Adaptive LDPC coding in L-band outperforming adaptive optics in C/L-bands in free-space optical communication under strong turbulence regime

In our pursuit to make improvements in free-space optical (FSO) communication over turbulent optical channels, we focus our attention on traditional adaptive coding techniques that are often used in wireless communication systems. Here we propose to use the adaptive coding based on quasi-cyclic LDPC codes of large girth. Our previous studies indicate that FSO systems operated over longer wavelengths outperform the FSO schemes operated over shorter wavelengths. The key idea of our proposal is to apply adaptive quasi-cyclic LDPC coding and operate the FSO communication system over the L-band, instead of C-band or visible band. To study the improvement of the proposed FSO communication system over traditional adaptive optics based FSO communication system, we developed an outdoor FSO testbed at the University of Arizona campus, and corresponding FSO link is over 1.5 km in total length. Our experimental results indicate that the proposed adaptive LDPC coding scheme operated in L-band in strong turbulence regime significantly outperforms the adaptive optics based counterpart operated in either C- or L-band. The proposed low-cost adaptive coding solution represents an excellent alternative to the expensive and bulky adaptive optics approaches. In the conference presentation, we will present experimental results for different code rates of the proposed adaptive LDPC coding scheme by employing the binary phase-shift keying (BPSK) at 10 Gb/s.