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Bu bildiride, görünür ışık haberleşme sistemi için opto-elektronik alıcı devre tasarımı yapılarak veri hızında önemli bir artış gerçekleştirilmiştir. Görünür haberleşme sisteminde verici taratfa kullanılan LED’ler sınırlı bant genişliğine sahip olduklarından dolayı yüksek hızlı veri aktarımına imkan vermemektedir. Denkleştrici yardımıyla LED üzerinden aktarılabilecek veri hızı artabilmektedir. Tasarlanan optoelektronik alıcı, fotodiyot, trans-impedans kuvvetlendirici ve denkleştirici bloklarını içermektedir. Öncelikle, var olan LED’in frekans yanıtı çıkarılmış ve bant genişliği 700 kHz civarında ölçülmüştür. 1 Mbit/s hızında veri gönderilerek belirgin bir göz açıklığı elde edildikten sonra 50 Mbit/s hızında veri gönderilerek simgelerarası karışmadan ötürü işaretin alıcı tarafta büyük oranda bozulduğu gözlenmiştir. Kurulan düzenekte tasarlanan opto-elektronik alıcı kullanılarak aynı LED ile 50 Mbit/s hızında veri gönderilmiş ve işaret denkleştirici çıkışında düzgün biçimde alınmıştır. Son olarak, 50 Mbit/s’den 120 Mbit/s’e kadar yüksek hızlardaki veri için bit hata olasılığı elde edilmiştir. Sadece denkleştirici kullanılarak 700 kHz bant genişliğine sahip LED ile 90 Mbit/s için 10-2 bit hata olasılığı elde edilmiştir. In this work, A significant increase in the data rate is achieved by designing an opto-electronic receiver for the visible light communication system. LEDs used at the transmitter side in the visible light communication system do not allow to transmit high data rates due to limited bandwidth. With the help of an equalizer, the data rate to be transmitted through LED can be increased. The designed opto-electronic receiver includes photodiode, trans-impedance amplifier (TIA), and equalizer blocks. First, the frequency response of the existing LED is extracted and its bandwidth is measured to be around 700 kHz. After obtaining a clear eye opening by transmitting a data at 1 Mbit/s, it is observed that the signal at the receiver side is impaired due to the intersymbol interference when the data rate is 50 Mbit/s. In the established setup, a data rate of 50 Mbit/s is achieved by using the same LED and the signal is properly received at the output of equalizer. Last, the bit error rate is obtained for higher data rates from 50 Mbit/s to 120 Mbit/s. By using only equalizer, a bit error rate of 10-2 is achieved for the data rate of 90 Mbit/s by using LED with the bandwidth of 700 KHz.

27th Signal Processing and Communications Applications Conference, SIU 2019 April 2019, Article number 8806280 27th Signal Processing and Communications Applications Conference, SIU 2019; Sivas; Turkey; 24 April 2019 through 26 April 2019; Category numberCFP19559-ART; Code 151073 Bu bildiride, görünür ışık haberleşme sisteminde direkt görüş hattı için kanal modellemesi problemi ele alınmıştır. Sunulan kanal modelinde LED ve fotodiyot arasında filtre veya mercek gibi herhangi bir optik komponent bulunmamaktadır. Sonrasında, sözü edilen kanal modelinin benzetimi için uygun bir deney düzeneği kurulmuştur. Kurulan deney düzeneğinde LED ve fotodiyot karşılıklı olarak konumlandırılmıştır. LED ve fotodiyot arasındaki mesafe değiştirilerek fotodiyota ulaşan optik gücündeğişimi ölçülmüştür. Yapılan ölçüm kanal modeli ile karşılaştırılmıştır. Kanal modeli ile ilgili düzenekte yapılan ölçümsonuçlarının örtüştüğü gösterilmiştir. Daha sonra, alıcı çıkışındaki işaret gürültü oranı mesafeye bağlı olarak ölçülmüştür. Yapılan ölçümlerde kullanılan LED’ler için işaret-gürültü oranı 20 cm mesafeden sonra 15 dB seviyesinin altına inmiştir ve aradaki mesafe 50 cm olduğunda çıkıştaki sinyal seviyesi gürültü ile aynı düzeye erişmiştir. In this work, a channel model for line-of-sight (LOS) in visible light communication is elaborated. In the proposed channel model, there is no optical component, such as filter or lens, between the transmitting LED and the receiving photodiode. Later, a suitable setup is built for emulation of the mentioned channel model. In the setup, the LED and the photodiode are located in a line-of-sight orientation. The change of the optical power on the photodiode is measured by varying the distance between the LED and photodiode. The measurement result is compared with the channel model. The channel model is in good agreement with the measurement results. Following that, the signal-to-noise ratio (SNR) at the output of the receiver, as a function of the distance, is measured. Measurement results indicate that the SNR value remains below 15 dB after exceeding a distance of 20 cm for the specific LED-Photodiode pair used in the experiment. The optical signal on photodiode drops to the noise level when the distance exceeds 50 cm. IEEE Turkey Sect Turkcell Türk Havacılık Uzaysanayii Turitak Bilgem Gebze Teknik University SAP Detaysoft NETAS Havelsan

