1
$\begingroup$

To reduce out of band energy, it is common to apply a Raised Cosine filter to the front and back of an OFDM symbol. It is also common to overlap the windowed portions of adjacent symbols to eliminate amplitude dips.

For an OFDM symbol consisting of a Cyclic Prefix followed by NFFT points from an IFFT, is it common to insert an additional prefix and suffix for the windowed portion? If that isn't done then the CP and the IFFT data are shaped by the window, distorting the constellation. A pretty good description of what I am talking about is here:

http://zone.ni.com/reference/en-XX/help/373725A-01/wlangen/windowing/

$\endgroup$
2
$\begingroup$

Yes, it is necessary to add an additional pre/suffix to the OFDM symbol that corresponds to the window length when windowing is applied. As the length of the already existing cyclic prefix is usually chosen as the maximum delay spread of the channel no further Inter-symbol interference (ISI) must be introduced or otherwise orthogonality is lost. The windowing method described in the link you referenced is introducing additional ISI by overlapping consecutive symbols.

Furthermore, weighting some samples of the OFDM time-domain signal with different factors is a distortion that also distorts the constellation in frequency domain, as you say.

Note that windowing is not filtering. Windowing is the multiplication of the OFDM signal $x_k$ with some window function $w_k$. In contrast, a filter has a memory.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.