Experimental Characterization of RGB LED Transceiver in Low-Complexity LED-To-LED Link

Letter Experimental Characterization of RGB LED Transceiver in Low-Complexity LED-to-LED Link

Mariam Galal 1,2 , Wai Pang Ng 1,* , Richard Binns 1 and Ahmed Abd El Aziz 2

1 Faculty of Engineering and Environment, Northumbria University, Newcastle upon Tyne NE1 8ST, UK; [email protected] (M.G.); [email protected] (R.B.) 2 College of Engineering and Technology, Arab Academy for Science, Technology & Maritime Transport, Alexandria P.O. 1029, Egypt; [email protected] * Correspondence: [email protected]

Received: 5 August 2020; Accepted: 9 October 2020; Published: 10 October 2020

Abstract: This paper proposes a low-complexity and energy-efficient light emitting diode (LED)-to-LED communication system for Internet of Things (IoT) devices with data rates up to 200 kbps over an error-free transmission distance up to 7 cm. The system is based on off-the-shelf red-green-blue (RGB) LEDs, of which the red sub-LED is employed as photodetector in photovoltaic mode while the green sub-LED is the transmitter. The LED photodetector is characterized in the terms of its noise characteristics and its response to the light intensity. The system performance is then analysed in terms of bandwidth, bit error rate (BER) and the signal to noise ratio (SNR). A matched filter is proposed, which optimises the performance and increases the error-free distance.

Keywords: LED photodetector; LED-to-LED communication; BER; matched ﬁlter; visible light communication (VLC)

1. Introduction The Internet of Things (IoT) requires that more and more devices be connected to each other. These devices are not just limited to computers and smartphones, but also include common household appliances such as washing machines, coffee machines, refrigerators as well as personal clothes and toys [1]. It is common nowadays to have tens or even hundreds of devices connected to the internet in one household. The connectivity for those smart appliances and devices is putting a great pressure on the spectrum of the radio frequency (RF) communication, which is already overcrowded [2]. Many researchers are therefore focused on finding out-of-the-box solutions for this so-called ‘spectrum crunch’ by proposing smaller cell-sizes, where frequency reuse can be employed. In these nano and femto-cells, the short-range and low-bandwidth communications between IoT devices can be achieved without overstressing the RF spectrum [3]. It remains a challenge however to find the appropriate low-power, low-complexity and low-cost communication technology to serve the needs for IoT devices. wireless fidelity (Wi-Fi) and Bluetooth low energy (BLE) are the “go-to” option for many of those smaller devices. While Wi-Fi operates on relatively high power and has an extended range, BLE is favoured as standard in many IoT applications because of its relatively high data rate (up to 2 Mbps) and its range extending to 200 m [4]. However, both Wi-Fi and Bluetooth are prone to eavesdropping and can cause interference with other RF technologies [5]. They are therefore not used in RF sensitive environments such as in some hospitals and control rooms. Similarly, many users refuse to rely on them for transmission of secure data such as during contactless payment or personal identification because they are easy to hack. These drawbacks provided a rich ground for the use of visible light communication (VLC) in IoT devices especially in indoor environments [6].

Sensors 2020, 20, 5754; doi:10.3390/s20205754 www.mdpi.com/journal/sensors Sensors 2020, 20, 5754 2 of 12

As a complementary technology to RF,VLC is gaining popularity due to its low energy consumption and inherent security against eavesdropping [6]. VLC signals cannot penetrate walls and obstacles and are therefore confined within a room, safe from hacking by outsiders not in line of sight with the transmitter. Light emitting diodes (LEDs) are at the core of contemporary energy efficient lighting, which at the same time, can be deployed as VLC transceivers, a poignant step towards green communications. Due to its low power consumption, its small cell size and its immunity to interference from RF, VLC is considered a major player in the era of 5G and IoT [7]. The research in visible light communication is currently divided into two paths: on the one hand, researchers are pushing the bandwidth, range, and speed limits of VLC to compete with and complement high-end RF communication technologies. On this path, VLC is used for high-speed data streaming, internet connection and telephony. For instance, the company PureLifi provides a VLC solution to equip homes and offices with high-speed internet and connectivity over visible light [8]. Researchers from Fraunhofer Heinrich Hertz Institute proposed a 500 Mbps bidirectional VLC system using orthogonal frequency division multiplexing (OFDM) as modulation technique [9]. A 5 m free-space link based on a blue laser diode is proposed in [10], which can provide up to 9-Gbps quadrature amplitude modulation (QAM)-OFDM communication over visible light. Those high-speed solutions mark VLC as a high-speed communication technology and enable it to compete with other RF technologies in this rank. However, they are expensive and complex and are therefore considered an overkill for simple IoT devices. On the other hand, the other path considers employing VLC in applications that require low cost of implementation, wide availability, and easy commercial deployment. These applications need only infrequent data transmission, low data throughput and low communication speeds over a short range [11]. The system proposed in this paper follows that low-cost-of-implementation path. A traditional VLC system transmitter consists of an LED controlled by a driver circuit to transmit modulated light at a higher flickering rate than the human eye can perceive. The most commonly used photodetector circuit is usually equipped with a PIN or avalanche photodiode followed by a transimpedance amplifier (TIA) [7]. Such VLC system configuration however has the limitation of having a single directional, simplex channel nature. If a bidirectional or duplex system is needed, another transmitter-receiver pair must be implemented, which doubles the cost, space and complexity of the system. This sets VLC back in comparison to other RF technologies such as Bluetooth or Wi-Fi. Those latter technologies possess a single antenna on each end of the communication system acting as both a transmitter and receiver for RF signals. LED-to-LED communication is a subcategory of VLC that employs the same LED for transmitting and receiving signals, decreasing thereby the cost, complexity, energy and space requirements even further. It reduces the cost of the system by replacing the costlier photodetector and upgrades the simplex channel of the VLC system to time-multiplexed half-duplex channel, hence halving the total number of transceiver devices [12]. The structure of a PN junction allows the LED to sense an incoming beam of light with zero or reverse bias [13]. Although its sensitivity is low compared to the PIN photodiode (due to the missing intrinsic layer), it can be employed in simple low-data-rate, short-distance and low-cost IoT devices [14,15]. Dietz et al., demonstrated the potential of using the LED as photosensor for ambient light by making use of its junction capacitor [12]. Employing such method, Disney Research Lab, Zurich, Switzerland, achieved a toy-to-toy communication channel at 1 kbps over small distances reaching 5 cm [8,9]. Giustiniano et al., proposed a similar system with data rates up to 14 kbps, reaching a distance of 20 cm based on the same principle of operation [16]. However, depending on the internal capacitor of the LED for photodetection is not a reliable solution. The capacitance varies according to the make and model of the LED. Furthermore, the long discharging time results in slow communication. Galal et al., showed the LED’s ability as photodetector where the sub-LEDs of an RGB LED can have a bandwidth up to 6 MHz when reverse biased [13]. The same study also concluded that such Sensors 2020, 20, 5754 3 of 12

