WO2020049347A1

WO2020049347A1 - User triggered wlan to cellular voice session transfer

Info

Publication number: WO2020049347A1
Application number: PCT/IB2018/056849
Authority: WO
Inventors: George Foti
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 2018-09-07
Filing date: 2018-09-07
Publication date: 2020-03-12

Abstract

Systems and methods are disclosed herein for seamless, User Equipment (UE) -initiated handover of an Internet Protocol (IP) Multimedia Subsystem (IMS) session for a voice call from a cellular access network to a Wireless Local Area Network (WLAN) access network. In some embodiments, a method of operation of a UE comprises establishing, over a cellular access network, a first Quality of Service (QoS) bearer/flow for an incoming IMS session associated with a first IP address. The method further comprises establishing, over a WLAN access network, an IMS registration with a second IP address. The method further comprises initiating establishment of a second QoS bearer/flow for the incoming IMS session over the WLAN access network, where the second QoS bearer/flow is associated with the second IP address. The method further comprises transferring the incoming IMS session from the first IP address to the second IP address.

Description

USER TRIGGERED WLAN TO CELLULAR VOICE SESSION TRANSFER

Technical Field

[0001 ] Internet Protocol (IP) Multimedia Subsystem (IMS) session continuity, handover.

Background

[0002] Voice over WiFi (VoWiFi) is a technology that is integrated with the Third Generation Partnership Project (3GPP) core network (e.g., the Evolved Packet Core (EPC) for Long Term Evolution (LTE) or the Fifth Generation Core (5GC) for Fifth Generation (5G) New Radio (NR)). VoWiFi is a complementary technology for, e.g., Voice over LTE (VoLTE) that utilizes Internet Protocol (IP) Multimedia Subsystem (IMS) technology to provide packet voice service that is delivered over WiFi using IP. In 3GPP terminology, VoWiFi is one type of Non- 3GPP (N3GPP) access that integrated with the 3GPP core network.

[0003] When a User Equipment (UE) supports Wireless Local Area Network (WLAN) and cellular access, the UE may receive a session on the cellular access and desire to initiate handover of the session to WLAN. This would be desirable if the UE has subscription to VoWiFi, is roaming, and knows that it has to access to good WLAN coverage where it is currently located such that it can benefit from a reduced charging rate by handing over the voice session to the WLAN since VoWiFi is typically charged at local rates when the UE is roaming. However, currently, no mechanisms exist that allow the UE to trigger a seamless handover of a voice session from a 3GPP access network to a WLAN in such a scenario.

[0004] As such, there is a need for systems and methods that enable seamless, UE (e.g., user) initiated handover of a voice session from a 3GPP access network to a WLAN (e.g., WiFi) access network. Summary

[0005] Systems and methods are disclosed herein for seamless, User Equipment (UE) -initiated handover of an Internet Protocol (IP) Multimedia Subsystem (IMS) session for a voice call from a cellular access network to a Wireless Local Area Network (WLAN) access network. In some embodiments, a method of operation of a UE in a wireless communication system comprises establishing, over a cellular communications access network, a first Quality of Service (QoS) bearer or QoS flow for an incoming IMS session having QoS requirements. The first QoS bearer or QoS flow for the incoming IMS session is associated with a first IP address assigned to the UE for the cellular

communications access network. The method further comprises establishing, over a WLAN access network, an IMS registration with a second IP address of the UE, where the second IP address is assigned to the UE for the WLAN access network. The method further comprises initiating establishment of a second QoS bearer or QoS flow for the incoming IMS session over the WLAN access network, where the second QoS bearer or QoS flow for the incoming IMS session is associated with the second IP address assigned to the UE for the WLAN access network. The method further comprises transferring the incoming IMS session from the first IP address of the UE associated with the first QoS bearer or QoS flow established for the incoming IMS session over the cellular communications access network to the second IP address of the UE associated with the second QoS bearer or QoS flow established for the incoming IMS session over the WLAN access network. In this manner, seamless, UE-initiated handover of the incoming IMS session for a voice call from the cellular access network to the WLAN access network is provided.

[0006] In some embodiments, initiating establishment of the second QoS bearer or QoS flow for the incoming IMS session over the WLAN access network comprises sending, over the cellular communications access network, a Session Initiation Protocol (SIP) message that initiates establishment of the second QoS bearer or QoS flow for the incoming IMS session over the WLAN access network. In some embodiments, the SIP message is a SIP OPTION message. In some embodiments, the cellular communications access network is a Third Generation Partnership Project (3GPP) access network, and the WLAN access network is a Non-3GPP (N3GPP) access network. Further, in some

embodiments, sending the SIP message comprises sending the SIP message to a Proxy Call Session Control Function (P-CSCF) associated with the UE.

[0007] In some embodiments, the cellular communications access network is a 3GPP access network, and the WLAN access network is a N3GPP access network.

[0008] In some embodiments, the IMS registration comprises an indication that the UE can perform IMS session transfer from the cellular communications access network to the WLAN access network.

[0009] In some embodiments, the method further comprises, prior to transferring the incoming IMS session, establishing a security association for the incoming IMS session with the WLAN access network based on the QoS requirements of the incoming IMS session.

[0010] In some embodiments, the method further comprises, prior to establishing the IMS registration with the second IP address of the UE, making a decision to switch the incoming IMS session from the cellular communications access network to the WLAN access network.

[0011] In some embodiments, establishing the first QoS bearer or QoS flow for the incoming IMS session comprises performing an attachment procedure to attach to the cellular communications access network wherein the first IP address is assigned to the UE during the attachment procedure; performing, over the cellular communications access network, an IMS registration procedure with the first IP address assigned to the UE for the cellular communications access network; and receiving a SIP INVITE message for a Voice over IP (VoIP) call terminating at the UE, where, during setup for the VoIP call, the first QoS bearer or QoS flow for the incoming IMS session is established to carry VoIP traffic for the VoIP call. In some embodiments, performing the IMS registration procedure with the first IP address assigned to the UE for the cellular communications access network comprises providing an indication that the UE can perform IMS session transfer between the cellular communications access network and the WLAN access network.

[0012] In some embodiments, establishing the IMS registration with the second IP address of the UE comprises performing a registration procedure over the WLAN access network wherein the second IP address is assigned to the UE during the registration procedure, and performing, over the WLAN access network, an IMS registration procedure with the second IP address assigned to the UE for the WLAN access network. In some embodiments, performing the IMS registration procedure with the second IP address assigned to the UE for the cellular communications access network comprises providing an indication that the UE can perform IMS session transfer from the cellular communications access network to the WLAN access network.

