  We exploit the redundancy of the language-based source to help polar
decoding. By judging the validity of decoded words in the decoded sequence with
the help of a dictionary, the polar list decoder constantly detects erroneous
paths after every few bits are decoded. This path-pruning technique based on
joint decoding has advantages over stand-alone polar list decoding in that most
decoding errors in early stages are corrected. In order to facilitate the joint
decoding, we first propose a construction of dynamic dictionary using a trie
and show an efficient way to trace the dictionary during decoding. Then we
propose a joint decoding scheme of polar codes taking into account both
information from the channel and the source. The proposed scheme has the same
decoding complexity as the list decoding of polar codes. A list-size adaptive
joint decoding is further implemented to largely reduce the decoding
complexity. We conclude by simulation that the joint decoding schemes
outperform stand-alone polar codes with CRC-aided successive cancellation list
decoding by over 0.6 dB.
