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[Discuss-gnuradio] Ofdm tramsmitter

From: GP 2014
Subject: [Discuss-gnuradio] Ofdm tramsmitter
Date: Sun, 13 Jul 2014 21:49:33 +0200

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, a stream to tagged stream block 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?

- 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 ?

-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 ?

Thanks for your help.

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