Performance of concatenated codes over an APD-based optical system with estimated channel-state information

In this paper, we investigate the efficiency of serially concatenated codes using a BCH (32400, 32208) inner code and an LDPC (64800, 32400) outer code over a free-space optical communication (FSOC) system with APD detectors. Based on the experimental optical signals recorded from a FSO (free-space optical) link with the distance z = 11.8 km, the bit error rate (BER) performance of the concatenated codes is evaluated from the aspect of simulation under weak turbulence conditions (for which the scintillation index σ _I ² = 0.18) and strong turbulence conditions (for which σ _I ² = 1.49). The results indicate that the fade level P _T can be set much lower for the FSOC system using the concatenated codes, leading to shorter fade length and interleavers in smaller size. The concatenated codes are shown to even outperform the (4, 1, 7) convolutional code in both cases. In addition, we analyze the BER performance of these concatenated codes on the basis of pilot-aided channel estimation with different numbers of pilot symbols.