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We propose and experimentally demonstrate a novel modulation scheme called superposed pulse amplitude modulation (SPAM) which is low-cost, insensitive to non-linearity of light emitting diode (LED). Multiple optical pulses transmit parallelly from different spatial position in the LED array and overlap linearly in free space to realize SPAM. With LED arrangement, the experimental results show that using the modulation we proposed the data rate of 120 Mbit/s with BER 1 × 10−3 can be achieved with an optical blue filter and RC post-equalization.
©2013 Optical Society of America
1. Introduction
Light-emitting diodes (LEDs) have the potential to become the dominant device for general illumination because of their energy efficiency, small size and reliability, compared with incandescent or fluorescent bulbs. Visible light communication (VLC) based on LED merges lighting and data communications in applications such as area lighting, streetlights, vehicles, and traffic signals [1–5]. VLC is potentially attractive since it can provide license-free, very high signal-to-noise rate (SNR) (> 50dB typically), high modulation power and low-cost front-ends [4]. However, experiments show that the system data rate is limited by the intrinsic modulation bandwidth of the white LED which is about several MHz or a few dozen MHz for reason of the slow time constant of the phosphor [6–10]. Therefore, using the simplest modulation mode of on-off-keying (OOK), the data rate reaches only to dozens Mbit/s. To improve the transmission capacity the main approach is spectrally utilizing efficient modulation. For example, quadrature-amplitude-modulation (QAM) with orthogonal frequency division multiplexing (OFDM) was used to achieve 513 Mbit/s [11]; [12] proposed LED transmitter using discrete power level stepping OFDM which enables a simple and power-efficient front-end design. However, these modulation formats need digital signal processing such as fast Fourier transform (FFT) and inverse fast Fourier transform (IFFT). To avoid complexity for transceiver, some modulation schemes have been used, such as pulse amplitude modulation (PAM) and pulse position modulation (PPM) [13–15]. [16] superposes the signals of pulse position modulation (PWM) and nonreturn-to-zero (NRZ)-OOK to realize LED dimming. Popoola et al have presented a scheme that combines the optical space shift keying (SSK) with pulse position modulation (PPM) to form spatial PPM (SPPM) [17]. Compared with schemes like OFDM, the SPPM has low complexity and is not as sensitive to device non-linearity. However, the PPM requires accurate synchronization. Therefore, the modulation of combining SSK with PAM can provide larger throughputs with low complexity for VLC system. Fath et al employ the SSK controlling multi-LEDs (distributed antennas) to form M-PAM, which needs complicated signal processing in multiple-input-multiple-output (MIMO) system [18]. The performances of multiple transmitters SSK, PAM and PPM are compared and investigated in detail in [19].
In this paper, we proposed a new scheme to enhance the transmission rate called superposed pulse amplitude modulation (SPAM). The N parallel signals of NRZ-OOK from LEDs of different position overlay in free space to form the signal of 2N-SPAM. Comparing to SSK mentioned above, SPAM requires LEDs transmitting different intensities simultaneously in which SM encoder is not needed to coordinate the LEDs. Furthermore, the SSK requires multiple photon detectors (PDs) to demodulate signals, and SPAM needs only one receiver without complicated signal processing. With commercially available white light LED, the experiment is operated with 2 × 2 LED arrangement. The experimental results show that the data rate of 120 MHz is achieved with indoor average illumination of 600 lx.
2. Modulation schemes and illumination design
For indoor scenario, the primary purpose for LED source is general lighting. With the commercially available LED, an amount of chips is needed to ensure sufficient brightness for the room illumination [20]. The LED array is always designed as lighting source in actual scenario, in which the LED chips are placed in space regularly. Therefore, we can utilize this spatial character of LED array in modulation mechanism. In VLC, optical signals transmit in line-of-sight (LOS) or non-line-of-sight (NLOS). The optical intensity is received by PD, which is known as intensity modulation (IM) and direct detection (DD) system. In this scheme, all the frequency and phase information of optical carrier is ignored and only the intensity of the optical signal is detected. Then the PD produces a current proportional to the received instantaneous optical power. When such optical intensity information sends in LOS with a very high SNR (> 50dB), the base-band signal could transmit directly. Therefore, multi signals transmitting in parallel can overlay linearly in free space.
