At present, Wi-Fi, as the main technology of wireless data transmission, utilizes radio frequency signals. As the demand for wireless Internet continues to grow, the available RF spectrum bandwidth is becoming less and less. The visible spectrum is not utilized, and its spectral width is 10,000 times that of the RF spectrum.
Light-fibre wireless communication (referred to as LiFi (Light Fidelity), is a new wireless transmission technology that uses visible light spectrum for data transmission, and has great development prospects. Visible light wireless communication uses wireless signals to transmit information wirelessly, encoding information in light at different rates. Due to the luminous intensity of the LED, the human eye does not notice a rapid change in light.
As long as a tiny chip is implanted in the lamp, it can become a device similar to an AP (WiFi hotspot), enabling the terminal to access the network at any time to achieve communication needs. From the available data, visible light communication technology is in its infancy, and LiFi has its unique advantages, but there is inevitably a problem that needs to be overcome in the future. This paper introduces the key technologies, their advantages and application fields of visible light wireless communication, analyzes the relationship between LiFi and Wifi, and finally discusses the problems and development trends that need to be overcome.
1 key technology
1.1 High-speed modulation drive circuit design
Modulation bandwidth is the main parameter to measure the LED modulation capability, which is related to the data transmission rate of LED in wireless optical communication. It is defined as the frequency corresponding to the _half-time (-3dB) of the _low-frequency reference frequency value when the modulation output is constant. From the microstructural analysis, there are two factors that affect the high-speed modulation of white LEDs: carrier lifetime and junction capacitance. Due to the limitation of the two LEDs, the highest frequency of modulation is usually only a few tens of MHz, which limits the application of LEDs in high bit rate systems . However, by properly designing and optimizing the drive circuit, LEDs can also be used in high-speed communication systems. Due to the simplicity of implementation, VLC systems are mostly designed as light intensity modulation/direct detection systems.
Guo Qian and others from Beijing Institute of Technology have designed avalanche photodiode (APD) detection circuit components for the application requirements of indoor visible light communication systems, as shown in Figure 1. The APD signal receiving subsystem has the characteristics of high gain, low power consumption, good reliability, small size, strong receiving ability for weak signals and long communication distance. It has high practical application value in the field of indoor visible light communication. Wang Ruiwei 0 proposed a low-cost integrated receiver production scheme, and proposed a new two-dimensional photodetection array preparation method based on wire bonding technology. Based on the research of indoor visible light communication system, Li Lianguan 0 designed a receiving circuit with _ fixed gain bandwidth, as shown in Figure 2. According to the modulation method, the drive circuit selects the current source to drive. A reasonable drive circuit design can maximize the output optical power and transmission distance to improve system performance. Shang Jianrong, Xi'an University of Posts and Telecommunications, studied the analog and digital modulation drive circuits for the current low-frequency modulation frequency, large power consumption and short transmission distance of visible light communication. A drive circuit based on DD311 was proposed, which can support multiple High power LED serial application.
1.2 white light led light source layout design
The luminous intensity and luminous power of a single LED are relatively small. In order to achieve both the indoor lighting and communication functions, the LED illumination source should be designed as an array of multiple white LEDs. In order to meet the basic lighting needs, the distribution of indoor illuminance should be considered first in the system design. In order to optimize the communication effect, it is necessary to make the distribution of light intensity in the room substantially unchanged according to the size of the room and the different layout of the indoor facilities, and try to avoid the occurrence of blind spots.
