Matlab based Platform for the Evaluation of Modulation Techniques used in VLC

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1 12 th International Conference on DEVELOPMENT AND APPLICATION SYSTEMS, Suceava, Romania, May 15-17, 2014 Matlab based Platform for the Evaluation of Modulation Techniques used in VLC Steven De Lausnay, Lieven De Strycker, Jean-Pierre Goemaere and Nobby Stevens Faculty of Engineering Science, DraMCo Research Group KU Leuven B-9000 Gent, Belgium Bart Nauwelaers Faculty of Engineering Science, TELEMIC, ESAT KU Leuven B Leuven, Belgium Abstract In this paper, we describe an experimental evaluation platform, which enables us to obtain the performance of baseband modulation techniques for usage in Visible Light Communication (VLC) applications. For the demonstration of the evaluation platform, we have chosen for Manchester encoding. This choice is exemplary, the described system is able to deal with a large set of analog and digital modulation techniques like Non-Return to Zero (NRZ), Return to Zero (RZ), Pulse Position Modulation (PPM), Binary Frequency Shift Keying, Dual Tone Multi-Frequency (DTMF), Phase Shift Keying (PSK), For the Manchester encoding, it is experimentally demonstrated that the statistical distribution of the received signal after matched filter operation, is Gaussian in nature. This allows us to make an estimation of the Bit Error Rate (BER) using Q-functions for Gaussian distributions. Our results demonstrate that the estimated BER strongly depends on the sampling rate at the receiver end. The sampling rate will therefor determine whether or not the receiver can receive Non- Line Of Side (NLOS) signals at normal light intensities. Keywords Baseband Modulation; Bit Error Rate; Evaluation Platform; Gaussian Noise distribution; Visible Light Communication I. INTRODUCTION Since a couple of years, the Light Emitting Diode (LED) has become a major player in the market of indoor and outdoor lighting applications. LEDs as a lighting technology offer a number of important advantages with regard to other lighting technologies, such as increased efficiency, high brightness, color selection without significantly compromising the efficiency, etc. [1]. Another distinctive characteristic of LEDs with regard to more classical lighting technologies is the larger bandwidth. This has opened the door for Visible Light Communication systems (VLC), in which the original function of lighting is combined with the creation of a wireless, optical communication system. An important boundary condition is of course that the lighting functionality is not jeopardized by the data communication system. Due to the large bandwidth of LEDs, this condition can be met. Because of the interesting properties of VLC, we see this technology as a new solution for localization in large indoor environments. The infrastructure for VLC is already in buildings because of the lighting functionality and the light has a high directivity compared to Radio Frequency (RF) technologies. These two advantages make VLC an interesting candidate for location-based services. With these services, the information on your smartphone would depend on your position within the building. This can be in the form of e.g., indoor guiding or location based information (you get extra information about the piece of art you are standing in front of at that moment). In order to consider VLC as an accurate localization system, the following conditions must be met: Positioning should be more accurate compared to the existing RF wireless indoor localization systems. The potential degradation of lighting function as a consequence of the overlay communication system should be held to a minimum, which corresponds to a unnoticeable difference for the observer. In order to get a rapid implementation, the infrastructural impact should be very low. For a localization system, every LED broadcasts its own unique identification (ID) and the user, equipped with a smartphone and photodiode, receives these IDs. Based on these IDs and a previously downloaded map of the indoor environment (containing the IDs of the LEDS and their position), the location can be calculated. For the transmission of the IDs, we want to explore a VLC system using Code Division Multiple Access (CDMA). As a first step, we have to evaluate the single source to single receiver behavior of different modulation techniques. With the proposed evaluation system, a wide range off modulation techniques can easily be evaluated. The goal of this paper is to describe this set-up and to demonstrate its functionality towards the BER evaluation for Manchester encoding. Specific emphasis is put on the discrete sampling property of the acquisition chart. The paper is organized as follows. In section II, the general description of the platform is given, with an emphasis on the specific hardware components that were used. In section III, we focus on one modulation technique, namely Manchester /14/$ IEEE 57

