Short-Length Raptor Codes for Free-Space Optical Communications

Free-space optical (FSO) links are competitive wireless links offering high data rate, security and low system complexity. Compared to radio-frequency (RF) links, FSO links offer high rates at gigabit-per-second (Gbps) level and relatively low cost. However, atmospheric scintillation and misalignment between optical transmitter and receiver impair data rates of FSO links. Scintillation and misalignment are slow fading processes with a fading interval of 10's ms. Conventional fixed-length channel coding which interleave data blocks are unrealistic to overcome this slow fading due to block length of 10's of megabits. Also, because of the Gbps data rate, data rate adaptation to channel conditions are expensive. In this work, short-length (16 1024) Raptor codes are designed to overcome the slow fading of FSO channels. These Raptor codes are applied at the packet-level with high data rate and low decoding complexity. The Raptor encoder and decoder can be easily implemented in any software or hardware form. The practicality of these Raptor codes is demonstrated by a Raptor encoder and decoder which are implemented in field-programmable gate array (FPGA) and shown to support a 1.22 Gbps encoding and 714 Mbps decoding rate with a 97 mW low power consumption and 26360 gate circuit scale. High-speed transmission at Gbps level is easily satisfied by the same design implemented in an application-specific integrated circuit (ASIC).