In OFDM some of the subcarriers are zero-padded at the edges for oversampling. Say, if there are 256-IFFT bins, then only 128 are used. How can I relate it to the sampling theorem.
OFDM symbol (in the time domain): the entity of all time domain samples that are obtained by taking the IDFT of all subcarriers plus the guard interval.
The OFDM symbol rate is given by
Rs=fs/(Nc+Ng)
where fs is the sampling rate, Nc is the number of subcarriers (equal to IDFT length) and Ng is the number of guard interval samples.
Note that the number of zero-padded subcarriers is irrelevant for the calculation of Rs.
Dear Gokul,
The basic parameter for ofdm system is the symbol time Tsy which is related to sub carrier frequency spacing deltaf. This time interval contains a number of samples equal to the total number of fft points comprising all the sub carriers whether it has a value or is nulled.
Therefore for 256 subcarrires one has 256 samples. In the frequency domain they cover a bandwidth= N deltaf, and in time domain then the sampling time will be Tsa = Tsy/N = 1/ N deltaf and the sampling frequency will be fs= 1/Tsa= N deltaf. The equality of the number of samples and the number of sub carriers is dictated by the equivalence of signal in the discrete time and frequency domain.i
wish you success
Thank you sir, Khalid Al-Hussaini and Abdelhalim Zekry, very much for helping me to understand it.
HI
N-IFFT is like multiplying a symmetric matrix N * N with our input N * 1.
2N-IFFT is like mutiplying a symmetric matrix 2N * 2N with our input 2N * 1. that has N zeros
This N*N matrix is nothing but sample points of the subcarriers say N sample points of N orthogonal subcarriers and 2N* 2N is 2N sample points of 2N orthogonal subcarriers arranged as frequency 0,1/T,2/T...(N-1)/T row wise or colloum wise.
As sample points are increasing it is called Oversampling.
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