LED could detect light in photovoltaic mode (with no bias) and when slightly forward biased up to 2V. Furthermore, the experimental results proved that the red LED has the highest responsivity out of all three sub-LEDs to the light emitted from the green sub-LED. Hence the combination of red-to-green has been identified as the best-case scenario for a successful LED-to-LED link and is therefore adopted in this paper. A 100 Mbps LED-to-LED communication system was designed by Stepniak et al., [17] and Chun et al., [18]. The high data rate was achieved through design optimization of a receiver circuit and the careful choice of high-quality communication LEDs along with complex modulation and wavelength division multiplexing techniques. The proposed systems in [17] and [18] are based on expensive and specifically designed single-colour LEDs, as well as complicated optics and heavy offline processing. Such a solution is too complex for IoT devices. Many researchers have already proposed VLC systems with RGB LEDs as transmitters and light sources, but not as photodetectors. For instance, Shrestha et al., proposed an indoor VLC system employing an array of RGB LEDs as transmitters on the ceiling, which has a range of 2 m and can provide data rates up to 19.2 kbps [19]. Tothe best of the authors’ knowledge, the gap in the field of LED-to-LED communication mentioned in literature is threefold: Firstly, only single-coloured (mostly red and amber) LEDs are employed. Secondly, the research focuses on increasing the data rate and system performance, increasing thereby the cost and complexity by using complex lenses and concentrators [18]. This disagrees with the nature of communication required by typical IoT devices. Finally, the energy response and noise characteristics of an off-the-shelf RGB LED as a photodetector have not been investigated. In order to fill those gaps, this paper proposes an LED-to-LED system based on off-the-shelf RGB white LEDs which are more favourable for lighting and indicators in IoT than single-coloured LEDs. It also offers reduced cost and complexity by stripping the system of all complicated circuitry, optics and processing-hungry techniques. This paper implements and experimentally verifies the proposed system with off-the-shelf components costing as low as USD 2.60 per communicating device. Finally, this paper studies the noise characteristics of the LED photodetector and verifies that it is additive white Gaussian noise (AWGN). For that reason, a matched filter is proposed to achieve error-free transmission over a 7 cm distance with a speed up to 200 kbps, reaching thereby better performance compared to a PIN photodiode employed in the same zero bias scenario and meeting the requirements for IoT devices. The proposed low-complexity LED-to-LED link can be applied to any application that requires low data rate and short-range line-of-sight communication. It is especially useful in applications where small chunks of secure data are transmitted, such as the exchange of biometric information, user identification, contactless payment as well as for communication between smart devices in a personal or body area network or for infotainment systems [20]. Other applications that could benefit from the LED-to-LED communication include sensing applications such as high-accuracy distance sensors (as replacement for ultrasonic distance sensors) and biomedical sensors [12]. Last but not least, LED-to-LED has a high potential for being used in bidirectional opto-coupling and opto-insulating applications.

2. Experimental Setup White light can be produced using either blue-phosphorus or RGB LEDs. While the former is not suitable for LED-to-LED communication because the phosphorus coating prevents the LED from receiving light, the latter can emit and detect simultaneously through three independent channels [13]. This paper utilizes common cathode RGB LEDs (Kingbright) on both ends of the communication link. The green and red sub-LEDs are employed for transmission and photodetection respectively, as recommended by the findings in [13]. The block diagram of the experimental setup is shown in Figure1. The transistor-based LED driver biases the green sub-LED with 15 mA while keeping the energy consumption at a minimum and allows fast on-off switching of the transmitter LED [21]. The waveform generator transmits a non-return-to-zero (NRZ) on-off-keying (OOK) modulated cyclic and pseudorandom binary sequence Sensors 2020, 20, 5754 4 of 12

(PRBS) of length 220 1 bits at 10, 50, 100 and 200 kbps. The sequence is generated using linear − feedback shift registers applying the monic polynomial:

20 3 Sensors 2020, 20, x FOR PEER REVIEW PRBS20 = x + x + 14 of (1) 12 Sensors 2020, 20, x FOR PEER REVIEW 4 of 12

FigureFigure 1. 1.Block Block diagram diagram of of the the proposed proposed experimental experimental setup. setup. Figure 1. Block diagram of the proposed experimental setup. ForFor testing testing the the LED’s LED’s response response and noiseand characteristicsnoise characteristics as photodetector, as photodetector, the waveform the generatorwaveform For testing the LED’s response and noise characteristics as photodetector, the waveform isgenerator replaced is with replaced a constant with (DC)a constant signal (DC) to keep signal the to transmitter keep the transmitter LED on. LED on. generator is replaced with a constant (DC) signal to keep the transmitter LED on. AtAt the the receiver receiver end end withwith aa varyingvarying channelchannel distancedistance dan an identical identical RGB RGB LED LED (to (to the the transmitter) transmitter) At the receiver end with a varying channel distance an identical RGB LED (to the transmitter) isis connectedconnected toto the TIA TIA based based on on an an operational operational am amplifierplifier chip chip (OPA380) (OPA380) as asseen seen in Figure in Figure 2. The2. is connected to the TIA based on an operational amplifier chip (OPA380) as seen in Figure 2. The Thetransmitter transmitter andand receiver receiver circuits circuits are aregalvanically galvanically completely completely separated. separated. ForFor comparison comparison purposes, purposes, the transmitter and receiver circuits are galvanically completely separated. For comparison purposes, the thereceiving receiving LED LED is replaced is replaced with with a PIN a PINphotodiode photodiode (BPW34 (BPW34 Vishay Vishay Semiconductors) Semiconductors) under under the same the receiving LED is replaced with a PIN photodiode (BPW34 Vishay Semiconductors) under the same samemeasurement measurement conditions. conditions. Since Since reverse reverse biasing biasing the theLED LED has hasonly only a negligible a negligible effect effect on on improving improving its measurement conditions. Since reverse biasing the LED has only a negligible effect on improving its itsbandwidth bandwidth and and sensitivity sensitivity [13] [13 ]un unlikelike PIN PIN photodiodes, photodiodes, the the photovoltaic mode isis chosenchosen forforthe the bandwidth and sensitivity [13] unlike PIN photodiodes, the photovoltaic mode is chosen for the detectiondetection circuit. circuit. The The detected detected signal signal from from the the TIA TIA feeds feeds into into an optimisedan optimised matched matched filter, filter, to mitigate to mitigate the detection circuit. The detected signal from the TIA feeds into an optimised matched filter, to mitigate LEDthe photodetectorLED photodetector noise enoiseffect and effect is captured and is captured on an oscilloscope on an oscilloscope (Picoscope (Picoscope 6404D). Offl 6404D).ine processing Offline the LED photodetector noise effect and is captured on an oscilloscope (Picoscope 6404D). Offline isprocessing for performance is for performance analysis. analysis. processing is for performance analysis.

