Blind channel estimation in CDMA are studied in this paper. MSE (mean squared error) of parameter estimation is important for evaluating the estimation algorithm and its lower bound is CRB (Cramer Rao bound). In this paper, FIM and CRB for CDMA complex channel estimation are analyzed. Furthermore FIM and CRB for CDMA blind complex channel estimation under linear constraints is studied in complex Gaussian signal model. In the end, subspace based blind channel estimation in CDMA are simulated and compared with CRB under linear constraints.