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Re: [Discuss-gnuradio] Viterbi--OFDM

From: Tobias Schmid
Subject: Re: [Discuss-gnuradio] Viterbi--OFDM
Date: Sat, 11 Sep 2010 20:18:28 +0200
User-agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv: Gecko/20100826 Thunderbird/3.0.7

Hello Achilleas,

that's exactly what I thought abaout as well. Because the part I discribed as channel in my last mail is a wireless transmission using usrp2. Not using channel coding, I have packet error rates of 1 to 2 % using bpsk subcarrier constellation and abaout 18 % using qpsk. And if I evaluate the packet error rates not as a mean value, but in smaller periodes, there are periodes where the viterbi correcty almost every error, but there are also periodes in which there are just erroneus packets produced.

So as you said, I thought about using an interveaver to reduce the problem of burst errors. If that doesn't work, it's no bigger problem, because I've implemented a selective repeat arq as well, so using this protocol, I'm getting good or even better performances, due to the reduced amount of data to transmit.

So thanks for your quick help, I'll try this out, when I'm back a university on monday.


Am 10.09.2010 20:47, schrieb Achilleas Anastasopoulos:

My guess is that the inner channel (ie the combination of OFDM modulator/channel/OFDM demodulator) is producing big bursts of errors. Essentially either the packet is correctly received or completely erroneously received. In that case the outer Convolutional code cannot do much; on the contrary it deteriorates performance because of the SNR loss due to coding. One way to verify this hypothesis is to measure the error statistics of the inner channel.

The way to improve is to interleave before the inner channel with sufficient depth (multiple OFDM symbols).


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