22A.1 LTE-WLAN Aggregation

36.3003GPPEvolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN)Overall descriptionRelease 17Stage 2TS

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

22A.1.1 General

E-UTRAN supports LTE-WLAN aggregation (LWA) operation whereby a UE in RRC_CONNECTED is configured by the eNB to utilize radio resources of LTE and WLAN. Two scenarios are supported depending on the backhaul connection between LTE and WLAN:

– non-collocated LWA scenario for a non-ideal backhaul;

– collocated LWA scenario for an ideal/internal backhaul;

The overall architecture for the non-collocated LWA scenario is illustrated in Figure 22A.1.1-1 below where the WLAN Termination (WT) terminates the Xw interface for WLAN.

Figure 22A.1.1-1: Non-collocated LWA Overall Architecture

22A.1.2 Radio Protocol Architecture

In LWA, the radio protocol architecture that a particular bearer uses depends on the LWA backhaul scenario and how the bearer is set up. Two bearer types exist for LWA: split LWA bearer and switched LWA bearer. Those two bearer types are depicted on Figure 22A.1.2-1 for the collocated scenario and on Figure 22A.1.2-2 for the non-collocated scenario.

Figure 22A.1.2-1: LWA Radio Protocol Architecture for the Collocated Scenario

Figure 22A.1.2-2: LWA Radio Protocol Architecture for the Non-Collocated Scenario

In the downlink, for PDUs sent over WLAN in LWA operation, the LWAAP entity, as specified in TS 36.360 [66] generates LWAAP PDU containing a DRB identity and the WT uses the LWA EtherType 0x9E65 for forwarding the data to the UE over WLAN. The UE uses the LWA EtherType to determine that the received PDU belongs to an LWA bearer and uses the DRB identity to determine to which LWA bearer the PDU belongs to.

In the uplink, for PDUs sent over WLAN in LWA operation, the LWAAP entity in the UE generates LWAAP PDU containing a DRB identity and the UE uses the LWA EtherType 0x9E65 for sending the data over WLAN.

LWA supports split bearer operation where the PDCP sublayer supports in-sequence delivery of upper layer PDUs based on the reordering procedure introduced for DC.

The UE supporting LWA may be configured by the eNB to send PDCP status report or LWA status report, as specified in TS 36.323 [15], in cases where feedback from WT is not available.

Only RLC AM and RLC UM can be configured for an LWA bearer.

E-UTRAN does not configure LWA with DC, LWIP or RCLWI simultaneously for the same UE.

If LWA and RAN assisted WLAN interworking are simultaneously configured for the same UE, in RRC_CONNECTED, the UE only applies LWA.

For LWA bearer, if the data available for transmission is equal to or exceeds the threshold configured by E-UTRAN the UE decides which PDCP PDUs are sent over WLAN or LTE. If the data available is below the threshold, the UE transmits PDCP PDUs on LTE or WLAN as configured by E-UTRAN.

For each LWA DRB, E-UTRAN may configure the IEEE 802.11 AC value to be used for the PDCP PDUs that are sent over WLAN in the uplink.

For LWA bearer, for routing of UL data over WLAN, the WT MAC address may be provided to the UE by the E-UTRAN or using other WLAN procedure.

22A.1.3 Network Interfaces

22A.1.3.1 General

In the non-collocated LWA scenario, the eNB is connected to one or more WTs via an Xw interface. In the collocated LWA scenario the interface between LTE and WLAN is up to implementation. For LWA, the only required interfaces to the Core Network are S1-U and S1-MME which are terminated at the eNB. No Core Network interface is required for the WLAN.

NOTE 1: WT is a logical node and 3GPP does not specify where it is implemented.

NOTE 2: LTE-WLAN aggregation support at a WLAN does not preclude the implementation of legacy WLAN interworking (e.g. S2a, S2b or NSWO) in the same WLAN.

22A.1.3.2 User Plane

In the non-collocated LWA scenario, the Xw user plane interface (Xw-U) is defined between eNB and WT. The Xw-U interface supports flow control based on feedback from WT.

The Flow Control function is applied in the downlink when an E-RAB is mapped onto an LWA bearer, i.e. the flow control information is provided by the WT to the eNB for the eNB to control the downlink user data flow to the WT for the LWA bearer. The OAM configures the eNB with the information of whether the Xw DL delivery status provided from a connected WT concerns LWAAP PDUs successfully delivered to the UE or successfully transferred toward the UE.

The Xw-U interface is used to deliver LWAAP PDUs between eNB and WT.

For LWA, the S1-U terminates in the eNB and, if Xw-U user data bearers are associated with E-RABs for which the LWA bearer option is configured, the user plane data is transferred from eNB to WT using the Xw-U interface.

Figure 22A.1.3.2-1 shows U-plane connectivity of eNB and WT involved in LWA for a certain UE: the S1-U is terminated at the eNB; the eNB and the WT are interconnected via Xw-U.

Figure 22A.1.3.2-1: U-Plane connectivity of eNB and WT for LWA

22A.1.3.3 Control Plane

In the non-collocated LWA scenario, the Xw control plane interface (Xw-C) is defined between eNB and WT. The application layer signalling protocol is referred to as Xw-AP (Xw Application Protocol).

