PATENT DOCUMENT

Publication Number: US-9949311-B2
Application Number: US-201514604351-A
Country: US
Kind Code: B2

Title: Apparatus, system and method for VoLTE call continuity

Abstract:
A system, apparatus and method to maintain continuity of a Voice over Long Term Evolution (LTE) (VoLTE) call. The system includes a first user equipment (UE) configured to perform a (VoLTE) call and a second UE configured to perform the VoLTE call with the first UE. The UE&#39;s are configured to maintain continuity of the VoLTE call by determining when one of a first dedicated bearer linked to a first default bearer of the first UE or a second dedicated bearer linked to a second default bearer of the second UE is lost and transmitting a signal to an Internet Protocol (IP) Multimedia Subsystem (IMS) server that prevents release of the determined dedicated bearer, the signal further triggering a re-activation of the determined dedicated bearer.

Claims:
What is claimed is: 
     
       1. A system, comprising:
 a first user equipment (UE) configured to perform a Voice over Long Term Evolution (LIE) (VoLTE) call; 
 a second UE configured to perform the VoLTE call with the first UE; 
 wherein the first UE is configured to maintain continuity of the VoLTE call by:
 determining when one of a first dedicated bearer linked to a first default bearer of the first UE or a second dedicated bearer linked to a second default bearer of the second UE is lost; and 
 transmitting a signal to an Internet Protocol (IP) Multimedia Subsystem (IMS) server that prevents release of the determined dedicated bearer, the signal further triggering a re-activation of the determined dedicated bearer. 
 
 
     
     
       2. A user equipment, comprising:
 a transceiver configured to establish a connection with a Long Term Evolution (LTE) Network; and 
 a processor configured to execute a Voice over LTE (VoLTE) call application with a further user equipment, 
 wherein the transceiver and processor are configured to maintain continuity of the VoLTE call by:
 determining when one of a first dedicated bearer linked to a first default bearer of the user equipment and a second dedicated bearer linked to a second default bearer of the further user equipment is lost; and 
 transmitting a signal to an Internet Protocol (IP) Multimedia Subsystem (IMS) server that holds the determined dedicated bearer from release, the signal further triggering a re-activation of the determined dedicated bearer. 
 
 
     
     
       3. The user equipment of  claim 2 , wherein one of the first or second dedicated bearers are lost from a radio link failure (RLF). 
     
     
       4. The user equipment of  claim 2 , wherein, when the first dedicated bearer is determined to be lost, the user equipment is further configured to perform a Radio Resource Control (RRC) reestablishment procedure. 
     
     
       5. The user equipment of  claim 4 , wherein, when the RRC reestablishment procedure succeeds, the user equipment remains in a RRC connected state to transmit the signal to the IMS server. 
     
     
       6. The user equipment of  claim 4 , wherein, when the RRC reestablishment procedure fails, the user equipment enters a RRC idle state and transmits the signal to the IMS server, the signal triggering a RRC connection request. 
     
     
       7. The user equipment of  claim 4 , wherein the signal is a Session Initiation Protocol (SIP) message invite. 
     
     
       8. The user equipment of  claim 2 , wherein, when the second dedicated bearer is determined to be lost, the user equipment is configured to determine whether downlink (DL) Real-time Transport Protocol (RTP) packets have been received within a predetermined timer. 
     
     
       9. The user equipment of  claim 8 , wherein the predetermined timer is less than a RTP timer between six and ten seconds. 
     
     
       10. The user equipment of  claim 8 , wherein the signal is a SIP message re-invite including proper parameter information associated with the VoLTE call, the signal triggering a dedicated bearer reestablishment procedure for the second UE. 
     
     
       11. A method comprising:
 at a first user equipment (UE) configured to perform a Voice over Long Term Evolution (LTE) (VoLTE) call with a second UE:
 determining when one of a first dedicated bearer linked to a first default bearer of the first UE or a second dedicated bearer linked to a second default bearer of the second UE is lost; and 
 transmitting a signal to an Internet Protocol (IP) Multimedia Subsystem (IMS) server that delays release of the determined dedicated bearer, the signal further triggering a re-activation of the determined dedicated bearer, the re-activation of the determined dedicated bearer maintaining continuity of the VoLTE call. 
 
 
     
     
       12. The method of  claim 11 , wherein, when the first dedicated bearer is determined to be lost, the method further comprises:
 performing a Radio Resource Control (RRC) reestablishment procedure. 
 