2 A. Transmitter At the transmitter side, random bits are generated at a bit rate R b. The bits are modulated with a baseband modulation technique, like Non-Return to Zero (NRZ), Return to Zero (RZ), Manchester Coding, Pulse Position Modulation (PPM), Binary Frequency Shift Keying (BFSK), etc. These are all baseband modulation techniques with low complexity, so they can be easily implemented. The modulated sequence is upsampled by N where N is the ratio of the bit period (T b ) to the sample period (T S ). The up-sampled sequence is fed to the pulse shaping, which produces a discreet time pulse train at the sample rate TS (Equation 1). The discrete time pulse train is directly fed to the LED driver through an analog or digital output port of the DAQ which results in an analog signal s(t) (Equation 2) [2]. The a(k) coefficients depends on the chosen modulation technique. Further details can be found in [2]. Fig. 1 Practical set-up room encoding. The sampling rate is left as a parameter in a non line of sight configuration. Based on our observation, it is clear that the BER strongly depends on the sampling rate at the receiver end. At the end, a summarized overview of the main topics covered in this paper can be found in the conclusions section IV. II. DESCRIPTION OF THE PLATFORM Fig. 1 gives an overview of the point-to-point VLC system under consideration. The system consists of three building blocks, namely the transmitter (LED) which is mounted at the ceiling, the receiver (photodiode) placed on a desk and the room (channel) with a window, to introduce additional noise. An overview of the different blocks of the evaluation platform is given in Fig. 2. For quick evaluation and no synchronization problems, the transmitter and receiver are both implemented in Matlab. The connection to the hardware is made with the National Instruments USB-6211 Data Acquisition box (DAQ). ( ) = a( k) p( nt S kt b ) s nt S s t (1) k (2) ( ) = a( k) p( t kt b ) k In order to accelerate the implementation of the technology, we use LEDs, which are currently used for lighting applications. High power LEDs currently used for lighting applications are composed of a blue LED with a phosphor layer. Previous research has show that the phosphor layer will limit the optical bandwidth of the LED and thus the maximum data rate for communication [3] [5]. For the experimental setup described in this paper, the Bridgelux BXRA-30E0740-A- 00 is used. This is a neutral white LED with a Lambertian C- plane and a total Luminous flux of 820 lm. For the tests, the LED is mounted onto the ceiling at a height of 3.5 m (Fig. 1). As driving circuit for the LED, there are two possibilities namely a LED driver that is currently used in lighting applications or a Trance Conductance Amplifier (TCA). When a LED driver is used, the digital Pulse Width Modulation (PWM) input can be used to inject the data into the LED current. To perform the measurements in this paper, a circuit was build with the LT3756 from Linear Technologies. This driver can control LEDs up to 30 W electrical power. It was Fig. 2 The practical evaluation system 58

