  Topological insulators have surface states with unique spin-orbit coupling.
With impurities on the surface, the quasiparticle interference pattern is an
effective way to reveal the topological nature of the surface states, which can
be probed by the scanning tunneling microscopy. In this paper, we present a
general analytic formulation of the local density of states using the
stationary phase approximation. The power laws of Friedel oscillations are
discussed for a constant energy contour with a generic shape. In particular, we
predict unique signature of magnetic impurities in comparison with nonmagnetic
impurities for a surface state trapped in a "magnetic wall".
