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I was trying to understand what exactly happens when you try to modulate an ofdm symbol in the downlink in LTE for multi-user case.

When we modulate, usually we allocate different sets of RBs for different users. Since an RB is 12 subcarriers in frequency, a given ofdm symbol (say 2048 SCs in a 20 MHz case) can contain data from different users. And we do take IFFT of this 2048 sized data vector comprising of different user data. Is that right?. OR do we append 2048-12 zeros to a specific user data and then take IFFT?

Thanks in advance.

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  • $\begingroup$ What is RB in your question? $\endgroup$ – BlackMath Sep 8 '19 at 16:22
  • $\begingroup$ Resource Block - It is an LTE terminology $\endgroup$ – Jyotish Robin Sep 8 '19 at 16:26
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The zero padding (or the cyclic prefix) takes care of inter-symbol interference that occurs due to delay spread. It is hence something you append to your signal in time domain. Hence the order is this:

  1. Assemble all the data symbols along the subcarriers (can contain data for multiple users, as you said).
  2. Compute DFT (via FFT, typically)
  3. Add cyclic prefix (or zero padding)

Then transmit.

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