A lower bound for the Lyapounov exponents of the random Schrodinger operator on a strip

We consider the random Schrodinger operator on a strip of width $W$, assuming the site distribution of bounded density. It is shown that the positive Lyapounov exponents satisfy a lower bound roughly exponential in $-W$ or $W\to \infty$. The argument proceeds directly by establishing Green's function decay, but does not appeal to Furstenberg's random matrix theory on the strip. One ingredient involved is the construction of `barriers' using the RSO theory on $\mathbb Z$.