In this paper, we consider multiple-input multiple-output (MIMO) visible light communication (VLC) systems. The physical layer of VLC system builds upon optical orthogonal frequency division multiplexing (O-OFDM). Repetition code (RC), spatial multiplexing (SMUX) and spatial modulation (SMUD) are considered as MIMO techniques. We evaluate the performance of three MIMO O-OFDM VLC systems under consideration through the derivation of bit error rate expressions. Istanbul Kalkınma Ajansı

Due to copyright restrictions, the access to the full text of this article is only available via subscription. Optical wireless communication (OWC) enables wireless connectivity using infrared, visible or ultraviolet bands. With its powerful features such as high bandwidth, low cost and operation in an unregulated spectrum, OWC can be, in some applications, a powerful alternative to and, in others, complementary to the existing wireless technologies. It is one of the most promising current areas of research with significant potentials for high-impact results which will considerably change the wireless market mostly dominated by the radio-frequency (RF) technologies. In this paper, we provide an overview of OWC highlighting the advantages and wide range of application areas of this emerging technology. European Commission

Autonomous underwater vehicles (AUVs) are instrumental for data offloading in underwater sensor networks (USNs). With high data rate capacity at transmission ranges in the order of several tens of meters, visible light communication (VLC) is well-positioned to serve as a wireless link between the AUV and sensor nodes. In this paper, we consider a USN network where an AUV is used for data retrieval from the sensors through VLC link. We formulate the design of optimal AUV trajectory as an optimization problem to minimize the AUV energy consumption under data rate constraints imposed by the VLC link and in the presence of ocean currents. Our numerical results demonstrate that our proposed trajectory is reactive to ocean currents and brings significant reductions in energy consumption and mission time of the AUVs, in particular for USN scenarios with a large number of sensor nodes.

17th International Conference on Transparent Optical Networks (ICTON 2015) -- 42190 -- Budapest, HUNGARY, WOS: 000380506700023, In this paper, we investigate the two single carrier frequency domain equalization (SC-FDE) techniques, namely on-off keying single carrier frequency domain equalization (OOK-SC-FDE) and decomposed quadrature optical (DQO)-SC-FDE, under realistic channel conditions with a practical LED model for visible light communications (VLC). We first compare their performances under dynamic range constraints of the LEDs and observe lower peak-to-average power ratio (PAPR) for SC-FDE schemes as compared to orthogonal division multiplexing (OFDM) techniques. We further introduce decision feedback equalizer (DFE) for SC-FDE schemes and demonstrate performance improvement in multi-tap indoor VLC channels., ERICSON, ADVA OPT NETWORKING, SILICON LABS, BHE, MobiTel, ROHDE & SCHWARZ

Adaptive spatial modulation (ASM) varies the modulation size across the transmit light-emitting diodes (LEDs) to improve the overall spectral efficiency of visible light communication (VLC) systems. This article proposes a novel ASM scheme, referred to as flexible generalized spatial modulation (FGSM), for VLC systems. The proposed FGSM system changes the modulation sizes over the LEDs and the number of active LEDs, to improve the average symbol error rate (SER) and spectral efficiency compared to ASM with a fixed overall number of LEDs. The modulation sizes are selected in order to optimize the average SER under a predefined spectral efficiency value. A closed-form expression of approximate SER is derived along with decoding complexity calculations for the proposed system. Numerical results are provided to confirm the superiority of the proposed system and to support theoretical derivations., Scientific and Technological Research Council of Turkey (TÜBİTAK)