Figure 2. Transimpedance ampliﬁer (TIA) circuit diagram. Figure 2. Transimpedance amplifier (TIA) circuit diagram. To achieve good synchronizationFigure 2. Transimpedance between amplifier the transmitter (TIA) circui andt diagram. receiver circuits and hence a minimumTo achieve BER, a preamblegood synchronization consisting of abetween sequence the of ‘1’transmitter and ‘0’ bits and is transmittedreceiver circuits at the and begin hence of the a transmissionminimumTo achieve BER, phase. gooda preamble The synchronization detecting consisting circuit ofbetween uses a sequence this the preamble transmitterof ‘1’ and to detect ‘0’ and bits the receiver is rising transmitted edge,circuits know at and the the hencebegin exact ofa bitminimumthe duration transmission BER, and a adjust phase.preamble the The threshold. consisting detecting After ofcircuit a thesequence uses preamble, this of preamble‘1’ the and detecting ‘0’ to bits detect processor is transmitted the rising can sample edge,at the know atbegin a time theof thattheexact transmission roughly bit duration falls phase. at and the middle adjustThe detecting the of thethreshold. received circuit Afteuses bitr (at thisthe the preamble,preamble widest eye tothe opening).detect detecting the rising processor edge, can know sample the exactat a Thetime bit proposedduration that roughly and system fallsadjust employsat thethe threshold.middle only of a singletheAfte receivedr the 5 V preamble, power bit (at supply the the widest detecting and consumeseye opening).processor a maximum can sample of 110at a mW.timeThe that Careproposed roughly has beensystem falls taken atemploys the to middle choose only of a components singlethe received 5 V power which bit (at supply arethe cheapwidest and consumes andeye availableopening). a maximum oﬀ-the-shelf. of 110 mW.The Care proposed has been system taken toemploys choose on componentsly a single 5whic V powerh are cheapsupply and and available consumes off-the-shelf. a maximum The of 110low mW.price Care and has power been takenconsumption to choose are components attractive whicfeaturesh are forcheap IoT and device available manufacturers off-the-shelf. and The mass low priceproducers. and power The experiments consumption take are place attractive in a well-lit features lab withfor IoToverhead device artificial manufacturers lighting placedand mass at a producers.distance of The approximately experiments 2 mtake from place the in testing a well-lit bench. lab Since with theoverhead ambient artificial light is perpendicularlighting placed to at the a distancetransmission of approximately direction of the2 m LED-to-LED from the testing link bench.and the Since cone the of acceptanceambient light of theis perpendicular LED is proven to to the be transmissiononly 18° according direction to [12],of the the LED-to-LED ambient light link has and no the effect cone on of the acceptance receiver LED. of the Furthermore, LED is proven the to LED be only 18° according to [12], the ambient light has no effect on the receiver LED. Furthermore, the LED

Sensors 2020, 20, x FOR PEER REVIEW 5 of 12 photodetectorSensors 2020, 20, 5754 sensitivity is too low to detect indirect ambient light from the ceiling lights [13,22].5 ofFor 12 those reasons, the ambient light is found to have a negligible effect on the experimental results presentedThe low price in this and paper. power consumption are attractive features for IoT device manufacturers and mass producers. The experiments take place in a well-lit lab with overhead artificial lighting placed at a 3.distance Results of and approximately Analysis 2 m from the testing bench. Since the ambient light is perpendicular to the transmissionThe compiled direction results of theare LED-to-LEDthe averages linkof the and series the of cone repeated of acceptance measurements of the LED in all is experiments. proven to be only 18◦ according to [12], the ambient light has no effect on the receiver LED. Furthermore, the LED 3.1.photodetector Response of RGB sensitivity LED Photodetector is too low to detect indirect ambient light from the ceiling lights [13,22]. For those reasons, the ambient light is found to have a negligible effect on the experimental results The aim of this experiment is to deduce the response of the RGB LED as photodetector to presented in this paper. incoming light at different energy levels and to relate the signal decay response to a well-known function.3. Results The and obtained Analysis energy levels at each distance are shown in Figure 3a. The graph shows a significant but expected drop up to a transmission distance of 1 cm and proves that the LED photodetectorThe compiled can be results described are the by averages the inverse of the square series law of repeatedof light 1/ measurements at distances in alllonger experiments. than 0.5 cm [10]. The measured signal energy also shows an overlap with the negative exponential function. 3.1. Response of RGB LED Photodetector Therefore, the response can be best described as a two-term exponential function as follows: The aim of this experiment is to deduce the response of the RGB LED as photodetector to incoming = 1.9 10 . +9.110 . (2) light at different energy levels and to relate the signal decay response to a well-known function. whereThe obtained is the energy transmission levels at distance. each distance are shown in Figure3a. The graph shows a significant but expected drop up to a transmission distance of 1 cm and proves that the LED photodetector can be 3.2.described LED Photodetector by the inverse Noise square Measurement law of light 1/r2 at distances longer than 0.5 cm [10]. The measured signalIn energyorder to also analyse shows the an noise overlap of the with LED the as negative a photodetector, exponential a DC function. signal is Therefore, transmitted. the Then, response the DCcan component be best described of the received as a two-term signal exponential is subtracted function to retrieve as follows: the receiver noise. Figure 3b depicts the resultant noise probability density function5 has2.8 ad normal distribution,7 0.89d as expected. The performed E = 1.9 10− e− + 9.1 10− e− (2) FFT indicates that the noise is white.× Hence,× the noise ×of the ×LED photodetector is defined to be additivewhere d iswhite the transmissionGaussian noise distance. (AWGN). obability density functiondensity obability (PDF) r P

(a) (b)

FigureFigure 3. CharacteristicsCharacteristics of of a a light light emitting diode (LED) photodetector: ( (aa)) received received signal signal energy energy decaydecay with with increasing increasing distance; distance; ( (bb)) histogram histogram of of LED LED photodetector photodetector noise noise signal signal and and its its fitting ﬁtting to to a a GaussianGaussian function. function.

