VLC Physical Layer Design based on Pulse Position Modulation (PPM) for Stable Illumination

VLC Physical Layer Design based on Pulse Position Modulation (PPM) for Stable Illumination Angga Pradana 1, Nur Ahmadi 2, Trio Adiono 3, Willy Anugrah Cahyadi 4, Yeon-Ho Chung 5 1,2,3 Department of Electrical Engineering, School of Electrical and Informatics Engineering Bandung Institute of Technology, Jl. Ganesha No. 10 Bandung, 40132, Indonesia 1 angga.pradana@students.itb.ac.id 2 nurahmadi@stei.itb.ac.id 3 tadiono@stei.itb.ac.id 4,5 Department of Information & Communication Engineering Pukyong National University, Busan, Republic of Korea 4 wac.zze@gmail.com, 5 yhchung@pknu.ac.kr Abstract Visible Light Communication (VLC) is promising new technology to be applied in the lighting system infrastructure. This communication function should not interfere with the existing lighting system. In order to prevent from this interference, the utilized modulation scheme should have a limited dynamic range to avoid flickering and dimming effect. We propose Pulse Position Modulation (PPM) technique in our VLC system due to its very minimum dynamic range feature. This paper describes the design and analysis of VLC physical layer using PPM scheme to obtain a stable illumination. The design of physical layer consists of analog front end (AFE) circuit and the data processing which is implemented in microcontroller and FPGA. Based on the test results of our VLC system using low cost photo detector, the system is able to provide stable illumination while reaching the maximum data transfer rate of 20 kbps. Keywords: VLC, optical communication, PPM, physical layer, analog front end. I. INTRODUCTION Visible Light Communication (VLC) is a new alternative for wireless communication besides radio frequency (RF). VLC is one type of optical communication which uses visible light spectrum in 390-780 nm, therefore the spectrum availability is very large compared to RF. Indoor VLC draws increasing attention along with the use of solid state lighting as entire lighting infrastructure replacing conventional lighting sources such as fluorescent or incandescent lamp. One example of a popular solid state lighting is Light Emitting Diode (LED). LEDs have several advantages over conventional lighting, such as more efficient (energy saving), longer lifetime, and cheaper. LED can be switched in very high speed so that it is potentially to be used as a communication media. One of important aspects which becomes the major focus of the researchers in VLC is the modulation scheme. Several modulation techniques have been proposed by many researchers, such as On-Off Keying (OOK) [1], Pulse Width Modulation (PWM) [2], and Pulse Position Modulation (PPM) [3,4]. OOK is the simplest type of modulation in which data '0' is represented by light off, while the data '1' is represented by lights on or vice versa. Since the data can only be encoded in two states (On/Off), OOK is only able to modulate the digital data. As consequence, it is less efficient in bandwidth utilization. Another issue in the use of OOK modulation is the effect of the lighting functions [5]. As much as possible LED should not be experiencing the effects of flickering and dimming during the communication process takes place. In OOK modulation, the state of LED will greatly depend on the transmitted data. As a result, if we have a lot of '0' data, the LED will be in off state for a long time. It could cause flickering effect of the lighting system in the room. In PWM modulation scheme, the data is represented by pulse width, or better known as the duty cycle (abbreviated in D ). PWM can modulate analog data, therefore PWM has better bandwidth efficiency than OOK. Due to the fact that the pulse width depends on the data to be sent, then the average power at the transmitter will fluctuate. Therefore this modulation can also cause a dimming effect on the lighting system [6]. Another modulation scheme is PPM. In this modulation technique, the data is represented on the pulse position where the pulse width is fixed (constant duty cycle). By varying the pulse position, it is possible to modulate analog data. Therefore this modulation technique has a better bandwidth efficiency [7]. While the use of a constant pulse width will make the average power of the LED becomes more constant (smaller dynamic range), as a result