No abstract

While free-space optical (FSO) links have many inherent advantages, such as high data rates and license-free operation, they are sensitive to atmospheric conditions. A promising solution to improve the reliability of such communication links is to combine FSO and radio frequency (RF) channels using Raptor codes. This paper presents a first in-field demonstration of real-time Raptor-coded FSO communication over a 1.87 km urban link. This work serves as an important initial step toward field-programmable gate array (FPGA)-based implementation of Raptor-coded hybrid FSO/RF links. The hardware architecture of a Raptor 10 decoder based on a novel resource-and time-efficient matrix inversion algorithm is presented and implemented in an FPGA platform. The receiver is implemented in a pipelined architecture to maximize link utilization. The throughput of the Raptorcoded FSO link is measured both experimentally and numerically under real-life weather conditions for a continuous period of 8 h. Experimental results demonstrate that Raptor-coded FSO links are capable of tracking changes in the channel state to maximize link utilization.

In this paper, a rate-adaptive free-space optical (FSO) communication system using systematic Raptor codes is investigated based on experimental data measured over a 1.87 km terrestrial FSO link in different weather conditions. Using Raptor codes, the FSO communication system can adapt to the channel gain fluctuations caused by atmospheric turbulence and/or adverse weather conditions. To alleviate the burden of synchronizing between transmitter and receiver, messages are packetized and sent in sequence. For each packet, a cyclicredundancy code (CRC) is added resulting in an effective packet erasure channel (PEC). Using experimental data and on-offkeying (OOK) modulation, the performance of the rate-adaptive FSO system is evaluated at different signaling rates. While an uncoded FSO system fails to efficiently operate during adverse weather conditions, the rate-adaptive technique provides reliable FSO communication under clear and rainy weather conditions.