3.3.3.2. Bandwidth LED Photodetector of the LED Noise Receiver Measurement Circuit TheIn order bandwidth to analyse of thethe noiseLED ofreceiver the LED circuit as a photodetector, with the proposed a DC signal TIA is is practically transmitted. investigated Then, the DC to identifycomponent the ofmaximum the received data signalrate and is subtracted its simulated to retrieve Bode plot the is receiver shown noise.in Figure Figure 4. 3Theb depicts Bode plot the suggestsresultant that noise the probability TIA circuit density acts as function a low pass has filter a normal with distribution, a 3 dB bandwidth as expected. of around The performed140 kHz. For FFT a non-return-to-zeroindicates that the noise (NRZ) is white.OOK signal Hence, with the one noise level of the per LED bit, photodetectorthis bandwidth is allows deﬁned a maximum to be additive bit white Gaussian noise (AWGN). rate of 280 kbps. To verify this result practically, the rise time of the received signal is measured

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3.3. Bandwidth of the LED Receiver Circuit Sensors 2020, 20, x FOR PEER REVIEW 6 of 12 The bandwidth of the LED receiver circuit with the proposed TIA is practically investigated to

(calculatedidentify the between maximum 10% data and rate 90% and on itsthe simulated rising voltage Bode edge) plot is[23]. shown is in found Figure to4 .be The 3.8 Bodeµs, which plot translatessuggests thatto a thebandwidth TIA circuit acts of as 90 a kHz low passaccording ﬁlter to with the aequation 3 dB bandwidth [23]: of around 140 kHz. For a non-return-to-zero (NRZ) OOK signal with one level per bit, this bandwidth allows a 0.35 maximum bit rate of 280 kbps. To verify this result practically,= the rise time tr of the received signal(3) is measured (calculated between 10% and 90% on the rising voltage edge) [23]. tr is found to be 3.8 µs, This 90 kHz bandwidth allows OOK data transmission at speeds up to 180 kbps. For that reason, which translates to a bandwidth BW3dB of 90 kHz according to the equation [23]: data rates of only up to 200 kbps are investigated. At higher frequencies, the TIA fails to saturate to the upper voltage rail, hence acting as a low pass filter/integrator0.35 and turning the received square BW3dB = (3) wave close to a triangular shaped waveform. tr

Figure 4. BodeBode plot plot of TIA receiver circuit.

This 90 kHz bandwidth allows OOK data transmission at speeds up to 180 kbps. For that reason, 3.4. BER Improvement with Matched Filter data rates of only up to 200 kbps are investigated. At higher frequencies, the TIA fails to saturate to the upperNow voltage that rail,the photodetector hence acting as noise a low has pass been ﬁlter determined,/integrator the and bit turning error rate the (BER) received and square the SNR wave of theclose system to a triangular can be deduced shaped to waveform. investigate the performance of the system. The BER can be measured using two methods: 3.4. BERThe Improvementerror bits are with deduced Matched from Filter the generated PRBS20 bits to calculate the BER: Now that the photodetector noise has been determined, the bit error rate (BER) and the SNR of = (4) the system can be deduced to investigate the performance of the system. The BER can be measured using two methods: In order to verify the measured BER results, the BER is also calculated via the quality factor (Q- The error bits are deduced from the generated PRBS20 bits to calculate the BER: factor) obtained from the captured eye diagram [24]:

|error bits| BER = = (5)(4) total number + o f bits whereIn order and to verify are the the mean measured voltages BER for results, the ‘1’-bit the voltage BER is rail also and calculated ‘0’-bit voltage via the rail quality and factor and (Q-factor) are the obtainedstandard fromdeviation the captured values of eye the diagram ‘1’ and ‘0’ [24 rails]: due to the AWGN respectively. Hence, the BER can be calculated using the following relation [24]:

µ1 µ0 1 =| − | | | = Q + (6)(5) 2 σ1 + σ0 where µ1and is theµ0 arethreshold the mean voltage voltages which for determines the ‘1’-bit voltage where railthe anddecision ‘0’-bit is voltage made. railIn this and scenarioσ1 and σ it0 isare taken the standard as the centre deviation rail between values of the the ‘0’ ‘1’ and and ‘1’. ‘0’ railsThe duehistograms to the AWGN for 0 and respectively. 10 cm at 10 Hence, kbps theas well BER ascan the be eye calculated diagrams using and theQ-factor’s following parameters relation [are24]: depicted in Figure 5a,b, respectively. For the back- to-back case in Figure 5a, the two rails of the signal are distinct and far away from each other, allowing      µ V V µ a right decision to be taken with 1 at the center.1 th This  agrees withth the0 wide eye opening. On the BER(Vth) = er f c −  + er f c −  (6) contrary, this distinction vanishes in 2the signal σconstellation1  at 10 cmσ0 distance as seen in Figure 5b, where the normal distribution functions of the ‘0’ and ‘1’ rails are overlapping. In agreement therewith, the eye is closed, which corresponds to a higher probability of error.

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where Vth is the threshold voltage which determines where the decision is made. In this scenario it is taken as the centre rail between the ‘0’ and ‘1’. The histograms for 0 and 10 cm at 10 kbps as well as the eye diagrams and Q-factor’s parameters are depicted in Figure5a,b, respectively. For the back-to-back case in Figure5a, the two rails of the signal are distinct and far away from each other, allowing a right decision to be taken with Vth at the center. This agrees with the wide eye opening. On the contrary, this distinction vanishes in the signal constellation at 10 cm distance as seen in Figure5b, where the normal distribution functions of the ‘0’ and ‘1’ rails are overlapping. In agreement therewith, the eye is Sensors 2020, 20, x FOR PEER REVIEW 7 of 12 closed,Sensors 2020 which, 20, x corresponds FOR PEER REVIEW to a higher probability of error. 7 of 12

