# Why DFT-SPREAD OFDM has lower PAPR than OFDM?

If the DFT size M equals the IDFT size N, the cascaded DFT/IDFT processing would obviously completely cancel each other out. However, if M is smaller than N and the remaining inputs to the IDFT are set to zero, the output of the IDFT will be a signal with “single-carrier” properties – that is, a signal with low power variations, and with a bandwidth that depends on M.

My question is that why DFT-SPREAD OFDM has lower variations in comparison with OFDM? The picture is DFT-SPREAD OFDM modulation scheme.

Thank you.

• This scheme is more commonly called single-carrier frequency division multiple access (SC-FDMA) and there are some interesting answers here already. Basically, SC-FDMA is a way of shifting a single-carrier signal in frequency by using a pair of DFT/IDFT with different lengths. – Deve Jul 23 '17 at 9:17

• thank you for your detailed explanation and now I understand the technique. In comparison with $M = N$, it to some extent decreased the PAPR with $M < N$. Many thanks for you~ – Charles Hou Jul 23 '17 at 4:59