[0013] Embodiments of a UE for a wireless communication system are also disclosed. In some embodiments, a UE for a wireless communication system is adapted to establish, over a cellular communications access network, a first QoS bearer or QoS flow for an incoming IMS session having QoS requirements, where the first QoS bearer or QoS flow for the incoming IMS session is associated with a first IP address assigned to the UE for the cellular

communications access network. The UE is further adapted to establish, over a WLAN access network, an IMS registration with a second IP address of the UE, wherein the second IP address is assigned to the UE for the WLAN access network. The UE is further adapted to initiate establishment of a second QoS bearer or QoS flow for the incoming IMS session over the WLAN access network, wherein the second QoS bearer or QoS flow for the incoming IMS session is associated with the second IP address assigned to the UE for the WLAN access network. The UE is further adapted to transfer the incoming IMS session from the first IP address of the UE associated with the first QoS bearer or QoS flow established for the incoming IMS session over the cellular communications access network to the second IP address of the UE associated with the second QoS bearer or QoS flow established for the incoming IMS session over the WLAN access network. [0014] In some embodiments, a UE for a wireless communication system comprises at least one processor and memory storing instructions executable by the at least one processor whereby the UE is operable to: establish, over a cellular communications access network, a first QoS bearer or QoS flow for an IMS session having QoS requirements, where the first QoS bearer or QoS flow for the incoming IMS session is associated with a first IP address assigned to the UE for the cellular communications access network; establish, over a WLAN access network, an IMS registration with a second IP address of the UE, wherein the second IP address is assigned to the UE for the WLAN access network; initiate establishment of a second QoS bearer or QoS flow for the incoming IMS session over the WLAN access network, wherein the second QoS bearer or QoS flow for the incoming IMS session is associated with the second IP address assigned to the UE for the WLAN access network; and transfer the incoming IMS session from the first IP address of the UE associated with the first QoS bearer or QoS flow established for the incoming IMS session over the cellular communications access network to the second IP address of the UE associated with the second QoS bearer or QoS flow established for the incoming IMS session over the WLAN access network.

[0015] Embodiments of a method of operation of a network entity are also disclosed. In some embodiments, a network entity for an IMS of a wireless communications system comprises receiving from a UE a message that requests transfer of an incoming IMS session established for the UE over a cellular communications access network from the cellular communications access network to a WLAN access network, and initiating establishment of a QoS bearer or QoS flow for the incoming IMS session over the WLAN access network.

[0016] In some embodiments, the cellular communications access network is a 3GPP access network, and the WLAN access network is a N3GPP access network. In some embodiments, the network entity is a P-CSCF. In some embodiments, initiating establishment of the QoS bearer or QoS flow for the incoming IMS session over the WLAN access network comprises transmitting, to a Policy and Charging Rules Function/Policy Control Function (PCRF/PCF), a request to establish a QoS bearer or QoS flow for the incoming IMS session over the WLAN access network.

[0017] In some embodiments, the method further comprises receiving, from the UE, an indication that the UE can perform IMS session transfer from the cellular communications access network to the WLAN access network. In some embodiments, the method further comprises making a decision to allow transfer of the incoming IMS session from the cellular communications access network to the WLAN access network based on the indication that the UE can perform IMS session transfer from the cellular communications access network to the WLAN access network. Further, initiating establishment of the QoS bearer or QoS flow for the incoming IMS session over the WLAN access network comprises initiating establishment of the QoS bearer or QoS flow for the incoming IMS session over the WLAN access network upon making the decision to allow transfer of the incoming IMS session from the cellular communications access network to the WLAN access network.

[0018] In some embodiments, receiving the message that requests transfer of the incoming IMS session established for the UE over the cellular

communications access network from the cellular communications access network to the WLAN access network comprises receiving, from the UE, a SIP message that requests establishment of the QoS bearer or QoS flow for the incoming IMS session over the WLAN access network. In some embodiments, the SIP message is a SIP OPTION message.

[0019] Embodiments of a network entity are also disclosed. In some embodiments, a network entity for an IMS of a wireless communications system is adapted to receive, from a UE, a message that requests transfer of an incoming IMS session established for the UE over a cellular communications access network from the cellular communications access network to a WLAN access network, and initiate establishment of a QoS bearer or QoS flow for the incoming IMS session over the WLAN access network.

[0020] In some embodiments, a network entity for an IMS of a wireless communications system comprises at least one processor and memory storing instructions executable by the at least one processor whereby the network entity is operable to receive from a UE a message that requests transfer of a incoming IMS session established for the UE over a cellular communications access network from the cellular communications access network to a WLAN access network and initiate establishment of a QoS bearer or QoS flow for the incoming IMS session over the WLAN access network.

Brief Description of the Drawings

[0021 ] The accompanying drawing figures incorporated in and forming a part of this specification illustrate several aspects of the disclosure, and together with the description serve to explain the principles of the disclosure.

[0022] Figure 1 illustrates one example of a wireless communication system in which a User Equipment (UE) has the capability to utilize both a cellular access network and a Wireless Local Area Network (WLAN) access network in accordance with some embodiments of the present disclosure;

[0023] Figure 2 illustrates the operation of the wireless communication system of Figure 1 to provide seamless, UE-initiated handover of an Internet Protocol (IP) Multimedia Subsystem (IMS) session for a voice call from cellular access network to the WLAN access network in accordance with some embodiments of the present disclosure;

[0024] Figures 3 and 4 illustrate two specific examples of the wireless communication system of Figure 1 in which the core network is a Fifth

Generation (5G) Core network (5GC) and a Evolved Packet Core (EPC), respectively;

[0025] Figures 5A through 5D illustrate the operation of the wireless communication system of Figures 3 and 4 to provide seamless, UE-initiated handover of an IMS session for a voice call from the cellular access network to the WLAN access network in accordance with some embodiments of the present disclosure;

[0026] Figure 6 is a schematic block diagram of a network node according to some embodiments of the present disclosure; [0027] Figure 7 is a schematic block diagram that illustrates a virtualized embodiment of the network node of Figure 6 according to some embodiments of the present disclosure;

[0028] Figure 8 is a schematic block diagram of the network node of Figure 6 according to some other embodiments of the present disclosure;

[0029] Figure 9 is a schematic block diagram of a UE according to some embodiments of the present disclosure; and

[0030] Figure 10 is a schematic block diagram of the UE of Figure 9 according to some other embodiments of the present disclosure.

Detailed Description

[0031 ] The embodiments set forth below represent information to enable those skilled in the art to practice the embodiments and illustrate the best mode of practicing the embodiments. Upon reading the following description in light of the accompanying drawing figures, those skilled in the art will understand the concepts of the disclosure and will recognize applications of these concepts not particularly addressed herein. It should be understood that these concepts and applications fall within the scope of the disclosure.

[0032] Radio Node: As used herein, a“radio node” is either a radio access node or a wireless device.

[0033] Radio Access Node: As used herein, a“radio access node” or“radio network node” is any node in a Radio Access Network (RAN) of a cellular communications network that operates to wirelessly transmit and/or receive signals. Some examples of a radio access node include, but are not limited to, a base station (e.g., a New Radio (NR) base station (gNB) in a Third Generation Partnership Project (3GPP) Fifth Generation (5G) NR network or an enhanced or evolved Node B (eNB) in a 3GPP Long Term Evolution (LTE) network), a high- power or macro base station, a low-power base station (e.g., a micro base station, a pico base station, a home eNB, or the like), and a relay node.

