  We propose an orthogonal series density estimator for complex surveys, where
samples are neither independent nor identically distributed. The proposed
estimator is proved to be design-unbiased and asymptotically design-consistent.
The asymptotic normality is proved under both design and combined spaces. Two
data driven estimators are proposed based on the proposed oracle estimator. We
show the efficiency of the proposed estimators in simulation studies. A real
survey data example is provided for an illustration.
