Continue to Site

Welcome to

Welcome to our site! is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

QAM demapping to LDPC decoder

Not open for further replies.


Advanced Member level 1
May 23, 2004
Reaction score
Trophy points
Activity points
soft bits demapping

Hi all,

LDPC decoder use soft information. But how to get the soft information from QAM?

BTW, DVB-S2 use ASK demapping, is it suitable for soft information demapping?

Best regards,

haykin is the solution of ur answer

QAM soft demapping is independent to the channel coding (LDPC Viterbi, Turbo). The best performance is LLR, using the max-log approximation, the LLR is equivalent to the slicer as the classical hard decision boundary, the only difference is that more quatization levels are used for soft demapping.

Hi davyzhu
Your question is attractive. I am waiting for good reply. If you find your answer, please share it.

Hi m-llaa,

I have a post before about a LDPC simulation environment. You can search it in this forum.

The environment contains the soft decoding algorithm.

Best regards,

Hi davyzhu
"But how to get the soft information from QAM?"
This question is interesting for me.
I know the best decoder like BP for LDPC uses soft information in each bit. But what happen when we use modulation like QAM? Is it possible to make the decoder better?


The QAM modulator receives the information from the channel and asume it is afected by noise. So for example if we use 4QAM, the symbols used to modulate the signals are +1,-1 for the I-channel and +1,-1 for the Q-channel. After the channel, we have the symbolos plus noise, so for example if +1 was Tx, we may receive 0.5456. We know that an error happens when we get a sign change i.ex -0.123. The key in soft demodulatipon (not soft decoding) is to pass to the decoder a quantization of the effect of the channel. If we use 3 bits quantization (with MSB for the sign) we get four levels of representation. That is:

00 ---- (for signals between 0 and 0.25)
01 ---- (for signals between 0.25 and 0.5)
10 ---- (for signals between 0.5 and 0.75)
11 ---- (for signals between 0.75 and 1)

and the third bit indicates if the symbol represents a one or a cero (used for hard decision). For 4QAM it is easy see that a greater magnitude on the symbol will not cause an error but for 16QAM the thing will go different.

I hope that can help you get started.


Not open for further replies.

Part and Inventory Search

Welcome to