Visible light communication system based on software defined radio: Performance study of intelligent transportation and indoor applications

electronics Article Visible Light Communication System Based on Software Defined Radio: Performance Study of Intelligent Transportation and Indoor Applications Radek Martinek *,† , Lukas Danys *,† and and Rene Jaros *,† Department of Cybernetics and Biomedical Engineering, Faculty of Electrical Engineering and Computer Science, VSB–Technical University of Ostrava, 17. listopadu 15, 708 33 Ostrava, Czech Republic *Correspondence: [email protected] (R.M.); [email protected] (L.D.); rene.jar[email protected] (R.J.); Tel.: +420-721-009-971 (R.M.); +420-734-239-361 (L.D.); +420-774-650-522 (R.J.) † These authors contributed equally to this work. Received: 28 February 2019; Accepted: 10 April 2019; Published: 15 April 2019   Abstract: In this paper, our first attempt at visible light communication system, based on software defined radio (SDR) and implemented in LabVIEW is introduced. This paper mainly focuses on two most commonly used types of LED lights, ceiling lights and LED car lamps/tail-lights. The primary focus of this study is to determine the basic parameters of real implementation of visible light communication (VLC) system, such as transmit speed, communication errors (bit-error ratio, error vector magnitude, energy per bit to noise power spectral density ratio) and highest reachable distance. This work focuses on testing various multistate quadrature amplitude modulation (M-QAM). We have used Skoda Octavia III tail-light and Phillips indoor ceiling light as transmitters and SI PIN Thorlabs photodetector as receiver. Testing method for each light was different. When testing ceiling light, we have focused on reachable distance for each M-QAM variant. On the other side, Octavia tail-light was tested in variable nature conditions (such as thermal turbulence, rain, fog) simulated in special testing box. This work will present our solution, measured parameters and possible weak spots, which will be adjusted in the future. Keywords: multistate quadrature amplitude modulation (M-QAM); visible light communication (VLC); software defined radio (SDR); sofware defined optics (SDO); LED tail-light; LED indoor ceiling light; vehicle-to-everything (V2X); nature conditions (thermal turbulence, rain, fog); bit-error ratio (BER) 1. Introduction In recent years, visible light communication (VLC) surfaced as an alternative to classical radio frequency (RF) technology [ 1 – 3 ]. Current communication bands often lack free channels, which is notable particularly in Wi-Fi or in industrial, scientific and medical bands. VLC is an optical wireless standard which operates from 380 to 780 nm, using a visible light source as a signal transmitter, free space environment as transmission medium and the appropriate photodiode/photodetector as a receiver. VLC seems to be capable technology for short-range or possibly in the future even long-range communications. Future appliances vary greatly, spanning from vehicle-to-vehicle [ 4 – 9 ] communications, infrastructure-to-vehicle communications or simply as an alternative to typical local area networks (LAN) [10,11]. A number of papers focused on multiple VLC technologies [ 12 ]. Light-fidelity (Li-Fi) [ 13 – 20 ] is slowly surfacing as commercially available alternative to Wi-Fi [ 21 ]. Orthogonal frequency division multiplexing in car-to-car was tested in real-world driving scenarios by Shen et al. [22]. Electronics 2019,8, 433; doi:10.3390/electronics8040433 www.mdpi.com/journal/electronics Electronics 2019,8, 433 2 of 36 VLC on software defined radio (SDR) [ 23 , 24 ] is evolving quickly. In 2011, a 1 Mbps video stream was achievable over 3 meters, when deployed on custom LED matrixes [ 25 ]. In 2015, Hussain et al. tested the implementation of IEEE 802.15.7, they achieved results according to this standard, however transmission distance was limited to 1 m [ 26 ]. Nowadays, we are testing longer distances and mainly higher data rates, even on commercial light sources. Rapid expansion of LED is crucial for this technology, as it offers multiple advantages such as long lifespan, low power consumption, high tolerance to humidity, high efficiency, and fast switching. However the main advantage of VLC based on LED is the use of the visible spectrum (380–780 nm). For this reason LED can perform communication functionality while maintaining the original function as illumination lighting. In this paper, we used commercially available and currently used light sources. For this purpose, a Skoda Octavia III tail-light without any modifications was chosen. To test indoor deployment, we have also used Phillips Fortimo DLM 300 44 W/840 Gen3 [ 27 ]. Avalanche photodiodes (APD) and positive-intrinsic-negative (PIN) detectors are commonly used as receivers. We used Thorlabs PDA36A-EC PIN [ 28 ] photodetector with 13 mm 2 of active area as receiver, since it was the most suitable candidate from available portfolio. Our work is aimed at implementation of a vehicle-to-everything (V2X) system with highly modular design [ 29 – 31 ]. For this reason, we have developed a system, based on SDR. Each individual component can be swiftly exchanged, without any necessary adjustments to original code. Concept of V2X is based on the passing of information from a vehicle to any