An apparatus, method and computer program is described comprising: receiving, at a receiver (46) of a transmissions system (40), transmitted signals from each of a plurality of transmitters (42a, 42b, 42c), wherein each transmitter communicates with the receiver over one of a plurality of channels (44a, 44b, 44c) of the transmission system, wherein each transmitter includes a transmitter algorithm having at least some trainable weights, wherein each transmitter algorithm has the same trainable weights and wherein each of the transmitted signals is based on a perturbed channel symbol generated at the respective transmitter, wherein the channel symbols and perturbations are known to the receiver; updating said weights of said transmitter algorithm, at the receiver, based on a loss function; providing said updated weights to each transmitter of the transmission system; and repeating the receiving and updating until a first condition is reached.
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