(a) (b) (a) (b) Figure 5. Eye diagram (inset) and signal constellation in (a) back-to-back configuration and (b) at 10 FigureFigure 5.5. Eye diagram diagram (inset) (inset) and and signal signal constellation constellation in in (a ()a back-to-back) back-to-back configuration conﬁguration and and (b) ( bat) at10 cm distance. 10cm cm distance. distance. The comparison between the measured BER resultsresults employing Equation (4) and the calculated The comparison between the measured BER results employing Equation (4) and the calculated BER from Equation (6) for all data rates shows lessless than 9% deviationdeviation between the two methods as BER from Equation (6) for all data rates shows less than 9% deviation between the two methods as seen in Figure6 6a.a. TheThe deviationdeviation isis calculatedcalculated byby subtractingsubtracting thethe measuredmeasured fromfrom thethe calculatedcalculated BERBER seen in Figure 6a. The deviation is calculated by subtracting the measured from the calculated BER and dividing by the measuredmeasured (true)(true) value.value. Hence, both approaches have similar responses which and dividing by the measured (true) value. Hence, both approaches have similar responses which validates the practical approach. validates the practical approach. ] ] dB [

s/bit] ) dB r [

o s/bit] ) r r r o SNR r ( r SNR ( atio r atio r ate (BER) [e ate (BER) r

r ate (BER) [e ate (BER) r o

r r r o r nal to noise r g nal to noise Bite Si g Bite Si

(a) (b) (a) (b) Figure 6. (a) Comparison between the measured and calculated BER; BER; ( (b)) BER and SNR at didifferentﬀerent Figure 6. (a) Comparison between the measured and calculated BER; (b) BER and SNR at different distances and data rates. distances and data rates. Figure6b shows the average BER at varying distances. As expected, the BER rises with longer Figure 6b shows the average BER at varying distances. As expected, the BER rises with longer transmissionFigure 6b distance shows and the higheraverage data BER rate. at varying Error-free dist dataances. transmission As expected, can bethe achieved BER rises at awith maximum longer transmission distance and higher data rate. Error-free data transmission can be achieved at a distancetransmission of 1cm distance at 200 kbps,and higher 1.5 cm atdata 100 rate. kbps Erro andr-free 2 cm atdata 50 kbpstransmission while it reaches can be 3 cmachieved at 10kbps. at a maximum distance of 1 cm at 200 kbps, 1.5 cm at 100 kbps and 2 cm at 50 kbps while it reaches 3 cm maximum distance of 1 cm at 200 kbps, 1.5 cm at 100 kbps and 2 cm at 50 kbps while it reaches 3 cm at 10 kbps. The BER stays below the acceptable 10−3 range [17] up to 7 cm for 10 and 50 kbps, up to 4 at 10 kbps. The BER stays below the acceptable 10−3 range [17] up to 7 cm for 10 and 50 kbps, up to 4 cm for 100 kbps and up to 3 cm for 200 kbps. cm for 100 kbps and up to 3 cm for 200 kbps. This low sensitivity is because an RGB LED has a much smaller active area for light emission This low sensitivity is because an RGB LED has a much smaller active area for light emission and detection per sub-LED compared to a regular single-colour LED. This is apparent after placing and detection per sub-LED compared to a regular single-colour LED. This is apparent after placing both LEDs under a microscope and comparing their active areas when dimly lit. The active area of both LEDs under a microscope and comparing their active areas when dimly lit. The active area of the single-colour LED is 1.99 times larger than that of the sub-LED of the RGB LED, which means the single-colour LED is 1.99 times larger than that of the sub-LED of the RGB LED, which means only 50% of light is captured compared to single-colour LED. Figure 7 compares the active areas only 50% of light is captured compared to single-colour LED. Figure 7 compares the active areas (circled in red) of the single-colour red LED in (a) with that of the red sub-LED in a typical off-the- (circled in red) of the single-colour red LED in (a) with that of the red sub-LED in a typical off-the- shelf RGB LED in (b) under the same scale. Moreover, it can be concluded that the small data rates shelf RGB LED in (b) under the same scale. Moreover, it can be concluded that the small data rates

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3 The BER stays below the acceptable 10− range [17] up to 7 cm for 10 and 50 kbps, up to 4 cm for 100 kbps and up to 3 cm for 200 kbps. This low sensitivity is because an RGB LED has a much smaller active area for light emission and detection per sub-LED compared to a regular single-colour LED. This is apparent after placing both LEDs under a microscope and comparing their active areas when dimly lit. The active area of the single-colour LED is 1.99 times larger than that of the sub-LED of the RGB LED, which means only 50% of light is captured compared to single-colour LED. Figure7 compares the active areas (circled in red) of the single-colour red LED in (a) with that of the red sub-LED in a typical oﬀ-the-shelf Sensors 2020, 20, x FOR PEER REVIEW 8 of 12 RGB LED in (b) under the same scale. Moreover, it can be concluded that the small data rates achieved byachieved this small-area by this small-area LED photodetector LED photodetector can be interpolated can be interpolated to match the to higher match datathe higher rates achieved data rates in Stepniak’sachieved in research Stepniak’s [17 research]. [17].

(a) (b)

Figure 7.7. Images of thethe activeactive areasareas ofof ((a)) single-coloursingle-colour (red) LEDLED andand ((b)) redred sub-LEDsub-LED ofof RGBRGB LEDLED whenwhen seenseen underunder thethe microscopemicroscope atat thethe samesame scale.scale. The signal-to-noise ratio (SNR) can be determined from the BER of the NRZ OOK modulation The signal-to-noise ratio (SNR) can be determined from the BER of the NRZ OOK modulation as follows [23]: as follows [23]: ! 1 1 BER = er f c √SNR (7) 21 1 = 2 √2 √ (7) 2 2√2 The graph in Figure6b also compares the SNR for the LED-to-LED communication link at di fferent distancesThe graph for data in ratesFigure of 6b 10, also 50, 100compares and 200 the kbps. SNR The for maximumthe LED-to-LED SNR achieved communication is 50 dB link at the at back-to-backdifferent distances configuration for data when rates transmittingof 10, 50, 100 at and 10 kbps.200 kbps. It drops The tomaximum 35 dB when SNR the achieved speed increases is 50 dB toat 200the kbps.back-to-back As expected, configuration the SNR decayswhen transmitting exponentially at along 10 kbps. the distance It drops and to at35 higherdB when speeds. the Inspeed the worst-caseincreases to scenario, 200 kbps. the As SNR expected, reaches the 2.3 SNR dB when decays transmitting exponentially at 200 along kbps the over distance a distance and ofat 10higher cm. speeds.The In matched the worst-case filter technique scenario, is athe special SNR type reache of maximums 2.3 dB when likelihood transmitting receivers at which 200 kbps is known over toa maximizedistance of the 10 signalcm. to noise ratio of a received signal suffering from AWGN [7], such as in the RGB LEDThe photodetector. matched filter technique is a special type of maximum likelihood receivers which is known to maximizeThe block the diagram signal into Figurenoise ratio8 demonstrates of a received a typical signal digital suffering communication from AWGN system. [7], such In orderas in the to minimizeRGB LED thephotodetector. BER of the system, the SNR needs to be maximized by a filter at the comparator stage. At theThe input block point diagram of the in threshold Figure 8 comparatordemonstrates block, a typical the AWGN digital power communicationNp can be derivedsystem. inIn termsorder ofto theminimize its power the spectralBER of the density system,PSD th,e which SNR isneeds equal to to beN maximized0/2 as follows by a [23 filter]: at the comparator stage. At the input point of the threshold comparator block, the AWGN power can be derived in terms of the its power spectral density Z, ∞which is equal to Z∞ ⁄2 as follows [23]: N0 2 Np = PSD d f = H( f ) d f (8) 2 −∞= = −∞ || (8) 2 where H( f ) is the transfer function of the filter. where is the transfer function of the filter.