appropriate entity and vice versa. It is also often divided into different subsections, such as vehicle-to-infrastructure (V2I) [ 32 , 33 ], vehicle-to-network (V2N) [34,35], vehicle-to-vehicle (V2V) [ 36 , 37 ], vehicle-to-pedestrian (V2P) [ 35 , 38 ], vehicle-to-device (V2D) [ 39 , 40 ] and vehicle-to-grid (V2G) [ 41 , 42 ], vehicle-to-home (V2H) [ 43 , 44 ]. To test these concepts, we have also built our own testing polygon named BroadBAND light. As Skoda cars are the most widespread vehicles in the Czech Republic, we have picked Skoda Octavia III tail-light as the transmitter in V2X scenarios. So far, the partnership with manufacturers of these lights have yielded results, as we have received multiple samples of planned or already available products for testing. Philips Fortimo DLM300 is the most deployed LED indoor ceiling light in Czech Republic, so we used it in indoor experiments. Most Czech public institutions deploy exactly this type–testing it is the most logical step, as we can use every ceiling light at our university as transmitters. Figure 1describes different ways of V2X communication [ 30 , 45 , 46 ], mentioned earlier. V2V is a system, which enables car to communicate with each other. Its main goal is to reduce vehicle collisions and crashes. It will be a backbone of multiple levels of autonomy, delivering assisted driver services like collision warnings [ 32 , 47 , 48 ]. One key issue with V2V is that to be most effective, it should reside in all cars on the road. However, this technology has to start somewhere, so car manufacturers are slowly introducing their solutions. V2D is a system, that links cars to many external receiving devices and will be particularly useful to bikers. Vehicles can communicate with V2D device on cycle to alert rider to potential danger or to avoid accidents [ 39 , 40 , 49 ]. V2P is a system which should be particularly useful to elderly persons, school kids and physically challenged persons. V2P maintains a link between pedestrian’s smart devices and vehicles to act as an advisory to avoid collisions [ 50 , 51 ]. V2H communications involve a link between a vehicle and the owner’s home, sharing the task of providing energy [ 52 , 53 ]. During power outages, a vehicles battery can be used as a critical power source. V2G is a system which can communicate with the power grid to adjust the vehicle’s charging rate [ 54 , 55 ]. It will be an element in some electric vehicles and is used as a power grid modulator to dynamically adjust to the energy demand [56]. Electronics 2019,8, 433 3 of 36 Vehicle-to-Vehicle Vehicle-to-Device Vehicle-to-Pedestrian Vehicle-to-Home Vehicle-to-Grid Vehicle-to-Infrastructure Figure 1. Vehicle-to-everything scenarios communication. Figure 2describes proposed different ways of indoor visible light communication (VLC) communication. Smart lighting inside smart buildings provides the infrastructure for illumination, control and communications and will greatly reduce energy consumption within a building. Smart appliances, meters or factory applications, especially in dangerous conditions are all possible target devices. Direct connection between mobile devices might be possible as well, since modern smartphones have illumination LEDs which might be used as transmitters and cameras, which could work as receivers. There are advantages of using VLC in hospitals and healthcare [ 57 – 60 ]. RF technologies are mostly undesirable in certain parts of hospitals, especially around MRI scanners and in operating theatres. VLC deployment in aviation is also desirable. Radio is undesirable in passenger compartments of aircrafts. Modern aircrafts already use LEDs for illumination, so VLC might replace wiring to passenger seats. This might reduce aircraft construction costs and weight [61,62]. Light-to-Smart Metering Light-to-Smart Appliances Light-for-Internet Access Light in Dangerous Environment Light-to-Medical Devices Light-to-Wearable Devices VLC-based positioning Light-to-Machine Figure 2. Indoor VLC scenarios. Factory applications are especially interesting. There are multiple benefits to machine to machine VLC solutions (machine-to-machine communication - M2M [ 20 , 63 – 66 ]). Smart factories could be built upon enhanced industrial instrumentation, such as advanced sensors or meters. VLC can be also used to coordinate the movement and timing of robots in manufacturing settings, such as automobile factories. It can provide location-based communications to automated guided carts and “smart cart” robots or provide drone-to-station communications for precise, interference-free movement, drop zones and landing sites [67,68]. Vehicular visible light communication (V-VLC) [ 46 , 69 – 73 ] is often described as supplementary or “sister” technology to planned 5G [ 31 ]. There is also a concept of social internet of vehicles (SIOV) [ 74 – 76 ], which is based on LTE/4G network, consisted of multiple road side units (RSU) based on eNodeB base stations and on-board units (OBU) with LTE/4G capabilities. SIOV consists of multiple entities, which are treated as nodes connected to each other via links. However this concept can be further expanded by VLC implementation. Daytime running lamps (DRL) are often mandatory in European countries and optional VLC capabilities would possibly save energy needed to maintain continual 4G/5G connection [77,78]. Electronics 2019,8, 433 