Methods and systems for transmitting a signal. A first signal from a first device operating in spatial multiplexing transmission is concatenated with a second signal from a second device operating without spatial multiplexing transmission to generate a concatenated signal in a non-linearly superpositioned constellation, in which a portion of the constellation corresponding to the first signal is symmetrical about each of the real and imaginary axes. The concatenated signal is processed according to transmission processing of the second device, to generate a processed signal. The processed signal is transmitted.