     
     
       13. The method of  claim 12 , wherein, when the RRC reestablishment procedure succeeds, the first UE remains in a RRC connected state to transmit the signal to the IMS server. 
     
     
       14. The method of  claim 12 , wherein, when the RRC reestablishment procedure fails, the first UE enters a RRC idle state and transmits the signal to the IMS server, the signal triggering a RRC connection request. 
     
     
       15. The method of  claim 12 , wherein the signal is a Session Initiation Protocol (SIP) message invite. 
     
     
       16. The method of  claim 11 , wherein, when the second dedicated bearer is determined to be lost, the method further comprises:
 determining whether downlink (DL) Real-time Transport Protocol (RTP) packets have been received within a predetermined timer. 
 
     
     
       17. The method of  claim 16 , wherein the predetermined timer is less than a RTP timer between six and ten seconds. 
     
     
       18. The method of  claim 16 , wherein the signal is a SIP message re-invite including proper parameter information associated with the VoLTE call, the signal triggering a dedicated bearer reestablishment procedure for the second UE. 
     
     
       19. A method, comprising:
 at a first user equipment (UE) configured to perform a Voice over Long Term Evolution (LTE) (VoLTE) call with a second UE: 
 performing the VoLTE call with the second UE using a first dedicated bearer linked to a first default bearer associated with a first evolved Node B (eNB) of the LTE network; 
 establishing a connection to the LTE network via a second eNB, the second eNB assigning a second default bearer to the first UE; 
 determining that the second eNB has not configured a second dedicated bearer linked to the second default bearer for the first UE; 
 transmitting a signal to an Internet Protocol (IP) Multimedia Subsystem (IMS) server that triggers a setup procedure for the second eNB to establish the second dedicated bearer for the first UE, the establishment of the second dedicated bearer maintaining a VoLTE call continuity. 
 
     
     
       20. The method of  claim 19 , further comprising:
 roaming to the second eNB of the LTE network.

Description:
BACKGROUND INFORMATION 
     A first station may be configured to communicate wirelessly with a second station. Specifically, the first station may transmit data to and receive data from the second station through a wired or wireless communications network. The first and second stations may use the network to communicate using a variety of different applications. For example, the first station may be a mobile originating (MO) user equipment (UE) while the second station may be a mobile terminating (MT) UE for a voice call. The voice call may be performed in a variety of different manners. For example, when the MO or MT UE is connected to a legacy network, the voice call may be performed using circuit switching. In another example, when the MO or MT UE is connected to an Internet Protocol (IP) data transmission network, the voice call may be performed using Voice over IP (VoIP). More specifically, when the network is a Long Term Evolution (LTE) network, the VoIP call may be a Voice over LTE (VoLTE) call. 
     When the VoLTE call is performed, the MO and MT UEs may perform a setup procedure. Initially, when the MO and MT UEs connect to the LTE network, each UE may be associated with one or more default bearers that provide a best effort service in the exchange of data with the LTE network. Furthermore, when a particular application is being utilized such as the VoLTE call, each UE may be associated with a dedicated bearer that provides a dedicated tunnel for data to be transmitted with regard to the VoLTE call (e.g., the voice data). The dedicated bearer may provide a variety of functionalities such as improving throughput or guaranteeing a bit rate for the data to be transmitted. However, during the VoLTE call, the MO and/or the MT UE may experience a disruption such as a radio link failure (RLF) that causes the dedicated bearer to be lost. When the dedicated bearer is lost, the VoLTE call may be dropped. 
