Irregular SPC and RI SPC PCs
More recent results involve the addition of irregularity to the structure of the product code. Irregular low density parity check (LDPC) codes are the best, currently known, codes for long blocklengths.
Multi-dimensional product codes are also a form of LDPC code so it is not too surprising that the performance of the SPC and RI SPC product codes can be improved by adding irregularity.
The complexity of encoding and decoding of the irregular codes is almost unchanged, however adding irregularity to the data bits results in a code with a lower rate (and smaller blocklength).