Noise and Interference Analysis of White Light LED Communication System

0 Introduction

As white light LED lighting technology develops, white light LED communication, as an emerging communication technology, has attracted great attention by utilizing the high modulation frequency of LED. According to communication theory, reducing the bit error rate in judgment can be done from two aspects: one is to increase its input optical power; the other is to improve the signal-to-noise ratio. In white light LED communication systems, since white light LED light sources emit visible light and have a large divergence angle, they are basically harmless to human eyes and have no electromagnetic wave damage. The reliability of the system can be improved by increasing the transmission power of the white light LED light source. Ultimately, it is the signal-to-noise ratio of the receiving system that determines the communication performance of the entire system [2]. Based on the analysis of noise and interference in white light LED communication systems, we introduced a bandpass filter and a new imaging optical system in the noise-limited system, designed a low-noise amplifier using noise matching, and used measures such as band-stop filters, subcarrier modulation, and additional filter capacitors in the interference-limited system. We proposed a new receiver design scheme based on dual noise and interference limitations.

White light LED communication technology refers to the use of the high-speed luminous response characteristics of LED devices to modulate and transmit information using the optical carrier signal emitted by the LED at a high rate that is imperceptible to the naked eye, and then use photoelectric conversion devices such as photodiodes to receive the optical carrier signal and obtain information to combine visible light communication with LED lighting to build a base station lamp that can be used for both LED lighting and communication.

As shown in Figure 1, the transmitting end of the white light LED communication system modulates the electrical signal according to the transmitted data, and then uses LED to convert it into an optical signal and send it out. The receiving end uses a photodetector to receive the optical signal, and then converts the optical signal into an electrical signal, which is demodulated and read as signal data.

Figure 1 White light LED communication system structure schematic

The receiving end mainly includes an optical system that can achieve optimal reception of the signal light source, a photodetector and preamplifier circuit that restores the optical signal to an electrical signal, and a signal processing and output circuit that converts the electrical signal into a signal that can be recognized by the terminal. The main task of the optical receiver is to restore the information carried by the optical carrier after transmission through the wireless optical channel with minimal additional noise and distortion. Therefore, the output characteristics of the optical receiver comprehensively reflect the performance of the entire visible light communication system.

2 Noise and Interference Analysis of White Light LED Communication System

2.1 White LED Communication Noise-Limited System

The white light LED communication system uses the light intensity modulation and direct detection (IM-DD) technology. The structure principle of the digital optical receiver of the white light LED communication system is shown in Figure 2.

Figure 2 Schematic diagram of the digital optical receiver structure

Here, it is assumed that the receiver of the white light LED communication system shown in Figure 2 uses a PIN photodetector. In the white light LED communication system, the fluctuation noise determines the transmission quality in the optical channel. The ideal signal should include a time-varying fluctuation noise process, which is about at least 104~105 photons per bit. Although a narrowband filter is used in the receiver, when strong light illuminates the detector, fluctuation noise of 107~108 photons/bit will still appear in the system. Therefore, strong background light constitutes the main noise source of the white light LED communication system.

Another part of the noise comes from inside the receiver system, mainly:

① Shot noise. It is mainly introduced by the noise current generated by the background light at the PIN tube. Because the optical device LED is biased, there will be a small amount of optical power, that is, DC optical power, even when the signal is not transmitted. Therefore, its noise mean square value is: , where Idc is the photon current generated by the background light and DC light in the PIN tube; It can be seen that the noise current caused by the background light current is proportional to the system bandwidth Δf. Under the premise of not affecting signal reception, a smaller Δf is beneficial to suppressing background noise and reducing the noise of the detector and amplifier.

At present, it is quite difficult to achieve a modulation rate of 100MHz for white light LEDs. In this case, PIN also generates some low-frequency flicker noise, which has a significant impact on the receiver sensitivity. This noise can be classified as shot noise . Adding a bandpass filter to the receiving system can effectively suppress the low-frequency flicker noise.

② Dark current noise. When there is no light, the photodetector will generate a "dark light current" of nanoamperes due to the negative bias voltage. The mean square value of the noise is: , where Idark is the dark current of the PIN tube.

③ Thermal noise. The noise caused by the detector load and amplifier heating. Its noise RMS value is: where RP is the reverse junction resistance and RS is the series resistance.

④ Amplifier noise. It is mainly the inherent noise of electrical components introduced by noise sources such as internal resistance and transistors of the amplifier. It can generally be analyzed and calculated using equivalent noise sources and is related to the type of amplifier device.

