How Random Access procedure works in LTE?

The RACH procedure in LTE is used for four cases: 

• Initial access from disconnected state (RRC_IDLE) or radio failure .
• Handover requiring random access procedure.
• DL or UL data arrival during RRC_CONNECTED after UL PHY has lost synchronization (possibly due to power save operation).
• UL data arrival when are no dedicated scheduling request (PUCCH) channels available.

Timing is critical because the UE can move different distances from the base station, and LTE requires microsecond level precision; the speed-of-light propagation delay alone can cause enough change to cause a collision or a timing problem if it is not maintained. 

There are two forms of the RACH procedure: Contention-based, which can apply to all four events above, and noncontention based, which applies to only handover and DL data arrival. The difference is whether or not there is a possibility for failure using an overlapping RACH preamble.

Read more from here.....