Dima Bykhovsky

Multiple Access Resource Allocation in Visible Light Communication Systems

Discrete multi-tone (DMT) modulation is known to be an efficient single-transmitter technique for visible-light communication. However, the use of this technique in a multiple transmitter environment requires effective subcarrier and power allocation design in order to exploit the full potential of spatial multiple-transmitter diversity. Spatial reuse of the subcarriers in the presence of interference and power constraints increases the efficiency of multiple access (MA) DMT communication. In this paper, we propose an algorithm that manages interference-constrained subcarrier reuse between different transmitters and power redistribution between different subcarriers in a heuristic manner. The algorithm simulation shows an improvement in the average bit-rate as compared with a conventional DMT method. Furthermore, the effectiveness of the proposed MA-DMT scheme increases with the number of users.

An Experimental Comparison of Different Bit-and-Power-Allocation Algorithms for DCO-OFDM

This research comprises an end-to-end experimental comparison of the bit-rates and bit error-rates of four major adaptive bit-and-power allocation algorithms for dc biased optical orthogonal frequency-division multiplexing. The comparison includes studying different channel conditions and different numbers of subcarriers. The experimental results show that all the methods compared display similar performance without significant superiority of a single method, even though in theory some difference is expected. Our analysis of the experimental data showed that the main reason for the uniformity in performance is the high variance in the channel estimation, especially at higher frequency subcarriers.

Electrical Network Frequency (ENF) Maximum-Likelihood Estimation Via a Multitone Harmonic Model

Estimation of the parameters of the electric network signal, usually present in many audio and video recordings, is known to have several important forensic applications. In this paper, we consider the problem of estimating the base frequency and signal to noise ratio (SNR). Although the electric network signal is present via its base frequency and its integer multiplies, recent estimators in the literature focus on single-tone models. In this work, we offer a multitone harmonic model for the electric network signal. We use the Cramer-Rao bound for the frequency estimation problem and show that this approach can lead to a theoretical O(M3) factor improvement in the estimation accuracy, where M is the number of harmonics. We then derive the computationally efficient form of the maximum-likelihood estimator, applicable in the limit of large number of measurements. The problem of estimating the SNR of the signal is also discussed. Through extensive tests on real data and data sets reported in the current literature, the performance of the new estimators is evaluated. Results indeed show a significant gain compared to the single-tone model, and are better than previously reported estimators in the literature for moderate and high SNR values.

OFDM Allocation Optimization for Crosstalk Mitigation in Multiple Free-Space Optical Interconnection Links

The growing demand for high interconnection speed in next-generation computers is driving the technology shift for communication from the electronic to the optic domain. One of the favored interconnection technologies for this task is the freespace optical interconnect (FSOI). FSOI technology uses laser links between computer components and provides a lower bound on propagation delay due to the low index of refraction of air, when compared with the indexes common in waveguide technologies. FSOIs based on dc-biased optical orthogonal frequency-division multiplexing (DCO-OFDM) may provide excellent data throughput in intensity modulation/direct detection systems. However, the main drawback limiting the implementation of FSOIs is the inevitable tradeoff between interconnection density and the crosstalk level, resulting from the diffraction effect and from optical misalignment. The purpose of this paper is to promote improved interconnection density of such FSOIs by use of inherent DCO-OFDM resource allocation capabilities. The crosstalk-resulted interference was formulated as joint multilink bit-and-power allocation optimization. The theoretical analysis reveals general guidelines for dense FSOI. Further, a reduced-complexity numerical suboptimal algorithm for joint multilink bit-and-power allocation was proposed. The simulation results show that the proposed suboptimal algorithm outcome is close to the theoretical optimal performance.

Simple Generation of Gamma, Gamma–Gamma, and K Distributions With Exponential Autocorrelation Function

The simulation of a free-space optical (FSO) communication channel in the presence of strong turbulence typically requires the generation of channel states with a K or gamma-gamma distribution and a predefined autocorrelation function. In this paper, we propose a simple and effective simulator of the strong-turbulence FSO channel that addresses the influence of the temporal covariance effect. Specifically, the proposed simulator provides K and gamma-gamma distributed values with the exponential autocorrelation function and a prescribed correlation time. This simulator is based on the numerical solution of the first-order stochastic differential equation. The simulated channel states are generated by a simple discrete-time differential equation and the simulator performance is analyzed in the paper.

Free-space optical channel simulator for weak-turbulence conditions.

Free-space optical (FSO) communication may be severely influenced by the inevitable turbulence effect that results in channel gain fluctuations and fading. The objective of this paper is to provide a simple and effective simulator of the weak-turbulence FSO channel that emulates the influence of the temporal covariance effect. Specifically, the proposed model is based on lognormal distributed samples with a corresponding correlation time. The simulator is based on the solution of the first-order stochastic differential equation (SDE). The results of the provided SDE analysis reveal its efficacy for turbulent channel modeling.

Unguided Optical Communication 2

This letter presents a new unguided free-space optical bus design concept. The major difference between the design presented in this letter and existing line-of-sight free-space solutions is the paradoxical employment of interfering effects, such as scattering, multipath diffusion, and crosstalk, for the purpose of communication performance improvement. The multiple-input multiple-output (MIMO) communication paradigm is used to explore the optical multipath that arises from these effects. The use of MIMO also diminishes the problems concerned with the alignment of communication devices over optical axes. A 2 × 2 optical bus model based on a Lambertian reflection scheme is analyzed. The maximum likelihood (ML) receiver is used to evaluate the bus performance. The outcome from this work is a mathematical model describing the performance of the MIMO bus as a function of the distance between the transmitter and receiver arrays.

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To further reduce the size of computers and increase their speed, optical technology has to replace electrical technology for motherboard-level interconnects. This letter presents a new design for motherboard-level interconnects based on the unguided optical interconnection board concept. The design includes chips with optical transceivers that communicate using diffused light through the same shared optical medium. In this letter, we derive a model of bit-error probability (BER) and find the order of the Lambertian reflection that maximizes the BER.

2 Mimo Bus Implementation

Free-space optical interconnection (FSOI) technology enables high interconnection speed in next-generation servers. FSOIs use laser links between server components and provide a lower bound on propagation delay due to the shortest line-of-sight path and the low index of refraction of air, when compared with the indices common in waveguide technologies. One of the main problems of FSOI systems is the inevitable turbulence effect that results from the air cooling of server components. Cooling fans produce a flow of hot air, due to excessive thermal emission from electronic components inside the chassis; this produces turbulence-induced fading and may influence the communication performance. The objective of this paper is to experimentally examine the influence of the turbulence effect on FSOI channels inside a server chassis. This includes studying scintillation statistics and characterizing temporal and spatial in-chassis turbulence. The results of the analysis reveal that the in-chassis optical channel preserves the well-known log-normal scintillation distribution. However, some temporal and spatial characteristics do not fit the commonly accepted turbulence theory predictions.

Design and Simulation of Optical Unguided Bus Interconnect

The orientation of remote objects can be easily measured optically and very accurately using optical technologies. In this paper, we address the accuracy of a roll angle estimation technique that is based on the estimation of the polarization of a retroreflected beam and is applied in a long-range measurement task. Over long transmission ranges, the optical power decreases significantly leading to severe SNR deterioration. As a result, the measurement accuracy decreases. In this study, the estimation is carried out using a maximum-likelihood estimator and its performance is evaluated using the analytical Cramer-Rao bound.