Due to the influence of the multipath effect caused by the distance difference between the transmitting end and the receiving end of each LED on the communication system, Zhang Li et al. of Jiangsu University proposed to use the guard interval Orthogonal Frequency Division Multiplexing (OFDM) modulation method to suppress the multipath effect. The impact on communication performance, the results show that OFDM signals with guard interval have strong anti-multipath effect capability. It is therefore easy to see that OFDM modulation is suitable for use in high speed indoor visible light communication systems. Qi Jingfeng 63 proposed the relationship between the layout design of the illumination source and the received optical power distribution, and analyzed the inter-code crosstalk caused by the signal-to-noise ratio and the multipath effect in the indoor VLC system with the direct detection method of intensity modulation, and proposed to use the optical diversity reception technology to overcome the code interval. Crosstalk and improve the signal to noise ratio and give a model of the photodetector array layout. Cao Jianian7 designed and studied the angle diversity system suitable for LED lighting communication system for background light noise and multipath effect, and compared the three main diversity combining methods. The MRC method showed the best performance. Ding Deqiang 8 On the basis of a kind of white LED illumination model with high similarity, the design method of multi-white LED array element transmitting antenna for indoor visible light communication system is given. Ding Deqiang also analyzed the relationship between layout design and received optical power distribution, and gave an optimal layout design of four communication sources in the room. LED lighting space layout, air environment, etc. will also have an impact on communication performance. In order to improve communication efficiency, it is necessary to make a reasonable choice of the number of LED lamps, spatial layout and brightness. How to carry out reasonable LED layout optimization and establish perfect visible light communication is the next big problem. The above research results have laid a good foundation for further research.
1.3 channel coding technology
Digital signals are inevitably subjected to various noise interferences during transmission, resulting in bit errors in the transmitted data stream, thereby enabling reception.
Anomalies occur at the end, such as image jumps, discontinuities, mosaics, and the like. The channel coding technology performs corresponding processing on the data stream, so that the system has _determined error correction capability and anti-interference ability, improves data transmission efficiency, reduces bit error rate, and finally improves data communication distance.
Chen Changyu and Zhao Jun of Jinan University proposed a mBnB block coding technology suitable for LED digital transmission. Generally speaking, the block code refers to grouping the original information code words in units of m bits, and using another code word of n bits per group according to the _determination rule, and then these new packets are in NRZ code or RZ code. Format to transfer. Commonly used channel codes are 182B (Manchester code), 384B, 586B, 688B, and the like. The advantages of the mBnB code are as follows: 1 the power spectrum shape is better; 2 〇 〇 1 is limited, there is no baseline drift problem; 3 provides reliable error detection and word synchronization means. The experiment proves that after 688B encoding, the optical signal is not affected by the number of LEDs, the resistance and the crossover of the serial port module in the communication distance r=0.5~2.5m. Using 688B encoding technology, it can ensure the high-speed transmission of data in this system, and the signal transmission distance exceeds 2.5m. Moreover, the disadvantage of increasing the error value of the mBnB code when decoding the mBnB code can be overcome by adopting two different code tables for the data. As shown in Figure 3, a 12-bit raw data is taken as an example to introduce the 688B encoding implementation process.
1.4 Orthogonal Frequency Division Multiplexing (OFDM) technology
Orthogonal frequency division multiplexing is a type of multi-carrier modulation. The main idea is to divide the channel into several orthogonal sub-channels, convert the high-speed data signals into parallel low-speed sub-data streams, and modulate them to transmit on each sub-channel. The orthogonal signals can be separated by using correlation techniques at the receiving end, which can reduce the mutual interference ISI between the subchannels. The signal bandwidth on each subchannel is smaller than the associated bandwidth of the channel, so that each subchannel can be seen as flatness fading, thereby eliminating intersymbol interference. And since the bandwidth of each subchannel is only a small fraction of the original channel bandwidth, channel equalization becomes relatively easy.
