The XK_INDEX field (if present in the Data Output channel) gives the sample number of the XK_RE/XK_IM data being presented at the same time. In the case of natural order outputs, XK_INDEX increments from 0 to (point size) -1. When bit reversed outputs are used, XK_INDEX covers the same range of numbers, but in a bit (or digit) reversed manner.
For example, when you have an 8 point FFT, XK_INDEX takes on the values in Table: XK _INDEX values for 8 point FFT .
If cyclic prefix insertion is used, the cyclic prefix is unloaded first and XK_INDEX counts from (point_size) - (cyclic prefix length) up to (point size) -1. After the cyclic prefix has been unloaded, or if the cyclic prefix length is zero, the whole frame of output data is unloaded. XK_INDEX counts from 0 up to (point size) -1 as before. Cyclic Prefix Insertion is only possible with natural order outputs.