AI BriefingCoherent Joint Transmission(CJT): A Key Technology Opening Up 6G High Capacity
Key point
CJT binds multiple TRPs together as if they were one, boosting 6G capacity and coverage.
Details
Coherent Joint Transmission(CJT) is a method that binds several TRPs together like a single large antenna, sending the same data stream simultaneously. Compared to Non-coherent joint transmission(NCJT), it enables stronger beamforming gain and transmission of more layers, which helps compensate for the layer-count limitations that tend to occur especially in FR3(around 7~24GHz).
The core is three elements that unlock CJT's potential in 6G.
- Coordination set adaptation: Even as the UE moves, transmission is not cut off at cell boundaries; instead, the TRP set is dynamically changed via PHY-layer control. Unlike 5G NR, which groups by cell unit, this can include TRPs from neighboring cells as well, maintaining a larger CJT set without handover interruption.
- CSI acquisition: CSI feedback is needed for joint beamforming across multiple TRPs. The Type-II codebook for CJT in Rel-18 5G NR showed promise but was complex, so for 6G a more flexible design is proposed that handles the port-group-level SD component and the common FD component more simply, and even includes AI-enabled CSI feedback.
- UE-assisted calibration: Time, frequency, and phase mismatches between non-co-located TRPs are corrected using UE feedback. In particular, delay offset reporting has two use cases—TRP selection and delay compensation—and even just 5-bit delay quantization can bring performance close to ideal.
Simulation results back this up. Dynamic coordination set adaptation showed an average UPT improvement of 9% under a 2-slot processing latency condition, and CSI feedback for CJT produced up to a 40% average UPT improvement compared to a single TRP. For UE-assisted calibration, an average UPT improvement of about 2.5~3x was observed, showing that a large perceived performance gain is possible with low overhead.
Ultimately, CJT in 6G is not simply about using multiple base stations together—it is evolving toward combining dynamic cooperation that crosses cell boundaries, low-overhead CSI design, and UE-assisted calibration to turn multiple TRPs into effectively a single high-performance transmitter.
