Smart codec: An adaptive packet data link

An adaptive convolutional code-rate system for packet data is presented which adapts the level of coding to overcome the effect of varying levels of noise. The system utilises the frame structure of packet data to synchronise adaption and uses a parallel decoding technique to ensure a hitless changeover between a number of different coding rates. The issues concerning the design are discussed in detail, including the resynchronisation of the Viterbi decoders, and the estimation of the level of noise. The system has been implemented using off-the-shelf components and tested over a range of error conditions and introduces only a low per-packet overhead. One important application is to counter the effect of a rain fade on a satellite communication link. A considerable improvement in the link throughput has been observed for a satellite link which suffers from typical rain fades.