the dimming effect can be minimized compared to PWM. PPM is also suitable to be applied in optical communication such as VLC, since it has small or no multipath interference [8]. In this paper, we propose the use of PPM scheme and implement it in microcontroller and FPGA to demonstrate the data communication system using VLC. This paper focuses on designing VLC physical layer utilizing PPM modulation to obtain 20 kbps data transfers without interfering with the function of lighting. The system is designed for text data transfer for digital signage application. The first step is to design AFE circuit on the receiver and transmitter. Furthermore, the AFE circuit is tested to evaluate its performance. The next step is designing data processing which is adjusted to the specifications of its AFE performance. PPM modulator, the data framing, and flow control are implemented on the microcontroller, whereas the PPM demodulator and synchronizer are implemented on FPGA. II. PWM VS PPM In order to propagate the data or information through the communication channel, they need to be modulated. A simple and reliable modulation technique which is suitable

for optical communications is pulse modulation. Pulse modulation is a type of digital transmission communication system. Digital transmission has advantages over analog transmission which is more resistant to noise. In digital transmission, the message signal represented in a discrete form in both amplitude and time. Two popular types of pulse modulation is pulse width modulation (PWM) and pulse position modulation (PPM). Both PWM and PPM are able to modulate the analog data. The working principle of these modulation is illustrated in Figure 1. According to (3), it is known that PWM average voltage will depend on duty cycle (D) variable. It means that PWM pulse in Figure 1 (c) has an average voltage D 1 v A during t 0 t 1, D 2 v A during t 1 t 2, D 3 v A during t 2 t 3, and D 4 v A during t 4 t 5. Because the value of D 1, D 2, D 3, and D 4 is different to each other, then the average voltage during the period of t 0 to t 4 will vary. Consequently, the intensity of the LED also changes. According to Figure 1, the higher the value of analog signal, the higher the LED intensity and vice versa. This varying average voltage causes dimming effect on the lighting system. This dimming effect causes discomfort to the human eye and should be minimized. Unlike PWM which has varying duty cycle, PPM has a fixed duty cycle. According to (3), the average voltage of PPM will always remain constant during t 0 until t 5 because the amplitude and the duty cycle are always constant. In other words, the transmitted data will not affect the LED voltage. It can be concluded that the intensity of the LED light will tend to be constant and not fluctuate as PWM. III. EXPERIMENT SETUP The overall block diagram of communication system used in this experiment is illustrated in Figure 2. The block diagram illustration of the system is depicted in Figure 2(a) while the real implementation is shown in Figure 2(b). The system design is point-to-point unidirectional communication. Fig. 1. Pulse modulation waveform. Figure 1 (a) and (b) show the shape of the signal information in analog form and the sampling pulse respectively. PWM and PPM signal are digital signals that have constant pulse amplitudes where the amplitude in Figure 1 is v A. In PWM, the pulse width on high level (referred to duty cycle) is proportional to the amplitude of the sampled analog signal. The larger the amplitude of the analog signal, the larger the duty cycle. Unlike PWM which has pulse with varying duty cycle, PPM signal has a pulse with a constant duty cycle. Sampled amplitude of the analog signal is mapped to the position of the pulse within a prescribed time slot. In Figure 1, the time slots for the PPM pulses is t s. The larger the amplitude of sampled analog signal then the PPM pulse position is located farther to the right in the time slot. Varying duty cycle (D) in PWM raises the issue of LED illumination function. The voltage average value of the PWM pulses in Figure 1 (c) will change every one cycle of the sampling pulses (t s ). ( ) (1) ( ) (2) because PWM signal is digital pulse where v high = v A and v low = 0, so (3) Fig. 2. Experiment setup The scenario in this experiment is sending the data text streaming from a personal computer (PC) in the transmitter

