1. Introduction

With the development of optical wireless communication (OWC), visible light communication (VLC) has been an emerging technology. VLC can use the high-power white light-emitting diode (LED), which is widely used in lighting, as the transmitting antenna and has extraordinary anti-electromagnetic interference properties [1]. Meanwhile, in contrast to radio frequency (RF), the frequency spectrum used by VLC is unregulated [2]. These advantages make VLC appropriate for Internet-of-Things (IoT), indoor localization, and next-generation communication [3,4].

Although promising, it is not easy to achieve high-speed data transmission under the bandwidth constraint of commercial white LEDs, which is the main challenge for VLC. The white LED used by VLC is mainly phosphor-coated LEDs and red-green-blue (RGB) LEDs. Due to the influence of the slow yellow light, the bandwidth of phosphor-coated LEDs is roughly 3-5 MHz [5]. Even for RGB LEDs, the bandwidth is about 20 MHz [6]. To support higher data rates, VLC systems need wider bandwidth. O’Brien et.al proposed a passive resonance circuit for pre-equalization, which made the bandwidth of the phosphor-coated LED be compensated from several megahertz to 45 MHz, and the data rate was 80 Mb/s [7]. In the following year, in [8], O’Brien et.al implemented post-equalization using a passive high-pass filter circuit at the receiver, improving the bandwidth of the VLC system to 50 MHz and data rates to 100 Mb/s. N.Fujimoto et.al used the red light of RGB LEDs to achieve 160 MHz bandwidth and 477 Mb/s real-time data transmission by combining LED driver circuit with pre-equalization function and operational amplifier circuit with peak characteristics at the receiver [9]. Li et.al designed an analog pre-equalization circuit based on NPN transistors and an active post-equalization circuit based on operational amplifiers for the phosphor-coated LED, and demonstrated a VLC system with 233 MHz bandwidth finally [10]. In [11], Huang et.al proposed a passive cascade bridge-T equalization circuit, and the bandwidth of the VLC system based on phosphor-coated LEDs was increased from 17 MHz to 366 MHz. Based on the bridge-T network structure, several passive equalization circuits have also been proposed, such as T-Bridge cascaded pre-equalization circuit [12] and Lattice pre-equalization circuit [13]. Zhou et.al implemented a VLC link with 450 MHz bandwidth based on the phosphor-coated LED by using a two-order linear software equalizer at the receiver [14]. Referring to the design method of filters, Min et.al proposed a novel equalization circuit design process and realized 1.35 Gb/s data rates with 600 MHz bandwidth using phosphor-coated LEDs [15]. In addition to using equalization technologies to expand the bandwidth of LEDs, there have been many researches on high-speed LED devices in recent years [16–18]. Chang et.al implemented 2.805 Gb/s phosphor white light visible light communication utilizing an InGaN/GaN semipolar blue micro-LED [16], and the micro-LED array has an electrical-to-optical (EO) bandwidth of 1042.5 MHz.

High-order modulation methods based on quadrature amplitude modulation (QAM), such as carrierless amplitude and phase (CAP) modulation, orthogonal frequency division multiplexing (OFDM) and discrete multi-tone (DMT), can be used in VLC systems to support high-speed data transmission [6,19–24] under bandwidth constraints. Equalization technologies can expand the bandwidth and high-order modulation technologies can improve the spectrum utilization. In order to achieve high-speed VLC systems, the combination of higher-order modulation and equalization is also widespread [11,14,21–23]. However, high-order modulation methods are usually more complex and power-consuming. In contrast, if using equalization technologies expand the bandwidth of LEDs, high-speed VLC systems can be realized using simpler on-off keying (OOK) modulation, and the VLC system using OOK modulation has advantages of low cost, low power consumption and small size. In addition, according to Shannon’s theorem, under the condition of the same channel capacity, the system with a wider bandwidth has a lower signal-to-noise ratio (SNR) requirement. The power of light propagating in the free space decreases rapidly as the distance increases. The increase of the distance leads to the decrease of the SNR of the VLC system, while the VLC system with wide bandwidths can tolerate the low SNR, which makes long transmission distances possible. Hence, for the high-speed VLC system, it is essential to expand the bandwidth using equalization technologies.