4 of 36 Multiple teams implemented VLC using SDR and LabVIEW, with different success. A turkish team successfully carried out a number of experiments, using simpler on-off keying (OOK) and variable pulse position modulation (VPPM) modulations [ 79 ]. Another team from Chile managed to implement their own system using similar hardware [ 7 ]. However, there are two main concerns. Both teams do not mention their maximal reached transmit speed and the used photodetector (PDA36A) is certainly a limiting factor, as we have run a number of tests on it as well. According to manufacturer’s datasheet, increasing gain of its trans-impedance amplifier significantly limits useable bandwidth. According to the state of art study, universal software radio peripherals(USRPs) are more than capable of being used in VLC experiments [ 80 ]. They also mention the necessary adjustments and possible limitations of lower end (USRP 292X) models. That’s why we switched to much more powerful and newer models which will be used in future experiments [7,79–82]. As mentioned before, the described system is our team’s first attempt at SDR VLC quadrature amplitude modulation(QAM) system. Different teams have already delved into different problematics, such as adaptive modulation schemes [ 83 ]. Multiple input multiple output design described by Deng et al. 2017 [ 84 ] is already implementing higher state QAM modulation schemes capable of adaptive switching. Designing similar systems based on orthogonal frequency division multiplexing (OFDM) will be a topic of imminent research, as we are already switching to more capable hardware components [85,86] . Khalid et al. investigated the implementation of a VLC system based on DAQ hardware [ 87 ]. This system is however very limited performance-wise. The second iteration of our prototype will be also based on LabVIEW, but we are focusing on field programmable gate array (FPGA) implementation, as it will increase performance dramatically. There are multiple advantages to practical implementation of VLC. Each of presented solution has its own use and advantages. When approaching the problems of V2V communication, the natural conditions are major concerning factor, as they significantly vary throughout the year. Carrying out a number of experiments in this area is a logical first step in implementation of channel equalization, as we estimate it will significantly improve transmit speeds or reachable distance. Also we tested our modular platform in previously mentioned conditions, as it will become a basic platform, which will be modified and improved in the future. Currently, we are exploring outdoor car lamps, tail-lights, and indoor ceiling lights, as these light sources that are the most perspective. In the future we will also include street lamps, as V2I and V2V outdoor experiments will be carried out in previously mentioned testing polygon, which is already running on LEDs, so every lamp is prepared for VLC. Tsiropoulou et al. [ 88 ] have investigated problematics of non-orthogonal multiple acces (NOMA) vs. orthogonal frequency division multiple access (OFDMA) approach [ 89 – 91 ]. According to her, NOMA offers us multiple advantages, such as considerable interference mitigation or simultaneous bandwidth utilization. Also due to the absence of resource block per user, NOMA can sufficiently accommodate more users than OFDMA. Modern mobile networks based on LTE use the OFDMA approach, as it is a basis of LTE standardization. Car manufacturers are currently enrolling LTE modules into vehicles and are preparing for 5G transfer. Currently, 5G is surfacing technology, which was deployed in only a limited number of countries. However 5G is based on NOMA, so its implementation in VLC is necessary if both technologies should coexist or work in conjunction. Lin et al. also tested hybrid NOMA/OFDMA approach with partial success [92]. Tsiropoulou et al. [ 93 ] propose a concept of visible light communication local area networks (VLC-LANs), where users are served by optical access points (OAPs). In this scenario, VLC-LANs [ 94 ] are presented as alternative to macrocell area coverage. Two-tier VLC topology is considered, mostly for indoor and outdoor coverage. The system is based on OFDMA, basically to be ready for LTE incorporation. Each user communicates directly with a single OAP via communication link. OAPs total bandwidth is divided into subcarriers, which are organized in resource block (RB). Each RB is occupied exclusively by one user. This concept could be easily adapted for 5G by introduction of NOMA, as discussed earlier. Electronics 2019,8, 433 5 of 36 2. Experimental Setup Experimental setup is based upon application created in LabVIEW, which was used for input/output signal processing and measurement. Application output was fed into NI USRP-2921 [ 95 ], which stands for transmitting element. Signal was then amplified by a 1.6 W amplifier, effectively working from 1 to 200 MHz. The amplified signal went through bias tee into the transmitting light source, which can be exchanged at will. A photodetector was located at a variable distance from the transmitting element. It was also possible to insert special measuring box, used for simulation of multiple nature conditions, such as fog, rain or thermal turbulence. Received signal was fed into NI USRP-2921 whose output is connected to the same computer