Realization of visible light communication on outdoor LED street lights

1 Introduction

In recent years, semiconductor (LED) lighting technology known as "green lighting" has developed rapidly. Compared with traditional illumination sources, white LEDs not only have low power consumption, long service life, small size, environmental protection, but also have good modulation performance and high response sensitivity. Using this characteristic of LED, it can be used as illumination, and can also modulate the signal onto the visible beam of LED for data transmission, realizing an emerging optical wireless communication technology, namely Visible Light Communication (VLC) technology. . Compared with traditional RF wireless technology, VLC can utilize high bandwidth, higher security and privacy, no electromagnetic interference, no license for the corresponding frequency band, and high bandwidth and high rate wireless at low cost. Communication access. It has good spatial reusability and greatly expands the coverage of the network. It is a good complement to the existing RF technology. These attractive features have made VLC a great concern worldwide.

VLC offers many applications such as LED lighting, information broadcasting and M2M (machine to machine). LED lighting can be used in office/home lighting, street lighting and automotive lighting. Information broadcasts can be applied to signage (eg billboards), offices, home lighting and street lights. M2M can be used in mobile phones to mobile phones, cars to cars, cars to traffic lights and car to street lights, and more. In the actual research direction, LED visible light communication is divided into outdoor communication and indoor communication. The research team of Keio University in Japan has proposed the LED-based visible light communication (VLC) system. At present, most of the domestic and international research directions are also in the mainstream. Focused on this, mainly including indoor positioning and navigation and high-speed network connection. Outdoor visible light communication was proposed by the University of Hong Kong G. Pang et al in 1998, and its application areas are mainly concentrated in Intelligent Transportation (ITS) and Outdoor Advertising. ITS applications mainly include: two-way communication from car to car, and two-way communication from car to transportation facility.

In recent years, the domestic LED street lamp market has accelerated its development. In order to comply with this situation, this study is mainly aimed at the realization of the visible light communication of urban LED street lamps and the basic application exploration. By selecting the appropriate optical modulation and demodulation method, the basic data of LED street lamps and The on-site VLC reading of the operating parameters verifies the feasibility and lays the foundation for the innovative intelligent management ideas and development direction of the urban LED street lamps.

2 Research content

2. 1 Composition and working principle

Figure 1 depicts a basic optical communication system consisting primarily of a signal transmitting portion and a signal receiving portion. At the transmitting end, the data encoded by the encoder is converted into a transmitting electrical signal by a signal modulation circuit, and then the LED street light source is driven to transmit the optical signal to the signal receiving end via the free-space optical path. At the receiving end, the optical signal is detected by the optical sensor and the amplifier and converted into an electrical signal. Finally, the corresponding data is obtained by the demodulation circuit and the decoder.

Figure 1 System composition

2. 2 signal modulation

Optical communication is communication in which light waves are used as carriers, that is, modulation of light waves by baseband signals. Common modulation methods include: OOK, CCM (Color Code Modulation), HHW (HighHamming Weight), VPM, R-RZ, etc. This study uses a relatively simple intensity modulation direct detection, ie IMDD, which is a non-coherent communication system. Usually the IMDD system is divided into a binary system and a multi-ary system. Here, the binary system is selected and encoded with OOK.

Due to the lighting function of the LED street lamp itself, if the OOK code is used alone, the LED street lamp will flicker during data transmission, which is unfavorable for normal illumination, so it is further optimized to the secondary modulation mode. The so-called secondary modulation is to first modulate the baseband signal to a carrier of lower frequency, the mode is 2FSK, the carrier itself is a square wave signal, and then the modulated square wave signal is used to modulate the light wave again, the mode is OOK.

For example, if you want to transmit data 1, the square wave signal f1 is output, and then the LED light switch is controlled by f1, that is, the LED light flashes at the frequency of f1. If data 0 is transmitted, the square wave signal f2 is output, and then the LED light is controlled by f2. The switch, ie the LED, flashes at the frequency of f2. Under normal circumstances, when the flicker frequency is lower than 50Hz, the human eye can recognize the flicker of the light source. When the light intensity is operated at a frequency greater than 50Hz, the response of the human eye can not keep up with the change of the light source, and most people will not be able to distinguish The light source flashes and the light source at this point will emit a steady, continuous light. For example: The human eye does not notice the flicker of fluorescent lights (100 Hz) that are 100 times per second. Therefore, when f1 and f2 select a higher frequency, the human eye cannot observe that the LED light is blinking. However, the brightness of the LED lamp will change at this time, and the degree of change depends on the duty ratio of the f1, f2 square wave.

The signal modulation circuit is shown in Figure 2. The baseband signal 1 selects the output f1 square wave, the signal 0 selects the output f2 square wave, then adds f1 and f2 to obtain the modulated 2FSK signal, and then converts the signal into LED light. The dimming signal output, in order to simplify the design, the LED lamp works in the switch mode.

Figure 2 Signal Modulation

2. 3 signal demodulation

The signal demodulation circuit is shown in Figure 3. After the changed optical signal is detected by the optical sensor, the DC and low frequency (mainly power frequency) components are filtered out by the "high-pass filter", and then amplified, and the amplified signal is output to the solution. The tuning circuit obtains the required data by coherent demodulation, low-pass filtering, and sampling decision.