Compared to RGB LEDs and micro-LEDs, phosphor-coated LEDs have a narrower modulation bandwidth due to slow yellow light components. However, phosphor-coated LEDs are more attractive for VLC systems, due to their low cost and low complexity [25]. And phosphor-coated LEDs are often used for indoor lighting, which paves the way for VLC systems combining lighting and communication. Many VLC systems based on phosphor-coated LEDs have been demonstrated [10–15]. In these phosphor-coated VLC systems, blue filters were used. Table 1 summaries of the recent achievements of different high-speed phosphor white light LED VLC systems, illustrating different equalization methods, filters used to remove the influence of the slow yellow light, modulation formats, achieved data rates, etc. The blue filter can suppress the slow yellow light generated by the phosphor-coated LED, which can improve the data rate of VLC systems. However, the blue filter inevitably introduces attenuation of blue lights when suppressing yellow lights [26], which reduces the amplitude of high-frequency signals in the VLC system. Meanwhile, the blue filter is a delicate optical device, and using blue filters would increase the cost and complexity of VLC systems.

Table 1. Summary of recent achievements of different high-speed phosphor white light LED VLC systems

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In this paper, we expand the bandwidth of VLC systems based on the commercial phosphor-coated LED by pre-emphasis technologies and demonstrate a VLC system without blue filter supporting high data rates. An analog folded equalization circuit based on a new equalization scheme is proposed. Compared with existing analog equalization circuits, the folded equalization circuit can provide a larger equalization response range and expand the 3 dB bandwidth of high-power LEDs more effectively. An analog bridged-T equalizer is used to eliminate the influence of yellow lights produced by phosphor-coated LEDs. The passive bridged-T equalizer is more suitable than blue filters for a high-speed VLC system since it does not cause the attenuation of blue lights like blue filters. Combining the folded equalization circuit and the bridged-T equalizer, a novel transmitter based on phosphor-coated LEDs is constructed, and the transmitter makes the 3 dB bandwidth of the VLC system using the phosphor-coated LED be extended from a few megahertz to 893 MHz without using blue filters. At the same time, the VLC system can support on-off keying non-return to zero (OOK-NRZ) data rates up to 1.9 Gb/s at the distance of 7 m, and the bit error rate (BER) is $3\times 10^{-5}$. To the best of our knowledge, this is the widest bandwidth and the uppermost data rate reached for OOK modulated VLC systems based on commercial phosphor-coated LEDs. The proposed transmitter makes a low-cost, low-power, and high-speed VLC system supporting real-time data transmission possible.

2. Proposed transmitter

Equalization technologies and circuit design are crucial for VLC systems. An appropriate emphasis method can effectively expand the 3 dB bandwidth of VLC systems, and rigorous circuit design can ensure the integrity of high-speed signals in data transmission. To solve the bandwidth limitation of VLC systems, we construct a novel transmitter based on phosphor-coated LEDs, as shown in Fig. 1. The folded equalization circuit consists of a NPN transistor and two equalization branches, $Z_c$ and $Z_e$, and it has a symmetrical-folded structure. The equalization branch $Z_c$ consists of an inductor and a resistor, and the equalization branch $Z_e$ consists of a capacitor and a resistor. The folded equalization is used to expand the narrow bandwidth of VLC systems using high-power LEDs. The bridged-T equalizer has a bridge-T network structure [27], and is used to suppress the influence of yellow lights produced by phosphor-coated LEDs. The bridged-T equalizer can cause less loss than blue filters on high-speed signals. Besides equalization circuits, an AC-coupled drive circuit is designed to load signals to high-power LEDs. Impedance matching is considered in the design of the transmitter to ensure the integrity of high-speed signals. The principle of these circuits will be described.

 

Fig. 1. VLC transmitter with bridged-T equalizer, folded equalization circuit, and AC-coupled drive circuit.

