependence on the output light. In order to achieve independent color point of operation temperature, it is necessary to separately control the feedback loop and drive current of each monochromatic LED. The preparation brings higher cost and complicated modulation circuits. The modulation bandwidth of the LED is limited by the response rate, which in turn is affected by the carrier lifetime. In addition to designing the modulation circuit and reducing the RC (resistance-times-capacitance) delay, the conventional method of increasing the modulation bandwidth of the device is to increase the radiation recombination rate of the electron holes and reduce the spontaneous emission lifetime of the carriers. The common carrier composite ABC model is shown in equation (1).
Where N represents the carrier concentration of the luminescent active layer in cm-3, A represents the Shockley-Read-Hall (SRH) dielectric defect composite coefficient, B represents the spontaneous emission (bimolecule) coefficient, and C represents the Auger composite The coefficient, BN2, represents the rate of spontaneous emission. The carrier spontaneous emission lifetime is reduced by increasing the injected carrier concentration, and the method of increasing the carrier concentration is to increase the injection current and the delta doping. At high currents, the concentration of injected carriers increases, so the probability of exciton recombination increases, the lifetime of radiative recombination carriers decreases, and electro-optical conversion responds quickly. Delta doping technology also enables a large number of carrier injections, which reduces carrier lifetime and achieves improved modulation bandwidth at the same current density.
The change in carrier concentration affects the internal quantum efficiency of the LED, as shown in equation (2):
Where εrad is the internal quantum efficiency. As shown in Table 1, the values ​​of A, B, and C are selected from the experimental values ​​in the literature, and the Auger composite coefficient in the conventional evaluation of the theory is four orders of magnitude smaller than the experimental results. The possible causes are impurities and phonons. Participating in the Auger recombination process as an intermediate medium makes the C value practically large. Another possibility is that the droop effect is the result of the action of carrier overflow, etc., independent of Auger radiation.
The important electro-optic characteristics that are difficult to measure by the test equipment, such as the carrier concentration-inner quantum efficiency curve, can be obtained from the carrier ABC model, as shown in FIG. In the calculation of the theoretical value, the internal quantum efficiency gradually tends to 100%, but in practice, the internal quantum efficiency of the LED device will have a droop effect that rises to the peak and then decreases, and the relationship between the output luminous flux and the injected current also has a droop effect. The modulation of the LED usually occurs in the working area, and modulation in the saturation region introduces a poor signal-to-noise ratio, so the range of the injected current is controlled.
2, LD (laser diode)
Since researchers did not meet the data transfer rates achieved by LED modulation, Professor HardalHass, the first proponent of LiFi, replaced existing LEDs with laser diodes, using the high energy and high efficacy of the laser to transmit data at a rate 10 times faster than LEDs. Laser illumination can mix light of different wavelengths to produce white light, similar to RGB LEDs. Although LED-based LiFi can achieve a data transmission rate of 10 Gb/s, which can improve the data transmission rate limit of 7 Gb/s in WiFi, the rate at which lasers transmit data can easily exceed 100 Gb/s. The latest report shows that the R&D team at the School of Electronics, Computer and Energy Engineering at Arizona State University has developed nano-scale white lasers that can be used more conveniently as LiFi sources.
In terms of communication, laser diodes have faster response speeds, direct modulation, and high coupling efficiency compared to LEDs. For a conventional electric injection semiconductor laser, when the injection current exceeds a certain value, the LD can emit modulated light controlled by the input current, and its modulation characteristics are as shown in FIG. 5. The current at this point is called a threshold current, and the threshold current is above. Until the saturation region belongs to the working area of ​​the LD, and the modulation range is preferably performed in the linear region, it is important to reduce the threshold current of the device and obtain a larger modulation working region.
The threshold current density is as shown in equation (3)
Where Jth is the threshold current density; e is the electron element charge; d is the active layer thickness; Iinj is the injection current; N' is the transparent carrier concentration; αm and αi are the mirror loss and optical absorption loss, respectively; Γg0 is Maximum mode gain; B and C are the radiation composite coefficient and the Auger composite coefficient, respectively.
