PDCP State Initialization for Active NR Multicast Sessions
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Solution Overview
Problem
In NR multicast systems, the synchronization of hyper frame number (HFN) and initialization of packet data convergence protocol (PDCP) state variables is challenging due to ongoing PDCP packet transmission before the UE enters the RRC CONNECTED state, preventing traditional initialization methods from being effective.
Innovation Solution
The UE sets initial PDCP state variables based on configuration values received from the network, using RRC signaling to initialize HFN and SN for PDCP PDUs, and stores received PDUs in a buffer until configuration values are received.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If traditional PDCP initialization methods are used (starting from zero), then the initialization is simple and deterministic, but synchronization fails when UE joins an active MBS session because PDCP packets have already been transmitted
Solution Approach 1:
The network performs preliminary action by determining and signaling the initial PDCP state variables (INITIAL_HFN and INITIAL_SN) to the UE before the UE joins the MBS session. This allows the UE to initialize its PDCP state variables correctly upon joining, ensuring synchronization without requiring complex real-time negotiation or buffer management during session establishment.
2Measurement precision
If the UE waits for configuration values before processing PDCP packets, then initialization accuracy is improved, but packet processing delay increases
Solution Approach 1:
The network determines and signals the initial PDCP state variables (INITIAL_HFN and INITIAL_SN) to the UE in advance via RRC signaling (RRCReconfiguration, RRCResume, or RRCSetup message) before the UE joins the MBS session. This preliminary action allows the UE to initialize its PDCP state variables correctly upon receiving the configuration, eliminating the need to wait and process packets in an uninitialized state, thus avoiding delay while ensuring accuracy.
3Adaptability or versatility
If the UE joins an active MBS session, then service availability is improved, but PDCP state synchronization becomes difficult because transmission has already started
Solution Approach 1:
The network performs preliminary action by determining and signaling the initial PDCP state variables (INITIAL_HFN and INITIAL_SN) to the UE before the UE joins the MBS session. This allows the UE to initialize its PDCP state variables correctly upon joining, ensuring synchronization with the network side even though the session is already active and transmission has begun. The network provides these configuration values through RRC signaling, enabling the UE to adapt to the active session while maintaining reliable state synchronization.
Data Source
AI summary
Apparatus and methods are provided for setting initial PDCP state variables for multicast services. In one novel aspect, the UE sets initial PDCP state variables for the MBS session based on configuration values received from the network. In one embodiment, the UE receives dedicated RRC signaling from the network, which contains initial HFN value and the SN of the next PDCP PDU to be transmitted. In one embodiment, the one or more configuration values are provided in RRC Reconfiguration, RRCResume, or RRCSetup message, according to different RRC states of UE. In one embodiment, the UE initializes the RX_DELIV value based on the configuration value of HFN and SN received from the wireless network. When receiving the one or more configuration values of initial PDCP state variables, UE sets HFN to initial HFN value and sets RX_DELIV to the COUNT value of next PDCP PDU to be transmitted by network.