Re: [Discuss-gnuradio] Ofdm tramsmitter

Hi GP 2014

On 13.07.2014 21:49, GP 2014 wrote:

Hi,  I hope if someone can help me, i have some questions related to the Ofdm  transmitter block:  - I take the input of the transmitter from a sip phone, i can find from  wireshark that the length of the packet of Sip protocol which contains a  registration request is of 485 bytes and the packet which represents an  invitation (SIP/SDP) to the other sip phone is of 724 bytes, i want to send  both kinds of packets using the OFDM transmitter block.

 I used a udp sink,

You mean a UDP source, to get the network packets into GNU Radio?

  a stream to tagged stream block

Well, you drop all the UDP header information and get a stream of bytes out of the UDP source, so this does not sound like it would work.

 ofdm transmitter block with fft len =128,  rate= 125 Kbps, and a usrp for which i used a UHD sink block. The only way  that made the transmitter to send all the data coming from the sip phone  correctly is the case where, i put the packet length in the stream to  tagged stream block =1, however i put the packet length in the transmitter  parameter =850 or 724, or whatever length.It seems, that it doesn't depend  on the packet length parameter which i enter to the transmitter block, if i  put the same parameters in the receiver block at the other end (the same  fft len ,the same occupied and pilots carriers). can someone explain me the  concept behind that?

The UDP source takes the *content* of UDP packets and treats that as samples. What you want is a network PDU to tagged stream block.

    - Also, i use a tag debug block before and after the transmitter block, i  find that the first one before the transmitter displays 442 successive  bytes each of 1 byte length as the packet length in the stream to tagged  stream block =1, and then the tag debug after the transmitter block  displays 255168 bytes of data  packets each of 576 byte, which means that  the transmitter buffer the packets till reach 442 packets and then output  them each of size 576 byte.why this happens ? why 442 bytes espicially ?  Note: in this case i put the packet length=850. can someone explain me what  happens here ?  

Sorry, I don't really understand your question. Could you rephrase for us?

  -My last question is :in my design for the OFDM system i want to send the  packet on 4 OFDM symbols: here i put the fft_len =128 and specified the  occupied and pilots carriers as:   occupied = (range(-61, -42) + range(-41, -14) + range(-13, -7) + range(-6,  0) + range(1, 7) + range(7, 14)+range(15,42)+range(43,61),)  pilot carriers=  ((-42,-14,14,42 ,),)  does this satisfy to distribute the packet on 4 OFDM symbols ?  

This is a bad idea, generally. Your system is designed to carry network packets, so it should be able to deal with a variable packet length. This means that you will need to add a header to your data on the first OFDM symbol, saying how long this transmission is going to be.
Have you had a look at the payload-header-demux architecture?

  Thanks for your help.    

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