     SUMMARY 
     A first exemplary embodiment is directed to a system having a first user equipment (UE) configured to perform a Voice over Long Term Evolution (LTE) (VoLTE) call and a second UE configured to perform the VoLTE call with the first UE. The first UE is configured to maintain continuity of the VoLTE call by determining when one of a first dedicated bearer linked to a first default bearer of the first UE or a second dedicated bearer linked to a second default bearer of the second UE is lost and transmitting a signal to an Internet Protocol (IP) Multimedia Subsystem (IMS) server that prevents release of the determined dedicated bearer, the signal further triggering a re-activation of the determined dedicated bearer. 
     Another exemplary embodiment is directed to a user equipment having a transceiver configured to establish a connection with a Long Term Evolution (LTE) Network and a processor configured to execute a Voice over LTE (VoLTE) call application with a further user equipment. The transceiver and processor are configured to maintain continuity of the VoLTE call by determining when one of a first dedicated bearer linked to a first default bearer of the user equipment and a second dedicated bearer linked to a second default bearer of the further user equipment is lost and transmitting a signal to an Internet Protocol (IP) Multimedia Subsystem (IMS) server that holds the determined dedicated bearer from release, the signal further triggering a re-activation of the determined dedicated bearer. 
     A further exemplary embodiment is directed to a method performed by a first user equipment (UE) configured to perform a Voice over Long Term Evolution (LTE) (VoLTE) call with a second UE. The method includes determining when one of a first dedicated bearer linked to a first default bearer of the first UE or a second dedicated bearer linked to a second default bearer of the second UE is lost and transmitting a signal to an Internet Protocol (IP) Multimedia Subsystem (IMS) server that delays release of the determined dedicated bearer, the signal further triggering a re-activation of the determined dedicated bearer, the re-activation of the determined dedicated bearer maintaining continuity of the VoLTE call. 
     A further exemplary embodiment is directed to a method performed by a first user equipment (UE) configured to perform a Voice over Long Term Evolution (LTE) (VoLTE) call with a second UE. The method includes performing the VoLTE call with the second UE using a first dedicated bearer linked to a first default bearer associated with a first evolved Node B (eNB) of the LTE network, establishing a connection to the LTE network via the second eNB, the second eNB assigning a second default bearer to the first UE, determining that the second eNB has not configured a second dedicated bearer linked to the second default bearer for the first UE, and transmitting a signal to an Internet Protocol (IP) Multimedia Subsystem (IMS) server that triggers a setup procedure for the second eNB to establish the second dedicated bearer for the first UE, the establishment of the second dedicated bearer maintaining a VoLTE call continuity. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an exemplary network arrangement. 
         FIG. 2  shows an exemplary user equipment configured with dedicated bearer functionalities. 
         FIG. 3  shows a first exemplary signaling diagram for re-activating a dedicated bearer. 
         FIG. 4  shows a second exemplary signaling diagram for re-activating a dedicated bearer. 
         FIG. 5  shows an exemplary LTE network arrangement. 
         FIG. 6  shows an exemplary signaling diagram for establishing a further dedicated bearer. 
         FIG. 7  shows a first exemplary method for re-activating a dedicated bearer when experiencing a radio link failure. 
         FIG. 8  shows a second exemplary method for re-activating a dedicated bearer when experiencing a radio link failure. 
         FIG. 9  shows an exemplary method for transferring a VoLTE call upon experiencing a roam. 
     
