Previous chapters have underlined the importance of inter-symbol interference (ISI) as channel bandwidths grow and symbol periods shrink. At the cost of some overhead, multicarrier transmissions allow the system to avoid ISI. In this approach, a data stream is divided into multiple parallel substreams which are transmitted over different orthogonal subchannels, each centered at a different subcarrier frequency. Each of these substreams has a narrow bandwidth, and consequently the associated symbol duration is relatively long. The number of substreams is chosen to make the symbol duration on each substream much greater than the delay spread of the channel or, equivalently, to make the substream bandwidth less than the channel coherence bandwidth. This design choice ensures that the substreams will not experience significant ISI. However, achieving this gain comes at the cost of an overhead equivalent to the worst case channel delay spread.