WO2021253263A1

WO2021253263A1 - Handover of voice over new radio (vonr) triggered by unsuccessful vonr attempt

Info

Publication number: WO2021253263A1
Application number: PCT/CN2020/096497
Authority: WO
Inventors: Haibo Liu; Bing LENG; Chaofeng HUI; Xuesong Chen; Xiaomeng Lu
Original assignee: Qualcomm Incorporated
Priority date: 2020-06-17
Filing date: 2020-06-17
Publication date: 2021-12-23

Abstract

Aspects of the disclosure relate to ways to reduce time for an inter-RAT handover for a voice call. In particular, a user equipment (UE) determines that a voice call by the UE via a new radio (NR) base station has failed to be established within a defined time limit. The UE further determines whether information on a neighbor base station is available at the UE from a past handover event specified between the NR base station and the neighbor base station in response to determining that the voice call has failed to be established via the NR base station, where the neighbor base station may utilize a RAT different from a RAT of the NR base station. The UE further performs a process for providing a voice service via the neighbor base station in response to determining that the information on the neighbor base station is available.

Description

HANDOVER OF VOICE OVER NEW RADIO (VONR) TRIGGERED BY UNSUCCESSFUL VONR ATTEMPT

TECHNICAL FIELD

The technology discussed below relates generally to wireless communication systems, and more particularly, to providing a voice service via wireless communication.

INTRODUCTION

Wireless communications systems are widely deployed to provide various types of communication content such as voice, video, messaging, and other types of data. With the development of a mobile device, such as a mobile phone, a voice service over wireless telecommunication has become more common. For example, fourth generation (4G) long-term evolution (LTE) networks may be utilized to allow a user of a mobile device to place a voice call. Recently, a next-generation evolution called New Radio (NR) , which may correspond to a fifth generation (5G) network, has been under active development. With the development of wireless communication via NR, providing a voice call over NR, often called voice over NR (VONR) , has been actively studied. As the LTE networks and the NR networks may coexist, improvements may be made to utilize both the LTE networks and the NR networks when providing a voice call service.

BRIEF SUMMARY OF SOME EXAMPLES

The following presents a simplified summary of one or more aspects of the present disclosure, in order to provide a basic understanding of such aspects. This summary is not an extensive overview of all contemplated features of the disclosure, and is intended neither to identify key or critical elements of all aspects of the disclosure nor to delineate the scope of any or all aspects of the disclosure. Its sole purpose is to present some concepts of one or more aspects of the disclosure in a simplified form as a prelude to the more detailed description that is presented later.

Various aspects pertain to ways to reduce time for a handover from a first base station of a first radio access technology (RAT) to a second base station of a second RAT different from the first RAT for a voice call. In one exemplary approach, a user equipment (UE) determines that a voice call by the UE via a new radio (NR) base station operating in an NR network has failed to be established within a defined time limit. The UE further determines whether information on a neighbor base station is available at the UE from a past handover event specified between the NR base station and the neighbor base station in response to determining that the voice call has failed to be established via the NR base station. The neighbor base station may utilize a RAT different from a RAT of the NR base station and may operate in a neighbor network different from the NR network. The UE further performs a process for providing a voice service via the neighbor base station in response to determining that the information on the neighbor base station is available.

In one example, a method for wireless communication performed by a UE is disclosed. The method includes determining that a voice call by the UE via an NR base station operating in an NR network has failed to be established within a defined time limit, determining whether information on a neighbor base station is available at the UE from a past handover event specified between the NR base station and the neighbor base station in response to determining that the voice call has failed to be established via the NR base station, the neighbor base station utilizing a RAT different from a RAT of the NR base station and operating in a neighbor network different from the NR network, and performing a process for providing a voice service via the neighbor base station in response to determining that the information on the neighbor base station is available. In an aspect, the method may further include performing a new public land mobile network (PLMN) search to search for a new network for a voice service in response to determining that the information on the neighbor base station is not available.

In an aspect, determining that the voice call via the NR base station has not been successfully performed within the defined time limit may include determining that a response to a session initiation protocol (SIP) invite message has not been successfully received within the defined time limit. In an aspect, the past handover event specified between the NR base station and the neighbor base station may be based on at least one of a present radio resource control (RRC) connection to the NR base station or a previous RRC connection to the NR base station whose cell identifier has not changed since the previous RRC connection.

In an aspect, performing the process for providing a voice service may include performing the process for providing a voice service, determining whether the signal strength measured is greater than a signal strength threshold, and performing a handover process from the NR base station to the neighbor base station in response to determining that the signal strength measured is greater than the signal strength threshold. In this aspect, the method may further include performing a new PLMN search to search for a new network for a voice service in response to determining that the signal strength measured is not greater than the signal strength threshold. In this aspect, the signal strength threshold may be less than or equal to a handover threshold associated with triggering a handover event and received from the NR base station.

Further, in this aspect, the handover process from the NR base station to the neighbor base station may be performed by determining whether a radio link failure with the NR base station has caused the voice call via the NR base station to fail, and performing a packet system fall back process to the neighbor base station with a radio link to the neighbor base station in response to determining that no radio link failure with the NR base station has caused the voice call via the NR base station to fail, and may be further performed by performing a cell search procedure to establish a new radio link to the neighbor base station in response to determining that the radio link failure with the NR base station has caused the voice call via the NR base station to fail. In such an aspect, performing the packet system fall back process to the neighbor base station may further include transmitting a handover report to the NR base station, transmitting a handover report to the NR base station, determining whether the packet system fall back has been successfully performed within a fall back time limit, and placing a voice call via the neighbor base station in response to determining that the packet system fall back has been successfully performed within the fall back time limit. In such an aspect, the handover process from the NR base station to the neighbor base station may be further performed by performing a cell search procedure to establish a new radio link to the neighbor base station in response to determining that the packet system fall back has not been successfully performed within the fall back time limit.

In another example, a UE for wireless communication is disclosed. The UE includes at least one processor, a transceiver communicatively coupled to the at least one processor, and a memory communicatively coupled to the at least one processor. The at least one processor is configured to determine that a voice call by the UE via an NR base station operating in an NR network has failed to be established within a defined time limit, determine whether information on a neighbor base station is available at the UE from a past handover event specified between the NR base station and the neighbor base station in response to determining that the voice call has failed to be established via the NR base station, the neighbor base station utilizing a RAT different from a RAT of the NR base station and operating in a neighbor network different from the NR network, and perform a process for providing a voice service via the neighbor base station in response to determining that the information on the neighbor base station is available.

In another example, a non-transitory computer-readable medium storing computer-executable code for a UE is disclosed. The non-transitory computer-readable medium includes code for causing a computer to determine that a voice call by the UE via an NR base station operating in an NR network has failed to be established within a defined time limit, determine whether information on a neighbor base station is available at the UE from a past handover event specified between the NR base station and the neighbor base station in response to determining that the voice call has failed to be established via the NR base station, the neighbor base station utilizing a RAT different from a RAT of the NR base station and operating in a neighbor network different from the NR network, and perform a process for providing a voice service via the neighbor base station in response to determining that the information on the neighbor base station is available.

In another example, a UE for wireless communication is disclosed. The UE includes means for determining that a voice call by the UE via a new radio (NR) base station operating in an NR network has failed to be established within a defined time limit, means for determining whether information on a neighbor base station is available at the UE from a past handover event specified between the NR base station and the neighbor base station in response to determining that the voice call has failed to be established via the NR base station, the neighbor base station utilizing a radio access technology (RAT) different from a RAT of the NR base station and operating in a neighbor network different from the NR network, and means for performing a process for providing a voice service via the neighbor base station in response to determining that the information on the neighbor base station is available.

These and other aspects of the invention will become more fully understood upon a review of the detailed description, which follows. Other aspects, features, and embodiments will become apparent to those of ordinary skill in the art, upon reviewing the following description of specific, exemplary embodiments in conjunction with the accompanying figures. While features may be discussed relative to certain embodiments and figures below, all embodiments can include one or more of the advantageous features discussed herein. In other words, while one or more embodiments may be discussed as having certain advantageous features, one or more of such features may also be used in accordance with the various embodiments discussed herein. In similar fashion, while exemplary embodiments may be discussed below as device, system, or method embodiments it should be understood that such exemplary embodiments can be implemented in various devices, systems, and methods.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a wireless communication system according to some aspects.

FIG. 2 is a conceptual illustration of an example of a radio access network according to some aspects.

FIG. 3 is a block diagram illustrating an example of a network architecture employing both next generation (e.g., 5G) and legacy (e.g., 3G and/or 4G) communication networks.

FIG. 4 is a block diagram illustrating a wireless communication system supporting multiple-input multiple-output (MIMO) communication.

FIG. 5 is an example diagram illustrating a process to establish a call via a wireless network.

FIG. 6 is an example diagram illustrating communications between a user equipment and various network nodes to perform a voice call with another user equipment, according to an aspect of the disclosure.

FIG. 7 is an example diagram illustrating a process to establish a call via a wireless network, according to an aspect of the disclosure.

FIG. 8 is an example diagram illustrating a process to establish a call via a wireless network, according to an aspect of the disclosure.

FIG. 9 is an example diagram illustrating a process to establish a call via a wireless network, according to an aspect of the disclosure.

FIG. 10 is a block diagram conceptually illustrating an example of a hardware implementation for a user equipment according to some aspects of the disclosure.

FIG. 11 is a flow chart illustrating an exemplary process for wireless communication according to some aspects of the disclosure.

DETAILED DESCRIPTION

The detailed description set forth below in connection with the appended drawings is intended as a description of various configurations and is not intended to represent the only configurations in which the concepts described herein may be practiced. The detailed description includes specific details for the purpose of providing a thorough understanding of various concepts. However, it will be apparent to those skilled in the art that these concepts may be practiced without these specific details. In some instances, well known structures and components are shown in block diagram form in order to avoid obscuring such concepts.

A device (e.g., user equipment) connected to a new radio (NR) base station may place a voice call to another device via the NR base station and an IP multimedia subsystem (IMS) network. In the past, when the device fails to establish a voice call via an NR base station, the device may perform a new public land mobile network (PLMN) search to search for a new network for a voice service. However, because the new PLMN search may consume a significant amount of time, the voice call is delayed and thus creating undesirable user experience.

The device may have information on a neighbor base station of a different RAT (e.g., LTE) , which is available from a past handover event specified between the NR base station and the neighbor base station. If this information is available, the device may perform a process for providing a voice service via the neighbor base station, which is less time consuming that performing a new PLMN search. The process for providing the voice service via the neighbor base station may involve a process of performing an evolved packet system fallback to the neighbor base station and/or a process of performing a cell search procedure to establish a new radio link to the neighbor base station.

