On Thu, Jun 2, 2011 at 3:10 AM, Usman Haider <usmanhaide...@gmail.com>wrote:
> 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: > It's been a long time since I've looked at these items, so my memory is a bit fuzzy on these things. But here goes, anyway. > 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 ?? > Yes, I believe you are correct. The extra 4 and 4 comes form the header and CRC. > 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? > > I think 1 is for the preamble and 1 is for that last packet. > 3) How many preamble symbols are inserted for one packet ? I think there > is one preamble symbol for one packet ? right ? > Yes, 1 preamble that is split into 2 internal repetitions used for correlation. > 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 ?? > I can't remember exactly. I think this had something to do with the USRP USB transport issues, and we had to pad it out with this. > 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. > It's like a seed value so that you aren't necessarily using the same whitening coefficients all the time if you don't want to. > 6) Which thing really invokes the call to rx_callback() ? > It's used in gnuradio-core/sry/python/gnuradio/blks2impl/ofdm.py in the ofdm_demod class. This sets up a "watcher" thread called _queue_watcher_thread (line 279), which waits for a message to be appended to the message queue. The message is appended in gr_ofdm_frame_sink when a packet is received. When the watcher thread gets the new message, it ships it off to the callback function for processing. > Thank you very much for precious time. waiting for your response > > > Best Regards > Usman > Hope this helps, Tom
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