hard and soft demappers

[rmreddy]

soft demapper

hi all,

can anyone explain me whats the differnece between hard and soft demappers.
which has the better performance and lower BER.

 

[amihomo]

soft demapper algorithm

hard demapper works on symbol by symbol basis
soft demapper uses the information from previous decisions (e.g. uses the viterbi algorithm)

the soft demapper has lower BER , hence better performance , but it is more complex

 

[bulx]

soft demapper

"amihomo" has described soft decoding, where a decision on a recived symbol is not taken immediately after receiving one symbol, but a whole sequence is examined before a decision is taken (viterbi decoding being the prime example).

However, I guess what the OP is asking about is soft demapper. The demapper associates the recieved signal to a constellation point. In hard decoding, it choses the nearest valid constellation point and decalres that as the recieved symbol. When soft decoding is done, the demapper gives out a set of values (each being a few bits), each being the quantised distance of the recieved signal from all, or a subset of, valid constellation points. The ECC decoder that follows the demapper usually gives a better performance if soft info is used.

So soft decoding can be done with both soft and hard demapping.
-b