Spatially coupled protograph-based LDPC codes, composed of many
identical protographs coupled with their neighboring protographs, have
recently attracted much attentions.
This is due to the fact that spatial coupling increases the belief
propagation threshold of coupled code to the maximum a-priori
threshold of the underlying protograph LDPC code.

We consider spatially coupled protograph-based LDPC codes for a three
terminal erasure relay channel.
It is observed that density evolution thresholds, the maximal erasure
probability of the channel for which decoding error probability
converges to zero, of spatially coupled codes, especially spatially
coupled Mackay-Neal codes, is very close to the theoretical limits for
the relay channel.
We believe that this empirical result suggests that spatially coupled
protograph-based LDPC codes have great potential to achieve the
theoretical limits of general relay channels.