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2.1 Principles of the folded equalization circuit

Considering the lighting requirements, a 1 W commercial phosphor-coated LED is used as the light source for the VLC system. However, for this high-power LED, the attenuation of high-frequency signals is serious. With the increase of frequency, the amplitude-frequency response of LEDs gradually decreases. In [15], this characteristic of LEDs was also described. The wider the bandwidth of VLC systems is expanded, the more gain compensation needs to be provided by equalization circuits. In the field of VLC, the analog equalization scheme for expanding the bandwidth of LEDs can be divided into two categories. One is the attenuating equalization scheme, and the cascaded bridged-T equalizer as shown in Fig. 2(a) is a typical attenuating equalization circuit. The other is the amplification equalization scheme, and the pre-emphasis circuit as shown in Fig. 2(c) is a representative amplification equalization circuit.

 

Fig. 2. (a) Cascaded bridged-T equalizer. (b) Amplitude-frequency responses of LED and LED with cascaded bridged-T equalizer (Schematic diagram). (c) Pre-emphasis circuit. (d) Amplitude-frequency responses of LED and LED with pre-emphasis circuit (Schematic diagram).

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The cascaded bridged-T equalizer was proposed in [11], and it is a passive circuit. The cascaded bridged-T equalizer is composed of two cascaded bridge-T network structure. Every bridge-T network structure contains two interdependent equalization branches $Z_{11}$ and $Z_{12}$. Figure 2(b) shows the amplitude-frequency responses of LED and LED with cascaded bridged-T equalizer (Schematic diagram). It can be seen that the cascaded bridged-T equalizer expands the 3 dB bandwidth of LEDs by suppressing low-frequency signals. The cascaded bridged-T equalizer has excellent impedance matching characteristics and does not require external energy supply, which is considered to be suitable for high-speed VLC systems. However, it is well known that the cascaded bridged-T equalizer would reduce the power of the output signal, which decreases the SNR of the VLC system. The pre-emphasis circuit consists of an RF NPN transistor and an equalization branch $Z_e$ composed of resistors and capacitors, and the equalization branch $Z_e$ is connected to the emitter of the transistor [10]. Figure 2(d) shows the amplitude-frequency responses of LED and LED with pre-emphasis circuit (Schematic diagram). And we can see that the pre-emphasis circuit based on RF NPN transistors expands bandwidth by enhancing high-frequency signals. The pre-emphasis circuit can expand the bandwidth of LEDs and have higher power of the output signal. But for VLC systems, the compensation of high-frequency signals increases with the increase of frequency, and the pre-emphasis circuit using discrete component is difficult to provide sufficient gain at high frequencies.

Since the bandwidth limitations, it is difficult to realize a high-speed VLC system based on high-power LEDs. In order to expand the bandwidth of high-power LEDs as much as possible, we propose a folded equalization circuit based on a novel equalization scheme, as shown in Fig. 3(a). The folded equalization circuit is composed of an NPN transistor and two equalization branches, $Z_c$ and $Z_e$. Different from the structure of the pre-emphasis circuit in [10], we add the $Z_c$ branch consisting of inductance $L_{b1}$, resistance $R_{b6}$, and capacitance $C_{b2}$ in series to the collector of the transistor, and add impedance matching circuits. As far as we know, the equalization branch $Z_c$ based on the inductance $L_{b1}$ is used by us in active equalization circuits for the first time. In the pre-emphasis circuit [10], the equalization branch $Z_e$ based on the capacitance is introduced to amplify the high-frequency signal, and in the proposed folded equalization circuit, the equalization branch $Z_c$ based on the inductance is introduced to attenuate the low-frequency signal. By using two equalization branches, $Z_c$ and $Z_e$, the folded equalization circuit can amplify high-frequency signals while suppressing low-frequency signals. Figure 3(b) shows the amplitude-frequency responses of LED and LED with folded equalization circuit (Schematic diagram). It can be seen that the folded equalization circuit expands the 3 dB bandwidth of LEDs by simultaneously amplifying the high-frequency signal and suppressing the low-frequency signal, which is different from the existing analog equalization methods. Combining the principle of the amplification equalization scheme and the attenuating equalization scheme, the folded equalization circuit can effectively expand the 3 dB bandwidth of high-power LEDs. Due to amplifying high-frequency signals, the folded equalization circuit has higher output signal power than the cascaded bridged-T equalizer proposed in [11] for the same input signal power. Due to suppressing low-frequency signals, the folded equalization circuit needs to provide less gain at high frequencies than the pre-emphasis circuit based on RF NPN transistors proposed in [10] for the same bandwidth of VLC systems. Utilizing two equalization branches, the folded equalization circuit can provide a greater emphasis response range than the pre-emphasis circuit.

