Since each MS has a unique distance from the base station, it is critical in the uplink to synchronize the symbols and equalize the received power levels among the various active MSs. This process is known as ranging; when initiated, ranging requires the BS to estimate the channel strength and the time of arrival for the MS in question.
Downlink synchronization is not needed, since this link is already synchronous, but in the uplink, the active users need to be synchronized to at least within a cyclic prefix guard time of one another. Otherwise, significant intercarrier and intersymbol interference can result. Similarly, although downlink power control is recommended in order to reduce spurious other-cell interference, it is not strictly required. Uplink power control is needed to (1) improve battery life, (2) reduce spurious other-cell interference, and (3) avoid drowning out faraway users in the same cell who are sharing an OFDM symbol with them. The third point arises from degraded orthogonality between cocell uplink users, owing to such practical issues as analog-todigital dynamic range, carrier offset from residual Doppler and oscillators mismatching that is not corrected by ranging, and imperfect synchronization. 
The uplink power-control problem in WiMAX is similar to the near/far problem in CDMA, although considerably less strict; in uplink CDMA, the power control must be extremely accurate.In WiMAX, four types of ranging procedures exist: initial ranging,  periodic ranging, bandwidth request, and handover ranging. Ranging is performed during two or four consecutive symbols with no phase discontinuity, which allows the BS to listen to a
misaligned MS that has a timing mismatch larger than the cyclic prefix. 
If the ranging procedure is successful, the BS sends a ranging response (RNG-RES) message that instructs the MS on the appropriate timing-offset adjustment, frequency-offset correction, and power setting. If ranging was unsuccessful, the MS increases its power level and sends a new ranging message, continuing this process until success .