Noise analysis in LNA, why can be expressed as...???

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In LNA noise analysis:

why the output noise power density arising from the "channel current noise" source can be expressed as:

why the channel noise is divided by (1+(W*Ls/Rs))^2 ???

PS: Equivalent circuit for input stage noise calculations is attached:
 

Do a full ac analysis,you will find it.It's not difficult,just be patient.
 

to thcm:

I know it is related with Gm(the transconductance of the input stage):

Gm=Wt/W0*Rs*(1+Wt*Ls/Rs)

and the output noise power density:

S(W0) = Vni * Gm
Vni = id*W*Cgs*Rs/gm1

Could you tell me why "Vni" equal to "id*W*Cgs*Rs/gm1"?
or give me some available link I could get some information.
 

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