 

Fig. 3. (a) Folded equalization circuit. (b) Amplitude-frequency responses of LED and LED with folded equalization circuit (Schematic diagram). (c) Simplified small signal equivalent circuit of the folded equalization circuit.

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To analyze the folded equalization circuit in detail, the simplified small-signal equivalent circuit of the folded equalization circuit is proposed, as shown in Fig. 3(c). The value of $R_{b0}$ is 50 $\varOmega$ for impedance matching. Resistances $R_{b1}$, $R_{b2}$, $R_{b3}$, and $R_{b4}$ are used for DC bias of the NPN transistor, setting the appropriate Quiescent point (Q-point). Resistances $r_{bb^{'}}$, $r_{b^{'}e}$, Miller capacitance $C^{'}_{\pi }$, and transconductance $g_m$ are basic parameters of the NPN transistor, which can be obtained from the instruction manual. Resistance $R_L$ is the load, and since the folded equalization circuit cascades the power amplifier (PA), as shown in Fig. 1, $R_L$ is equal to the input impedance of the PA, which is $Z_{in}$. $Z_c$ and $Z_e$ are impedance of equalization branches, and their expressions in the s-domain are

$Z_c$ and $Z_e$ are frequency dependent, which is the basic principle of the equalization circuit. In our experiment, the role of $C_{b2}$ is to isolate the DC component, and the value of $C_{b2}$ is $10 \mu F$. Even in the low-frequency range, the capacitive reactance of $C_{b2}$ is so small that it can be ignored. At the same time, the NPN transistor we used is an RF transistor with a small $C^{'}_{\pi }$ capacitance. We can ignore the influence of the $C^{'}_{\pi }$ capacitance because the equalization circuit operates at frequencies below 2 GHz. The transconductance $g_m$ is about $1 S$, and $r_{b^{'}e}$ is much larger than $r_{bb^{'}}$ and $R_{b4}//Z_e(s)$. Then, according to the simplified small signal equivalent circuit, combined with the Eq. (1), the system function of the folded equalization circuit could be approximately expressed as

Now, we can use the Eq. (2) to express the system function of the folded equalization circuit. According to the system function, the amplitude-frequency response of the folded equalization circuit is

From Eq. (3), it can be seen that the inductance $L_{b1}$ makes the numerator increase as the angular frequency $\omega$ increases and the capacitance $C_{b1}$ makes the denominator decrease as the angular frequency $\omega$ increases, which makes the amplitude-frequency response increase rapidly with the increase of the frequency. This characteristic makes the folded equalization circuit have strong equalization abilities. Meanwhile, we can set appropriate values of $L_{b1}$, $C_{b1}$, $R_{b5}$ and $R_{b6}$, so that the amplitude-frequency response is less than 0 dB at low frequencies and greater than 0 dB at high frequencies. In this case, the folded equalization circuit can simultaneously amplify high-frequency signals and suppress low-frequency signals. These advantages make the folded equalization circuit can effectively expand the bandwidth of VLC systems.

It is not difficult to choose the values of $L_{b1}$, $C_{b1}$, $R_{b5}$ and $R_{b6}$. A method of obtaining the system function of LEDs was introduced in [15]. We can determine the system function of the folded equalization circuit according to the system function of LEDs, and then calculate the value of circuit components from Eq. (2).

2.2 Bridged-T equalizer

Although the folded equalization circuits can significantly overcome bandwidth constraints of LEDs, the 1 W phosphor-coated LED has strong yellow light components. Just as the narrow 3 dB bandwidth of high-power LEDs seriously limits the data rate of VLC systems, the yellow light affects the data rate of VLC systems. The light of phosphor-coated LEDs mainly consists of blue lights generated by the spontaneous radiation of PN junctions and yellow lights generated by the stimulated radiation of phosphors. Due to the afterglow effect of phosphors, yellow lights only enhance the amplitude-frequency response of the VLC system at low frequencies, which further reduces the data rate of the VLC system. Blue filters are often used in the VLC system using phosphor-coated LEDs to suppress the yellow light. However, blue filters also cause attenuation of blue light components while suppressing yellow light components. Hence, in this paper, a bridged-T equalizer instead of the blue filter is used to eliminate the effect of yellow lights, and the role of the bridged-T equalizer will be explained in the following paragraphs.