A new modulation scheme is illustrated in Fig. 1. The LED chips are placed in the array of rectangular which includes two group LED denoted by different colors. Two NRZ-OOK signals with different amplitude are loaded onto the LED chips (Tx1 and Tx2) respectively. Then the optical signals are emitted from different LED and overlay in free space. Choosing suitable driving voltage for LED, we can get multilevel modulation. In our scheme, N LEDs have intensity ranges of V, V/2, V/4, …, V/2N−1, and 2N-SPAM can be obtained. For example, two NRZ-OOK signals of V1 and V1/2 drive the LED of Tx1 and Tx2, which produce optical signals and overlay to 4-SPAM in free space. It could be argued in amplitude modulation that using N-1 LEDs with the identical intensity of V, N-SPAM can be achieved. In this scheme, the average intensity contribution of each LED is V, which yields much higher SNR, but more number of LEDs and SSK encoder is needed for the same order of SPAM. Moreover, the first scheme we used can support color temperature shift, because different driving currents for LED result in different colors.
To keep the overall system complexity low in the experiment, we choose one cell from LED arrangement, which only contains 2 × 2 LEDs. Figure 2 displays the arrangement of LEDs. The space between the LED chips is 2 cm. In order to obtain a linear superposition for two OOK signals in free space, the optical distribution must provide the same coverage of Tx1 and Tx2. Commercial available LED (OSTAR Sl2n) provided by OSRAM is used.
We regard 600 lx as a typical brightness, and aim for a 200-800 lx span in the whole room [8].The optical efficacy of the LED chip of Sl2n is 100 lm/W in the typical working state. The maximum power of each LED chip is 11 W. With four LED chips sufficient illumination and communication purpose can achieve. The distribution of the luminous intensity is measured by an illuminometer. Figure 3 displays experimental measurement of the illuminance distribution at distance of 0.5 m. Figures 3(a) and 3(b) show the distribution of Tx1 and Tx2 respectively. The Figs. 3(c) and 3(d) shows the result of Tx1 + Tx2, and illumination performance with all LED chips turning on. We can see that the illumination distribution of Tx1 and Tx2 are almost the same (the central position with a little different distribution can be ignored), which can provide a linear superposition for optical signals.
Fig. 3 The measurement of illumination distribution (a) The distribution of Tx1. (b) The distribution of Tx2. (c) The distribution of Tx1 + Tx2. (d) All the LED chips turning on.
3. Experimental setup
The experimental setup is shown in Fig. 4. The light source is an array of 2 × 2 LED lamps based on the aforementioned LED chips. The modulation signals from the arbitrary waveform generator (AWG, Tektronix AWG5012) are amplified to 4 Vpp and 8 Vpp respectively, and then superimposed onto the LED by aid of a direct current bias (DC-bias). A band-pass blue-filter of 400-480 nm is utilized in front of the analogue receiver to filter out the slow yellow component which is excited by the component of blue light. A commercially available silicon positive intrinsic negative (PIN) detector (PDA10A THORLABS) is used to receive the optical signal. The detection wavelength range of PD is about 200-1100 nm and effective area is 0.8 mm2. The received signal is amplified by a wideband amplifier (AMP) and connected to a low pass filter (LPF), then equalized by a first-order equalizer (RC post-equalizer), and finally connected to an oscilloscope (LeCroy SDA760Zi).
