Performance Of Optical Wireless Communication Research Articles

Optical Uplink, D2D and IoT Links Based on VCSEL Array: Analysis and Demonstration

In this paper, vertical cavity surface emitting laser (VCSEL) array is used to establish gigabits/second (Gbps) optical uplink, device-to-device (D2D), and Internet-of-thing (IoT) links, as a supplementary for visible light communication (VLC) and ultra-low latency near-field communication in a typical indoor scenario. The mathematical model based on a modified Monte-Carlo ray-tracing (MMCR) algorithm for VCSEL-based optical wireless communication (OWC) is presented, which takes into account both accuracy and time complexity for the calculation of the channel characteristics including power distribution, impulse response, error analysis, and signal-to-noise ratio (SNR). Simulation firstly takes a global approach to the optical uplink lens design method, compared between Lambertian and Gaussian sources, and then extended six typical line-of-sight (LOS) or non-line-of-sight (NLOS) VCSEL-based OWC models. For demonstration, we adopted a 940-nm VCSEL array and 850-nm single-pixel VCSEL to establish LOS and NLOS systems after measuring the optics-electronics and bandwidth characteristics, respectively. Furthermore, multiple multi-carrier schemes are adopted to improve the OWC performance system based on a 940-nm VCSEL array including uniform-loading orthogonal frequency division multiplexing (OFDM), channel-coded OFDM, bit-loading/ power-allocation OFDM, and OFDM access (OFDMA). Results show that OFDM can effectively decrease the inter-symbol interference (ISI) of the indoor channel and increase the data rate, and the bit/ power-loading method achieves the highest 7.2 Gbps transmission with the bit error rate (BER) within the forward error correction (FEC). All the theoretical and experimental results, for the first time, provide a comprehensive design and optimizing process of VCSEL-based indoor high-capacity OWC systems for future optical uplink, D2D, and IoT applications.

Performance of Dual-Hop Relaying for OWC System Over Foggy Channel With Pointing Errors and Atmospheric Turbulence

Optical wireless communication (OWC) over atmospheric turbulence and pointing errors is a well-studied topic. Still, there is limited research on signal fading due to random fog in an outdoor environment for terrestrial wireless communications. In this paper, we analyze the performance of a decode-and-forward (DF) relaying under the combined effect of random fog, pointing errors, and atmospheric turbulence with a negligible line-of-sight (LOS) direct link. We consider a generalized model for the end-to-end channel with independent and not identically distributed (i.ni.d.) pointing errors, random fog with Gamma distributed attenuation coefficient, double generalized gamma (DGG) atmospheric turbulence, and asymmetrical distance between the source and destination. We develop density and distribution functions of signal-to-noise ratio (SNR) under the combined effect of random fog, pointing errors, and atmospheric turbulence (FPT) channel and distribution function for the combined channel with random fog and pointing errors (FP). Using the derived statistical results, we present analytical expressions of the outage probability, average SNR, ergodic rate, and average bit error rate (BER) for both FP and FPT channels in terms of OWC system parameters. We also develop simplified and asymptotic performance analysis to provide insight on the system behavior analytically under various practically relevant scenarios. We demonstrate the mutual effects of channel impairments and pointing errors on the OWC performance, and show that the relaying system provides significant performance improvement compared with the direct transmissions, especially when pointing errors and fog becomes more pronounced.

Adaptive optics effect on performance of BPSK-SIM oceanic optical wireless communication systems with aperture averaging in weak turbulence

Turbulence-induced wavefront deformations cause the irradiance of an optical signal to fluctuate resulting a in serious degradation in the bit-error-rate (BER) performance of optical wireless communication (OWC) system. Adaptive optics is an effective technique to compensate for the wavefront aberrations to reduce the fluctuations in the received intensity. In this paper, we investigate how the adaptive optics technique affects the BER performance of an oceanic OWC (OOWC) system employing binary phase shift keying-subcarrier intensity modulation (BPSK-SIM) and aperture averaging. To evaluate BER performance in weak oceanic turbulence, the required entities such as the received optical power captured by a circular aperture and the aperture averaged scintillation index measuring the fluctuations in the received irradiance are derived. The effect of adaptive optics correction of various wavefront aberrations (i.e., tilt, defocus, astigmatism and the coma) on the BER performance is illustrated and the performance of the adaptive optics-OOWC system is compared to that of a non-adaptive optics OOWC system by the metric defined.

