Analysis and optimization of iteration schedules for LDPC coded modulation and detection

In this paper, we consider iterative detection in combination with an outer low-density parity-check (LDPC) code, and present new EXIT curves that help to optimize the decoding schedule among these blocks. For this, we compute a set of (inner) detector and variable node decoder curves depending on whether or not the detector update is included into the iteration loop; interestingly, this involves the differentiation of EXIT curves. We illustrate the schedule optimization using examples such as an inner equalizer for partial response channels. The new design insights are confirmed by trajectory simulations and are used to find the detector execution order minimizing the bit error rate at a given channel quality.