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.
Zhongshan Wu <wuzhongshan@GMAIL.COM>
2006/09/30 06:45 AM
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主旨： [SPAM] [STDS-802-16] two simple questions.
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.
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