While aspects and embodiments are described in this application by illustration to some examples, those skilled in the art will understand that additional implementations and use cases may come about in many different arrangements and scenarios. Innovations described herein may be implemented across many differing platform types, devices, systems, shapes, sizes, packaging arrangements. For example, embodiments and/or uses may come about via integrated chip embodiments and other non-module-component based devices (e.g., end-user devices, vehicles, communication devices, computing devices, industrial equipment, retail/purchasing devices, medical devices, AI-enabled devices, etc. ) . While some examples may or may not be specifically directed to use cases or applications, a wide assortment of applicability of described innovations may occur. Implementations may range a spectrum from chip-level or modular components to non-modular, non-chip-level implementations and further to aggregate, distributed, or OEM devices or systems incorporating one or more aspects of the described innovations. In some practical settings, devices incorporating described aspects and features may also necessarily include additional components and features for implementation and practice of claimed and described embodiments. For example, transmission and reception of wireless signals necessarily includes a number of components for analog and digital purposes (e.g., hardware components including antenna, RF-chains, power amplifiers, modulators, buffer, processor (s) , interleaver, adders/summers, etc. ) . It is intended that innovations described herein may be practiced in a wide variety of devices, chip-level components, systems, distributed arrangements, end-user devices, etc. of varying sizes, shapes and constitution.

The various concepts presented throughout this disclosure may be implemented across a broad variety of telecommunication systems, network architectures, and communication standards. Referring now to FIG. 1, as an illustrative example without limitation, various aspects of the present disclosure are illustrated with reference to a wireless communication system 100. The wireless communication system 100 includes interacting domains: a first core network 102, a first radio access network (RAN) 104, and a user equipment (UE) 106. The wireless communication system 100 may further include a second core network 152 and a second RAN 154. In some examples, the first RAN 104 may utilize a different radio access technology from that of the second RAN 154. By virtue of the wireless communication system 100, the UE 106 may be enabled to carry out data communication with an external data network 110, such as (but not limited to) the Internet.

The first RAN 104 may implement any suitable wireless communication technology or technologies to provide radio access to the UE 106. As one example, the first RAN 104 may operate according to 3 ^rd Generation Partnership Project (3GPP) New Radio (NR) specifications, often referred to as 5G. As another example, the first RAN 104 may operate under a hybrid of 5G NR and Evolved Universal Terrestrial Radio Access Network (eUTRAN) standards, often referred to as LTE. The 3GPP refers to this hybrid RAN as a next-generation RAN, or NG-RAN. Of course, many other examples may be utilized within the scope of the present disclosure.

As illustrated, the first RAN 104 includes at least one base station 108 and the second RAN 154 includes at least one base station 158. Broadly, a base station is a network element in a radio access network responsible for radio transmission and reception in one or more cells to or from a UE. In different technologies, standards, or contexts, a base station may variously be referred to by those skilled in the art as a base transceiver station (BTS) , a radio base station, a radio transceiver, a transceiver function, a basic service set (BSS) , an extended service set (ESS) , an access point (AP) , a Node B (NB) , an eNode B (eNB) , a gNode B (gNB) , or some other suitable terminology.

The first RAN 104 is further illustrated supporting wireless communication for multiple mobile apparatuses. A mobile apparatus may be referred to as user equipment (UE) in 3GPP standards, but may also be referred to by those skilled in the art as a mobile station (MS) , a subscriber station, a mobile unit, a subscriber unit, a wireless unit, a remote unit, a mobile device, a wireless device, a wireless communications device, a remote device, a mobile subscriber station, an access terminal (AT) , a mobile terminal, a wireless terminal, a remote terminal, a handset, a terminal, a user agent, a mobile client, a client, or some other suitable terminology. A UE may be an apparatus (e.g., a mobile apparatus) that provides a user with access to network services.

Within the present document, a “mobile” apparatus need not necessarily have a capability to move, and may be stationary. The term mobile apparatus or mobile device broadly refers to a diverse array of devices and technologies. UEs may include a number of hardware structural components sized, shaped, and arranged to help in communication; such components can include antennas, antenna arrays, RF chains, amplifiers, one or more processors, etc. electrically coupled to each other. For example, some non-limiting examples of a mobile apparatus include a mobile, a cellular (cell) phone, a smart phone, a session initiation protocol (SIP) phone, a laptop, a personal computer (PC) , a notebook, a netbook, a smartbook, a tablet, a personal digital assistant (PDA) , and a broad array of embedded systems, e.g., corresponding to an “Internet of things” (IoT) . A mobile apparatus may additionally be an automotive or other transportation vehicle, a remote sensor or actuator, a robot or robotics device, a satellite radio, a global positioning system (GPS) device, an object tracking device, a drone, a multi-copter, a quad-copter, a remote control device, a consumer and/or wearable device, such as eyewear, a wearable camera, a virtual reality device, a smart watch, a health or fitness tracker, a digital audio player (e.g., MP3 player) , a camera, a game console, etc. A mobile apparatus may additionally be a digital home or smart home device such as a home audio, video, and/or multimedia device, an appliance, a vending machine, intelligent lighting, a home security system, a smart meter, etc. A mobile apparatus may additionally be a smart energy device, a security device, a solar panel or solar array, a municipal infrastructure device controlling electric power (e.g., a smart grid) , lighting, water, etc. ; an industrial automation and enterprise device; a logistics controller; agricultural equipment; military defense equipment, vehicles, aircraft, ships, and weaponry, etc. Still further, a mobile apparatus may provide for connected medicine or telemedicine support, e.g., health care at a distance. Telehealth devices may include telehealth monitoring devices and telehealth administration devices, whose communication may be given preferential treatment or prioritized access over other types of information, e.g., in terms of prioritized access for transport of critical service data, and/or relevant QoS for transport of critical service data.

Wireless communication between the first RAN 104 and the UE 106 (or between the second RAN 154 and the UE 106) may be described as utilizing an air interface. Transmissions over the air interface from a base station (e.g., base station 108) to one or more UEs (e.g., UE 106) may be referred to as downlink (DL) transmission. In accordance with certain aspects of the present disclosure, the term downlink may refer to a point-to-multipoint transmission originating at a scheduling entity (described further below; e.g., base station 108) . Another way to describe this scheme may be to use the term broadcast channel multiplexing. Transmissions from a UE (e.g., UE 106) to a base station (e.g., base station 108) may be referred to as uplink (UL) transmissions. In accordance with further aspects of the present disclosure, the term uplink may refer to a point-to-point transmission originating at a scheduled entity (described further below; e.g., UE 106) .

In some examples, access to the air interface may be scheduled, wherein a scheduling entity (e.g., a base station 108) allocates resources for communication among some or all devices and equipment within its service area or cell. Within the present disclosure, as discussed further below, the scheduling entity may be responsible for scheduling, assigning, reconfiguring, and releasing resources for one or more scheduled entities. That is, for scheduled communication, UEs 106, which may be scheduled entities, may utilize resources allocated by the scheduling entity 108.

A base station 108 is not the only entity that may function as a scheduling entity. That is, in some examples, a UE may function as a scheduling entity, scheduling resources for one or more scheduled entities (e.g., one or more other UEs) .

As illustrated in FIG. 1, a scheduling entity 108 may broadcast downlink traffic 112 to one or more scheduled entities 106. Broadly, the scheduling entity 108 is a node or device responsible for scheduling traffic in a wireless communication network, including the downlink traffic 112 and, in some examples, uplink traffic 116 from one or more scheduled entities 106 to the scheduling entity 108. On the other hand, the scheduled entity 106 is a node or device that receives downlink control information 114, including but not limited to scheduling information (e.g., a grant) , synchronization or timing information, or other control information from another entity in the wireless communication network such as the scheduling entity 108.

In general, a base station 108 may include a backhaul interface for communication with a backhaul portion 120 of the wireless communication system. The backhaul 120 may provide a link between the base station 108 and the first core network 102. Further, in some examples, a backhaul network may provide interconnection between the base station 108 and another base station in the first RAN 104. Various types of backhaul interfaces may be employed, such as a direct physical connection, a virtual network, or the like using any suitable transport network.

The first core network 102 may be a part of the wireless communication system 100, and may be independent of the radio access technology used in the first RAN 104. In some examples, the first core network 102 may be configured according to 5G standards (e.g., 5GC) . In other examples, the first core network 102 may be configured according to a 4G evolved packet core (EPC) , or any other suitable standard or configuration.

Similarly, a scheduling entity/base station 158 may communicate with the UE 106 via uplink communication and/or downlink communication. The base station 158 may include a backhaul interface for communication with a backhaul portion 170 of the wireless communication system. The backhaul 170 may provide a link between the base station 158 and the second core network 152. Further, in some examples, a backhaul network may provide interconnection between the base station 108 and another base station in the second RAN 154. Various types of backhaul interfaces may be employed, such as a direct physical connection, a virtual network, or the like using any suitable transport network.

The second core network 152 may be a part of the wireless communication system 100, and may be independent of the radio access technology used in the second RAN 154. In some examples, the second core network 152 may be configured according to a 4G evolved packet core (EPC) . In other examples, the second core network 152 may be configured according to 5G standards (e.g., 5GC) , or any other suitable standard or configuration.

Referring now to FIG. 2, by way of example and without limitation, a schematic illustration of a RAN 200 is provided. In some examples, the RAN 200 may be the same as the first RAN 104 or the second RAN 154 described above and illustrated in FIG. 1. The geographic area covered by the RAN 200 may be divided into cellular regions (cells) that can be uniquely identified by a user equipment (UE) based on an identification broadcasted from one access point or base station. FIG. 2 illustrates

macrocells

202, 204, and 206, and a small cell 208, each of which may include one or more sectors (not shown) . A sector is a sub-area of a cell. All sectors within one cell are served by the same base station. A radio link within a sector can be identified by a single logical identification belonging to that sector. In a cell that is divided into sectors, the multiple sectors within a cell can be formed by groups of antennas with each antenna responsible for communication with UEs in a portion of the cell.

In FIG. 2, two base stations 210 and 212 are shown in

cells

202 and 204; and a third base station 214 is shown controlling a remote radio head (RRH) 216 in cell 206. That is, a base station can have an integrated antenna or can be connected to an antenna or RRH by feeder cables. In the illustrated example, the

cells

202, 204, and 126 may be referred to as macrocells, as the

base stations

210, 212, and 214 support cells having a large size. Further, a base station 218 is shown in the small cell 208 (e.g., a microcell, picocell, femtocell, home base station, home Node B, home eNode B, etc. ) which may overlap with one or more macrocells. In this example, the cell 208 may be referred to as a small cell, as the base station 218 supports a cell having a relatively small size. Cell sizing can be done according to system design as well as component constraints.

It is to be understood that the radio access network 200 may include any number of wireless base stations and cells. Further, a relay node may be deployed to extend the size or coverage area of a given cell. The

base stations

210, 212, 214, 218 provide wireless access points to a core network for any number of mobile apparatuses. In some examples, the

base stations

210, 212, 214, and/or 218 may be the same as the base station/scheduling entity 108 described above and illustrated in FIG. 1.