At first, we measured the amplitude-frequency response of phosphorescent white LED and phosphorescent white LED with blue filter, as shown in Fig. 4(a). In order to analyze the principle of blue filters more intuitively, we calculated the difference between white LED and white LED with blue filter (DHC GCC-2030), and then plotted it in Fig. 4(a). It can be found that the frequency response of the white LED is extremely high when the frequency is below 8 MHz due to the yellow light. And the frequency response of the white LED with blue filter is flatter than the white LED at low frequencies since the yellow light is filtered by the blue filter. Meanwhile, we can distinctly see from the response of white LED with blue filter minus white LED that the blue filter only causes 3.6 dB attenuation when the frequency is beyond 47 MHz. Therefore, for the VLC system using the phosphor-coated LED, blue filters can be used to filter out yellow lights, but the use of blue filters inevitably causes attenuation of high-frequency signals because of the loss of blue lights.

 

Fig. 4. (a) Amplitude-frequency responses of white LED, white LED with blue filter, and white LED with blue filter minus white LED. Measurements are at a distance of 7 m. (b) Amplitude-frequency responses of white LED, white LED with ideal filter, and white LED with ideal filter minus white LED.

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The amplitude response of high-frequency signals is crucial for high-speed VLC systems. Figure 4(b) illustrates the amplitude-frequency response of an ideal filter for the VLC system based on phosphor-coated LEDs. It can be seen from the difference curve of white LED with ideal filter and white LED that the ideal filter attenuates signals with frequencies below 47 MHz and does not attenuate signals with frequencies above 47 MHz. The ideal filter can suppress the high amplitude-frequency response of phosphor-coated LEDs at low frequency. Different from the blue filters, the ideal filter does not cause the loss of blue lights which avoids the attenuation of high-frequency signals. Therefore, compared with blue filters, the ideal filter are more suitable for high-speed VLC systems.

The bridged-T equalizer can be used to perform the function of the ideal filter. The bridged-T equalizer is a passive circuit and has the same bridge-T network structure as the cascaded bridged-T equalizer proposed in [11]. The cascaded bridged-T equalizer could be used to extend the 3 dB bandwidth of LEDs, and the function of the ideal filter can be equivalent to expanding the bandwidth of the phosphor-coated LEDs from a few megahertz to 47 MHz. Therefore, it is feasible to realize the ideal filter using the bridged-T equalizer.

As shown in Fig. 1, $Z_{11}$ and $Z_{12}$ are the impedance of equalization branches. $R_{a3}$ and $R_{a4}$ are the resistance on the branch of the bridge-T network structure, and $R_{b0}$ is the load resistance of the bridged-T equalizer. According to the article [27], in order to ensure the impedance matching characteristics, the bridged-T equalizer needs to meet

Combined with the Eq. (4), the system function of the bridged-T equalizer can be expressed as

As shown in Fig. 4(b), the amplitude-frequency response of the ideal filter is known. According to the filter synthesis method [15], the system function $H_b(s)$ of the bridged-T equalizer can also be obtained from the known amplitude-frequency response of the ideal filter. And then $Z_{11}$ and $Z_{12}$ can be calculated according to Eq. (5).

By setting the appropriate values of $Z_{11}$ and $Z_{12}$, the bridged-T equalizer can effectively reduce the amplitude-frequency response of phosphor-coated LEDs at low frequencies and cause no significant attenuation at high frequencies. Compared with blue filters, using the bridged-T equalizer can effectively avoid the loss of amplitude-frequency response of VLC systems at high frequencies. It can not be denied that the bridged-T equalizer also attenuate the electrical signal at higher frequencies. However, the 3 dB bandwidth of VLC systems using high-power LEDs is normally less than 800 MHz. The bridged-T equalizer has good impedance matching characteristic, and it does not significantly attenuate signals with frequencies below 800 MHz through rigorous circuit design. Hence, the bridged-T equalizer is more suitable for high-speed VLC systems than blue filters. And not using blue filters can reduce the complexity and the cost of VLC systems.