Similarly, the following Equation displays the power of the signal at the input of the threshold comparator block:

= || = (9)

where is the sampled filter output signal at .

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FigureFigure 8. BlockBlock diagram diagram of of a a digital digital communication communication system. system.

DividingSimilarly, the the signal following power Equation by the displaysnoise power the powerto get ofthe the signal signal to Snoisep at the ratio input of: the threshold comparator block: = = Z 2 2 ∞ (10) j2π f Tb Sp = ys(Tb) = |S(f)|H ( f )e− (9) 2 To minimize the SNR, Schwartz inequality −∞is employed. It states that for any two conjugate where yS(Tb) is the sampled filter output signal at Tb. functions and : Dividing the signal power by the noise power to get the signal to noise ratio SNR: | | ∙ | | (11) R j2π f T 2 Sp ∞ S( f )H( f )e− b d f Applying Schwartz inequalitySNR =to the Equation= −∞ (10) and considering to be and to(10) be N R 2 Np 0 ∞ , the following Equation for the SNR is2 derived: H( f ) d f −∞ || ∙ To minimize the SNR, Schwartz inequality is employed. It states that for any two conjugate (12) functions f1(t) and f2(t): || 2 Z 2 2 Z Z ∞ |∞ | 2 ∞ 2 f1(t) f2(t)dt f1(t) dt f2(t) dt (13)(11) ≤ × −∞ −∞ −∞ where || is the energy of the bit signal and can be therefore written as . Hence, the Applying Schwartz inequality to the Equation (10) and considering f to be H( f ) and f to be SNR is expressed as: 1 2 S( f )e j2π f Tb , the following Equation for the SNR is derived: − 2 (14) R 2 R j2π f T 2 Sp ∞ H( f ) d f ∞ S( f )e− b d f According to the Schwartz inequality,−∞ this only× −∞ holds when (12) N∗ R 2 Np ≤ = 0 ∞ (15) 2 H( f ) d f This means that the filter transfer function shou−∞ ld have the exact shape and size of the logic ‘1’ Z bit. In the case of the proposed experiment,Sp this2 translates∞ to 2 a square-shaped signal of magnitude 4.5 S( f ) d f (13) V and duration . In case of a logic ‘1’ Nbitp ≤beingN0 transmitted as , the output of the matched filter −∞ is a triangleR of base 2 width 2, with a peak at of magnitude . Hence, the maximum output is ∞ ( ) achievedwhere every S f d, f correspondingis the energy of to the the bit peak signal of andthe triangle. can be therefore written as Es. Hence, the SNR is −∞ expressedFor the as: proposed system, a matched filter is designed to improve the BER at every speed and Sp 2 increases the error-free distance as depicted in Figure 9a.Es A 90% BER improvement is obtained at (14)200 N ≤ N kbps when the distance is 5 cm. The most significp ant0 improvement of the matched filter technique is measuredAccording at the to200 the kbps, Schwartz with an inequality, improvement this only averaging holds when60% for all distances. Figure 9b compares the maximum error-free transmission distances with and without matched filter as well as with those h(t) = s (T t) (15) achieved when employing a regular PIN photodiode∗ bas− photodetector. The bar chart demonstrates that applying the matched filter technique increases the error-free transmission distances for the 10 kbps Thistransmission means that data the rate filter case transfer from functionaround 3 should cm to havealmost the 7 exactcm. This shape matches and size the of performance the logic ‘1’ bit.of theIn the LED case photodetector of the proposed to that experiment, of a PIN photodiode. this translates At to200 a square-shapedkbps the LED-to-LED signal oflink magnitude with matched 4.5 V T s(t) filterandduration exceeds theb. Inerror-free case of a transmission logic ‘1’ bit being distance transmitted achieved as by the, the LED-to-PIN output of the photodiode matched filter link is at a T T E photovoltaictriangle of base mode. width 2 b, with a peak at b of magnitude s. Hence, the maximum output is achieved every Tb, corresponding to the peak of the triangle. For the proposed system, a matched filter is designed to improve the BER at every speed and increases the error-free distance as depicted in Figure9a. A 90% BER improvement is obtained at

Sensors 2020, 20, 5754 10 of 12

200 kbps when the distance is 5 cm. The most significant improvement of the matched filter technique is measured at the 200 kbps, with an improvement averaging 60% for all distances. Figure9b compares the maximum error-free transmission distances with and without matched filter as well as with those achieved when employing a regular PIN photodiode as photodetector. The bar chart demonstrates that applying the matched filter technique increases the error-free transmission distances for the 10 kbps transmission data rate case from around 3 cm to almost 7 cm. This matches the performance of the LED photodetector to that of a PIN photodiode. At 200 kbps the LED-to-LED link with matched filter exceeds the error-free transmission distance achieved by the LED-to-PIN photodiode link at photovoltaicSensors 2020, 20 mode., x FOR PEER REVIEW 10 of 12

10 kbps with matched filter 8 0.2 10 kbps without matched filter Without matched filter 50 kbps with matched filter 7 With matched filter 50 kbps without matched filter PIN Photodiode 100 kbps with matched filter 6 0.15 100 kbps without matched filter 5 200 kbps with matched filter 200 kbps without matched filter 0.1 4

