The Random-Access Channel (RACH) Procedure in 5G networks is essential for synchronizing a UE with the gNB for initial uplink access or during other events requiring re-synchronization.
The RACH procedure enables UEs to initiate communication with the network and establish timing alignment for uplink transmission. RACH operates on the Physical Random-Access Channel (PRACH), where UEs send a RACH preamble to request resources and initiate synchronization.
Why RACH?
RACH is essential in 5G NR for several reasons:
Initial Access: When a UE is first powered on, it uses RACH to establish communication with the gNB. Without this step, the UE cannot join the network.
Uplink Synchronization: RACH helps in aligning the UE’s timing to the network’s, enabling efficient uplink communication.
Mobility Support: During handovers or beam failures, the UE may need to resynchronize with a new gNB or beam; RACH facilitates this process.
Efficient Resource Allocation: By allowing the UE to communicate its presence and request resources, RACH helps the gNB allocate resources accurately, avoiding unnecessary interference.
When is RACH Triggered?
RACH can be triggered in various scenarios within 5G NR networks:
Initial Access from RRC_IDLE: When a UE first joins the network or reestablishes a connection.
RRC Connection Re-establishment: If the UE loses connection and needs to reestablish its link.
Handover: During mobility events, the UE may need to transition to a new cell or beam, requiring re-synchronization.
Out-of-Sync in RRC_CONNECTED: If the uplink or downlink synchronization is lost while the UE is already connected, RACH is triggered to re-sync.
Beam Failure Recovery: If the current beam degrades due to signal loss or blockage, RACH enables the UE to find a new beam to connect to the gNB.
Transition from RRC_INACTIVE: When the UE transitions from an inactive state to an active connection, requiring re-synchronization with the gNB.
Time Alignment for SCell Addition: When adding a Secondary Cell (SCell) in carrier aggregation, RACH is used to establish timing alignment for the new cell.
Request for Other System Information (SI): When the UE requests additional system information beyond what is provided in the initial connection.
Types of RACH Procedure
There are two types of RACH procedures in 5G:
Contention-Based Random Access (CBRA)
Multiple UEs can initiate the same preamble, which might lead to contention, and a contention resolution is performed.
When a UE wants to access the network, it transmits a specific signal, known as a PRACH Preamble. This preamble acts like a "signature" to identify the UE’s access request. Each cell has 64 unique preamble signatures available, and each UE randomly picks one.
Since multiple UEs may pick the same preamble at the same time, there is a chance of collision or contention. When contention happens, the network must perform additional steps to resolve which UE will gain access.
Steps in the Contention-Based RACH Procedure
This process generally involves four steps, as follows:
UE → Network: RACH Preamble Transmission (Step 1 - Msg1)
The UE randomly selects a preamble and transmits it to the network on the Physical Random-Access Channel (PRACH).
The network receives this request but doesn’t yet know if multiple UEs used the same preamble.
Beam and Preamble: During synchronization, the UE finds a suitable beam and initiates random access by sending a RACH preamble identified by a Random-Access Preamble ID (RAPID) using a Zadoff-Chu sequence.
Preamble Selection: The UE selects a preamble from the CBRA pool based on totalNumberOfRA-Preambles or contention-based preambles linked to the SS/PBCH block.
SSB and System Info: The UE receives SSB, measures RSRP across beams, picks one, and retrieves system information.
RMSI for PRACH Configuration: In standalone or NSA scenarios, RMSI provides PRACH configuration, including time-frequency resources and preamble format.
RA-RNTI Calculation and Transmission
s_id: Index of the first OFDM symbol in PRACH (0 to 13).
t_id: Index of the first slot in PRACH within a system frame (0 to 79).
f_id: Index of the PRACH in the frequency domain (0 to 7).
ul_carrier_id: UL carrier for Msg1 transmission (0 for NUL, 1 for SUL).
RA-RNTI Calculation: The UE calculates an RA-RNTI for Msg1 transmission if conditions are met.
Transmission and Power: The UE calculates transmission power and sends the preamble.
Frequency and Time Domain Setup: PRACH preamble frequency and time locations are determined by msg1-FDM, msg1-FrequencyStart, and prach-ConfigurationIndex.
RA-RNTI and PDCCH: The gNB calculates RA-RNTI based on the PRACH occasion for addressing the UE on PDCCH.
MAC Layer Role: MAC instructs the physical layer for PRACH transmission using specified resources and RA-RNTI.
