A method for transmitting a plurality of input streams from a transmitting unit to a receiving unit receives the plurality of input data streams and each of the plurality of input data streams are processed to generate a parallel pair of data streams including an in-phase stream (I) and a quadrature-phase stream (Q) for each of the plurality of input data streams. A first and second parallel pair of data streams are modulated with a selected one of at least three mutually orthogonal functions at a first and second signal widths, respectively, using a predefined modulation process to generate a plurality of first data sub-layers and a plurality of second data sub-layers. A plurality of composite data stream are generated by overlaying the first data subs-layers with the second data sub-layers at a preconfigured overlay offset. The plurality of composite data streams are overlapped by a predetermined overlap amount and placed on a wavelength of a communications link. Intersymbol interference is controlled between the plurality of composite data streams responsive to the predefined modulation process used to create the first and second data sub-layers.