This figure has two parts, labelled A and B.

Part A is a representation of frequency division multiplexing (FDM). There is a horizontal axis labelled ‘frequency’, and six equally spaced marks on the frequency axis are labelled ‘carrier frequencies’. These marks are quite far apart. Centred on each of the carrier frequencies is a wave shape that has a positive peak at the carrier frequency. On either sides of the carrier frequency, the height of the wave falls away symmetrically down to the axis, then drops below the axis to a small negative peak, then rises to a small positive peak, and the wave makes a couple of further, smaller ripples above and below the frequency axis. The ripples merge into those of the wave shapes centred on adjacent carrier frequencies.

Part B is a representation of orthogonal frequency division multiplexing (OFDM). Again there is a horizontal frequency axis marked with six equally spaced markers as in part A. The six marks are very close, unlike those in part A, and are labelled ‘subcarrier frequencies’. Centred on each frequency is a wave shape identical to those in part A; that is, a high positive peak centred on the marked frequency, falling away sharply on either side in ripples of decreasing amplitude. Because the frequency markers are so close, there is much more overlapping of the wave shapes than in part A, so that as the wave shape falls away on either side of the peak, at a value of about three-quarters of its peak value it intersects the wave shape centred on the adjacent subcarrier frequency. Each subcarrier peak coincides with zeros on all the other waveforms it overlaps with. An arrow from the end of the last subcarrier to the end of the frequency axis is labelled ‘bandwidth reduction’. It shows that the frequency range of part B of the diagram is a lot narrower than that of part A.

4.2 Orthogonal frequency division multiplexing (OFDM)