Comparison of RACH Procedures in LTE and 5G-NR

Random Access Channel (RACH) is one of the first steps in establishing communication between User Equipment (UE) and the base station. While LTE introduced robust mechanisms for initial access and handovers using RACH, 5G-NR has significantly evolved these procedures to cater to new paradigms such as beamforming, mmWave operations, massive machine-type communications (mMTC), and ultra-reliable low-latency communication (URLLC).

Purpose and Use Cases of RACH

LTE:

In LTE, the RACH procedure allows UEs to:

• Initiate network access
• Perform handover
• Achieve uplink synchronization
• Recover from Radio Link Failure (RLF)

It supports both contention-based and contention-free access. The UE may trigger access during power-on or during mobility procedures, while the eNodeB may initiate contention-free access during handovers.

5G-NR:

In 5G-NR, these functions remain but with extended capabilities. Besides the standard access cases:

• RACH in 5G is vital for beam-based initial access, especially in mmWave (FR2) where directional communication is key.
• It supports Network Slicing, allowing differentiated RACH parameters for different slices or services.
• Contention-free RACH becomes more important, supporting scenarios like beam failure recovery, triggered by the gNB.

Types of Random Access Procedures

In LTE and 5G-NR, random access procedures are categorized into:

• Contention-Based RACH: UE selects a random preamble; contention resolution is needed.
• Contention-Free RACH: Pre-configured preamble is assigned to UE to avoid collision.

While LTE used contention-free primarily during handovers, 5G NR leverages it broadly—for example, when recovering from beam failure or when gNB triggers an action in URLLC setups. This improves reliability and response time.

Preamble Formats and Subcarrier Spacing

LTE:

• Uses Zadoff-Chu sequences for 64 preambles.
• Limited to narrowband allocations (1.08 MHz) with fixed subcarrier spacing.

5G NR:

• Each beam can have 64 unique preambles, significantly expanding the contention domain in dense deployments.
• Multiple preamble formats are defined based on subcarrier spacing (SCS) and frequency range (FR1/FR2).
• Supports larger bandwidths and flexible SCS to match deployment scenarios and reduce latency.

Preamble Timing and Power Control

5G introduces flexibility in preamble transmission:

• Preambles in FR2 are significantly shorter, allowing faster access.
• Power ramping can adapt based on beamforming direction, improving success probability in varying path loss conditions.
• gNB can configure backoff and retry mechanisms specific to beams, improving performance in multi-beam scenarios.

Step RACH Procedure – Detailed View

SSB Index Association: Msg1 is associated with a specific SSB/beam, critical in directional transmission.

Flexible Scheduling: Msg3 and Msg4 allow dynamic scheduling for different slices or latency profiles.

RACH Configuration: SIB vs Beam-Specific Adaptations

LTE:

RACH parameters are broadcast in SIB2 and are common across all UEs and cells.

5G NR:

RACH can be configured per beam, SSB, or even per UE group, enabling extreme flexibility and QoS-based access control.

Advanced Features in 5G NR RACH

1. Beam Management:

• Not available in LTE.
• SSB-index association is vital in 5G for beam-specific random access, particularly in FR2.

2. URLLC Enhancements:

• 5G NR introduces early data transmission by combining Msg1 and Msg3 into MsgA, and Msg2/Msg4 into MsgB.
• Reduces latency dramatically to <1 ms.

3. 2-Step RACH Procedure (Rel-16):