15.3. FFT Design Elements Library
The radix-22 architecture is a serial version of the radix-4 architecture. It computes a radix-4 butterfly over four (not necessarily consecutive) inputs and produces four (not necessarily consecutive) outputs.
For more information about the radix-22 algorithm, refer to A New Approach to Pipeline FFT Processor – Shousheng He & Mats Torkleson, Department of Applied Electronics, Lund University, Sweden.