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Why do we have to consider GSM Power Amplifier Linearity?

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criterion456

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Hi,

Of course, the better the linearity of PA is, the better the TX performance will be.

Nevertheless, GSM adopts GMSK modulation, which is constant envelope.

In other words, it doesn't need back-off to maintain linearity.

It can even operate in saturation mode.




But, I'm confused.

Now that it can even operate in saturation mode,

why does its linearity affect TX performance, such as harmonics and ACPR ?




Thanks a lot~!!
 

Among things a comms designer is "on the hook for"
is adjacent channel interference. Nonlinearity is what
makes mixers work and intermodulation of local tones
(clocks, data) with the carrier can spread it into the
nearby channels. Or just add phase error / noise.

If you fail type acceptance for that, somebody gonna
pay.
 

This question, why not use the GSM GMSK PA in saturation mode (or to operate in Class-C because GMSK modulation is constant envelope), was keep asking by people by more than 20 years, since starting of the GSM network development.
The answer is simple. If the GSM PA is working in a very nonlinear region the transmitter will never pass the Spectrum due to Switching requirements (ETSI TS 145 005 - Ch. 4.2.2 Spectrum due to switching transients).
 

Depends on the type of nonlinearity. For constant envelope modulation, P1dB isn't an issue itself since it refers to the carrier output power. But emission at other frequencies is important in order to meet regulatory requirements. So intermodulation specs can still be critical.
 

Hi,
When u operate GSM PA in saturation mode, it bound to have harmonics , since its operating in saturation(means u are starting to clip waveform in time domin). The generate harmonics may create problem to other devices which are operating at that frequency.
Regarding ACPR, when u operate GSM in multi carrier operation, this parameter is needed to be evaluated.

Regards
 

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