In this correspondence, we investigate in a comprehensivefashion a one-layer coding/shaping scheme resembling a perfectly cooperatedmultiple-access system. At the transmitter, binary data areencoded by either single-level or multilevel codes. The coded bits are firstrandomly interleaved and then entered into a signal mapper. At each time,the signal mapper accepts as input multiple binary digits and deliversas output an amplitude signal, where the input are first independentlymapped into 2-PAM signals (possibly having different amplitudes) andthen superimposed to form the output. The receiver consists of an iterativedecoding/demapping algorithm with an entropy-based stopping criterion.In the special cases when all the 2-PAM signals have equal amplitudes,based on an irregular trellis, we propose an optimal soft-input–soft-output(SISO) demapping algorithm with quadratic rather than exponentialcomplexity. In the general cases, when multilevel codes are employed,we propose power-allocation strategies to facilitate the iterative decoding/dempaping algorithm. Using the unequal power-allocations andthe Gaussian-approximation-based suboptimal demapping algorithm(with linear complexity), coded modulation with high bandwidth efficiencycan be implemented.