Short-Length Raptor Codes for Free-Space Optical Communications

Abstract

<p>Free-space optical (FSO) links are competitive wireless links offering high data rate,<br />security and low system complexity. Compared to radio-frequency (RF) links, FSO<br />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<br />processes with a fading interval of 10's ms. Conventional fixed-length channel coding<br />which interleave data blocks are unrealistic to overcome this slow fading due to block<br />length of 10's of megabits. Also, because of the Gbps data rate, data rate adaptation<br />to channel conditions are expensive.<br />In this work, short-length (16 - 1024) Raptor codes are designed to overcome the<br />slow fading of FSO channels. These Raptor codes are applied at the packet-level with<br />high data rate and low decoding complexity. The Raptor encoder and decoder can be<br />easily implemented in any software or hardware form. The practicality of these Raptor<br />codes is demonstrated by a Raptor encoder and decoder which are implemented in<br />field-programmable gate array (FPGA) and shown to support a 1.22 Gbps encoding<br />and 714 Mbps decoding rate with a 97 mW low power consumption and 26360 gate<br />circuit scale. High-speed transmission at Gbps level is easily satisfied by the same<br />design implemented in an application-specific integrated circuit (ASIC).</p> <p>Two applications of these short-length Raptor codes in FSO links are presented<br />in this work. Firstly, these Raptor codes are applied to hybrid FSO/RF links to<br />achieve high data rate by sending Raptor encoded packets simultaneously over the<br />FSO and RF links which we term such links as Raptor-coded soft-switching hybrid<br />FSO/RF links. The performance of these Raptor codes in the hybrid FSO/RF links<br />is simulated in a realistic channel model based on climate data of three Canadian<br />cities. For a 1 Gbps FSO link combined with a 96 Mbps WiMAX RF link, the softswitching<br />system achieves an average rate of 472 Mbps using the implemented Raptor<br />code while hard-switching technique achieves only 112 Mbps on average.<br />Secondly, these Raptor codes are applied in mobile FSO links for an unmanned<br />aerial vehicle (DAV). This mobile FSO link suffers from severe instantaneous misalignment. For packet-level transmission, the time varying misalignment is unknown<br />to the transmitter and causes data packet corruption and erasure. As a result, the<br />application of conventional fixed-rate erasure coding techniques is difficult. In this<br />work, short-length Raptor codes are applied in such mobile FSO channels. A key advantage of Raptor codes is their independence on channel state, no matter how large<br />the misalignment. With a 1 Gbps transmitter, the designed Raptor code with k = 64<br />message packets offers 650 Mbps data rate when transmitting power is 20 dEmo In<br />contrast, a traditional automatic repeat-request (ARQ) algorithm technique on the<br />same FSO jitter channel achieves a rate of 70 JVlbps.</p>