Understanding DFT-s-OFDM

Published On: Jun 27, 2021

I was reading a comment on one of my youtube video on DFT-s-OFDM which asked about subcarrier mapping. Thinking of that I got one question myself that how exactly PAPR reduction is achieved in DFT-s-OFDM.

In this article, I will try to answer both of these questions.

OFDM has many advantages but few disadvantages also. One of them is high PAPR (Peak to Average Power Ratio).

First, let's try to understand why low PAPR is needed in communication systems.

There are multiple advantages of low PAPR

  • Low PAPR allows PA to operate with less nonlinear distortion at same output backoff (OBO)
  • Lower PAPR signals can exploit more power efficient PA operating point: Increase Power Efficiency
  • Lower PAPR signals can operate PA at higher average power at same distortion level​ : Improve Cell Edge Performance

This figure shows the relation in Input and output power of a Power Amplifier (PA) and also explains the above three points.

The DFT-s-OFDM transmitter chain is as follows:

The two extra blocks from an OFDM Transmitter chain are 

  1. DFT block
  2. Subcarrier Allocation (or mapping)
In the DFT-spreading process, n subcarriers are divided into k sub-bands and all subcarriers in each sub-band will goes through m-point DFT. Therefore, you can easily understand that, n=k×m.
  •  After the DFT on each sub-band, the n subcarriers will be re-allocated into a vector.
  • Subcarriers are allocated one sub-band by one sub-band → localized allocation.
  • Subcarriers are chosen from different sub-band in turn, or in other word, subcarriers are allocated interleaved → interleaved allocation.

I ran a MATLAB simulation to see the PAPR comparison between OFDM and DFT-s-OFDM different techniques and the plot is as follows:

The above plot clearly shows that DFT-s-OFDM has lower PAPR than OFDM and PAPR is lower for lesser number of Sub Bands.

To further understand the PAPR the CCDF(linear) curve is as follows

To answer the question on "how exactly PAPR reduction is achieved in DFT-s-OFDM", I have explained this with an easy example in this videohttps://youtu.be/f7UWbazSfWk 

For any questions or doubts, you can either put a comment on the above video or send an email to sigpcomm@gmail.com.