A −70 dBm High-Sensitivity AGC Algorithm for PPM-APD Low-Power FSOC Systems

Automatic gain control (AGC) is essential for improving the sensitivity and dynamic range of receiver in free space optical communication (FSOC) systems. In this paper, we investigate an AGC algorithm for a typical low-power FSOC system with pulse position modulation (PPM) and avalanche photodiode (APD). Conventional average/peak detection methods are inadequate for measuring signal strength in the PPM-APD FSOC systems due to the low received power and pulse sparsity. To overcome this issue, we propose a high-sensitivity signal strength estimation (SSE) method based on correlation detection. Then, we design an AGC algorithm that maximizes the output signal-to-noise ratio (SNR) of the APD. The proposed SSE algorithm can retain a sensitivity of -70 dBm. The designed AGC algorithm achieves a dynamic range of at least 20 dBm and reduces the bit error rate (BER) of demodulation by 1 to 4 orders of magnitude under a 20 Mbps rate and 16-PPM format near the decoding threshold.