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Re: [Discuss-gnuradio] How to decode the convolutional code, in the GRC

From: Achilleas Anastasopoulos
Subject: Re: [Discuss-gnuradio] How to decode the convolutional code, in the GRC
Date: Mon, 14 Mar 2011 10:11:44 -0400
User-agent: Mozilla/5.0 (Windows; U; Windows NT 6.0; en-US; rv: Gecko/20110303 Thunderbird/3.1.9


as i said in my previous email, you just have to consider the equivalent
channel (the fact that it is noiseless is irrelevant in this discussion)
from the output of the FSM encoder to the input of the VA.
It seems you prefer to call it equivalent "modulator"; that's fine!
Once you realize what this equivalent channel/modulator is
(what is its input/output alphabet and what kind of symbols it
inputs/outputs) then it will be strainghtforward to see what the viterbi
decoder block should look like.
If I were to guess from your description, i would say that your
equivallent channel
has input/output alphabet equal to the output alphabet of the FSM.
Its input is a coded symbol from the FSM encoder and its output is also
such a symbol from the packet decoder.
So all you need is to do VA with symbol-wise hamming distance as your
metric. You can achieve that using the viterbi_combined block with
a trivial 1-D chunks to symbols block (identity) and the appropriate
parameter for the metric_type.


On 3/14/2011 6:45 AM, intermilan wrote:
> Achilleas:
> What I want to do is want to see the BER when the convolutional code is 
> used in different modulator in the GRC,but I had not put the noise 
> source in the flow graph so there is no noise between the modulator and 
> demodulator.
> Besides,in my opinion, we can consider the packet_encoder and 
> packet_decoder block is part of the modulation.In other words,in the 
> flow graph I mentioned ,the data input the packet_encoder is the same as 
> the packet_decoder block(because I did not put the noise block between 
> the modulator and demodulator ).
> But I still not understand how to set the parameters in the viterbi 
> block in this case, particularly the parameter constellation if I do not 
> use the chunks_to_symbols block.
> Thanks for your help.
> inter
>  > Date: Fri, 11 Mar 2011 17:33:25 -0500
>  > From: address@hidden
>  > To: address@hidden; di! address@hidden
>  > Subject: RE: [Discuss-gnuradio] How to decode the convolutional code, 
> in the GRC
>  >
>  >
>  > inter,
>  >
>  > think of whatever is AFTER the trellis encoder and BEFORE the viterbi
>  > decoder as an equivalent channel.
>  >
>  > What is the equivalent channel in your case?
>  > What is its input alphabet? what is its output alphabet?
>  >
>  > From your description, it seems that the input alphabet is the same as
>  > the output alphabet of your FSM.
>  >
>  > I am not familiar with the block "packet decoder" so i don't know what
>  > it outputs, but whatever it is it should be the output alphabet of your
>  > "equivalent" channel.
>  >
>  > Once you figure out these things it will be straightforward to see what
>  > the viterbi block should be...
>  > If you gather these pieces of information and still don't see how to
>  > implement the viterbi decoder let me know ! and i will provide 
> further help.
>  >
>  > Achilleas

Achilleas Anastasopoulos                        
Associate Professor
EECS Department               Voice : (734)615-4024
UNIVERSITY OF MICHIGAN        Fax   : (734)763-8041
Ann Arbor, MI 48109-2122      E-mail: address@hidden
URL: http://www.eecs.umich.edu/~anastas/        

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