Belief Propagation Decoding of Finite-Length Polar Codes
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Polar codes, recently invented by Arikan, are the first class of codes known to achieve the symmetric capacity for a large class of channels. The symmetric capacity is the highest achievable rate subject to using the binary input letters of the channel with equal probability. Polar code construction is based on a phenomenon called channel polarization. The encoding as well as the decoding operation of polar codes can be implemented with O(N logN) complexity, where N is the blocklength of the code. In this work, we study the factor graph representation of finite-length polar codes and their effect on the belief propagation (BP) decoding process over Binary Erasure Channel (BEC). Particularly, we study the parity-check-based (H-Based) as well as the generator based (G-based) factor graphs of polar codes. As these factor graphs are not unique for a code, we study and compare the performance of Belief Propagation (BP) decoders on number of well-known graphs. Error rates and complexities are reported for a number of cases. Comparisons are also made with the Successive Cancellation (SC) decoder. High errors are related to the so-called stopping sets of the underlying graphs. we discuss the pros and cons of BP decoder over SC decoder for various code lengths.