Vertical-cavity Surface-Emitting Lasers (VCSELs) have the advantages of low threshold, wide modulation and high photoelectric conversion efficiency. PrincetonOptronics has developed a laser illumination module that integrates multiple VCSELs arrays. More than 650W, as shown in Figure 6. However, as the injection current increases, high-power VCSELs excite multiple transverse modes, resulting in an increase in bit error rate when the device is used in a communication source. Therefore, polarization selection is required for the exit mode of high-power VCSELs.
The light output of a semiconductor laser can be modulated directly. The most common laser output modulation is to control the current flowing through the device for amplitude modulation or pulse modulation. The modulation bandwidth of the laser diode is B < ω0, where ω0 is the resonance-like frequency, and above the threshold, the modulation bandwidth can be approximated by the formula (4)
Where τ is the carrier lifetime; τS is the photon lifetime; J is the injected current density; J is the transparent current density; σ is the spontaneous emission factor; Γ is the light limiting factor, and
Where c is the speed of light and n is the refractive index. If the spontaneous emission is neglected in the lasing region of the LD, that is, σ=0, it is found from equation (4) that the modulation bandwidth and the injection current density are positively correlated linear relations, but the actual semiconductor laser has a droop effect, and in addition to the main mode In addition, the side mode also has a strong relaxation oscillation. Therefore, in the micro volume of the microcavity, the spontaneous emission factor is large, and the σ of the ordinary laser is 10-5 to 10-4, and the σ of the microcavity laser may increase to 0.1 or even close to 1.
Although LiFi's light source can choose laser diodes, and one of the Nobel Prize winners in 2014, Nakamura Shuji is predicting that laser lighting may replace LED lighting in the future, the current mainstream lighting technology still promotes high cost performance and technology relative Mature LEDs, and for the characteristics of LiFi sources, the development of LED devices with high brightness, high efficiency and high speed modulation can drive the commercialization of LiFi technology faster.
The color of LiFi light source <br> Unlike WiFi, which only pays attention to the improvement of communication performance, LiFi lighting system must consider improving the communication performance while ensuring the quality of lighting. Therefore, LiFi's light source, whether LED or LD, is to output white light, and the color quality of white light is very important for lighting.
LED luminaire color characteristic parameters can be calculated from the spectral power distribution (SPD). SPD is a mathematical expression of the output intensity distribution relative to the wavelength of light, which can provide detailed information about the spectral components. In the LiFi system, the SPD may vary as the drive current of the LED changes. Deviated SPD can cause perceived color point drift and can affect color rendering characteristics, while special modulation techniques in LiFi are more susceptible to color quality degradation. The color quality of LiFi modulation can be evaluated by measuring the SPD deviation caused by the change in drive current with the SPD model.
However, using the SPD model to characterize the color quality of LiFi has many disadvantages: a large number of fitting parameters in the model can only be obtained through the experience of LED testing; the SPD model design is based on relatively static conditions and cannot explain LiFi under high-frequency current oscillation. The situation; it is difficult to apply an SPD model to all LED types, for example, it cannot explain the additional effects of phosphor materials in PC-LEDs. On the other hand, it is possible to detect the real-time color characteristics of LiFi under working conditions. For high-brightness LED products, LED manufacturers need to provide color data at different drive currents and modulation frequencies, such as SPD, color coordinates, and color rendering index (CRI). .
Because LiFi needs to provide flicker-free lighting services with sufficient brightness when transmitting data or idle state, LiFi devices need to have flicker removal and brightness adjustment. In the IEEE PAR 1789 "Potential Health Effects of LED Lighting Blinking (Draft)", the fluctuation depth is used to evaluate the flicker problem. The LiFi source has a modulation frequency of at least millions of times per second, so the flicker of the LiFi source is risk-free. In terms of brightness adjustment, in addition to OOK (on-off key control) and VPPM (variable pulse position modulation), there is CSK (color drift keying) adjustment.
In September 2011, the IEEE802.15.7, an international standard for visible light communication with a transmission speed of up to 95 Mbit/s, was established, and the primary task of the Standards Development Committee was to implement “Lighting First, Communication Secondâ€.