running LabVIEW application. Signal was then evaluated, and parameters were displayed accordingly. This whole setup was designed with the highest possible modularity in mind. We wanted to switch each component at will and observe their direct impact on whole prototype. We had to swap NI USRP-2921 transmitting and receiving boards with Ettus LFRX/LFTX Daughterboards, which were operating at 0-30 MHz. This band was approximately what we had in mind when designing whole prototype, as USRP capabilities were one of the biggest limiting factors. Original boards from 2921 were designed for 2.4–2.5 GHz and 4.9–5.9 GHz, which was completely unsuitable for our needs. We had also adjusted connectors on the Octavia tail-light, as they were designed for car engine control unit. To further improve RSL when transmitting using tail-light, we had mounted a planoconvex lens on a photodetector to focus received light into the converging beam with lens focus at the active area of photodiode. However, since we measured RSL at variable receiving angle when using stationary ceiling light as source, it was undesirable to use lens in this different scenario. We have chosen ZX85-12G+ bias tee [ 96 ], which operates from 0.2 MHz to 12 GHz, its maximal current is 0.4 A and maximal input voltage is 25 V. There are multiple parameters, that can be configured in LabVIEW application, such as: • Carrier frequency: max 30 MHz • Bandwidth • Sample width • Number of states: max 4096-QAM • Message symbols • Used TX filter • TX gain • RX gain • TX device IP address • RX device IP address Tests wre carried out using static modulation formats. Long term measurements were essential to specify threshold for successful modulation switching in adaptive modulation. We were aiming for a similar system, which is used in case of microwave point-to-point links, where both units are capable of quickly changing modulation scheme according to natural conditions and measured parameters. Sacrificing part of the transmission speed in favor of link robustness is the main concept of this system. Evaluation Parameters Received signal level (RSL), which signalize signal strength received at second, or receiver, USRP. It is the sum of all losses and gains on the receiver input. Our LabVIEW application use channel coding with hard decision forward error correction (FEC) threshold of 3.8 × 10 −3 . Data with bit-error ratio (BER) below this threshold can be repaired by FEC codes. This threshold was appropriate for our M-QAM modulation. LabVIEW Modulation Tookit [ 97 ] also includes multiple different types of channel coding [ 98 ]. The influence of these techniques on transmission quality will be a topic of further research. As FEC was not the main topic of this paper, Electronics 2019,8, 433 6 of 36 we used predetermined threshold and functions already present in LabVIEW libraries. Problematics of FEC in QAM VLC was investigated by team in Edinburg [99]. Modulation error ratio (MER) [ 100 ] is defined as a relationship between error vector magnitude (EVM) and signal-to-noise ratio (SNR). It was used to quantify the performance of transmitter/receiver in the system, which used digital modulations (in this case QAM). It is influenced by various imperfections in the implementation (such as noise, phase noise, distortion, right focusing) and characteristics of signal path which cause the actual constellation point to deviate from ideal locations. EVM [ 101 ] is a measurement of demodulator performance in the presence of impairments (Figure 3, where −→ v is the ideal symbol vector, −→ w is the measured symbol vector, −→ w – −→ v is the magnitude error, Φ is the phase error, and −→ e=−→ w−−→ v is the error vector). The soft symbol decisions obtained after decimating the recovered waveform at the demodulator output were compared against ideal symbol locations. The root mean square error vector magnitude and phase error were then used in determining the EVM measurement over a window of N demodulated symbols. EVM was related to the modulation error ratio. There is one-to-one relationship between EVM and MER. Q I e v w θ Ideal Symbol Location Measured Symbol Location Magnitude Error Figure 3. Error vector magnitude. Energy per bit to noise power spectral density ratio (E b /N 0 ) was an important parameter in digital communications. It is a normalized SNR measure, also known as SNR per bit. It is useful when comparing BER performance of different digital modulation schemes without taking bandwidth into account. To calculate this value, SNR must be periodically obtained at receiver USRP. E b /N 0 was calculated at the receiver from the estimation of the SNR, the relationship between them is the following: Eb N0 |dB =SNR|dB +10log10(n)−10log10(sps), (1) where n is the amount of information bits per symbol. Both modulation order and the code rate (in case that channel coding is employed) have an influence on this value. E b /N 0 measurement on USRP is further expanded in paper by Alonso et al. [102]. 