  In this work, we show that polar belief propagation (BP) decoding exhibits an
error floor behavior which is caused by clipping of the log-likelihood ratios
(LLR). The error floor becomes more pronounced for clipping to smaller
LLR-values. We introduce a single-value measure quantifying a "relative error
floor", showing, by exhaustive simulations for different lengths, that the
error floor is mainly caused by inadequate clipping values. We propose four
modifications to the conventional BP decoding algorithm to mitigate this error
floor behavior, demonstrating that the error floor is a decoder property, and
not a code property. The results agree with the fact that polar codes are
theoretically proven to not suffer from error floors. Finally, we show that
another cause of error floors can be an improper selection of frozen bit
positions.