Performance of Opportunistic Beam Selection for OWC System Under Foggy Channel with Pointing Error

Performance of optical wireless communication (OWC) in outdoor environments can be significantly degraded due to the fog and pointing errors. In this letter, we consider a multi-aperture detector which employs a low complexity opportunistic beam selection (OBS) in order to improve the performance of OWC system. We analyze the OBS scheme by deriving computable bounds on the average signal-to-noise ratio (SNR), ergodic rate, and diversity order and compare its performance by deriving exact analytical expressions of a single-aperture system. The derived bounds provide insights on the system behavior depicting significant benefit for OWC systems with small link distances which are typical in foggy environments. We also demonstrate the effect of various parameters of fog and pointing error on the OWC performance and validate the derived analytical expressions using numerical and simulation analysis. A two-aperture OBS achieves an SNR gain of approximately 4 dB and improves the outage performance by more than 20 folds for a half-km OWC link comparing to the single-aperture system without acquiring the channel state information.

Performance analysis of OCDMA wireless communication system based on double length modified prime code for security improvement

In this study, a construction analysis and design of a new spreading code set are presented. This code is designed to be used as a source code in optical wireless communication (OWC) system to achieve better performance and security enhancement. The new proposed code is called double length modified prime code (DLMPC), which is constructed based on the popular modified prime code that is commonly applied in the optical code division multiple access (OCDMA) networks. The correlation properties and characteristics of the DLMPC are investigated in this study. Based on the new code, the OWC system architecture and channel model are illustrated and further analysed. Additionally, the transmitted and received data bits encoding and decoding scenarios are discussed. Moreover, a low power consumption optical light source and photo-detector are utilised for a transmitter and receiver, respectively. The overall OWC performance in terms of the bit error rate (BER) has been evaluated considering the multi-user interference as the only source of noise. Finally, the results show that the proposed DLMPC outperforms the popular codes with respect to the channel capacity, security, and system BER performance.

Enhanced Visible Light Communication Modulator With Dual-Feedback Control

This article presents a modulator for optical wireless communication (OWC) via LEDs, controlled by a dual closed loop. While the potential communication speed and performance of OWC have received a lot of research attention, the design of appropriate modulator circuits has not been studied so widely. This article addresses the control aspects to minimize power losses and to ensure automatic adaptation to component spreads or aging, for “high-power” LED emitters. We initially address a “low-bandwidth– system, particularly suited for indoor camera-based detection, and then a “higher bandwidth” one for Li-Fi data communication. In contrast to maximizing data throughput as it applies to high-speed modulators, low-speed modulators aim at high power and high efficiency in general lighting applications. Specifically, for the high-power modulator, we propose a control strategy that allows a wide range of combinations of LED types and LED power supplies. The power consumption of the modulator is minimized by voltage margin setting that is automatically adapt to the modulation and the LED type using the nonlinear detector. The common industry practice is to use constant-current LED drivers for powering LEDs. After creating a theoretical framework of some generality to evaluate designs and control loops with rationals and verified models, we realize modulator designs that are supplied by constant current LED drivers and verify their performance by simulation and measurements.

Effect of Environmental Parameters on the Performance of Optical Wireless Communications

Free space optical communication systems are the future of the communication systems which can meet the needs of the end users who are demanding high bandwidths to support high-speed communication. These merits are due to its numerous advantages as well as their better performance. This paper studies the effect of environmental parameters like rain, fog, haze, snow, and dust on the performance of optical wireless communications using Opti-system program. This work considers the effect of visibility as well as operating wavelengths on atmospheric attenuation in different weather conditions for free space optical link. For different weather conditions the max distance between transmitter and receiver can also be estimated. The system analysis depicted the better performance of the Multi-Input/Multi-Output (MIMO) configurations compared to Single Input/Single Output (SISO) at different weather conditions.