The physical layers PHYI and II in the standard support OOK adjustment and VPPM adjustment, respectively, while the physical layer PHY III uses CSK modulation to support multiple source bandwidths. The visible light is divided into 7-segment optical bands, and the different optical band ID numbers are identified by 3-bit bits. The CSK modulates the data according to the optical band ID number and transmits them in parallel on the optical waves of different wavelengths to improve the spectral utilization rate, and selects the ID of the color by the ID. Change the combination to achieve the purpose of brightness adjustment. For the LED light source, the physical layer PHYIII only works under the RGB-LED device and is suitable for short frame transmission, so the LiFi light source with CSK adjustment can select RGB-LED or RGBLD, which is suitable for indoor communication.
Light source layout for LiFi systems
With its unique advantages, LiFi can be used in a wide range of applications: intelligent lighting , vehicle transportation, hospitals, offices, aircraft, defense security, underwater communications, indoor positioning and hazardous environments (such as mines, power plants and gas stations). Especially for indoor positioning, ByteLight Corporation of the United States and Huace Optical Communication of the United States have developed an indoor positioning system based on white LEDs, which can realize one-way transmission of LiFi for indoor information push and positioning services.
However, the indoor LiFi system faces many technical problems, such as the safety of the system, if the light is blocked, the signal will be broken; LiFi's two-way data transmission problems. Professor HardalHass also believes that LiFi will not replace WiFi. For indoor communication, LiFi can be a good complement to WiFi, but LiFi has a good application space in certain places where radio waves are limited. Due to lighting and the effect of preventing shadowing effects, multiple LED lights need to be installed indoors, so the proper layout of the light source is a key factor affecting lighting and system performance.
In order to meet the requirements of indoor lighting, the layout of the light source must not only make the indoor illumination and illumination uniformity meet the corresponding standard requirements, but also contribute to the safety and comfort of people's activities. The light source should be selected for products with high luminous efficiency, suitable color temperature, long life and reliability. The lighting layout of the interior needs to consider the requirements of basic lighting, accent lighting, decorative lighting and emergency lighting.
Considering the inter-symbol interference caused by different paths in the LiFi system, the influence of indoor personnel walking and physical shadow effects on the communication system, OFDM (Orthogonal Frequency Division Multiplexing) scheme can be adopted while taking care of the LiFi communication of the key illumination part. Improve the overall performance of the LiFi system and achieve efficient use of bandwidth resources. For example, the PC-LED-based LiFi system uses OFDM modulation technology to filter out the slower response fluorescence component, expand the modulation bandwidth, and combat multipath effects for high-speed data transmission and communication, but whether such a system satisfies The uniformity of illumination has not been confirmed.
Summary <br> As a communication technology compared to WiFi, LiFi has also received more and more attention and research. Based on the requirements of LiFi's light source, this paper describes the research of LiFi light source from three aspects: current light source selection, light source color and light source layout.
In the light source selection of LiFi, the factors affecting the modulation characteristics of the LED were analyzed from the carrier injection angle of the LED device. At present, RGB-LED has a good data transmission rate relative to PC-LED, but it needs to reduce the cost and simplify the circuit design. In the case where a laser diode is used for a LiFi light source, the modulation characteristics of the semiconductor laser are analyzed from the aspect of the working area above the threshold current.
The effect of SPD model and CSK modulation on the color quality of LiFi source was analyzed on the color of LiFi light source. In the layout of the LiFi light source, not only the OFDM modulation is used to reduce the inter-symbol interference of the LiFi system, but also the data transmission rate is improved, and the uniformity of the indoor illumination should be paid attention to.
With the development of high-brightness, high-efficiency and high-speed modulation LED devices, LiFi technology based on white LEDs will become more and more mature, which will bring new development opportunities for LEDs. As Nakamura said, LiFi may become Another killer of LEDs. In addition, with the continuous advancement of laser lighting research, whether laser lighting will replace LED in LiFi technology in the future is also very worthy of expectation.
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