[0034] Core Network Entity: As used herein, a core network entity is an entity in a core network. In other words, a core network entity is an entity that implements a function in the Evolved Packet Core (EPC) network (e.g., a Mobility Management Entity (MME), a Packet Data Network (PDN) Gateway (PGW), or the like) or a network function in the 5G Core network (5GC) (e.g., an Access and Mobility Management Function (AMF), a Session Management Function (SMF), a Policy Control Function (PCF), or the like). A core network entity may be implemented either as a network element on a dedicated hardware, as a software instance running on a dedicated hardware, or as a virtualized function instantiated on an appropriate platform, e.g., a cloud infrastructure.

[0035] Internet Protocol (IP) Multimedia Subsystem (IMS) Entity: As used herein, an“IMS entity” is an entity in an IMS. In other words, an IMS entity is an entity that implements a function in an IMS. Some example IMS entities include a Proxy Call Session Control Function (P-CSCF), a Serving Call Session Control Function (S-CSCF), an Access Transfer Control Function (ATCF), and the like.

[0036] Wireless Device: As used herein, a“wireless device” is any type of device that has access to (i.e., is served by) a cellular communications network by wirelessly transmitting and/or receiving signals to a radio access node(s). Some examples of a wireless device include, but are not limited to, a User Equipment device (UE) in a 3GPP network and a Machine Type Communication (MTC) device.

[0037] Network Node: As used herein, a“network node” is a general term used to refer to any of a radio access node, a node implementing a core network entity, or a node implementing an IMS entity.

[0038] Wireless Access Node: As used herein, an“access node” is an access node in a Wireless Local Area Network (WLAN) such as, e.g., a WiFi Access Point (AP).

[0039] Note that the description given herein focuses on a 3GPP cellular communications system and, as such, 3GPP terminology or terminology similar to 3GPP terminology is oftentimes used. However, the concepts disclosed herein are not limited to a 3GPP system.

[0040] As discussed above in the background section, when a UE supports WLAN and cellular access, the UE may receive a session on the cellular access and desire to initiate handover of the session to WLAN. This would be desirable if the UE has a subscription to Voice over WiFi (VoWiFi), is roaming, knows that it has access to good WLAN coverage where it is currently located, and can benefit from a reduced charging rate by handing over the voice session to the WLAN since VoWiFi is typically charged at local rates when the UE is roaming. However, currently, no mechanisms exist that allow the UE to trigger a seamless handover of a voice session from a 3GPP access network to a WLAN in such a scenario. The embodiments described provide a solution for a seamless session transfer that is initiated by the UE and performed with the help of the network to enable such a scenario.

[0041] The embodiments described herein propose a simple solution to enable a UE to accept an incoming IMS session for a Voice over IP (VoIP) call on a cellular access network (e.g., a 3GPP access network) and then move the IMS session to a WLAN access network (e.g., a Non-3GPP (N3GPP) access network), e.g., for a cheaper rate. As described below in detail, while the call is ongoing, the UE sends a request to the network to initiate establishment of a dedicated voice bearer over the WLAN access network. Once the dedicated bearer is established, the UE switches the call to WLAN.

[0042] In some embodiments, the UE provides an indication, which is sometimes referred to herein as a new feature tag, to the network, e.g., during IMS registration, that indicates that the UE supports seamless, UE-internal session transfer. Preferably, this indication is included both in the IMS registration over the cellular access network and the IMS registration over the WLAN access network. This indication is known by the P-CSCF and all IMS entities involved in IMS registration, and its usage may be shown later at session tear down. In addition, this indication enables the P-CSCF to deploy an ATCF in the IMS session.

[0043] The embodiments described herein propose a simple way to support seamless session transfer for a UE without needing to clear the ongoing call.

[0044] In this regard, Figure 1 illustrates one example of a wireless communication system 100 in which a UE 102 has the capability to utilize both a cellular access network, which is shown as a 3GPP (R)AN 104, and a WLAN access network, which is shown as a N3GPP AN 106. The 3GPP (R)AN 104 may be a Fourth Generation (4G) RAN (such as a LTE or LTE-Advanced RAN, including a number of base stations which are referred to as eNBs) or a 5G RAN (such as a NR RAN, including a number of base stations which are referred to as gNBs). The 3GPP (R)AN 104 is connected to a core network, which is shown as a 3GPP Core Network (CN) 108 (e.g., an EPC or 5GC). The N3GPP AN 106 is connected to the 3GPP core network 108 via a gateway or interworking function, which is shown as an evolved Packet Data Gateway (ePDG) / N3GPP Inter- Working Function (N3IWF) 1 10. Notably, the term“ePDG” is used for 4G, and the term“N3IWF” is used for 5G. The 3GPP core network 108 is connected to an IMS 1 12, as will be appreciated by one of skill in the art.

[0045] As discussed below in detail, the UE 102 receives a voice call over an incoming IMS session established over the 3GPP (R)AN 104. During the call, the UE 102 sends a request to initiate transfer of the incoming session from the 3GPP (R)AN 104 to the N3GPP AN 106. In the preferred embodiments described herein, this request is made via a Session Initiation Protocol (SIP) message sent to the IMS 1 12. Upon receiving the request, the IMS 1 12 sends a request to the 3GPP core network 108 for establishment of a dedicated bearer or dedicated flow for the IMS session over the N3GPP AN 106. The dedicated bearer or dedicated flow is also referred to herein as a Quality of Service (QoS) bearer or QoS flow. For shorthand, a QoS bearer or QoS flow is denoted herein as“QoS bearer/flow.” Once the QoS bearer/flow for the IMS session is established over the N3GPP AN 106, the UE 102 transfers the IMS session from the 3GPP (R)AN 104 to the N3GPP AN 106. In this manner, a seamless, UE- initiated handover is performed.

[0046] Figure 2 illustrates the operation of the wireless communication system 100 to provide seamless, UE-initiated handover of an IMS session for a voice call from the 3GPP (R)AN 104 to the N3GPP AN 106 in accordance with some embodiments of the present disclosure. As illustrated, the UE 102 establishes a first QoS bearer/flow for an incoming IMS session having QoS requirements over the 3GPP (R)AN 104 (step 200). The first QoS bearer/flow is associated with a first IP address assigned to the UE 102 for the 3GPP (R)AN 104. In this example, the incoming IMS session having QoS requirements is an IMS session for a voice call terminating at the UE 102. The first QoS bearer/flow is

established using any suitable QoS bearer/flow establishment procedures for an IMS session. For example, as discussed below, the UE 102 may first perform an attachment procedure by which the UE 102 attaches to the 3GPP (R)AN 104 and is assigned a first IP address and a default bearer/flow. The UE 102 may then initiate setup of a PDN connection or PDU session with the IMS 1 12 over the 3GPP (R)AN 104, and perform IMS registration over the 3GPP (R)AN 104 with the first IP address. The UE 102 then receives a SIP INVITE message for the voice call and establishes the first QoS bearer/flow over the 3GPP (R)AN 104 for the voice call.