Fig. 3. Analog Front End (AFE) schematic to the PC in receiver. User enters the data of text in ASCII format via user interface application which is installed on TX PC. This data is sent to microcontroller via UART. Microcontroller serves as data processor in transmitter. Furthermore, the microcontroller will modulate the data using the PPM encoder. PPM signal is fed to the TX AFE circuit that serves as the LED driver. LED converts PPM signal from electrical into light energy and propagated in free space. In receiver, photodiode (PD) converts the emitted PPM optical signal back into electrical signals. RX AFE is used for signal conditioning so that the signal can be read by FPGA properly. FPGA serves as data processing at receiver. PPM decoder design is implemented in FPGA. The output of PPM decoder is text data in ASCII format. This data is sent into RX PC via UART. The stream of this data is then displayed on RX PC and subsequently stored for analysis purpose of whole communication systems. Data processor at transmitter uses microcontroller for PPM modulation process because the process tends to be simpler. At the receiver side, the data processor uses FPGA for PPM demodulation process because it is more complicated and requires higher accuracy of PPM pulse timing. A. Design of Analog Front End (AFE) The front end part in transmitter is LED while in receiver is photodiode (PD). The technical specifications of LED and PD used in this experiment are shown in Table 1. TABLE I. Component LED Photodiode LED AND PHOTODIODE SPECIFICATION Specification voltage rating : 12V power : 2W view angle : 100 o luminous intensity: 180-300 lx wavelength : 380 760 nm (white) spectral response : 400 1000 nm dark current : 10 μa rise/fall time : 5 μs When the input of LED driver receives logic '1' from microcontroller, the transistor will saturate and the LED turns on. In contrast, when it receives logic '0' from microcontroller, the transistor will cut-off and LED turns off. Figure 3(b) shows the AFE circuit at the receiver and the various signals formed in several test points. This circuit is used for signal conditioning which will make sure that the signal generated by PD can be read properly by FPGA. Because the output of PD is electrical current, it is necessary to convert the current into voltage using op-amp U1 in Figure 3(b). This op-amp circuit is also known as trans impedance amplifier (TIA) or inverting amplifier. Capacitor C1 (also called feedback capacitor) is used for opamp compensation so that the op-amp circuit becomes more stable. Afterwards, the signal is passed to the three stages of transistor circuit. PNP-type BJT in first stage serves to change the polarity of the op-amp output signal. The output signal of the op-amp has a negative amplitude, so it needs to be clamped into positive in order to be able read by FPGA correctly. Transistor circuit at the next stage shifts the voltage level of amplitude signal to 3.3V so that it complies with I/O voltage reference in FPGA. At the last stage, the transistor circuit is used for impedance matching between the AFE circuit with FPGA I/O. B. Data Processor Design The design of data processor is implemented in microcontroller at the transmitter side, while the data processor at the receiver side is implemented in FPGA. The flow chart of microcontroller program i s illustrated in Figure 4. At the transmitter, the AFE is formed by LED driver circuit. LED driver circuit is an amplifier with one transistor as shown in Figure 3 (a). PPM is a digital pulse signal, so that only required two conditions of the LEDs: on and off state. Fig. 4. The flow chart of microcontroller program

Fig. 6. Data processor core design for receiver Microcontroller will be in stand-by mode waiting for the arrival of data from TX PC. As soon as the data is received from the TX PC via USART, the data is converted into a bit stream and modulated by PPM modulator. Each 8-bit data will be given a header for the synchronization process. The output signal from microcontroller is fed to the TX AFE for subsequently transmitted to the receiver. As explained in Section II, PPM can modulate analog signal. It means that pulse signal can carry N messages bits which are encoded by transmitting a single pulse in one of 2 N possible required time shifts (N-ary PPM). The pulse is repeated