This paper deals with synchronization problem in IEEE 802.11a wireless system. In addition to traditional training sequences, the SIGNAL field of the physical frame can be considered as a new source of information. Indeed the receiver is able to predict the SIGNAL unknown parts relying on the knowledge provided by the CSMA/CA protocol during the negotiation of the transmission medium reservation. The exchanged RtS control frame used jointly with the bit-rate adaptation algorithm to the channel helps the receiver not only to predict the SIGNAL field but also to get information on the channel state. Based on this knowledge, joint MAP channel, time and frequency synchronization algorithm is carried out. Moreover to estimate the residual time offset, a timing metric in frequency domain is performed by minimizing the average of transmission errors in the presence of all channel estimation errors. The performance in terms of probability of synchronization failure is shown to be improved compared to existing algorithms.