Computer-vision-based intelligent adaptive transmission for optical wireless communication.

Optical wireless communication (OWC) has been presented as a promising candidate for future space-air-ground-ocean-integrated communication. However, the OWC is quite sensitive to the variation of the channel transmission characteristics. The light beam absorption and the scattering in the transmission media affect not only the channel feature, but also the imaging quality. Thus, there is an inherent relationship between the OWC performance and the optical imaging quality. Based on this consideration, we firstly present the idea of introducing computer vision mechanisms into the OWC systems, and then propose a computer vision-based multi-domain cooperative adjustment (CV-MDCA) mechanism's functional modules to realize the intelligent adaptive transmission in OWC systems. The CV-MDCA mechanism are specifically designed, with the emphasis on how to quantitatively determine the exact on-line channel quality from the captured images by using effective computer vision schemes. Two groups of experiments, the indoor-simulated underwater visible light communication and the outdoor-practical atmospheric free-space optics, are implemented in order to evaluate the presented CV-MDCA mechanism's performance. The results not only validate the feasibility to determine the channel quality, according to the captured channel images, but also reveal the presented three computer vision-based criteria's limitations.

Error performance of optical wireless communication systems exercising BPSK subcarrier intensity modulation in non-Kolmogorov turbulent atmosphere

Abstract Subcarrier intensity modulation (SIM) scheme is preferred due to efficient bandwidth usage superiority over other modulation techniques such as on–off keying (OOK), pulse position modulation (PPM). In this paper, we investigate the bit error rate (BER) performance of optical wireless communication (OWC) system using binary phase shift keying (BPSK) SIM in non-Kolmogorov turbulent atmosphere. We pay attention to the weak turbulence conditions by using Rytov approximation and considering that the receiver is a PIN photodetector. Propagating beam type is Gaussian. It is seen that BER performance of the BPSK SIM OWC is significantly affected from non-Kolmogorov power law exponent, load resistor, responsivity of the PIN photodetector, bandwidth, beam source size, turbulence strength and noise factor.

PPM‐ and GMSK‐based hybrid modulation technique for optical wireless communication cellular backhaul channel

In this study, a hybrid modulation technique called PPM–GMSK, based on the pulse position modulation (PPM) and phase-sampled Gaussian minimum shift keying (GMSK) technique, is investigated by considering log-normal and gamma–gamma turbulence as the channel-fading statistics. The analytical bit error rate (BER) and outage probability of the proposed hybrid modulation are derived. The results show that the hybrid modulation technique is an efficient technique to improve the performance of optical wireless communication for a cellular backhaul channel. Compared to 2-PPM and minimum shift keying (MSK), the proposed hybrid scheme has a less BER and a reduced amount of outage probability. Moreover, to achieve a reliable voice communication at 2 Gbps bit rate and BER of under a weak turbulence effect, the PPM–GMSK has a coverage distance of about 3 km, whereas for PPM and MSK it is 2.6 and 2.4 km, respectively.

On Performance of Optical Wireless Communication With Spatial Multiplexing Towards 5-G

Optical wireless communication (OWC) with orbital angular momentum (OAM)-based spatial multiplexing enables to offer ultra-high information capacity as well as high spectral and energy efficiencies, which is a promising alternative to establish the fronthaul toward the fifth generation (5-G) wireless communication networks. The air turbulence in the optical wireless channel, however, would result in adverse effect in the performance of OWC with spatial multiplexing. We study the impact of air turbulence on the purities of multiplexed OAM states. The split step propagation scheme combining with the Monte-Carlo phase screen method is employed to accurately emulate the OWC link with OAM-based spatial multiplexing. The numerical results reveal that the air turbulence-induced cross-talk among the parallel OAM spatial channels would severely degrades the performance of the fronthaul in 5-G which is established by the OWC with OAM-based spatial multiplexing.