3 synchronization issues. Remark that the delay due to the channel is in the order of several nanoseconds for indoor environments. This can be neglected for data rates up to several Mbps [9]. The sampled version of the received signal (r(nts)) is filtered by a matched filter with a impulse response which is the time reversed version of the pulse shape filter. The output of the matched filter is given by Equation 3 [2]. x nt S mt S ( ) = r( ) p m n m (( )T S ) (3) The matched filter output is down-sampled to one sample/bit at n = kt b / T S which produces a sequence x(ktb). The decision block will estimated the received bits and compare them with the send bits to calculate the BER. For the performed test in Section III, the receiver is placed on a desk at a distance of 2.7 m from the LED. Furthermore, we have rotated the receiver over 90 degrees, in order to have a non line of sight (NLOS) configuration. As can be seen on Fig. 3, there is at least one reflection required for the transmitted data to reach the photodiode. Fig. 3: Practical set-up room NLOS possible to get bit rates up to 2 Mbps before the functionality of the driver was corrupted. The driver is limited for the number of modulation types possible namely only digital modulation types with full On-Off Keying (OOK). When analog modulation techniques or digital modulation techniques with more flexibility in modulation depth want to be evaluated, a TCA can be used. In [6] they proposed a block diagram of a TCA transmitter for VLC. This block diagram leaves a lot of possibilities for the implementation design, where we build several transmitters. The final design has an electrical bandwidth of about 20 MHz and can handle digital and analog signals [7]. The TCA can drive the LED with a current swing from 0 ma up to 1000 ma which is enough to drive LEDs currently used in lighting applications which have typical currents from 350 ma or 700 ma. B. Receiver The receiver part starts at the photo diode (Fig. 2) where the incoming light is transformed to a current. The generated current will depend on the responsivity curve of the photodiode. Because the LED has a high blue component in the spectrum [7], the responsivity curve of the photo diode should be high in the wavelength region of 400 nm. This specification was crucial for the selection of the appropriate detector. After some research, a detector from Thorlabs was found to be the most suited to fulfill this requirement. The PDA10A is a fixed amplified detector with a Si-photo diode which is sensitive in the visible light region and has a active area of 0.8 mm2. The current from the photodiode is converted to a voltage with a Trans-Impedance Amplifier (TIA). The PDA10A has a build in TIA with a gain of 10 kv/a [8]. The output voltage of the TIA is amplified a last time with a voltage amplifier, resulting in the analog signal r(t). The signal r(t) is the transmitted signal s(t) plus noise from the surrounding light. The received signal r(t) is sampled using the DAQ at the same rate of the transmitted signal with synchronized clocks. This way, there are no C. Room The room is a normal working office with gray plaster walls and a large window over the entire length of one side of the office. The room will have an impact on the channel impulse response and will allow noise to intrude the signal for an optical communication link. Because the data is traveling at light speed, the delay spread will be in the order of nanoseconds. For localization purposes, a high bit rate is not necessary, so the influence of Inter Symbol Interference (ISI) can be neglected [9]. Two important noise sources in a wireless optical link are shot noise and terminal noise. Terminal noise is introduced by the pre-amplifier electronics and is thereby signal independent Gaussian noise [9] [11]. Terminal noise will be the dominant noise source when there is less ambient light (sun light, other light sources, etc.). When a well-developed pre-amplifier is used, the limiting noise factor is shot noise. This noise source is introduced by ambient light as well as the received signal power and can be modeled as Additive White Gaussian Noise (AWGN) [9] [11]. As shown in Fig. 1 the room has a window that will allow sunlight to fall in onto the receiver, which will introduce shot noise. III. RESULTS A. Modulation techniques The modulation technique we have chosen is the Manchester encoding. Remark that our set-up is not limited to Manchester encoding; other binary line codes can be easily implemented in the Matlab environment as well analog modulation techniques. Manchester encoding is a split-phase code, which is well suited for our purpose. In case of nonreturn to zero codes, a long sequence of zero s could introduce flickering of the light intensity, which violates the compatibility with the normal lighting function. Remark that the bandwidth of the human eye is around 60 Hz. The zero 59

Other advantages of Manchester coding compared to other baseband modulation techniques, are the easy clock recovery (bit synchronization) and the received signal can be ACcoupled at the receiver.

4 Fig. 7 Distribution x(kt b) for N = 50 Fig. 5 The average value µ as function of N Fig. 6 Distribution x(kt b) for N = 200 crossing bandwidth of Manchester encoding is twice the inverse of the bit time Tb, which has a positive impact on flickering, even at lower data rates. Other advantages of Manchester coding compared to other baseband modulation techniques, are the easy clock recovery (bit synchronization) and the received signal can be ACcoupled at the receiver. In a VLC system the transmitted signal can not be negative (s(t) > 0) and so also the received signal at the photo diode. In the specific experiment we performed, 106 bits were sent at a data rate of 1 kbps by use of Manchester encoding. The zero crossing bandwidth is 2 KHz and the duration of the experiment was 1000 seconds, or almost 17 minutes. B. Influence of the sample rate One parameter that is left undetermined right now is the ratio N, defined as the sample rate (1/T S ) to the bit rate (1/T b ). According to the zero crossing bandwidth and the Nyquist sampling theorem, a value for N of 4 should be sufficient. The critical component in the practical evaluation system of Fig. 2 is the discrete calculation of the matched filter output. The matched filtering operation, which is basically the calculation of an integral in the continuous time domain, is approximated by a finite sum. It is clear that this sum is a better Fig. 4 The standard deviation σ as function of N approximation as the number of sample points increases. In order to evaluate the impact of the sampling rate, we have chosen a high sample rate, namely 200 ksps, which results in 200 (= N) samples every bit time Tb. Once we have this high sample ratio, we can easily reduce N by only selecting a reduced set of these samples. On Fig. 4 and Fig. 5, one can observe the distribution of the measured x(ktb) samples for N = 50 and N = 200 when the set-up is used of Fig. 3. An observation one can make is that the scaled histograms can be well approximated by a Gaussian distribution. We have plotted the magnitude of the average value µ and the standard deviation σ as a function of N. The result can be found on Fig. 6 and Fig. 7. There is no distinction between these trends for a 0 or 1-bit sent. To determine whether the histograms of the received data have a Gaussian distribution, a KolmogorovSmirnov test (KS-test) was performed. This test confirmed that the received data has a Gaussian distribution, so proves that the noise introduced in a VLC is AWGN. Based on the histogram plots, it is clear that lower values of N will lead to an increased BER. This trend is clearly 60

