A hybrid ARQ scheme for resilient packet header compression

In this work we address resilient packet header compression in a setup similar to RFC 3095 (RObust Header Compression - ROHQ, where a noisy feedback channel is available. We propose a new predictive hybrid ARQ (IIARQ), using a systematic convolutional code with delay-limited interleaving. The compressed packet headers are bit interleaved, encoded, and the parity bits are loaded onto the packets in a manner that is consistent with existing standards. Our HARQ design is distinct from previous ones in that it cannot wait until the end of a codeword for retransmission, rather, ARQ's corresponding to individual packet headers arrive continually during a convolutional codeword and must be processed at that time. In our system, each time a NACK is encountered, the encoder estimates the state of the Viterbi decoder, deciding when a retransmission is necessary. The use of coding in conjunction with a specially designed interleaver, provides a flexible and powerful design methodology that makes it possible to improve the throughput via FEC and interleaving, while strictly limiting the delay to avoid timeout in higher layers. Simulations show that the throughput of the resulting system is superior to ROHC, and the delay is less sensitive to the channel conditions. Furthermore, over a large part of the operating range the delay is comparable to, or smaller than, the delay of ROHC.