    
    
     DETAILED DESCRIPTION 
     The exemplary embodiments may be further understood with reference to the following description and the related appended drawings, wherein like elements are provided with the same reference numerals. The exemplary embodiments are related to a system and method for maintaining a call continuity during a Voice over Long Term Evolution (LTE) (VoLTE) VoLTE call when experiencing a variety of conditions where a dedicated bearer may be lost. Specifically, one of the user equipment (UE) in the VoLTE call may be configured to perform a respective functionality based upon the different conditions to maintain the call continuity where the conditions may relate to the UE itself or to the other UE of the VoLTE call. 
       FIG. 1  shows an exemplary network arrangement  100 . The exemplary network arrangement  100  includes UEs  110 - 114 . In this example, it is assumed that a respective, different user is using each of the UEs  100 - 114 . For example, a first user may be utilizing the UE  110 , a second user may be utilizing the UE  112 , and a third user may be utilizing the UE  114 . Those skilled in the art will understand that the UEs  110 - 114  may be any type of electronic component that is configured to communicate via a network, e.g., mobile phones, tablet computers, desktop computers, smartphones, phablets, embedded devices, etc. It should also be understood that an actual network arrangement may include any number of UEs being used by any number of users and being associated with any number of these users where the user may be associated with one or more of the UEs. That is, the example of three (3) UEs  110 - 114  is only provided for illustrative purposes. However, as will be understood from the description herein, the exemplary embodiments may relate to when at least two UEs  110 - 114  are present in the network arrangement  100 . 
     Each of the UEs  110 - 114  may be configured to communicate with one or more networks. In this example, the networks with which the UEs  110 - 114  may communicate are a legacy radio access network (RAN)  120 , a LTE RAN (LTE-RAN)  122 , and a wireless local area network (WLAN)  124 . In this example, each of the networks  120 - 124  is a wireless network with which the UEs  110 - 114  may communicate wirelessly. However, it should be understood that the UEs  110 - 114  may also communicate with other types of networks using a wired connection. With regards to the exemplary embodiments, the UEs  110 - 114  may establish a connection with the LTE-RAN  122  to perform VoLTE calls with other UEs. For example, the UEs  110 - 114  may have a LTE chipset to communicate with the LTE-RAN  122 . Again, the use of three (3) networks is only exemplary and there may be any other number of networks with which the UEs  110 - 114  may communicate. 
     The legacy RAN  120  and the LTE-RAN  122  are portions of cellular networks that may be deployed by cellular providers (e.g., Verizon, AT&amp;T, Sprint, T-Mobile, etc.). These networks  120  and  122  may include, for example, base client stations (Node Bs, eNodeBs, HeNBs, etc.) that are configured to send and receive traffic from UEs that are equipped with the appropriate cellular chip set. Examples of the legacy RAN  120  may include those networks that are generally labeled as 2G and/or 3G networks and may include circuit switched voice calls and packet switched data operations. Those skilled in the art will understand that the cellular providers may also deploy other types of networks, including further evolutions of the cellular standards, within their cellular networks. The WLAN  124  may include any type of wireless local area network (WiFi, Hot Spot, IEEE 802.11x networks, etc.). Those skilled in the art will understand that there may be thousands, hundreds of thousands or more of different WLANs deployed in the United States alone. For example, the WLAN  124  may be the user&#39;s home network, the user&#39;s work network, a public network (e.g., at a city park, coffee shop, etc.). Generally, the WLAN  124  will include one or more access points that allow the UEs  110 - 114  to communicate with the WLAN  124 . However, as noted above, the exemplary embodiments relate to the UEs  110 - 114  utilizing the LTE-RAN  122  to perform VoLTE calls. 
     In addition to the networks  120 - 124 , the network arrangement  100  also includes a cellular core network  130  and the Internet  140 . The cellular core network  130 , the legacy RAN  120 , and the LTE-RAN  122  may be considered a cellular network that is associated with a particular cellular provider (e.g., Verizon, AT&amp;T, Sprint, T-Mobile, etc.). The cellular core network  130  may be considered to be the interconnected set of components that manages the operation and traffic of the cellular network. The interconnected components of the cellular core network  130  may include any number of components such as servers, switches, routers, etc. The cellular core network  130  also manages the traffic that flows between the cellular network and the Internet  140 . 
     The network arrangement  100  also includes an IP Multimedia Subsystem (IMS)  150 . The IMS  150  may be generally described as an architecture for delivering multimedia services to the UEs  110 - 114  using the IP protocol. The IMS  150  may include a variety of components to accomplish this task. For example, a typical IMS  150  includes a Home Subscriber Server (HSS) that stores subscription information for a user of the UEs  110 - 114 . This subscription information is used to provide the correct multimedia services to the user such as a VoLTE call. The IMS  150  may communicate with the cellular core network  130  and the Internet  140  to provide the multimedia services to the UEs  110 - 114 . The IMS  150  is shown in close proximity to the cellular core network  130  because the cellular provider typically implements the functionality of the IMS  150 . However, it is not necessary for this to be the case such as when the IMS  150  is provided by another party. 
     Thus, the network arrangement  100  allows the UEs  110 - 114  to perform functionalities generally associated with computers and cellular networks. For example, the UEs  110 - 114  may perform the VoLTE calls to other parties, may browse the Internet  140  for information, may stream multimedia data to the UEs  110 - 114 , etc. 