2.3 AC-coupled drive circuit

For the VLC system, besides using the equalization technology to expand the bandwidth, loading signals to the high-power LED efficiently is crucial. An AC-coupled drive circuit is designed to modulate a 1 W LED. The inductor $L_{c1}$ and constant current supply $I_{bias}$ (AMS7135) provide a direct current (DC) bias component to the LED. The constant current supply $I_{bias}$ provides a constant current of 350 mA to the LED, which allows the LED to shine steadily and operate in a linear region. The inductor $L_{c1}$ is used to isolate the alternating current (AC) component, which ensures that the DC component in the constant current supply $I_{bias}$ branch remains stable at high frequencies. The AC component of equalization signals can be loaded to the LED through the AC branch consisting of the capacitor $C_{c1}$ and the resistor $R_{c1}$. The capacitor $C_{c1}$ is used to isolate the DC component. Since the equivalent impedance of the LED is less than 50 $\varOmega$, the resistor $R_{c1}$ is used for impedance matching. A PA is used to amplify the signal passing through the equalization circuit to enhance the drive capability. The pattern of ac coupling can significantly reduce the power consumption of the drive circuit.

2.4 Value of key components

Utilizing the folded equalization circuit, bridged-T equalizer, and AC-coupled drive circuit proposed above, we built a VLC transmitter circuit based on phosphor-coated LEDs and implemented it on the circuit board. Table 2 lists the value of the key components employed in the transmitter. We did plenty of experiments to test the performance of the transmitter, and then the experiment and results will be described.

Table 2. Value of the key components employed in the transmitter

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3. Experiment and results

To test the performance of the proposed transmitter, a VLC system was demonstrated. The VLC system schematic is illustrated in Fig. 5(a). The bridged-T equalizer cascades the folded equalization circuit to form pre-emphasis circuits. The input signal is processed by pre-emphasis circuits and modulated to a LED by the AC-coupled drive circuit. To improve the transmission distance of the VLC system, convex lenses are used to converge the light beam generated by the LED. A silicon PIN photodetector (PD) is used to convert optical signals into electronic signals at the receiver. The electronic signal generated by the PIN PD is weak, hence, a power amplifier (PA) is used to enhance the power of output signals. At the receiver, the post-equalization technology is not used.

 

Fig. 5. VLC system. (a) VLC system schematic. (b) VLC system experimental link at a distance of 7 m. (c) Photographs of the bridged-T equalizer and the folded equalization circuit. (d) Photographs of the AC-coupled drive circuit and the 1 W phosphor-coated LED.

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Figure 5(b) shows the VLC system experimental link at a distance of 7 m. A 1 W commercial phosphor-coated LED (OSRAM LUW CN7N) was used as the light source of the VLC system experimental link, and a silicon PIN photodetector module (818-BB-21A) with 1.2 GHz bandwidth was used to receive optical signals. Meanwhile, the PA (MPA-10-40) we used can provide 40 dB power gains and the bandwidth is 1 GHz. The power supply voltage of the PA is 12 V, the operating current is 360 mA, and the power is 4.32 W. Two convex lenses were used to collimate the light beam generated by the LED, and a convex lens was used to converge the light beam to the PIN PD. Due to the limited space of the experimental platform, two flat mirrors were used to reflect the light beam to increase the distance of the link. Finally, a 7 m VLC system experimental link based on a phosphor-coated LED was demonstrated. Figure 5(c) shows the photograph of the pre-emphasis circuits consisting of the bridged-T equalizer and the folded equalization circuit. In the folded equalization circuit, a wideband NPN transistor (NXP BFR520) was used. Figure 5 (d) shows the photographs of the AC-coupled drive circuit and the 1 W phosphor-coated LED. The transmitter based on phosphor-coated LEDs is composed of the pre-emphasis circuits and the AC-coupled drive circuit, and it can be seen that the transmitter has a simple structure and small size.

