Rate-Equivocation Optimal Spatially Coupled LDPC Codes for the BEC Wiretap Channel

We consider transmission over a wiretap channel where both the main channel and the wiretapper's channel are Binary Erasure Channels (BEC). We use regular convolutional LDPC ensembles, introduced by Felstrom and Zigangirov, together with Wyner's coset encoding scheme. We show that such a construction achieves the whole rate-equivocation region of the BEC wiretap channel. This result is based on the recent observation by Kudekar, Richardson, and Urbanke who proved that convolutional LDPC ensembles exhibit a "threshold saturation" phenomenon which converts the MAP threshold into the BP threshold for transmission over the BEC. Although our present result is less general (since we only consider the BEC) than the elegant code constructions based on polar codes which were recently introduced by several research groups, we see two potential advantages which we believe makes our construction worth considering. First, the proposed codes have a significantly better performance already for moderate lengths. Second, and perhaps more importantly, the proposed construction has the potential of being universal. More precisely, the phenomenon of spatial coupling has been observed empirically to hold for general binary memoryless symmetric channels as well. Hence, we conjecture that our construction is a universal rate-equivocation achieving construction when the main channel and wiretapper's channel are binary memoryless symmetric channels, and the wiretapper's channel is degraded with respect to the main channel.