     The network arrangement  100  may also include a network services backbone  160  that is in communication either directly or indirectly with the Internet  140  and the cellular core network  130 . The network services backbone  160  may be generally described as a set of components (e.g., servers, network storage arrangements, etc.) that implement a suite of services that may be used to extend the functionalities of the UEs  110 - 114  in communication with the various networks. The network services backbone  160  may interact with the UEs  110 - 114  and/or the networks  120 ,  122 ,  124 ,  130 ,  140  to provide these extended functionalities. 
     The network services backbone  160  may be provided by any entity or a set of entities. In one example, the network services backbone  160  is provided by the supplier of one or more of the UEs  110 - 114 . In another example, the network services backbone  160  is provided by the cellular network provider. In still a further example, the network services backbone  160  is provided by a third party unrelated to the cellular network provider or the supplier of the UEs  110 - 114 . 
     The exemplary embodiments relate to the UEs  110 - 114  performing a VoLTE call. For example, the UE  110  may be a mobile originating (MO) UE that invites a further UE such as the UE  112 , which may be a mobile terminating (MT) UE. Initially, the UEs  110 - 114  establish a connection to the LTE-RAN  122 . Those skilled in the art will understand that any association procedure may be performed for the UEs  110 - 114  to connect to the LTE-RAN  122 . For example, as discussed above, the LTE-RAN  122  may be associated with a particular cellular provider where the UE  110 - 114  and/or the user thereof has a contract and credential information (e.g., stored on a SIM card). Upon detecting the presence of the LTE-RAN  122 , the UEs  110 - 114  may transmit the corresponding credential information to associate with the LTE-RAN  122 . More specifically, the UEs  110 - 114  may associate with a specific base station (e.g., an eNB of the LTE-RAN  122 ). 
     When the UEs  110 - 114  associate and establish a connection with the LTE-RAN  122 , one or more default bearers may be established for the UE  110 - 114 . Initially, a bearer may define how data is treated when transmitted through the network. That is, the bearer may be a set of network parameters that define data specific treatment by type or association with an application. Accordingly, the LTE-RAN  122  may treat a first type of data in a first way and treat a second type of data in a second way. For example, the first type of data may be prioritized so that the LTE-RAN  122  treats this data in a special manner. It should be noted that the bearer may also define the treatment based upon user identity or other identification parameters. 
     One type of bearer that is established when initially connecting to the LTE-RAN  122  is a default bearer. The LTE-RAN  122  via the associated eNB may assign the default bearer, which remains so long as the UE is connected to the LTE-RAN  122 . The default bearer may be a best effort service for the data transmitted through the LTE-RAN  122 . Thus, depending upon various network conditions currently being experienced by the LTE-RAN  122 , the data being transmitted through the default bearer may be given whatever resources are available. The default bearer may be associated with a particular IP address and a single UE may have further default bearers established. Each default bearer may be assigned a quality of service (QoS) class indicator (QCI) of 5 to 9 that relates to non-guaranteed bit rate (GBR) bearers. Applications that may utilize the default bearer may be those in which the delivery of data is less time sensitive. For example, signaling messages such as Session Initiation Protocol (SIP) may utilize the default bearer. Other examples include smartphone traffic including video, chat, email, browsing, etc. 
     A further type of bearer that may be established between the UE and the LTE-RAN  122  is a dedicated bearer. The dedicated bearer may provide a dedicated tunnel to specific traffic. One application that utilizes the dedicated tunnel is VoLTE voice data used when performing the VoLTE call. The dedicated bearer may be an additional bearer over the default bearer that is established at a time subsequent to the default bearer being established (e.g., upon performing the VoLTE call). Since only default bearers require separate IP addresses and the dedicated bearer is provided over an established default bearer, the dedicated bearer does not require a separate IP address. However, the dedicated bearer is linked to the default bearer established previously. Specifically, a value defined during setup of the dedicated bearer may be used to link the dedicated bearer to the default bearer. The dedicated bearer may utilize the same QCI as the default bearer but may also utilize a different QCI that relates to a GBR bearer. The dedicated bearer may use traffic flow templates (TFT) to provide the special treatment to specific services such as the VoLTE call. That is, the TFT may also define the rules of when the dedicated bearer is to be used based upon the application being executed. 
     When considering the operation of the default bearer and the dedicated bearer, the default bearer is established upon the UE connecting to the LTE-RAN  122 . Specifically, the LTE-RAN  122  assigns the default bearer to the UE. Subsequently, while still connected to the LTE-RAN  122 , the UE may execute the VoLTE call functionality. For example, the UE  110  may be the MO UE while the UE  112  may be the MT UE. As such, the MT UE may also be connected to the LTE-RAN  122  and have a default bearer assigned and established. When the VoLTE call functionality is performed, various signaling messages are transmitted between the UE  110  and the UE  112  via the LTE-RAN  122  and the IMS  150  through the respective default bearer. Once the VoLTE call has been established from successfully transmitting the different signaling messages, the UE  110  and the UE  112  may be connected to each other to perform the VoLTE call. Specifically, the dedicated bearer may be established for each of the UE  110  and the UE  112  for the VoLTE data to be transmitted. 