Optical wireless communication using positive real-valued orthogonal frequency-division multiplexing and optical beamforming

The relatively unsatisfactory performance of optical wireless communication (OWC) with respect to WiFi and millimeter-wave communications has formed a key issue preventing its commercialization. We experimentally demonstrate an OWC technology using a combination of positive real-valued orthogonal frequency-division multiplexing (OFDM) and optical beamforming (OB). Due to the intensity-modulation and direct-detection aspects of OWC systems, a positive real-valued OFDM signal can be suitably utilized to maximize the OWC data rate. Further, the OB technique, which can focus laser light on a desired target, can be utilized to increase the OWC data rate and transmission distance. Our experimental results show that the received optical signal power and electrical signal increase by up to 42 and 25 dB, respectively. Further, the data rate increases by a factor of 200 with OB over the conventional approach.

Effects of fog-haze random media on the short-range optical wireless communications link

A model for short-range optical wireless communication (OWC) link under fog-haze random media is presented based on the consideration of beam pointing error. The performance of OWC is analyzed in terms of received power, link margin, signal to noise ratio (SNR) and bit error rate (BER) with different track length, visibility of atmosphere, strength of the atmospheric turbulence, operating wavelength, emitting beam width, and variance of pointing error. It is found that we can ignore the impact of emitting beam width on the link margin, however, there is a optimizing emitting beam width wop for the SNR performance of the channel. The link margin and SNR decrease as the increasing of the track length and the turbulence strength. The link margin and SNR decrease as the meteorological conditions transform from clear, haze to heavy fog. The longer operating wavelength can increase the link margin and SNR. The pointing error caused by beam wander is the dominant factor and it seriously reduces the link margin and SNR, and increases the BER.

Air turbulence effects on performance of optical wireless communication with crosstalk in server backplane

Free space optical interconnections (FSOIs) are anticipated to become a prevalent technology for short-range high-speed communication. FSOIs use lasers in board-to-board and rack-to-rack communication to achieve improved performance in next generation servers and are expected to help meet the growing demand for massive amounts of inter-card data communication. An array of transmitters and receivers arranged to create an optical bus for inter-card and card-to-backplane communication could be the solution. However, both chip heating and cooling fans produce temperature gradients and hot air flow that results in air turbulence inside the server, which induces signal fading and, hence, influences the communication performance. In addition, the proximity between neighboring transmitters and receivers in the array leads to crosstalk in the received signal, which further contributes to signal degradation. In this Letter, the primary objective is to experimentally examine the off-axis crosstalk between links in the presence of turbulence inside a server chassis. The effects of geometrical and inter-chassis turbulence characteristics are investigated and first-and second-order statistics are derived.

Beam wander of Gaussian-Schell model beams propagating through oceanic turbulence

For Gaussian-Schell model beams propagating in the isotropic turbulent ocean, theoretical expression of beam wander is derived based on the extended Huygens–Fresnel principle. The spatial coherence radius of spherical waves propagating in the paraxial channel of turbulent ocean including inner scale is also developed. Our results show that the beam wander decreases with the increasing rate of dissipation of kinetic energy per unit mass of fluid ϵ, but it increases as the increasing of the dissipation rate of temperature variance χt and the relative strength of temperature and salinity fluctuations ϖ. The salinity fluctuation has greater influence on the beam wander than that of temperature fluctuations. The model can be evaluated submarine-to-submarine/ship optical wireless communication performance.