[0047] During the call, the UE 102 decides to initiate transfer of the call from the 3GPP (R)AN 104 to the N3GPP AN 106 and, in response to making this decision, initiates a second connection with the IMS 1 12 over the N3GPP AN 106 and establishes an IMS registration with a second IP address assigned to the UE 102 for the N3GPP AN 106 (step 202), and sends a message to the IMS 1 12 to initiate transfer of the incoming IMS session for the call from the 3GPP (R)AN 104 to the N3GPP AN 106 (step 204).

[0048] Upon receiving the request from the UE 102, the IMS 1 12 sends a request to the 3GPP CN 108 to establish a second QoS bearer/flow over the N3GPP AN 106 for the incoming IMS session (step 206). The 3GPP CN 108 then initiates a procedure to establish a second QoS bearer/flow over the N3GPP AN 106 for the incoming IMS session (step 208). The procedure for establishing the second QoS bearer/flow is performed by the appropriate nodes in the 3GPP CN 108 and the N3GPP AN 106 as well as the UE 102. During this procedure, a Security Association (SA) for the second QoS bearer/flow based on the QoS requirements of the incoming IMS session.

[0049] Once the second QoS bearer/flow for the incoming IMS session is established over the N3GPP AN 106, the UE 102 transfers the incoming IMS session from the 3GPP (R)AN 104 to the N3GPP AN 106 (i.e., transfer the incoming IMS session from the first IP address associated with the first QoS bearer/flow established over the 3GPP (R)AN 104 to the second IP address associated with the second QoS bearer/flow established over the N3GPP AN 106) (step 210). As discussed below, in some embodiments, this transfer includes sending a SIP UPDATE message to the IMS 1 12 as well as internally moving the incoming IMS session at the UE 102 from the first IP address to the second IP address.

[0050] Figures 3 and 4 illustrate two specific examples of the wireless communication system 100 of Figure 1 in which the 3GPP core network 108 is a 5GC 300 in Figure 3 and an EPC 400 in Figure 4.

[0051] Looking first at Figure 3, as will be appreciated by one of skill in the art, the 5GC 300 includes a number of Network Functions (NFs) connected by service-based interfaces in the control plane. An NF may be implemented either as a network element on a dedicated hardware, as a software instance running on a dedicated hardware, or as a virtualized function instantiated on an appropriate platform, e.g., a cloud infrastructure. As illustrated, the 5GC 300 includes a User Plane Function (UPF) 302, an SMF 304, an AMF 306, an

Authentication Server Function (AUSF) 308, a Network Slice Selection Function (NSSF) 310, a Network Exposure Function (NEF) 312, a Network Repository Function (NRF) 314, a PCF 316, a Unified Data Management (UDM) 318, and an Application Function (AF) 320.

[0052] Note that while Figure 3 illustrates the 5GC 300 as a service-based architecture, a reference point representation may alternatively be used.

Reference point representations of the 5G network architecture are used to develop detailed call flows in the normative standardization.

[0053] The 5GC 300 network aims at separating a user plane and a control plane. The user plane carries user traffic while the control plane carries signaling in the network. In Figure 3, the UPF 302 is in the user plane and all other NFs, i.e., the SMF 304, AMF 306, AUSF 308, NSSF 310, NEF 312, NRF 314, PCF 316, UDM 318, and AF 320, are in the control plane. Separating the user and control planes guarantees each plane resource to be scaled independently. It also allows UPFs 302 to be deployed separately from control plane functions in a distributed fashion. In this architecture, the UPFs 302 may be deployed very close to UEs 102 to shorten the Round Trip Time (RTT) between the UEs 102 and data network for some applications requiring low latency.

[0054] The core 5G network architecture is composed of modularized functions. For example, the AMF 306 and SMF 304 are independent functions in the control plane. Separated AMF 306 and SMF 304 allow independent evolution and scaling. Other control plane functions like the PCF 316 and AUSF 308 can be separated as shown in Figure 3. Modularized function design enables the 5G core network to support various services flexibly.

[0055] Each NF interacts with another NF directly. It is possible to use intermediate functions to route messages from one NF to another NF. In the control plane, a set of interactions between two NFs is defined as service so that its reuse is possible. This service enables support for modularity. The user plane supports interactions such as forwarding operations between different UPFs 302.

[0056] The service(s) that a NF provides to other authorized NFs can be exposed to the authorized NFs through the service-based interface. In Figure 3, the service based interfaces are indicated by the letter“N” followed by the name of the NF (e.g., Namf for the service based interface of the AMF 306 and Nsmf for the service based interface of the SMF 304, etc.).

[0057] Some properties of the NFs shown in Figure 3 may be described in the following manner. The AMF 306 provides UE-based authentication,

authorization, mobility management, etc. A UE 102 even using multiple access technologies is basically connected to a single AMF 306 because the AMF 306 is independent of the access technologies. The SMF 304 is responsible for session management and allocates IP addresses to UEs 102. It also selects and controls the UPF 302 for data transfer. If a UE 102 has multiple sessions, different SMFs 304 may be allocated to each session to manage them individually and possibly provide different functionalities per session. The AF 320 provides information on the packet flow to the PCF 316 responsible for policy control in order to support QoS. Based on the information, the PCF 316 determines policies about mobility and session management to make the AMF 306 and SMF 304 operate properly. The AUSF 308 supports authentication function for the UEs 102 or similar and thus stores data for authentication of the UEs 102 or similar while the UDM 318 stores subscription data of the UE 102.

[0058] In addition to the NFs described above, the 5GC 300 includes a N3IWF 322 that provides an interface between the N3GPP AN 106 and the 5GC 300. While not necessary for understanding the present disclosure, for additional details regarding the N3IWF 322, the interested reader is directed to 3GPP TS 23.501 and 23.502.

[0059] As will be understood by those of skill in the art, the IMS 1 12 includes various IMS entities such as, for example, a P-CSCF 324, an Interrogating Call Session Control Function (l-CSCF) 326, a S-CSCF 328, a ATCF 330, and an Access Gateway 332. The operational details of the P-CSCF 324, an l-CSCF 326, a S-CSCF 328, and a ATCF 330 are well known to those of skill in the art and are therefore not described here.

[0060] Now turning to Figure 4, as will be appreciated by one of skill in the art, the EPC 400 includes a number of core network entities such as, e.g., a Serving Gateway (SGW) 402, a PGW 404, an MME 406, a Flome Subscriber Server (FISS) 408, and a Policy and Charging Rules Function (PCRF) 410. The operational details of the SGW 402, the PGW 404, the MME 406, the FISS 408, and the PCRF 410 are well known to those of skill in the art and therefore are not repeated here.

[0061 ] In addition, the EPC 400 includes an ePDG 412 that provides an interface between the EPC 400 and the N3GPP AN 106. While not necessary for understanding the present disclosure, for additional details regarding the ePDG 412, the interested reader is directed to 3GPP TS 23.402.

