The simplest form of multicarrier modulation divides the data stream into multiple substreams to be
transmitted over different orthogonal subchannels centered at different subcarrier frequencies. The number
of substreams is chosen to make the symbol time on each substream much greater than the delay
spread of the channel or, equivalently, to make the substream bandwidth less then the channel coherence
bandwidth. This insures that the substreams will not experience significant ISI.