SASTRA University
Question
Asked 15th Jun, 2016
In experiment,how to calculate the bandwidth of the OFDM signal?
For a OFDM signal, after DAC with 12.5Gsa/s sampling rate, what is the bandwith? How to calculate?
If the OFDM signal with Hermitian symmetry, what is the bandwith? The Hermitian symmetry will reduce the bandwith?
For a OFDM signal with DAC sampling rate of 12.5Gsa/s, after ADC with 25Gsa/s, what is the bandwith? After down converter, what is the bandwith?
I'm very confused by the bandwith calcluation of the OFDM signal? How to calcluate the bandwith?
Most recent answer
how to calculate minimum and maximum throughput in OFDM scheme?
All Answers (7)
University of Electronic Science and Technology of China
I'm grateful for your answer. I understand your anser as follows. The frequency spacing delta_f=1/Td, Td is the symbol duration. Td=1/Rs, Rs is the symbol rate. After obtained the frequency spacing, the bandwidth can be obtained. Is this right?But the symbol rate is related to sampling rate. Is it right?
University of Electronic Science and Technology of China
I think I have known the bandwidth calculation of the OFDM signal. In order to better understand the bandwidth calculation process for OFDM signal, we should better understand the OFDM theory and the IFFT implementation process of OFDM. The schematic diagram of the OFDM is shown in Fig1. We can see that the OFDM signal cantains the negative frequency carriers ,positive frequency carriers,DC carriers. In IFFT implementation process of OFDM, the negative frequency carriers ,positive frequency carriers,DC carriers distribution is changed. Assumping that the IFFT size is N, the carrier index 0 is the DC carrier, the carrier indices 1~N/2 are the positive carriers, the carrier indices N/2+1~N are negative carriers. On the orther hand, the bandwidth calculation for OFDM baseband signal, the negative carriers are excluded. Thus the bandwidth of the OFDM baseband signal is N/2*delta_f,delta_f is the sub-carrier frequency spacing. The sub-carrier frequency spacing delta_f =1/Ts= 1/(Td * N), Ts is the OFDM symbol duration,Td is the sampling symbol duration and equal to the smapling time interval。Thus Td=1/fs, fs is the DAC sampling rate. Then we can calculate the bandwidth of the fignal. W=N/2*delta_f=N/2/(Td*N)=fs/2. Of course, this calculation process assumes that all carriers are data carriers expect carrier 0.
KU Leuven
In an article we wrote ten years ago, "Intra-Symbol Windowing for Egress Reduction in DMT Transmitters", the relation between the carrier spacing, symbol length and the spectrum is elaborated. Maybe this can help you.
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