The free space optical communication information transmission rate is high, which is attractive for high-speed communication scenarios. Compared with the near-Earth communication link, the satellite-to-ground laser communication link has the characteristics of long link distance, cloud layer and atmospheric turbulence. Aiming at the problem that the spread of star-to-ground laser communication pulses produces inter-code crosstalk and reduces the quality of the communication system, this paper analyzes the relationship between gauss optical pulse time domain widening and intercode crosstalk, and uses adaptive filters to balance the channels. The results show that the laser pulse broadening amount increases with the decrease of the input light pulse half wide, and the use of adaptive filter can effectively reduce the influence of code-to-code crosstalk caused by pulse broadening on the communication quality.
The previous researches on signal compensation technology based on feedforward neural network (FNN) are all in underwater channel and fiber channel. In this paper, the signal compensation technology based on FNN is applied to 8Gb/s 4-PAM indoor free space optical communication (FSOC) system for the first time. Under 7% forward error correction (FEC) threshold, the FNN algorithm is compared with direct detection and traditional LMS filtering algorithm. The FNN-based method can significantly improve the receiver sensitivity and improve the performance of the communication system.
In a free-space optical communication system with an avalanche photodiode (APD) detector, the average bit error rate (ABER) is studied based on pulse position modulation (PPM) considering Gamma–Gamma atmospheric turbulence and fiber coupling efficiency (FCE). By analyzing the shot noise and thermal noise, the approximate analytical expression of the ABER of binary PPM is theoretically derived. Then, the approximate analytical expression of ABER union bound is derived for L-ary PPM. The FCE has a greater impact on the ABER of the APD detection system than it does on the PIN detection system. By adopting the adaptive optics technology, compared with the PIN detection system, the communication performance of the APD detection is greatly improved. Moreover, the optimal average APD gain of the APD detection system is strongly correlated to the detector temperature but weakly dependent on the atmospheric turbulence intensity, FCE, number of wavefront compensation terms, average received photon number, and receiving aperture size. By optimizing the receiving aperture and designing an accurate temperature control system, the communication performance of the APD detection system can be further improved.
The transmission characteristics of 384 Gbit / s free-space laser link with dense wavelength division multiplexing (DWDM) are demonstrated, with channel spacing of 100 GHz. We use a simulated atmosphere channel with a tunable weak turbulence, and bit error rate (BER) curves of back-to-back transmission, which are measured and contrasted, respectively. To our knowledge, it is the first demonstration in the research of DWDM-DP-QPSK high-speed free-space optical (FSO) transmission in an atmospheric channel with tunable turbulence. For 100 GHz channel spacing, receiving sensitivities can be −33.6, −32.7, and −32.1 dBm for 300, 336, and 384 Gbit / s DWDM-PM-QPSK signals, respectively, at BER of 3.8 × 10 − 3 (FEC limit). In comparison with −37.6, −36.1, and −35.1 dBm for B2B case, the power penalties are 4, 3.4, and 3.0 dB, respectively. This study indicates that high-speed coherent DWDM-DP-QPSK FSO transmission is feasible.
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