Network → UE: Random Access Response (Step 2 - Msg2)
The network replies with a Random-Access Response (RAR), which includes:
Timing Advance: Helps the UE synchronize its transmission timing with the network.
Temporary C-RNTI: A temporary identifier to help distinguish between UEs during further steps.
Uplink Grant: Resources allocated for the UE’s next message.
If multiple UEs used the same preamble, they all receive this RAR message and move to the next step using the same resources.
After sending the Random Access (RA) preamble, the UE waits for an acknowledgment from the gNB via the Random-Access Response (RAR).
The UE checks for RAR within a window (ra-ResponseWindow), specifically looking for a DCI format 1_0 with CRC scrambled by RA-RNTI or C-RNTI.
RAR Window: Configured by rar-WindowLength IE in a SIB message or in rrcReconfiguration for contention-free RACH.
ra-ResponseWindow: Sourced from RACH-ConfigCommon, indicating when the UE should monitor for RAR on the PDCCH.
If RAR includes a MAC subPDU with a matching RAPID (Random Access Preamble Identifier), the UE considers the RAR successful.
RAR Content (if not initiated for "SI request"):
RAPID: Identifies the RA preamble received by the network.
Timing Advance Command: Adjusts the UE’s timing for uplink.
UL Grant: Grants uplink resources for Msg3.
Temporary C-RNTI: A temporary identifier for UE communication with gNB.
Upon successful decoding of PDCCH, the UE reads the PDSCH carrying RAR data.
UE → Network: L2/L3 Message Transmission (Step 3 - Msg3)
Each UE sends its unique Layer 2 or Layer 3 (L2/L3) message on the allocated resources. This message can include information like connection requests or initial registration details.
If two or more UEs are using the same resources (due to contention), their signals may interfere with each other, potentially causing the network to fail in decoding any of the messages.
For Contention-Based RACH (CBRA):
UE sends Msg3 on the PUSCH using the uplink grant from the MAC RAR.
UE includes its identity in Msg3 to aid in contention resolution.
A Temporary C-RNTI is used for Msg3 transmission.
Msg3 Content Varies:
Case 1: If UE already has a C-RNTI (e.g., in RRC_CONNECTED state):
RA Initiated for Beam Failure Recovery (BFR): UE includes C-RNTI MAC CE in Msg3.
RA Initiated by PDCCH Order: If preamble ID in PDCCH is 0b0000000, UE includes C-RNTI MAC CE.
RA Initiated by MAC or RRC:
During CBRA in synchronized RRC reconfiguration (e.g., handover), UE includes RRCReconfigurationComplete in Msg3.
If SR (Scheduling Request) is not configured, has failed, or if uplink is Out-of-Sync, UE may include Buffer Status Report (BSR) in Msg3 for uplink resource requests.
Network → UE: Contention Resolution (Step 4 - Msg4)
The network processes the received messages:
If it successfully decodes a message from only one UE, it sends a confirmation message to that UE. The successful UE receives a HARQ ACK (Hybrid Automatic Repeat Request Acknowledgement), which indicates it has successfully accessed the network.
If both messages collide and cannot be decoded, no UE receives the HARQ ACK, and they both restart the process.
In some cases, if the network decodes multiple messages, it can send additional steps to handle the conflict.
Case 1: UE with C-RNTI:
If the UE already has a C-RNTI, the network resolves contention by addressing the UE’s C-RNTI on the PDCCH.
The UE considers the RA process successful, discards Temporary C-RNTI, and no explicit contention resolution message is required.
If no PDCCH addressed to C-RNTI is detected during the ra-ContentionResolutionTimer, the UE assumes contention resolution failed.
Case 2: UE without C-RNTI:
If the UE doesn’t have a valid C-RNTI, the gNB sends Msg4 with a “UE Contention Resolution Identity” using Temporary C-RNTI.
This identity is checked against the UE’s transmitted identity. If it matches, contention resolution is successful.
If contention resolution succeeds (except for SI requests), the UE promotes Temporary C-RNTI to C-RNTI.
If no resolution identity is received within the timer, the UE retries the RA procedure by discarding Temporary C-RNTI and starting over.
Contention-Free Random Access (CFRA)
Specific RACH preambles are assigned to individual UEs by the network, reducing collision chances and eliminating contention.
In specific cases, such as during a handover, the network needs to ensure there is no contention. Here’s how the non-contention RACH procedure works:
UE Preparation: The network assigns a specific preamble to the UE, ensuring no other UEs will use the same preamble, eliminating any risk of collision.