FIG. 2 further includes a quadcopter or drone 220, which may be configured to function as a base station. That is, in some examples, a cell may not necessarily be stationary, and the geographic area of the cell may move according to the location of a mobile base station such as the quadcopter 220.

Within the RAN 200, the cells may include UEs that may be in communication with one or more sectors of each cell. Further, each

base station

210, 212, 214, 218, and 220 may be configured to provide an access point to a core network (e.g., first core network 102 of FIG. 1) for all the UEs in the respective cells. For example, UEs 222 and 224 may be in communication with base station 210;

UEs

226 and 228 may be in communication with base station 212;

UEs

230 and 232 may be in communication with base station 214 by way of RRH 216; UE 234 may be in communication with base station 218; and UE 236 may be in communication with mobile base station 220. In some examples, the

UEs

222, 224, 226, 228, 230, 232, 234, 236, 238, 240, and/or 242 may be the same as the UE/scheduled entity 106 described above and illustrated in FIG. 1.

In some examples, a mobile network node (e.g., quadcopter 220) may be configured to function as a UE. For example, the quadcopter 220 may operate within cell 202 by communicating with base station 210.

In a further aspect of the RAN 200, sidelink signals may be used between UEs without necessarily relying on scheduling or control information from a base station. For example, two or more UEs (e.g., UEs 226 and 228) may communicate with each other using peer to peer (P2P) or sidelink signals 227 without relaying that communication through a base station (e.g., base station 212) . In a further example, UE 238 is illustrated communicating with

UEs

240 and 242. Here, the UE 238 may function as a scheduling entity or a primary sidelink device, and

UEs

240 and 242 may function as a scheduled entity or a non-primary (e.g., secondary) sidelink device. In still another example, a UE may function as a scheduling entity in a device-to-device (D2D) , peer-to-peer (P2P) , or vehicle-to-vehicle (V2V) network, and/or in a mesh network. In a mesh network example,

UEs

240 and 242 may optionally communicate directly with one another in addition to communicating with the scheduling entity 238. Thus, in a wireless communication system with scheduled access to time–frequency resources and having a cellular configuration, a P2P configuration, or a mesh configuration, a scheduling entity and one or more scheduled entities may communicate utilizing the scheduled resources.

In the radio access network 200, the ability for a UE to communicate while moving, independent of its location, is referred to as mobility. The various physical channels between the UE and the radio access network are generally set up, maintained, and released under the control of an access and mobility management function (AMF, not illustrated, part of the first core network 102 in FIG. 1) , which may include a security context management function (SCMF) that manages the security context for both the control plane and the user plane functionality, and a security anchor function (SEAF) that performs authentication.

In various aspects of the disclosure, a radio access network 200 may utilize DL-based mobility or UL-based mobility to enable mobility and handovers (i.e., the transfer of a UE’s connection from one radio channel to another) . In a network configured for DL-based mobility, during a call with a scheduling entity, or at any other time, a UE may monitor various parameters of the signal from its serving cell as well as various parameters of neighboring cells. Depending on the quality of these parameters, the UE may maintain communication with one or more of the neighboring cells. During this time, if the UE moves from one cell to another, or if signal quality from a neighboring cell exceeds that from the serving cell for a given amount of time, the UE may undertake a handoff or handover from the serving cell to the neighboring (target) cell. For example, UE 224 (illustrated as a vehicle, although any suitable form of UE may be used) may move from the geographic area corresponding to its serving cell 202 to the geographic area corresponding to a neighbor cell 206. When the signal strength or quality from the neighbor cell 206 exceeds that of its serving cell 202 for a given amount of time, the UE 224 may transmit a reporting message to its serving base station 210 indicating this condition. In response, the UE 224 may receive a handover command, and the UE may undergo a handover to the cell 206.

In a network configured for UL-based mobility, UL reference signals from each UE may be utilized by the network to select a serving cell for each UE. In some examples, the

base stations

210, 212, and 214/216 may broadcast unified synchronization signals (e.g., unified Primary Synchronization Signals (PSSs) , unified Secondary Synchronization Signals (SSSs) and unified Physical Broadcast Channels (PBCH) ) . The

UEs

222, 224, 226, 228, 230, and 232 may receive the unified synchronization signals, derive the carrier frequency and slot timing from the synchronization signals, and in response to deriving timing, transmit an uplink pilot or reference signal. The uplink pilot signal transmitted by a UE (e.g., UE 224) may be concurrently received by two or more cells (e.g., base stations 210 and 214/216) within the radio access network 200. Each of the cells may measure a strength of the pilot signal, and the radio access network (e.g., one or more of the base stations 210 and 214/216 and/or a central node within the core network) may determine a serving cell for the UE 224. As the UE 224 moves through the radio access network 200, the network may continue to monitor the uplink pilot signal transmitted by the UE 224. When the signal strength or quality of the pilot signal measured by a neighboring cell exceeds that of the signal strength or quality measured by the serving cell, the network 200 may handover the UE 224 from the serving cell to the neighboring cell, with or without informing the UE 224.

Although the synchronization signal transmitted by the

base stations

210, 212, and 214/216 may be unified, the synchronization signal may not identify a particular cell, but rather may identify a zone of multiple cells operating on the same frequency and/or with the same timing. The use of zones in 5G networks or other next generation communication networks enables the uplink-based mobility framework and improves the efficiency of both the UE and the network, since the number of mobility messages that need to be exchanged between the UE and the network may be reduced.

FIG. 3 is a block diagram illustrating an example of a network architecture 300 employing both next generation (e.g., 5G) and legacy (e.g., 3G and/or 4G) communication networks. The network architecture 300 may include one or more user equipment (UE) 302, an NR (e.g., next generation, 5G) RAN 304, a legacy (e.g., 3G or 4G) RAN 354, a next generation core network 306, and a legacy (3G or 4G) core network 356. By virtue of the network architecture 300, the UE 302 may be enabled to carry out data communication with an external data network 314, such as (but not limited to) the Internet, an Ethernet network, an IP multimedia subsystem (IMS) network or a local area network.

In this illustration, any signal path between a UE and a core network is presumed to be passed between these entities by a RAN, as represented by an illustrated signal path crossing the RAN. Here, the

RANs

304 and 354 may each be the RAN 100 described above and illustrated in FIG. 1. In the description that follows, when reference is made to a RAN or actions performed by the RAN, it may be understood that such reference refers to one or more network nodes in the RAN that is or are communicatively coupled to a core network e.g., via a backhaul connection. As one nonlimiting example, for clarity of description, such reference to the RAN may be understood as referring to a base station. However, those of ordinary skill in the art will comprehend that this is may not always be the case, for example, as in certain 3G RANs where base stations are under the control or direction of centralized radio network controllers within their RAN. In addition, both user plane (UP) and control plane (CP) functionality may be supported by the UE 302, the

RANs

304 and 354 and the

core networks

306 and 356.

In some examples, the legacy RAN 354 may provide an access point to both the legacy core network 356 and the next generation core network 306, while the NR RAN 304 may provide an access point to the next generation core network 306. In other examples, the legacy RAN 354 and the NR RAN 304 may each provide respective access points to both the legacy core network 356 and the next generation core network 306.

In various aspects of the present disclosure, each RAN (NR RAN 304 and legacy RAN 354) may utilize a different respective radio access technology (RAT) to access a core network (e.g., next generation core network 306 and/or legacy core network 356) . For example, the legacy RAN 354 may utilize a first (e.g., legacy) RAT to access a core network (e.g., either the next generation core network 306 or the legacy core network 356) , while the NR RAN 304 may utilize a second (e.g., next generation) RAT to access a core network.

The next generation core network 306 may include, for example, an access and mobility management function (AMF) 308, a session management function (SMF) 310, and a user plane function (UPF) 312. The AMF 308 and SMF 310 employ control plane (e.g., Non Access Stratum (NAS) ) signaling to perform various functions related to mobility management and session management for the UE 302. For example, the AMF 308 provides connectivity, mobility management and authentication of the UE 302, while the SMF 310 provides session management of the UE 302 (e.g., processes signaling related to protocol data unit (PDU) sessions between the UE 302 and the external data network 314) . The UPF 312 provides user plane connectivity to route 5G (NR) packets to/from the UE 302 via the NR RAN 304.

The next generation core network 306 may further include other functions, such as a policy control function (PCF) 316, authentication server function (AUSF) 318, unified data management (UDM) 320, network slice selection function (NSSF) 322, and other functions (not illustrated, for simplicity) . The PCF 316 provides policy information (e.g., rules) for control plane functions, such as network slicing, roaming, and mobility management. In addition, the PCF 316 supports 5G quality of service (QoS) policies, network slice policies, and other types of policies. The AUSF 318 performs authentication of the UE 302. The UDM 320 facilitates generation of authentication and key agreement (AKA) credentials, performs user identification and manages subscription information and UE context. In some examples, the AMF 308 includes a co-located security anchor function (SEAF) that allows for re-authentication of a UE 302 when the UE moves between different NR RANs (e.g., between the NR RAN 304 and another NR RAN) without having to perform a complete authentication process with the AUSF 318. The NSSF 322 redirects traffic to a network slice. Network slices may be defined, for example, for different classes of subscribers or use cases, such as smart home, Internet of Things (IoT) , connected car, smart energy grid, etc. Each use case may receive a unique set of optimized resources and network topology (e.g., a network slice) to meet the connectivity, speed, power, and capacity requirements of the use case.

To establish a connection to the next generation core network 306 via the NR RAN 304, the UE 302 may transmit a registration request and PDU session establishment request to the next generation core network 306 via the NR RAN 304. The AMF 308 and SMF 310 may process the registration request and PDU session establishment request and establish a PDU session between the UE 302 and the external data network 314 via the UPF 312. A PDU session may include one or more sessions (e.g., data sessions or data flows) and may be served by multiple UPFs 312 (only one of which is shown for convenience) . Examples of data flows include, but are not limited to, IP flows, Ethernet flows and unstructured data flows.

The legacy RAN 354 may be, for example, an Evolved UMTS Terrestrial Radio Access Network (E-UTRAN) within a Long Term Evolution (LTE) network, a Universal Mobile Telecommunications System (UMTS) Terrestrial Radio Access Network (UTRAN) , a Wireless Local Area Network (WLAN) or other type of legacy access network. The NR RAN 304 may be, for example, a 5G Radio Access Network (RAN) or Evolved E-UTRAN (i.e., an E-UTRAN enhanced to natively connect to the next generation core network 306 with the same interface as the 5G RAN) . In other examples, the NR RAN 304 may be a next generation Wireless Local Area Network (WLAN) , a next generation fixed broadband Internet access network or other type of next generation access network that utilizes a next generation RAT to access the next generation core network 306.

