Short-Length Raptor Codes for Free-Space Optical Communications
Wenzhe Zhang · MacSphere (McMaster University) · 2009
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). Two applications of these short-length Raptor codes in FSO links are presented in this work. Firstly, these Raptor codes are applied to hybrid FSO/RF links to achieve high data rate by sending Raptor encoded packets simultaneously over the FSO and RF links which we term such links as Raptor-coded soft-switching hybrid FSO/RF links. The performance of these Raptor codes in the hybrid FSO/RF links is simulated in a realistic channel model based on climate data of three Canadian cities. For a 1 Gbps FSO link combined with a 96 Mbps WiMAX RF link, the softswitching system achieves an average rate of 472 Mbps using the implemented Raptor code while hard-switching technique achieves only 112 Mbps on average. Secondly, these Raptor codes are applied in mobile FSO links for an unmanned aerial vehicle (DAV). This mobile FSO link suffers from severe instantaneous misalignment. For packet-level transmission, the time varying misalignment is unknown to the transmitter and causes data packet corruption and erasure. As a result, the application of conventional fixed-rate erasure coding techniques is difficult. In this 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 the misalignment. With a 1 Gbps transmitter, the designed Raptor code with k = 64 message packets offers 650 Mbps data rate when transmitting power is 20 dEmo In contrast, a traditional automatic repeat-request (ARQ) algorithm technique on the same FSO jitter channel achieves a rate of 70 JVlbps.