  We consider transmission of two independent and separately encoded sources
over a two-user binary-input Gaussian multiple-access channel. The channel
gains are assumed to be unknown at the transmitter and the goal is to design an
encoder-decoder pair that achieves reliable communication for all channel gains
where this is theoretically possible. We call such a system \emph{universal}
with respect to the channel gains.
  Kudekar et al. recently showed that terminated low-density parity-check
convolutional codes (a.k.a. spatially-coupled low-density parity-check
ensembles) have belief-propagation thresholds that approach their maximum
a-posteriori thresholds. This was proven for binary erasure channels and shown
empirically for binary memoryless symmetric channels. It was conjectured that
the principle of spatial coupling is very general and the phenomenon of
threshold saturation applies to a very broad class of graphical models. In this
work, we derive an area theorem for the joint decoder and empirically show that
threshold saturation occurs for this problem. As a result, we demonstrate
near-universal performance for this problem using the proposed
spatially-coupled coding system.
