In fact, the syndrome is a characteristic of the error pattern.
Another topic related to decoding of the FEC code words is erasure decoding [9]. The FEC decoding
operation can be improved if the decoder can recognize a bit of questionable reliability and then
declare that bit as an erasure. Here a bit position is identified and an exhaustive search among the possible
combinations of bits is conducted.
Recall the number of detectable errors is given by dmin 1, then a pattern of or fewer erasures
can be corrected if the following condition is met
(5.44)
Consider this practical example, the received signal strength (i.e., RSSI) is sampled and used as
channel measurement information to identify unreliable (or questionable) bits. The purpose here is to
weaken their contribution in the decoding phase, because these bits are most likely in error. Once
erasures are selected then the FEC decoding operations are conducted for 2 iterations.
In fact, this is called ???Chase??™s Second Algorithm??? and is given below
Step 1: Determine the bits (actual positions within the code word) with the lowest signal strength.
Step 2: Apply 2 patterns to the erasure bits and decoding is performed for each candidate.
b dmin 1
+
1
S0 +
1
S1 S2 y(p)
2
1
Output
Syndrome
FIGURE 5.
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