The two primary RACH procedures: Contention-Based Random Access and Contention-Free Random Access.

Understanding Preambles: The Signatures of Random Access

Before exploring the RACH procedures, it’s crucial to understand preambles. These act as unique “signatures” transmitted by the UE to the eNodeB to signal its access request. There are 64 preambles available in an LTE system, and UEs can randomly choose one during the access process.

Two Groups of Preambles:

Contention-Free Preambles: These are preambles specifically allocated to UEs undergoing handover between cells. This allocation ensures prioritized access for handover UEs, minimizing delays.

Contention-Based Preambles: The remaining preambles are used by UEs during the initial access procedure or when they lack dedicated resources for uplink transmission. These scenarios involve multiple UEs potentially contending for access, hence the name.

Contention-Based Random Access

As the name suggests, this procedure comes into play when multiple UEs attempt to access the network simultaneously. Here’s a breakdown of the steps involved:

• Step 1: UE Transmits Preamble: The UE selects a random preamble from the contention-based pool and transmits it on the RACH channel. The preamble selection also carries information about the required scheduling resources based on the data payload in Layer 3 (L3) packets the UE intends to send.

• Step 2: eNB Response and Collision Potential: The eNodeB attempts to decode the received preambles. If successful, it sends a Random-Access Response (RAR) to all UEs that transmitted a preamble, regardless of their identities at this stage. This RAR is broadcasted on the Downlink Shared Channel (DL-SCH) and includes an RA-RNTI (Random Access Response Temporary C-RNTI). The RA-RNTI identifies the resource block where the eNodeB decoded the preamble. Each UE is allocated a specific time slot to receive the RAR. If a UE doesn’t receive a response within its designated time window, it assumes a collision and retransmits a preamble with increased power in the next available slot. It’s important to note that if multiple UEs transmitted the same preamble and the eNodeB successfully decoded all of them, all UEs will receive an RAR, indicating they have a scheduled slot for sending an RRC (Radio Resource Control) connection request in the next step.

• Step 3: UE Sends RRC Request and Contention Resolution: Utilizing the resource block allocated in Step 2, the UE transmits an RRC connection request message on the Uplink Shared Channel (UL-SCH). This message includes the UE’s identifier, allowing the eNodeB to distinguish between UEs that might have collided in the previous step. Here’s where contention resolution takes place. If multiple UEs transmitted on the same resource block due to a collision, only the UE with the best radio conditions (strongest signal) will have its message successfully decoded by the eNodeB. The eNodeB then broadcasts a contention resolution message on the DL-SCH, which includes the identifier of the successful UE. UEs that cannot decode the message or whose identifiers don’t match back off and wait for the next access attempt. The UE that successfully transmitted its RRC request message can then proceed with data transmission on the allocated uplink resource block.

Contention-Free Random Access

In contrast to the contention-based approach, contention-free random access is initiated by the network, specifically during a UE handover scenario. Here’s a simplified view of the process:

• Step 1: eNodeB Reserves and Assigns Preamble: The serving eNodeB reserves a set of preambles dedicated to handover and assigns a specific preamble from this pool to the UE.

• Step 2: UE Responds with Preamble and ID: Since the eNodeB controls the process, there’s no contention here. The UE transmits the assigned preamble along with its identifier in response.

• Step 3: eNodeB Sends Random Access Response: Upon receiving the UE’s response, the eNodeB confirms the successful handover by sending an RAR that includes information about the resources allocated to the UE on the target cell.