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davyzhu
Joined: 23 May 2004
Posts: 521
Helped: 3
Location: oriental
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 06 Apr 2006 7:27 Preamble and pilot in OFDM WLAN? |
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Hi all,
I am new to OFDM WLAN.
What's Preamble and pilot difference in OFDM WLAN? It seems they are all for synchronization.
I read a "book" about softwear defined radio and they said pilot is to estimate the channel's phase shift.
But what's Preamble's function?
Is there some fundamental book on paper on this subject?
Best regards,
Davy
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mshareef
Joined: 18 Feb 2006
Posts: 203
Helped: 45
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| 06 Apr 2006 10:47 Re: Preamble and pilot in OFDM WLAN? |
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The preamble field is present for the receiver to acquire an incoming OFDM signal and synchronize the demodulator. The preamble consists of 12 symbols. Ten of the symbols are short for establishing automatic gain control (AGC) and the coarse frequency estimate of the carrier signal. The receiver uses the long symbols for fine-tuning. With this preamble, it takes 16 microseconds to train the receiver after first receiving the frame.
For more details: http://www.commsdesign.com/story/OEG20010122S0078
I have found a book on OFDM and CDMA at http://download.hadown.com/uploadfile/2005/12/12/18223231335.zip
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shedeed
Joined: 26 Sep 2004
Posts: 70
Helped: 1
Location: Egypt
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| 09 Apr 2006 1:56 Re: Preamble and pilot in OFDM WLAN? |
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in general: A preamble is used in Digital Communication Systems to train the VCO of the receiver to the incoming signals's clock so as to produce a clocking in the reciver that is synchronized to the received signal, and so a perfect sampling and/or demodulation can be done.
Secondly: The pilot symbols are used in wireless communication systems for the sake of channel estimation and correction purposes.
Where the wireless channel have great effect on the phase and the frequency of the wireless signal and alters them by some value, which have many bad side effects and may affect the process of demoudulation
I can list some of these effects as :Phase rotation, doppler frequency shift, degradation of the amplitude and phase distortion.
All of these cause poor SNR,SIR and one important thing is it may alter the place of the frequncy of some subcarriers in an OFDM which may cause the loss of othogonality between them and the OFDM symbol is destructed.
So, at the transmitter a well known symbol (its freqency,amplitude and phase) is inserted among the subcarriers to carry the effects of the channel, and at the receiver it demodulated and then all the effects are calculated and the received signal is corrected by those calculated amounts
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