3. A Feasibility Study on Indoor Visible Light Communication Measurements using ceiling light were carried out under laboratory conditions (calm wind, 24 ◦ C). Figure 4describes tested setup. Our tested light source is capable to cover a conical area with radius of approximately 350 cm, as is described on Figure 5. We began our measurements at a right center of this covered area, directly under light source. Then, we periodically repeated measurements but moved to the edge of covered area with a step of 25 cm. The distance between receiver and transmitter was 202 cm. Electronics 2019,8, 433 7 of 36 The following configuration was used in this scenario: • Carrier frequency: 3 MHz • Bandwidth: 1–4 MHz • Modulation type: M-QAM • TX/RX gain: 0 dB • Message symbols: 10,000 • Tx filter: root raised cosine • Sample width: 16-bit • Receiver-transmitter distance: 202 cm • Measured distance: 0–350 cm (step of 25 cm) • Photodetector without planoconvex lens →more suitable for this scenario • Measured parameters: Eb/N0, BER, EVM and MER USRP 1 USRP 2 Light Photodetector Amplifier LabVIEW application Tx Rx Normal state * *(24 °C, without wind, direct visibility) Bias tee Philips - Fortimo LED DLM 3000 44 W/840 Gen3 1 MHz - 500 MHz 1,6 W/32 dB1,6 W/32 dB PDA36A-EC - without lenses Gain: 10 dB Stabilized source RF DC RF + DC Figure 4. Ceiling light setup. measured section 202 cm 0 cm 350 cm (center) Fortimo LED DLM 3000 44 W/840 Gen3 Figure 5. Setup with measured distance. Electronics 2019,8, 433 8 of 36 In Table 1you can see parameters of light of Fortimo LED DLM 3000 44 W/840 Gen3 and in Table 2you can see chosen parameters of photodetector PDA36A-EC. Table 1. Parameters of light of Fortimo LED DLM 3000 44 W/840 Gen3. Initial lumens 3000 lm Color rendering index 80 Correlated color temperature 4000 K System input power 46.0 W System efficiency 68.0 Lm/W Input voltage 220–240 V Max. vitality 5000 h Umax dc 80 V Max operating temperature 65 ◦C Pmax 40 W Table 2. Chosen parameters of photodetector PDA36A-EC. Type of detector Si PIN Active area 3.6 ×3.6 mm (13 mm2) Wavelength range 350–1100 nm Gain adjustment range 70 dB Gain step 8 ×10 dB Operating temperature 0–40 ◦C Max output current 100 mA Before proceeding with the main part, we measured attenuation characteristics of our prototype by using the vector network analyzer. Figure 6represents measured data. It is visible that attenuation quickly increased with longer distances. Even the sample with the best conditions (directly under light source) showed an increase in attenuation by 47 dB relative to the reference of 0 dB. By increasing distance to 3 m, attenuation reached 68 dB, which negatively impacted prototype capabilities, mainly the constellation decoding. We have chosen carrier frequency of 3 MHz, so that we can increase bandwidth up to 4 MHz without getting into sub 1 MHz band. However, increasing bandwidth also increases the difference in attenuation between highest and lowest frequency. This fact negatively impacted constellation decoding as well. The resulting theoretical transmit speed was influenced by many factors, such as modulation scheme, used bandwidth and mainly by the distance of photodetector from center of measurements, which affected attenuation. We were able to reach 2 Mbps at 325 cm, by using 4-QAM with bandwidth of 1 MHz. The highest achieved transmit speed was 20 Mbps (32-QAM, 4 MHz), which could be maintained up to 90 cm from the center. Figure 7represent possible maximal reachable transmission speeds for different M-QAM and bandwidths. Further implementation of adaptive modulation seems like a good way to further push this prototype forward. By defining strict rules for modulation switching, we could seamlessly maintain the highest possible transmit speed while having reliable connection. For our testing, we have used BER = 10 −5 as a threshold value. The valubble in Figure 7are theoretically achievable transmit speeds. They were calculated in LabVIEW software, but a number of measurements were carried out to verify them. Effective transmit speed varied a bit (by approximately hundreds of kbps), because it was practically impossible to design an ideal channel. However deviations were so small, it was possible to neglect them. Electronics 2019,8, 433 9 of 36 012345678910 Frequency (Hz) 106 -100 -90 -80 -70 -60 -50 -40 Attenuation (dB) Carrier frequency 3 MHz BW 2MHz BW 3 MHz BW 4 MHz BW 1 MHz Distance 3 m from the center Distance 2 m from the center Distance 1 m from the center Center x Figure 6. Attenuation characteristics of ceiling setup. 0 25 50 75 100 125 150 175 200 225 250 275 300 325 350 Distance (cm) 0 2 4 6 8 10 12 14 16 18 20 22 64-QAM 4-QAM 8-QAM 16-QAM 4-QAM 4-QAM 8-QAM 4-QAM 32-QAM 64-QAM 16-QAM 8-QAM 8-QAM 16-QAM 16-QAM 32-QAM 32-QAM 32-QAM 64-QAM Transmission speed (Mb/s) Bandwidth: 3 MHz Bandwidth: 1 MHz Bandwidth: 2 MHz Bandwidth: 4 MHz Figure 7. Transmit speed/distance from center relationship for ceiling light with different M-QAM and bandwidths. Figure 8represents a constellation diagrams of 4-QAM modulation measured at different distances and channel widths. It is noticeable that constellation points of the 4 MHz channel width measurements were more spread from their ideal position. These diagrams did not exceed the BER threshold mentioned earlier, but implementation of higher state modulation at limiting distance would result in rapid increase of BER and might even end in complete link degradation. Electronics 2019,8, 433 16 of 36 4-QAM 8-QAM 16-QAM 32-QAM 64-QAM 128-QAM 256-QAM 512-QAM 1024-QAM 2048-QAM M-QAM 10-5 10-4 10-3 10-2 10-1 100 BER (-) Thermal turbulence, bandwidth 4 MHz Thermal turbulence, bandwidth 3 MHz Thermal turbulence, bandwidth 2 MHz Thermal turbulence, bandwidth 1 MHz Figure 19. BER/distance relationship for Octavia tail-light with different M-QAM and bandwidths—scenario 2—thermal turbulence. 