Why do we need an oversampling in OFDM signal? and how to do it? If around half of the Sub-carriers are only used to send data, then do we need to perform oversampling to the signal? Is it necessary to have LPF in OFDM transceiver?

  • $\begingroup$ Could you provide some more detail about what you're asking specifically? Sampled OFDM signals require the same Nyquist criterion that any other discrete-time signal would in order to avoid aliasing. I would say that in general, though, using oversampling in OFDM systems wouldn't be typical. $\endgroup$ – Jason R Feb 18 '14 at 13:02
  • $\begingroup$ I have seen some implementation of OFDM where they insert an interpolation and windowing block. I don't understand the need for this block? Is it a must to have it or I can eliminate it if I am using less number of data sub-carriers compared to IFFT size? I am new to communication field so can you please clarify to me what to do with OFDM signal after IFFT block in transmitter side if I am going to send it at IF or RF frequency. $\endgroup$ – Sara Feb 18 '14 at 13:13
  • $\begingroup$ If possible you should provide a reference to what you're asking about then so it will be easier to provide an answer. $\endgroup$ – Jason R Feb 18 '14 at 13:16
  • $\begingroup$ In the book of "OFDM for Wireless Communications Systems" by Ramjee Prasad 2004. There is a block diagram where they include windowing block. $\endgroup$ – Sara Feb 18 '14 at 13:19
  • $\begingroup$ Also, in this reference they are using raised cosine function cresis.ku.edu/~rvc/documents/862/862_ofdmreport.pdf $\endgroup$ – Sara Feb 18 '14 at 13:20

Oversampling in OFDM is usually implemented by modulating some subcarriers at the spectrum margins with zero. This zero-padding in frequency domain corresponds to oversampling in time domain. If only 50% percent of the subcarriers are used, then this corresponds to an oversampling factor of 2. Specifically, if the total number of subcarriers is $N$ and if $M$ of these subcarriers are set to zero, then the oversampling factor is $N/(N-M)$. Note, that the $M$ zero subcarriers have to be arranged in a row, cyclic shifting allowed. Normally, they're arranged symmetrically at the lowest and highest frequencies.

An OFDM transmitter might require a low pass filter in order to keep out-of-band radiation low. The cut-off frequency is usually half of the sampling frequency ($f_\mathrm{s}$).

An OFDM receiver requires a low pass filter before A/D conversion (sampling) with cut-off frequency $f_\mathrm{s}/2$. Otherwise aliasing can occur.

| improve this answer | |
  • $\begingroup$ So, the LPF is optional in transmitter to remove out of band radiation while its a must in the receiver to avoid aliasing? In a transmitter the LPF is placed before DAC and in receiver before ADC? $\endgroup$ – Sara Feb 19 '14 at 11:32
  • $\begingroup$ I would say the transmitter LPF is optional, yes. The filter is analogue and has to be placed after the DAC and before the ADC. $\endgroup$ – Deve Feb 19 '14 at 13:58
  • $\begingroup$ I am confused why in a transmitter the LPF is placed after the DAC? can you please clarify. In the book I referred in my previous comment in section 5.2.1, figure 5.1 they are putting windowing block before the DAC! many thanks $\endgroup$ – Sara Feb 19 '14 at 15:26
  • $\begingroup$ The LPF that I'm talking about is an analogue device that suppresses alias spectra caused by the digital-to-analog conversion. It therefore must be placed after the DAC. The windowing block in the figure you're referring to is for spectral shaping. $\endgroup$ – Deve Feb 19 '14 at 16:13
  • $\begingroup$ Thanks. Just to clarify if I got it right. In this case the windowing block is optional in the transmitter while the LPF after the DAC is a must to avoide aliasing? or the analog LPF after the DAC is also optional since we are using LPF in receiver before the ADC? $\endgroup$ – Sara Feb 19 '14 at 17:00

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.