Cost-efficient FPGA layered LDPC decoder with serial AP-LLR processing

This paper proposes an FPGA based layered architecture for quasi-cyclic (QC) irregular LDPC decoder. Our approach is based on merging variable and check node processing into one single variable-check node (VCN) unit. Layer message computation is done using a parallel scheme of a number of VCNs equal to the expansion factor of the QC matrix. The proposed architecture is characterized by the serial processing of the a posteriori LLRs by an FPGA specific high frequency VCN unit implementation using ROM memories. In our approach data conversions as well as additions and comparators are replaced by look-up-tables implemented using distributed RAM. In addition to this, other techniques such as: efficient packaging of LLRs messages and check-node message compression as well as the configurable port width of the FPGA's BRAM are used to reduce BRAM block utilization. Throughput increase is achieved by utilizing techniques such as pipelining, parallel processing of multiple VCNs, as well as relatively high working frequency. Implementation results for the WiMAX (1152, 2304) QC irregular LDPC code indicate that the proposed architecture has up to 3x less slices resource utilization and up to 1 order of magnitude less BRAM blocks with respect to other approaches, while maintaining a throughput of several hundreds of Mbps (800 Mbps coded bits). We achieved this without sacrificing flexibility; therefore we can easily adapt our design to accommodate different code rates.