Need Help: Subcarrier frequency in OFDM

[srk.amt]

Hi,

I want to know frequency of each subcarriers of OFDM symbol in matlab.

After doing IFFT operation, is there any way to know frequency of each subcarriers.

Is there any other method?

Please reply.

Thanks.

 

[Mityan]

1. Please check out: **broken link removed**
If you want to know the particular frequencies of all subcarriers, the table in the above link contains the values of subcarrier spacing, so in baseband representation these frequencies will be (0...fft_length-1)*spacing.

2. If you input data sample period is T, then after splitting to N channels you get symbol rate = 1/(NT). So this is the frequency spacing. Frequencies are from 0 to N-1.

 

[srk.amt]

1. Please check out: **broken link removed**
If you want to know the particular frequencies of all subcarriers, the table in the above link contains the values of subcarrier spacing, so in baseband representation these frequencies will be (0...fft_length-1)*spacing.

2. If you input data sample period is T, then after splitting to N channels you get symbol rate = 1/(NT). So this is the frequency spacing. Frequencies are from 0 to N-1.

Hi Mityan,

Thanks for your suggestions but i want to do it in Matlab and in matlab for ifft, direct command is present then how can i know the frequency of each subcarrier generated by ifft command.

 

[Mityan]

Hi Mityan,

Thanks for your suggestions but i want to do it in Matlab and in matlab for ifft, direct command is present then how can i know the frequency of each subcarrier generated by ifft command.

According to the given formula. In matlab you just take a vector and do the ifft, but in real world you have data at some rate (period T). So you may estimate those particular frequencies.

 

[srk.amt]

Thanks to Mityan and humourmind for your prompt reply.

Now i need to use null subcarriers in ofdm. How can i use it. Actually i am confused because when we perform ifft on modulated symbol then we get ifft data and this data doesnt contain any null values.

How can i use null subcarriers?

 

[Mityan]

IFFT input is your channel signal frequency domain representation. Clearing some of the ifft input values (after mapping your data to constellation points according to the chosen modulation type) just do the ifft and dont look that the result have no zeros. This samples are of time domain and flushing them consequently do DAC gives the OFDM channel symbol, which spectrum will look like ifft input. So there must be zeros in frequency domain during one channel OFDM symbol.