The investigation is concerned with various synchronous multiplexing and demultiplexing processes suitable for use with serial baseband data-transmission systems. The multiplexed signals are transmitted in orthogonal groups over a channel which introduces additive white Gaussian noise but no signal distortion. Techniques are considered for increasing both the capacity and tolerance to additive noise, when the number of multiplexed signals may vary with time, and may exceed the maximum number of orthogonal multiplexed signals.