FPGA Implementation of Rate-Adaptable Prefix-Free Code Distribution Matching for Probabilistic Constellation Shaping

In this paper, we implement rate-adaptable (RA) prefix-free code distribution matching (PCDM) in a field-programmable gate array (FPGA). The implemented RA-PCDM supports a wide range of information rates from 1.6 to 4.8 bit/symbol with a fine granularity of 0.2 bit/symbol in probabilistic constellation shaping (PCS) systems. The shaping performance of the implemented RA-PCDM is within 0.2 to 0.5 dB of the theoretical limit. In a universal architecture for all PCDM codebooks, massively parallel processing is demonstrated with various parallel factors, using a sliding window processor. It is shown that the RA-PCDM achieves fine-grained rate adaptation using a substantially less hardware area than a traditional rate adaptation scheme based on low-density parity-check (LDPC) codes, hence LDPC coders are a predominant consumer of area in PCS transceivers. The RA-PCDM is therefore a practical PCS technology that enables high-throughput optical communications to approach channel capacity more closely at a lower cost than RA-LDPC.