On Thu, Jun 2, 2011 at 12:00 PM, Usman Haider <[email protected]>wrote:
> Hi Tom > Thanks a lot. Yes, that did help me :). One more question: what is the > purpose of probe at the end of OFDM chain? > > alpha=0.001 > thresh=30 #in dB > self.probe = gr.probe_avg_mag_sqrd_c(thresh,alpha) > self.connect(self.ofdm_rx, self.probe) > > I do understand that probe computes the " running average of the magnitude > squared of the the input", but I am not getting its purpose at the end of > the chain. > > Best Regards > Usman That was just used as an indicator or the received signal strength but doesn't really serve much of a purpose. Tom > > On Thu, Jun 2, 2011 at 7:03 PM, Tom Rondeau <[email protected]>wrote: > >> On Thu, Jun 2, 2011 at 3:10 AM, Usman Haider <[email protected]>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 >> >> > >
_______________________________________________ Discuss-gnuradio mailing list [email protected] https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
