  We consider a signaling format where the information to be communicated from
one or multiple transmitters to a receiver is modulated via a superposition of
independent data streams. Each data stream is formed by error-correction
encoding, constellation mapping, replication and permutation of symbols, and
application of signature sequences. The relations between the data bits and
modulation symbols transmitted over the channel can be represented by a sparse
graph. In the case where the modulated data streams are transmitted with time
offsets the receiver observes spatial coupling of the individual graphs into a
graph chain enabling efficient demodulation/decoding. We prove that a two-stage
demodulation/decoding method, in which iterative demodulation based on symbol
estimation and interference cancellation is followed by parallel error
correction decoding, achieves capacity on the additive white Gaussian noise
(AWGN) channel asymptotically. We compare the performance of the two-stage
receiver to the receiver which utilizes hard feedback between the
error-correction encoders and the iterative demodulator.
