Towards zero retransmission overhead: A symbol level network coding approach to retransmission

We present SYNC, a physical layer transmission scheme that drastically reduces the cost of retransmission by introducing network coding concepts to symbol level operation. It piggybacks a new packet on each retransmitted packet, and exploits the previously received packet (possibly with error) at the receiver to recover both the retransmitted packet and the piggybacked packet. The piggybacking is achieved through higher modulation, but it does not decrease the decodability of the mixed packets owing to the previously received packet at the receiver, which can be analytically shown. SYNC works independently of other PHY level performance boosting schemes such as channel coding and spatial diversity. The proof-of-concept SYNC prototype has been implemented on a software defined radio (SDR) platform. The measurement data shows that under the same channel condition SYNC achieves 110 percent and 42 percent median throughput gain over traditional retransmission and SOFT, respectively. We also show that SYNC can be used proactively where the feedback as to the success of the previous transmission is not available, such as in broadcast.