Optical Wireless Transmitter Employing Discrete Power Level Stepping

A major shortcoming of light-emitting diodes (LEDs) is their highly non-linear optical-power-versus-current characteristic. This non-linearity largely restricts the dynamic range and the transmission power of common optical wireless transmitters. This restriction degrades the performance of optical wireless communication (OWC) systems. In this paper, a novel transmitter concept for OWC is proposed which employs discrete power level stepping. The transmitter consists of several on-off-switchable emitter groups. These groups are individually controlled and emit fixed specific optical intensities in parallel. As optical intensities constructively add up, the total emitted intensity is generated by the sum of the emitted intensities of all activated emitter groups. Therefore, the proposed transmitter solution can generate several discrete optical intensity levels which can be used for optical wireless signal transmission. The transmitter design allows the utilisation of the full dynamic range of LEDs or laser diodes by avoiding non-linearity issues. Moreover, costs and complexity of the optical front-end are significantly reduced as neither a (DAC) nor high-speed current controllers are required. This simple design also provides improved power efficiency. Transmission experiments prove the functionality of the implemented optical transmitter. It is shown that the practical performance of the transmitter closely matches the expected performance determined by computer simulations. Moreover, the implemented optical transmitter is compared to an electrical transmission which provides ideal linearity characteristics, and therefore corresponds to an ideal conventional optical transceiver.

Non-Uniform Spectral Amplitude Encoding for Optical Wireless Communication Links through Atmospheric Turbulence

The performance of optical wireless communication (OWC) links based on the spectral amplitude encoding (SAE) of a broad-band spatially partially coherent beam through atmospheric turbulence is analyzed. It has been found that the power efficiency of transmitters of OWC links based on the non-uniform SAE is higher than that based on the uniform SAE. Further, the spectrum division method for the non-uniform SAE of broad-band beams has been proposed. The non-uniform SAE of a broad-band spatially partially coherent beam is an effective method to reduce the performance degradation of OWC links caused by atmospheric turbulence.

A closed-form solution of the bit-error rate for optical wireless communication systems over atmospheric turbulence channels

Atmospheric turbulence is a major limiting factor in an optical wireless communication (OWC) link. The turbulence distorts the phase of the propagating optical fields and limits the focusing capabilities of the telescope antennas. Hence, a detector array is required to capture the widespread signal energy in the focal-plane. This paper addresses the bit-error rate (BER) performance of optical wireless communication (OWC) systems employing a detector array in the presence of turbulence. Here, considering the gamma-gamma turbulence model, we propose a blind estimation scheme that provides the closed-form expression of the BER by exploiting the information of the data output of each pixel, which is based on the singular value decomposition of the sample matrix of the received signals after the code-matched filter. Instead of assuming spatially white additive noise, we consider the case where the noise spatial covariance matrix is unknown. The new method can be applied to either the single transmitter or the multi-transmitter cases. Simulation results for different Rytov variances are presented, which conform closely to the results of the proposed model.

Look-up table error correcting multiple pulse PPM codes for wireless optical communication channels

Performance of optical wireless communication (OWC) systems can be optimised using constant optical power transmission in conjunction with forward error control coding. A new family of constant power error correcting codes using multiple pulse position modulation optical transmission are introduced. This doubly featured technique may extend the capability of many difficult wireless optical links beyond their existing barriers and may enable a higher error performance for OWC networks. The potential and design of look-up table codes are investigated. The Monte Carlo methods provide supportive data for the performance and code word search.

Optical wireless communication through fog in the presence of pointing errors.

Terrestrial optical wireless communication (OWC) is emerging as a promising technology, which makes connectivity possible between high-rise buildings and metropolitan and intercity communication infrastructures. A light beam carries the information, which facilitates extremely high data rates. However, strict alignment between the transmitter and the receiver must be maintained at all times, and a pointing error can result in a total severance of the communication link. In addition, the presence of fog and haze in the propagation channel hampers OWC as the small water droplets scatter the propagating light. This causes attenuation due to the resultant spatial, angular, and temporal spread of the light signal. Furthermore, the ensuing low visibility may impede the operation of the tracking and pointing system so that pointing errors occur. We develop a model of light transmission through fogs of different optical densities and types using Monte Carlo simulations. Based on this model, the performance of OWC in fogs is evaluated at different wavelengths. The handicap of a transceiver pointing error is added to the model, and the paradoxically advantageous aspects of the transmission medium are exposed. The concept of a variable field of view receiver for narrow-beam OWC is studied, and the possibility of thus enhancing communication system performance through fog in an inexpensive and simple way is indicated.