WHAT IS RANDOM ACCESS PROCEDURE (PHASE 1)?

Before sending a preamble, the UE synchronize to the downlink transmissions and read the system information Broadcast control channel (BCCH). The BCCH will reveal where the random access (RA) time slots are located, which frequency bands can be used, and which preamble (sequences) are available.

At the next RA slot, the UE will send the preamble. If multiple RA frequency bands have been defined, the UE randomly selects one of them. The group of sequences allocated to a cell is partitioned into two subgroups. By selecting a preamble sequence from a specific subgroup, the UE can give a single-bit indication of its resource requirement and/or link quality.

The particular sequence used for the preamble is randomly selected within the desired subgroup. This sequence implicitly contains a random ID which serves as a UE identifier. The eNodeB estimates the UL timing of the UE based on the timing of the received preamble.