  We investigate spatially coupled code ensembles. For transmission over the
binary erasure channel, it was recently shown that spatial coupling increases
the belief propagation threshold of the ensemble to essentially the maximum
a-priori threshold of the underlying component ensemble. This explains why
convolutional LDPC ensembles, originally introduced by Felstrom and Zigangirov,
perform so well over this channel. We show that the equivalent result holds
true for transmission over general binary-input memoryless output-symmetric
channels. More precisely, given a desired error probability and a gap to
capacity, we can construct a spatially coupled ensemble which fulfills these
constraints universally on this class of channels under belief propagation
decoding. In fact, most codes in that ensemble have that property. The
quantifier universal refers to the single ensemble/code which is good for all
channels but we assume that the channel is known at the receiver. The key
technical result is a proof that under belief propagation decoding spatially
coupled ensembles achieve essentially the area threshold of the underlying
uncoupled ensemble. We conclude by discussing some interesting open problems.
