Soft demodulation of 16-QAM/64-QAM in OFDM/LTE

Hi,
I am implementing an OFDM modulation scheme as part of an LTE project in C/C++ and then testing it on a DSP board. I am using turbo coding with soft decision decoding. On the receiver side of the LTE system, I need to perform a soft demodulation (16-QAM and 64-QAM) so I can feed real values to the turbo coder for decoding. I have looked for information about soft demodulation on the Internet but didn't find any helpful ones. Does anyone know of a book or journal paper or any source of information that explains the soft demodulation (QPSK, 16-QAM, or 64-QAM)?

Any input is much appreciated. Thanks.

mrajib said:

Hi,
I am implementing an OFDM modulation scheme as part of an LTE project in C/C++ and then testing it on a DSP board. I am using turbo coding with soft decision decoding. On the receiver side of the LTE system, I need to perform a soft demodulation (16-QAM and 64-QAM) so I can feed real values to the turbo coder for decoding. I have looked for information about soft demodulation on the Internet but didn't find any helpful ones. Does anyone know of a book or journal paper or any source of information that explains the soft demodulation (QPSK, 16-QAM, or 64-QAM)?

Any input is much appreciated. Thanks.

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But usually demodulation is done after decoding. This is the point of codec: to increase the reliability of the received bits so that they can be decoded with lower probability of error. Why do you need to demodulate before decoding?

Hi,
Thanks for your reply. The size of symbols to be fed to the decoder has to match the size of the encoded bits at the transmitter. Here is what I mean.

At the transmitter side, suppose the data block to be encoded is of length 64 bits. Applying a turbo coder with rate 1/3, the size of the coded data is now 192 bits (excluding the trellis termination). Now the code word is fed into a modulation mapper to convert the bits into complex valued symbols. In the case of 16-QAM modulation scheme, the size of the modulation symbols becomes 48 symbols (4 bits -> 1 symbol). At the receiver side (I am excluding other OFDM processing blocks such as cyclic prefix, IFFT, channel model, etc...), the turbo decoder expects a data size of 192 (systematic, parity1, and parity2) which can only be achieved by demodulation. So, if we apply decoding before demodulation, the size of the symbols is 48 and not 192.