A SEQUENTIAL SCHEME FOR ADAPTIVE ERROR CONTROL OVER SLOWLY VARYING CHANNELS

Improved performance of error control techniques over slowly varying channels can be realized by a system which adapts itself to changes in the channel conditions. Such action requires a method for monitoring the channel state and detecting a change in it. The frequency of retransmission requests in ARQ and type-I hybrid-ARQ error control strategies provides a natural source of channel state information. The proposed scheme incorporates the retransmission requests into the scoring of a statistical sequential inspection scheme proposed by E. Page in 1954. Each transmitted packet is scored based on the outcome of the decoding process (i.e., whether it is accepted or rejected). When the cumulative score crosses a decision boundary, the coding strategy is altered and the sequential inspection scheme is restarted using the same scoring routine with different weighting and boundary constants. In this way the channel encoder/decoder is able to alter its strategy and adapt itself to changes in the channel. To be effective, the channel needs to vary slowly in comparison to the signaling rate. This gives the system time to detect a change in the channel and compensate for it before the next change in the channel occurs. The analysis of this system is based on the average run length of the tests in the various channel states. These values indicate the delay the system experiences in reacting to a change in the channel and the tendency the test has to terminate prematurely. These factors are a function of the test constants which are determined algorithmatically by maximizing the system throughput using the system reliability as a constraint. Results show that the performance of this system is close to that of the ideal system with perfect channel state information.