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why LTE does not use all subcarriers

why LTE does not use all subcarriers# EE - 电子工程

c*92008-03-13 07:03

1 楼

In the DL transmission of LTE, not all subcarriers are used. E.g., when FFT
size is 128, only 72 or 76 subcarriers are used. Why is that? Thx.

r*m2008-03-13 07:03

2 楼

My guess is (1) pilot sbucarriers (2) guard subcarriers to prevent ICI. not
sure

S*p2008-03-13 07:03

3 楼

Because the system BW is limited!
The availabe BW configurations are:
1.25MHz, 2.5MHz,5MHz, 10MHz, 20MHz
When FFT size is 128, if you use 75 subcarriers, the
BW will be:
75*15kHz=1.125MHz, which effectively utilize 90% of the
availabe 1.25MHz BW.

FFT

【在 c*****9 的大作中提到】

: In the DL transmission of LTE, not all subcarriers are used. E.g., when FFT
: size is 128, only 72 or 76 subcarriers are used. Why is that? Thx.

c*92008-03-13 07:03

4 楼

Thanks. I think the argument of the BW makes sense.

F*M2008-03-13 07:03

5 楼

This is correct. In order to utilize the FFT size that are powers of two
for implementation simplicity and to maintain sub-carrier spacing wide
enough to prevent inter-carrier interference at high Doppler channels (LTE
uses 15 KHz sub-carrier spacing), oversampling has to be done, which results
in a larger initial bandwidth, which needs to be filtered (i.e. effectived
truncate the edge carriers) to fit the BW requirement.

【在 S*******p 的大作中提到】

: Because the system BW is limited!
: The availabe BW configurations are:
: 1.25MHz, 2.5MHz,5MHz, 10MHz, 20MHz
: When FFT size is 128, if you use 75 subcarriers, the
: BW will be:
: 75*15kHz=1.125MHz, which effectively utilize 90% of the
: availabe 1.25MHz BW.
:
: FFT