Before revision:  By estimating the large error covariance matrix consistently, the proposed feasible GLS (FGLS) estimator is robust to heteroskedasticity, serial correlation, and cross-sectional correlation.
After revision:  By estimating the large error covariance matrix consistently, the proposed feasible GLS (FGLS) estimator is more efficient than the ordinary least squares (OLS) in the presence of heteroskedasticity, serial correlation, and cross-sectional correlation.