5 For an indoor localization system, this will be an imported factor, which will determine if the positioning algorithm could take into account reflections. When there are a lot of LOS signals, it will be better to decrease the sample rate, so reflections don t influence the positioning algorithm. When there are no LOS signals, the sample rate could be increased to still receive an ID from LEDs so a rough estimation of the position could be made. By changing the sample rate, a hybrid system could me made where a rough position estimation could be made when there are no LOS signal and an accurate position could be calculated when different LOS signals are received. ACKNOWLEDGMENT The authors would like to thank the Agency for Innovation by Science and Technology in Flanders (IWT). Fig. 8 The estimated BER as function of N demonstrated on Fig. 8. The test was also repeated with the setup from Fig. 1. These results showed the same trend for the BER as the one shown on Fig. 8 but a couple of orders lower. This shows that the sample rate (1/T S ) will strongly determine if the receiver will be able to also receive NLOS signals in a VLC system. This can be an imported factor when the system is used as an indoor localization system. IV. CONCLUSION In this paper we have presented a platform that allows us to evaluate analog and digital modulation techniques. With this platform, modulation techniques like NRZ, RZ, PPM, Manchester Coding, BFSK, DTMF, PSK, etc. can easily be synthesized and analyzed using Matlab. The system allows us to quickly compare the different modulation techniques and to evaluate which encoding technique is most suitable to be used in an indoor localization system based on LED lighting. In this paper, Manchester Coding is chosen because it doesn t introduce flickering at low bit rates, an easy bit synchronization is possible and the received signal can be amplified with an AC- coupled amplifier. The results of the practical set-up have shown that the BER of a VLC system will depend on the sample rate of the received signal. When the ratio of the sample rate to bit rate increases, this decreases exponentially the BER of the system. For e.g., a NLOS signal, the ratio sample rate over bit rate will determine whether the BER becomes acceptable or not. REFERENCES [1] Company McKinsey, Lighting the way: Perspectives on the global lighting market, McKinsey & Company, August, [2] M. Rice, Digital Communications: A Discrete-Time Approach, Pearson Education, ISBN: , [3] D. C. O Brien and L. Zeng and H. Le-Minh and G. Faulkner and J. W. Walewski and S. Randel, Visible Light Communications challenges and possibilities, Personal, Indoor and Mobile Radio Communications, [4] L. Zeng and D. O Brien and H. Le-Minh and K. Lee and D. Jung and Y. Oh, Improvement of Date Rate by using Equalization in an Indoor Visible Light Communication System, IEEE, [5] H.Le-Minhand and D. O Brienand and G. Faulknerand and L. Zengand and K.Lee and D. Jung and Y. Oh, 80 Mbits/s Visble Light Communications Using Pre-Equalized White LED, IEEE ECOC, [6] R.Baumgather, A. Kornbichler, J. W. Walewski, High-power highbandwidth linear driving circuit for VLC applications, IEEE P Task group Visible-Light Communication. March 2010 [7] Steven De Lausnay, Lieven De Strycker, Jean-Pierre Goemaere, Bart Nauwelaers and Nobby Stevens, Design of a Visible Light Communication Transmitter for the Evaluation of a Wide Range of Modualtion Techniques, IWOW 2013, October 2013 [8] Thorlabs, PDA10A, [9] T. Komine, Visible Light Communication and Its Fundamental Study, PhD Thesis, 2005 [10] T. Komine and M. Nakagawa, Performance Evaluation of Visible-Light Wireless Communication System using White LED Lightings, IEEE, [11] J. M. Kahn and J. R. Barry, Wireless Infrared Communications, Proceedings of the IEEE, February, No. 2, Vol. 85,

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