Demonstrating capacity-approaching FSO communications

Abstract

Atmospheric turbulence causes the receive signal intensity on free space optical (FSO) communication links to vary over time. Scintillation fades can stymie connectivity for milliseconds at a time. To approach the information-theoretic limits of communication in such time-varying channels, it necessary to either code across extremely long blocks of data – thereby inducing unacceptable delays – or to vary the code rate according to the instantaneous channel conditions. We describe the design, hardware implementation, and system performance of an FSO modem that employs low-density parity-check (LDPC) coding in an incremental redundancy (IR) hybrid automatic repeat request (HARQ) protocol. Independent tests performed by the U.S. Government demonstrate that our protocol effectively adapts the LDPC code rate to match the instantaneous channel conditions. For links with fixed throughput, this translates to the longest possible range in the presence of optical scintillation; for links with fixed range, this translates to the highest possible average throughput. By leveraging an LDPC that is amenable to low-complexity, high-throughput implementation in hardware, our modem is able to provide throughputs in excess of 850 Mbps on links with ranges greater than 15 kilometers.