For VLC systems, the 3 dB bandwidth is a critical parameter. We tested the amplitude-frequency response of the VLC system using a network analyzer (Agilent E5071B) and set the output signal power of the network analyzer to -20 dBm. Pre-emphasis circuits are composed of the bridged-T equalizer and the folded equalization circuit. Figure 6(a) shows amplitude-frequency responses of white LED, white LED with pre-emphasis, folded equalization circuit, and bridge-T equalizer. We can see that the 3 dB bandwidth of white LED is only a few megahertz, and due to the yellow light, white LED has extremely high amplitude-frequency responses when the frequency is lower than 8 MHz. Meanwhile, it can be found that the folded equalization circuit can suppress low-frequency signals and amplify high-frequency signals simultaneously and can provide an emphasis response range of about 30 dB. To the best of our knowledge, 30 dB emphasis response range is the highest among existing equalization circuits. Hence, the folded equalization circuit can significantly expand the bandwidth of high-power LEDs. It can be seen that the bridge-T equalizer can partially attenuate low-frequency signals, and has less attenuation on high-frequency signals, which can be used to removing the influence of the slow yellow light. Figure 6(a) also shows that with pre-emphasis circuits, the 3 dB bandwidth of the VLC system using a phosphorescent white LED can be extended from a few megahertz to 893 MHz.

 

Fig. 6. (a) Amplitude-frequency responses of white LED, white LED with pre-emphasis, folded equalization circuit, and bridge-T equalizer. (b) Amplitude-frequency responses of white LED, white LED with blue filter, white LED with bridged-T equalizer, white LED with bridged-T equalizer minus white LED, and white LED with blue filter minus white LED. Measurements are at a distance of 7 m.

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To compare the effect of the bridge-T equalizer with that of blue filters, amplitude-frequency responses of white LED, white LED with blue filter (DHC GCC-2030), and white LED with bridged-T equalizer was measured as shown in Fig. 6(b). It can be found from the response of white LED with blue filter minus white LED that the blue filter only causes 3.6 dB attenuation at high frequencies. The amplitude-frequency response of white LED with bridged-T equalizer is decreased a lot when the frequency is less than 47 MHz, and the attenuation of the high-frequency response is less than white LED with the blue filter, which proves that the bridged-T equalizer can be used to replace the blue filter and bring little loss to the VLC system at high frequencies. According to the curve of white LED with bridged-T equalizer minus white LED in Fig. 6(b), there is no denying that the bridged-T equalizer causes some attenuation to the high-frequency signal, when the frequency is higher than 500 MHz. But in the frequency range of 893 MHz, the attenuation of the bridged-T equalizer for high-speed signals is less than that of the blue filter. Since the amplitude response of high-frequency signals is crucial for high-speed VLC systems, the bridge-T equalizer is more suitable than the blue filter (DHC GCC-2030) in the VLC system proposed in this paper. However, the bridged-T equalizer cannot completely suppress the high amplitude-frequency response of the white LED at low frequencies. As shown in Fig. 6(a), the curve of white LED with pre-emphasis still has high responses when the frequency is lower 6 MHz. But the VLC system proposed in this article mainly transmits high-speed data, and in high-speed communication protocols, balancing codes such as 8B\10B code are often used to eliminate long consecutive zeros and long consecutive ones. Hence, the high response when the frequency is below 6 MHz has little influence on the high-speed VLC system. To sum up, the folded equalization circuit can provide an emphasis response range of 30 dB, which can be used to significantly expand the bandwidth of LEDs. According to the amplitude-frequency response of white LED with bridged-T equalizer shown in Fig. 6(b), the bridged-T equalizer can replace the blue filter to reduce the influence of slow yellow light at low frequencies. Utilizing the bridged-T equalizer and the folded equalization circuit, the bandwidth of the phosphorescent white LED can be extended from a few megahertz to 893 MHz.