The legacy RAN 354 may include an evolved Node BS (eNB) that provides user and control plane protocol terminations toward the UE 302. The eNB may also be referred to by those skilled in the art as a base station, a base transceiver station, a radio base station, a radio transceiver, a transceiver function, a basic service set (BSS) , an extended service set (ESS) , or some other suitable terminology. The eNB may be connected to the other eNBs via an X2 interface (i.e., backhaul) .

The eNB provides an access point to the legacy core network 356, such as an Evolved Packet Core (EPC) network. In addition, although not shown, the NR RAN 304 may also provide an access point to the legacy core network 356. The legacy core network 356 may include, for example, a Serving Gateway (SGW) 360, a Packet Data Network (PDN) Gateway 362 and a Mobility Management Entity (MME) 358. All user IP packets are transferred through the SGW 360, which itself is connected to the PDN Gateway 362. The PDN Gateway 362 provides UE IP address allocation as well as other functions.

The MME 358 is the control node that processes the signaling between the UE 302 and the legacy core network 356. Generally, the MME 358 provides bearer and connection management for the UE 302 according to mechanisms defined for the legacy core network 356. For example, the MME 358 may manage security when a UE 302 connects to the legacy RAN 354 by using information provided by a Home Subscriber Server (HSS, not shown) to authenticate UEs and update UEs location information in the HSS. The MME 358 may further maintain the tracking area identity (TAI) of the current tracking area (e.g., group of neighboring cells/eNBs) within which the UE 302 is located to enable paging of the UE 302 when the UE is in idle mode. In some examples, the legacy RAN 354 may include a single tracking area. In other examples, the legacy RAN 354 may include two or more tracking areas. Moreover, the MME 358 may manage connectivity via Packet Data Connections (PDNs) between the UE 302 and the PDN Gateway 362, and determine and provide a set of legacy Quality of Service (QoS) parameters to the eNB.

When the NR (e.g., 5G) RAN 304, the legacy (e.g., 3G or 4G) RAN 354, the next generation core network 306, and the legacy core network 356 coexist, an inter-RAT handover may be performed to perform a handover between the NR RAN 304 and the legacy RAN 354. The inter-RAT handover may be performed via a common subscription data access function (HSS/UDM, not shown) , the SMF 310, the UPF 312, the SGW 360, the PDN Gateway 362 and an N26 interface between the MME 358 and the AMF 308. A handover from a 4G core network to a 5G core network may also be referred to as an evolved packet system (EPS) fallback.

In an aspect, to perform the inter-RAT handover between the NR RAN 304 and the legacy RAN 354, the UE 302 may obtain measurements associated with a target cell to ensure that the target cell is a suitable for the UE 302. For example, the UE 302 may measure a signal strength associated with the target cell to determine whether the signal strength is sufficiently high (e.g., greater than a certain threshold) to reliably serve the UE 302. The UE 302 may rely on an RRC connection reconfiguration message and a measurement report. In an example, if the UE 302 is camped on the NR RAN304, the NR RAN 304 may send the RRC connection reconfiguration message to the UE 302, where the RRC connection reconfiguration message specifies a type of event for the UE 302 to report. A type of event specified in the RRC connection reconfiguration message may be an inter-RAT handover event. Hence, if a condition for the specified inter-RAT handover event is satisfied, the UE 302 may send a measurement report to the NR RAN 304 to trigger an inter-RAT handover, where the measurement report is associated with the inter-RAT handover event. The condition for the specified inter-RAT handover event may be associated with a signal strength of a target cell.

3GPP specification 38.331 specifies Events A1, A2, A3, A4, A5, A6, B1, and B2 for 5G NR. For example, the RRC connection reconfiguration message may specify Event B1, whose condition is that an inter-RAT neighbor cell becomes better than a B1 threshold. In this example, if a reference signal received power (RSRP) value of the inter-RAT neighbor cell (e.g., legacy RAN 354) becomes better than the B1 threshold, then the condition for Event B1 is satisfied and the UE 302 sends a B1 measurement report to the NR RAN 304 to trigger an inter-RAT handover from the NR RAN 304 to the legacy RAN 354.

In some aspects of the disclosure, the scheduling entity and/or scheduled entity may be configured for beamforming and/or multiple-input multiple-output (MIMO) technology. FIG. 4 illustrates an example of a wireless communication system 400 supporting MIMO. In a MIMO system, a transmitter 402 includes multiple transmit antennas 404 (e.g., N transmit antennas) and a receiver 406 includes multiple receive antennas 408 (e.g., M receive antennas) . Thus, there are N × M signal paths 410 from the transmit antennas 404 to the receive antennas 408. Each of the transmitter 402 and the receiver 406 may be implemented, for example, within a scheduling entity 108, a scheduled entity 106, or any other suitable wireless communication device.

The use of such multiple antenna technology enables the wireless communication system to exploit the spatial domain to support spatial multiplexing, beamforming, and transmit diversity. Spatial multiplexing may be used to transmit different streams of data, also referred to as layers, simultaneously on the same time-frequency resource. The data streams may be transmitted to a single UE to increase the data rate or to multiple UEs to increase the overall system capacity, the latter being referred to as multi-user MIMO (MU-MIMO) . This is achieved by spatially precoding each data stream (i.e., multiplying the data streams with different weighting and phase shifting) and then transmitting each spatially precoded stream through multiple transmit antennas on the downlink. The spatially precoded data streams arrive at the UE (s) with different spatial signatures, which enables each of the UE (s) to recover the one or more data streams destined for that UE. On the uplink, each UE transmits a spatially precoded data stream, which enables the base station to identify the source of each spatially precoded data stream.

The number of data streams or layers corresponds to the rank of the transmission. In general, the rank of the MIMO system 400 is limited by the number of transmit or receive

antennas

404 or 408, whichever is lower. In addition, the channel conditions at the UE, as well as other considerations, such as the available resources at the base station, may also affect the transmission rank. For example, the rank (and therefore, the number of data streams) assigned to a particular UE on the downlink may be determined based on the rank indicator (RI) transmitted from the UE to the base station. The RI may be determined based on the antenna configuration (e.g., the number of transmit and receive antennas) and a measured signal-to-interference-and-noise ratio (SINR) on each of the receive antennas. The RI may indicate, for example, the number of layers that may be supported under the current channel conditions. The base station may use the RI, along with resource information (e.g., the available resources and amount of data to be scheduled for the UE) , to assign a transmission rank to the UE.

In Time Division Duplex (TDD) systems, the UL and DL are reciprocal, in that each uses different time slots of the same frequency bandwidth. Therefore, in TDD systems, the base station may assign the rank for DL MIMO transmissions based on UL SINR measurements (e.g., based on a Sounding Reference Signal (SRS) transmitted from the UE or other pilot signal) . Based on the assigned rank, the base station may then transmit the CSI-RS with separate C-RS sequences for each layer to provide for multi-layer channel estimation. From the CSI-RS, the UE may measure the channel quality across layers and resource blocks and feed back the CQI and RI values to the base station for use in updating the rank and assigning REs for future downlink transmissions.

In the simplest case, as shown in FIG. 4, a rank-2 spatial multiplexing transmission on a 2x2 MIMO antenna configuration will transmit one data stream from each transmit antenna 404. Each data stream reaches each receive antenna 408 along a different signal path 410. The receiver 406 may then reconstruct the data streams using the received signals from each receive antenna 408.

In order for transmissions over the radio access network 200 to obtain a low block error rate (BLER) while still achieving very high data rates, channel coding may be used. That is, wireless communication may generally utilize a suitable error correcting block code. In a typical block code, an information message or sequence is split up into code blocks (CBs) , and an encoder (e.g., a CODEC) at the transmitting device then mathematically adds redundancy to the information message. Exploitation of this redundancy in the encoded information message can improve the reliability of the message, enabling correction for any bit errors that may occur due to the noise.

In early 5G NR specifications, user data is coded using quasi-cyclic low-density parity check (LDPC) with two different base graphs: one base graph is used for large code blocks and/or high code rates, while the other base graph is used otherwise. Control information and the physical broadcast channel (PBCH) are coded using Polar coding, based on nested sequences. For these channels, puncturing, shortening, and repetition are used for rate matching.

However, those of ordinary skill in the art will understand that aspects of the present disclosure may be implemented utilizing any suitable channel code. Various implementations of scheduling entities 108 and scheduled entities 106 may include suitable hardware and capabilities (e.g., an encoder, a decoder, and/or a CODEC) to utilize one or more of these channel codes for wireless communication.

With the development of the new radio (NR) technology in wireless communication, voice over new radio (VONR) has also been under development. To initiate a voice call via VONR, a user may place a call using a UE, which then transmits a SIP invite message to an NR base station serving the UE. The NR base station forwards the SIP invite message to the IMS network, which then forwards the SIP invite to a recipient UE via a base station.

Referring to FIG. 5, by way of example and without limitation, an example diagram 500 illustrating a process to establish a call via a wireless network is provided. The example diagram 500 of FIG. 5 involves communication among a first UE 502, a first RAN 504 where the first UE 502 is camped, a first IMS network 506 providing a multimedia service to the first RAN 504, a second IMS network 508, a second RAN 510, where the second IMS network 508 provides the multimedia service to the second RAN 510, and a second UE 512 camped on the second RAN 510. In an example, the first RAN 504 may be an NR RAN, which enables a voice call via VONR. In an example, the second RAN 510 may be an NR RAN or a legacy RAN.

Once the first UE 502 successfully performs an IMS registration at 520 with the first IMS network 506, a user may use the first UE 502 to initiate a voice call. When the voice call is initiated, at 522, the first UE 502 transmits the SIP invite to the second UE 512 via the first RAN 504, the first IMS network 506, the second IMS network 508, and the second RAN 510. In response to the SIP invite, at 524, the second IMS network 508 transmits a 100 Trying response to the first UE 502 via the first IMS network 506 and the first RAN 504. The 100 Trying response may indicate that the SIP invite message has been received and may cause the first UE 502 to avoid re-transmission of the SIP invite message. Further, in response to the SIP invite, at 516, the second UE 512 transmits a 180 Ringing message to the first UE 502, via the second RAN 510, the second IMS network 508, the first IMS network 506, and the first RAN504. The 180 Ringing response may indicate that the second UE 512 has received the SIP invite and is ringing. When the call from the first UE 502 is answered by the second UE 512, at 528, the second UE 512 transmits a 200 OK response to the first UE 502 via the second RAN 510, the second IMS network 508, the first IMS network 506, and the first RAN504. In response, at 530, the first UE 502 transmits an acknowledgement message to the second UE 512 via the first RAN 504, the first IMS network 506, the second IMS network 508, and the second RAN 510, to acknowledge that the 200 OK response has been received. Simultaneously or shortly thereafter, at 532, a session is established and a voice call is performed by the first UE 502 and the second UE 512 by communicating voice data packets to each other.