The Xw-AP protocol supports the following functions:

– Transfer of WLAN metrics (e.g. bss load) from WT to eNB;

– Support of LWA for UE in ECM-CONNECTED:

– Establishment, Modification and Release of a UE context at the WT;

– Control of user plane tunnels between eNB and WT for a specific UE for LWA bearers.

– General Xw management and error handling functions:

– Error indication;

– Setting up the Xw;

– Resetting the Xw;

– Updating the WT configuration data.

eNB-WT control plane signalling for LWA is performed by means of Xw-C interface signalling.

There is only one S1-MME connection per LWA UE between the eNB and the MME. Respective coordination between eNB and WT is performed by means of Xw interface signalling.

Figure 22A.1.3.3-1 shows C-plane connectivity of eNB and WT involved in LWA for a certain UE: the S1-MME is terminated in eNB; the eNB and the WT are interconnected via Xw-C.

Figure 22A.1.3.3-1: C-Plane connectivity of eNB and WT for LWA

22A.1.4 Mobility

A WLAN mobility set is a set of one or more WLAN Access Points (APs) identified by one or more BSSID/HESSID/SSIDs, within which WLAN mobility mechanisms apply while the UE is configured with LWA bearer(s), i.e., the UE may perform mobility between WLAN APs belonging to the mobility set without informing the eNB.

The eNB provides the UE with a WLAN mobility set. When the UE is configured with a WLAN mobility set, it will attempt to connect to a WLAN whose identifiers match the ones of the configured mobility set. UE mobility to WLAN APs not belonging to the UE mobility set is controlled by the eNB e.g. updating the WLAN mobility set based on measurement reports provided by the UE. A UE is connected to at most one mobility set at a time.

All APs belonging to a mobility set share a common WT which terminates Xw-C and Xw-U. The termination endpoints for Xw-C and Xw-U may differ.The WLAN identifiers belonging to a mobility set may be a subset of all WLAN identifiers associated to the WT.

22A.1.5 WLAN Measurements

The UE supporting LWA may be configured by the E-UTRAN to perform WLAN measurements. WLAN measurement object can be configured using WLAN identifiers (BSSID, HESSID and SSID), WLAN carrier information and WLAN band (2.4GHz, 5GHz and 60GHz). WLAN measurement reporting is triggered using RSSI. WLAN measurement report contains, for each included WLAN, RSSI and WLAN identifier, and may contain WLAN carrier information, WLAN band, channel utilization, station count, admission capacity, backhaul rate and an indication whether the UE is connected to the WLAN.

WLAN measurements may be configured to support the following:

1. LWA activation;

2. Inter WLAN mobility set mobility;

3. LWA deactivation.

UE is configured with measurements for WLAN using IEEE terminology, as specified in IEEE 802.11, Part 11 [65], (e.g. ‘Country’, ‘Operating Class’, and/or ‘Channel Number’).

22A.1.6 Procedure for WLAN Connection Status Reporting

The purpose of the WLAN Connection Status Reporting procedure is to provide feedback to the eNB related to the WLAN status and operation. The WLAN Connection Status Reporting procedure supports the following indications:

1. WLAN connection failure;

2. WLAN connection success;

3. WLAN temporary suspension;

4. WLAN connection resumption.

When a UE configured with at least one LWA bearer becomes unable to establish or continue LWA operation, the UE sends the WLANConnectionStatusReport message to indicate "WLAN connection failure" to the eNB. When a UE configured with at least one LWA bearer is not able to support the LWA operation for a temporary duration, the UE may suspend the LWA operation by sending the WLANConnectionStatusReport message to indicate "WLAN temporary suspension" to the eNB.

The criteria to determine WLAN connection failure or WLAN temporary suspension is left for UE implementation. Upon WLAN connection failure or WLAN temporary suspension, the UE RRC connection re-establishment is not triggered, data reception on WLAN is suspended, and there is no impact to LTE part of the LWA split bearer. Upon WLAN temporary suspension, UE keeps the LWA configuration including LWA bearer configuration.

When a UE configured with at least one LWA bearer successfully connects to an AP, the UE sends the WLANConnectionStatusReport message to indicate "WLAN connection success", if configured by the eNB. When a UE configured with at least one LWA bearer that has previously indicated WLAN temporary suspension is able to resume the LWA operation, the UE shall send the WLANConnectionStatusReport message to indicate "WLAN connection resumption" to the eNB.

22A.1.7 LTE-WLAN Aggregation Operation

22A.1.7.1 WT Addition

The WT Addition procedure is initiated by the eNB and is used to establish a UE context at the WT in order to provide WLAN resources to the UE.

Figure 22A.1.7.1-1: WT Addition procedure

1. The eNB sends the WT Addition Request message to request the WT to allocate WLAN resources for specific E-RABs, indicating E-RAB characteristics. The WT may reject the request.