RACH Preamble Transmission: The UE transmits the preamble on the PRACH.
Network Response: The network responds with the RAR message, which includes timing advance, a unique identifier (C-RNTI), and uplink grant information.
Since each UE is assigned a unique preamble, no contention resolution is necessary in this method. This approach is typically used when the network anticipates uplink data, such as during handovers or high-priority situations.
What is a Preamble?
In the context of 5G NR), a preamble is a unique sequence used by the User Equipment (UE) to initiate communication with the network by requesting access to the gNB. The preamble acts as a form of identity and helps the gNB identify and allocate resources to individual UEs attempting access. In 5G NR, the PRACH is used to transmit these preambles, which are generated based on specific sequence designs.
Preamble Structure
A 5G NR preamble is divided into two main parts:
Cyclic Prefix (CP): A guard interval at the start of the preamble to prevent inter-symbol interference.
Preamble Sequence: The unique sequence based on a Zadoff–Chu (ZC) design, optimized for high detection accuracy and minimal interference.
The cyclic prefix length, sequence length, guard period, and number of repetitions vary based on the preamble format, which is selected based on the cell’s radius, speed of UE movement, and other conditions.
Preamble Format in 5G NR
Preambles in 5G NR are categorized into two main types to address different network conditions and cell sizes:
Long Sequence Preambles
Short Sequence Preambles
These preamble formats vary in terms of sequence length, bandwidth, subcarrier spacing, and application, allowing 5G networks to adapt to both small and large cell environments.
Long Sequence Preambles
Long Sequence Preambles are typically used in larger cells where coverage range is crucial. They are derived from a root sequence with a length of 839 symbols. The main formats under long sequence preambles are Format 0, Format 1, Format 2, and Format 3.
Format 0 and Format 1:
Subcarrier Spacing: Both use a subcarrier spacing of 1.25 kHz.
Bandwidth: The occupied bandwidth is 1.08 MHz.
Applications: These are suitable for both low-speed UEs and cells with wide coverage areas (Format 1 supports larger cells with a sequence duration of 3 ms).
Format 3:
Subcarrier Spacing: This uses a wider subcarrier spacing of 5 kHz.
Bandwidth: The occupied bandwidth increases to 4.32 MHz.
Applications: Format 3 is optimal for high-speed UEs due to the higher subcarrier spacing, which supports a shorter transmission time and reduces delay in fast-moving environments.
These formats can also support restricted sets (Type A and Type B), which are configurations used to mitigate Doppler shift effects, particularly in high-mobility scenarios.
Short Sequence Preambles
Short Sequence Preambles are derived from a root sequence of 139 symbols, making them suitable for small cell environments or high-density deployments. These are categorized as Formats A1, A2, A3, B1, B2, B3, B4, C0, and C2. Each format within short sequences has its specific properties to suit various small cell scenarios and beam management needs.
Subcarrier Spacing: Short sequence preambles offer higher subcarrier spacing options of 15 kHz, 30 kHz, 60 kHz, and 120 kHz.
Applications: These formats are optimized for compact cells, providing flexibility in high-density areas or for small cells deployed in urban environments where shorter sequences and higher subcarrier spacing are advantageous for managing interference and maintaining signal quality.
PRACH Config Index
The PRACH Configuration Index is a parameter used in 5G NR to determine when and how the PRACH sequences are transmitted in the time domain. It specifies various aspects of the PRACH operation, such as the timing, duration, and frequency of the PRACH transmission occasions. The purpose of PRACH Configuration Index is to guide the UE on when to send a random-access preamble, ensuring proper synchronization with the gNB.
The process of generating the PRACH time-domain sequence includes these steps:
Frequency Domain Data Generation: A frequency-domain sequence is created based on subcarrier spacing and preamble format.
IFFT Transformation: This sequence is converted to the time domain using IFFT, enabling transmission over the air.
Complex Equation: The equation for this sequence, detailed in 3GPP TS 38.211, Section 5.3.2, is complex, but not every term is essential for practical understanding.
Key Parameters: Important factors like preamble format, cyclic prefix length, and guard time shape the PRACH transmission structure, while smaller parameters are often omitted in simplified explanations.
Random access configurations for FR1 and paired spectrum/supplementary uplink.
RACH Configuration Index settings, which control the timing and structure of PRACH transmissions in 5G NR:
PRACH Configuration Index: Determines the specific PRACH setup for time and frequency resources.
