Rapidity dependence of particle densities in pp and AA collisions

We use multiple scattering and energy conservation arguments to describe $dn/d\eta_{NANA}$ as a function of $dn/d\eta_{pp}$ in the framework of string percolation. We discuss the pseudo-rapidity $\eta$? and beam rapidity Y dependence of particle densities. We present our results for pp, Au- Au, and Pb-Pb collisions at RHIC and LHC.