In the paper, there are reviewed pre-correction and pre-equalization schemes used for the correction of VLC LEDs frequency response/nonlinearity. Moreover, a theoretical analysis of pre- and post-equalization schemes efficiency was performed for a generic linear channel. It turned out that the most effective in terms of the receiver SNR is a distributed scheme when the equalization is performed at both the transmitter and receiver. The SNR gain of the distributed scheme may exceed 2 dB as compared to either pre- or post- equalization. The obtained generic results well apply to the VLC LED case.
KEYWORDS: Light emitting diodes, LED lighting, Light sources and illumination, Modulation, Linear filtering, Fourier transforms, Telecommunications, Systems modeling, Data modeling, Wireless communications
In this paper, we attempted to match an analytical model to the observed dynamic nonlinearity of the optical power versus driving current characteristic of lighting LED. Among the models tested were the (generalized) Wiener model, the Hammerstein model, and a variant of the generalized Wiener-Hammerstein model. Unfortunately, it turned out that neither model fits the data obtained from LED power versus current dynamic measurements.
KEYWORDS: Ray tracing, Monte Carlo methods, Transmitters, Reflection, Telecommunications, Channel projecting optics, Wireless communications, Systems modeling, Visibility
The article presents methodology of obtaining the channel impulse responses (CIRs) on a way of numerical calculation for optical wireless transmission systems working indoor, including also those which work in scope of visible spectrum of light (VLC), based on the modified Monte Carlo ray tracing algorithm (MMC), which allows to determine them with really good quality for any room conditions in a short time. The influence of transmission channel is one of the most important factors, which is taken into consideration when design of wireless transmission systems is done. Using of the CIR is commonly applicable technique for this purpose. Based on the channel impulse response we can among others analyze and predict the link performance in the presence of noise and other factors, which can have significant impact on it final performance.
In the paper, there is analyzed the signal quality assessment method based on delay tap asynchronous sampling, both for binary and multilevel PAM signals. The obtained multilevel phase diagrams are far more complicated than binary ones. The phase diagrams are affected by the signal distortions but it is difficult to relate reliably the phase diagram form to the distortion type and its influence on the signal quality.
Communications application of chaotic oscillations of lasers with optoelectronic feedback was discussed. The possibility of eavesdropping of the transmission was analyzed. It was proved that if the rogue party precisely knows parameters of the chaotic system it may recreate the entire signals solely by observation of the optical signal power causing security breach.
Continuous increasing share of light emitting diodes (LEDs) in a lighting market, which we observe during the last couple years, opens new possibilities. Especially, when we talk about practical realization the concept of visible light communications (VLC), which gains on popularity recently. The VLC concept presupposes utilization of illumination systems for a purpose of data transmission. It means, the emitters, in this case the LEDs, will not of a light source only, but also the data transmitters. Currently, most of the conducted researches in this area is concentrated on achievement of effective transmission methods. It means a transmission only in one direction. This is not enough, when we talk about the fully functional transmission system. Ensuring of feedback transmission channel is a necessary also. One of the ideas, which was postulated by authors of this article, is using for this purpose the LEDs in a double role. A utilization of LEDs as photo-detectors requires a reverse polarization, in contrast to a forward bias, which has a place when they work as light emitters. Ensuring of proper polarization get significant meaning. The article presents the investigations results on the influence of reverse bias on photo-receiving properties of LEDs used as light detectors. The conducted research proved that an improvement of sensitivity and bandwidth parameters are possible by application of appropriate value of the reverse voltage in a receiver.
The paper analyzes suitability of various spectrally efficient modulations (PAM, CAP, OFDM/DMT) in a VLC system using lighting LEDs as a transmitter. Although under ideal conditions all modulation have similar efficiency i.e. they produce similar throughputs with a given BER, their practical performances are different. For example, the level of nonlinear distortions generated by each modulation is the least for PAM and by far the greatest for OFDM/DMT locating CAP in the middle. The suitability of various OFDM/DMT variants in a VLC LED link was also analyzed proving that the asymmetrically clipped (ACO) OFDM has a worse performance as compared with DC biased (DCO) OFDM.