As explained above, in a successful VONR voice call via an NR base station operating in an NR RAN, a UE receives a response to the SIP invite message sent by the UE, where the response to the SIP invite message may include a 100 Trying response sent from an IMS network and/or a 180 Ringing response sent from the recipient UE. The voice call via the NR base station may fail due to a timeout error for failing to successfully receive a response to a SIP invite message within a T-call timer value, where the T-call timer value is a time limit within which the response to the SIP invite message is to be received for successful reception of the response. At least the following three cases may contribute to a failure to establish the voice call via the NR base station within the T-call timer value. In one case, an IMS network may fail to send a response to the SIP invite message, for example, due to issues within the IMS network. In one case, because a SIP invite message is generally sent via a UL grant and a size of the UL grant is small, the UE may not be able to successfully send the SIP invite message via the UL grant. In one case, the UE may experience a radio link failure, which causes the UE to fail to send the SIP invite message successfully. When the SIP invite message is not successfully received by the IMS network or the recipient UE, the IMS network or the recipient UE may not send a response to the SIP invite message.

If the UE has not successfully received a response to the SIP invite message within the T-call timer value, the UE determines that a voice call via the NR base station has failed. In response to the failure to establish the voice call via the NR base station, the UE may trigger a new PLMN search to find a new available cell of a base station to place a voice call. However, a new PLMN search is generally very time-consuming, which may cause a long delay in placing a voice call. Further, a new PLMN search may find a cell that is not a cell of a neighbor base station to the UE, and thus may not find the most desirable base station/network. Therefore, in case of the failure to establish a voice call via the NR base station, an approach to reduce a call delay in attempting the voice call with another cell is desired.

According to an aspect of the disclosure, when a voice call via an NR base station operating in an NR network fails, the UE may perform a process for providing a voice service via a neighbor base station utilizing a different RAT from the NR base station if information on the neighbor base station is available at the UE, instead of automatically performing a new PLMN search. The information on the neighbor base station may be available from a past handover event specified between the NR base station and the neighbor base station. The process for providing the voice service via the neighbor base station may include a process to perform an EPS fallback from the NR base station to the neighbor base station and/or a process to perform a cell search procedure to establish a new radio link to the neighbor base station, as described in more detail below.

Referring to FIG. 6, by way of example and without limitation, an example diagram 600 illustrating communications between a user equipment and various network nodes to perform a voice call with another user equipment, according to an aspect of the disclosure, is provided. In the example diagram 600, a UE 602 may be initially camped on an NR base station 604 and a recipient UE 612 may be camped on a base station 614. The UE 602 may initiate a voice call with a recipient UE 612 via the NR base station 604 and a first IMS network 608 providing a multimedia service to the NR base station 604. For example, when the UE 602 initiates the voice call, a SIP invite message may be forwarded to the recipient UE 612 via the NR base station 604, the first IMS network 608, a second IMS network 616 proving a multimedia service to the base station 614, and the base station 614.

The UE 602 may determine that the voice call with the recipient UE 612 via the NR base station 604 has failed to be established if the voice call is not established within a defined time limit, such as the T-call timer value. The voice call via the NR base station 604 by the UE 602 may fail due to a variety of reasons. For example, the UE 602 may determine that the voice call via the NR base station 604 has failed to be established within the defined time limit if the UE 602 fails to successfully receive a response the SIP invite message sent to recipient UE 612. As discussed above, the response to the SIP invite message may include a 100 Trying message and/or a 180 Ringing message.

When the UE 602 determines that the voice call with the recipient UE 612 has failed to be established via the NR base station 604, the UE 602 may determine whether information on a neighbor base station is available at the UE 602 from a past handover event specified between the NR base station 604 and a neighbor base station 606 that utilizes a RAT different from a RAT of the NR base station 604 and operates in a neighbor network different from the NR network of the NR base station 605. The past handover event may be a completed handover event or an incomplete handover event that was specified in a measurement configuration but not completed. If the information on the neighbor base station 606 is available from the past handover event, the UE 602 may perform a process for providing a voice service via the neighbor base station 606 (e.g., using the information on the neighbor base station) . Otherwise, the UE 602 may perform a new PLMN search to search for a new network to camp on, for a voice service.

In an aspect, the information on the neighbor base station may be obtained from the past handover event as described below. When the UE 602 receives a measurement configuration (e.g., via an RRC connection reconfiguration message) from the NR base station 604 that specifies a type of handover event to report, the UE 602 may make measurements (e.g., signal strength measurements) of signals associated with the neighbor base station 606 based on the measurement configuration. For example, the UE 602 may measure the signal strength (e.g., RSRP or RSRQ) associated with the neighbor base station 606, e.g., based on a reference signal sent to the neighbor base station 606. Thus, the NR base station 604 may send the UE 602 configuration information on the specified handover event via the measurement configuration, which includes information on the neighbor base station 606. The information about the neighbor base station 606 may be from a measurement configuration received in the present RRC connection with the NR base station 604 or may be from a measurement configuration received in a previous RRC connection with the NR base station 604 if the NR cell identifier has not been changed. This information on the neighbor base station 606 from the past handover event may be available at the UE 602, which can utilize such information on the neighbor base station 606 to perform a process for providing a voice service via the neighbor base station 606 without performing a new PLMN search.

The measurement configuration received at the UE 602 (e.g., via the RRC connection reconfiguration message) may include a condition for triggering the specified type of handover event. The condition for triggering the specified type of handover event may be a handover threshold, where the specified type of handover event is triggered when the handover threshold is exceeded. Thus, the handover threshold may be received from the NR base station 604 via the measurement configuration. If the condition for triggering the specified type of handover event is satisfied, the UE 602 may send a measurement report associated with the specified handover event to the NR base station 604, to trigger the handover event from the NR base station 604 to the neighbor base station 606.

In an aspect, the condition for triggering a specified handover event may be that a signal strength measured for a neighboring inter-RAT cell becomes better than the handover threshold. For example, the specified handover event may be Event B1 and the handover threshold may be a B1 threshold associated with Event B1. If the NR base station 604 receives the measurement report associated with the specified handover event, then the NR base station 604 may determine to handover the UE to the neighbor base station 606. For example, when the NR base station 604 is heavily loaded (e.g., due to many subscribers) and there is no other NR base station nearby, and a signal strength associated with the neighbor base station 606 is good (e.g., greater than the handover threshold) , the NR base station 604 may trigger the handover event to hand over to the neighbor base station 606. Hence, this type of handover may achieve load balancing of the NR base station 604, especially when there is no other NR base station nearby.

If the UE 602 has the information about the neighbor base station 606 from the past handover event, as a part of the process for providing a voice service via the neighbor base station 606, the UE 602 may perform the following features to check that the neighbor base station 606 is a suitable base station for the UE 602. In particular, the UE 602 may measure a signal strength (e.g., an RSRP value or an RSRQ value) associated with the neighbor base station 606. Subsequently, the UE 602 may determine whether the signal strength measured on the neighbor base station 606is greater than a signal strength threshold R1. The signal strength threshold R1 may be determined by the UE and may be set to be lower or equal to the handover threshold associated with triggering a handover event (e.g., B1 threshold associated with the LTE event B1) and received from the NR base station 604, where the handover event is triggered when the signal strength associated with the neighbor base station 606exceeds the handover threshold. In an example where the handover event is Event B1, the B1 threshold is generally considered very high, and thus may not represent a minimum signal strength for a successful voice call via the neighbor base station 606. For example, even if a signal strength associated with the neighbor base station 606 is 2-6 dB lower than the B1 threshold, a successful voice call may be made via the neighbor base station 606, which may be because a voice call generally takes significantly less data than other types of services such as video streaming, etc. Thus, in an aspect, by defining the signal strength threshold R1 that represents a sufficient signal strength for the voice call and is lower than the handover threshold, a handover to the neighbor base station 606may be performed even when the signal strength does not exceed the B1 threshold. To make sure UE can dial a voice call on this LTE cell with high successful call setup rate. In one example, if B1 threshold is -100dBm, then the signal strength threshold R1 may be -105dBm.

If the signal strength associated with the neighbor base station 606 does not exceed the signal strength threshold R1, then the neighbor base station 606 may not be a desirable base station for providing a voice service for the UE 602. Thus, if the UE 602 determines that the signal strength associated with the neighbor base station 606 does not exceed the signal strength threshold R1, then the UE 602 may perform a new PLMN search to search for an optimal cell with a sufficient signal strength to successfully provide a voice service, and may not perform a hand over to the neighbor base station 606.

If the UE 602 determines that the signal strength associated with the neighbor base station 606 exceeds the signal strength threshold R1, the UE may proceed to attempt to provide a voice service via the neighbor base station 606. In an aspect, as a part of the process for providing a voice service via the neighbor base station 606, the UE 602 may determine if a reliable radio link is present between the UE 602 and the NR base station 604. If a reliable radio link with the NR base station 604 is available, performing the EPS fallback to connect to the neighbor base station 606 may be less time consuming and thus may be more desirable than performing a cell search procedure to establish a new radio link to the neighbor base station 606. Therefore, if the UE 602 determines that the failure to establish the voice call via the NR base station 604 is not due to a radio link failure with the NR base station 604, then a radio link with the NR base station 604 may be reliable and thus the EPS fallback to the neighbor base station 606may be performed via the radio link to the NR base station 604. In an aspect, the UE 602 may determine that the failure to establish the voice call via the NR base station 604 is not due to the radio link failure if a block error rate (BLER) associated with the NR base station 604 does not exceed an error threshold (e.g., 10%) . In an aspect, the UE 602 may determine that there is a radio link failure with the NR base station 604 if UE 602 sends a transmission with 1 RRC PDU to the NR base station 604 and receives no response.

To trigger the EPS fallback, the UE 602 may transmit the handover event report to the NR base station 604 in response to a measurement configuration received (e.g., via an RRC connection reconfiguration message) from the NR base station 605. When the event report is received, the NR base station 604 triggers a handover from the NR base station 604 to the neighbor base station 606, such that the UE 602, the NR base station 604, the neighbor base station 606, and the first IMS network 608 may perform an EPS fallback from the NR base station 604 to the neighbor base station 606. In an aspect, a fallback time limit T1 may be implemented to ensure that an unsuccessful attempt to perform the EPS fallback is addressed. In such an aspect, if the EPS fallback is successfully performed within the fallback time limit T1, the UE 602 may start performing a voice service (e.g., voice service with the recipient UE 612) via the neighbor base station 606. If the EPS fallback is not successfully performed within the PSFB time limit T1, the UE 602 may stop performing the EPS fallback to the neighbor base station 606 and instead perform a cell search procedure to establish a new radio link to the neighbor base station 606, as described in more detail below.

