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QAM demapping & simulation results. Advisor : Yung-An Kao Student : Chi-Ting Wu 2005.03.31. Outline. Mean offset 16 QAM demapping Simulation results. Mean value offset. Mean value offset. Mean offset.
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QAM demapping & simulation results Advisor : Yung-An Kao Student : Chi-Ting Wu 2005.03.31
Outline • Mean offset • 16 QAM demapping • Simulation results
Mean value offset Mean value offset Mean offset • In OFDM system, the mean value of signal after compensation is not {+1,-1} because of channel and noise effect • We have to shift the mean value back to {+1,-1} then feed into Viterbi decoder Original mean value Shifted mean value
Mean offset calculation • From Gaussian distribution, ideally we could write it as but in practical situation ( channel effects ), the Gaussian distribution is written as
Mean offset calculation • To obtain the mean value offset, we use the time average method and then in Viterbi algorithm, we could use the following formula
Block diagram for mean value calculation Channel state information from LMS weighting values Time average method Data from FFT Mean offset recovery for each subcarrier Mean offset calculation for each subcarrier Decoded bits Viterbi decoder
16 QAM demapping • In 16 QAM or 64 QAM modulation, 2 or 3 bits are mapped onto a value. • But in soft decision Viterbi decoding, we have to give the soft value of each bit • Here comes the question, in 16 QAM, how to demapping a value to two bit soft value?
. . 16 QAM demapping Example : { 3.2 + 0.9i } 3.2 => { b0, b1}={ 3.2, -1.2}, 0.9 => { b2, b3}={ 0.9, -1.1} { -0.7 – 1.4i } -0.7 => { b0, b1}={ -0.7, 1.4}, -1.4 => { b2, b3}={ -1.4, -0.6}
Quantization after demapping • After demapping, we have to quantize again by using the following algorithm ( floating point ) -1 0 +1 …………………………… -1 0 +1
Simulation results Soft decision 6-bit，固定100組channel,10dB的情況下,100個OFDM symbol,time average version1,QPSK modulation, alfa=0.3 Soft decision 6-bit，固定100組channel,10dB的情況下,100個OFDM symbol,time average version1,QPSK modulation, alfa=0.7
Simulation results Soft decision, 固定100組channel,10dB的情況下,100各OFDM symbol, time average version1, QPSK modulation, 理論值 Soft decision, 固定100組channel,10dB的情況下,100各OFDM symbol, time average version1, QPSK modulation, 理論值+square weighting value Soft decision, 固定100組channel,10dB的情況下,100各OFDM symbol, time average version1, QPSK modulation, 實際值+square weighting value Soft decision, 固定100組channel,10dB的情況下,100各OFDM symbol, time average version 2, QPSK modulation, 實際值+square weighting value
