Dense Maritime Fog Attenuation Prediction from Measured Visibility Data

The benefits of Free Space Optics (FSO) motivate to use it for future high data rate demanding communication applications. However, widespread growth of the technology has been hampered by reduced availability due to weather influences on the link. The fog has been analyzed as the most detrimental for FSO communication. There are some models that predict fog attenuation in terms of visibility. These models are compared with measured attenuation data of dense maritime fog. The comparison has been in terms of Sum of Square Error (SSE) and Root Mean Square Error (RMSE). A new model has been proposed that gives the least SSE and RMSE for the measured data

The Effects of Weather on the Life Time of Wireless Sensor Networks Using FSO/RF Communication

The increased interest in long lasting wireless sensor networks motivates to use Free Space Optics (FSO) link along with radio frequency (RF) link for communication. Earlier results show that RF/FSO wireless sensor networks have life time twice as long as RF only wireless sensor networks. However, for terrestrial applications, the effect of weather conditions such as fog, rain or snow on optical wireless communication link is major concern, that should be taken into account in the performance analysis. In this paper, life time performance of hybrid wireless sensor networks is compared to wireless sensor networks using RF only for terrestrial applications and weather effects of fog, rain and snow. The results show that combined hybrid network with three threshold scheme can provide efficient power consumption of 6548 seconds, 2118 seconds and 360 seconds for measured fog, snow and rain events respectively resulting in approximately twice of the life time with only RF link

Free-space optical communication employing subcarrier modulation and spatial diversity in atmospheric turbulence channel

An expression for the bit error rate of a multiple subcarrier intensity-modulated atmospheric optical communication system employing spatial diversity is derived. Spatial diversity is used to mitigate scintillation caused by atmospheric turbulence, which is assumed to obey lognormal distribution. Optimal but complex maximum ratio, equal gain combining (EGC) and relatively simple selection combining spatial diversity techniques in a clear atmosphere are considered. Each subcarrier is modulated using binary phase shift keying. Laser irradiance is subsequently modulated by a subcarrier signal, and a direct detection PIN receiver is employed (i.e. intensity modulation/direction detection). At a subcarrier level, coherent demodulation is used to extract the transmitted data/information. The performance of on–off-keying is also presented and compared with the subcarrier intensity modulation under the same atmospheric conditions

MIMO free-space optical communication employing subcarrier intensity modulation in atmospheric turbulence channels

In this paper, we analyse the error performance of transmitter/receiver array free-space optical (FSO) communication system employing binary phase shift keying (BPSK) subcarrier intensity modulation (SIM) in clear but turbulent atmospheric channel. Subcarrier modulation is employed to eliminate the need for adaptive threshold detector. Direct detection is employed at the receiver and each subcarrier is subsequently demodulated coherently. The effect of irradiance fading is mitigated with an array of lasers and photodetectors. The received signals are linearly combined using the optimal maximum ratio combining (MRC), the equal gain combining (EGC) and the selection combining (SelC). The bit error rate (BER) equations are derived considering additive white Gaussian noise and log normal intensity fluctuations. This work is part of the EU COST actions and EU projects

Performance Analysis of Throughput Efficient Switch-over between FSO and mmW Links

Free Space Optics (FSO) links provide usage of high bandwidth and the flexibility of wireless communication links. However, weather patterns like fog and heavy snow fall limit the availability of FSO. Another technology providing similar properties regarding offered data rates and flexibility of setup is Millimeter Wave Technology (mmW), operating at several tens of GHz. In this case, heavy rain limits mmW link availability. A combination of both technologies had been proved to be very effective to achieve very high availability. Different hybrid architectures of these two links and switch-over techniques had been proposed in the recent years. All of these techniques require redundant transmission on either both transmission links or waste bandwidth of backup link when main FSO link is operational. In this paper, a switch-over between these technologies is proposed, to maintain high availability without the loss of transmission bandwidth. The performance of this switch-over has been simulated for more than one year measured availability data for hybrid network of mmW link and FSO link. The switch over behavior has also been simulated for fog, rain and snow events. It has been shown that the availability with switch-over reaches the redundant link availability but switchover can save more than 90% redundant transmission and increase the hybrid network throughput significantly

Probabilistic Model for Free-Space Optical Links Under Continental Fog Conditions

The error characteristics of a free-space optical (FSO) channel are significantly different from the fiber based optical links and thus require a deep physical understanding of the propagation channel. In particular different fog conditions greatly influence the optical transmissions and thus a channel model is required to estimate the detrimental fog effects. In this paper we shall present the probabilistic model for radiation fog from the measured data over a 80 m FSO link installed at Graz, Austria. The fog events are classified into thick fog, moderate fog, light fog and general fog based on the international code of visibility range. We applied some probability distribution functions (PDFs) such as Kumaraswamy, Johnson SB and Logistic distribution, to the actual measured optical attenuations. The performance of each distribution is evaluated by Q-Q and P-P plots. It is found that Kumaraswamy distribution is the best fit for general fog, while Logistic distribution is the optimum choice for thick fog. On the other hand, Johnson SB distribution best fits the moderate and light fog related measured attenuation data. The difference in these probabilistic models and the resultant variation in the received signal strength under different fog types needs to be considered in designing an efficient FSO system

How to find an attractive solution to the liar paradox

The general thesis of this paper is that metasemantic theories can play a central role in determining the correct solution to the liar paradox. I argue for the thesis by providing a specific example. I show how Lewis’s reference-magnetic metasemantic theory may decide between two of the most influential solutions to the liar paradox: Kripke’s minimal fixed point theory of truth and Gupta and Belnap’s revision theory of truth. In particular, I suggest that Lewis’s metasemantic theory favours Kripke’s solution to the paradox over Gupta and Belnap’s. I then sketch how other standard criteria for assessing solutions to the liar paradox, such as whether a solution faces a so-called revenge paradox, fit into this picture. While the discussion of the specific example is itself important, the underlying lesson is that we have an unused strategy for resolving one of the hardest problems in philosophy

Comprehensive vascular imaging using optical coherence tomography-based angiography and photoacoustic tomography

Studies have proven the relationship between cutaneous vasculature abnormalities and dermatological disorders, but to image vasculature noninvasively in vivo, advanced optical imaging techniques are required. In this study, we imaged a palm of a healthy volunteer and three subjects with cutaneous abnormalities with photoacoustic tomography (PAT) and optical coherence tomography with angiography extension (OCTA). Capillaries in the papillary dermis that are too small to be discerned with PAT are visualized with OCTA. From our results, we speculate that the PA signal from the palm is mostly from hemoglobin in capillaries rather than melanin, knowing that melanin concentration in volar skin is significantly smaller than that in other areas of the skin. We present for the first time OCTA images of capillaries along with the PAT images of the deeper vessels, demonstrating the complementary effective imaging depth range and the visualization capabilities of PAT and OCTA for imaging human skin in vivo. The proposed imaging system in this study could significantly improve treatment monitoring of dermatological diseases associated with cutaneous vasculature abnormalities