every T seconds so the bit transfer rate is N / T bits per seconds. For testing purposes, the design of PPM in this experiment employs 1 bit and 2 bits per pulse modulation as illustrated in Figure 6. For 1-PPM, there are 2 1 possibilities time shifts. They are t 1 and t 2 to represent data '0' and '1' respectively as shown in Figure 5(a). For 2-PPM, there are 2 2 possibilities time shifts. They are t 1, t 2, t 3, and t 4 to represent data '00', '01', '10' and '11' respectively as shown in Figure 5(b). IV. TEST AND ANALYSIS The first testing needs to be done is part of AFE. It is necessary to know the capability and performance of the AFE circuit so that some design parameters on data processor can adjust the ability of the AFE. Afterwards, the whole communication system as shown in block diagram in Figure 3 is tested. A. Analog Front End Test (AFE) AFE testing includes the testing against the effect of frequency and distance. Frequency test needs to be carried out to determine the bandwidth of the AFE. VLC is a type of wireless communication so that it should be good enough to support mobility. Therefore, testing against distance is also required to be perfomed. To perform frequency response test, the microcontroller in transmitter is programmed to generate square wave with a frequency of 1 khz to 222 khz. Then the output signal from RX AFE observed with an oscilloscope. According to the observation, the use of frequencies above 20 khz begins to cause trans impedance amplifier (TIA) circuit on receiver unstable. The output signal suffers an oscillation as shown in Figure 7(a). It can be overcome by using a feedback capacitor (C1) in TIA circuit as a compensator. As a result, the output signal from the TIA becomes more stable as shown in figure 7(b). (b) 2-PPM Fig. 5. N-ary PPM. The data processor at the receiver is implemented on FPGA. The block diagram of the data processor core is depicted in Figure 6. The output signal from RX AFE is connected to an FPGA I/O (incoming signal). Header detector serves to detect the start bit of incoming data. Synchronizer block serves to determine the header bits, the first bit, second bit, and so on. The synchronizer controls PPM decoder, serial to parallel and USART module. Fig. 7. TIA output signal waveform TX input waveform and RX output waveform for frequency test of 1 khz to 222 khz is shown in Figure 8. Based on the observations, the higher the frequency of square wave signal, then the amplitude of the output signal of RX AFE getting smaller and more distorted.

Fig. 8. TX (yellow) and RX (blue) square wave signal at several frequencies Full graph plot for frequency response (khz) against gain amplifier (db) is presented in Figure 9. It is known that the AFE bandwidth is about 132 khz. Fig. 10. TX (yellow) and RX (blue) PPM waveform at several distance The complete graph plot for relation between LED- PD distance (cm) against gain amplifier (db) is presented in Figure 11. It is known that the maximum limit of the optimal range for this communication is 67 cm. Fig. 9. Square wave frequency response for 1 khz to 222 khz To obtain the best performance, PPM modulation requires sharp rise time and fall time pulse to keep the PPM pulse can be read correctly on the receiver. From the observation of the signal captured by the oscilloscope, note that the rise time (t r ) and fall time of the system transceiver is 10 µs. The rise time of this analog circuit should be much smaller than the period of the pulse PPM (T s ). To obtain the best performance, we select T s ten times t r, so ( ) (4) In this test, if we use a 2-PPM pulse period (T s ) of 100 μs, then transmit bit rate obtained is (5) Fig. 11. Effect of variable distance from 25 cm to 78 cm against received power at the receiver For transmission at longer distances (in this case more than 50 cm), the effect of ambient light cannot be ignored. Because this device is designed for indoor applications, the most likely ambient light in the room is type of artificial light source such as a fluorescent lamp which operates at a frequency of 50-100 Hz. To overcome this problem, it would require the addition of a type of filter circuit to eliminate the influence of ambient light. The next AFE testing is the effect of distance between LED and PD against gain amplifier. In this test, the microcontroller on transmitter is programmed to generate test vectors of PPM signal. Then the RX AFE output is observed on oscilloscope. The distance between LED and PD is varied from 25 to 80 cm. PPM signal waveform in TX and RX for several different distances which is captured by oscilloscope are shown in Figure 10. It can be observed that the greater the distance of LED - PD, then the amplitude of RX signal weakened. Fig. 12. Screen shoot of user application

