Zabih Ghassemlooy

BPSK Subcarrier Intensity Modulated Free-Space Optical Communications in Atmospheric Turbulence

Free-space optical communications (FSO) propagated over a clear atmosphere suffers from irradiance fluctuation caused by small but random atmospheric temperature fluctuations. This results in decreased signal-to-noise ratio (SNR) and consequently impaired performance. In this paper, the error performance of the FSO using a subcarrier intensity modulation (SIM) based on a binary phase shift keying (BPSK) scheme in a clear but turbulent atmosphere is presented. To evaluate the system error performance in turbulence regimes from weak to strong, the probability density function (pdf) of the received irradiance after traversing the atmosphere is modelled using the gamma-gamma distribution while the negative exponential distribution is used to model turbulence in the saturation region and beyond. The effect of turbulence induced irradiance fluctuation is mitigated using spatial diversity at the receiver. With reference to the single photodetector case, up to 12 dB gain in the electrical SNR is predicted with two direct detection PIN photodetectors in strong atmospheric turbulence.

Digital pulse interval modulation for optical communications

This article presents a study of digital pulse interval modulation (DPIM) for optical wireless communications using intensity modulation with direct detection. The DPIM code properties are discussed, and expressions for the transmission capacity, power spectrum, and error probability are presented. We show that for a given number of bits per symbol, DPIM has a higher transmission capacity, a similar spectral profile, and only a marginally inferior error probability performance compared with pulse position modulation, and is rather less complex to implement. Finally, problems associated with the nonuniform symbol length characteristics of DPIM together with possible solutions are discussed.

Experimental Demonstration of 50-Mb/s Visible Light Communications Using 4

This letter reports the experimental demonstration of an indoor visible light nonimaging multiple-input multiple-output system with an aggregate error free bit rate of 50 Mb/s over a distance of 2 m. The system uses four independent white LED transmitters, each transmitting 12.5 Mb/s of data in the ON-OFF keying nonreturn zero format, and four independent nonimaging optical receivers. The performance of four detection methods ranging from the basic channel inversion to the more advanced space time techniques is compared experimentally. The results gathered demonstrate that the simplest technique is capable of the same bit error rate as the most complex scheme. The system also provides full illumination with a mean level of 350 Lux satisfying the ISO lighting standards for home and office environments.

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In this paper, we experimentally demonstrate for the first time an on off keying modulated visible light communications system achieving 170 Mb/s using an artificial neural network (ANN) based equalizer. Adaptive decision feedback (DF) and linear equalizers are also implemented and the system performances are measured using both real time (TI TMS320C6713 digital signal processing board) and offline (MATLAB) implementation of the equalizers. The performance of each equalizer is analyzed in this paper using a low bandwidth (4.5 MHz) light emitting diode (LED) as the transmitter and a large bandwidth (150 MHz) PIN photodetector as the receiver. The achievable data rates using the white spectrum are 170, 90, 40 and 20 Mb/s for ANN, DF, linear and unequalized topologies, respectively. Using a blue filter to isolate the fast blue component of the LED (at the cost of the power contribution of the yellowish wavelengths) is a popular method of improving the data rate. We further demonstrate that it is possible to sustain higher data rates from the white light with ANN equalization than the blue component due to the high signal-to-noise ratio that is obtained from retaining the yellowish wavelengths. Using the blue component we could achieve data rates of 150, 130, 90 and 70 Mb/s for the same equalizers, respectively.

4 MIMO

This paper investigates the use of a pilot signal in reducing the electrical peak-to-average power ratio (PAPR) of an orthogonal frequency division multiplexing (OFDM) intensity-modulated optical wireless communication system. The phase of the pilot signal is chosen based on the selected mapping (SLM) algorithm while the maximum likelihood criterion is used to estimate the pilot signal at the receiver. Bit error rate (BER) performance of the pilot-assisted optical OFDM system is identical to that of the basic optical OFDM (with no pilot and no PAPR reduction technique implemented) at the desired BER of less than 10-3 needed to establish a reliable communication link. The pilot-assisted PAPR reduction technique results in higher reduction in PAPR for high order constellations than the classical SLM. With respect to a basic OFDM system, with no pilot and no PAPR reduction technique implemented, a pilot-assisted M-QAM optical OFDM system is capable of reducing the electrical PAPR by over about 2.5 dB at a modest complementary cumulative distribution function (CCDF) point of 10-4 for M = 64. Greater reductions in PAPR are possible at lower values of CCDF with no degradation to the systems error performance. Clipping the time domain signal at both ends mildly (at 25 times the signal variance level) results in a PAPR reduction of about 6.3 dB at the same CCDF of 10-4 but with an error floor of about 3 ×10-5. Although it is possible to attain any desired level of electrical PAPR reduction with signal clipping, this will be at a cost of deterioration in the systemss bit error performance.

