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 the following table.
| XK_INDEX with Natural Outputs | XK_INDEX with Bit Reversed Outputs |
|---|---|
| 0 (‘b000) | 0 (‘b000) |
| 1 (‘b001) | 4 (‘b100) |
| 2 (‘b010) | 2 (‘b010) |
| 3 (‘b011) | 6 (‘b110) |
| 4 (‘b100) | 1 (‘b001) |
| 5 (‘b101) | 5 (‘b101) |
| 6 (‘b110) | 3 (‘b011) |
| 7 (‘b111) | 7 (‘b111) |
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.