Ioannis Neokosmidis

Impact of LED Nonlinearity on Discrete Multitone Modulation

In the context of communications based on light-emitting diodes (LEDs), spectrally efficient modulation has been considered for overcoming their limited bandwidth, and one scheme under investigation is quadrature-amplitude modulation on discrete multitones. The dependence of the output optical power on the driving current of practical LEDs is nonlinear, which distorts the transmitted signal. We investigate the impact of the nonlinear LED transfer function, i.e., the dependence of the emitted optical power on the driving current, on discrete multitone modulation. The effect incurred by this distortion was analyzed by using detailed numerical simulations addressing the impact of clipping, individual subcarriers, signal-to-noise ratio, and bit-error ratio. The approach was generalized to describe the impact of the nonlinearity of arbitrary LEDs and laser diodes, resulting in a powerful tool for assessing the impact of the nonlinearity on the link performance. This approach was applied to three types of LED, showing anything from a minuscule effect to the case in which error-free data transmission is made impossible by the transfer-function nonlinearity.

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.

Optical wireless communications for broadband access in home area networks

As a part of the EU-FP7 R&D programme, the OMEGA project (hOME Gigabit Access) aims at bridging the gap between mobile broadband terminals and the wired backbone network in homes. To provide Gb/s connectivity a combination of various technologies is considered. Beside radio frequencies, the wireless links will use infrared and visible light. Combined with power-line communications this enables a home area network (HAN) that meets the vision of broadband home networking dasiawithout new wirespsila. A technology-independent MAC layer is foreseen to control such network and to provide services as well as connectivity to any device the user wishes to connect. Moreover, this MAC layer should allow the service to follow the user from device to device in any room of a building /apartment. The contribution presents ideas and approaches for broadband optical wireless (OW) communications using infrared Gb/s hotspots and 100 Mb/s information broadcasting by means of interior lighting based on white-light LEDs. Important issues concerning the physical layer are discussed.

New techniques for the suppression of the four-wave mixing-induced distortion in nonzero dispersion fiber WDM systems

The performance of a wavelength-division multiplexing (WDM) optical network can be severely degraded due to fiber nonlinear effects. In the case where nonzero dispersion (NZD) fibers are employed, the four-wave mixing (FWM) effect sets an upper limit on the input power, especially in the case of narrow channel spacing. In order to reduce FWM-induced distortion two new techniques, the hybrid amplitude-/frequency-shift keying (ASK/FSK) modulation and the use of prechirped pulses are investigated. It is shown that both techniques can greatly improve the Q-factor in a 10 Gb/s WDM system. This happens even for very high input powers (/spl sim/10 dBm), where the degradation of the conventional WDM system is prohibitively high. The proposed methods are also applied and tested in higher bit rates (40 Gb/s). It is deduced that although the hybrid ASK/FSK modulation technique marginally improves the system performance, the optical prechirp technique can still be used to greatly increase the maximum allowable input power of the system.

Hybrid Free Space Optical / Millimeter Wave Outdoor Links for Broadband Wireless Access Networks

As the backbone and the metropolitan area network technologies can increasingly provide unprecedented bandwidth capacities, the focus is being gradually shifted toward broadband access technologies capable of connecting the customer premises to the local exchange. This paper reviews the prospects of state-of-the art Free Space Optical (FSO) technology, as a last mile solution, combined with Millimeter Wave (MMW) systems aiming at increasing both the link range and service availability while providing reliable broadband connections at distances above 1 Km. An overview of the hybrid FSO/MMW system that has been deployed in Athens, Greece, is provided and a brief introduction to the trials that are to be conducted is given.

Estimation of the four-wave mixing noise probability-density function by the multicanonical Monte Carlo method

The performance of high-powered wavelength-division multiplexed (WDM) optical networks can be severely degraded by four-wave-mixing- (FWM-) induced distortion. The multicanonical Monte Carlo method (MCMC) is used to calculate the probability-density function (PDF) of the decision variable of a receiver, limited by FWM noise. Compared with the conventional Monte Carlo method previously used to estimate this PDF, the MCMC method is much faster and can accurately estimate smaller error probabilities. The method takes into account the correlation between the components of the FWM noise, unlike the Gaussian model, which is shown not to provide accurate results.

Optical Delay Lines Based on Soliton Propagation in Photonic Crystal Coupled Resonator Optical Waveguides

In this paper, a study of optical delay lines based on soliton propagation in coupled resonator optical waveguides is performed. For a given bit rate and required delay, design equations are given that relate the soliton peak power and collision period to the soliton width. To study the influence of higher order linear and nonlinear dispersion, a continuous wave propagation model incorporating these effects is also derived. Using this model, the soliton stability in the presence of higher order dispersion, optical loss and adjacent soliton pulses is numerically verified. It is also shown that soliton-based delay lines can achieve nanosecond delay at a propagation length of a few millimeters due to the high slow down factors that can be obtained.

A System of Systems Framework for the Reliability Assessment of Telecommunications Networks

In this paper, a system of systems (SoS) framework for the reliability analysis of telecommunication networks is proposed. In this framework, two hazard analysis techniques, hazard and operability analysis and fault tree analysis, are combined in a hybrid scheme. This is further enhanced using the Bayesian network model along with sensitivity analysis in order to answer complex probability queries and to estimate the impact of residual mishap risks, unknown events, or events that cannot easily be modeled. The SoS emergent behavior is further revealed using exploratory modeling. The proposed SoS framework is applied in the case of a fiber-to-the-curb VDSL telecommunication network.

Wireless optical network for a home network

During the European collaborative project OMEGA, two optical-wireless prototypes have been developed. The first prototype operates in the near-infrared spectral region and features Giga Ethernet connectivity, a simple transceiver architecture due to the use of on-off keying, a multi-sector transceiver, and an ultra-fast switch for sector-to-sector hand over. This full-duplex system, composed by one base station and one module, transmits data on three meters. The second prototype is a visible-light-communications system based on DMT signal processing and an adapted MAC sublayer. Data rates around to 100 Mb/s at the physical layer are achieved. This broadcast system, composed also by one base station and one module, transmits data up to two meters. In this paper we present the adapted optical wireless media-access-control sublayer protocol for visible-light communications. This protocol accommodates link adaptation from 128 Mb/s to 1024 Mb/s with multi-sector coverage, and half-duplex or full-duplex transmission.

Risks associated with next generation access networks investment scenarios

The deployment of next generation access networks (NGAs) is investigated from technical, regulatory and investment perspectives. A brief review of the possible network architectures and deployment scenarios of NGAs is provided. Risk calculations of these NGA scenarios are performed based on a fully detailed techno-economic model. Furthermore, the effectiveness of the European Commissions Recommendation on regulated access to NGAs, aiming to tackle the regulatory trade-off between encouraging investments and promoting competition, is discussed.