15 20 25 30 35 40 45 4-QAM 8-QAM 16-QAM 32-QAM 64-QAM 128-QAM 256-QAM 512-QAM 1024-QAM 2048-QAM M-QAM Empty box, bandwidth: 4 MHz Empty box, bandwidth: 3 MHz Empty box, bandwidth: 2 MHz Empty box, bandwidth: 1 MHz Thermal turbulence, bandwidth 4 MHz Thermal turbulence, bandwidth 2 MHz Thermal turbulence, bandwidth 1 MHz Thermal turbulence, bandwidth 3 MHz Eb/N0(dB) Figure 20. E b /N 0 /distance relationship for Octavia tail-light with different M-QAM and bandwidths—scenario 2—thermal turbulence. Electronics 2019,8, 433 17 of 36 0 2 4 6 8 10 12 14 4-QAM 8-QAM 16-QAM 32-QAM 64-QAM 128-QAM 256-QAM 512-QAM 1024-QAM 2048-QAM M-QAM EVM (%) Empty box, bandwidth: 4 MHz Empty box, bandwidth: 3 MHz Empty box, bandwidth: 2 MHz Empty box, bandwidth: 1 MHz Thermal turbulence, bandwidth 4 MHz Thermal turbulence, bandwidth 2 MHz Thermal turbulence, bandwidth 1 MHz Thermal turbulence, bandwidth 3 MHz Figure 21. EVM/distance relationship for Octavia tail-light with different M-QAM and bandwidths—scenario 2—thermal turbulence. 4.3. Scenario 3-Rain 42 L/min The first part of measurements with rain were carried out in modified box as well. In this scenario, we have removed the top part of the box and exchanged it with three water nozzles. The box was equipped with compressor which pumped water from the bottom of the box back into nozzles. The transmitter and receiver were located outside the box, which was opened from both sides. In this scenario, the water flow was set to 42 L/min. Setup can be seen on Figure 22. Figure 23 shows a photo of this setup. Measurements were carried out in a room at 22–25 ◦ C. Water temperature was stabilized at 22–25 ◦ C before each measurement. Both transmitter and receiver were located 25 cm from open side of the box. This way, it was impossible for them to get fogged up. To investigate the influence of walls or partitions a series of tests without a box and in a completely dark room were carried out before. The box had minimal influence, as its construction was adjusted to prevent it. Transmitting light was precisely focused into the box to avoid interference, which could be caused by possible reflections. Incoming light was also focused into photodetector by planoconvex lens, as mentioned earlier. Box construction was spacious enough to avoid unnecessary reflections on running water. However, the influence of water particles on detector/transmitter itself will definitely be a topic of further research. Rain (42 l/min) Partition bar (transparent) Nozzles Box - 50x50x500 cm Measuring distance 550 cm Photodetector Light Figure 22. Octavia tail-light setup adjusted for scenario 3—Rain 42 L/min. Electronics 2019,8, 433 18 of 36 Figure 23. Photo of setup for scenario 3 and 4—rain. Comparison of BER values with different bandwidths can be seen on Figure 24. This time, up to 8-QAM, BER stayed below 10e −5 . On the contrary, 16-QAM and higher modulations suffer from much higher BER. 4 - Q A M 8 - Q A M 1 6 - Q A M 3 2 - Q A M 6 4 - Q A M 1 2 8 - Q A M M - Q A M BER (-) 10-5 10-4 10-3 10-2 10-1 100 Rain 42 l/min, bandwidth 1 MHz Rain 42 l/min, bandwidth 2 MHz Rain 42 l/min, bandwidth 3 MHz Rain 42 l/min, bandwidth 4 MHz Figure 24. BER/distance relationship for Octavia tail-light with different M-QAM and bandwidths—scenario 3—Rain 42 L/min. E b /N 0 and EVM values are displayed on Figures 25 and 26. By comparing these values to reference setup, there was a significant drop in signal quality. For example, in case of 4-QAM modulation and 1 MHz bandwidth, SNR was nearly 10 dB lower than the reference. Significant decrease in signal quality was observed, which also led to lower maximal reachable modulation/bandwidth combination, which was 128-QAM/1 MHz. Electronics 2019,8, 433 19 of 36 15 20 25 30 35 40 45 4-QAM 8-QAM 16-QAM 32-QAM 64-QAM 128-QAM 256-QAM 512-QAM 1024-QAM 2048-QAM M-QAM Empty box, bandwidth: 4 MHz Empty box, bandwidth: 3 MHz Empty box, bandwidth: 2 MHz Empty box, bandwidth: 1 MHz Rain 42 l/min, bandwidth 4 MHz Rain 42 l/min, bandwidth 2 MHz Rain 42 l/min, bandwidth 1 MHz Rain 42 l/min, bandwidth 3 MHz Eb/N0(dB) Figure 25. E b /N 0 / distance relationship for Octavia tail-light with different M-QAM and bandwidths —scenario 3—Rain 42 L/min. 4-QAM 8-QAM 16-QAM 32-QAM 64-QAM 128-QAM 256-QAM 512-QAM 1024-QAM 2048-QAM M-QAM EVM (%) 0 2 4 6 8 10 12 14 Empty box, bandwidth: 4 MHz Empty box, bandwidth: 3 MHz Empty box, bandwidth: 2 MHz Empty box, bandwidth: 1 MHz Rain 42 l/min, bandwidth 4 MHz Rain 42 l/min, bandwidth 2 MHz Rain 42 l/min, bandwidth 1 MHz Rain 42 l/min, bandwidth 3 MHz Figure 26. EVM/distance relationship for Octavia tail-light with different M-QAM and bandwidths —scenario 3—Rain 42 L/min. This setup was also influenced by the drops of water on the sides of the box. Mainly the “separation wall” between box sections caused concerns. To analyze this concerning issue, we have carried out several measurements with different box setups, which can be seen on Figure 27. The first curve corresponds to the empty box, second one to the box with rain but without partition and the last one was the original measured setup. By comparing these values, there is a noticeable 3 dB increase in attenuation between second and third curve. Electronics 2019,8, 433 20 of 36 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Frequency (Hz) x106 -110 -100 -90 -80 -70 -60 -50 -40 -30 Attenuation (dB) Carrier frequency 3 MHz Box without rain and without partition Rain - max. flow (42 l/min) without partition Rain - max. flow (42 l/min) Figure 27. Attenuation characteristics of Octavia tail-light for scenario 3—Rain 42 L/min—comparison of intended setup and adjusted setup without partition. 