In this paper, we explore the use of visible light communication as a means of wireless monitoring in gas pipelines. In an effort to shed light on the communication limits in the presence of gas, we create a three-dimensional simulation platform where the pipeline size/shape, the reflection characteristics of the interior coating, gas specifications (i.e., temperature, density, refractive index, transmittance, etc.) and the specifications of the light sources and detectors (i.e., field of view, lighting pattern, etc.) are precisely defined. Based on ray tracing, we obtain channel impulse responses within the gas pipeline considering the deployment of different colored LEDs with various viewing angles. We further investigate the maximum achievable link range to ensure a given bit error rate. NPRP award under the Qatar National Research Fund (a member of the Qatar Foundation) Publisher version

Visible light communication (VLC) builds upon the dual use of existing lighting infrastructure for wireless data transmission. VLC has recently gained interest as cost-effective, secure, and energy-efficient wireless access technology particularly for indoor user-dense environments. While initial studies in this area are mainly limited to single-user point-to-point links, more recent efforts have focused on multi-user VLC systems in an effort to transform VLC into a scalable and fully networked wireless technology. In this paper, we provide a comprehensive overview of multi-user VLC systems discussing the recent advances on multi-user precoding, multiple access, resource allocation, and mobility management. We further provide possible directions of future research in this emerging topic., Comment: Version 3: Accepted for publication in IEEE Access, Nov. 2018

In this paper, we present a comprehensive channel modeling and characterization study for underwater visible light communications. Our study is based on the advanced ray tracing, which allows for an accurate description of the interaction of rays emitted from the lighting source within an underwater environment. Contrary to existing works, which are mainly limited to simplified underwater scenarios, i.e., empty sea, we take into account the presence of human and man-made objects to investigate the effects of shadowing and blockage. The reflection characteristics of the sea surface and sea bottom as well as the water characteristics, i.e., extinction coefficient and scattering phase function of particles, are precisely considered. As case studies, we consider various underwater scenarios with different transmitter/receiver specifications (i.e., viewing angle, aperture size) and different depths from the sea surface. For each environment, we obtain channel impulse responses and present a characterization study where channel parameters, such as channel DC gain, path loss, and delay spread, are obtained. publisher version

In this paper, underwater visible light communication (UWVLC) vertical links are modeled and evaluated, taking account of the inhomogeneous nature of underwater (UW) environment. An equivalent simplified model of stratified N layers is employed in which variations in refractive index and attenuation profiles across UW depth are considered. A generalized path loss expression is deduced which allows estimation of the vertical link loss prior to link design. Mathematical expressions of the received power for line-of-sight (LOS) and non-line-of-sight (NLOS) links between transmitters (Tx) and receivers are deduced. We evaluate the performance by computing the received power and bit error rate for inhomogeneous underwater links. Numerical examples are used to illustrate the proposed models. Deviations from expected results when considering a homogeneous underwater model are discussed. A simple underwater bilayer model is then introduced, which is considered a rough approximate model in comparison to the stratified N layers model. This model assists in estimating the UW link behavior without extensive calculations. The conditions necessary for applying this model are discussed and justified. Moreover, the effects of Tx orientation along with narrowing the transmitted light cone on LOS and NLOS vertical links are examined. A Tx power saving of 30%-50% in UWVLC links is proved to be achieved by rotating the Tx and narrowing its emitted light cone. NPRP award from the Qatar National Research Fund (Qatar Foundation) Publisher version

Due to copyright restrictions, the access to the full text of this article is only available via subscription. In this letter, we propose a realistic channel model for visible light communication (VLC) assuming a mobile user. Based on non-sequential ray tracing, we first obtain channel impulse responses for each point over the user movement trajectories, and then express path loss and delay spread as a function of distance through curve fitting. Our results demonstrate large variations in received power. In system design, this necessitates the use of adaptive schemes, where transmission parameters can be selected according to channel conditions. To demonstrate the benefits of link adaptation over a mobile VLC channel, we propose an adaptive system with luminary selection and demonstrate improvements in spectral efficiency over non-adaptive systems. Istanbul Development Agency ; TÜBİTAK Istanbul Development Agency (ISTKA) under Innovative Istanbul Financial Support Program; TÜBİTAK

