LTE UE Uplink Network Entry: Random Access Procedure

This article describes the LTE UE network entry procedure, covering the cell search procedure, system information decoding, and random access procedure. These steps are essential for an LTE UE to successfully establish network entry with an eNB.

LTE UE uplink network entry random access procedure

The figure above illustrates the entire process necessary for an LTE UE to camp on to an LTE-compliant eNB or LTE cell. Let’s break down each step:

Step 1: Cell Search

The LTE UE begins by using the Primary Synchronization Signal (PSS) and Secondary Synchronization Signal (SSS) to determine the Physical Layer Cell Identity (PCI). There are 504 possible PCI values, and the UE needs to identify the correct one for the cell it’s trying to connect to.

Step 2: System Information Decoding

After successfully completing the cell search, the LTE UE decodes critical system information using the Physical Broadcast Channel (PBCH). This information includes:

• Master Information Block (MIB): Transmitted on the BCH (Broadcast Channel) with a periodicity of 40ms. The MIB contains essential parameters like:

  • System bandwidth
  • Physical Hybrid ARQ Indicator Channel (PHICH) configuration
  • System Frame Number (SFN)
  • Number of transmit antennas

• System Information Block Type 1 (SIB1): Transmitted on the DL-SCH (Downlink Shared Channel) with a periodicity of 80ms. SIB1 carries the following information:

  • PLMN IDs (Public Land Mobile Network Identities)
  • Tracking Area Code (TAC)
  • Cell Identity
  • Access restrictions
  • Scheduling information
  • And more.

The SI-RNTI (System Information Radio Network Temporary Identifier) is used on the PDCCH (Physical Downlink Control Channel) to address System Information Block Type 1 and SI Messages.

Step 3: Random Access Procedure

The random access procedure is the mechanism by which the UE initially contacts the eNB.

(a) Random Access Initiation: The UE initiates the random access procedure by sending a random access preamble on the PRACH (Physical Random Access Channel). The eNB then responds with a random access response on the PDSCH (Physical Downlink Shared Channel).

(b) Data Transmission and Contention Resolution: Data transmission is scheduled on the PUSCH (Physical Uplink Shared Channel). Contention resolution, which handles situations where multiple UEs attempt to access the network simultaneously, is managed by the MAC (Medium Access Control) layer. The outcome of the contention resolution process is communicated to the UE by the eNB on the DL-SCH.