Description
The Uplink Shared Channel (UL-SCH) is a fundamental transport channel defined in both LTE (E-UTRA) and NR (New Radio) standards. It is the principal conduit for transmitting user data, higher-layer control information (e.g., RRC messages, NAS messages), and some physical layer control information from the UE to the gNB (in NR) or eNB (in LTE). The channel is characterized by its shared nature, meaning its physical resources are dynamically allocated to different UEs by the network scheduler on a per-subframe or per-slot basis. This dynamic scheduling, signaled via the Physical Downlink Control Channel (PDCCH), allows for highly efficient statistical multiplexing of uplink traffic, adapting to the bursty nature of data applications.
From a procedural standpoint, the UL-SCH transport block processing involves several key physical layer steps. For a scheduled transmission, the MAC layer delivers a transport block to the physical layer. This block undergoes processes including transport block CRC attachment, code block segmentation and CRC attachment, channel coding (typically LDPC in NR, Turbo coding in LTE), rate matching, and code block concatenation. The resulting codeword is then mapped to the Physical Uplink Shared Channel (PUSCH) for transmission. A critical aspect of UL-SCH is the support for Hybrid Automatic Repeat Request (HARQ). Each transmission is associated with a HARQ process, allowing for rapid retransmissions in case of decoding failure at the receiver, which is essential for achieving high reliability and low latency.
The role of UL-SCH extends beyond mere data delivery. It is tightly integrated with uplink control signaling. For instance, Uplink Control Information (UCI), such as HARQ acknowledgments for downlink data (ACK/NACK) and Channel State Information (CSI) reports, can be multiplexed with uplink data on the PUSCH when the UE has a valid UL-SCH grant. This piggybacking improves resource efficiency. Furthermore, the UL-SCH supports adaptive modulation and coding (AMC), where the modulation scheme (e.g., QPSK, 16QAM, 64QAM, 256QAM) and coding rate are adjusted based on the instantaneous uplink channel quality reported by the UE or estimated via sounding reference signals (SRS). This ensures optimal spectral efficiency under varying radio conditions.
Purpose & Motivation
The UL-SCH was created to address the need for a flexible, efficient, and high-capacity uplink transport mechanism for packet-switched services in 3GPP's 4G LTE and 5G NR systems. Prior 3G systems like UMTS relied on dedicated channels (DCH) for user data, which were inefficient for bursty internet traffic as they reserved resources for a user even during idle periods. The shared channel paradigm, introduced with the High-Speed Uplink Packet Access (HSUPA) Enhanced Dedicated Channel (E-DCH), was fully realized and optimized in LTE with the UL-SCH.
The primary motivation was to maximize spectral efficiency and system capacity in the uplink direction, which is often the limiting factor due to UE power constraints and the need for orthogonal multiple access. By allowing the base station scheduler to dynamically assign time-frequency resources to UEs precisely when they have data to send, the UL-SCH eliminates the waste associated with permanently allocated circuits. This dynamic allocation also enables advanced techniques like frequency-selective scheduling, where the scheduler can assign resources in frequency bands where the specific UE experiences good channel conditions, thereby improving link robustness and data rates.
Furthermore, the design of UL-SCH, with its support for fast HARQ and AMC, was crucial for meeting the low latency and high reliability requirements of real-time services envisioned for LTE and, later, enhanced for NR. It provides the foundational transport layer upon which all uplink-centric services—from web browsing and file uploads to VoIP and ultra-reliable low-latency communications (URLLC)—are built, making it a cornerstone of modern cellular uplink architecture.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (236 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
Studied in Rel-8, normative work from Rel-15.
In Release 15, the UL-SCH function was updated with corrections and clarifications to its associated control procedures, including specific fixes for PUSCH power scaling and the handling of dynamic HARQ-ACK codebooks. The release also introduced corrections for the multiplexing of HARQ-ACK information within a single slot and provided clarifications on the use of the RNTI for scrambling PUSCH transmissions scheduled by a Random Access Response uplink grant.
- Clarification on CRC attachment for DL-SCH and PCH transport channels in NB-IoT TS 36.212CR0285
- Correction on the interpretation of HARQ-ACK bitmap for FeLAA in 36.212 TS 36.212CR0297
- Correction on the partial PUSCH mode field for FeLAA in 36.212 TS 36.212CR0300
- Handling of Pmax for PC2 and uplink intra-band contiguous CA capable UEs TS 36.331CR3394
- Corrections to mpdcch-UL-HARQ-ACK-FeedbackConfig TS 36.331CR3840
- Correction to dynamic HARQ codebook in NR TS 38.213CR0014
+ 26 more changes
In Release 16, key enhancements for the UL-SCH included new procedures for Uplink Power Sharing and PUSCH processing capability during Dual Active Protocol Stack (DAPS) handover, alongside corrections for uplink transmission cancellation in the same scenario. The release also introduced and refined procedures for shared spectrum channel access, impacting uplink transmissions, and addressed the prioritization between uplink and sidelink transmissions. Furthermore, corrections were made to the PUSCH Repetition Adjustment and Zero Padding Procedures for Preconfigured Uplink Resources (PUR).