4.4. Scenario 4 -Rain 22 L/min The second part of measurements with rain were carried out in the modified setup from scenario 3. The main difference was a different water flow of 22 L/min. Setup can be seen on Figure 28. Rain (22 l/min) Partition bar (transparent) Nozzles Box - 50x50x500 cm Measuring distance 550 cm Photodetector Light Figure 28. Octavia tail-light setup adjusted for scenario 4-Rain 22 L/min. Comparison of BER values with different bandwidths can be seen on Figure 29. This time, up to 16-QAM, BER stayed below 10e −5 . On the contrary, 32-QAM and higher modulations suffer from much higher BER. 4-QAM 8-Q AM 16-QAM 32-QAM 64-QAM 128-QAM 256-QAM M - Q A M 10-5 10-4 10-3 10-2 10-1 100 BER (-) Rain 22 l/min, bandwidth 1 MHz Rain 22 l/min, bandwidth 2 MHz Rain 22 l/min, bandwidth 3 MHz Rain 22 l/min, bandwidth 4 MHz Figure 29. BER/distance relationship for Octavia tail-light with different M-QAM and bandwidths —scenario 4-Rain 22 L/min. Electronics 2019,8, 433 21 of 36 E b /N 0 and EVM values (Figures 30 and 31) tended to follow the trend from previous scenario. This time, in case of 4-QAM modulation and 1 MHz bandwidth, SNR is nearly 7 dB lower than the reference scenario 1. Significant decrease in signal quality was observed, which also led to lower maximal reachable modulation/bandwidth combination, which was 256-QAM/1 MHz. 15 20 25 30 35 40 45 4-QAM 8-QAM 16-QAM 32-QAM 64-QAM 128-QAM 256-QAM 512-QAM 1024-QAM 2048-QAM M-QAM Empty box, bandwidth: 4 MHz Empty box, bandwidth: 3 MHz Empty box, bandwidth: 2 MHz Empty box, bandwidth: 1 MHz Rain 22 l/min, bandwidth 4 MHz Rain 22 l/min, bandwidth 2 MHz Rain 22 l/min, bandwidth 1 MHz Rain 22 l/min, bandwidth 3 MHz Eb/N0(dB) Figure 30. E b /N 0 / distance relationship for Octavia tail-light with different M-QAM and bandwidths —scenario 4—Rain 22 L/min. EVM (%) 0 2 4 6 8 10 12 14 4-QAM 8-QAM 16-QAM 32-QAM 64-QAM 128-QAM 256-QAM 512-QAM 1024-QAM 2048-QAM M-QAM Empty box, bandwidth: 4 MHz Empty box, bandwidth: 3 MHz Empty box, bandwidth: 2 MHz Empty box, bandwidth: 1 MHz Rain 22 l/min, bandwidth 4 MHz Rain 22 l/min, bandwidth 2 MHz Rain 22 l/min, bandwidth 1 MHz Rain 22 l/min, bandwidth 3 MHz Figure 31. EVM/distance relationship for Octavia tail-light with different M-QAM and bandwidths —scenario 4—Rain 22 L/min. Scenario 4 was also influenced by water drops. By carrying out the same test as in scenario 3, we have measured a 4 dB difference in attenuation, which was caused by “partition bar”’. Measured values are displayed on Figure 32. Electronics 2019,8, 433 22 of 36 -100 -90 -80 -70 -60 -50 -40 -30 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Frequency (Hz) x106 Attenuation (dB) Carrier frequency 3 MHz Box without rain and without partition Rain - flow rate 22 l/min without partition Rain - flow rate 22 l/min Figure 32. Attenuation characteristics of Octavia tail-light for scenario 4-Rain 22 L/min—comparison of intended setup and adjusted setup without partition. 4.5. Scenario 5-Fog The last scenario consisted of a sealed box connected to the fog machine. This measurement differed from the others in that we did not always have the same conditions in the box, as fog inside gradually evaporated. That is why we had to add a 4 mW laser and optical power meter to our experiment (Figure 33). Figure 34 shows a photo of this setup. During measurements, we have monitored and gathered optical power levels to use them as reference values. Due to the slow fog evaporation, higher modulations gradually reached the desired BER threshold of 10 −3 . As soon as it happened, we carried out our main measurements. In this case we have measured all bandwidths at once, to preserve credibility. Figure 35 describes a slow dissipation of fog inside box with gathered laser optical power levels and BER values for 1 MHz bandwidth. Fog generator Fog Laser (4 mW) Optical power meter Measuring distance 550 cm Box - 50x50x500 cm Photodetector Light Figure 33. Octavia tail-light setup adjusted for scenario 5–fog. Figure 34. Photo of setup for scenario 5-fog. Electronics 2019,8, 433 23 of 36 0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 10-5 10-4 10-3 10-2 Optical performance (W) 4-QAM BER: 0 16-QAM BER: 1.653E-03 8-QAM BER: 2.507E-04 32-QAM BER: 4.922E-04 64-QAM BER: 1.335E-03 128-QAM BER: 1.591E-03 256-QAM BER: 2.684E-03 1024-QAM BER: 7.048E-02 512-QAM BER: 1.866E-02 Time (s) Figure 35. Fog dissipation with laser power levels and modulations with appropriate BER values. The Laser itself was mainly used to exactly determine how fast the fog dissipated (concentration) and if it dissipated similarly in multiple measurements, which proved to be true. High directionality of laser beam and planoconvex lens on photodetector helped us to avoid any unnecessary effects of second light source. As mentioned the box was big enough to host both devices, which can work independently. In this case, the laser was the available device which we already had and could be used. Table 4consists of measured optical power level