when doing s parameter analysis in cadence simulation, S11 generated during simulation is w.r.to voltage or w.r.to Power?
I'm not sure this question makes sense. S parameters are defined as in this Wikipedia entry. Assuming you're talking about spectre ("cadence simulation" doesn't really mean very much, since Cadence is the name of the company and we have a number of simulators...), then the sp analysis does this:
The S-parameter analysis linearizes the circuit about the DC operating point and computes S-parametersof the circuit taken as an N-port. The port statements define the ports of the circuit. Each activeport is turned on sequentially, and a linear small-signal analysis is performed. Spectre converts theresponse of the circuit at each active port into S-parameters and outputs these parameters. There mustbe at least one active port statement in the circuit.
As far as I know there are not two types of S11 - it's computed according to the definition of what S11 means.