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Re: [Discuss-gnuradio] Ofdm tramsmitter

From: GP 2014
Subject: Re: [Discuss-gnuradio] Ofdm tramsmitter
Date: Tue, 15 Jul 2014 05:05:22 +0200


2014-07-15 4:22 GMT+02:00 Sara Chérif <address@hidden>:

---------- Forwarded message ----------
From: Marcus Müller <address@hidden>
Date: 2014-07-13 22:37 GMT+02:00
Subject: Re: [Discuss-gnuradio] Ofdm tramsmitter
To: address@hidden

Hi GP 2014

On 13.07.2014 21:49, GP 2014 wrote:
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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thanks, for your help.
According to the fourth part of my question: i put two tag debug blocks, one after the transmitter block (Name: apres txer)and one before it (Name:avant txer),i get the input from a file source where i wrote ("this is a test message.")which is a 23 bytes file, then i used a stream to tagged stream block with packet _len=1, then connect it to the transmitter input. When i put the fft_len=64, the transmitter outputs also 23 packets but each is of 288 bytes, the tag debug which is after the transmitter block displays for example for the first packet :
 offset:0  source:n/a  key:packet_Len  value:288.
also when i try for fft_len=128, each packet at the output of the transmitter is of 576 bytes.
It seems to me that each input byte has been outputted 288 bytes at the output of the transmitter, is this true ? why this happens? i know that the crc32 block adds 4 bytes to an input packet, what other blocks in the transmitter architecture add all of these bytes? i hope the question is clear now.
Note: i attached a screenshot of the output displayed by the tag debug blocks and the grc file i used.
thanks for your help

Attachment: test.grc
Description: application/gnuradio-grc

Attachment: Selection_015.png
Description: PNG image

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