Unipolar orthogonal frequency division multiplexing (U-OFDM) appears as an attractive optical OFDM solution for emerging visible light communication (VLC) systems. This paper proposes spectral efficiency improvement for U-OFDM systems by applying adaptive transmission over realistic VLC links. This adaptive transmission includes switching among a number of multiple-input multiple-output (MIMO) modes combined with appropriate modulation size selection. The considered MIMO modes are repetition coding, spatial modulation, and spatial multiplexing, where each mode supports different modulation sizes. The selection of the corresponding MIMO mode and its modulation size is based on the received signal-to-noise ratio and target bit error rate. The proposed U-OFDM system is applied over different VLC MIMO setups with realistic channel models for 8 x 8, 4 x 4 and 2 x 2 MIMO systems. Our simulation results show that the proposed adaptive system provides a significant spectral efficiency improvement over stand-alone U-OFDM MIMO modes/setups., Scientific and Technological Research Council of Turkey (TÜBİTAK)

In this paper, we present a unified resource allocation and mobility management algorithm based on particle swarm optimization (PSO) for indoor visible light communication (VLC) networks. We consider a VLC network where multiple LEDs serve as access points (APs). A centralized controller collects channel state information, quality of service requirements of the users, and the overload status of the APs. Based on the available information, in each time interval, the controller decides which user is served by which AP and assigns subcarriers to the users with the objective of maximizing both the system throughput and user satisfaction. We formulate the resource allocation problem as a constrained nonlinear integer programming problem and solve it using meta-heuristic PSO. Through an extensive simulation study, the superiority of the proposed algorithm in terms of system throughput and user satisfaction over round robin, best channel quality information, and genetic algorithms is demonstrated. Council for Scientific and Industrial Research Publisher version

Estimation of the time-varying optical wireless channel response is crucial in order to decode received signals coherently. In this work, we investigate symbol-error-rate and mean absolute error performance of different interpolation techniques including linear, nearest, spline, and piece-wise cubic Hermite interpolating polynomial (pchip), which are used in pilot-aided channel estimation process for visible light communication. The performance of interpolators is evaluated in realistic time-varying channel model, generated on Zemax software and compared with each other under the consideration of different modulation orders, different pilot symbol periods, and different user equipment (UE) speeds through Monte Carlo simulations. The results reveal that spline and pchip techniques are more robust to low pilot symbol transmission rate and fast time-varying channel conditions as a consequence of high UE speeds. However, low complex linear interpolation technique can be chosen for highly rated pilot signal transmission cases or when optical channel varies slowly over the time., Scientific and Technological Research Council of Turkey (TÜBİTAK); Turkish Academy of Sciences (TÜBA); Young Scientist Award Program (GEBIP); Dr. Mustata Parlar Foundation Research Encouragement Award

Due to copyright restrictions, the access to the full text of this article is only available via subscription. Inter-vehicular connectivity to enhance road safety and support highly autonomous driving is increasingly becoming popular. Despite the prevalent works on radio-frequency (RF) based vehicular communication schemes, visible light communication (VLC) is considered to be a promising candidate for vehicular communications due to its low complexity and RF interference-free nature. Deployment of multiple light emitting diodes (LEDs) enables multiple-input multiple-output (MIMO) transmission in the context of vehicular VLC. This paper investigates applicability of both point-to-point (direct) vehicular VLC and decode-and-forward relaying based cooperative vehicular VLC including relay terminals between source and destination terminals to enhance road safety based on real world measurements. We consider direct current biased optical orthogonal frequency division multiplexing (DCO-OFDM) based MIMO transmission scheme and evaluate the performances of different MIMO modes including repetition code (RC) and spatial multiplexing (SM), different modulation orders with different transmitter–receiver selection mechanisms to support line-of-sight (LoS) and beyond LoS multi-hop vehicular VLC. The results reveal that the selection of the closest transmitters to the receivers provides better performance due to high signal-to-noise-ratio requirements for RC mode whereas SM suffers from channel correlation. Usage of all possible transmitters does not always yield better performance due to the power division at the transmitter side. On the other hand, the performance of RC shows more degradation on higher-order modulations that are required to yield the same throughput with SM. Therefore, considering the higher order modulation requirement for RC based VLC, SM is concluded to be a favorable MIMO scheme for cooperative vehicular VLC. We further demonstrate the benefits of multi-hop transmission over direct transmission with respect to different number of relay vehicles as a consequence of varying inter-vehicular distances between source and destination vehicles. Turkish Academy of Sciences (TUBA) within the Young Scientist Award Program (GEBIP) ; METU-Prof.Dr. Mustafa Parlar Foundation Research Encouragement Award ; TÜBİTAK