If the UE determines that the failure to establish the voice call via the NR base station 604 is due to a radio link failure with the NR base station 604or that the EPS fallback was not successfully performed within the fallback time limit T1, then the EPS fallback may not be successfully performed and thus the UE 602 may perform a cell search procedure to establish a new radio link to the neighbor base station 606. In particular, the UE 602 may first search and camp on a neighbor cell of the neighbor base station 606. After camping on the neighbor cell, the UE 602 may initiate an attach process with the neighbor base station 606by transmitting an attach request to the neighbor base station 606. After the attach request is accepted by the neighbor base station 606 and an attach response sent from the neighbor base station 606 is received, a new radio link is established with the neighbor base station 606 and thus the UE 602 may perform a voice service (e.g., voice service with the recipient UE 612) via the neighbor base station 606 using the new radio link.

Therefore, according to an aspect of the disclosure, the UE 602 may re-dial the voice call quickly via the neighbor base station 606 after the voice call via the NR base station 604 has failed to be established within a defined time limit, instead of performing a new PLMN search, e.g., using the information on the neighbor base station 606 available from the past handover event. If the UE 602 has the information on the neighbor base station 606 available from the past handover event, this information may indicate that the neighbor base station 606 has a neighbor relationship with the NR base station 604 and thus a handover to the neighbor base station 606 has a high success rate. On the contrary, if a new PLMN search is performed, there is a chance that the UE 602 may find a base station that is different from the neighbor base station 606 and provides a worse voice service than the neighbor base station 606.

Referring to FIG. 7, by way of example and without limitation, an example diagram 700 illustrating a process to establish a call via a wireless network, according to an aspect of the disclosure, is provided. The example diagram 700 of FIG. 7 involves communication among a UE 702, an NR network 704 provided by an NR base station, an IMS network 706, and a neighbor network 708 provided by a neighbor base station of a different RAT from that of the NR network 704. In an aspect, the neighbor network 708 may be an LTE network provided by an LTE base station.

At 712, the UE 702 successfully performs an NR registration with the NR base station of the NR network 704 to camp on the NR network 704. At 714, the UE 702 successfully performs an IMS registration with the IMS network 706 via the NR network 705. Subsequently, at 716, the UE 702 initiates a voice call with a second UE (not shown) via the NR network 704 by sending a SIP invite to the NR network 704. When the NR network 704 receives the SIP invite from the UE 702, at 718, the NR network 704 forwards the SIP invite to the IMS network 706. When the IMS network 706 receives the SIP invite, the IMS network 706 forwards the SIP invite to the second UE via a network where the second UE is camped.

In this example diagram 700, at 720, the UE 702 determines that the voice call via the NR network 704 has failed to be established within a defined time limit. For example, the UE 702 may determine that the voice call via the NR network 704 has failed if a response to the SIP invite has not been received within the defined time limit (e.g., T-call timer value) . As discussed above, the response to the SIP invite may include a 100 Trying response or a 180 Ringing response.

When the UE 702 determines that the voice call via the NR network 704 has failed, the UE 702 may determine whether the UE 702 has information about the neighbor network 708 from a past handover event between the NR network 704 and the neighbor network 708. If the UE 702 does not have information about the neighbor network 708 from the past handover event, then the UE 702 may perform a new PLMN search to search for a new network for a voice service. If the UE 702 has information about the neighbor network 708 from the past handover event, the UE 702 may proceed to perform a process for providing a voice service via the neighbor network 708, as described below.

If the UE 702 has information about the neighbor network 708 from the past handover event, at 722, the UE 702 measures a signal strength of the neighbor network 708. In an aspect, the signal strength of the neighbor network 708 may be a signal power such as an RSRP value or a signal quality such as an RSRQ value associated with the neighbor network 708. If the signal strength of the neighbor network 708 does not exceed a signal strength threshold R1, then the UE 702 may perform a perform a new PLMN search to search for a new network for a voice service.

If the signal strength of the neighbor network 708 exceeds the signal strength threshold R1, the UE 702 may perform an EPS fallback to the neighbor network 708 via an existing radio link and/or establish a new radio link to the neighbor network 708 to provide a voice service via the neighbor network 708. At 724, the UE 702 checks whether the voice call via the NR network 704 has failed due to a radio link failure associated with a radio link to the NR network 704. If the voice call via the NR network 704 has failed due to the radio link failure with the NR network 704, then there may be no radio link that the UE 702 may utilize to perform an EPS fallback, and thus establishes a new radio link to the neighbor network 708 without performing an EPS fallback, as described in more detail with reference to FIG. 9.

If no radio link failure the voice call via the NR network 704 caused the voice call via the NR network 704 to fail, then the UE 702 may proceed to attempt to perform an EPS fallback to the neighbor network 708. In particular, at 726, the UE 702 may receive an RRC connection reconfiguration message that specifies an inter-RAT handover event and a condition (e.g., handover threshold) for triggering the inter-RAT handover event. When the UE 702 determines that the condition for triggering the inter-RAT handover event is satisfied, the UE 702 sends a measurement report 726 associated with the inter-RAT handover event to the NR network 704, which triggers an inter-RAT handover from the NR network 704 to the neighbor network 708.

When the NR network 704 receives the measurement report 728, the NR network 704 triggers an EPS fallback to the neighbor network 708 and performs the EPS fallback by communicating with the UE 702, the neighbor network 708, and/or the IMS network 706. When the EPS fallback to the neighbor network 708 is successfully performed within a fallback time limit, at 732, the UE 702 may initiate the voice call via the neighbor network 708 on the RAT (e.g., LTE) of the neighbor network 708. For example, at 732, the UE 702 may initiate the voice call by transmitting a SIP invite to the neighbor network 708. Subsequently, at 734, the neighbor network 708 forwards the SIP invite to the IMS network 706, which then forwards the SIP invite to a recipient UE. On the other hand, if the EPS fallback to the neighbor network 708 is not successfully performed within the fallback time limit, the UE 702 may perform a cell search procedure to establish a new radio link with the neighbor network 708, as described with reference to FIG. 8.

Referring to FIG. 8, by way of example and without limitation, an example diagram 700 illustrating a process to establish a call via a wireless network is provided, according to an aspect of the disclosure. The example diagram 800 of FIG. 8 involves communication among a UE 802, an NR network 804 provided by an NR base station, an IMS network 806, and a neighbor network 808 provided by a neighbor base station of a different RAT from that of the NR network 804. In an aspect, the neighbor network 808 may be an LTE network provided by an LTE base station. In an aspect, the UE 802, the NR network 804, the IMS network 806, and the neighbor network 808 may be similar to the UE 702, the NR network 704, the IMS network 706, and the neighbor network 708 of FIG. 7, respectively. Further, in the example diagram 800 of FIG. 8, features performed at or associated with 812-828 are similar to the features performed at or associated with 712-728 in the example diagram 700 of FIG. 7. Hence, detailed descriptions related to the features performed at or associated with 812-828 are omitted for brevity.

After the voice call via the NR network 804 fails and the UE 802 determines that information on the neighbor network 808 is available from the past handover event, the signal strength associated with the neighbor network 808 is greater than the signal strength threshold, and no radio link failure with the NR network 804 caused the voice call via the NR network 804 to fail, then the UE 802 may proceed to attempt to perform an EPS fallback to the neighbor network 808. At 830, the UE 802 performs the EPS fallback to the neighbor network 808, but the EPS fallback to the neighbor network 808 is not successfully performed within the fallback time limit. Hence, at 832, the UE 802 may perform a cell search procedure to establish a new radio link with the neighbor network 808 and camp on the neighbor network 808. To establish a new radio link with the neighbor network 808, at 834, the UE 802 transmits an attach request to the neighbor network 808. In response, at 836, the neighbor network 808 responds by sending an attach accept message to the UE 802, and a new radio link between the UE 802 and the neighbor network 808 is established.

Via the new radio link, at 838, the UE 802 may initiate the voice call via the neighbor network 808 on the RAT (e.g., LTE) of the neighbor network 808. For example, at 838, the UE 802 may initiate the voice call by transmitting a SIP invite to the neighbor network 808. Subsequently, at 834, the neighbor network 808 forwards the SIP invite to the IMS network 806, which then forwards the SIP invite to a recipient UE.

Referring to FIG. 9, by way of example and without limitation, an example diagram 700 illustrating a process to establish a call via a wireless network is provided, according to an aspect of the disclosure. The example diagram 900 of FIG. 9 involves communication among a UE 902, an NR network 904 provided by an NR base station, an IMS network 906, and a neighbor network 908 provided by a neighbor base station of a different RAT from that of the NR network 904. In an aspect, the neighbor network 908 may be an LTE network provided by an LTE base station. In an aspect, the UE 902, the NR network 904, the IMS network 906, and the neighbor network 908 may be similar to the UE 702, the NR network 704, the IMS network 706, and the neighbor network 708 of FIG. 7, respectively. Further, in the example diagram 900 of FIG. 9, features performed at or associated with 912-922 are similar to the features performed at or associated with 712-7228 in the example diagram 700 of FIG. 7. Hence, detailed descriptions related to the features performed at or associated with 812-822 are omitted for brevity.

After the voice call via the NR network 904 fails and the UE 902 determines that information on the neighbor network 908 is available from the past handover event and the signal strength associated with the neighbor network 908 is greater than the signal strength threshold, at 924, the UE 902 checks whether the voice call via the NR network 804 has failed due to a radio link failure associated with a radio link to the NR network 804. In the example diagram 900, the UE 902 determines that the voice call via the NR network 804 has failed due to a radio link failure with the NR network 804. The voice call failure due to the radio link failure with the NR network 904 may imply that there may be no radio link with the NR network 904 that the UE 902 may utilize to perform an EPS fallback. Thus, the UE 902 proceeds to establish a new radio link to the neighbor network 908 without performing an EPS fallback. To establish a new radio link with the neighbor network 908, at 926, the UE 902 transmits an attach request to the neighbor network 908. In response, at 928, the neighbor network 908 responds by sending an attach accept message to the UE 902, and a new radio link between the UE 902 and the neighbor network 908 is established.

Via the new radio link, at 930, the UE 902 may initiate the voice call via the neighbor network 908 on the RAT (e.g., LTE) of the neighbor network 908. For example, at 932, the UE 902 may initiate the voice call by transmitting a SIP invite to the neighbor network 908. Subsequently, at 934, the neighbor network 908 forwards the SIP invite to the IMS network 906, which then forwards the SIP invite to a recipient UE.

FIG. 10 is a block diagram illustrating an example of a hardware implementation for a UE 1000 employing a processing system 1014. For example, the UE 1000 may be a UE as illustrated in any one or more of FIGs. 1, 2, 3, 5, 6, 7, 8, and/or 9.

The UE 1000 may be implemented with a processing system 1014 that includes one or more processors 1004. Examples of processors 1004 include microprocessors, microcontrollers, digital signal processors (DSPs) , field programmable gate arrays (FPGAs) , programmable logic devices (PLDs) , state machines, gated logic, discrete hardware circuits, and other suitable hardware configured to perform the various functionality described throughout this disclosure. In various examples, the UE 1000 may be configured to perform any one or more of the functions described herein. That is, the processor 1004, as utilized in the UE 1000, may be used to implement any one or more of the processes and procedures described below and illustrated in FIG. 11.

