An LTE User Equipment (UE) can only be scheduled for uplink transmission if its uplink transmission timing is synchronized. The LTE Random Access CHannel (RACH) therefore plays a key role as an interface between non-synchronized UEs and the orthogonal transmission scheme of the LTE uplink radio access. In this chapter, the main roles of the LTE RACH are elaborated, together with its differences from the Wideband Code Division Multiple Access (WCDMA) RACH. The rationale for the design of the LTE Physical Random Access CHannel (PRACH) is explained, and some possible implementation options are discussed for both the UE and the eNodeB.
17.2 Random Access Usage and Requirements in LTE
In WCDMA, the RACH is primarily used for initial network access and short message transmission. LTE likewise uses the RACH for initial network access, but in LTE the RACH cannot carry any user data, which is exclusively sent on the Physical Uplink Shared CHannel (PUSCH). Instead, the LTE RACH is used to achieve uplink time synchronization for a UE which either has not yet acquired, or has lost, its uplink synchronization. Once uplink synchronization is achieved for a UE, the eNodeB can schedule orthogonal uplink transmission resources for it. Relevant scenarios in which the RACH is used are therefore:
1. (1) A UE in RRC_CONNECTED state, but not uplink-synchronized, needing to send new uplink data or control information (e.g. an ...