Comparison of different models for ground-level atmospheric turbulence strength (Cn^2) prediction with a new model according to local weather data for FSO applications

Abstract

Atmospheric parameters strongly affect the performance of free-space optical communication (FSOC) systems when the optical wave is propagating through the inhomogeneous turbulence transmission medium. Developing a model to get an accurate prediction of the atmospheric turbulence strength (C(n)(2)) according to meteorological parameters (weather data) becomes significant to understand the behavior of the FSOC channel during different seasons. The construction of a dedicated free-space optical link for the range of 0.5 km at an altitude of 15.25 m built at Thanjavur (Tamil Nadu) is described in this paper. The power level and beam centroid information of the received signal are measured continuously with weather data at the same time using an optoelectronic assembly and the developed weather station, respectively, and are recorded in a data-logging computer. Existing models that exhibit relatively fewer prediction errors are briefed and are selected for comparative analysis. Measured weather data (as input factors) and C(n)(2) (as a response factor) of size [177,147×4] are used for linear regression analysis and to design mathematical models more suitable in the test field. Along with the model formulation methodologies, we have presented the contributions of the input factors' individual and combined effects on the response surface and the coefficient of determination (R(2)) estimated using analysis of variance tools. An R(2) value of 98.93% is obtained using the new model, model equation V, from a confirmatory test conducted with a testing data set of size [2000×4]. In addition, the prediction accuracies of the selected and the new models are investigated during different seasons in a one-year period using the statistics of day, week-averaged, month-averaged, and seasonal-averaged diurnal Cn2 profiles, and are verified in terms of the sum of absolute error (SAE). A Cn2 prediction maximum average SAE of 2.3×10(-13) m(-2/3) is achieved using the new model in a longer range of dynamic meteorological parameters during the different local seasons.

Quality Metrics and Reliability Analysis of Laser Communication System

“…During data transmission, the angle between the T x and R x line of sight and the transmitting beam axis must be kept within a fraction of the transmitting beamwidth which may be as smaller as a few micro-radians as discussed in Section-I. In laser communication, the fluctuations of the P r have to be related with the A att and C n 2 to characterise the turbulent channel effects on the propagated data carrying optical beam 8,10,[18][19][20][21][22][23] . The data carrying modulated optical beam cannot be used to measure the A att and the C n 2 since it bears data (information) and its average power get fluctuated at the receiver plane due to the modulation of the beam even under a favorable atmospheric condition 21,23 .…”

“…In laser communication, the fluctuations of the P r have to be related with the A att and C n 2 to characterise the turbulent channel effects on the propagated data carrying optical beam 8,10,[18][19][20][21][22][23] . The data carrying modulated optical beam cannot be used to measure the A att and the C n 2 since it bears data (information) and its average power get fluctuated at the receiver plane due to the modulation of the beam even under a favorable atmospheric condition 21,23 . One way of simultaneously measuring the A att and C n 2 is using a separate unmodulated optical setups/link, i.e., beacon beam, parallel to the data bearing optical path as a transmissometer for A att and scintillometer for C n 2 as detailed 6,18,21,23 .…”

“…The data carrying modulated optical beam cannot be used to measure the A att and the C n 2 since it bears data (information) and its average power get fluctuated at the receiver plane due to the modulation of the beam even under a favorable atmospheric condition 21,23 . One way of simultaneously measuring the A att and C n 2 is using a separate unmodulated optical setups/link, i.e., beacon beam, parallel to the data bearing optical path as a transmissometer for A att and scintillometer for C n 2 as detailed 6,18,21,23 . This approach will increase the overall cost of the system.…”

“…Furthermore, the atmospheric effect mitigation system is required to stabilize this beam also. Therefore, models yielding more accurate estimation of A att (dB/km) and C n 2 (m -2/3 ) according to the local weather data at our test field are formulated as described 21,23 , and given in Eqns. (2) …”

“…A widely using OOK modulation scheme is used to directly modulate the optical signal 6,8,10,15 . Transmitting optics is used to increase the beam diameter from 3 mm to 9 mm, such as to significantly reduce the beam divergence at the aperture of the receiver 18,21 . Intensity modulated optical beam is made to propagate through real-world open atmosphere and to be available at the aperture of the receiver.…”