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HELP:Limitation on QAM-on Pilot subcarrier in OFDM ?

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bts_dev

bts_dev

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I m working on OFDM based system. Where tehres no pilots available for tracking frequency offsets. I plan to use some of high SNR subcarriers as for the same - using data aided CFO estimation.
I want to theoretically determine the limitation on the highest QAM which can be used for a given frequency offset.

Degradation due to CFO = ICI + Phase rotation

degradation due to ICI is easy to evaluate. But Can someone provide me a theoretical bound on the phase rotations that can be tolerated by a QAM ??
 

bts_dev said:
Degradation due to CFO = ICI + Phase rotation

degradation due to ICI is easy to evaluate. But Can someone provide me a theoretical bound on the phase rotations that can be tolerated by a QAM ??
Click to expand...

i once made made a matlab program showing transmitted QAM symbols phases in phasor chart. on receiver, u can perform a calculations where u take a range for the symbol. Suppose i say that my Receiver will consider that the symbol is on zero degree, if it lies between +45 dgree and -45 degree, in phasor chart...

by using these concept, one can say that QAM can tolerate phase rotation upto Plus-Minus 45 degrees...

am i clear???



Regards
Naveed
 

Hi Naveed,

I guess the 45degrees is too large. What you basically need to do is write down the signal space diagram, detection boundaries and assuming zero amplitude imbalance find the phase error that can be tolerated. remember that this values changes with the constellation.

Hope this helps,
brmadhukar
 

brmadhukar said:
Hi Naveed,

I guess the 45degrees is too large. What you basically need to do is write down the signal space diagram, detection boundaries and assuming zero amplitude imbalance find the phase error that can be tolerated. remember that this values changes with the constellation.

Hope this helps,
brmadhukar
Click to expand...
hi,
i just wanted to give an idea, of having a range for angle. One can select any suitable. i for got to add that for QAM in addition to the angle range one should select magnitude as well.

Naveed
 

For QPSK constellation, decision boundary is +-45 degree, which is different from the phase error. For analysis of phase error on the performance of QAM, the phase noise could be modeled as Gaussian noise. Then you need evaluate the conditional error probablity due to the phase noise. It is more compilcated if the magnitude imbalance is also considered.
 

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