Preamble Format: Defines the PRACH preamble structure and timing, with Format 0 shown here.
nSFN mod x=y: Specifies that PRACH occurs only when the System Frame Number (SFN) modulo x=16 equals y=1.
Subframe Number: Indicates the subframe in which the PRACH transmission is scheduled.
Starting Symbol: Shows the starting OFDM symbol; here, PRACH begins at symbol 0 in each subframe.
Number of PRACH Slots within a Subframe: Indicates the count of PRACH slots in a subframe; a dash (–) means not applicable.
Number of Time-Domain PRACH Occasions within a PRACH Slot: Shows PRACH occasions per slot; a dash (–) indicates none for these configurations.
PRACH Duration: Specifies PRACH duration in OFDM symbols, with 0 indicating no additional duration.
Example 1: PRACH Config Index 21
For PRACH Configuration Index 21, let’s calculate the starting symbol index l using the formula:
Note l = starting symbol index for the PRACH transmission within the subframe
From the configuration and the table, we know:
Subframe Number: 3 and 8 (PRACH transmissions can happen in these subframes).
Starting Symbol ( l0 ): 0 (PRACH starts at the beginning of the subframe).
Number of PRACH Slots within a Subframe: Slots 3 and 8.
Duration (NRA,dur ): 0 (no extra duration beyond the slot timing).
Now, let's calculate l for each PRACH slot within the designated subframes (3 and 8).
Calculation of l for Each PRACH Slot
For Slot 3 in Subframe 3:
l=0+0+14×3+0=42
For Slot 8 in Subframe 8:
l=0+0+14×8+0=112
For PRACH Configuration Index 21:
In Subframe 3, using PRACH Slot 3, the starting symbol index l is 42.
In Subframe 8, using PRACH Slot 8, the starting symbol index l is 112.
Example 2: PRACH Config Index 87
Preamble Format A1: This format is commonly used for PRACH with a specific duration and bandwidth requirement, compatible with higher frequencies.
Starting Symbol (0): Indicates that the PRACH starts at the very beginning of the designated subframe.
Subframe Numbers (4, 9): PRACH occasions occur in subframes 4 and 9, meaning that within each radio frame, PRACH transmissions are scheduled at these subframes.
Number of PRACH Slots within a Subframe (1): Only one slot is available within each of these subframes for PRACH.
Number of Time-Domain PRACH Occasions within a PRACH Slot (6): This means there are 6 distinct PRACH transmission opportunities within each PRACH slot in these subframes.
PRACH Duration (2): This duration refers to the length of the PRACH transmission, calculated in milliseconds or symbols based on the configuration.
Calculation of PRACH Occasions per Frame
Since each frame has two subframes (4 and 9) allocated for PRACH, and each subframe provides 1 slot with 6 PRACH occasions, the total PRACH occasions per frame are calculated as follows:
Total PRACH Occasions per Frame= (PRACH Slots per Subframe) × (PRACH Occasions per Slot) ×(Number of Subframes)
Substitute values:
PRACH Occasions per Frame=1×6×2=12
Thus, there are 12 PRACH occasions per frame for PRACH Configuration Index 87.
Random access configurations for FR1 and paired spectrum/supplementary uplink.
zeroCorrelationZoneConfig and Ncs
Ensures collision-free PRACH preambles, allowing multiple UEs to access the network simultaneously. Minimizes interference, leading to reliable preamble detection by the network.
zeroCorrelationZoneConfig:
A parameter set by the network in the RRC message to define the minimum correlation separation needed between preamble sequences.
It determines the cyclic shift amount (Ncs) required to minimize interference between UEs.
Ncs (Cyclic Shift Length):
The cyclic shift applied to the base preamble sequence, calculated based on zeroCorrelationZoneConfig.
Ensures that each UE has a unique PRACH preamble by separating sequences to avoid collisions.
Relationship between zeroCorrelationZoneConfig and Ncs:
Ncs is derived by looking up zeroCorrelationZoneConfig in mapping tables (e.g., Table 6.3.3.1-5 and Table 6.3.3.1-6) for Long Sequence RACH Preambles.
A higher zeroCorrelationZoneConfig results in a larger Ncs value, increasing sequence separation and reducing interference.
Root Sequence Index
Root Sequence Index is used to generate PRACH preambles. This index determines the base sequence from which UEs derive their unique preamble sequences by applying cyclic shifts.