In summary, a system like this, which limits the communication performance of white light LEDs due to the noise introduced in the receiving part by noise sources such as inherent circuit noise and external environment of the circuit, can be regarded as a white light LED communication noise-limited system.

2.2 White LED Communication Interference Limited System

As mentioned above, the main noise source of the white light LED communication system is the background light. In the noise-limited system, the background light mainly introduces shot noise in the PIN tube through the optical power, and the amplifier itself will introduce inherent noise. In addition, since the white light LED communication system cannot use a band-stop filter to filter out the interference of modulated visible light, if the circuit design is unreasonable, it may also introduce external modulated visible light to interfere with the receiving system. The modulated background light of different frequencies will interfere with the system output signal. At this time, the system becomes an interference-limited system.

Some artificial light sources, such as incandescent lamps and fluorescent lamps, emit a lot of visible light components. Sunlight also contains a lot of visible light, which together constitute background light noise. The sun is the main source of background light interference. Generally speaking, sunlight is stronger than artificial light. It is a non-modulated light source with a wide spectral bandwidth, and the maximum power spectrum density is at a wavelength of 500nm. All artificial light sources are modulated, and the frequency is generally 50Hz (equal to the power frequency), so as to introduce power frequency interference into the system through the detection circuit. Some fluorescent lamps are modulated by high-frequency through-band signals. The frequency of these signals is as high as tens or even hundreds of kHz, which also makes the fluorescent lamps contain harmonic energy of up to tens of kHz. Such a high frequency will have a great impact on visible light communication. In addition, the switching of high-frequency power supply will also generate spike pulse interference, which is introduced into the detection circuit through the power supply system.

2.3 Spectral distribution of noise and interference in white light LED communication system

Based on the above, the spectrum distribution of noise and interference in the white light LED communication system can be obtained, as shown in Figure 3.

Fig. 3 Spectral distribution of noise and interference in white light LED communication system.

3 Design of optical receiver based on dual limitation of noise and interference

3.1 Design of a new imaging optical receiving system

This design divides the actual area of ​​the detector into several independent small units. In this way, each small unit has its own receiving angle of view FOV, which can realize independent signal reception in space.

Assuming the focal length of the lens is f, as shown in Figure 4, the receiving angle of view of the small unit is FOV = arctan (x/f). If the detector area is a × a, the maximum receiving angle of view of the entire detector is FOVmax = arctan [(2a) 1/2/2f].

Fig. 4 Design of new imaging optical receiving system.

When the detector is divided into several small units, the average receiving angle of the detector becomes smaller, thus reducing the intensity of the received background light. At the same time, the detector is divided into several small units to reduce the equivalent input capacitance of the photodetector, thereby improving the receiver sensitivity.

3.2 High-frequency low-noise amplifier design

In white light LED lighting communication systems, there is strong background light noise in the channel, so the preamplifier should be designed as a low-noise amplifier. The noise of the preamplifier has a great influence on the sensitivity of the optical receiver, and its equivalent input noise current density is an important indicator. In order to reduce the noise, it is necessary to effectively design the noise matching circuit between the photodiode and its subsequent active devices, that is, to obtain the minimum equivalent input noise current through the optimal noise matching network [11].

3.3 Interference Suppression Measures for Interference-Limited Systems

① Use a band-stop electronic filter to suppress background light interference at a certain frequency, such as the power frequency interference generated by fluorescent lamps; ② Use subcarrier modulation to move the optical signal pulse to a higher frequency band, and use circuit filtering in the receiver to eliminate the background light noise, which is usually low-frequency; ③ Add power supply filter capacitors and amplifier bias circuit filter capacitors to the circuit to suppress power supply noise.

4 Conclusion

In view of the problem of noise and interference coexisting in white light LED communication systems, noise and interference are analyzed in noise-limited systems and interference-limited systems respectively, and corresponding measures are proposed in the optical and electrical domains to suppress noise and weaken the influence of interference. Combining the advantages of each, a new receiver design scheme based on a noise and interference dual-limited system is proposed, which can achieve good receiving performance in white light LED communication systems where noise and interference coexist. The prototype developed according to this scheme has been publicly exhibited at the Shanghai World Expo as the only white light LED communication system in my country so far, and it is also one of the only two similar systems in the world that can be demonstrated to the audience on site at a world-class expo.