  The key to successive cancellation (SC) flip decoding of polar codes is to
accurately identify the first error bit. The optimal flipping strategy is
considered difficult due to lack of an analytical solution. Alternatively, we
propose a deep learning aided SC flip algorithm. Specifically, before each SC
decoding attempt, a long short-term memory (LSTM) network is exploited to
either (i) locate the first error bit, or (ii) undo a previous `wrong' flip. In
each SC attempt, the sequence of log likelihood ratios (LLRs) derived in the
previous SC attempt is exploited to decide which action to take. Accordingly, a
two-stage training method of the LSTM network is proposed, i.e., learn to
locate first error bits in the first stage, and then to undo `wrong' flips in
the second stage. Simulation results show that the proposed approach identifies
error bits more accurately and achieves better performance than the
state-of-the-art SC flip algorithms.