B. Measurement of BER performance The last section of the experiment is a testing for the whole communication system. This test scenario is carried out by entering a few paragraphs of text via user applications installed on the TX PC as shown in Figure 12(a) and then transmit and display it on user application in RX PC as shown in Figure 12(b). Fig. 13. Bit error rate (BER) against LED-PD disance The received data on RX PC then stored and compared with data on the TX PC so that BER (bit error rate) of the communication system can be calculated. BER measurements is performed for distances of 25 to 65 cm. PPM modulation scheme which is used in this test are 1- and 2-PPM. PPM pulse frequency is set to 1 khz and 10 khz. BER measurement results against the distance for several data transfer rate is presented in Figure 13. Based on the graph, it is known that the higher speed of data transfer, then the data error is also higher. The farther the distance between LED-PD, the BER tends to get worsen. At the same frequency, 2-PPM tends to have a higher BER than 1 PPM. This is because the 2-PPM has bit resolution per-pulse lower than 1-PPM hence less tolerant of error time shift calculation in the demodulator. V. CONCLUSION One PPM pulse can bring up a few bits of data (N-ary PPM). The higher N is used, then the baud rate is also higher. Besides, PPM pulse has a smaller dynamic range of amplitude than other digital pulse modulations such as OOK and PWM. With a smaller dynamic range, PPM is able to minimize the effect of flickering and dimming that could disturb the lighting system. In this experiment, VLC physical layer system has been built successfully using PPM modulation scheme. The maximum data transfer rate up to 20 kbps is obtained. According to the experiment results, the higher the data transfer rate and the further the distance between LED-PD, the BER is getting worse. Therefore, for further development, we recommend to employ error detection or error correction method on this physical layer to improve the performance of this VLC system REFERENCE [1] D. Zhang, Y. Zhu, and Y. Zhang, Multi-LED Phase- Shifted OOK Modulation Based Visible Light Communication Systems, in IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 25, NO. 23, DECEMBER 1, 2013. [2] M. Saadi, L. Wuttisittikulkij, Y. Zhao, et al., Performance analysis of optical wireless communication system using pulse width modulation, in 2013 10th International Conference on Electrical Engineering/Electronics, Computer, Telecommunications and Information Technology (ECTI-CON), IEEE, 2013, pp. 1-5. [3] Ali, A.Y., Zaichen Zhang ; Baiqing Zong, Pulse position and shape modulation for visible light communication system, in 2014 International Conference on Electromagnetics in Advanced Applications (ICEAA). IEEE, 2014, pp. 546-549. [4] Shinwasusin, E.-A., Charoenlarpnopparut, C., Suksompong, P., Taparugssanagorn, A., Modulation performance for visible light communications, in 2015 6th International Conference of Information and Communication Technology for Embedded Systems (IC-ICTES). IEEE, 2015, pp. 1-4. [5] Hyung-Joon J., Jooj-Ho C., Eun Byeol C., Chung Ghiu L., Simulation of a VLC system with 1 Mb/s NRZOOK data with dimming signal, in International Conference on Advanced Infocom Technology 2011 (ICAIT 2011). IEEE, 2011, pp. 1-3. [6] M. Doshi and R. Zane, Control of Solid-State Lamps Using a Multiphase Pulsewidth Modulation Technique, IEEE Transactions on Communications, vol. 25, no. 7, pp. 1894 1904, 2010. [7] Rimhwan L., Kyungsu Y., Jong-Ho Y., et al. Performance analysis of M-ary PPM in dimmable visible light communications, in 2013 Fifth International Conference on Ubiquitous and Future Networks (ICUFN). IEEE, 2013, pp. 380-383. [8] Nan Wu, Xudong Wang, Hao Dai, Performance of indoor visible light systems using OOK and PPM modulations under multipath channels, in 2013 2nd International Workshop on Optical Wireless Communications (IWOW). IEEE, 2013, pp. 84-88.