A method for communication includes defining N respective symbol constellations in a signal space for N transmitters (24), N(2, subject to a predetermined power constraint, so as to produce a joint constellation including superpositions of symbols selected from the N symbol constellations. Data is transferred from the N transmitters by simultaneously transmitting from the N transmitters N respective symbols selected respectively from the N symbol constellations. A composite symbol including a superposition of the N transmitted symbols is received. The received composite symbol is demodulated using the joint constellation so as to reconstruct the data transferred from the N transmitters.