Re: [Discuss-gnuradio] How to count number of samples in a packet?

[Michael Dickens]

Hi Dara - Welcome to GNU Radio! Sounds like you have a fun project to get yourself motivated.

It sounds like what you're looking for is, when using the provided tx_ofdm.grc flowgraph, how to convert from an input packet of data of some arbitrary length into the OFDM frames so-as to add on the USRP tags for burst Tx ... yes?

Assuming you're using the default values (48 carriers, 3 header frames, CRC32) along with (as you say) BPSK for header & QPSK for payload & FFT size == 64, then you end up with the following equation for an input data of length "packet_len":

#output OFDM frames = int(ceil(float((packet_len+4)*8)/float(48*2))+3)

--> where the "4" is for the CRC32 (4 bytes); the "8" is bits / byte; the "48" is the number of active carriers (out of a possible 64, though the DC bin isn't used so really 63 of fft_len-1); the "2" is the number of bits / payload symbol (QPSK -> 2; so if you used 16QAM then this would be 4 instead of 2), and "3" is the number of header OFDM frames (regardless of the constellation chosen: bpsk, qpsk, whatever).

You can multiply the output OFDM frames value by "fft_len+cyclic_prefix_len" (which for fft_len == 64 -> cyclic_prefix_len == fft_len / 4 == 16 ==> == 80 total by default) to get the total number of output complex symbols to add the burst tags too.

I've used this equation to compute sample rate changes and it is correct. Is this what you're looking for? Hope this helps & good luck! - MLD

On Thu, Jan 18, 2018, at 5:16 AM, Khajavi, Dara wrote:

I am new to gnuradio and don't know much about it. I  would like to make a burst transmission system based on OFDM. Regarding the previously discussed emails, I learned that the first and the last samples in a burst should be tagged with "tx_sob" and "tx_eob" , but I don't know how to count the number of samples I have in one packet. Could anyone kindly help me with that? I actually want to send one packet per each burst. The FFT size is 64 and sampling rate is 1MSps. Payload is qpsk modulated and packet header is bpsk modulated 

I am using the already exist tx_ofdm.grc file but I added the 2 Tag Strobes and one Delay block to see if the tags are recognized by the USRP X300.

Thanks a lot in advance