thresholds, after passing through fog. It is visible that at least a quarter of transmitted optical power is needed for higher modulation formats (128-QAM+) to work. That is why following the figures display minimal values for successful initiation of communication between transmitter and receiver for multiple modulation formats. Table 4. Minimal optical power levels of laser for multiple bandwidth and modulations measured after passing through fog. M-QAM Bandwidth - 1 MHz 2 MHz 3 MHz 4 MHz 4-QAM 101 µW 95 µW 97 µW 93 µW 8-QAM 112 µW 135 µW 175 µW 189 µW 16-QAM 154 µW 205 µW 250 µW 276 µW 32-QAM 275 µW 308 µW 463 µW 550 µW 64-QAM 525 µW 780 µW 930 µW 951 µW 128-QAM 905 µW 1.095 mW 1.399 mW 1.550 mW 256-QAM 2.180 mW 2.394 mW 2.545 mW 2.585 mW 512-QAM 2.475 mW 2.522 mW 2.735 mW 2.930 mW 1024-QAM 2.640 mW 2.955 mW 3.120 mW 3.172 mW Measured BER values are displayed on Figure 36. This figure consists of all modulation formats, where BER values were better than 1. However, it is necessary to compare these values to Table 4, as different optical power levels were needed for every modulation scheme. For example, at least a quarter of transmitted optical power had to be received for 128-QAM/2MHz combination to work. Eb/N0and EVM values displayed on Figures 37 and 38 also must be compared to Table 4. Electronics 2019,8, 433 24 of 36 4-QAM 8-QAM 16-QAM 32-QAM 64-QAM 128-QAM 256-QAM 512-QAM 1024-QAM M-QAM 10-5 10-4 10-3 10-2 10-1 100 BER (-) Fog, bandwidth 4 MHz Fog, bandwidth 3 MHz Fog, bandwidth 2 MHz Fog, bandwidth 1 MHz Figure 36. BER/distance relationship for Octavia tail-light with different M-QAM and bandwidths—in scenario 5—fog. 4-QAM 8-QAM 16-QAM 32-QAM 64-QAM 128-QAM 256-QAM 512-QAM 1024-QAM M-QAM 18 20 22 24 26 28 30 32 Eb/N0(dB) Fog, bandwidth 2 MHz Fog, bandwidth 1 MHz Fog, bandwidth 3 MHz Fog, bandwidth 4 MHz Figure 37. E b /N 0 / distance relationship for Octavia tail-light with different M-QAM and bandwidths —in scenario 5—fog. 0 5 10 15 20 25 EVM (%) Fog, bandwidth 2 MHz Fog, bandwidth 1 MHz Fog, bandwidth 3 MHz Fog, bandwidth 4 MHz 4-QAM 8-QAM 16-QAM 32-QAM 64-QAM 128-QAM 256-QAM 512-QAM 1024-QAM M-QAM Figure 38. EVM/distance relationship for Octavia tail-light with different M-QAM and bandwidths —scenario 5—fog. Electronics 2019,8, 433 25 of 36 From all the meteorological phenomena, fog had the greatest influence on visible light communication and will be the hardest to overcome. Figure 39 displays attenuation characteristics for different laser power levels. For example, at 1 mW output power, attenuation increased by 63 dB in comparison with reference values. 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Frequency (Hz) x106 -110 -100 -90 -80 -70 -60 -50 -40 -30 -20 Carrier frequency 3 MHz Reference laser power 17 μW Reference laser power 30 μW Reference laser power 43 μW Reference laser power 63 μW Reference laser power 53 μW Reference laser power 88 μW Reference laser power 107 μW Reference laser power 300 μW Reference laser power 500 μW Reference laser power 800 μW Reference laser power 1 mW Reference laser power 1.5 mW Reference laser power 2 mW Reference laser power 2.5 mW Reference laser power 3 mW Reference laser power 183 μW Attenuation (dB) Figure 39. Attenuation characteristics of Octavia tail-light during fog dissipation –in scenario 5 - fog. 5. Future Research and Discussion During testing we have encountered several shortcomings, we will try to overcome in the future. As our prototype is designed with the highest possible modularity in mind, we want to aim for better parts especially tailored for our needs. We have identified amplifier nonlinearity and photodetector as our biggest limitation. We were able to acquire two APD modules from Hamamatsu Photonics Japan, which will be tested, evaluated and possibly incorporated into our prototype [ 103 , 104 ]. There are also multiple companies we are currently in contact with to manufacture custom amplifiers such as Mini Circuits and Ophir RF. Figures 6and 13 show trend of increasing attenuation below 1 MHz and above 5 MHz. It is caused either by insufficient optimization of inbuild pre or post equalizers or by an amplifier in combination with insufficient impedance matching. Nonlinearity at 1 MHz to 5 MHz did not cause any significant changes to transmission quality. However we are currently working on optimization and have ordered a number of specialized parts. We would like to propose a new concept of software defined optics (SDO). SDO is a visible light communication system, in which the critical part of signal processing is implemented by software programmable circuits. As a result, a variety of software changes can be used to swiftly modify the communication system, such as used modulation, channel coding etc. The transmitter consists of D/A converter and appropriate transmitter (LED or laser diode), which is modulated by a bias-tee. The receiver consists of a photodiode or camera and A/D converter. The main advantage of our experiment was deployment of commercially available lights as transmitters. We avoided excessive modifications of original designs and tested them to their thresholds. However, we estimate, that modification of an Octavia tail-light LED cluster might increase effective maximal communication distance by 20%, as original LED matrixes tend to be of lower quality. 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