[0062] Figures 5A through 5D illustrate the operation of the wireless communication system 100 of Figures 3 and 4 to provide seamless, UE-initiated handover of an IMS session for a voice call from the 3GPP (R)AN 104 to the N3GPP AN 106 in accordance with some embodiments of the present disclosure. This process can be understood as one detailed example of the process of Figure 2. Note that only relevant nodes of the IMS 1 12 are shown.

The S-CSCF 328 and l-CSCF 326 are not shown.

[0063] As illustrated, the UE 102 establishes a first QoS bearer/flow for an incoming IMS session having QoS requirements over the 3GPP (R)AN 104, where the first QoS bearer/flow is associated with a first IP address assigned to the UE 102 for the 3GPP (R)AN 104. In this example, the incoming IMS session having QoS requirements is an IMS session for a voice call terminating at the UE 102. Specifically, the UE 102 performs an attachment procedure by which the UE 102 attaches to the 3GPP (R)AN 104 and is assigned a first IP address (IP1 ) and a default bearer/flow (step 500). It is assumed the UE 102 established a PDN connection with the IMS 1 12 during the attachment procedure. Also, the UE 102 obtains the address of the P-CSCF 324 during the attachment procedure. The UE 102 then performs IMS registration over the 3GPP (R)AN 104 with the first IP address (step 502). During IMS registration, the UE 102 provides an indication to the P-CSCF 324 that indicates that the UE 102 is capable of performing internal IMS session transfer from the 3GPP (R)AN 104 to the N3GPP AN 106. This indication is referred to herein as a new feature tag. The SMF/PGW-C 304/404 records the access network used for the UE 102 (step 504). Note that“PGW-C” refers to the control plane function of the P-GW when Control Plane And User Plane Separation (CUPS) is utilized. Specifically, the SMF/PGW-C 304/404 records that the first IP address (IP1 ) belongs to EPC in this example and possibly also record that the first IP address (IP1 ) belongs to the 3GPP (R)AN 104 and that the 3GPP RAN is used for the first IP address (IP1 ).

[0064] A UE (UEb) originates a voice call and sends a SIP INVITE message to the P-CSCF 324 (step 506), and the P-CSCF 324 sends the SIP INVITE message to the UE 102 (UEa) (step 508). Once the call is answered, the UE 102 (UEa) sends an OK message to the P-CSCF 324, which is forward to UEb (step 510). UEb sends an Acknowledgement (ACK) to the P-CSCF 324, which is forwarded to the UE 102 (UEa) (step 512). The first QoS bearer/flow for the voice call (i.e., the voice traffic) over the incoming IMS session is established over the 3GPP (R)AN 104 for the voice call. Note that the first QoS bearer/flow is established during session setup, as will be appreciated by one of ordinary skill in the art.

[0065] During the call, the UE 102 decides to initiate transfer of the call from the 3GPP (R)AN 104 to the N3GPP AN 106 (step 514). This decision may be made automatically based on a predefined profile or rule or may be made in response to input from a user (e.g., the user inputs a command instructing the UE 102 to handover the call to the N3GPP AN 106). In response to making this decision, the UE 102 establishes a Virtual Private Network (VPN) connection with the N3IWF/ePDG 322/412, performs a registration procedure with the 3GPP core network 108 over the N3GPP AN 106, and subsequently establishes a PDU session with the IMS 1 12 (step 516). As part of the PDU session establishment with the IMS 1 12, the UE 102 is assigned a second IP address (IP2) for access via the N3GPP AN 106. In addition, during the process of step 516, the UE 102 also obtains the address of the P-CSCF 324. The SMF/PGW-C 304/404 records, for the same UE 102, the two IP addresses (IP1 and IP2) and

associates each of these IP addresses with the respective access network (step 518). The UE 102 performs an IMS registration with the second IP address (IP2) assigned to the UE 102 for the N3GPP AN 106 (step 520). During this IMS registration, the UE 102 provides an indication to the P-CSCF 324 that indicates that the UE 102 is capable of performing internal IMS session transfer from the 3GPP (R)AN 104 to the N3GPP AN 106. Again, this indication is referred to herein as a new feature tag. At this point, the same UE 102 now has two simultaneous sessions/connections with the IMS 1 12 over different access networks and, potentially, different core networks. The table below illustrates different cases that are covered by this solution. Note that it is assumed that the UE 102 is served by the same PCF/PCRF 316/410 and the same user plane node regardless of the cases in the table below.

Table 1 : Possible Scenarios to which the UE-Triggered Handover Solution

Applies

[0066] The UE 102 (UEa) then sends a SIP OPTION message to the P-CSCF 324 to initiate transfer of the incoming IMS session for the call from the 3GPP

(R)AN 104 to the N3GPP AN 106 (step 522). The SIP OPTION message is sent over the 3GPP (R)AN 104. Note that while a SIP OPTION message is used in this example, other types of SIP messages may alternatively be used. Upon receiving the SIP OPTION message from the UE 102, the P-CSCF 324 optionally decides whether to grant the request based, e.g., on whether the IMS

registrations for the UE 102 indicate that the UE 102 is capable of internal session transfer from the 3GPP (R)AN 104 to the N3GPP AN 106 and/or one or more preconfigured policies and/or rules (e.g., one or more preconfigured operator policies). Assuming that the transfer is allowed, the P-CSCF 324 sends a request to the PCF/PCRF 316/410 to establish a second QoS bearer/flow over the N3GPP AN 106 for the incoming IMS session (step 524). The PCF/PCRF 316/410 then sends a session modification request with QoS information (e.g., the QoS requirements for the incoming IMS session corresponding to the new N3GPP AN) to the N3IWF/ePDG 322/412 via the SMF/PGW-C 304/404, and AMF 306 (step 526). A dedicated bearer/flow establishment procedure is then performed to establish a second QoS bearer/flow over the N3GPP AN 106 for the incoming IMS session (step 528). The procedure for establishing the second QoS bearer/flow is performed by the appropriate nodes in the 3GPP CN 108 and the N3GPP AN 106, as well as the UE 102. During this procedure, a SA for the second QoS bearer/flow is established based on the QoS requirements of the incoming IMS session. Note that the procedure for establishing the QoS bearer/flow over the N3GPP AN 106 is described in 3GPP Technical Specification (TS) 24.302 section 4.8 for stage 3 and 3GPP TS 23.402 section 4.10.5 for stage 2 for EPC and in 3GPP TS 23.502 section 4.12.5 for 5GC.

[0067] Once the second QoS bearer/flow for the incoming IMS session is established over the N3GPP AN 106, the N3IWF/ePDG 322/412 sends a session modification response to the PCF/PCRF 316/410, which then sends a response to the P-CSCF 324. The P-CSCF 324 then ends a 200 OK message to the UE 102 in response to the SIP OPTION message of step 522 (step 532).