Xiong Feijun's research shows that the visible light data transmission system based on white LED and SC^FDMA technology is technically feasible. In terms of peak power ratio, the low PAPR characteristics of SC^FDMA not only avoid distortion of the LED transmission system. It has better performance than OFDM systems and also protects eyes &1. Based on OOK, M-PPM, DPPM and OPPM modulation structures, Li Shiliang of Harbin Engineering University proposed differential overlapable pulse position modulation (DOPPM), and compared the illumination power and transmission rate of each modulation method. The results show that the transmission rate of DOPPM is significantly higher than other modulation and coding modes while maintaining high illumination power. M 1.5 Optical Code Division Multiple Access (OCDMA) technology
Optical code division multiple access technology 141 is used to distinguish information of different users. In the visible light wireless local area network, all end users share the same main light source, so different user signals must have different characteristics, so that the different user signals can be separated when the adapter receives. OCDMA assigns each user a separate address code, and the digital signals are encoded on their respective address codes and decoded at the receiver by the corresponding sequence. The use of OCDMA technology can also greatly improve the system's anti-noise capability. Signals can be detected from very noisy environments.
Fang Anan et al. based on optical code division multiple access (OCDMA) technology, uses optical orthogonal code (OOC) as the address code to encode the signal. The simulation results show that the improved system error rate significantly reduces M. Zhang Baofu et al. studied the error performance of optical code division multiple access (OCDMA) system. The analysis shows that the multi-user fiber CDMA system is a system with elastic capacity. When the number of users is small, the system error performance is mainly affected by its receiver. The influence of noise, when there are many users, is mainly determined by the multiple access interference between users.
1.6 diversity receiving technology
The diversity receiving technique is proposed to improve the signal-to-noise ratio of the VLC system and overcome the influence of inter-symbol interference in high-speed communication. The idea of ​​diversity reception is to install multiple photodetectors in different directions at the receiver, compare the signals received by multiple detectors, and select the signal with the largest signal to noise ratio for communication.
In the diversity receiving system, two key tasks are: the way the signal is selected and the layout of the photodetector. In the selection of signals, for low-rate white LED communication systems, the signals received by multiple detectors are directly added by _ adders, and then the added signals are sent to the receiver for filtering and solution. and decoding processing such as modulation, greatly improving the signal to noise ratio; communication system when the transmission rate is higher than 100Mb / s, due to the influence of intersymbol interference, the signal can not be added up, the control circuit must be designed special automated channel The principle of judgment and choice is shown in Figure 4.
The voltage signals converted by each detector are collected and sampled in real time, and then sent to a voltage comparator for comparison to find the channel with the largest voltage value, which is the channel for communication transmission. At the same time, the comparator output control signal strobes the corresponding channel. For the layout of the photodetector, a plurality of photodetectors are installed in different directions of the receiver and uniformly distributed on the hemispherical surface, thereby reducing the number of detectors and improving the receiving effect. Thus, communication is not interrupted as long as the entire receiver is not blocked. The number and layout of the detectors need to be determined according to the specific environment and communication performance requirements. By adopting diversity reception technology in high-speed communication, the signal-to-noise ratio of the system is improved by 2dB on average, and the influence of receiver position change, indoor walking and object shadow on the communication system is effectively overcome.
