Performance comparison of NRZ and RZ modulations with and without forward error corrections for free-space optical communication

Our free-space optical (FSO) communication experiments show that, compared to non-return-to-zero (NRZ) encoding, return-to-zero (RZ) data format is more robust to atmospheric turbulence. A forward error correction coding gain up to 5 dB is obtained at a bit error-rate of 1x10-15 for FSO link with Reed-Solomon (255,239) generic FEC (GFEC) which is specified in ITU-T G.709 standards. The coding gain increases further up to 1.6 dB with enhanced FEC (EFEC) that adds no additional overheads. Still, the link-margin with both GFEC and EFEC decreases with atmospheric turbulence.