How FFT size affect the signal?

[Ryan]

Hi Experts.

In 5G 2x2 we use on phy side FFT with size 2048. Is it spec definition?

Why not let’s say 4096?

How FFT size affect the signal and if there any minimum we shall avoid it for not damage the signal?

Admin note: this post was updated with image below.

[Vijay_Varma]

As per my understanding:

FFT size depends upon the number of sub carriers.

As in 5G max possible sub carriers is 3300 (max 275RB*12) that’s why FFT size will be 4096 to cover 3300.

3GPP TS 38.104

Please correct me if my understanding is wrong.

[Ryan]

Hmmm, got you!

But why with 5G 2x2 MIMO then 2048 FFT size used and not 4096 then?

[Sakshama_Ghoslya]

FFT side is not at all related to MIMO config.

It only depends on number of subcarriers.

Therefore it only depends on Bandwidth & Subcarrier Spacing.

[Ryan]

Thanks, got it.

Now assume that I used FFT size less than number of subcarriers, then what will happen to signal?

Data loss?

[Vijay_Varma]

If you use less FFT size 2048 for mapping 3300 sub carriers then obviously remaining subcarriers can’t be mapped.

It means out of 100 Mhz BW you are able to use only 2048 subcarriers instead of 3300.

[Sakshama_Ghoslya]

@Ryan

Now assume that I used FFT size less than number of subcarriers, then what will happen to signal?

Data loss?

2 things that happen in that case:

• At transmitter, you are not using all the subcarriers, they are mapped with zero, this decrease your sampling rate at the transmitter.

• 2nd is your data mapping to subcarriers becomes different than what is expected at the receiver end, since receiver might be using the right mapping.
  For symbol zero it might be correct but not with remaining symbols,
  So nothing will be decoded.

[Ryan]

@Vijay_Varma

If you use less FFT size 2048 for mapping 3300 sub carriers then obviously remaining subcarriers can’t be mapped.

It means out of 100 Mhz BW you are able to use only 2048 subcarriers instead of 3300.

So its like using 50 Mhz bandwidth?

@Sakshama_Ghoslya

2 things that happen in that case:

• At transmitter, you are not using all the subcarriers, they are mapped with zero, this decrease your sampling rate at the transmitter.
• 2nd is your data mapping to subcarriers becomes different than what is expected at the receiver end, since receiver might be using the right mapping.
  For symbol zero it might be correct but not with remaining symbols,
  So nothing will be decoded.

Got it, thanks!

First point understood.

Second point not totally… Why receiver may won’t be able to decode successfully?

[Sakshama_Ghoslya]

You have 8000 IQ samples for an examples to be transmitted.

You were suppose to transmit with 100 MHz bandwidth and 30 kHz subcarrier spacing = 3276 subcarriers.

So you were ideally mapping/transmitting 3276 out of 8000 data samples at ofdm symbol 0
And next 3276 (3277 to 6556) at next ofdm symbol 1.

But now you decided to use lower number of subcarriers let’s say not 3276 but 2048.

So you will map/transmit 2048 out of 8000 at ofdm symbol 0 and next 2048 (2049 to 4096) at ofdm symbol 1.

So the subcarriers at ofdm symbol 1 specially that were supposed to carry right data now carrying something else.

And this thing is not known to the receiver.

If you reduce your data mapping.

Let’s say out of 3276 you are going to use middle 2000 subcarriers only in every case.

Then it won’t be an issue.

Only issue will be using the lower sampling rate.

And receivers need not to have the same sampling rate at transmitter.

[Vijay_Varma]

@Ryan

So its like using 50 Mhz bandwidth?

It depends upon SCS for 30 kHz yes.

But for 15 kHz 4096 FFT size required.