In this example, the processing system 1014 may be implemented with a bus architecture, represented generally by the bus 1002. The bus 1002 may include any number of interconnecting buses and bridges depending on the specific application of the processing system 1014 and the overall design constraints. The bus 1002 communicatively couples together various circuits including one or more processors (represented generally by the processor 1004) , a memory 1005, and computer-readable media (represented generally by the computer-readable medium 1006) . The bus 1002 may also link various other circuits such as timing sources, peripherals, voltage regulators, and power management circuits, which are well known in the art, and therefore, will not be described any further. A bus interface 1008 provides an interface between the bus 1002 and a transceiver 1010. The transceiver 1010 provides a communication interface or means for communicating with various other apparatus over a transmission medium. Depending upon the nature of the apparatus, a user interface 1012 (e.g., keypad, display, speaker, microphone, joystick) may also be provided. Of course, such a user interface 1012 is optional, and may be omitted in some examples, such as a base station.

In some aspects of the disclosure, the processor 1004 may include a voice call initiation circuit 1040 configured for various functions, including, for example, determining that a voice call by the UE via an NR base station operating in an NR network has failed to be established within a defined time limit. An information management circuit 1042 may serve to determine whether information on a neighbor base station is available at the UE from a past handover event specified between the NR base station and the neighbor base station in response to determining that the voice call has failed to be established via the NR base station, the neighbor base station utilizing a RAT different from a RAT of the NR base station and operating in a neighbor network different from the NR network. A voice service management circuit 1044 may serve to perform a process for providing a voice service via the neighbor base station in response to determining that the information on the neighbor base station is available.

The processor 1004 is responsible for managing the bus 1002 and general processing, including the execution of software stored on the computer-readable medium 1006. The software, when executed by the processor 1004, causes the processing system 1014 to perform the various functions described below for any particular apparatus. The computer-readable medium 1006 and the memory 1005 may also be used for storing data that is manipulated by the processor 1004 when executing software.

One or more processors 1004 in the processing system may execute software. Software shall be construed broadly to mean instructions, instruction sets, code, code segments, program code, programs, subprograms, software modules, applications, software applications, software packages, routines, subroutines, objects, executables, threads of execution, procedures, functions, etc., whether referred to as software, firmware, middleware, microcode, hardware description language, or otherwise. The software may reside on a computer-readable medium 1006. The computer-readable medium 1006 may be a non-transitory computer-readable medium. A non-transitory computer-readable medium includes, by way of example, a magnetic storage device (e.g., hard disk, floppy disk, magnetic strip) , an optical disk (e.g., a compact disc (CD) or a digital versatile disc (DVD) ) , a smart card, a flash memory device (e.g., a card, a stick, or a key drive) , a random access memory (RAM) , a read only memory (ROM) , a programmable ROM (PROM) , an erasable PROM (EPROM) , an electrically erasable PROM (EEPROM) , a register, a removable disk, and any other suitable medium for storing software and/or instructions that may be accessed and read by a computer. The computer-readable medium 1006 may reside in the processing system 1014, external to the processing system 1014, or distributed across multiple entities including the processing system 1014. The computer-readable medium 1006 may be embodied in a computer program product. By way of example, a computer program product may include a computer-readable medium in packaging materials. Those skilled in the art will recognize how best to implement the described functionality presented throughout this disclosure depending on the particular application and the overall design constraints imposed on the overall system.

In one or more examples, the computer-readable medium 1006 may include voice call initiation software or instructions 1052 configured for various functions, including, for example, determining that a voice call by the UE via an NR base station operating in an NR network has failed to be established within a defined time limit. Information management software or instructions 1052 may serve to determine whether information on a neighbor base station is available at the UE from a past handover event specified between the NR base station and the neighbor base station in response to determining that the voice call has failed to be established via the NR base station, the neighbor base station utilizing a RAT different from a RAT of the NR base station and operating in a neighbor network different from the NR network. Voice service management software or instructions 1054 may serve to perform a process for providing a voice service via the neighbor base station in response to determining that the information on the neighbor base station is available. Of course, in the above examples, the circuitry included in the processor 1004 is merely provided as an example, and other means for carrying out the described functions may be included within various aspects of the present disclosure, including but not limited to the instructions stored in the computer-readable medium 1006, or any other suitable apparatus or means described in any one of the FIGs. 1, 2, 3, 5, 6, 7, 8, and/or 9 and utilizing, for example, the processes and/or algorithms described herein.

FIG. 11 is a flow chart illustrating an exemplary process 1100 for wireless communication by a UE in accordance with some aspects of the present disclosure. As described below, some or all illustrated features may be omitted in a particular implementation within the scope of the present disclosure, and some illustrated features may not be required for implementation of all embodiments. In some examples, the process 1100 may be carried out by the UE 1000 illustrated in FIG. 10. In some examples, the process 1100 may be carried out by any suitable apparatus or means for carrying out the functions or algorithm described below.

At block 1102, the process 1100 includes determining that a voice call by the UE via an NR base station operating in an NR network has failed to be established within a defined time limit. In an aspect, determining that the voice call via the NR base station has not been successfully performed within the defined time limit may include determining that a response to a SIP invite message has not been successfully received within the defined time limit.

At block 1104, the process 1100 includes determining whether information on a neighbor base station is available at the UE from a past handover event specified between the NR base station and the neighbor base station in response to determining that the voice call has failed to be established via the NR base station, the neighbor base station utilizing a RAT different from a RAT of the NR base station and operating in a neighbor network different from the NR network. In an aspect, the past handover event specified between the NR base station and the neighbor base station may be based on at least one of a present RRC connection to the NR base station or a previous RRC connection to the NR base station whose cell identifier has not changed since the previous RRC connection.

If the information on the neighbor base station is available, at 1106, the process 1100 includes performing a process for providing a voice service via the neighbor base station. On the other hand, if the information on the neighbor base station is not available, at 1108, the process 1100 includes performing a new PLMN search to search for a new network for a voice service.

In an aspect, performing the process for providing a voice service may include measuring a signal strength of a signal associated with the neighbor base station in response to determining that the information on the neighbor base station is available, and determining whether the signal strength measured is greater than a signal strength threshold. In such an aspect, performing the process for providing a voice service may further include performing a handover process from the NR base station to the neighbor base station if the signal strength measured is greater than the signal strength threshold. On the other hand, if the signal strength measured is not greater than the signal strength threshold, the process 1100 includes performing a new public land mobile network (PLMN) search to search for a new network for a voice service. In an aspect, the signal strength threshold may be less than or equal to a handover threshold associated with triggering a handover event and may be received from the NR base station.

In an aspect, the handover process from the NR base station to the neighbor base station may be performed by determining whether a radio link failure with the NR base station has caused the voice call via the NR base station to fail. In such an aspect, the handover process from the NR base station to the neighbor base station may be performed further by performing a packet system fall back (e.g., EPS fallback) process to the neighbor base station with a radio link to the neighbor base station if no radio link failure with the NR base station has caused the voice call via the NR base station to fail. On the other hand, if the radio link failure with the NR base station has caused the voice call via the NR base station to fail, the handover process from the NR base station to the neighbor base station may be performed further by performing a cell search procedure to establish a new radio link to the neighbor base station.

In an aspect, performing the packet system fall back process to the neighbor base station may include transmitting a handover report to the NR base station, performing a packet system fall back from the NR base station to the neighbor base station in response to the handover report, and determining whether the packet system fall back has been successfully performed within a fall back time limit. In such an aspect, performing the packet system fall back process to the neighbor base station may further include placing a voice call via the neighbor base station if the packet system fall back has been successfully performed within the fall back time limit. On the other hand, if the packet system fall back has not been successfully performed within the fall back time limit, performing the packet system fall back process to the neighbor base station may further include performing a cell search procedure to establish a new radio link to the neighbor base station.

Several aspects of a wireless communication network have been presented with reference to an exemplary implementation. As those skilled in the art will readily appreciate, various aspects described throughout this disclosure may be extended to other telecommunication systems, network architectures and communication standards.

By way of example, various aspects may be implemented within other systems defined by 3GPP, such as Long-Term Evolution (LTE) , the Evolved Packet System (EPS) , the Universal Mobile Telecommunication System (UMTS) , and/or the Global System for Mobile (GSM) . Various aspects may also be extended to systems defined by the 3rd Generation Partnership Project 2 (3GPP2) , such as CDMA2000 and/or Evolution-Data Optimized (EV-DO) . Other examples may be implemented within systems employing IEEE 802.11 (Wi-Fi) , IEEE 802.16 (WiMAX) , IEEE 802.20, Ultra-Wideband (UWB) , Bluetooth, and/or other suitable systems. The actual telecommunication standard, network architecture, and/or communication standard employed will depend on the specific application and the overall design constraints imposed on the system.

Within the present disclosure, the word “exemplary” is used to mean “serving as an example, instance, or illustration. ” Any implementation or aspect described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other aspects of the disclosure. Likewise, the term “aspects” does not require that all aspects of the disclosure include the discussed feature, advantage or mode of operation. The term “coupled” is used herein to refer to the direct or indirect coupling between two objects. For example, if object A physically touches object B, and object B touches object C, then objects A and C may still be considered coupled to one another-even if they do not directly physically touch each other. For instance, a first object may be coupled to a second object even though the first object is never directly physically in contact with the second object. The terms “circuit” and “circuitry” are used broadly, and intended to include both hardware implementations of electrical devices and conductors that, when connected and configured, enable the performance of the functions described in the present disclosure, without limitation as to the type of electronic circuits, as well as software implementations of information and instructions that, when executed by a processor, enable the performance of the functions described in the present disclosure.

One or more of the components, steps, features and/or functions illustrated in FIGs. 1–11 may be rearranged and/or combined into a single component, step, feature or function or embodied in several components, steps, or functions. Additional elements, components, steps, and/or functions may also be added without departing from novel features disclosed herein. The apparatus, devices, and/or components illustrated in FIGs. 1–11 may be configured to perform one or more of the methods, features, or steps described herein. The novel algorithms described herein may also be efficiently implemented in software and/or embedded in hardware.

It is to be understood that the specific order or hierarchy of steps in the methods disclosed is an illustration of exemplary processes. Based upon design preferences, it is understood that the specific order or hierarchy of steps in the methods may be rearranged. The accompanying method claims present elements of the various steps in a sample order, and are not meant to be limited to the specific order or hierarchy presented unless specifically recited therein.

