Soft decoding in FEC block about openwifi-hw HOT 8 CLOSED

Hello Hyanki,
The goal of soft decoding is to limit the quantization error of IQ demodulation so that the Viterbi decoder will make a better estimate during the decoding process.

Furthermore, at any given constellation point and modulation scheme (i.e. BPSK, QPSK, 16QAM, 64QAM), only a single bit per phase (I.e in-phase, quadrature) will experience quantization error. You can check it yourself on the constellation diagram (https://openofdm.readthedocs.io/en/latest/_images/mod.png).

This means, we only need to save the soft decoding information for two bits and the position within the constellation diagram. Now the Viterbi decoder from Xilinx by default uses 3-bit width for soft-decoding representation and that makes the total bits 6 = 2*3.

So for your question, yes the number of bits in soft and hard decoding is the same but they have a different meaning

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Hi mmehri
In 64QAM we are packing 6 bit in one symbol then modulate. In receiver we demodulate it and feeded to viterbi but i think viterbi is giving one bit output.
Here how these packed bits are reterved using viterbi?
Regards
hyanki

from openwifi-hw.

In hard decoding mode, the Viterbi decoder accepts two bits and produces 1 bit. Whereas in the soft decoding mode, the Viterbi decoder accepts 6 bits (2*3) and produces 1 bit.

Inside openofdm, the Viterbi decoder from Xilinx was always enabled to work in soft decoding mode but the original author @jhshi has not made the module to take advantage of this. That means, the module was working in hard decoding mode and that is what we have updated in our development.

As to the principle of working of the Viterbi decoder, you can look at the following youtube movies
https://www.youtube.com/watch?v=kRIfpmiMCpU
https://www.youtube.com/watch?v=dKIf6mQUfnY

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Where did you get the information that 16QAM uses convolution rate 2/3?`

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First of all, we haven't implemented the Viterbi decoder, except using the IP core from Xilinx.

Second, the Viterbi decoder works on a sequence of inputs before producing an output, rather than 4bits in the case of 16 QAM. The operation of the Viterbi decoder does not depend on the modulation schemes. All it cares about is the bits.

So I would advise you to watch the youtube videos I have attached previously.

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