  We report on an orbital-angular-momentum-enhanced scheme for angular
displacement estimation based on two-mode squeezed vacuum and parity detection.
The sub-Heisenberg-limited sensitivity for angular displacement estimation is
obtained in an ideal situation. Several realistic factors are also considered,
including photon loss, dark counts, response-time delay, and thermal photon
noise. Our results indicate that the effects of the realistic factors on the
sensitivity can be offset by raising orbital angular momentum quantum number
$\ell$. This reflects that the robustness and the practicability of the system
can be improved via raising $\ell$ without changing mean photon number $N$.
