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[Discuss-gnuradio] Viterbi--OFDM

From: Tobias Schmid
Subject: [Discuss-gnuradio] Viterbi--OFDM
Date: Thu, 9 Sep 2010 16:10:12 +0200 (CEST)


I've got another question. I implemented an OFDM-transmission eviroment and now I additionally added channel coding. It works fine, but when using trellis modulator and viterbi decoder, the packeterrorrate increases.

I realized the blockbuilding, using a message_queue counting the incomming items and when reached a certain limit, push it into an message source.
To realize the defined initial and final state, I added zeros at the end of each block using the python --struct-- module. These data blocks are converted to chunks and handed over to the trellis_modulator.
The outcoming chunks are, depending on there number of bits packt to shorts to transmit it using the ofdm_mod block.

Obn the receiver side the recieved block is unpackt using the inverse operation of the block after the trellis_mod und the tcm output symbols are handed over to the combined_viterbi afterwards packt to the original data stream. This stream is catched by a message queue as well and packets of the original length are reconstructed.

So my question is, what did I do wrong, to get an packet error rate, that is worse than the uncoded transmission. It seems to me, that some times in the transmission the viterbi works fine and corrects all the errors, but sometimes, it doesn't output any correct data.
Is there a solution to this problem?

Thanks alot for your help.


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