SC-FDMA in LTE system

Question

I am trying to understand how SC-FDMA is used in LTE system. Every description I found so far refers to DFT and IDFT.

However, when I checked 3GPP TS 36.211 chapter 5.4 (SC-FDMA baseband signal generation) and 6.12 (OFDM baseband signal generation) I found that there is hardly any difference between given equations (I did not copy the equations here because they are quite long) - OFDMA takes into consideration DC subcarrier, in case of SC-FDMA term "k + 1/2" is used wheres OFDMA uses simply term "k" (k can be denoted as number of subcarrier). Moreover the document does not say anything about neither DFT nor IDFT.

In case of OFDMA, I can imagine how the given equation can be considered as IDFT. But what about SC-FDMA? It looks like that modulated symbol is mapped in between two subcarrier. Can somebody explain me this issue in details?

Edit:

I was interested in how DFT and IDFT can be mapped onto 3GPP TS 36.211. In other words, which part of the standard implies usages of DFT and IDFT. Based on the answer given by AlexTP I understand that DFT usage is implied by chapter 5.3.3, whereas IDFT is described in chapter 6.12 (I am refering to version 13.4 of the standard). My first guess was that both DFT and IDFT are implied by chapter 6.12. Extra information that only PUSCH uses SC-FDMA was very useful.

There are some subtleties though. However, it is out of the scope of this question.

Answer 1

First of all, in refering to standard specification, please mention the version because the numeration between versions can be different.

My answer should be only a comment but I think with images it would be easier for you. Everything in my answer is for 36.211 v8.7.0.

The section 5.6 SC-FDMA baseband signal generation and 6.12 OFDM baseband signal generation are IDFT. The difference is the values of $\left\lbrace a_{k,n} \right\rbrace$. Ignoring MIMO stuffs in DL, in OFDM case, $\left\lbrace a_{k,n} \right\rbrace$ are mapped directly to subcarrier. In SC-FDMA case, $\left\lbrace a_{k,n} \right\rbrace$ are outputs of a supplementary block Transform precoder as in the image below.

This transform precoder is implemented as a DFT and its formula can be found in the section 5.3.3 of 36.211 v8.7.0. It is specified only for PUSCH data (if my memory is correct, among UL physical channels, only PUSCH data uses frequency-generated SC-FDMA).

I hope my answer covers what have been asked in your question. For more details (k+1/2, null DC, ...) I suggest a very good book "LTE – The UMTS Long Term Evolution From Theory to Practice" which is surprisingly can be found by a simple Google search.