New ARQ based on code combining applicable to HF channels

The highly time varying nature of the HF radio channel makes the design of coding schemes extremely difficult. The designer must make the choice between a code which is powerful enough to allow error free communication in the worst channel state or a code which will allow communication only when the channel is good and risk the introduction of errors in to the received data. Clearly, what is required is an adaptive system which uses a high rate code for high throughput when channel conditions are good and a more powerful, low rate code to ensure reliable reception when the channel is poor. An adaptive system must monitor the channel state and ensure that the correct code is selected, this requires accurate real time channel evaluation data. Code combining hybrid ARQ systems provide continuous adaptive operation, without relying on channel evaluation data, ensuring that the correct amount of error control is added to any transmitted data. This is achieved using a system of incremental redundancy where progressively more parity symbols are transmitted as errors are detected. This paper describes a code combining, hybrid ARQ system based on the Reed Muller code using a full, trellis based, soft maximum likelihood decoder. The system is capable of providing reliable communication over widely varying channel conditions using a low complexity decoder.