Re: [Discuss-gnuradio] Viterbi--OFDM

[Tobias Schmid]

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.

Tobias

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).
>
> Achilleas
>
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