Coherent Joint Transmission

Description

Coherent Joint Transmission (CJT) is a sophisticated downlink multi-point coordination scheme standardized in 3GPP Release 18. It operates within the framework of multi-TRP (mTRP) deployments, where a User Equipment (UE) is served simultaneously by multiple Transmission Reception Points. Unlike non-coherent joint transmission or coordinated scheduling, CJT's defining characteristic is the precise phase alignment of the transmitted signals from the participating TRPs. This requires advanced channel state information (CSI) feedback and tight synchronization between the cooperating TRPs, often facilitated through high-capacity, low-latency fronthaul connections, typically within a centralized or distributed unit (CU/DU) architecture.

The technical implementation hinges on the UE's ability to measure and report detailed CSI for each TRP involved in the potential CJT set. The network, using this feedback, calculates complex precoding weights for each TRP's antennas. These weights adjust the phase (and optionally amplitude) of the transmitted signal such that the multiple signal paths arrive at the UE's receiver coherently—that is, in-phase. This constructive superposition transforms what would be multi-path interference into a powerful, combined signal, dramatically improving the received Signal-to-Interference-plus-Noise Ratio (SINR). The transmission is managed as a single Physical Downlink Shared Channel (PDSCH) from a network perspective, but with resources and layers mapped across the coordinated TRPs.

Key components enabling CJT include the enhanced CSI framework for multi-TRP operation, specified in 38.214, and the corresponding RRC signaling for configuring CJT hypotheses and resource sets, detailed in 38.331. The UE must support capabilities for processing quasi-co-location (QCL) assumptions related to multiple TRPs and for calculating combined channel quality indicators (CQIs) under a CJT hypothesis. From a network perspective, a central scheduling entity (e.g., within the CU) is responsible for dynamic TRP selection, precoding calculation, and joint resource allocation, requiring real-time coordination and data sharing between the TRPs.

CJT's role is to push the boundaries of spectral efficiency and coverage in dense 5G-Advanced networks. It is particularly effective in scenarios with high line-of-sight probability between TRPs and the UE, such as indoor hotspots or street-level deployments. By turning interference into useful signal energy, CJT directly increases cell-edge user throughput and reliability, making it a cornerstone technology for achieving consistent high-performance service delivery across the entire network footprint, a critical requirement for advanced mobile broadband and ultra-reliable low-latency communication (URLLC) use cases.

Purpose & Motivation

CJT was developed to address fundamental limitations in traditional single-cell and basic multi-point transmission schemes. As networks densify with more small cells and TRPs, interference management becomes paramount. Simple cell selection or non-coordinated transmission leads to severe inter-cell interference at boundaries, capping throughput for edge users. Earlier multi-TRP techniques like Dynamic Point Selection (DPS) or non-coherent JT improved reliability by providing macro-diversity but did not maximize the potential signal power gain from having multiple transmission points.

The primary motivation for CJT is to unlock the full potential of network densification and massive MIMO. While adding more TRPs increases capacity, without coherent coordination, the returns diminish due to increased interference. CJT solves this by enabling the dense set of TRPs to act as a geographically distributed, phased-array antenna system. This transforms the interference-limited regime into a power-limited one, allowing the network to focus radio frequency energy precisely on the user. It addresses the critical challenge of providing uniformly high data rates and low latency, not just in the cell center but throughout the entire service area, which is essential for future applications like immersive XR and industrial automation.

Historically, achieving such coherence was considered impractical due to the stringent requirements on synchronization and CSI feedback overhead. However, advancements in fronthaul technology (e.g., enhanced Common Public Radio Interface, eCPRI), more powerful UE processing capabilities, and sophisticated reference signal design (like CSI-RS for multi-TRP) in 5G NR made CJT feasible. Its introduction in Rel-18 represents a significant step beyond the foundational multi-connectivity features of earlier releases, moving from diversity-oriented transmission to true beamforming gain aggregation across multiple network nodes.