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Rate-adaptive Concatenated Multilevel Coding with Fixed Decoding Complexity

In this work, a low-complexity multilevel coding (MLC) based coded modulation approach having rate flexibility with two degrees of freedom is proposed. It consists of an inner soft-decision (SD) polar code serially concatenated with an outer hard-decision (HD) code. For a given HD outer code, a method is developed to optimize a multi-stage decoding based MLC scheme so that it will require a fixed amount of inner decoding operations and thereby having constant complexity while changing the transmission rate with near-continuous granularity. Over the range of studied transmission rates, the proposed MLC scheme with outer staircase code and zipper code utilizes 75%, and 73% fewer inner SD decodings, respectively, compared to BICM. Compared to the existing MLC approaches, the proposed MLC scheme, with same inner and outer codes, provides up to 72% reduction in the number of required inner SD decodings.