NOTE: The eNB may either decide to request resources from the WT of such an amount, that the QoS for the respective E-RAB is guaranteed by the exact sum of resources provided by the eNB and the WT together, or even more. The eNB’s decision may be reflected in step 1 by the E-RAB parameters signalled to the WT, which may differ from E-RAB parameters received over S1.

2. If the WT is able to admit the full or partial WLAN resource request, it responds with the WT Addition Request Acknowledge message.

3. The eNB sends the RRCConnectionReconfiguration message to the UE including the new radio resource configuration. The eNB may include the Access Categories for uplink E-RABs, if received from the WT in step 2.

4. The UE applies the new configuration and replies with the RRCConnectionReconfigurationComplete message.

5. The UE performs WLAN Association.

6. The WT, if supported, sends the WT Association Confirmation message.

7. If configured by the eNB, the UE may send the WLANConnectionStatusReport message.

22A.1.7.2 WT Modification

The WT Modification procedure may be initiated either by the eNB or by the WT and be used to modify, establish or release bearer contexts or to modify other properties of the UE context within the same WT.

The WT Modification procedure does not necessarily need to involve signalling towards the UE.

eNB initiated WT Modification

Figure 22A.1.7.2-1: WT Modification procedure – eNB initiated

1. The eNB sends the WT Modification Request message to request the WT to modify the WLAN resources for specific E-RABs.

2. If the WT accepts the request, it applies the modified WLAN resource configuration and responds with the WT Modification Request Acknowledge message.

3. If the modification requires RRC configuration, eNB sends the RRCConnectionReconfiguration message to the UE including the new WLAN radio resource configuration. The eNB may include the Access Categories for uplink E-RABs, if received from the WT in step 2.

4. The UE applies the new RRC configuration and replies with the RRCConnectionReconfigurationComplete message.

5. The UE starts utilizing the new LWA configuration.

WT initiated WT Modification

Figure 22A.1.7.2-2: WT Modification procedure – WT initiated

1. The WT sends the WT Modification Required message to the eNB to modify the WLAN resources for specific E-RABs.

2. The eNB replies with the WT Modification Confirm message.

4. The UE applies the new RRC configuration and replies with the RRCConnectionReconfigurationComplete message.

5. The UE starts utilizing the new LWA configuration.

22A.1.7.3 WT Release

The WT Release procedure may be initiated either by the eNB or by the WT and is used to initiate the release of the UE context at the WT. The recipient node of this request cannot reject.

The WT Release procedure does not necessarily need to involve signalling towards the UE.

eNB initiated WT Release

Figure 22A.1.7.3-1: WT Release procedure –eNB initiated

1. The eNB sends the WT Release Request message to request WT to release the allocated WLAN resources.

2. The WT initiates release of all allocated WLAN resources.

3. If required, the eNB sends the RRCConnectionReconfiguration message to the UE indicating the release of WLAN radio resource configuration.

4. The UE releases the LWA configuration.

5. The UE replies with the RRCConnectionReconfigurationComplete message.

NOTE 1: It is up to UE implementation what happens with WLAN association after LWA configuration has been released.

WT initiated WT Release

Figure 22A.1.7.3-2: WT Release procedure –WT initiated

1. The WT sends the WT Release Required message to the eNB to request the release of the allocated WLAN resources.

2. The eNB replies with the WT Release Confirm message.

3. The WT initiates release of all allocated WLAN resources.

4. If required, the eNB sends the RRCConnectionReconfiguration message to the UE indicating the release of WLAN radio resource configuration.

5. The UE releases the LWA configuration.

6. The UE replies with the RRCConnectionReconfigurationComplete message.

NOTE 2: It is up to UE implementation what happens with WLAN association after LWA configuration has been released.

22A.1.7.4 Change of WT

The change of WT procedure is initiated by eNB and used to transfer a UE context from a source WT to a target WT. This procedure can be realized using WT Release and WT Addtion procedures.

22A.1.8 WLAN Authentication

For a UE supporting LWA, WLAN authentication is performed as follows:

If WT Counter is included in LWA Configuration in the RRC Connection Reconfiguration message, the UE shall start using the S-K_WT derived using the WT Counter value and K_eNB as PMK or PSK as specified in TS 33.401 [22], clause G and TS 36.331 [16], clause 5.6.14.2. For a UE already authenticated with WLAN, configuration of a new PMK or PSK triggers refreshing the IEEE 802.11 security using the new PMK or PSK.

If WT Counter is not included in LWA Configuration in the RRC Connection Reconfiguration message:

– if WT Counter has not been previously configured for the UE, the UE which is not already authenticated with a WLAN in the WLAN mobility set shall use authentication methods specified in TS 33.402 [70], clause 6;

– if WT Counter has been previously configured for the UE, the UE which is not already authenticated with a WLAN in the WLAN mobility set shall keep using the S-K_WT previously derived using the WT Counter value and KeNB as PMK or PSK as specified in TS 33.401 [22], clause G and TS 36.331 [16], clause 5.6.14.2;

– the UE which is already authenticated with a WLAN in the WLAN mobility set continues using the previously configured authentication method and is not required to refresh IEEE 802.11 security.