Description
The Physical Random Access Channel (PRACH) is a fundamental uplink channel in 3GPP wireless technologies, including UMTS (UTRA) and LTE/5G NR (E-UTRA/NR). Its primary function is to allow a User Equipment (UE) to achieve uplink synchronization with the network and request an initial allocation of resources when it has no dedicated scheduling request channel available. The PRACH procedure, often called the Random Access (RA) procedure, is the entry point for a UE to transition from an idle or inactive state to a connected state, enabling it to transmit data or signaling.
The operation of the PRACH involves the transmission of a specific preamble sequence. In LTE and 5G NR, the network configures a set of available preamble sequences, which are derived from Zadoff-Chu sequences known for their good auto-correlation and cross-correlation properties. The UE randomly selects one preamble from a designated subset (contention-based) or uses a specifically assigned one (contention-free, e.g., for handover). The UE then transmits this preamble on a specific time-frequency resource defined by the PRACH configuration index, which dictates the system frame number, subframe number, and frequency location. The preamble format defines the duration and structure of the transmission, accommodating different cell sizes and scenarios.
Upon transmitting the preamble, the UE listens for a Random Access Response (RAR) from the network within a configured window. The RAR, sent on the PDCCH and PDSCH, contains a timing advance command to adjust the UE's transmission timing, an initial uplink grant for the subsequent Message 3 transmission (e.g., an RRC Connection Request), and a temporary Cell Radio Network Temporary Identifier (C-RNTI). If the UE receives a RAR corresponding to its transmitted preamble, it proceeds with the remaining steps of the RA procedure. In a contention-based scenario, if multiple UEs select the same preamble, a collision occurs, requiring a backoff and retransmission mechanism.
Architecturally, the PRACH is a physical layer channel defined in the PHY specifications (TS 25.211, 36.211, 38.211). Its configuration and parameters are managed by higher layers via RRC signaling, detailed in the RRC protocol specifications (TS 25.331, 36.331, 38.331). The PRACH configuration includes parameters like the root sequence index, preamble format, time/frequency resources, and power ramping parameters. The eNodeB/gNB's receiver performs correlation detection on the received signal to identify the transmitted preamble and estimate the timing offset, which is crucial for establishing and maintaining uplink orthogonality in OFDMA/SC-FDMA systems.
Purpose & Motivation
The PRACH exists to solve the fundamental problem of initial access and uplink synchronization in a shared wireless medium. Before a UE can engage in scheduled communication, it must first alert the network to its presence and align its transmission timing to prevent interference with other users. In the absence of a dedicated control channel, a random access mechanism is necessary for a UE to request the establishment of such a channel.
Historically, in pre-3GPP systems and early cellular networks, initial access methods were often simpler but less efficient and scalable. The design of PRACH in UMTS and its evolution through LTE and 5G NR was motivated by the need for a robust, low-latency, and capacity-scalable access method suitable for dense networks and a wide range of deployment scenarios. It addresses the limitations of fixed-access slots and non-orthogonal preambles by introducing configurable Zadoff-Chu sequences with zero auto-correlation zones, improving detection performance and reducing false alarm rates in high-interference environments.
The evolution of PRACH also supports new use cases. For example, in LTE-A and 5G NR, new preamble formats were introduced for very large cells (e.g., for rural coverage) and for high-speed scenarios (e.g., high-speed trains). Furthermore, the PRACH design in NR supports flexible numerology and wide bandwidths, enabling efficient access in millimeter-wave spectrum and for diverse services like massive IoT and ultra-reliable low-latency communication (URLLC), where fast and reliable access is paramount.
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (86 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
In Release 15, specific corrections and enhancements were introduced for the PRACH function, including a correction on Random Access Preamble groups for Early Data Transmission (EDT). Furthermore, a Late Drop Change Request specifically addressed PRACH Power Ramping Counter Suspension, and another correction was made to the time gap definition for the random access procedure.
- Clarification on CRC attachment for DL-SCH and PCH transport channels in NB-IoT TS 36.212CR0285
- 36.300 CR on Correction of Physical Layer Resource to Cell Resource TS 36.300CR1211
- Minor corrections to services provided by physical layer TS 36.302CR1195
- Correction on the logical channel selection in sidelink LCP TS 36.321CR1330
- Corrections to random access power control for TDD in 36.321 TS 36.321CR1362
- Correction on Random Access Preamble groups for EDT TS 36.321CR1446
+ 17 more changes
In Release 16, key enhancements for the PRACH function included the introduction and finalization of specific performance requirements for an enhanced High-Speed Train (HST) scenario. These were detailed in updates to the relevant test specifications (TS 36.104 and TS 36.141). Additionally, corrections were made to UE assumptions regarding resource block set configuration for PRACH.
