DC-bias for Optical OFDM in Visible Light Communications

The requirement of a DC-bias is known to make DC-biased Optical Orthogonal Frequency Division Multiplexing (DCO-OFDM) less energy efficient. This can be improved by Asymmetrically Clipped Optical OFDM (ACO-OFDM), Pulse Amplitude Modulated OFDM (PAM-OFDM) or Flip-OFDM, but these variants use the bandwidth inefficiently. Our trade-off between energy and spectrum efficiency considers a given limited channel bandwidth of the Light Emitting Diode (LED) and then attempts to get the highest throughput per unit of energy. We investigate previous findings that clipped OFDM can be more attractive in a low-SNR regime. More specifically, we consider Visible Light Communication (VLC) in which the average light level, i.e., the bias, is prescribed by illumination requirements, thus comes for free. ACO/PAM/Flip-OFDM can convert the DC-bias into power for communication, but all variants of OFDM, including DCO-OFDM consume extra electrical power. We conclude that in this scenario, advantages attributed to ACO/PAM/Flip-OFDM vanish, as DCO-OFDM outperforms its variants in all SNR conditions, in terms of achieved throughput over a bandlimited channel as a function of extra electrical power required. For hybrid solutions, such as Asymmetrically clipped DC biased Optical OFDM (ADO-OFDM) and Hybrid ACO-OFDM (HACO-OFDM), we optimize a new adaptive power and rate splitting between odd (clipped) and even (biased/clipped) subcarriers to balance power and bandwidth efficiency

A Space-frequency Power Allocation Algorithm for MIMO OWC Systems over Low-Pass Channels

In the last two decades, an unprecedented spread of communication systems has been witnessed. While at the beginning these systems were only able to support a small number of devices with limited data services, they have now matured to high speed networks that are densely populated. Society is increasingly connected, with different types of applications running on, by now Billions of devices, and this trend drives the use of communication systems. The growth is so fast that the Radio Frequency (RF) spectrum is already overcrowded. In future, it is expected that many applications will require speeds far beyond a Gbit/s. In order to achieve this capacity and, at the same time, to off load the pressure on RF systems, higher spectral bands and optical frequencies are currently being explored.Exploring higher frequencies in the electromagnetic spectrum, optical wireless communication (OWC) systems have recently gained great interest [1,2]. Due its many advantages, such as low cost, high energy efficiency, and minimal heat generation, LEDs are commonly used for illumination and are strong candidates to drive data transmission in OWC systems [2-4]. However, the modulation bandwidth of this source is limited and there is still the need to increase data throughput [4,5]. An alternative is to deploy multiple LEDs in a Multiple Input Multiple Output (MIMO) scheme [2-6]. MIMO is a well-known technology which explores the additional spatial dimension in order to provide a degree-of-freedom gain. By transmitting multiple data-streams over the light channel in a Spatial Multiplexing (SM) scheme from multiple spatially separated locations, Distributed-MIMO technology offers higher data throughput without the need of additional power or bandwidth. An important additional advantage of MIMO in OWC systems is that communication still works even when one line-of-sight link is blocked. In further boosting the bits rate, the low-pass frequency response of the LEDs poses further limitations. The low-pass behaviour of this source was pointed out in [6-9], but its impact on the performance of LED-based MIMO OWC systems still not fully addressed. To compensate the low-pass effect, Orthogonal Frequency Division Multiplexing (OFDM) is often used. OFDM is a robust and effective technology commonly used in RF systems to suppress inter-symbol interference (ISI) and to convert a frequency-selective fading channel into multiple parallel flat-fading, i.e., non-dispersive channels. In an OFDM scheme the spectrum bandwidth is divided into a set of orthogonal subcarriers in order to support high data rates through parallel transmission. By using OFDM, power loading strategies can be used to appropriately distribute power over the subcarriers in order to reduce the performance degradation caused by the low-pass effect of the LEDs [8]. Different power loading strategies are proposed to allocate power resources in the frequency domain, mainly the uniform loading and the optimized waterfilling loading [6-8]. In this paper, we consider the transmission mode of an indoor LED-based MIMO OWC system with SM and OFDM. We present an analytical model for the channel and we derive expressions for the achievable rate of the system considering common low-pass channel frequency responses: Gaussian, exponential and first-order [6-9]. Based on an indoor LED-based MIMO OWC setup, we investigate through analytical and simulation results the system performance for different power loading strategies. Through simulation results, we point out that the resource allocation optimization only in the frequency domain may not be satisfactory and we propose a new algorithm that considers both spatial and frequency domains to load power over the MIMO channels and OFDM subcarriers. With the singular value decomposition (SVD) applied to the channel frequency response matrix, the proposed space-frequency power allocation algorithm allocates more power to subchannels with larger gains considering all subchannels available for transmission in space and frequency domains.<br/

Productization Experiences of G.vlc (ITU) based LiFi System for high Speed Indoor Wireless Access

We describe the first productization of bidirectional LiFi architecture based on latest ITU-T standard G.9991 (G.vlc).We report experiences in-offering a gross data-rate of 250 Mbps downlink and 200 Mbps uplink in point-to-multipoint system configuration

Enhancement of low-mass dileptons in heavy-ion collisions

Using a relativistic transport model for the expansion stage of S+Au collisions at 200 GeV/nucleon, we show that the recently observed enhancement of low-mass dileptons by the CERES collaboration can be explained by the decrease of vector meson masses in hot and dense hadronic matter.Comment: 12 pages, RevTeX, 3 figures available from [email protected]

