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 IMî“”DD, which is a non-coherent communication system. Usually the IMî“”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