[0068] The UE 102 then transfers the incoming IMS session from the 3GPP (R)AN 104 to the N3GPP AN 106 (i.e., transfer the incoming IMS session from the first IP address associated with the first QoS bearer/flow established over the 3GPP (R)AN 104 to the second IP address associated with the second QoS bearer/flow established over the N3GPP AN 106). Specifically, the UE 102 sends a SIP UPDATE message to the P-CSCF 324 that requests that the incoming IMS session message be moved to the second IP address (IP2) associated with the second QoS bearer/flow established over the N3GPP AN 106 (step 534). The P-CSCF 324 sends a SIP UPDATE message to the

ATCF/AGW 330/332, where the SIP UPDATE message includes the second IP address (IP2) (step 536). The ATCF/AGW 330/332 does not forward the SIP UPDATE, modifies the IP address of the UE 102 for the incoming IMS session to be IP2 (step 538), and responds with an SIP 200 OK (step 540).

[0069] The UE 102 internally moves the incoming IMS session from IP1 to IP2 (step 542). At that point, QoS bearer/flow passes through the N3IWF/ePDG 322/412 and the N3GPP AN 106. Two options are available following the session transfer, the UE 102 or the network can maintain the first QoS

bearer/flow associated with IP1 and can transfer the IMS session back to cellular or they can release the first QoS bearer/flow and re-establish the first QoS bearer/flow later if need be. If the UE 102 initiates release of the first QoS bearer/flow, the UE 102 will send a SIP OPTION request to the P-CSCF 324 to that effect (step 544). The P-CSCF 324 informs the PCF/PCRF 316/410 to release the bearer/flow associated with IP1 (step 545). The PCF/PCRF 316/410 then initiates IP_CAN session modification request (step 546), and the standard procedure for bearer/flow de-activation is performed (step 548). Once the P- CSCF receives the outcome (step 550), it sends back a SIP 200 OK to the UE (step 552). The PCF/PCRF 316/410 also removes the entry associated with IP1 that it created.

[0070] Note that the process of Figures 5A through 5D assumes that the same P-CSCF 324 is used for both IMS registrations; however, different P- CSCFs 324 may be used for the two IMS registrations. Also note that Figures 5A through 5D do not show tearing down of the IMS session since this is orthogonal to the embodiments described herein.

[0071] Figure 6 is a schematic block diagram of a network node 600 according to some embodiments of the present disclosure. The network node 600 may be a node that implements a core network entity (e.g., a node implementing a core network function such as, e.g., any one of those illustrated in Figure 3 or Figure 4), a node implementing an IMS function such as, e.g., any one of those illustrated in Figures 3 or 4, or a radio access node (e.g., a base station). As illustrated, the network node 600 includes a control system 602 that includes one or more processors 604 (e.g., Central Processing Units (CPUs), Application Specific Integrated Circuits (ASICs), Field Programmable Gate Arrays (FPGAs), and/or the like), memory 606, and a network interface 608. The one or more processors 604 are also referred to herein as processing circuitry.

In addition, if the network node 600 is a radio access node, the network node 600 may also include one or more radio units 610 that each includes one or more transmitters 612 and one or more receivers 614 coupled to one or more antennas 616. The radio units 610 may be referred to or be part of radio interface circuitry. In some embodiments, the radio unit(s) 610 is external to the control system 602 and connected to the control system 602 via, e.g., a wired connection (e.g., an optical cable). Flowever, in some other embodiments, the radio unit(s) 610 and potentially the antenna(s) 616 are integrated together with the control system 602. The one or more processors 604 operate to provide one or more functions of a network node 600 as described herein. In some embodiments, the function(s) are implemented in software that is stored, e.g., in the memory 606 and executed by the one or more processors 604.

[0072] Figure 7 is a schematic block diagram that illustrates a virtualized embodiment of the network node 600 according to some embodiments of the present disclosure. As used herein, a“virtualized” network node is an implementation of the network node 600 in which at least a portion of the functionality of the network node 600 is implemented as a virtual component(s) (e.g., via a virtual machine(s) executing on a physical processing node(s) in a network(s)). As illustrated, in this example, the network node 600 optionally includes the control system 602 that includes the one or more processors 604 (e.g., CPUs, ASICs, FPGAs, and/or the like), the memory 606, and the network interface 608. If the network node 600 is a radio access node, the network node 600 may also include the one or more radio units 610 that each includes the one or more transmitters 612 and the one or more receivers 614 coupled to the one or more antennas 616, as described above. The control system 602 is connected to the radio unit(s) 610 via, for example, an optical cable or the like. The control system 602 is connected to one or more processing nodes 700 coupled to or included as part of a network(s) 702 via the network interface 608. Each processing node 700 includes one or more processors 704 (e.g., CPUs, ASICs, FPGAs, and/or the like), memory 706, and a network interface 708.

[0073] In this example, functions 710 of the network node 600 described herein are implemented at the one or more processing nodes 700 or distributed across the control system 602 and the one or more processing nodes 700 in any desired manner. In some particular embodiments, some or all of the functions 710 of the network node 600 described herein are implemented as virtual components executed by one or more virtual machines implemented in a virtual environment(s) hosted by the processing node(s) 700. Notably, in some embodiments, the control system 602 may not be included, in which case the radio unit(s) 610 communicate directly with the processing node(s) 700 via an appropriate network interface(s). [0074] In some embodiments, a computer program including instructions which, when executed by at least one processor, causes the at least one processor to carry out the functionality of network node 600 or a node (e.g., a processing node 700) implementing one or more of the functions 710 of the network node 600 in a virtual environment according to any of the embodiments described herein is provided. In some embodiments, a carrier comprising the aforementioned computer program product is provided. The carrier is one of an electronic signal, an optical signal, a radio signal, or a computer readable storage medium (e.g., a non-transitory computer readable medium such as memory).

[0075] Figure 8 is a schematic block diagram of the network node 600 according to some other embodiments of the present disclosure. The network node 600 includes one or more modules 800, each of which is implemented in software. The module(s) 800 provide the functionality of the network node 600 described herein. This discussion is equally applicable to the processing node 700 of Figure 7 where the modules 800 may be implemented at one of the processing nodes 700 or distributed across multiple processing nodes 700 and/or distributed across the processing node(s) 700 and the control system 602.

[0076] Figure 9 is a schematic block diagram of a UE 900 according to some embodiments of the present disclosure. As illustrated, the UE 900 includes one or more processors 902 (e.g., CPUs, ASICs, FPGAs, and/or the like), memory 904, and one or more transceivers 906 each including one or more transmitters 908 and one or more receivers 910 coupled to one or more antennas 912. The transceiver(s) 906 includes radio-front end circuitry connected to the antenna(s) 912 that is configured to condition signals communicated between the antenna(s) 912 and the processor(s) 902, as will be appreciated by on of ordinary skill in the art. The processors 902 are also referred to herein as processing circuitry. The transceivers 906 are also referred to herein as radio circuitry. In some

embodiments, the functionality of the UE 900 described above may be fully or partially implemented in software that is, e.g., stored in the memory 904 and executed by the processor(s) 902. Note that the UE 900 may include additional components not illustrated in Figure 9 such as, e.g., one or more user interface components (e.g., an input/output interface including a display, buttons, a touch screen, a microphone, a speaker(s), and/or the like and/or any other components for allowing input of information into the UE 900 and/or allowing output of information from the UE 900), a power supply (e.g., a battery and associated power circuitry), etc.

