What does it mean?

Stands for the O in de DAB-modulation mnemonic COFDM.

Two things are “orthogonal” if they will not influence each other.

The X-axis and Y-axis in a cartesian plane are said to be orthogonal, meaning that both have an angle of 90° towards each other. But it means that when you move horizontally parallel to X-axis the Y-value will not be influenced. Moving vertically parallel to the Y-axis, the X-value will not be influenced. Orthogonal axes mean that the X- and Y-axis do not influence each other.

In the DAB context, orthogonal indicates that the number of independent carriers (1.536 for Mode I) are placed in the spectrum so they do not influence each other.

The rule is that nearby carriers will not influence each other when the modulation frequency is equal to the frequency distance between the carriers.

The DAB-symbols modulated on the multitude of carriers are modulated during 1 ms (this duration is sufficient to avoid phase distortion). A time-lapse of 1 ms is related to a frequency of 1 kHz. Modulating the carriers with a symbol duration of 1 ms and a carrier distance of 1 kHz will realize orthogonal carriers.

In DAB mode I, the 1.536 carriers are spaced 1 kHz, giving a spectrum of 1,536 MHz and bringing us to the range of a DAB channel.