因为对OFDM中时域和频域的处理有些困惑，查到网上有篇如下的问答，可以帮助理解，所以转载至此。
问：
Hello,
I was following the chart below to understand the basics of LTE bandwidth configuration and put me in total confusion.
Could you please assist me in understanding the questions below. My apologizes if questions are too naive for experts in this community.
I want to understand 20Mhz FDD to start with and hopefully the rest will be easily followed. As seen in the chart below
1) what does sample rate( Mhz) mean/ signify?
2) how does sampling rate 30.72( 2048*15Hz) fit within 20Mhz bandwidth ?
3) 100 Resource blocks = 1200*15khz = 18Mhz.. Where is the ramining 2 Mhz( 20-18mhz) gone ?
4) 2048 IDFT size. Does it mean the total subcarriers as 2048 ?
5) what is FFT /IFFT/DFT/IDFT size mean/signify ?
6) 1200 number of occupied subcarriers. what are the unoccupied carriers( is it 2048-1200 = 848 ?)
7) if I imaging that the 20Mhz bandwidth allocated as 1760-1780(Rx) and 2160-2180( Tx). The subcarriers generated by IFFT operation should be integral multiples . Therefore what will be the individual frequencies in time domain if 2048 subcarriers are generated.
答：
Hello,
Let me try to answer the questions. Please correct me if needed.
4) 2048 IDFT size. Does it mean the total subcarriers as 2048 ?
>> Yes total sub-carriers are 2048 but only 1200 are occupied. Rest are zero padded.
5) what is FFT /IFFT/DFT/IDFT size mean/signify ?
>> IFFT size means number of sine waves the IFFT takes as input to construct the time domain signal. In case of 20 MHz the IFFT size is 2048 that means 2048 sub-carriers are used to construct the time domain signal. (Note: not all are occcupied).
6) 1200 number of occupied subcarriers. what are the unoccupied carriers( is it 2048-1200 = 848 ?)
>> Occupied carriers correspond to number of Physical resource elements. In 20MHz there are 1200 PREs. Unoccupied sub-carriers are zero padded.
3) 100 Resource blocks = 100*180( 12*15khz) = 18Mhz.. Where is the ramining 2 Mhz( 20-18mhz) gone ?
>> I think 2 MHz is the guard band.
1) what does sample rate( Mhz) mean/ signify?
>> Sample rate calculation for 20 MHz.
Let me first give you the thumb rule of OFDM system: The symbol period is reciprocal of the sub-carrier spacing. So in case of LTE where 15KHz is sub-carrier spacing the symbol length is 1/(15KHz).
2048 time samples from IFFT module fit into symbol period of 1/(15KHz). The sampling frequency hence is (15KHz * 2048) for 20 MHz.
Similarly by changing the number of sub-carriers you will get the sampling freq for other band-widths
2) how does sampling rate 30.72( 2048*15Hz) fit within 20Mhz bandwidth ?
>> Sampling rate is in time domain.
Regds,
Raj.
转载自 http://lteuniversity.com/ask_the_expert/f/59/t/2636.aspx
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