[0077] In some embodiments, a computer program including instructions which, when executed by at least one processor, causes the at least one processor to carry out the functionality of the UE 900 according to any of the embodiments described herein is provided. In some embodiments, a carrier comprising the aforementioned computer program product is provided. The carrier is one of an electronic signal, an optical signal, a radio signal, or a computer readable storage medium (e.g., a non-transitory computer readable medium such as memory).

[0078] Figure 10 is a schematic block diagram of the UE 900 according to some other embodiments of the present disclosure. The UE 900 includes one or more modules 1000, each of which is implemented in software. The module(s) 1000 provide the functionality of the UE 900 described herein.

[0079] Any appropriate steps, methods, features, functions, or benefits disclosed herein may be performed through one or more functional units or modules of one or more virtual apparatuses. Each virtual apparatus may comprise a number of these functional units. These functional units may be implemented via processing circuitry, which may include one or more

microprocessor or microcontrollers, as well as other digital hardware, which may include Digital Signal Processors (DSPs), special-purpose digital logic, and the like. The processing circuitry may be configured to execute program code stored in memory, which may include one or several types of memory such as Read Only Memory (ROM), Random Access Memory (RAM), cache memory, flash memory devices, optical storage devices, etc. Program code stored in memory includes program instructions for executing one or more telecommunications and/or data communications protocols as well as instructions for carrying out one or more of the techniques described herein. In some implementations, the processing circuitry may be used to cause the respective functional unit to perform corresponding functions according one or more embodiments of the present disclosure.

[0080] While processes in the figures may show a particular order of operations performed by certain embodiments of the present disclosure, it should be understood that such order is exemplary (e.g., alternative embodiments may perform the operations in a different order, combine certain operations, overlap certain operations, etc.).

[0081 ] At least some of the following abbreviations may be used in this disclosure. If there is an inconsistency between abbreviations, preference should be given to how it is used above. If listed multiple times below, the first listing should be preferred over any subsequent listing(s).

3GPP Third Generation Partnership Project

4G Fourth Generation

5G Fifth Generation

5GC Fifth Generation Core

ACK Acknowledgement

AF Application Function

AMF Access and Mobility Management Function

AP Access Point

ASIC Application Specific Integrated Circuit

ATCF Access Transfer Control Function

AUSF Authentication Server Function

CN Core Network

CPU Central Processing Unit

DSP Digital Signal Processor

eNB Enhanced or Evolved Node B

EPC Evolved Packet Core

ePDG Evolved Packet Data Gateway

FPGA Field Programmable Gate Array • gNB New Radio Base Station

• HSS Home Subscriber Server

• l-CSCF Interrogating Call Session Control Function

• IMS Internet Protocol Multimedia Subsystem

• IP Internet Protocol

• LTE Long Term Evolution

• MME Mobility Management Entity

• MTC Machine Type Communication

• N3GPP Non-Third Generation Partnership Project

• N3IWF Non-Third Generation Partnership Project Inter-

Working Function

• NEF Network Exposure Function

• NF Network Function

• NR New Radio

• NRF Network Repository Function

• NSSF Network Slice Selection Function

• PCF Policy Control Function

• PCRF Policy and Charging Rules Function

• P-CSCF Proxy Call Session Control Function

• PDN Packet Data Network

• PGW Packet Data Network Gateway

• QoS Quality of Service

• RAM Random Access Memory

• RAN Radio Access Network

• ROM Read Only Memory

• RTT Round Trip Time

• SA Security Association

• S-CSCF Serving Call Session Control Function

• SGW Serving Gateway

• SIP Session Initiation Protocol SMF Session Management Function

TS Technical Specification

UDM Unified Data Management

UE User Equipment

UPF User Plane Function

VoIP Voice over Internet Protocol

VOLTE Voice over Long Term Evolution

VoWiFi Voice over WiFi

VPN Virtual Private Network

WLAN Wireless Local Area Network

[0082] Those skilled in the art will recognize improvements and modifications to the embodiments of the present disclosure. All such improvements and modifications are considered within the scope of the concepts disclosed herein.

Claims

Claims What is claimed is:

1 . A method of operation of a User Equipment, UE, in a wireless

communication system, comprising:

establishing (200; 500-504, 508), over a cellular communications access network, a first Quality of Service, QoS, bearer or QoS flow for an incoming Internet Protocol Multimedia Subsystem, IMS, session having QoS requirements, the first QoS bearer or QoS flow for the incoming IMS session being associated with a first Internet Protocol, IP, address assigned to the UE for the cellular communications access network;

establishing (202; 516-520), over a Wireless Local Area Network, WLAN, access network, an IMS registration with a second IP address of the UE, the second IP address being assigned to the UE for the WLAN access network; initiating (204; 522) establishment of a second QoS bearer or QoS flow for the incoming IMS session over the WLAN access network, the second QoS bearer or QoS flow for the incoming IMS session being associated with the second IP address assigned to the UE for the WLAN access network; and

transferring (210; 534, 542) the incoming IMS session from the first IP address of the UE associated with the first QoS bearer or QoS flow established for the incoming IMS session over the cellular communications access network to the second IP address of the UE associated with the second QoS bearer or QoS flow established for the incoming IMS session over the WLAN access network.

2. The method of claim 1 wherein initiating (204; 522) establishment of the second QoS bearer or QoS flow for the incoming IMS session over the WLAN access network comprises sending (522), over the cellular communications access network, a Session Initiation Protocol, SIP, message that initiates establishment of the second QoS bearer or QoS flow for the incoming IMS session over the WLAN access network.

3. The method of claim 2 wherein the SIP message is a SIP OPTION message.

4. The method of claim 2 or 3 wherein the cellular communications access network is a Third Generation Partnership Project, 3GPP, access network, and the WLAN access network is a non-3GPP access network.

5. The method of claim 4 wherein sending the SIP message comprises sending the SIP message to a Proxy Call Session Control Function, P-CSCF, associated with the UE.

6. The method of claim 1 wherein the cellular communications access network is a Third Generation Partnership Project, 3GPP, access network, and the WLAN access network is a non-3GPP access network.

7. The method of any one of claims 1 to 6 wherein the IMS registration comprises an indication that the UE can perform IMS session transfer between the cellular communications access network and the WLAN access network.

8. The method of any one of claims 1 to 7 further comprising, prior to transferring the incoming IMS session, establishing (208; 528) a security association for the incoming IMS session with the WLAN access network based on the QoS requirements of the incoming IMS session.

9. The method of any one of claims 1 to 8 further comprising, prior to establishing (202; 516-520) the IMS registration with the second IP address of the UE, making (514) a decision to switch the incoming IMS session from the cellular communications access network to the WLAN access network.

