Free Space Optical (FSO) Communication systems use the atmosphere as a propagation medium. Hence the atmospheric turbulence effects lead to multiplicative noise related with signal intensity. In order to suppress the signal fading induced by multiplicative noise, we propose a fast Quasi-Cyclic (QC) Low-Density Parity-Check (LDPC) code for FSO Communication systems. As a linear block code based on sparse matrix, the performances of QC-LDPC is extremely near to the Shannon limit. Currently, the studies on LDPC code in FSO Communications is mainly focused on Gauss-channel and Rayleigh-channel, respectively. In this study, the LDPC code design over atmospheric turbulence channel which is nether Gauss-channel nor Rayleigh-channel is closer to the practical situation. Based on the characteristics of atmospheric channel, which is modeled as logarithmic-normal distribution and K-distribution, we designed a special QC-LDPC code, and deduced the log-likelihood ratio (LLR). An irregular QC-LDPC code for fast coding, of which the rates are variable, is proposed in this paper. The proposed code achieves excellent performance of LDPC codes and can present the characteristics of high efficiency in low rate, stable in high rate and less number of iteration. The result of belief propagation (BP) decoding shows that the bit error rate (BER) obviously reduced as the Signal-to-Noise Ratio (SNR) increased. Therefore, the LDPC channel coding technology can effectively improve the performance of FSO. At the same time, the BER, after decoding reduces with the increase of SNR arbitrarily, and not having error limitation platform phenomenon with error rate slowing down.
KEYWORDS: Turbulence, Filtering (signal processing), Signal to noise ratio, Receivers, Wireless communications, Adaptive optics, Atmospheric optics, Digital signal processing, Channel projecting optics, Detector development
A model for received signals has been developed for detection of binary data bits in the
presence of atmospheric turbulence noise. The model was used to analyze the likelihood ratio test
(LRT) according to optimal maximum a posterior (MAP) probability criterion. The adaptive
decision thresholding method was developed to maintain the detection threshold near optimum. The
detection threshold that was updated adaptively as the
signal-to-noise ratio (SNR) was changed.
Kalman filter and homomorphic filter were used to predict statistical variable of the bit and
turbulence parameters, respectively. Simulation and the experimental results indicate that bit-error
probability (BEP) performance of the system is greatly improved by an order of magnitude or more.
KEYWORDS: Free space optics, Telecommunications, Promethium, Receivers, Free space optical communications, Picosecond phenomena, Signal attenuation, Signal detection, Signal processing, Semiconductor lasers
Commercial and military interest in free-space optical communication (FSO) is growing due to the high bandwidths, portability and high security. FSO systems with proper radio-frequency backup can provide a viable solution for the last mile problem The communication reliability of FSO has been being pay attention to. In this paper, an innovation solution of light power equalization is put forward and some technologies such that enlarging the dynamic range of light power gain and realizing communication for FSO in the condition of all-weather are studied, a method to implement the power equalization is also given in term of system-embedded, some experiments are implemented with a system self-fabricated, too, the experimental results shown that a wide range power equalization system on FSO can effectively reduce the influences from the random variation of
atmospheric medium and other factors on the communication link path.
Optical wireless communication (OWC) systems use the atmosphere as a propagation medium. In optical wireless communication links, atmospheric turbulence causes distortions in the laser beam wavefront, leading to fluctuations in both the intensity and the phase of the received light signal performance, i.e. signal fading. This paper presents a suppression method of turbulence noise on the basis of analyzing the optical signal effect of turbulence.In this paper, according to probability distributions of turbulence-induced intensity, we describe a BEP model. From the model, we develop an adaptive threshold detector technique to decrease the BEP. The BEP increases from 10-11 to 10-7 for a linear increase of normalized standard deviation σx of the log-amplitude from 0 to 0.5 which is shown in fig.1 at a stationary threshold. Finally, we describe the matched receiver to maximize the output signal-to-noise ratio (SNR). We also numerically compute the average bit-error probabilities for the filter case.
The Free Space optical communication (FSO) or wireless optical communication, utilizes the atmospheric medium as transmission channel, where random variety such as fog, atomy and atmosphere flash and the atmospheric turbulence will badly affect the propagation of light, the receiving signal is easily swung and drifted with the change of weather. In this paper, we discussed the attenuation of the atmospheric channel and analyzed the signal characteristics in the condition of the atmospheric overfall, for the OOK modulation, discussed the receiving signal distribution in the atmospheric channel taking account for the noise gain of the light detector, and based on the principle of the Hartman-Shack sensor, we designed a wave-front distortion compensation system with fiber coupler. The signal fading resulted from wave-front phase distortion was compensated effectively by using the compensation system.
A way to overcome the fading caused by atmosphere is discussed, and the power gain equalization method for the
wireless optical communication system is studied, the results for the system are also given. In the scheme of wide-range power-gain equalization, the power gain of the wireless optical system is controlled automatically in accordance with
visibility change of propagation channel by means of sampling the propagation channel, which has also been shown.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.