WOS: 000431488400001 In this paper, we investigate link adaptation for an orthogonal frequency division multiplexing (OFDM)-based multiple-input multiple-output (MIMO) visible light communication (VLC) system. The proposed adaptive OFDM VLC system supports both repetition coding (RC) and spatial multiplexing (SM) as MIMO modes and allows spatial mode switching based on channel conditions. Regarding to the instantaneous signal-to-noise ratio for both RC and SM modes, the maximum constellation size that can be supported for each MIMO mode on each subcarrier is determined. The MIMO mode that gives the highest spectral efficiency (SE) is then selected. The proposed joint MIMO mode selection and bit loading scheme maximizes the SE while satisfying a target bit error rate. Our numerical results reveal that a peak data rate up to 18.3 Gb/sec can be achieved in a 16 x 16 MIMO setting using light emitting diodes with cut-off frequency of 10 MHz in typical indoor environments. Nazarbayev University; Turkish Scientific and Research Council (TUBITAK) [215E311] The work of R. C. Kizilirmak was supported by the research grant from Nazarbayev University. The work of M. Uysal was supported by the Turkish Scientific and Research Council (TUBITAK) under Grant 215E311.

Optical wireless communication (OWC) systems provide many advantages over radio frequency (RF) wireless technologies in some scenarios, including significantly higher data rates and a large amount of available license-free frequency spectrum. Recently, OWC has also been proposed in fifth generation 5G standard as a tool to augment capacity due to RF spectrum crunching challenges. Various forms of OWC can be used to augment RF capacity at both access and backhaul/fronthaul levels. Despite the major advantages and various application areas, the widespread deployment of OWC is delayed by several challenges, such as the demand to maintain strict line-of-sight alignment between transmitter and receiver in long range outdoor applications; the need to combat attenuation due to adverse weather conditions such as fog, cloud, and turbulence; to modulate light emitting diodes at high frequencies without distortion at indoor visible light communication (VLC) applications and retaining power levels within the eye safety limits for laser transmitters and comfortable illumination levels for LED transmitters.

Due to copyright restrictions, the access to the full text of this article is only available via subscription. New data services and applications are emerging continuously and enhancing the mobile broadband experience. The ability to cope with these varied and sophisticated services and applications will be a key success factor for the highly demanding future network infrastructure. One such technology that could help address the problem would be optical wireless communications (OWC), which presents a growing research interest in the last few years for indoor and outdoor applications. This paper is an overview of the OWC systems focusing on visible light communications, free space optics, transcutaneous OWC, underwater OWC, and optical scattering communications. European Commission ; National Key Basic Research Program of China ; National Natural Science Foundation of China ; Shenzhen Peacock Plan

Due to copyright restrictions, the access to the full text of this article is only available via subscription. In this paper, we present a comprehensive channel modeling and characterization study for visible light communications. Our study is based on ray tracing, which allows for an accurate description of the interaction of rays emitted from the lighting source within a specified confined space. Contrary to existing works, which are mainly limited to ideal Lambertian sources and purely diffuse reflections, our approach is capable of obtaining channel impulse responses (CIRs) for any nonideal sources, as well as specular and mixed specular-diffuse reflections. Furthermore, we can precisely reflect the presence of objects (e.g., furniture) and wavelength-dependent reflection characteristics of surface materials (e.g., ceilings, floor, walls, and furniture) in a channel study. As case studies, we consider a number of indoor environments with various dimensions and different surface materials, i.e., plaster, gloss paint, wood, aluminum metal, and glass. We further consider various scenarios with different transmitter specifications (i.e., single versus multiple transmitters and array type) and receiver specifications (i.e., location and rotation). For each environment, we obtain CIRs and present a channel characterization study where channel parameters, such as channel DC gain, root mean square (RMS) delay spread, coherence bandwidth, and mean excess delay, are obtained. We also make one-to-one comparisons between infrared and visible-light CIRs for the same environments to emphasize the differences between two optical bands. Qatar Foundation