Dilepton Production at SPS-energy Heavy Ion Collisions

The production of dileptons is studied within a hadronic transport model. We investigate the sensitivity of the dilepton spectra to the initial configuration of the hadronic phase in a ultrarelativistic heavy ion collision. Possible in medium correction due to the modifications of pions and the pion form factor in a hadronic gas are discussed.Comment: Dedicated to Gerry Brown in honor of the 32nd celebration of his 39th birthday. 31 pages Latex including 13 eps-figures, uses psfig.sty and epsf.st

J/ψ+c+cˉJ/\psi + c + \bar{c} Photoproduction in e+e−e^+ e^- Scattering

We investigate the $J/\psi$ + c + $\bar{c}$ photoproduction in $e^+ e^-$ collision at the LEP II energy. The physical motivations for this study are: 1) such next-to-leading order(NLO) process was not considered in previous investigations of $J/\psi$ photoproduction in $e^+ e^-$ interaction, and it is worthwhile to do so in order to make sound predictions for experimental comparison; 2) from recent Belle experiment results, the process with same final states at the $B$ factory has a theoretically yet unexplainable large fraction; hence it is interesting to see what may happen at other colliders; 3) the existing LEP data are marginal in observing such process, and at the planed Linear Colliders(LCs) this process can be measured with high accuracy; 4) it is necessary to take this process into consideration in the aim of elucidating the quarkonium production mechanism, especially in testing the universality of NRQCD nonperturbative matrix elements via $J/\psi$ photoproduction in electron-position collisions.Comment: 15 pages, 3 figure

Electric charge quantization and the muon anomalous magnetic moment

We investigate some proposals to solve the electric charge quantization puzzle, which simultaneously explain the recent measured deviation on the muon anomalous magnetic moment. For this we assess extensions of the Electro-Weak Standard Model spanning modifications on the scalar sector only. It is interesting to verify that one can have modest extensions which easily account for the solution for both problems.Comment: 20 pages, 1 figures, needs macro axodraw.st

Cross Section Measurements of Charged Pion Photoproduction in Hydrogen and Deuterium from 1.1 to 5.5 GeV

The differential cross section for the gamma +n --> pi- + p and the gamma + p --> pi+ n processes were measured at Jefferson Lab. The photon energies ranged from 1.1 to 5.5 GeV, corresponding to center-of-mass energies from 1.7 to 3.4 GeV. The pion center-of-mass angles varied from 50 degree to 110 degree. The pi- and pi+ photoproduction data both exhibit a global scaling behavior at high energies and high transverse momenta, consistent with the constituent counting rule prediction and the existing pi+ data. The data suggest possible substructure of the scaling behavior, which might be oscillations around the scaling value. The data show an enhancement in the scaled cross section at center-of-mass energy near 2.2 GeV. The differential cross section ratios at high energies and high transverse momenta can be described by calculations based on one-hard-gluon-exchange diagrams.Comment: 18 pages, 19 figure

Formation of superdense hadronic matter in high energy heavy-ion collisions

We present the detail of a newly developed relativistic transport model (ART 1.0) for high energy heavy-ion collisions. Using this model, we first study the general collision dynamics between heavy ions at the AGS energies. We then show that in central collisions there exists a large volume of sufficiently long-lived superdense hadronic matter whose local baryon and energy densities exceed the critical densities for the hadronic matter to quark-gluon plasma transition. The size and lifetime of this matter are found to depend strongly on the equation of state. We also investigate the degree and time scale of thermalization as well as the radial flow during the expansion of the superdense hadronic matter. The flow velocity profile and the temperature of the hadronic matter at freeze-out are extracted. The transverse momentum and rapidity distributions of protons, pions and kaons calculated with and without the mean field are compared with each other and also with the preliminary data from the E866/E802 collaboration to search for experimental observables that are sensitive to the equation of state. It is found that these inclusive, single particle observables depend weakly on the equation of state. The difference between results obtained with and without the nuclear mean field is only about 20\%. The baryon transverse collective flow in the reaction plane is also analyzed. It is shown that both the flow parameter and the strength of the ``bounce-off'' effect are very sensitive to the equation of state. In particular, a soft equation of state with a compressibility of 200 MeV results in an increase of the flow parameter by a factor of 2.5 compared to the cascade case without the mean field. This large effect makes it possible to distinguish the predictions from different theoretical models and to detect the signaturesComment: 55 pages, latex, + 39 figures available upon reques

Hydrodynamical Description of 200 A GeV/c S+Au Collisions: Hadron and Electromagnetic Spectra

We study relativistic S+Au collisions at 200 A GeV/c using a hydrodynamical approach. We test various equations of state (EOSs), which are used to describe the strongly interacting matter at densities attainable in the CERN-SPS heavy ion experiments. For each EOS, suitable initial conditions can be determined to reproduce the experimental hadron spectra; this emphasizes the ambiguity between the initial conditions and the EOS in such an approach. Simultaneously, we calculate the resulting thermal photon and dielectron spectra, and compare with experiments. If one allows the excitation of resonance states with increasing temperature, the electro-magnetic signals from scenarios with and without phase transition are very similar and are not resolvable within the current experimental resolution. With regard to the CERES dilepton data, none of the EOSs considered, in conjunction with the standard leading order dilepton rates, succeed in reproducing the observed excess of dileptons below the rho peak. Our work, however, suggests that an improved measurement of the photon and dilepton spectra has the potential to strongly constrain the EOS.Comment: Uses REVTeX, 48 pages, 13 Postscript figure