Need PA math Model Equation

I have a PA non-linear operator as following:

Ymp(n)=E(0:K-1)E(0:M-1) Akm*x(n-m)*|x(n-m)|^k

But I found that PA model is not so good.
I have PA input and output signals, but this model can balance passband and stopband at the same time.
Who can share a math model?
I need an equation. Thanks.

Nonlinear model can not distinguish the frequency information.It's effective on signal processing in time domain/instantaneous signal domain only.
What is your target ? Can you clarify a bit more ?/

I am simulating PA DPD, and I have gotten a DPD algorithm, but PA model is not so good.
So I need it.
I only have one input signal and one output signal at one power level.
So I need a group of input signals and output signals so that I can do some AMAM/AMPM simulation.
This is only for study, not for project.

If you're able to extract X-Parameters in ADS by simulating the real PA ( measurments are also welcome), they will represent your PA being as nonlinear equations to be able to simulate the whole system.