I am allowed to enter only one input port on the LSSP window in Cadence Virtuoso when I actually have two ports at the input (one for positive terminal and other one for negative terminal which as a phase difference of 180 with respect to the positive terminal).
LSSP in Cadence ADE simply invoke two one-tone-PSS(Shooting or HB) for forward and reverse direction.
One is for forward direction.
The other is for reverse direction.
Don't use Cadence ADE blindly without understanding background theory.
And equations in Cadence ADE have many mistakes especially for RF.
Don't use LSSP.
Simply do one-tone PSS.
If you can understand S-parameters definition, reference impedance concept and differential/common modes correctly, you can evaluate anything.
If you need only forward direction characteristics, two PSS are not required.
In differential DUT, characteristics measures are sixteens :
S11(DD), S12(DD)
S21(DD), S22(DD)
S11(DC), S12(DC)
S21(DC), S22(DC)
S11(CC), S12(CC)
S21(CC), S22(CC)
S11(CD), S12(CD)
S21(CD), S22(CD)
However, your schematic is configured for only differential output mode.
So you can not evaluate DC and CC.
https://en.wikipedia.org/wiki/Scattering_parameters#4-Port_Mixed-Mode_S-Parameters
If you require only DD characteristics, use one port for differential input same as output port.
Or use rfLib/balun or rfLib/balun_com.
I don't recommend analogLib/ideal_balun
See
https://www.edaboard.com/threads/352410/