The reliability function of memoryless channels with noiseless feedback and variable-length coding has been found to be a linear function of the average rate in the classic work of Burnashev. In this work we consider unifilar channels with noiseless feedback and study specific transmission schemes, the performance of which provides lower bounds for the channel reliability function. In unifilar channels the channel state evolves in a deterministic fashion based on the previous state, input, and output, and is known to the transmitter but is unknown to the receiver. We consider two transmission schemes with increasing degree of complexity. The first one is a single-stage sequential transmission scheme, where both transmitter and receiver summarize their common information in an M-dimensional vector with elements in the state space of the unifilar channel and an M-dimensional probability mass function, with M being the number of messages. The second one is a two-stage transmission scheme where the first stage is as described above, and the second stage, which is entered when one of the messages is sufficiently reliable, is resolving a binary hypothesis testing problem. This stage is different from the one employed by Burnashev for discrete memoryless channels. The analysis assumes the presence of some common randomness shared by the transmitter and receiver, and is based on the study of the log-likelihood ratio of the transmitted message posterior belief, and in particular on the study of its multi-step drift. Simulation results for the trapdoor, chemical, and other binary input/state/output unifilar channels confirm that the bounds are tight compared to the upper bounds derived in a companion paper.
['Achilleas Anastasopoulos', 'Jui Wu']
Variable-length codes for channels with memory and feedback: error-exponent lower bounds