21-08-2012, 04:09 PM
TURBO DECODERS USING VLSI IMPLEMENTATION
TURBO DECODERS.ppt (Size: 1.04 MB / Downloads: 52)
WHY TURBO CODING?
LDPC’s replacement
LDPC has set of disadvantages
Inflexibility
Large encoding
Utilizes forward error correction
Used in high-speed wireless systems
TURBO CODING
Redundancy introduction
Original data attained from the output
Achieve nearest shannon performance
-Greater coding gain
2 standards
- 3gpp
- Ccsds
Low signal to noise ratio
Simple hardware implementation
ITERATIVE TURBO DECODER
Output sent back to siso1
Here soft output detection takes placeSoft bit
-Probability of getting ‘0’ or ‘1’
Via interleaver soft output is sent to siso2
The estimation of soft outputs from siso1 and
parsing and channel metric block.
Output is sent to siso1 via de-interleaver
SISO1 has more information
accuracy is achieved in a better way
The process is continued
-Fixed number of times
-Criteria is satisfied
MAP ALGORITHM
Maximum A Posteriori algorithm
calculation of soft outputs
weight of each data symbol is calculated
-forward direction
-reverse direction
results in forward and reverse metrics
probabilities are compared
With respect to the higher probability the ‘0’ or ‘1’ is
ransmitted
reduction of computation is done
log-likelihood ratio or LLR
LLR=probability of receiving ‘0’
probability of receiving ‘1’