- CR to TS 36.104: Introduction of PRACH performance requirements for enhanced HST scenario TS 36.104CR4884
- CR to TS 36.141: Introduction of PRACH performance requirements for enhanced HST senario TS 36.141CR1242
- Introduction of Physical Layer Enhancements for URLLC TS 38.202CR0012
- Introduction of Physical Layer Enhancements for NR URLLC TS 38.212CR0026
- Introduction of shared spectrum channel access TS 38.213CR0071
- Mapping of Uplink Traffic to Backhaul RLC Channels TS 38.300CR0255
+ 24 more changes
In Release 17, key PRACH enhancements included corrections to enable the parallel transmission of PRACH with SRS, PUCCH, or PUSCH, and corrections for random-access based small data transmission (SDT). The release also introduced specific test cases for 2-Step PRACH and provided corrections for power scaling of PRACH on a Secondary Cell during uplink carrier aggregation.
- Correction on simultaneous reception of SDT and other channels in TS 38.202 TS 38.202CR0026
- CR on ChannelAccess-Cpext in Fallback DCI TS 38.212CR0118
- CR on channel access type indication in non-fallback DCI TS 38.212CR0125
- Corrections on intra-UE multiplexing and semi-static channel occupancy TS 38.212CR0136
- Corrections of random-access based small data transmission TS 38.213CR0358
- CR for ChannelAccess-CPext in RAR UL grant in FR2-2 TS 38.213CR0370
+ 7 more changes
In Release 18, the PRACH enhancements primarily focused on corrections and clarifications for specific operational scenarios, particularly for LTM (Limited Tx Mode) and CFRA (Contention-Free Random Access). Key updates included refined procedures for PRACH collision handling in LTM, adjustments for PRACH retransmission and repetition timing, and specific rules for managing collisions between PRACH and SSB transmissions in Two TA (Timing Advance) scenarios. These changes aimed to improve the reliability and coexistence of the random access procedure under more complex conditions.
- Introduction of sidelink channel access procedures for Rel-18 NR sidelink evolution TS 38.201CR0003
- Correction on determination of restricted type for candidate cell PRACH transmission in LTM TS 38.211CR0137
- Correction on mapping PSFCH to physical resources TS 38.211CR0141
- Corrections to PRACH transmission for LTM TS 38.211CR0147
- Corrections on PRACH association indicator in PDCCH order in 38.212 TS 38.212CR0192
- CR on the PRACH retransmission indicator field included in the PDCCH order TS 38.212CR0213
+ 7 more changes
In Release 19, key PRACH enhancements included clarifications and corrections for operation with channel bandwidths below 10 MHz, such as the specific introduction and restriction of a 7MHz channel bandwidth. Furthermore, the release introduced a correction to PRACH signal generation for LTM (Layer-to-Mapping) and provided a CR on the determination of the cyclic shift for PRACH transmission in 2TA (Two-Timing Advance) scenarios.
- CR to TS 38.176-2: restriction of 7MHz channel bandwidth introduction TS 38.176CR0087
- (NR_FR1_7MHz_BW-Perf) CR to TS 38.176-2 with clarification for channel bandwidths below 10 MHz TS 38.176CR0094
- Corrections on R19 NES adaptation of common channel/signals TS 38.212CR0243
- Corrections on R19 NES adaptation of common channel/signals TS 38.213CR0753
- CR to 38.174 with clarification for channel bandwidths below 10 MHz TS 38.174CR0136
- CR on determination of cyclic shift for PRACH transmission in 2TA TS 38.211CR0161
+ 1 more changes
Explore further
Broader topics and technologies where PRACH plays a role.