4. Experimental results
Firstly, the frequency response of electro-optical-electrical (EOE) channel is measured. A small-signal sine wave of 1 MHz to 115 MHz from AWG was employed to the LED, and the magnitude of the channel’s frequency response was measured. Figure 5 presents the normalized frequency response of measured about LED of S2ln. The blue circle denotes the measured 3 dB bandwidth of EOE channel, which is about 8 MHz. The solid line is a fitted curve by least square curve fitting using measured data. The EOE response is similar to a low pass response, and can be compensated by the first-order RC filter. The dashed line indicates the response of EOE after the 1st order equalizer. The equalizer includes a capacitor in parallel with a resistor (C = 12pF, R = 1.3kΩ) as shown in [9]. The bandwidth of the EOE achieves 20 MHz and the magnitude of the EOE response decays after the equalizer.
Fig. 5 The EOE channel response of measured (circle), fitted curve (solid line) and equalized by 1st order equalizer (dashed line).
The performances about OOK, 4-PAM and 4-SPAM are compared in experiment which is presented in Fig. 6. The results of BER and eye-diagram measurements when the data rate is varied for L = 0.5 m with 400mA of the DC-bias for the LEDs. The bit rate of OOK reaches 80Mbit/s. With 4-SPAM the bit rate of 120 Mbit/s is achievable with the BER of 1 × 10−3. Increasing the bit rate to 150 Mbit/s the system BER rises to 3 × 10−2. Furthermore, the BER performance of 4-PAM is very similar with 4-SPAM. Three illustrative eye-diagrams are inset in the BER graph. A clear and open eye-diagram can be obtained when bit rates reach to 80Mbit/s of OOK and 120Mbit/s of 4-SPAM (the same as 4-PAM). We can see that the 4-SPAM supports higher transmission rate than the modulation of OOK. Comparing to 4-PAM, 4-SPAM has the same performance, but 4-SPAM is low cost and is very simple for LED transmitter. Each LED emitter accepts the simplest signal of NRZ-OOK. Therefore, the 4-SPAM can reduce the complexity of the circuit for transmitter comparing to 4-PAM.
Finally, we have measured the dependence of the 4-SPAM system error performance on the illuminance level with the receiver in different position of the plane. Figures 7(a) and 7(b) show the BER of 4-SPAM at various points for L = 0.5 m, corresponding to the transmission rate of 100 Mbit/s and 120 Mbit/s respectively. The coverage within the target BER of 1 × 10−3 is measured. The coverage areas reach about 40cm × 40cm = 1600 cm2 and 10 cm × 10 cm = 100 cm2 with the bit rate of 100 Mbit/s and 120Mbit/s. The system error performance increases when the PD moves away from the centre of the coverage area, due to lower illumination intensity. Essentially, the receiver has a field of view that degrades by the case when the receiver is away from the centre. Employing a smaller L will increase the illumination intensity as well as lower error performance. If a larger LED arrangement is used, the coverage area will be improved.
Fig. 7 BER at various points on a plane at 0.5 m range for spacing = 14 cm. (a) The BER distribution of 100 Mbit/s. (b) 120 Mbit/s.
5. Conclusion
We propose and experimentally demonstrate the SPAM modulation with the RC post-equalization to compensate the channel fading for white-light LED VLC system. The commercially available LED with the modulation bandwidth of 8 MHz (with optical blue filter) is used. The experiment is operated using 2 × 2 LEDs and the transmission distance is about 0.4 m with the average illuminance level of 600 lx. The 2 × 2 LED array is designed to realize the 4-SPAM and indoor lighting. The base-band signal of NRZ-OOK is loaded onto the LED directly for each LED chip. The scheme presented is proved simple and low-cost without complex digital signal processing. The system shows error-free operation at 120 Mbit/s at the center of the coverage area with our scheme. It is expected that the higher transmission rate can be achieved with more LED emitters to achieve higher order SPAM such as 8-SPAM, 16-SPAM or 64-SPAM.
Acknowledgments
This research was supported in part by National 973 Program (No. 2013CB329204), National 863 Program (No.2013AA013601), National Science and Technology Major Project (No. 2010ZX03004-002-02) and Fundamental Research Funds for the Central Universities (BUPT2012RC0117), China.
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