The previous description is provided to enable any person skilled in the art to practice the various aspects described herein. Various modifications to these aspects will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other aspects. Thus, the claims are not intended to be limited to the aspects shown herein, but are to be accorded the full scope consistent with the language of the claims, wherein reference to an element in the singular is not intended to mean “one and only one” unless specifically so stated, but rather “one or more. ” Unless specifically stated otherwise, the term “some” refers to one or more. A phrase referring to “at least one of” a list of items refers to any combination of those items, including single members. As an example, “at least one of: a, b, or c” is intended to cover: a; b; c; a and b; a and c; b and c; and a, b and c. All structural and functional equivalents to the elements of the various aspects described throughout this disclosure that are known or later come to be known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed by the claims. Moreover, nothing disclosed herein is intended to be dedicated to the public regardless of whether such disclosure is explicitly recited in the claims.

Claims

A method of wireless communication performed by a user equipment (UE) , comprising:

determining that a voice call by the UE via a new radio (NR) base station operating in an NR network has failed to be established within a defined time limit;

determining whether information on a neighbor base station is available at the UE from a past handover event specified between the NR base station and the neighbor base station in response to determining that the voice call has failed to be established via the NR base station, the neighbor base station utilizing a radio access technology (RAT) different from a RAT of the NR base station and operating in a neighbor network different from the NR network; and

performing a process for providing a voice service via the neighbor base station in response to determining that the information on the neighbor base station is available.
The method of claim 1, further comprising:

performing a new public land mobile network (PLMN) search to search for a new network for a voice service in response to determining that the information on the neighbor base station is not available.
The method of claim 1, performing the process for providing a voice service comprises:

measuring a signal strength of a signal associated with the neighbor base station in response to determining that the information on the neighbor base station is available;

determining whether the signal strength measured is greater than a signal strength threshold; and

performing a handover process from the NR base station to the neighbor base station in response to determining that the signal strength measured is greater than the signal strength threshold.
The method of claim 3, further comprising:

performing a new public land mobile network (PLMN) search to search for a new network for a voice service in response to determining that the signal strength measured is not greater than the signal strength threshold.
The method of claim 3, wherein the signal strength threshold is less than or equal to a handover threshold associated with triggering a handover event and received from the NR base station.
The method of claim 3, wherein the handover process from the NR base station to the neighbor base station is performed by:

determining whether a radio link failure with the NR base station has caused the voice call via the NR base station to fail; and

performing a packet system fall back process to the neighbor base station with a radio link to the neighbor base station in response to determining that no radio link failure with the NR base station has caused the voice call via the NR base station to fail.
The method of claim 6, wherein the handover process from the NR base station to the neighbor base station is further performed by:

performing a cell search procedure to establish a new radio link to the neighbor base station in response to determining that the radio link failure with the NR base station has caused the voice call via the NR base station to fail.
The method of claim 6, wherein performing the packet system fall back process to the neighbor base station further comprises:

transmitting a handover report to the NR base station;

performing a packet system fall back from the NR base station to the neighbor base station in response to the handover report;

determining whether the packet system fall back has been successfully performed within a fall back time limit; and

placing a voice call via the neighbor base station in response to determining that the packet system fall back has been successfully performed within the fall back time limit.
The method of claim 8, wherein the handover process from the NR base station to the neighbor base station is further performed by:

performing a cell search procedure to establish a new radio link to the neighbor base station in response to determining that the packet system fall back has not been successfully performed within the fall back time limit.
The method of claim 1, wherein determining that the voice call via the NR base station has not been successfully performed within the defined time limit comprises determining that a response to a session initiation protocol (SIP) invite message has not been successfully received within the defined time limit.
The method of claim 1, wherein the past handover event specified between the NR base station and the neighbor base station is based on at least one of a present radio resource control (RRC) connection to the NR base station or a previous RRC connection to the NR base station whose cell identifier has not changed since the previous RRC connection.
A user equipment (UE) for wireless communication, comprising:

at least one processor;

a transceiver communicatively coupled to the at least one processor; and

a memory communicatively coupled to the at least one processor,

wherein the at least one processor is configured to:

determine that a voice call by the UE via a new radio (NR) base station operating in an NR network has failed to be established within a defined time limit;

determine whether information on a neighbor base station is available at the UE from a past handover event specified between the NR base station and the neighbor base station in response to determining that the voice call has failed to be established via the NR base station, the neighbor base station utilizing a radio access technology (RAT) different from a RAT of the NR base station and operating in a neighbor network different from the NR network; and

perform a process for providing a voice service via the neighbor base station in response to determining that the information on the neighbor base station is available.
The UE of claim 12, wherein the at least one processor is further configured to:

perform a new public land mobile network (PLMN) search to search for a new network for a voice service in response to determining that the information on the neighbor base station is not available.
The UE of claim 12, wherein the at least one processor configured to perform the process for providing a voice service is configured to:

measuring a signal strength of a signal associated with the neighbor base station in response to determining that the information on the neighbor base station is available;

determining whether the signal strength measured is greater than a signal strength threshold; and

performing a handover process from the NR base station to the neighbor base station in response to determining that the signal strength measured is greater than the signal strength threshold.
The UE of claim 14, wherein the at least one processor is further configured to:

perform a new public land mobile network (PLMN) search to search for a new network for a voice service in response to determining that the signal strength measured is not greater than the signal strength threshold.
The UE of claim 14, wherein the signal strength threshold is less than or equal to a handover threshold associated with triggering a handover event and received from the NR base station.
The UE of claim 14, wherein the at least one processor configured to perform the handover process from the NR base station to the neighbor base station is configured to:

determine whether a radio link failure with the NR base station has caused the voice call via the NR base station to fail; and

perform a packet system fall back process to the neighbor base station with a radio link to the neighbor base station in response to determining that no radio link failure with the NR base station has caused the voice call via the NR base station to fail.
The UE of claim 17, wherein the at least one processor configured to perform the handover process from the NR base station to the neighbor base station is further configured to:

perform a cell search procedure to establish a new radio link to the neighbor base station in response to determining that the radio link failure with the NR base station has caused the voice call via the NR base station to fail.
The UE of claim 17, wherein the at least one processor configured to perform the packet system fall back process to the neighbor base station is further configured to:

transmit a handover report to the NR base station;

perform a packet system fall back from the NR base station to the neighbor base station in response to the handover report;

determine whether the packet system fall back has been successfully performed within a fall back time limit; and

place a voice call via the neighbor base station in response to determining that the packet system fall back has been successfully performed within the fall back time limit.
The UE of claim 19, wherein the at least one processor configured to perform the handover process from the NR base station to the neighbor base station is further configured to:

perform a cell search procedure to establish a new radio link to the neighbor base station in response to determining that the packet system fall back has not been successfully performed within the fall back time limit.
The UE of claim 12, wherein the at least one processor configured to determine that the voice call via the NR base station has not been successfully performed within the defined time limit is configured to determine that a response to a session initiation protocol (SIP) invite message has not been successfully received within the defined time limit.
The UE of claim 12, wherein the past handover event specified between the NR base station and the neighbor base station is based on at least one of a present radio resource control (RRC) connection to the NR base station or a previous RRC connection to the NR base station whose cell identifier has not changed since the previous RRC connection.
A non-transitory computer-readable medium storing computer-executable code for a user equipment (UE) , comprising code for causing a computer to:

determine that a voice call by the UE via a new radio (NR) base station operating in an NR network has failed to be established within a defined time limit;

determine whether information on a neighbor base station is available at the UE from a past handover event specified between the NR base station and the neighbor base station in response to determining that the voice call has failed to be established via the NR base station, the neighbor base station utilizing a radio access technology (RAT) different from a RAT of the NR base station and operating in a neighbor network different from the NR network; and

perform a process for providing a voice service via the neighbor base station in response to determining that the information on the neighbor base station is available.
The non-transitory computer-readable medium of claim 23, wherein the code further causes the computer to:

perform a new public land mobile network (PLMN) search to search for a new network for a voice service in response to determining that the information on the neighbor base station is not available.
The non-transitory computer-readable medium of claim 23, wherein the code for causing the computer to perform the process for providing a voice service causes the computer to:

measuring a signal strength of a signal associated with the neighbor base station in response to determining that the information on the neighbor base station is available;

determining whether the signal strength measured is greater than a signal strength threshold; and

performing a handover process from the NR base station to the neighbor base station in response to determining that the signal strength measured is greater than the signal strength threshold.
The non-transitory computer-readable medium of claim 25, wherein the code further causes the computer to:

perform a new public land mobile network (PLMN) search to search for a new network for a voice service in response to determining that the signal strength measured is not greater than the signal strength threshold.
The non-transitory computer-readable medium of claim 25, wherein the signal strength threshold is less than or equal to a handover threshold associated with triggering a handover event and received from the NR base station.
The non-transitory computer-readable medium of claim 25, wherein the code for causing the computer to perform the handover process from the NR base station to the neighbor base station causes the computer to:

determine whether a radio link failure with the NR base station has caused the voice call via the NR base station to fail; and

perform a packet system fall back process to the neighbor base station with a radio link to the neighbor base station in response to determining that no radio link failure with the NR base station has caused the voice call via the NR base station to fail.
The non-transitory computer-readable medium of claim 28, wherein the code for causing the computer to perform the handover process from the NR base station to the neighbor base station further causes the computer to:

perform a cell search procedure to establish a new radio link to the neighbor base station in response to determining that the radio link failure with the NR base station has caused the voice call via the NR base station to fail.
The non-transitory computer-readable medium of claim 28, wherein the code for causing the computer to perform the packet system fall back process to the neighbor base station further causes the computer to:

transmit a handover report to the NR base station;

perform a packet system fall back from the NR base station to the neighbor base station in response to the handover report;

determine whether the packet system fall back has been successfully performed within a fall back time limit; and

place a voice call via the neighbor base station in response to determining that the packet system fall back has been successfully performed within the fall back time limit.
The non-transitory computer-readable medium of claim 30, wherein the code for causing the computer to perform the handover process from the NR base station to the neighbor base station further causes the computer to:

perform a cell search procedure to establish a new radio link to the neighbor base station in response to determining that the packet system fall back has not been successfully performed within the fall back time limit.
The non-transitory computer-readable medium of claim 23, wherein the code for causing the computer to determine that the voice call via the NR base station has not been successfully performed within the defined time limit further causes the computer to determine that a response to a session initiation protocol (SIP) invite message has not been successfully received within the defined time limit.
The non-transitory computer-readable medium of claim 23, wherein the past handover event specified between the NR base station and the neighbor base station is based on at least one of a present radio resource control (RRC) connection to the NR base station or a previous RRC connection to the NR base station whose cell identifier has not changed since the previous RRC connection.
A user equipment (UE) for wireless communication, comprising:

means for determining that a voice call by the UE via a new radio (NR) base station operating in an NR network has failed to be established within a defined time limit;

means for determining whether information on a neighbor base station is available at the UE from a past handover event specified between the NR base station and the neighbor base station in response to determining that the voice call has failed to be established via the NR base station, the neighbor base station utilizing a radio access technology (RAT) different from a RAT of the NR base station and operating in a neighbor network different from the NR network; and

means for performing a process for providing a voice service via the neighbor base station in response to determining that the information on the neighbor base station is available.