Due to copyright restrictions, the access to the full text of this article is only available via subscription. In this paper, we investigate cooperative visible light communication (VLC) system where an intermediate light source acts as a relay terminal. We assume that relay terminal operates in full-duplex mode. In contrast to radio frequency counterparts, full-duplex VLC terminal is relatively easier to implement due to directive propagation characteristic of light. We first model VLC relay terminal taking into account loop interference channel based on ray tracing simulations. Then, we investigate error rate performance of the relay-assisted VLC system. Our performance evaluations demonstrate the superiority of full-duplex relaying over half-duplex counterpart especially for high modulation sizes. Nazarbayev University; The Turkish Scientific and Research Council

Due to copyright restrictions, the access to the full text of this article is only available via subscription. Visible light communications (VLC) is a promising and uncharted new technology for the next generation of wireless communication systems. This paper proposes a novel generalized light emitting diode (LED) index modulation method for multiple-input-multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM)-based VLC systems. The proposed scheme avoids the typical spectrum efficiency losses incurred by time- and frequency-domain shaping in OFDM signals. This is achieved by exploiting spatial multiplexing along with LED index modulation. Accordingly, real and imaginary components of the complex time-domain OFDM signals are separated first, then resulting bipolar signals are transmitted over a VLC channel by encoding sign information in LED indexes. As a benchmark, we demonstrate the performance analysis of our proposed system for both analytical and physical channel models. Furthermore, two novel receiver designs are proposed. Each one is suitable for frequency-flat or selective channel scenarios. It has been shown via extensive computer simulations that the proposed scheme achieves considerably better bit error ratio versus signal-to-noise-ratio performance than the existing VLC-MIMO-OFDM systems that use the same number of transmit and receive units [LEDs and photo diodes (PDs)]. Compared with the single-input single-output (SISO) DC biased optical (DCO)-OFDM system, both spectral efficiency and DC bias can be doubled and removed respectively simply by exploiting a MIMO configuration. European Cooperation in Science and Technology (COST); The Scientific and Technological Research Council of Turkey (TUBITAK) Research; EPSRC under Established Career Fellowship

WOS: 000394680300033 The IEEE has established the standardization group 802.15.7r1 "Short Range Optical Wireless Communications", which is currently in the process of developing a standard for visible light communication (VLC). As with any other communication system, realistic channel models are of critical importance for VLC system design, performance evaluation, and testing. This article presents the reference channel models that were endorsed by the IEEE 802.15.7r1 Task Group for evaluation of VLC system proposals. These were developed for typical indoor environments, including home, office, and manufacturing cells. While highlighting the channel models, we further discuss physical layer techniques potentially considered for IEEE 802.15.7r1. Istanbul Development Agency (ISTKA) under the Innovative Istanbul Financial Support Program [TR10/15/YNK-72 OKATEM]; TUBITAK [113E307] This work is carried out as an activity of the "Optical Wireless Communication Technologies Excellence Centre" funded by the Istanbul Development Agency (ISTKA) under the Innovative Istanbul Financial Support Program 2015 (TR10/15/YNK-72 OKATEM). The statements made herein are solely the responsibility of the authors and do not reflect the views of ISTKA and/or the T.R. Ministry of Development. The works of Farshad Miramirkhani and Erdal Panayirci are supported by TUBITAK Research Grant No. 113E307.

