5G NR Random Access Process

Article from Abhijeet Kumar

Imagine you're trying to make the first contact with a radio tower from your smartphone. There’s no personal connection yet, but you must synchronize with the network before any data can flow. It's like trying to introduce yourself in a crowded room, everyone is talking, but you need the perfect moment to stand out.

The Random Access Procedure is that perfect introduction, where your phone raises its hand in the crowd to get the network's attention.

Let’s break down the four essential flows of the Random Access Procedure:

• Preamble Transmission (Msg1):

The UE sends a "preamble" to the network to indicate its desire to connect. Think of it as saying "hello" to the network.

Message Content:

It contains a randomly selected preamble from a pool (for contention-based access) or a dedicated preamble (for contention-free access). This message helps the network identify which device is trying to connect.

• Random Access Response (Msg2):

The gNB (network node) responds with a Random Access Response (RAR).

Message Content:

Timing advance: This helps the UE adjust its transmission timing so it syncs perfectly with the network.

Uplink grant: This allows the UE to send data.

Temporary C-RNTI: A temporary identifier given to the UE during the procedure.

• Scheduled Transmission (Msg3):

The UE sends its data over the granted resources, introducing itself fully to the network.

Message Content: Includes UE identity, which could be a temporary identifier or the full UE identifier (e.g., IMSI), especially in contention-based access.

• Contention Resolution (Msg4):
• The gNB resolves which UE should continue with the connection if multiple devices used the same preamble.
• Message Content: If the UE receives its identifier back from the network, it knows the connection was successful. If not, it retries the process.

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Let’s imagine a real-world scenario:

You’re at a crowded concert, and you want to catch the attention of your friends in a different section. You wave your hand (Preamble Transmission), and they respond by calling out your name (Random Access Response). But the problem is, a few other people wave at the same time. When your friends respond, you shout back, “It’s me!” (Scheduled Transmission), and your friends finally recognize you (Contention Resolution).

In the same way, your smartphone waves at the network, and the network responds—but it might have to figure out exactly which phone was waving, and that’s where contention resolution comes in.

I'd like to learn more about Random access.

1. Random Access Preamble (Msg1) - Section 8.1 38.213 3GPP

The first step in the random access procedure is the transmission of the Random Access Preamble.

• Purpose: The UE (User Equipment) uses this message to initiate communication with the network. It’s like knocking on the door before entering.
• Preamble Selection: The UE selects a preamble from a pool, either randomly (contention-based access) or as directed by the network (contention-free access).
• The formula for Timing:

Tpreamble=NCS+NGuard+NFormat

Where:

NGuard is the guard period between preambles.

NFormat defines the format and duration of the preamble.

• Transmission: The preamble is transmitted on the Physical Random Access Channel (PRACH) using different formats defined in the 3GPP specification. Each format determines how the preamble is structured (e.g., duration and bandwidth).

The formula for Preamble Format:

Tpreamble=NPRACH×Duration?per?preamble?symbol

2. Random Access Response (RAR - Msg2) - Section 8.2 38.213 3GPP (Type-1 Random Access Procedure)

After the UE sends the preamble, the gNB (5G base station) responds with the Random Access Response (RAR), which includes several critical parameters.

• Purpose: The RAR contains information for the UE to proceed with uplink synchronization.
• Content: Timing Advance Command (TAC):

Instructs the UE to adjust its transmission timing to synchronize with the network.

Formula for Timing Advance:

TTA=2×Dpropagation/C

Where: Dpropagation is the round-trip propagation delay.

C is the speed of light (299,792,458 m/s).

Uplink Grant: Provides the UE with the resources (frequency and time slots) to transmit its data.

Temporary C-RNTI: A temporary identifier for the UE during the access procedure.

• Response Window: The UE must respond within the RAR response window.
• Formula for Response Window:

TRAR?window=Tinitial+NRAR

• Where NRAR is the number of RARs within the window.

3. PUSCH Scheduled by RAR UL Grant (Msg3) - Section 8.3 38.213 3GPP

Once the UE receives the RAR with the uplink grant, it uses this to transmit its data on the Physical Uplink Shared Channel (PUSCH).

1. Purpose: The UE sends its identity to the network for contention resolution.
2. Content:
3. The UE identity can either be the Temporary C-RNTI (assigned in RAR) or its S-TMSI/IMSI for identification.
4. The PUSCH transmission follows the scheduling parameters defined in the RAR.
5. Transmission Power Control (TPC): The UE follows a power control mechanism to ensure the signal strength is optimal.
6. The formula for Transmission Power:

PUE=P0+α?PL+ΔTPC

• Where:

P0 is the initial power.

α is the path loss compensation factor.

PL is the path loss between UE and gNB.

ΔTPC is the power control command from the gNB.

Configuring the PUSCH Transmission

Once the UE receives the uplink grant from the RAR, it prepares to send Msg3, which includes its identifier (C-RNTI or S-TMSI). The PUSCH transmission follows specific steps, based on the parameters defined in the Random Access Response:

• Subcarrier Spacing (SCS): The UE uses the subcarrier spacing indicated by the uplink BWP (Bandwidth Part). Example: The SCS could be 15 kHz, 30 kHz, or even 60 kHz depending on whether the communication is in FR1 or FR2.
• Resource Allocation: The UE determines its resource allocation based on the frequency domain resource assignment in the RAR. For example, if the Bandwidth Part (BWP) size is less than or equal to 180 RBs (Resource Blocks), the frequency domain resource assignment field is truncated to a certain number of bits.

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The formula for determining the number of bits required for the resource allocation field is:

PUSCH Transmission (Msg3)

Once the UE configures the frequency and timing parameters, it sends Msg3 over the PUSCH using the uplink resources granted by the gNB. This message contains critical information, such as the UE's identifier (C-RNTI or S-TMSI), which is needed for contention resolution.

Repetitions and Redundancy Versions

In certain cases, the UE may be required to repeat its PUSCH transmission (for example, in poor signal conditions). The number of repetitions can be indicated in the RAR UL grant. The modulation and coding scheme (MCS) used for each repetition is determined by fields in the RAR UL grant or DCI format 0_0. If the RAR does not specify the number of repetitions, the UE assumes a default value of 1 repetition .

4. PDSCH with UE Contention Resolution Identity (Msg4) - Section 8.4

The final step in the contention-based random access procedure is resolving contention, where multiple UEs might have used the same preamble.

• Purpose: The gNB sends this message to finalize the connection with the UE, confirming its identity.
• Content:
• The PDSCH (Physical Downlink Shared Channel) contains the UE's identifier (e.g., C-RNTI or S-TMSI).
• If the identifier matches the one sent by the UE in Msg3, the contention is resolved, and the UE can continue communication. Otherwise, the UE restarts the random access procedure.

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