0.05 2

1 0 0 45678910 10 kbps 50 kbps 100 kbps 200 kbps Distance [cm] Data rate (a) (b)

Figure 9. a Figure 9. EEffectffect ofof thethe matchedmatched filterfilter onon improvingimproving thethe systemsystem performanceperformance regardingregarding ((a)) thethe bitbit errorerror rate (BER) at different distances and (b) the maximum error-free transmission distance. rate (BER) at different distances and (b) the maximum error-free transmission distance. The power consumption of the whole system is measured at 110 mW in the worst-case scenario The power consumption of the whole system is measured at 110 mW in the worst-case scenario when the transmitter LED is turned on for emitting (which consumes almost 100 mW) and the when the transmitter LED is turned on for emitting (which consumes almost 100 mW) and the photodetection circuit is detecting (consuming the remaining 10 mW). The average power consumption photodetection circuit is detecting (consuming the remaining 10 mW). The average power is expected to be half of this value assuming equal probabilities of ‘1’ and ‘0’ bits because the transmitter consumption is expected to be half of this value assuming equal probabilities of ‘1’ and ‘0’ bits because LED is then switched off half of the transmission time. The power consumed by the photodetection the transmitter LED is then switched off half of the transmission time. The power consumed by the circuit can also be slightly reduced by programming an event-driven turning on of the detector circuit: photodetection circuit can also be slightly reduced by programming an event-driven turning on of With the help of low-power comparators, the detection circuit is turned on only in response to system the detector circuit: With the help of low-power comparators, the detection circuit is turned on only interrupts and stays off otherwise [25]. in response to system interrupts and stays off otherwise [25]. 4. Conclusions 4. Conclusions This paper proves the feasibility of a low-complexity LED-to-LED communication link based entirelyThis on paper RGB proves LEDs asthe transmitters feasibility of and a low-complexity receivers. The LED-to-LED novelty of communication this research is link threefold: based First,entirely it on proposes RGB LEDs a low-complexityas transmitters and design receivers. based Th onlye novelty on off of-the-shelf this research RGB is LEDs threefold: to keep First, the it overallproposes cost a low-complexity and power consumption design based at only a minimum on off-the-shelf to allow RGB for LEDs easy integrationto keep the overall in IoT devices.cost and Second,power consumption it derives a formula at a minimum for the LEDto allow photodetector for easy integration response at in varying IoT devices. distances Second, and empiricallyit derives a provesformula that for the the noise LED of photodetector such detector isresponse AWGN. at Finally, varying knowing distances the and noise empirically power and proves its distribution, that the itnoise proposes of such a detector matched is filter AWGN. technique Finally, to knowing increase thethe systemnoise power performance and its distribution, even above it that proposes using aa regularmatched PIN filter photodiode. technique to increase the system performance even above that using a regular PIN photodiode. Without matched filter, successful OOK modulated signal detection is possible at data rates up to Without matched filter, successful OOK modulated3 signal detection is possible at data rates up 200 kbps over a distance up to 3 cm with BER < 10− and the distance rises to 6 cm at a transmission to 200 kbps over a distance up to 3 cm with BER < 10−3 and the distance rises to 6 cm at a transmission speed of 10 kbps. Employing the proposed matched filter technique results in a maximum error-free speed of 10 kbps. Employing the proposed matched filter technique results in a maximum error-free transmission distance comparable to that of a regular PIN photodiode as detector. The results prove transmission distance comparable to that of a regular PIN photodiode as detector. The results prove that using that technique, the BER improves by up to 90% at a transmission speed of 200 kbps over a that using that technique, the BER improves by up to 90% at a transmission speed of 200 kbps over a 5 cm distance. 5 cm distance. Although the achieved data rates are considered low in comparison to other VLC systems, it is sufficient for most small IoT devices that require sending only status flags and small chunks of information, such as toy-to-toy communication. Moreover, the paper proves that the system performance lags due to the much smaller active area of the RGB LED compared to a single-coloured red LED. On the cost of increasing the complexity, the system performance could be further optimized using lenses, concentrators, pre-processing, and further post-processing techniques along with more complex modulation schemes such as OFDM.

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Although the achieved data rates are considered low in comparison to other VLC systems, it is suﬃcient for most small IoT devices that require sending only status ﬂags and small chunks of information, such as toy-to-toy communication. Moreover, the paper proves that the system performance lags due to the much smaller active area of the RGB LED compared to a single-coloured red LED. On the cost of increasing the complexity, the system performance could be further optimized using lenses, concentrators, pre-processing, and further post-processing techniques along with more complex modulation schemes such as OFDM.

Author Contributions: Conceptualization, M.G., W.P.N. and R.B.; data curation, M.G.; formal analysis, M.G.; investigation, M.G.; methodology, M.G.; project administration, W.P.N.; resources, W.P.N.; software, M.G.; supervision, W.P.N., R.B. and A.A.E.A.; validation, M.G., W.P.N. and A.A.E.A.; visualization, M.G.; writing—original draft, M.G.; writing—review and editing, W.P.N. and A.A.E.A. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