2 other research progress
Hainan Lu et al. proposed a LiFi transmission system based on a two-stage injection drive system that locks a 680 nm vertical cavity surface emitting laser (VCSEL) transmitter with higher data rates, providing a communication link for accelerating the deployment of visible laser communications. Favorable conditions & 4. Emilie Bialic et al. introduced the signal-to-noise ratio response of two different types of PV modules in frequency, and analyzed the LiFi performance under different lighting conditions, demonstrating that the available bandwidth is highly dependent on the ambient lighting configuration. Under certain lighting conditions, the 4 MHz bandwidth can achieve a corresponding data rate of 8 Mbit/s. In addition, the illumination saturation effect was introduced and the study of translucent solar cells was demonstrated to improve bandwidth and data rate performance under high ambient lighting conditions. Cheng Ling Ying et al. proposed a 20Gbps LiFi transmission system using vertical cavity surface emitting laser (VCSEL) and 16 QAM (quadrature amplitude modulation) external optical injection technology - Orthogonal Frequency Division Multiplexing (OFDM) modulation signal method Good bit error rate (BER) performance and clear constellation M can be obtained. Jiun-Yu Sung et al. proposed an Orthogonal Frequency Division Multiple Access (OFDMA) system based on visible light communication (VLC), discussed the user location monitoring strategy to solve the problem of regional division, and analyzed the characteristics of the system in detail. . Zhong Zheng et al., State Key Laboratory of Communication Systems and Networks, Peking University, proposed a free-space optical communication and visible light communication integrated system for indoor wireless broadband access. The system has an all-optical link and a high-capacity M. Borja Genoves Guzman et al. proposed a new collaborative transmission and reception scheme for the field of visible light communication (VLC). The solution improves the reliability of large indoor scenes such as corridors, laboratories, shops, conference rooms, and coverage requires different VLC access points 1]. Shih-lao Chen et al. first proposed and demonstrated a visible light wireless communication system that uses indoor light-emitting diodes, mobile phone cameras and photodetectors to support single-input and multi-output quality of service. The proposed system can support 2 fixed rate quality of service transmissions M for 2 different applications. Chen et al. proposed a wireless image transmission system based on visible light communication (VLC). The system consists of OV7670 image acquisition module, STM32, FPGA-based coding module, modulation drive circuit and LED. Reverse pulse position modulation (IPPM) has the advantages of high average power and large signal-to-noise ratio. Univ Sheffield Ravinder Singh et al. proposed a new color-keyed (CSK) wireless visible-light communication modulation format that replaces the three colors 122 used in the existing IEEE 802. 15. 7 CSK physical layer standard based on four colors. Liang-Bi Chen et al. proposed a wireless digital conference system that uses dual-mode (white LED and infrared) visible light communication (VLC) wireless technology. The downlink communication uses white LED transmission, and the uplink communication selects infrared transmission M. Tadahiro Wada et al. constructed a new parallel transmission visible light wireless communication system (VLC) M using color keying (CSK). Yiguang Wang, Department of Communication Science and Engineering, Fudan University, proposed and experimentally verified a new type of high-speed LED-based VLC LAN, providing wireless optical access over 10Gb/s star-based topology for a large number of users. Sung-Man Kim of Qingxing University in South Korea proposed and demonstrated a new wireless visible light communication (VLC) technology that uses optical beamforming to improve signal-to-noise ratio (SNR) and transmission distance. Thomas Q.
Wang et al. use a hemispherical lens at the receiver that has a broad field of view and provides a significant diversity of MIMO wireless light applications27. Liwei Ding of the Communication Technology Research Institute of Nanjing University of Posts and Telecommunications reported that a wireless access system passes the visible light communication (VLC) technology M. In Hwan Park et al. analyzed and simulated a visible light communication interference cancellation scheme for aircraft wireless communication environments, using a continuous interference cancellation (SIC)-based interference cancellation scheme and evaluating the bit error rate performance (4). Sang Burm Ryu introduces the LED nonlinear transfer function into the OFDM system, reduces the PAPR (Peak to Average power ratio) and suppresses the ICI in the receiver's frequency domain, and proposes a new PAPR-reduction technique to reduce the nonlinear distortion of the LED, and An adaptive inter-carrier interference suppression algorithm to improve the bit error rate performance M. Keio Univ, Toshihiko Komine et al. proposed an adaptive equalization system to overcome the crosstalk between codes, and clarified the most effective training sequence interval 61 for channel estimation in the mobile environment. It proves that the adaptive equalization system with effective spacing mitigates the effects of shadows. . A link M based on white light emitting diode (LED) and photodetector (PD) visible light communication (VLC) is proposed.
3 Advantages and disadvantages in application
Visible light wireless communication can directly use the existing energy consumption output tools such as street lights, indoor lighting and public lighting to accomplish dual tasks. Compared with wireless transmission technology, it has its own advantages such as low radiation, low energy consumption and low carbon environmental protection. For example, mobile phones, millions of base stations around the world help them enhance their signals, but most of the energy is consumed by cooling, which is inefficient. The advantages of LED's own low power consumption occupy a huge market, laying the foundation for the popularity of LiFi. Adding a microchip to the LED bulb turns the bulb into a wireless network transmitter.
