Thomas Kamalakis

Combining Illumination Dimming Based on Pulse-Width Modulation With Visible-Light Communications Based on Discrete Multitone

In the field of indoor wireless networks, visible-light communications is garnering increasing attention. One of the type of emitters used in this technology is white light-emitting diodes, which can synergistically provide both illumination and data transmission. Discrete multitone modulation is attractive for visible-light communications. One of the issues to be addressed in these synergetic use cases is how to incorporate light dimming while not corrupting the communication link. In this paper, the performance of a visible-light communication system combining pulse-width modulation for dimming and discrete multitone for data transmission was investigated. Performance indicators were addressed, i.e., the signal-to-interference ratio due to dimming and the achievable bit-error ratio in the absence of additional noise. By aid of simulations it was shown that practical communication is only feasible when the line rate of the dimming modulation is at least twice the frequency assigned to the largest multitone subcarrier frequency. The results demonstrate that under this constraint and when using a suitably modified demodulation scheme, dimming does not influence the data transmission.

Impact of Nonlinear LED Transfer Function on Discrete Multitone Modulation: Analytical Approach

Light-emitting diodes constitute a low-cost choice for optical transmitters in medium-bit-rate optical links. An example for the latter is local-area networks. However, one of the disadvantageous properties of light-emitting diodes is their nonlinear characteristic, which may limit the data transmission performance of the system, especially in the case of multiple subcarrier modulation, which is starting to attract attention in various applications, such as visible-light communications and data transmission over polymer optical fibers. In this paper, the influence of the nonlinear transfer function of the light-emitting diodes on discrete multitone modulation is studied. The transfer function describes the dependence of the emitted optical power on the driving current. Analytical expressions for an idealized link were derived, and these equations allow the estimation of the power of the noise-like, nonlinear crosstalk between the orthogonal subcarriers. The crosstalk components of the quadrature and in-phase subcarrier components were found to be independent and approximately normally distributed. Using these results, the influence of light-emitting-diode nonlinearity on the performance of the system was investigated. The main finding was that systems using a small number of subcarriers and/or high QAM level exhibit a large signal-to-noise-ratio penalty due to the nonlinear crosstalk. The model was applied to systems with white and resonant-cavity light-emitting diodes. It is shown that the nonlinearity may severely limit the performance of the system, particularly in the case of resonant-cavity light-emitting diodes, which exhibit a strong nonlinear behavior.

Slow light in insulator–metal–insulator plasmonic waveguides

We study numerically the slow-light capability of insulator–metal–insulator (IMI) plasmonic waveguides. Metal-induced losses are included in the calculation of the dispersion relations, and their effect on the slow-light properties of the waveguide is investigated. In addition to reducing the propagation lengths of surface plasmon polaritons, losses are found to limit the achievable slowdown factors and the practical potential of the device. To alleviate the problem, we consider active materials. Using realistic parameters, we find that a spectral region is then formed where a slow-light pulsed signal can achieve infinite propagation lengths or be amplified. The optical buffering capabilities of the IMI waveguide with losses are analyzed, and we conclude that while losses limit the buffering capabilities of the passive device, the use of active materials may combat the problem effectively from an application point of view.

Empirical Volterra-Series Modeling of Commercial Light-Emitting Diodes

Light-emitting diodes (LEDs) constitute a low-cost alternative for optical data transmission of up to ~ 1 Gb/s. What differentiates such applications from, e.g., backhaul optical networks, is the fact that apart from their data throughput, LEDs are generally not as well characterized by the manufacturer as, for example, optical fiber amplifiers. While for simple modulation formats, this lack of knowledge is not a severe impediment; in any other situation, one may face rather complex behaviors of commercial LEDs. In this paper, the main electro-optical characteristics of LEDs are discussed, and it is shown that some popular simple nonlinear models available in the literature are inadequate in describing their dynamics. As a way out of this malady, we present a reverse-engineering approach that is based on Volterra expansions of the electro-optical characteristic function of LEDs, enabling the introduction of a realistic empirical model for commercial devices.

