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[Discuss-gnuradio] OFDM Help


From: Usman Haider
Subject: [Discuss-gnuradio] OFDM Help
Date: Thu, 2 Jun 2011 12:10:23 +0500

Hi all,

I am working on the OFDM in GnuRadio. After reading the code files and going through the presentation on the OFDM implementation I did understand most part of OFDM. But I still have few questions to ask:

1)   how symbols_per_packet is calculated ? I know the following formula 
          symbols_per_packet = math.ceil(((4+options.size+4) * 8) / options.occupied_tones)
      but why 4+4 ? why these 8 bytes are added to calculate the symbol per packet?
      If these 8 bytes are for packet header and CRC32 ??

2)  Why 2 is added to following formula while calculating samples_per_packet ?
          samples_per_packet = (symbols_per_packet+2) * (options.fft_length+options.  cp_length). If this 2 is for preamble symbols?  

3)  How many preamble symbols are inserted for one packet ? I think there is one preamble symbol for one packet ? right ?

4)  What is the format of OFDM packet transmitted. I got the following idea after reading the code
      ------------------------------------------------------------------------------------------------------------------
      packet Head(4bytes)  | payload(option.size bytes)   | CRC32(4bytes)  | something extra(1byte)  
      --------------------------------------------------------------------------------------------------------------------
      my question is what is the purpose of packet head ? And what is this last byte for ? I know this byte comes from the following code
         pkt_dt = ''.join((payload_with_crc, '\x55'))
         what is its purpose ??

5)   I know from the source code that whitening is done in order to ensure transition in data. am I right? But what is effect of variable "whitener_offset" ?it is set to 0. 

6)    Which thing really invokes the call to rx_callback() ?


Thank you very much for precious time. waiting for your response


Best Regards
Usman

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