- Mobility to NR operating with shared spectrum access TS 36.331CR4263
- Introduction of shared spectrum channel access TS 38.213CR0071
- Introduction of NR operation with Shared Spectrum Access to Stage 2 TS 38.300CR0199
- Mapping of Uplink Traffic to Backhaul RLC Channels TS 38.300CR0255
- PUR correction on PUSCH Repetition Adjustment and Zero Padding Procedures TS 36.212CR0361
- Correction on Sidelink Broadcast channel TS 38.212CR0062
+ 68 more changes
In Release 17, key enhancements for the UL-SCH included the introduction of uplink RRC segmentation capability and refinements to PUSCH power control parameters for multi-TRP transmissions. The release also brought corrections and clarifications for procedures involving multiple PUSCH scheduling, HARQ process numbers for higher frequency ranges, and HARQ-ACK codebook generation for features like multicast.
- Introduction of uplink RRC Segmentation capability TS 36.306CR1853
- Introduction of uplink RRC Segmentation capability TS 36.331CR4826
- CR on DCI size for Rel-17 NTN HARQ in 38.212 TS 38.212CR0116
- CR on the description of the SRS resource set indication for PUSCH repetition TS 38.212CR0117
- CR on ChannelAccess-Cpext in Fallback DCI TS 38.212CR0118
- CR on channel access type indication in non-fallback DCI TS 38.212CR0125
+ 71 more changes
In Release 18, key enhancements for the UL-SCH function included the introduction of HARQ-ACK multiplexing on a PUSCH with repetitions, allowing uplink control information to be combined with data transmissions more efficiently. Furthermore, support for oversize uplink data was introduced for early data transmission, and new reference signal procedures were defined for pathloss determination in configured grant PUSCH transmissions.
- Introduction of Rel-18 MIMO Evolution for Downlink and Uplink TS 38.212CR0145
- Introduction of MIMO Evolution for Downlink and Uplink TS 38.213CR0504
- Introduction of RS for pathloss determination of Type 1 CG PUSCH [PL RS Type 1 CG] TS 38.213CR0567
- Introduction of multiplexing in a PUSCH with repetitions HARQ-ACK associated with DL assignments received after an UL grant for the PUSCH [HARQ-ACK MUX on PUSCH] TS 38.213CR0568
- Introduction of MIMO evolution for Downlink and Uplink TS 38.300CR0742
- Support of oversize UL SDT Data Arrival [Large SDT Uplink Data] TS 38.300CR0748
+ 38 more changes
In Release 19, key enhancements for the UL-SCH include the introduction of 32 HARQ process numbers to increase uplink scheduling flexibility and efficiency. Furthermore, the release specifies the multiplexing of UEIRI (Uplink Early Indication of Receiver Information) into the PUSCH, refining uplink control information handling. These updates build upon the shared channel's role as a radio resource dynamically allocated between users.
- Introduction of 32 HARQ process numbers in Rel-19 [TN32HARQ] TS 38.212CR0222
- Introduction of PDCCH repetitions for Type0-PDCCH CSS set in TNs [Common_PDCCH_Rep_TN] TS 38.213CR0748
- Introduction of common PDCCH repetition (Rel-19 NTN) for TN [Common_PDCCH_rep_TN] TS 38.300CR1058
- Corrections on R19 NES adaptation of common channel/signals TS 38.212CR0243
- CR on UEIRI multiplexed into PUSCH in TS 38.212 TS 38.212CR0244
- Corrections on R19 NES adaptation of common channel/signals TS 38.213CR0753
+ 3 more changes
Explore further
Broader topics and technologies where UL-SCH plays a role.
Defining Specifications
3GPP specifications that define or reference UL-SCH, 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 36.212 vj10 | LTE Multiplexing and Channel Coding | Rel-19 |
| TS 36.213 vj10 | LTE Physical Layer Procedures | Rel-19 |
| TS 36.306 vj00 | E-UTRA UE Radio Access Capability Parameters | Rel-19 |
| TS 36.331 vj00 | LTE RRC Protocol Specification | 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 38.523 vj20 | 5G NR UE Conformance Testing: Idle/Inactive | Rel-19 |
| TR 38.889 vg00 | NR-based access to unlicensed spectrum study | Rel-16 |