implementation of turbo decoder using max-log-map ... - ijater

International Journal of Advanced Technology & Engineering Research (IJATER)
Fig. 6 Branch Metric Unit.
Fig. 9 Conventional LCU Units
Fig. 7 SMU Unit
2. Normalization / Saturation:
To avoid overflow metrics, Normalization is usually
employed as shown in figure 5 and 6. We have adopted a
very efficient normalization scheme where at each time
instant we check if any of the state metrics is greater than
2,[5] then a fixed value 2 is subtracted from all
state metrics. This is shown by normalization (N) block
shown in figure 5 and 6[3]. The block comprises of a subtractor
that subtracts a fixed value (2) from state metrics
and a multiplexer that selects the subtracted value if
the normalization has to be employed. The multiplexer select
signal is provided by each ACS block and in case of
state serial architecture mappings (states >8) the select
signal is provided after all the states are processed [5].
The LLR values (L1 or L0) are calculated using forward
(α0-7) and backward (β0-7) states and branch metric
(γ0-1) values of all states. The LLR computation unit
(LCU) is similar to the SMU which consists of 3-stage
compare and select process results long critical path delay.
In order to reduce the critical path delay LCU is pipelined
[6].
E. LLR Output Iterations
The output L (dˆ) of the decoder in Figure 10 is made
up of the LLR from the detector, L′ (dˆ), and the extrinsic
LLR output, Le (dˆ), representing knowledge gleaned
from the decoding process. As illustrated in Figure 10, for
iterative decoding, the extrinsic likelihood is fed back to
the decoder input, to serve as a refinement of the a priori
probability of the data for the next iteration [9].
3. LLR COMPUTATION UNIT:
Fig. 10 LLR iteration output values
ISSN No: 2250-3536 Volume 2, Issue 4, July 2012 249