Defining Specifications
3GPP specifications that define or reference PRACH, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TR 21.905 vj00 | 3GPP Technical Terms and Definitions | Rel-19 |
| TS 25.202 vj00 | 7.68Mcps TDD Option Technical Specification | Rel-19 |
| TS 25.211 vj00 | UTRA FDD Layer 1: Transport & Physical Channels | Rel-19 |
| TS 25.212 vj00 | UTRA FDD Layer 1 Multiplexing & Channel Coding | Rel-19 |
| TS 25.213 vj00 | UTRA FDD Spreading and Modulation | Rel-19 |
| TS 25.214 vj00 | UTRA FDD Physical Layer Procedures | Rel-19 |
| TS 25.221 vj00 | UTRA TDD Physical Layer Specification | Rel-19 |
| TS 25.223 vj00 | UTRA Physical Layer TDD Spreading & Modulation | Rel-19 |
| TS 25.224 vj00 | UTRA TDD Physical Layer Procedures | Rel-19 |
| TS 25.225 vj00 | UTRA TDD Physical Layer Measurements | Rel-19 |
| TS 25.331 vj00 | UTRAN RRC Protocol Specification | Rel-19 |
| TS 25.423 vj00 | UTRAN RNSAP Specification | Rel-19 |
| TS 25.430 vj00 | Introduction to Iub Interface Specifications | Rel-19 |
| TR 25.931 vj00 | UTRAN Signalling Procedures Examples | Rel-19 |
| TS 36.104 vj10 | Base Station (BS) radio transmission and reception | Rel-19 |
| TS 36.116 vj00 | E-UTRA Relay RF Requirements | Rel-19 |
| TS 36.117 vj00 | E-UTRA Relay RF Test Methods & Requirements | Rel-19 |
| TS 36.133 vj20 | E-UTRA RRM Requirements | Rel-19 |
| TS 36.141 vj00 | E-UTRA BS Conformance Testing | Rel-19 |
| TS 36.201 vj00 | LTE Physical Layer General Description | Rel-19 |
| TS 36.211 vj10 | LTE Physical Layer Specification | Rel-19 |
| TS 36.212 vj10 | LTE Multiplexing and Channel Coding | Rel-19 |
| TS 36.213 vj10 | LTE Physical Layer Procedures | Rel-19 |
| TS 36.300 vj00 | E-UTRAN Radio Interface Protocol Architecture Overview | Rel-19 |
| TS 36.302 vj00 | E-UTRA Physical Layer Services | Rel-19 |
| TS 36.321 vj00 | E-UTRA MAC Protocol Specification | Rel-19 |
| TS 36.878 vd00 | LTE Performance Enhancements for High Speed Scenarios | Rel-13 |
| TR 37.911 vj00 | 3GPP 5G NTN Self-Evaluation Report | Rel-19 |
| TS 38.133 vj20 | 5G UE Radio Requirements for RRC_IDLE Mobility | Rel-19 |
| TS 38.174 vj10 | NR Integrated Access and Backhaul Radio Spec | Rel-19 |
| TS 38.176 vj20 | IAB Conformance Testing Specification | Rel-19 |
| TS 38.201 vj00 | NR Physical Layer General Description | Rel-19 |
| TS 38.202 vj00 | 5G NR Physical Layer Services | Rel-19 |
| TS 38.211 vj10 | NR Physical Channels and Modulation | Rel-19 |
| TS 38.212 vj10 | NR Multiplexing and Channel Coding | Rel-19 |
| TS 38.213 vj10 | NR Physical Layer Control Procedures | Rel-19 |
| TS 38.300 vj00 | NG-RAN Overall Description | Rel-19 |
| TS 38.521 vj20 | NR Physical Layer UE Conformance Testing | Rel-19 |
| TS 38.522 vj11 | UE Conformance Test Applicability Statement | Rel-19 |
| TS 38.523 vj20 | 5G NR UE Conformance Testing: Idle/Inactive | Rel-19 |
| TR 38.808 vh00 | Study on NR above 52.6 GHz to 71 GHz | Rel-17 |
| TS 38.811 vf40 | Study on NR Support for Non-Terrestrial Networks | Rel-15 |
| TR 38.830 vh00 | NR Coverage Enhancements Study | Rel-17 |
| TR 38.869 vi00 | Study on low-power wake up signal and receiver for NR | Rel-18 |
| TR 38.889 vg00 | NR-based access to unlicensed spectrum study | Rel-16 |
| TR 38.903 vj00 | Test Tolerances & Measurement Uncertainties | Rel-19 |
| TS 43.064 vj00 | GPRS Radio Interface Lower-Layer Functions | Rel-19 |
| TS 45.820 vd10 | CIoT for Internet of Things | Rel-13 |