10. The method of any one of claims 1 to 9 wherein establishing (200; 500, 502, 508) the first QoS bearer or QoS flow for the incoming IMS session comprises:

performing (500) an attachment procedure to attach to the cellular communications access network, wherein the first IP address is assigned to the UE during the attachment procedure;

performing (502), over the cellular communications access network, an IMS registration procedure with the first IP address assigned to the UE for the cellular communications access network; and

receiving (508) a SIP INVITE message for a Voice over IP, VoIP, call terminating at the UE;

wherein, during setup for the VoIP call, the first QoS bearer or QoS flow for the incoming IMS session is established to carry VoIP traffic for the VoIP call.

1 1 . The method of claim 10 wherein performing (504) the IMS registration procedure with the first IP address assigned to the UE for the cellular

communications access network comprises providing an indication that the UE can perform IMS session transfer between the cellular communications access network and the WLAN access network.

12. The method of any one of claims 1 to 1 1 wherein establishing (202; 516- 520) the IMS registration with the second IP address of the UE comprises:

performing (516) a registration procedure over the WLAN access network, wherein the second IP address is assigned to the UE during the registration procedure; and

performing (520), over the WLAN access network, an IMS registration procedure with the second IP address assigned to the UE for the WLAN access network.

13. The method of claim 12 wherein performing (520) the IMS registration procedure with the second IP address assigned to the UE for the cellular communications access network comprises providing an indication that the UE can perform IMS session transfer between the cellular communications access network and the WLAN access network.

14. The method of any one of claims 1 to 13 further comprising sending a Session Initiation Protocol, SIP, message for initiating release of the first QoS bearer or QoS flow.

15. The method of claim 14 wherein the SIP message for initiating release of the first QoS bearer or QoS flow is a SIP OPTION message.

16. A User Equipment, UE, for a wireless communication system, the UE adapted to:

establish, over a cellular communications access network, a first Quality of Service, QoS, bearer or QoS flow for an incoming Internet Protocol Multimedia Subsystem, IMS, session having QoS requirements, the first QoS bearer or QoS flow for the incoming IMS session being associated with a first Internet Protocol, IP, address assigned to the UE for the cellular communications access network; establish, over a Wireless Local Area Network, WLAN, access network, an IMS registration with a second IP address of the UE, the second IP address being assigned to the UE for the WLAN access network;

initiate establishment of a second QoS bearer or QoS flow for the incoming IMS session over the WLAN access network, the second QoS bearer or QoS flow for the incoming IMS session being associated with the second IP address assigned to the UE for the WLAN access network; and

transfer the incoming IMS session from the first IP address of the UE associated with the first QoS bearer or QoS flow established for the incoming IMS session over the cellular communications access network to the second IP address of the UE associated with the second QoS bearer or QoS flow

established for the incoming IMS session over the WLAN access network.

17. The UE of claim 16 wherein the UE is further adapted to perform the method of any one of claims 2 to 15.

18. A User Equipment, UE, for a wireless communication system, comprising: at least one processor; and

memory storing instructions executable by the at least one processor whereby the UE is operable to:

establish, over a cellular communications access network, a first Quality of Service, QoS, bearer or QoS flow for an incoming Internet Protocol Multimedia Subsystem, IMS, session having QoS requirements, the first QoS bearer or QoS flow for the incoming IMS session being associated with a first Internet Protocol, IP, address assigned to the UE for the cellular communications access network;

establish, over a Wireless Local Area Network, WLAN, access network, an IMS registration with a second IP address of the UE, the second IP address being assigned to the UE for the WLAN access network;

transfer the incoming IMS session from the first IP address of the UE associated with the first QoS bearer or QoS flow established for the incoming IMS session over the cellular communications access network to the second IP address of the UE associated with the second QoS bearer or QoS flow established for the incoming IMS session over the WLAN access network.

19. The UE of claim 18 wherein, via execution of the instructions by the at least one processor, the UE is further operable to perform the method of any one of claims 2 to 15.

20. A method of operation of a network entity in an Internet Protocol

Multimedia Subsystem, IMS, of a wireless communications system, comprising: receiving (204, 522) from a User Equipment, UE, a message that requests transfer of an incoming IMS session established for the UE over a cellular communications access network from the cellular communications access network to a Wireless Local Area Network, WLAN, access network; and

initiating (206, 524) establishment of a Quality of Service, QoS, bearer or QoS flow for the incoming IMS session over the WLAN access network.

21 . The method of claim 20 wherein the cellular communications access network is a Third Generation Partnership Project, 3GPP, access network, and the WLAN access network is a non-3GPP access network.

22. The method of claim 21 wherein the network entity is a Proxy Call Session Control Function, P-CSCF.

23. The method of claim 22 wherein initiating (206, 524) establishment of the QoS bearer or QoS flow for the incoming IMS session over the WLAN access network comprises transmitting (524), to a Policy and Charging Rules

Function/Policy Control Function, PCRF/PCF, a request to establish a QoS bearer or QoS flow for the incoming IMS session over the WLAN access network.

24. The method of any one of claims 20 to 23 further comprising receiving, from the UE, an indication that the UE can perform IMS session transfer between the cellular communications access network and the WLAN access network.

25. The method of claim 24 further comprising:

making a decision to allow transfer of the incoming IMS session from the cellular communications access network to the WLAN access network based on the indication that the UE can perform IMS session transfer between the cellular communications access network and the WLAN access network;

wherein initiating (206, 524) establishment of the QoS bearer or QoS flow for the incoming IMS session over the WLAN access network comprises initiating (206, 524) establishment of the QoS bearer or QoS flow for the incoming IMS session over the WLAN access network upon making the decision to allow transfer of the incoming IMS session from the cellular communications access network to the WLAN access network.

26. The method of any one of claims 20 to 25 wherein receiving (204, 522) the message that requests transfer of the incoming IMS session established for the UE over the cellular communications access network from the cellular

communications access network to the WLAN access network comprises:

receiving (522), from the UE, a Session Initiation Protocol, SIP, message that requests establishment of the QoS bearer or QoS flow for the incoming IMS session over the WLAN access network.

27. The method of claim 26 wherein the SIP message is a SIP OPTION message.

28. A network entity for an Internet Protocol Multimedia Subsystem, IMS, of a wireless communications system, the network entity adapted to:

receive from a User Equipment, UE, a message that requests transfer of an incoming IMS session established for the UE over a cellular communications access network from the cellular communications access network to a Wireless Local Area Network, WLAN, access network; and

initiate establishment of a Quality of Service, QoS, bearer or QoS flow for the incoming IMS session over the WLAN access network.

29. The network entity of claim 28 wherein the network entity is further adapted to perform the method of any one of claims 21 to 27.

30. A network entity for an Internet Protocol Multimedia Subsystem, IMS, of a wireless communications system, comprising:

at least one processor; and

memory storing instructions executable by the at least one processor whereby the network entity is operable to:

receive from a User Equipment, UE, a message that requests transfer of an incoming IMS session established for the UE over a cellular communications access network from the cellular communications access network to a Wireless Local Area Network, WLAN, access network; and initiate establishment of a Quality of Service, QoS, bearer or QoS flow for the incoming IMS session over the WLAN access network.

31 . The network entity of claim 30 wherein, via execution of the instructions by the at least one processor, the network entity is further operable to perform the method of any one of claims 21 to 27.