The quality of multiple access interference (MAI), which can be improved by using channel estimation and data estimation of all active users, effects considerably the performance of PIC detector. Therefore, data and channel estimation performance is obtained in the initial stage has a significant relationship with the performance of PIC. So obviously it is necessary to make excellent joint data and channel estimation for initialization of PIC detector. In this work, to initiate PIC module efficiently a joint MAP channel estimation and data detection technique based on the Expectation Maximization (EM) method has been proposed. Moreover, the MAP-EM approach considers the channel variations as random processes and applies the Karhunen-Loeve (KL) orthogonal series expansion. The performance of the proposed approach are studied in terms of bit-error rate (BER). Throughout the simulations, extensive comparisons with previous works in literature are performed, showing that the new scheme can offer superior performance.