Highest Received PDCP SN on WLAN

Description

The Highest Received PDCP SN on WLAN (HRW) is a specific variable maintained within the Packet Data Convergence Protocol (PDCP) entity of a User Equipment (UE) when the UE is configured for LTE-WLAN Aggregation (LWA). LWA is a feature introduced in 3GPP Release 13 that allows the simultaneous use of LTE and WLAN radio resources for a single data bearer to increase user throughput and network efficiency. The PDCP layer is the aggregation point where data from the core network is split for transmission over the LTE-Uu interface (via the eNB) and/or the WLAN link (via a WT - WLAN Termination).

In the downlink, the eNB (or the WT, depending on the LWA architecture) can route PDCP Protocol Data Units (PDUs) to the UE via either the LTE path or the WLAN path. The UE receives these PDUs on two independent radio links. The PDCP entity in the UE is responsible for reassembling the data stream in the correct order before delivering it to the upper layers (RLC). To perform this reordering and to avoid processing duplicate packets, the PDCP entity maintains several state variables, including the HRW.

Specifically, the HRW variable stores the highest PDCP Sequence Number (SN) among all PDCP PDUs that have been successfully received and processed from the WLAN link. This variable is compared against the SN of incoming PDUs from the LTE link. When a PDU is received over LTE, its SN is checked against HRW. If the LTE PDU's SN is less than or equal to HRW, it indicates that a PDU with this SN (or a higher one) has already been received via WLAN, and the LTE PDU may be treated as a duplicate and discarded, depending on the exact reordering and duplicate detection algorithm. This prevents the upper layers from receiving the same data packet twice.

The maintenance of HRW is integral to the PDCP's reordering function in LWA. Without it, the UE would be unable to correctly sequence packets arriving from two heterogeneous links with potentially different latencies and loss characteristics. The variable is updated whenever a new PDCP PDU is delivered from the lower layers associated with the WLAN bearer (i.e., from the WLAN adapter to the PDCP entity). Its value is monotonically increasing as higher SNs are received. This mechanism ensures the reliability and seamlessness of the aggregated connection, making the dual connectivity transparent to the application layer.

Purpose & Motivation

HRW was created to solve the fundamental challenge of data aggregation over two disparate radio access technologies: LTE and WLAN. Prior to LWA, LTE and WLAN operated independently, often leading to inefficient use of available radio resources. A UE might be connected to both but could not combine their capacities for a single data flow. LWA was introduced to leverage the ubiquity and high capacity of WLAN to offload and boost LTE performance.

The core technical problem in aggregation is maintaining data integrity—specifically, ensuring in-order delivery and eliminating duplicates when packets for a single logical channel can take two different physical paths with independent packet loss and delay profiles. The PDCP layer, being the common endpoint before the data is split in the downlink (or after it is combined in the uplink), is the natural place for this coordination. HRW, as part of the PDCP state machine, provides the essential memory of what has been received on the WLAN leg, enabling the PDCP entity to make intelligent decisions about packets arriving on the LTE leg.

This mechanism addresses the limitations of simpler aggregation or offloading techniques that might operate at the IP layer (e.g., MP-TCP) which are not radio-aware and can be less efficient. By integrating the control at the PDCP layer, 3GPP ensured tight coupling with the radio link control, QoS handling, and security procedures of the LTE system, providing a carrier-grade aggregation solution. HRW is a key enabler that allows the network to dynamically and seamlessly split data between LTE and WLAN based on real-time radio conditions without compromising service quality.