Visible Light Communications: 170 Mb/s Using an Artificial Neural Network Equalizer in a Low Bandwidth White Light Configuration

New data services and applications are emerging continuously and enhancing the mobile broadband experience. The ability to cope with these varied and sophisticated services and applications will be a key success factor for the highly demanding future network infrastructure. One such technology that could help address the problem would be optical wireless communications (OWC), which presents a growing research interest in the last few years for indoor and outdoor applications. This paper is an overview of the OWC systems focusing on visible light communications, free space optics, transcutaneous OWC, underwater OWC, and optical scattering communications.

Pilot-Assisted PAPR Reduction Technique for Optical OFDM Communication Systems

In this paper we introduce a new mathematical model, which can be used to investigate the response of a long-period fibre grating to changes in ambient refractive indices of values both greater and less than that of the cladding. A numerical method to calculate the HE core and cladding modes is presented. The results show that even when the ambient refractive index is higher than that of the cladding, the coupling (resonance) wavelengths experience a measurable shift (a few nm). The result obtained suggests that LPGs coated with a material of higher refractive index than the cladding may be used as index sensors.

Emerging Optical Wireless Communications-Advances and Challenges

This paper theoretically and experimentally investigate the spectrum attenuation of free space optical (FSO) communication systems operating at visible and near infrared (NIR) wavelengths (0.6 μm <; λ <; 1.6 μm) under fog and smoke in a controlled laboratory condition. Fog and smoke are generated and controlled homogeneously along a dedicated atmospheric chamber of length 5.5 m. A new wavelength dependent empirical model is proposed to predict the fog and smoke attenuation operating at visible and NIR wavelengths. Comparison of the new proposed model with the measured continuous attenuation spectrum from visible-NIR in the fog and smoke channels shows a close relationship than the semi-empirical Kim and Kruse fog models. The experimental results also show the selection for the possible appropriate wavelengths from visible-NIR for FSO links to achieve the maximum link span in dense fog conditions.

Modelling of long-period fibre grating response to refractive index higher than that of cladding

This paper proposes and analyzes the performance of the multihop free-space optical (FSO) communication links using a heterodyne differential phase-shift keying modulation scheme operating over a turbulence induced fading channel. A novel statistical fading channel model for multihop FSO systems using channel-state-information-assisted and fixed-gain relays is developed incorporating the atmospheric turbulence, pointing errors, and path-loss effects. The closed-form expressions for the moment generating function, probability density function, and cumulative distribution function of the multihop FSO channel are derived using Meijers G-function. They are then used to derive the fundamental limits of the outage probability and average symbol error rate. Results confirm the performance loss as a function of the number of hops. Effects of the turbulence strength varying from weak-to-moderate and moderate-to-strong turbulence, geometric loss, and pointing errors are studied. The pointing errors can be mitigated by widening the beam at the expense of the received power level, whereas narrowing the beam can reduce the geometric loss at the cost of increased misalignment effects.

Modeling of Fog and Smoke Attenuation in Free Space Optical Communications Link Under Controlled Laboratory Conditions

The paper experimentally investigates the performance of the Ethernet and Fast-Ethernet free space optical (FSO) communications link under the influence of the weak atmospheric turbulence. A dedicated experimental indoor atmospheric chamber is used to generate and control the turbulence conditions that affect the FSO link performance. The chamber is calibrated and the measured data are verified with the theoretical prediction. We also demonstrate methods to control the turbulence levels and determine the equivalence between the indoor and outdoor FSO links. It is also observed that the connectivity of Ethernet and Fast-Ethernet links are highly sensitive to atmospheric conditions.