The paper presentes a two way open space optical link ready for 10/100 Mbit/s Ethernet signal transmission. It uses lighting LEDs as light sources in both transmission directions. A detailed description of used circuitry is given together with preliminary results.
Efficiency of PAM, CAP/QAM and OFDM/DMT modulation for Step Index Polymer Optical Fiber channel is analyzed
theoretically. It is shown that for the same transmitted optical power and same BER they offer similar data throughputs.
However, when the light source has limited dynamics the crucial factor is the peak to average power ratio of the
modulating signal. This is the greatest for OFDM/DMT and smaller for CAP and PAM. Consequently, the efficiency for
PAM and CAP should be comparable whereas that of DMT – inferior. This conclusion was confirmed by experimental
results.
In this paper there are presented measurements concerning performance analysis of the IEEE 802.11 signal distribution
over multimode fiber based passive optical network. In the paper there are addressed three main sources of impairments:
modal noise, frequency response fluctuation of the multimode fiber and non-linear distortion of the signal in the receiver.
The article was focused on presentation the achievements in a scope of experimental research on transmission of digital
video streams in the frame of specially realized for this purpose ROF (Radio over Fiber) network. Its construction was
based on the merge of wireless IEEE 802.11 network, popularly referred as Wi-Fi, with a passive optical network PON
based on multimode fibers MMF. The proposed approach can constitute interesting proposal in area of solutions in the
scope of the systems monitoring extensive, within which is required covering of a large area with ensuring of a relatively
high degree of immunity on the interferences transmitted signals from video IP cameras to the monitoring center and
a high configuration flexibility (easily change the deployment of cameras) of such network.
Interference sources specific for multimode fiber Passive Optical network are investigated. These include modal noise,
changes and instabilities of the frequency responses, and optical beat interference. All these interferences should be
reduced when the number of modes propagating in fiber is increased. Since the influence of optical beat interference in
multimode fiber is reduced modal noise appears to be the most important adverse effect.
The multimode optical fiber maximum operational range is defined by the fiber bandwidth (related to the intermodal dispersion) rather than by the fiber attenuation. The relation among the modal bandwidth of the fiber, the launching condition and mode coupling is fairly complicated. Presented is a theoretical and experimental study on the modal bandwidth of the multimode fiber. Theory is based on the numerical solution of coupled mode diffusion equation that allows for the bandwidth calculation of the MM optical fiber. The results are backed by measurements. In the Telecommunications Institute, Warsaw, such measurements may be performed up to ~600 MHz with a set up based on the frequency response measurement. The MM fibers are excited via a single mode patchcord with mechanically adjusted offset from the center. The patchcords with various MFDs can be used.
In the paper, there is investigated the interaction between the dispersion in optical fiber and the chirp of gaussian impulses. Formulas are presented for the impulse from at the fiber far end. The presented results indicate that this interaction may lead to significant distortion of optical impulses at the fiber output even when the second order dispersion is entirely cancelled and the input impulse has no linear chirp.
In this paper, we present an infrared transmission system meant for indoor applications. It is based on CDMA technology and therefore it is highly resistant to environmental interference such as fluorescent light. A very simple synchronization scheme is employed based on a slight detuning of quartz oscillators in the terminals and central station. The following issues are discussed: principle of operation, link configuration, code selection, synchronization, and range of optical. Measurement results are also presented.
Described is an optoelectronic system that makes it possible to measure the position and velocity of a ship model. It operates over a 200 m by 200 m area with a fundamental error less than 0.1 m and consists of an optoelectronic transmitter aboard the model and two stations ashore which are connected to a computer. The ground stations measure angles between a base and the mobile transmitter. Based on the angle data the position and velocity are calculated by the computer. The simultaneous measurements of two models are also possible.
An optoelectronic system intended for observing and recording trajectories of ship models is described. It allows to determine and record in real time the actual position of the model over an area exceeding 150 m X 150 m with an intrinsic error less than 0.1 m.
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