Downgrading from iOS 6.0.1 to iOS 5.1.1?
You cannot downgrade your device. It is forever locked to iOS 6.0.1.

A downgrade for certain devices (those that run on the A4 processor, and yours does) is only possible should you have the SHSH blobs (using Redsnow). However, since you do not have them, there is no way to revert to any version prior to 6.0.1 (unless you saved the blobs for 6.0, then you could certainly revert to the prior version).