Convergence properties and practical estimation of the probability of rank reversal in pairwise comparisons for multi-criteria decision making problems

Abstract In this paper, we address the impact of uncertainty introduced when the experts complete pairwise comparison matrices, in the context of multi-criteria decision making. We first discuss how uncertainty can be quantified and modeled and then show how the probability of rank reversal scales with the number of experts. We consider the impact of various aspects which may affect the estimation of probability of rank reversal in the context of pairwise comparisons, such as the uncertainty level, alternative preference scales and different weight estimation methods. We also consider the case where the comparisons are carried out in a fuzzy manner. It is shown that in most circumstances, augmenting the size of the expert group beyond 15 produces a small change in the probability of rank reversal. We next address the issue of how this probability can be estimated in practice, from information gathered simply from the comparison matrices of a single expert group. We propose and validate a scheme which yields an estimate for the probability of rank reversal and test the applicability of this scheme under various conditions. The framework discussed in the paper can allow decision makers to correctly choose the number of experts participating in a pairwise comparison and obtain an estimate of the credibility of the outcome.

Designing slow-light photonic crystal waveguides for four-wave mixing applications

We discuss the optimization of photonic crystal waveguides for four-wave mixing (FWM) applications, taking into account linear loss and free-carrier effects. Suitable figures of merit are introduced in order to guide us through the choice of practical, high-efficiency designs requiring relatively low pump power and small waveguide length. In order to realistically perform the waveguide optimization process, we propose and validate an approximate expression for the FWM efficiency, which significantly alleviates our numerical calculations. Promising waveguide designs are identified by means of an exhaustive search, altering some structural parameters. Our approach aims to optimize the waveguides for nonlinear signal-processing applications based on the FWM.

Theoretical estimation of the probability of weight rank reversal in pairwise comparisons

Pairwise comparison is a key component in multi-criteria decision making. The probability of rank reversal is a useful measure for evaluating the impact of uncertainty on the final outcome. In the context of this paper the type of uncertainty considered is related to the fact that experts have different opinions or that they may perform inconsistent pairwise comparisons. We provide a theoretical model for estimating the probability of the consequent rank reversal using the multivariate normal cumulative distribution function. The model is applied to two alternative weight extraction methods frequently used in the literature: the geometric mean and the eigenvalue method. We introduce a reasonable framework for incorporating uncertainty in the decision making process and calculate the mean value and cross-correlation of the average weights which are required in the application of the model. The theoretical results are compared against numerical simulations and a very good agreement is observed. We further show how our model can be extended in applications of a full multi-criteria decision making analysis, such as the analytic hierarchy process. We also discuss how the theoretical model can be used in practice where the statistical properties of the uncertainty-induced perturbations are unknown and the only information provided by the pairwise comparison matrices of a small group of experts. The methodology presented here can be used to extend the pairwise comparison framework in order to provide some information on the credibility of its outcome.

Optimization of the storage capacity of slow light photonic crystal waveguides.

The storage capacity of slow light photonic crystal waveguides is maximized using a systematic procedure based on the optimization of various parameters of the structure. Both optical loss and dispersion-induced broadening are incorporated into the model. The results indicate that this procedure can provide up to a threefold increase in storage capacity.

Enabling transistor-like action in photonic crystal waveguides using optical event horizons

We show that dispersion and loss-engineered photonic crystal waveguides can support optical event horizons enabling the manipulation of a strong soliton pulse by a weaker control pulse within a 3 mm waveguide. Depending on the launching frequency of the weak control pulse, both blue and red soliton shifts are observed, while the soliton appears to be delayed by several pulse widths.

Light-emitting diodes: The unknown entities

Light-Emitting Diodes (LEDs) constitute a low-cost choice for optical transmitters in medium-bit-rate. What differentiates such applications from, e.g., backhaul optical networks, is that for low-cost applications, the LED used is generally not as well characterised by the manufacturer as, are for example, optical fibre amplifiers. While for simple modulation formats like on-off keying (OOK) this lack of knowledge is not always an impediment, in any other situation one faces the often puzzling behaviour of commercial LEDs. In this tutorial the main electro-optical characteristics of LEDs are introduced and the models available in the literature are shown to be inadequate when designing optical links based on advanced modulation techniques or even OOK at very high data rates. As a way out of this grid lock a reverse-engineering approach is presented, enabling the introduction of a realistic model for commercially available LEDs.