Re: [STDS-802-16] two simple questions.
Hi Lepidus,
Thank you so much for your helpful input!! Here is my understanding.
1) The BW is (or roughly) the actual bandwidth that the data and pilot sub-carriers occupied. The Fs is kind of virutal bandwidth which includes the guard band at both ends.
2) I am not very clear about why the Fs is NOT the sampling frequency for ADC after the IFFT. For example, Fs/N is the sub-carrier spacing and so its inverse, i.e., N/Fs is the symbol duration (cyclic prefix is not considered here) and (N/Fs)/N is the sampling period and it is actually 1/Fs.
3) why is the bandwidth for data and pilots ROUGHLY equal to BW? some practical considerations?
Thanks for sharing your knowledge with me. I guess your reply can help some others too.
yours,
Zhongshan
On 9/30/06, Lepidus_Chang@mtk.com.tw <Lepidus_Chang@mtk.com.tw
> wrote:
Hello Zhongshan,
For an OFDM (or OFDMA) system whose FFT/IFFT size is N, we usually do not use all of the
N sub-carriers for data and pilots. Instead, we would leave some sub-carriers unused, called
the guard sub-carriers (or guard band), on the left and right side of the data and pilot sub-carriers.
For example, in the WirelessMAN-OFDMA (also known as 16e-2005) with N=1024, there are 86
guard sub-carriers at left side and right side of spectrum.
In this case, in order to utilize the N-point IFFT to generate transmit signal or N-point FFT to demodulate
signal, the sampling rate considered should be Fs. However, the actual null-to-null bandwidth occupied
by data and pilot sub-carriers is usually smaller than Fs and is roughly the value BW in your description.
In order not to be confused by the strange equation, I would suggest you to treat the BW value as the tag
while Fs is the fundamental bandwidth to compose the transmit OFDM signal
s(t) = 1/N \sum_{k=0}^{N-1} d_{k} exp{j 2\pi k Fs t/N}.
e.g. The 5 signal bandwidths considered in the current Mobilie WiMax System Profile (v1.2.0) are
Tag0: BW=3.0 MHz => Fs = 4.0 MHz;
Tag1: BW=5.0 MHz => Fs = 5.6 MHz;
Tag2: BW=7.0 MHz => Fs = 8 MHz;
Tag3: BW=8.75 MHz => Fs = 10 MHz;
Tag4: BW=10.0 MHz => Fs = 11.2 MHz.
In this case, you do not have to remember the value of n and the strange equation between BW and Fs.
Remarks:
1. Please note that Fs is not the sampling frequencies we considered in specifying analog-to-digital converters (ADC/DAC);
2. The actual signal banwidth occupied by data and pilot sub-carriers is also a bit different from the BW.
3. I do not directly answer your two questions, because I think it is easier to interpretate the meaning of BW and Fs this way.
Best regards,
Lepidus.
Hi,
I am reading the WiMax standard 802.16-2004 together with 802.16-2005. Here are two simples questions.
In section
8.4.2.4 of
802.16-2004, the sampling frequency Fs is defined as Fs = floor(n * BW / 8000) * 8000. The parameter "n' is defined as sampling factor. So, what is the purpose to use this sampling factor ? and what do we choose the constant 8000 in the previous definition ?
Maybe these are so simple for you. Please give me some hints. Thanks.
yours,
Zhongshan
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