In free space optical communications, long-distance transmission leads to the attenuation of beacon light, where we adopt a quadrant detector (QD) to receive the weak signal. However, the background light interferes so strongly that the output signal-to-noise ratio (SNR) of QD is at a low level, which causes a decrease in accuracy of the direct detection method. This requires finding a new light spot detection method, so an improved detection method is proposed. Because the dark current noise and the background light noise are both white noise, we adopt a Kalman filter to estimate the real output of four electric signals of QD. Unfortunately, running these through an amplifier introduces some direct current (DC) offsets into the signals. In order to balance the effect of the DC offsets, we consider using the modulation method, where we employ a sine signal to modulate the intensity of the beacon light at the transmitting end, after which we can give an inverse gain to move the center of signals to near zero to eliminate the DC offsets when we calculate the data. In Kalman filtering, we use the peak values of the signals in every period after the analog to digital converter (ADC) as the elements of the measurement matrix. Experimental results show that even when QD output SNR is about −10 dB, the detection root-mean-square errors decrease by 51.5% using the improved detection method compared with the direct detection method. Moreover, Kalman filtering does not require a large amount of data, which means it works efficiently, can reduce the cost of hardware resources, and is available for the real-time calculation of spot position.

The satellite-to-ground optical communication system suffers from atmosphere turbulence severely. It is well-known that the coherent detection can increase the receiver sensitivity and performing aperture averaging can reduce the scintillation caused by the atmosphere turbulence. In this paper, the bit error rate, the outage probability and the average capacity of a coherent satellite-to-ground optical communication downlink with aperture averaging are analyzed. The Log-normal atmosphere turbulence model and BPSK (Binary Phase Shift Keying) modulation is employed. The analyzing focuses on the improvement of aperture averaging with different atmospheric conditions and zenith angles of the satellite. The bit error rate performance based on measuring data is given too. The results demonstrate the bit error rate and the outage probability can be reduced and the average capacity can be improved by aperture averaging efficiently. When the turbulence is stronger and the zenith angle is larger, the effect of aperture averaging is more obvious. The aperture averaging effect on BER (Bit Error Rate) is better than the effect on average capacity.

The coherent receiving method can improve the sensitivity of an optical signal receiver for free space optical communication system effectively. To implement coherent receiving, the phase offset between the local laser in the receiver and the received optical signal through the atmosphere needs to be measured and estimated. The commonly used algorithm is the Viterbi-Viterbi phase offset estimation method (VVPE) and this method always produces great errors especially with low SNR (signal to noise ratio). To improve the estimation performance, we present a new method combing the VVPE with the Kalman filter (VVPE-KF) to estimate the phase offset. This method can lower the estimation error by no less than 60%, when the SNR is low. To verify the performance of this new method, the constant parameter channel and atmosphere turbulence channel are employed to evaluate the algorithm. The impact of the atmosphere turbulence intensity on the tracking error is discussed.