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Output Power Amlifier Back-off or OBO

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mashhur

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power amlifier

hi guys,

i need ur idea again.

we identify OBO with this formula OBO = log(Pout/Pin) right?

in many tutorials such as Shinsuke Hara and Ramjee Prasad titled Multicarrier Techniques for 4G Mobile Communications, published 2003 (p.85) there is a graph illustrates power spectrum of OFDM on OBO (-5 dB), that is x and y axises define bandwidth and psd (power spectral density) properly.

My question is where is the connection of OBO with psd, i mean can somebody give me mathematical formulas of their relationship?

thanks a lot!
 

mashhur

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output back off

it is so sad, no one replied :(
 

mowgli

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output back off

Hello Mashur,


I don't think that the formula OBO = log(Pout/Pin) is correct: Gain = Pout/Pin.

Said that, output back off means setting the working point of power amplifier at a gain OBO dB below its Gmax value. This is done to avoid that the intermodulation products originating from the input multicarrier signal raise over a certain level, causing excessive interference in the adjacent bands.
With OBO the psd of the output signal has better spectral characteristic in the out of band region, where typically lie the spurious components originating from non linearity of the device.
This topic is a crucial point in OFDM systems because in this case the modulated signal is characterized by a great PAPR and to avoid saturation of the PA you have to apply a OBO at least equal to the PAPR of the input signal and this means that the device is more expensive, bulky and in general less efficient.
To reduce the OBO necessary to match the requirements on maximum adjacent channel interference level several algorithm have been developed for OFDM to keep the PAPR of the modulated signal by means of smart coding of the baseband digital stream before mapping symbols on the carriers and you can find many papers on this topic.

Regards
Mowgli
 

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