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In OFDM,If the size of FFT is not in power of 2,what will happen?


It is possible to use non-power-of-2 FFT length in OFDM applications. For example, in Digital Radio Mondiale (DRM) receivers, a variety of both power-of-2 (N=256,512) and non-power-of-2 FFTs (N=1920, 576, 352, 288, 224, 176 and 112) are employed [1]

Adjusting the FFT length is related to implementation of the OFDM technique and hence will not alter the behavior of OFDM signal.

If you employ non-power-of-2 FFT, you have that much subcarriers in the OFDM signal. If you choose N=1920, that will render 1920 subcarriers. If you choose N=512 (power-of-2), that will render 512 subcarriers.

Usually FFT that are based on power-of-2 execute faster and are efficient. Efficiency comes from the fact that the FFT exploits the twiddle factors and hence the symmetry in the structure when the length is set to power-of-2.

Non-power-of-2 FFTs can be implemented by using a mix of other algorithms. For example Prime Factor Algorithm mapping [2] can be used to implement non-power-of-2 Example: FFT-288 can be expressed with a radix-2 and radix-3 FFT (FFT-32 x FFT-9)


[1] European Telecommunication Standard Institute (ETSI), “Digital Radio Mondiale (DRM),” System Specification ETSI ES 201 980, ETSI, Sophia Antipolis, France, April 2003. View at Google Scholar

[2] R. C. Singleton, “An algorithm for computing the mixed radix fast Fourier transform,” IEEE Transactions on Audio and Electroacoustics, vol. 17, no. 2, pp. 93–103, 1969. View at Google Scholar



I need the reason only,why FFT is in power of 2.