PDCP Poll Bit for HFN De-synchronization in LTE WLAN Aggregation
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Solution Overview
Problem
In wireless communication systems supporting LTE WLAN Aggregation, there is a challenge in ensuring lossless data transmission in uplink direction due to HFN De-Synchronization issues, particularly when using unlicensed spectrum, as the network cannot identify whether transmitted PDCP SDUs are new or old, leading to potential data loss.
Innovation Solution
A method where the user equipment (UE) sets a poll bit in PDCP PDUs to request a status report including a First Missing Number (FMN) from the network, configuring the FMN based on previous status reports and RRC signaling, to prevent transmission of PDCP SDUs with SN greater than or equal to (Maximum_PDCP_SN+1)/2 until a new FMN is received, thereby avoiding HFN de-synchronization.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If PDCP SDUs are transmitted continuously in uplink direction, then transmission efficiency is improved, but HFN de-synchronization occurs causing data loss
Solution Approach 1:
The network performs preliminary actions by sending downlink PDCP status reports containing FMN (First Missing Number) information to the UE before uplink transmission continues. This allows the UE to proactively adjust its transmission window and prevent HFN de-synchronization before it occurs, maintaining both transmission efficiency and data integrity.
Solution Approach 2:
A feedback mechanism is established where the network sends status reports with FMN information to the UE based on received uplink PDCP SDUs. The UE uses this feedback to adjust its transmission window (setting new FMN values) and control subsequent uplink transmissions, creating a closed-loop system that prevents HFN de-synchronization while maintaining high transmission efficiency.
2Reliability
If the network requests status reports frequently, then HFN synchronization is maintained, but uplink transmission delay increases
Solution Approach 1:
Instead of requiring frequent full status reports, the system uses partial status reports containing only the critical FMN information when needed. The network requests status reports selectively based on transmission conditions, and the UE sends only the essential FMN update rather than complete status information, reducing overhead and delay while maintaining HFN synchronization.
3Reliability
If the transmission window is restricted to prevent HFN de-synchronization, then data loss is avoided, but transmission throughput decreases
Solution Approach 1:
The transmission window (defined by FMN values) is made dynamic rather than static. The UE adjusts its uplink transmission window boundaries based on the FMN received in downlink status reports. When the network confirms receipt of PDCP SDUs, the UE expands its transmission window by updating FMN, allowing larger batches of data to be transmitted without increasing HFN de-synchronization risk, thus maintaining high throughput while ensuring data integrity.
Data Source
AI summary
A method and apparatus for a user equipment (UE) to transmit data to a network is disclosed. The UE sets a poll bit in a PDCP (Packet Data Convergence Protocol) PDU (Protocol Data Unit) to a first value to request a status report including a second FMN (First Missing Number) from the network, if a predetermined number of PDCP SDUs (Service Data Units) are transmitted after a PDCP SDU corresponding to a first FMN.