To evaluate the communication performance of the VLC system, a BER tester (Agilent 81250) was utilized to measure the BER at different data rates. The BER tester generated an OOK-NRZ data stream consisting of a pseudo-random binary sequence (PRBS)-5 ($2^5-1$) with a peak-to-peak voltage of 140 mV. The BER of the VLC system at different data rates is shown in Fig. 7. At the same time, using an oscilloscope (Agilent 86100C), eye diagrams and the signal-to-noise ratio (SNR) of the VLC system were measured when the data rates were 0.1, 1, 1.5, and 1.9 Gb/s. Due to the high amplitude-frequency response of the VLC system when the frequency is below 6 MHz, the BER increases sharply when the data rate is lower than 60 Mb/s, as shown in Fig. 7. But the BER of the VLC system is below $10^{-9}$ in the data rate ranging from 80 Mb/s to 1.3 Gb/s. Therefore, the high amplitude-frequency response when the frequency is below 6 MHz has no significant impact on the communication capacity of the high-speed VLC system. At the 1.5 Gb/s data rate, the BER of the VLC system is $9.8\times 10^{-7}$, and some distortion of signals can be seen from the eye diagram. The decrease in communication performance at the 1.5 Gb/s data rate is due to the jitter of high-speed signals. When the data rate is beyond 1.9 Gb/s, the BER of the VLC system increases sharply due to the limitation of the 3 dB bandwidth. Hence, the VLC system can support data rates of 1.9 Gb/s at a distance of 7 m with a BER of $3\times 10^{-5}$, and the BER at the 1.9 Gb/s data rate is much less than the forward error correction (FEC) threshold $3.8\times 10^{-3}$.

 

Fig. 7. BER of the VLC system vs. data rates. Insets: Eye diagram at 0.1, 1, 1.5, and 1.9 Gb/s. Measurements are at a distance of 7 m.

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The transmitter circuit was tested indoors, and the ambient illumination was recorded. The above measurements were performed at an illumination level of 221.8 lx. At the same time, the power dissipation of the pre-emphasis circuits was also measured. Since the bridged-T equalizer is passive, the power dissipation of the pre-emphasis circuits is equal to the power dissipation of the folded equalization circuit. To ensure that the NPN transistor can operate in the linear region, the voltage of the folded equalization circuit is set to 10 V. The operating current of the folded equalization circuit is 22 mA. Hence, the power of the pre-emphasis circuits is 0.22 W.

4. Conclusion

A novel transmitter based on commercial phosphor-coated LEDs has been proposed in this article. The transmitter consists of the folded equalization circuit, the bridged-T equalizer, and the AC-coupled drive circuit. By simultaneously amplifying high-frequency signals and suppressing low-frequency signals, the folded equalization circuit can provide an up to 30 dB emphasis response range, and significantly expand the bandwidth of VLC systems. The bridged-T equalizer can reduce the influence of yellow lights generated by the phosphor-coated LED. Compared with blue filters, the bridged-T equalizer is more suitable for high-speed VLC systems, due to less loss to high-frequency signals. Utilizing the proposed transmitter, a high-speed VLC system without blue filter is demonstrated. The 3 dB bandwidth of the VLC system is extended from several megahertz to 893 MHz. The VLC system can support 1.9 Gb/s OOK-NRZ data transmission at the distance of 7 m with the BER of $3\times 10^{-5}$.

The VLC system supports real-time data transmission at high speed. When the data rate is in the range of 80 Mb/s to 1.3 Gb/s, the BER of the VLC system is below $10^{-9}$, which makes Gigabit optical wireless networks possible. Although the BER of the VLC system is $3\times 10^{-5}$ when the data rate is 1.9 Gb/s, it is acceptable for the MPEG-4 video according to the standard [28] published by 3GPP, and it can also be reduced by introducing the error control coding into the VLC system. The VLC system has very low complexity and power consumption, and it can be used for high-speed communication networks in the future.

Although the bridged-T equalizer does not cause the attenuation of blue lights like blue filters, it can not completely eliminate the influence of yellow lights at low frequency duo to the finite ability of equalization. Hence, we will continue to study the bridged-T equalizer to perfectly deal with the yellow light component in the VLC system. The structure of the folded equalization circuit will also be improved to enhance the ability to expand the bandwidth of VLC systems using high-power LEDs. In addition, the combination of MIMO technology and high-order modulation technologies with the proposed VLC transmitter is recommended, which will vastly improve the data transmission rate of VLC systems.