Due to copyright restrictions, the access to the full text of this article is only available via subscription. In this paper, we consider a relay-assisted visible light communication (VLC) system where an intermediate light source cooperates with the main light source. Following the IEEE 802.15.7r1 VLC reference channel model, we assume the presence of two different light sources in an office space. The first one is the ceiling light that serves as the source terminal while the second one is the desk lamp that serves as the relay terminal. Our system builds upon unipolar optical orthogonal frequency division multiplexing (U-OFDM) where the desk light performs amplify-and-forward relaying to assist the ceiling light and operates in a half-duplex mode. In addition, we consider the use of enhanced U-OFDM (eU-OFDM) which doubles the spectral efficiency of U-OFDM at the expense of additional computational complexity. We analyze the bit error rate performance of the relay-assisted VLC system building on these two OFDM types and quantify performance improvements over point-to-point (i.e., no relaying) VLC systems. In an effort to further improve system performance, we investigate optimal power allocation between source and relay terminals. TÜBİTAK

Due to copyright restrictions, the access to the full text of this article is only available via subscription. In this paper, we investigate the performance of a direct current biased optical orthogonal frequency division multiplexing (DCO-OFDM) based adaptive transmission in the context of visible light communication (VLC). We consider the reference channel model (Scenario I) of IEEE 802.15.7r1 where an indoor office room is considered and channel impulse responses are presented for 24 different points. Our results reveal that high data rates such as 100 Mbps can be achieved with the choice of optimal modulation order according to channel conditions, while non-adaptive transmission based on the worst-case design provides only 14 Mbps under the consideration of target bit-error-rate (BER) of 10-3.

Visible light communication (VLC) builds upon the idea of using existing lighting infrastructure for wireless data transmission. In a conventional VLC network, each light fixture acts as an access point (AP) which are connected to each other through electrical grid as well as data backbone. These VLC-enabled fixtures consist baseband unit (BBU) followed by the optical front-end (OFE). In this paper, we propose the so-called centralized light access network (C-LiAN) which aggregates all AP computational resources into a central pool that is managed by a centralized controller. Unlike the distributed architecture where each light fixture performs both baseband processing and optical transmission/reception, the centralized architecture employs “dummy”fixtures with a VLC OFE. Moving the baseband processing to a central pool reduces the associated cost and complexity of each VLC-enabled LED luminary. It further enables joint processing of signals from different APs making possible an efficient implementation of joint processing, offloading, handover, interference management, scheduling, and resource management algorithms. As an example to demonstrate the virtues of C-LiAN, we further present the performance of coordinated multipoint transmission and enhanced inter-cell interference coordination with almost blank subframe techniques originally proposed for Long Term Evolution-Advanced in the context of indoor VLC networks.

Vehicular connectivity is envisaged to extend vehicle on-board camera videos to the nearby vehicles in order to increase drivers' road awareness. This paper demonstrates the possibilities of video transmission captured by vehicle on-board forward-looking camera via visible light communications (VLC) using taillights to provide see-beyond the front vehicle assistance. Line of sight (LoS) inter-vehicular video transmission using direct current biased optical orthogonal frequency division multiplexing (DCO-OFDM) based VLC with the data rate of 6.42 Mbps and lower latency than 32 msec is validated through experiments.

Due to copyright restrictions, the access to the full text of this article is only available via subscription. This paper discusses the impact of pulse shaping filter on the performance of orthogonal frequency division multiplexing (OFDM) based visible light communication (VLC) systems. Particularly, direct current biased optical OFDM (DCO-OFDM) with square-root-raised-cosine (SRRC) pulse shaping filters is investigated. The bit-error-rate (BER) performance of the system is evaluated for different excess bandwidth parameters (roll-off factors) and compared with the performance with rectangular pulse shaping filter through Monte Carlo simulations.

Due to copyright restrictions, the access to the full text of this article is only available via subscription. In this paper, we propose a novel protocol for cooperative multiple access in visible light communication (VLC) networks. The proposed protocol exploits the features of enhanced unipolar orthogonal frequency division multiplexing (eU-OFDM) scheme in order to enable user terminals to sequentially access the multi-hop network. The signal-to-noise ratios (SNRs) for user terminals within the network are derived and the individual bit-error-rate (BER) expressions are presented. We evaluate the BER performances of each user in the network through Monte Carlo simulation. The theoretical results validates the simulation results.

Visible light communication (VLC) has recently gained popularity as a complementary technology to radio frequency (RF) based alternatives for vehicular communications as a low-cost, secure and RF interference free technology. In this paper, we propose IEEE 802.15.7 standard-compliant physical layer (PHY) implementation and experimental evaluation, using commercial off-the-shelf (COTS) automotive light emitting diode (LED) fog light for the purpose of low-latency safety message dissemination. We first show that the standard is applicable to line of sight (LoS) vehicle-to-vehicle (V2V) VLC. We then demonstrate that the proper selection of modulation coding schemes (MCS) plays an important role in order to minimize bit-error- rate (BER) for the reliable transmission with varying inter-vehicle distances. We also addressed the angular limitations of COTS automotive LED light for viable vehicular VLC.

