Non-Coherent Joint Transmission

Description

Non-Coherent Joint Transmission (NCJT) is an advanced multi-Transmission/Reception Point (multi-TRP) physical layer technology introduced in 3GPP Release 17 for 5G-NR. It operates under the framework of multi-TRP and multi-panel user equipment (UE) enhancements. Unlike Coherent Joint Transmission (CJT), which requires precise phase synchronization and channel state information (CSI) sharing among TRPs to form a coherent beam, NCJT allows multiple, geographically separated TRPs (or panels within a TRP) to transmit independent data streams to the same UE simultaneously on the same time-frequency resources. These transmissions are not phase-aligned; they are treated as separate spatial layers or transmission occasions. The UE receives these multiple streams and must separate them using advanced receiver processing, such as interference suppression combining or successive interference cancellation, leveraging the spatial characteristics and potentially different quasi-co-location (QCL) assumptions of the channels from each TRP.

Architecturally, NCJT relies on network coordination, typically managed by a central unit (CU) or a coordinating node, which schedules the transmissions from the participating TRPs. The TRPs may be connected via ideal (e.g., fronthaul) or non-ideal backhaul (Xn interface). Key components include the scheduling entity, the participating TRPs (which can be gNBs or remote radio heads), and a UE capable of multi-panel reception or advanced multi-stream decoding. The physical downlink shared channel (PDSCH) is the primary channel for NCJT, where multiple PDSCHs can be transmitted from different TRPs. The network configures the UE via RRC signaling and dynamic scheduling (DCI) with parameters like transmission configuration indicator (TCI) states, each associated with a specific TRP, to indicate the QCL reference signals for each stream.

NCJT's role is to improve spectral efficiency, reliability, and user experience, especially for medium to high mobility users and at cell boundaries. It is a key enabler for ultra-reliable low-latency communication (URLLC) and enhanced mobile broadband (eMBB) scenarios. By not requiring tight phase coherence, it reduces the stringent synchronization and CSI feedback requirements compared to CJT, making it more practical for deployments with non-ideal backhaul. The technology is specified in detail in 3GPP TS 38.214 for physical layer procedures and TS 38.306 for UE radio access capabilities.

Purpose & Motivation

NCJT was created to address the growing demand for higher data rates, improved coverage, and ultra-reliable communications in 5G-Advanced networks, particularly in challenging radio environments. Prior to Release 17, multi-TRP operation primarily focused on schemes like coordinated multipoint (CoMP) with joint transmission requiring coherent combining, which imposed significant overhead for channel estimation, feedback, and tight synchronization between TRPs. This limited its practicality, especially in deployments with non-ideal backhaul links typical in heterogeneous networks.

The motivation for NCJT stems from the need for a more robust and implementable multi-TRP solution that sacrifices some beamforming gain for reduced coordination complexity and enhanced diversity gain. It solves problems like coverage holes, inter-cell interference at edges, and the reliability requirements of industrial IoT and URLLC services. By allowing non-coherent transmission, it enables networks to exploit spatial diversity from multiple TRPs without the need for perfect channel knowledge alignment, thus improving resilience against blockages and fast fading. This evolution reflects a shift towards leveraging multiple, possibly less coordinated, transmission points to boost system performance in real-world deployment scenarios.