Therefore, the visible light communication system can be applied indoors. The indoor visible light wireless communication is composed of a terminal, a visible light communication adapter, a visible light communication hub, a white light LED light source, a photodetector and a corresponding signal processing unit. The system is divided into two parts: the forward link and the reverse link. Each part includes the transmitting and receiving parts. The transmitting part is mainly composed of a white LED light source and a corresponding signal processing unit, and the receiving part is mainly composed of a photodetector and a corresponding signal processing unit. The visible light wireless hub receives information from the end user while simultaneously transmitting the received information through the primary light source in a broadcast manner. The visible light communication adapter is responsible for modulating the information of the end user into an optical signal and receiving an optical signal from the reverse link. A photodetector mounted on the ceiling receives the optical signal from the user and converts it into an electrical signal for transmission to the visible light communication hub. After the electrical signal is simply processed by the visible light communication hub, it is modulated into a white LED light source and becomes an optical signal, which is transmitted in a broadcast manner. At the receiving end, the visible light adapter of the terminal demodulates the information sent to itself and sends it to the end user, realizing indoor visible light wireless communication based on the VLC system. For example, Ding Deqiang of Xi'an University of Technology proposed a wireless LAN system based on visible light communication, as shown in Figure 5. According to the system, it can be a potential source of LiFi data transmission in any place with light in the future, especially in radio-sensitive applications. It is also worth mentioning that future visible light communication is safer and more economical than existing WiFi. WiFi relies on invisible radio wave transmission, the power of the device is getting larger and larger, and the local electromagnetic radiation is bound to increase. Moreover, the spectrum is larger than the radio spectrum, and the network setup requires almost no new infrastructure. But it is undeniable that LiFi technology relies on visible light, so the limitations of visible light itself are bound to hinder the further development of LiFi. For example, the penetration of visible light, the refraction, reflection, scattering, etc. of various media. In addition, considering the demand for light sources and the existing mass production of LED lamps do not have the ability to implant chips called LiFi technology requirements, that is to say, LiFi has many technologies to solve in thousands of households.
4 development trends
The key points and breakthroughs of the future work of white LED wireless communication are as follows:
1) Bandwidth expansion technology for white LED light sources. LED lamps currently used for visible light communication have a limited modulation bandwidth of only about 3 to 50 MHz, which is mainly used for illumination. If you want to use it for communication, you must develop an LED light source with a higher modulation bandwidth. Since LED communication has dual functions of illumination and communication, 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 high reliability. The indoor lighting layout needs to consider the requirements of basic lighting, accent lighting, decorative lighting and emergency lighting, so the reasonable layout of the light source is especially critical for lighting and system performance.
2) Fusion technology of power line communication and VLC. If the LED lighting system is combined with the power line communication, the communication signal is transmitted through the power line, which can effectively reduce the investment cost. The PLC technology, which is a combination of power line communication technology and VLC technology, is a communication method that uses power lines to transmit data and voice signals. From the perspective of the development trend of wireless communication technology, PLC technology has increasingly shown its strong vitality and laid a solid foundation for communication.
3) Make full use of the advantages of VLC to overcome the unfavorable factors to improve the performance of VLC communication. The biggest problem facing LEDs is the data communication rate of VLC. In order to increase the communication rate, in addition to the breakthrough on the LED device, it can be upgraded by other technical means. In order to overcome the limitation of white LED modulation bandwidth, it is necessary to deeply explore the modulation multiplexing technology with high frequency band utilization and good anti-interference performance. Research high-efficiency filtering technology and new filters to improve optical communication performance. In addition, how to make better use of the space resources of indoor VLC systems and obtain higher multiplexing gains is also worth pondering.
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