Due to copyright restrictions, the access to the full text of this article is only available via subscription. Vehicular hotspots for on-board Internet access using Long Term Evolution (LTE) as the backhaul network has recently gained popularity. Currently, Wi-Fi is the most common technology to provide in-vehicle access, where data has been relayed through on board LTE receiver. Despite its wide acceptance, coexistence and contention based data rate limitations with Wi-Fi necessitates alternatives for in-vehicle data access schemes. This paper investigates the performance of hybrid LTE and visible light communication (VLC) networks using LTE as the backhaul and VLC as the on-board access network.Under the consideration of vehicle interior unique channel characteristics and light emitting diode (LED) deployment flexibility, best transmitter configuration using repetition coding (RC) and spatial multiplexing (SM) multiple input multiple output (MIMO)modes is determined. Proposed configurations based on direct current biased optical orthogonal frequency-division multiplexing(DCO-OFDM) are compared with respect to their bit-error-rate (BER) performances. Furthermore, the performance of intravehicular VLC networks for single and multi-user scenarios is investigated. Argela ; Turk Telekom ; TÜBA

Due to copyright restrictions, the access to the full text of this article is only available via subscription. Visible light communications (VLC) is a high-speed wireless communication technology that has recently emerged. VLC uses the illumination infrastructure to meet broadband needs of bandwidth-hungry devices. Its high data rate, immunity to electromagnetic interference, ability to operate in an unlicensed band and high reuse factor position VLC as a strong alternative to existing communication technologies. The physical and medium access control (MAC) layers of VLC have been standardised in IEEE 802.15.7. Yet, theoretical performance bounds of the MAC layer have not been explored in detail so far. In this paper, we derive Markov-based performance bounds for the MAC layer of IEEE 802.15.7. We provide a thorough study of throughput, delay, power consumption, collision probability, transmission probability, access probability and packet discard probability using analytical analysis and simulations. Istanbul Development Agency

Due to copyright restrictions, the access to the full text of this article is only available via subscription. Visible light communication (VLC) involves the dual use of illumination infrastructure for high speed wireless access. Designing such optical based communication systems, realistic indoor optical channel modeling becomes an important issue to be handled. In this paper, first we obtain new realistic indoor VL channel characterizations and models, in a multipleinput multiple-output (MIMO) transmission scenario, using nonsequential ray tracing approach for the channel impulse responses (CIRs). Practical issues such as number of light emitting diode (LED) chips per luminary, spacing between LED chips, objects inside the room and cabling topology are also investigated. On the other hand, since indoor optical channels exhibit frequency selectivity, multi-carrier communication systems, particularly orthogonal frequency division multiplexing (OFDM) is used to handle the resulting inter-symbol interference in VLC systems. Hence, we propose a new MIMO-OFDM based VLC system, called MIMO enhanced unipolar OFDM (MIMOeU-OFDM) by combining MIMO transmission techniques with the recently proposed eU-OFDM scheme. The bit error rate (BER) performance of the proposed system is investigated in the presence of the 2 × 2 and 4 × 4 realistic MIMO VLC channels and its BER performance is compared with the reference optical MIMO-OFDM systems. TÜBİTAK ; Istanbul Development Agency

In this chapter, we explore the concept of cooperative transmissions in the context of visible light communications (VLC). An indoor office space is considered with two light sources; the one at the ceiling is connected to the backbone network and provides ambient light to the environment while the other one is mounted on the desk and used for task lighting. The system architecture builds upon DC biased optical orthogonal frequency-division multiplexing (DCO-OFDM). The task light performs relaying operation in either amplify-and-forward (AF) or decode-and-forward (DF) mode. Illumination constraints for task lighting are further considered in order to design a cooperative VLC system that provides satisfactory lighting. The communication performance of the system is optimized through a subcarrier-based power allocation mechanism. Numerical results incorporating practical issues such as band-limited channel process and imperfect channel estimation reveal that cooperative VLC systems can significantly outper-form the conventional point-to-point VLC systems.