At the same output power level of a power amplifier and for the same number of subcarriers would the measured EVM be different for a QPSK - OFDM signal than a 64 - QAM OFDM signal?
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Your question is interesting. – Enrico Migliore Nov 26 '21 at 07:55
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Should be almost the same since the peak to average power ratio (PAPR) of an OFDM signal (for a large number of subcarriers) does not depend on the modulation associated with the subcarriers, i.e. it is almost the same for QPSK-OFDM and 64 QAM-OFDM
cesar
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Actually, 64QAM constellations are more densely packed when compared to QPSK constellation,
- So, 64QAM is more sensitive to same amount of noise,when compared to QPSK.
- Also, the PAPR for 64QAM is higher than QPSK are there are more amplitude levels in 64QAM compared to QPSK. So at higher PAPR at PA input, we drive it to saturation and generate distortion(noise) in output.
Imagine a imaginary OFDM system where is no modulation on subcarriers and all the subcarrier have constant amplitude and phase (CW tones). Here the average power of the OFDM system is equal to the peak power , PAPR ~ 1.
Now scale it to as many sub carriers as needed, PAPR will still be 1 as there is no amplitude or phase modulation applied on the sub carriers .
Once we change the phase and amplitude profile of the sub carriers, we can assert that in time domain the different OFDM sub carriers will constructively interfere and lead to high peak power.
To be more simple.
QPSK has only one amplitude level.
16 QAM has four amplitude levels.(so when in time domain they constructively add), we have peak power = amplitude level 1 + lev 2 + lev 3 + lev 4.
so 16 QAM has higher peak power compared to QPSK.
Ananth Tenappan
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