According to the communication theory, reducing the bit error rate in the decision can be started from two aspects: one is to increase the input optical power; the other is to improve the signal to noise ratio. For LED street lights, because the LED light source emits visible light, and the divergence angle is large, it is basically harmless to the human eye, and there is no electromagnetic wave damage. The high power of the LED light source has guaranteed the reliability of the system to a certain extent. The result will be that the signal-to-noise ratio at the receiving end determines the communication performance of the entire system.

Figure 3 signal demodulation

2. 4 Matlab simulation (Figure 4, Figure 5, Figure 6)

Figure 4 baseband signal, inverse signal, carrier signal

Figure 5 baseband signal multiplication, 2FSK, noise signal generated in superimposed transmission

Figure 6 Coherent demodulation and sampling decision results

The above research design idea is simulated by Matlab. The baseband signal rate is 100bps. Considering the limitation of the optional components, f1 selects 800Hz and f2 is 1000Hz. The simulation results prove that the scheme is feasible.

3 test system and verification

3. 1 test system

As shown in Figure 7, the field test system consists of an LED street light with built-in optical communication controller, an optical receiver, and a smartphone device (with built-in read data display application). Data transmission between the optical receiver and the smartphone via Bluetooth ensures the convenience of field test at the street light.

Figure 7 Field Test System

In the field test, the LED street light selects a brand of high-power LED street light, which supports the supporting power supply of the PWM dimming input. The optical communication controller directly adopts the single-light controller of Shenzhen Zhihao Technology Co., Ltd., which supports PWM dimming output, and can collect real-time status data of voltage, current, active power, power factor and temperature of LED street lamp in real time, due to the scene. In the test and verification, the LED street lamp works in the switch mode, and the selected signal modulation operation rate is low. The signal modulation part will be completed by the single lamp controller internal software, and the 2FSK signal is directly output to control the LED light switch. Due to the limitations of the selected device, the baseband signal rate used in this test is 100bps, f1 is 800Hz, and f2 is 1000Hz. If PIN diode is used, it should work at a higher frequency. Of course, the corresponding circuit design will be more complicated.

3. 2 optical receiver

The optical receiver is designed to be hand-held and sized to fit the field and to be mobile. There are two key indicators for the choice of light sensor, one is the range of light intensity of the response, and the other is the response speed. Since the LED street lamp works outdoors, and the outdoor is full of various other visible light, and the ambient light brightness varies greatly during the day and night, the light sensor operating in this environment needs to have an extremely wide response range and a high range. Sensitivity. Since the LED street light flashes at high frequencies f1 and f2, the response speed of the light sensor should be higher than this frequency. This test system uses Owen Optoelectronics' ON9658, its measurement range: 0 ~ 1000lx, response time reaches Tr + Tf = 4μS (Figure 8).

Figure 8 ON9658

Considering that the street lamp only works at night, the optical receiver is only designed to work at night, and in order to avoid the influence of other visible light and adjacent street lamps on communication, the optical receiver increases the hood design, and in practical applications only needs to point to the corresponding street lamp.

The demodulation part of the optical receiver signal directly uses the general FSK demodulation chip XR2211 to perform coherent demodulation, and the XR2211 is an audio phase-locked loop circuit.

Bluetooth communication uses a universal Bluetooth to serial communication module to facilitate the transmission of read data to smartphone devices for display. The built-in smartphone built-in program is based on Android and developed using JAVA technology.

3. 3 Testing and verification

The field test selects the road section with less than one car, and installs the LED street light (160W) with the optical communication controller installed. 10 盏, the street light is 11 meters high, and the street light is 35 meters. The real-time reading of street light data through optical communication includes: the controller UID, the street lamp brand, the nominal power, the brand and type of supporting electrical appliances, the street lamp operating voltage, current, active power, power factor, temperature (Figure 9, Table 1) .

Figure 9 Schematic diagram of field test

Table 1 Effect of optical communication on road illumination

When the optical communication is turned on, the LED light source can recognize the flicker, but the illumination of the road surface is reduced to 50% when the dimming is performed. This is caused by the low power utilization of the modulation method used in the test system. When using more advanced PPM, or even DPPMDPIM, better power utilization and band utilization will be achieved (Table 2).

Table 2 Relationship between read success rate and R value

After testing, when located under the lamp head of the tested LED street light, the data reading success rate of all street lamps is 100%, and the successful reading time is less than 1S. Due to the hood design, the R receiver is always pointing to the position of the light source while the R value is constantly increasing. The interference of the optical receiver with ambient light or other street lamps can be neglected. The receiving reliability and the radiant power density of the light source and the receiving of the light sensor. Sensitivity is directly related.

It can be seen that although there are problems in the implementation scheme that the communication rate is too low, affecting the illumination of the street lamp, etc., it is completely feasible for the LED street lamp to realize visible light communication. And for daily street lighting inspection and management, the visible light communication method undoubtedly provides a new way and a new way to understand the real-time running status of the street lamp.

4 Summary

Visible light communication is still in its infancy and exploration stage at home and abroad, but its application prospects are very promising. In this study, the feasibility of visible light communication of LED street lamps was quickly verified by using the corresponding modulation and demodulation technology, and the real-time data transmission of LED street lamps was completed by optical communication. It provides a new idea and reference for the intelligent management of street lamps. At the same time, with the advancement of technology and the optimization of the scheme, the significance of LED street light to realize optical communication will not be limited to this, it will completely subvert the original value definition of urban street lamps, and the street lamps will not be limited to providing road lighting functions, but will be used as a The existence of communication access points throughout the city provides new directions and impetus for the realization of intelligent transportation and smart cities.

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