help needed in viterbi decoding

[shameem]

hi,
i have implemented a viterbi decoder with constraint length 7. My input buffer to the vitebi decoder is of length 768 bits. When i traceback after processing all the 768 bits, the output of viterbi is correct, but when i traceback after processing 64 bits at a time, a few bits in the output of viterbi are corrupted. I think in both the cases, the result should be same untill my traceback lenth is 5 times the constraint length.
If anybody have any idea, please help me.

thanks

shameem

 

[navid1314]

salam

How many bits are corrupted?
traceback length of 5 times the constraint length makes the error rate low enough, but the more traceback length , the less error bit rate

bye
navid

 

[shameem]

hi,
two or three bits at the boundry of the trace back lengths (i.e 64 at the discontinuity of the traceback and decoding) are corrupted. if i process only 64 bits at a time and traceback and decode upto 73 no of bits(i.e i overwrite some of the output bits) the output is correct. But i do not want to do unnecessary calculation to overwrite the output buffer.

 

[navid1314]

Hi

if the signal have memory of L, you can make a decision on a symbol after 5L, it means if L=7 and your buffer length is 64, you can only peak 29=64-5L symbols after doing the viterbi procedure on 64 symbols, then you shift them left and get in 29 new symbols from right.

 

[jimmylin0226]

The concept of the tracebakc is similare to "sliding window".
That's to say,
Assume constraint length=7,buffer for 64 bits.
Some textbooks said that 5 times constraint length is good enough for traceback to reach some reasonable BER.
So u first traceback 35 stages and then find a merge point.
From the merge point to the final stage, u can get 29 decoded bit.
After this, u store another 29 bits and do the same procedure as before.

 

[navid1314]

thanks jimmy
you explain better than me, what i've in mind.

 

[shameem]

hi,
guys thank u very much for the discussion. But i think u people have taken me wrong. My input buffer to the viterbi decoder is of length 768 bits. To avoid maintain the history for 768 bits, i want to process the input buffer in the bunches of 64 bits. In this way i have to keep the history for only 64 bits.

thanks

shameem