In both LTE and NR, the Root Sequence Index selects a specific base sequence for PRACH preambles.
NR uses different numbering systems at the RRC layer and the Physical layer for this index.
The RRC layer assigns the PRACHRootSequenceIndex to each UE, which needs to be translated into a sequence number (u) at the Physical layer.
Table 6.3.3.1-3 in 3GPP 38.211 defines the mapping between PRACHRootSequenceIndex (i) and sequence number (u) for NR preamble formats with L_RA = 839 (long sequence length).
For L_RA = 1151, a separate mapping table (defined in 3GPP 38.211, Table 6.3.3.1-4) is used to convert PRACHRootSequenceIndex (i) from the RRC layer to sequence number (u) at the Physical layer.
This mapping ensures consistency between the logical (RRC) and physical layers in PRACH preamble generation, allowing the network to properly identify and decode the UE’s preamble.
Both L_RA = 839 and L_RA = 1151 use similar mechanisms but have distinct tables for mapping PRACHRootSequenceIndex to sequence numbers, as defined in 3GPP 38.211.
Likewise L_RA 139 and 571 also there
Long vs Short PRACH Sequences: Role of zeroCorrelationZoneConfig and Cyclic Shift (Ncs)
Unlike LTE, where a single sequence length is used, NR supports two types of sequence lengths for PRACH: Long Sequence and Short Sequence. These sequences serve different deployment scenarios, and each is influenced by parameters like zeroCorrelationZoneConfig and Ncs.
Long Sequences target large cells, while Short Sequences are optimized for smaller cells and indoor environments.
zeroCorrelationZoneConfig and Ncs parameters define cyclic shifts and restricted sets, ensuring collision-free PRACH access and better signal detection across UEs.
Long Sequence (L_RA = 839):
Length: 839
Preamble Formats: Four, based on LTE preamble formats, for large cell deployments.
Deployment: Only in FR1 with subcarrier spacings of 1.25 kHz or 5 kHz.
zeroCorrelationZoneConfig and Ncs: Used to define restricted sets, minimizing interference and optimizing unique cyclic shifts for each UE.
Short Sequence (L_RA = 139):
Length: 139
Preamble Formats: Nine, introduced in NR for smaller cells, normal cells, and indoor deployments.
Deployment: Usable in both FR1 (15 or 30 kHz) and FR2 (60 or 120 kHz).
RACH Occasion in 5G NR
In 5G NR, a RACH Occasion (RO) is a specific time-frequency resource block allocated for receiving PRACH preambles from UEs. The concept of RACH Occasion is more complex in NR compared to LTE due to the use of multiple beams and the mapping of Sync Signal Blocks (SSB) to RACH occasions.
An area in both time and frequency where the network listens for RACH preambles from UEs.
In NR, multiple RACH occasions can exist to correspond with different beams.
SSBs are transmitted by the gNB (5G base station) with specific beams, allowing UEs to choose the strongest beam.
When a UE selects an SSB, it sends its PRACH preamble in a specific RACH Occasion mapped to that SSB, helping the network identify the chosen beam.
msg1-FDM: Specifies the number of RACH occasions allocated in the frequency domain within a single time slot.
ssb-perRACH-OccasionAndCB-PreamblesPerSSB: Determines the number of SSBs mapped to each RACH Occasion and the number of unique preamble indices assigned per SSB.
Mapping Logic:
The RACH preambles are organized based on a specific order as outlined in 3GPP 38.213 - Section 8.1:
Preamble Indexes: Start with the lowest preamble index within each RACH occasion.
Frequency Resources: Increase the frequency index if multiple RACH occasions exist in the frequency domain.
Time Resources: Proceed with time indexing for RACH occasions within the same slot.
PRACH Slots: Increase the slot index in case of multiple PRACH slots.
References:
3GPP TS 38.211 - Physical Channels and Modulation
3GPP TS 38.213 - Physical Layer Procedures for Control
3GPP TS 36.211 - E-UTRA; Physical Channels and Modulation
5G NR PRACH Design and Random Access Procedure (Keysight, Qualcomm, Nokia white papers)
"5G NR: The Next Generation Wireless Access Technology" by Erik Dahlman, Stefan Parkvall, and Johan Sköld
"Fundamentals of 5G Mobile Networks" by Jonathan Rodriguez
3GPP R1-1801040: RAN1 Meeting Contributions
Academic Research Papers on PRACH in 5G NR (IEEE Xplore, Elsevier, Springer)
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