Cyclic prefix insertion takes a section of the output of the FFT and prefixes it to the beginning of the transform. The resultant output data consists of the cyclic prefix (a copy of the end of the output data) followed by the complete output data, all in natural order. Cyclic prefix insertion is only available when output ordering is Natural Order.
When cyclic prefix insertion is used, the length of the
cyclic prefix can be set frame-by-frame without interrupting frame processing. The cyclic
prefix length can be any number of samples from zero to one less than the point size. The
cyclic prefix length is set by the CP_LEN field in the Configuration channel.
For example, when N = 1024, the cyclic prefix length can be from 0 to 1023 samples, and a
CP_LEN value of 0010010110 produces a cyclic prefix consisting of the last
150 samples of the output data.
The initial value and reset value of
CP_LEN is 0 (no cyclic prefix). The core uses the log2(point size) MSBs of
CP_LEN for the cyclic prefix length. So, when the point size decreases, the
leftover LSBs are ignored. This effectively scales the cyclic prefix length with the point
size, keeping them in approximately constant proportion. However, all bits of
CP_LEN are programmed into the core and are used in later transforms if the
point size increases.