1. Kumar, P.; Pati, U.C. IoT based monitoring and control of appliances for smart home. In Proceedings of the 2016 IEEE International Conference on Recent Trends in Electronics, Information & Communication Technology (RTEICT), Bengaluru, India, 20–21 May 2016; pp. 1145–1150. 2. Alimi, I.A.; Abdalla, A.M.; Rodriguez, J.; Pereira Monteiro, P.; Teixeira, A.L.; Zvánovec, S.; Ghassemlooy, Z. Enabling VLC and WiFi Network Technologies and Architectures Toward 5G; John Wiley & Sons, Ltd.: Chichester, UK, 2019. 3. Jara, A.J. Wearable Internet: Powering Personal Devices with the Internet of Things Capabilities. In 2014 International Conference on Identiﬁcation, Information and Knowledge in the Internet of Things; IEEE Computer Society: Beijing, China, 2014; p. 7. [CrossRef] 4. Karvonen, H.; Pomalaza-Ráez, C.; Mikhaylov, K.; Hämäläinen, M.; Iinatti, J. Experimental Performance Evaluation of BLE 4 Versus BLE 5 in Indoors and Outdoors Scenarios BT—Advances in Body Area Networks I; Fortino, G., Wang, Z., Eds.; Springer International Publishing: Cham, Switzerland, 2019; pp. 235–251. 5. Teli, S.R.; Zvanovec, S.; Ghassemlooy, Z. Optical Internet of Things within 5G: Applications and Challenges. In Proceedings of the 2018 IEEE International Conference on Internet of Things and Intelligence System (IOTAIS), Bali, Indonesia, 1–3 November 2018; pp. 40–45. 6. Katz, M.; O’Brien, D. Exploiting novel concepts for visible light communications: From light-based IoT to living surfaces. Optik 2019, 195, 163176. [CrossRef] 7. Ghassemlooy, Z.; Alves, L.N.; Zvanovec, S.; Khalighi, M.-A. Visible Light Communications: Theory and Applications; CRC Press Taylor & Francis Group: Florida, FL, USA, 2017; ISBN 1498767540. 8. Burchardt, H.; Seraﬁmovski, N.; Tsonev, D.; Videv, S.; Haas, H. VLC: Beyond point-to-point communication. IEEE Commun. Manag. 2014, 52, 98–105. [CrossRef] 9. Grobe, L.; Paraskevopoulos, A.; Hilt, J.; Schulz, D.; Lassak, F.; Hartlieb, F.; Kottke, C.; Jungnickel, V.; Langer, K. High-speed visible light communication systems. IEEE Commun. Manag. 2013, 51, 60–66. [CrossRef] 10. Chi, Y.-C.; Hsieh, D.-H.; Tsai, C.-T.; Chen, H.-Y.; Kuo, H.-C.; Lin, G.-R. 450-nm GaN laser diode enables high-speed visible light communication with 9-Gbps QAM-OFDM. Opt. Express 2015, 23, 13051–13059. [CrossRef][PubMed] 11. Galisteo, A.; Juara, D.; Wang, Q.; Giustiniano, D. OpenVLC1.2: Achieving higher throughput in low-end visible light communication networks. In Proceedings of the 2018 14th Annual Conference on Wireless On-demand Network Systems and Services (WONS), Isola, France, 6–8 February 2018; pp. 117–120. 12. Dietz, P.; Yerazunis, W.; Leigh, D. Very Low-Cost Sensing and Communication Using Bidirectional LEDs. In UbiComp 2003: Ubiquitous Computing. UbiComp 2003; Lecture Notes in Computer Science; Dey, A.K., Schmidt, A., McCarthy, J.F., Eds.; Springer: Berlin/Heidelberg, Germany, 2003; Volume 2864. [CrossRef] Sensors 2020, 20, 5754 12 of 12

13. Galal, M.; Ng, W.P.; Binns, R.; Abd El Aziz, A. Characterization of RGB LEDs as Emitter and Photodetector for LED-to-LED Communication. In Proceedings of the 12th IEEE/IET International Symposium on Communication Systems, Networks and Digital Signal Processing—CSNDSP, Porto, Portugal, 20–22 July 2020. 14. Tippenhauer, N.O.; Giustiniano, D.; Mangold, S. Toys communicating with LEDs: Enabling toy cars interaction. In Proceedings of the 2012 IEEE Consumer Communications and Networking Conference (CCNC), Las Vegas, NV, USA, 14–17 January 2012; pp. 48–49. 15. Corbellini, G.; Aksit, K.; Schmid, S.; Mangold, S.; Gross, T.R. Connecting networks of toys and smartphones with visible light communication. IEEE Commun. Manag. 2014, 52, 72–78. [CrossRef] 16. Giustiniano, D.; Tippenhauer, N.O.; Mangold, S. Low-complexity Visible Light Networking with LED-to-LED communication. In Proceedings of the 2012 IFIP Wireless Days, Dublin, Ireland, 21–23 November 2012; pp. 1–8. [CrossRef] 17. Stepniak, G.; Kowalczyk, M.; Maksymiuk, L.; Siuzdak, J. Transmission Beyond 100 Mbit/s Using LED Both as a Transmitter and Receiver. IEEE Photonics Technol. Lett. 2015, 27, 2067–2070. [CrossRef] 18. Chun, H.; Rajbhandari, S.; Faulkner, G.; Tsonev, D.; Haas, H.; O’Brien, D. Demonstration of a Bi-directional visible light communication with an overall sum-rate of 110 Mb/s using LEDs as emitter and detector. In Proceedings of the 2014 IEEE Photonics Conference, San Diego, CA, USA, 12–16 October 2014; pp. 132–133. 19. Shrestha, N.; Sohail, M.; Viphavakit, C.; Saengudomlert, P.; Mohammed, W.S. Demonstration of visible light communications using RGB LEDs in an indoor environment. In Proceedings of the ECTI-CON2010: The 2010 ECTI International Confernce on Electrical Engineering/Electronics, Computer, Telecommunications and Information Technology, Chiang Mai, Thailand, 19–21 May 2010; pp. 1159–1163. 20. Rehman, S.U.; Ullah, S.; Chong, P.H.J.; Yongchareon, S.; Komosny, D. Visible Light Communication: A System Perspective-Overview and Challenges. Sensors 2019, 19, 1153. [CrossRef][PubMed] 21. Burton, A.; Bentley, E.; Minh, H.L.; Ghassemlooy, Z.; Aslam, N.; Liaw, S. Experimental demonstration of a 10BASE-T Ethernet visible light communications system using white phosphor light-emitting diodes. IET Circuits Devices Syst. 2014, 8, 322–330. [CrossRef] 22. Galal, M.; Ng, W.P.; Aziz, A.A.E.; Binns, R. Characterisation and Interference Model of Contemporary Artiﬁcial Light Sources Noise on a VLC channel. In Proceedings of the 2018 11th International Symposium on Communication Systems, Networks and Digital Signal Processing, CSNDSP 2018, Budapest, Hungary, 18–20 July 2018. 23. Haykin, S.S.; Simon, S. Communication Systems, 4th ed.; John Wiley & Sons: New York, NY, USA, 2001. 24. Grupp, W. Chapter 14—Optical Performance Monitoring in Optical Transport Networks; Chan, C.C.K., Ed.; Academic Press: Oxford, UK, 2010; pp. 385–421. ISBN 978-0-12-374950-5. 25. Li, J.; Liu, A.; Shen, G.; Li, L.; Sun, C.; Zhao, F. Retro-VLC: Enabling Battery-Free Duplex Visible Light Communication for Mobile and IoT Applications. In Proceedings of the 16th International Workshop on Mobile Computing Systems and Applications; Association for Computing Machinery: New York, NY, USA, 2015; pp. 21–26.

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