Introduction
This article explains the concept and working principles of 5G super uplink, a technique for improving uplink coverage and throughput by coordinating TDD and FDD carriers and combining high- and low-frequency bands.
Current 5G uplink status
The current 5G uplink is limited by terminals, frame structure, and frequency bands, so uplink experience is generally inferior to downlink.
In the 3.5 GHz band, uplink has relatively weak coverage and low capacity. Traditional uplink enhancement techniques have been used to address these issues.
Traditional uplink enhancement techniques
3GPP proposed two uplink enhancement schemes: SUL and UL CA.
The essence of SUL and UL CA is to enhance uplink using coordination between TDD/FDD and high/low frequency bands. Super uplink emerged to address limitations of those approaches.
Limitations of SUL and UL CA
- SUL
- At any given time, uplink data can be transmitted on only one carrier.
- Primarily used to improve cell-edge rates; it cannot increase uplink capacity near the cell.
- UL CA
- Allows concurrent uplink on two carriers.
- For 2Tx UEs, capacity improvement near the cell may be limited or even reduced.
Super uplink is designed to address these shortcomings effectively.
Definition and standardization
China Telecom led the R16 standardization effort for "super uplink". The timeline includes:
- June 2019: China Telecom presented a super uplink technical proposal at a Shanghai event.
- September 2019: Super uplink was approved for R16 work items, including CA, SUL, and EN-DC scenarios.
- H2 2020: R16 super uplink specifications were frozen.
Definition used by China Telecom:
Super uplink is an uplink enhancement technique that coordinates TDD and FDD and complements high and low frequency bands. It uses the terminal's uplink selection transmission function to choose transmission across TDD/FDD based on channel conditions to improve uplink coverage and rate.
Principle: Uplink Tx Switching
Super uplink is realized via Uplink Tx Switching, which enables terminals to switch between two operation modes, case1 and case2.
- Switching between case1 option1 and case2 is referred to as Uplink Tx Switching option1; switching between case1 option2 and case2 is option2.
- For SUL and UL CA, 3GPP defined three levels of switching delay: 35 μs, 140 μs, and 210 μs.
- The switching period is configurable and typically resides on the FDD carrier.
- This mechanism enables TDM transmission across two uplink carriers.
SUL-based super uplink: implementation
Implementation based on SUL typically includes the following elements:
1. SUL parameter configuration
- The cell's SUL carrier frequency information is provided in the system information SIB1.
- SUL carrier parameters such as subcarrier spacing, PUCCH resources, and SRS resources are provided via the cell's RRC reconfiguration messages.
2. UE access method
- The UE can initiate access on the non-SUL carrier.
- If the cell's SIB1 configures an SSB threshold and the UE's measured SSB-RSRP is below that threshold, the UE may initiate access on the SUL carrier.
3. Activation method
- If super uplink is enabled, the UE will activate super uplink when initiating access on the non-SUL carrier, regardless of traffic volume.
- If the UE initiates access on the SUL carrier, the default behavior is single-carrier transmission on the SUL carrier.