     When a connection to the LTE-RAN  122  with more particular emphasis on the eNB to which the UE is associated is maintained, the VoLTE call may be performed without any issues until its completion. However, those skilled in the art will understand that there may be any number of conditions that may be present while the VoLTE call is being performed. The conditions may relate to a variety of physical or environmental factors or may relate to procedural aspects of network management. 
     In a first example, the UE  110  may experience a connection failure such as when a radio link failure (RLF) occurs. When the RLF happens, the dedicated bearer may become lost. When the dedicated bearer is lost, the VoLTE call may be dropped. The UE  110  may attempt to re-establish a connection with the LTE-RAN  122  and/or the dedicated bearer. If the reestablishment fails, the UE  110  may enter a radio resource control (RRC) idle state in which the LTE-RAN  122  releases the dedicated bearer. With no dedicated bearer, the VoLTE call drops. Even though the UE  110  may be capable of establishing a RRC connection at a later time, the LTE-RAN  122  may not configure the UE  110  with the dedicated bearer. If the reestablishment succeeds, the UE  110  may remain in a RRC connected state. However, the providers of the LTE-RAN  122  may have configured the network to still release the dedicated bearer context on the network side. When released, the UE  110  waits without any ability to perform an action until the Real-Time Transport Protocol (RTP)/RTP Control Protocol (RTCP)) times out that results in the VoLTE call being dropped. 
     In a second example, the UE  112  (i.e., the other UE involved in the VoLTE call) may experience a connection failure such as the RLF. In a substantially similar situation, when the UE  112  has lost its dedicated bearer, the VoLTE call may still be dropped despite the UE  110  maintaining its established dedicated bearer. With no recourse, the UE  110  may again be forced to wait without performing an action until the RTP/RTCP times out on the UE  112  side that results in the VoLTE call being dropped. 
     In a third example, the UE  110  may remain connected to the LTE-RAN  122  but may re-associate with a different eNB. For example, the UE  110  may move and perform a roaming procedure. However, when roaming and changing the eNB to which the UE  110  is associated, a new default bearer is established. Furthermore, because any dedicated bearer that is established is linked to the default bearer, a new default bearer requires a new dedicated bearer as well. Thus, when performing the VoLTE call and performing a roam, the dedicated bearer being used may be lost. When lost, again, the VoLTE call may be dropped. 
     The exemplary embodiments incorporate these various conditions and provide a mechanism to maintain VoLTE call continuity. As will be described in further detail below, the UE  110  (acting as the MO UE) may be configured with a first mechanism when experiencing its own RLF, a second mechanism when determining a likelihood that the UE  112  (acting as the MT UE) is experiencing a RLF, and a third mechanism when the UE  110  (acting as the MO UE) is performing a roam procedure. In each of these scenarios, the respective mechanism may enable the UE side and the network side to be prepared to maintain the VoLTE call continuity, particularly through re-activating the dedicated bearer when lost. 
       FIG. 2  shows an exemplary UE  200  configured with dedicated bearer functionalities. Specifically, the UE  200  is configured to execute a plurality of applications that perform the respective functionalities when experiencing a potential loss of the dedicated bearer. For exemplary purposes, the UE  200  may represent the UE  110  that is one party to the VoLTE call. Those skilled in the art will understand that the UE  200  may also represent the other UEs  112 ,  114 . However, it should be noted that the other UEs  112 ,  114  may not necessarily be capable of performing the functionalities described below with regard to the UE  110 . 
     The UE  200  may represent any electronic device that is configured to perform wireless functionalities and may be representative of one or more of the UEs  110 - 114 . For example, the UE  200  may be a portable device such as a smartphone, a tablet, a phablet, a laptop, etc. In another example, the UE  200  may be a client stationary device such as a desktop terminal. The UE  200  may be configured to perform cellular and/or WiFi functionalities. The UE  200  may include a processor  205 , a memory arrangement  210 , a display device  215 , an input/output (I/O) device  220 , a transceiver  225 , and other components  230 . The other components  230  may include, for example, an audio input device, an audio output device, a battery, a data acquisition device, ports to electrically connect the UE  200  to other electronic devices, etc. 
     The processor  205  may be configured to execute a plurality of applications of the UE  200 . For example, the applications may include a web browser when connected to a communication network via the transceiver  225 . As such, when connected to the LTE-RAN  122 , the data for the web browser may utilize the default bearer. In another example, the processor  205  may execute a VoLTE call application  235  that enables the UE  200  to perform a VoLTE call functionality such as with the UE  112 . In yet another example, the processor  205  may execute a reestablishment application  240 . As will be described in further detail below, the reestablishment application  240  may provide a mechanism to transmit a preparation signal so that the dedicated bearer is re-activated despite the UE  110  experiencing a RLF. In a further example, the processor  205  may execute a bearer loss application  245 . As will be described in further detail below, the bearer loss application  245  may provide a mechanism for the UE  110  to determine whether the UE  112  is experiencing a RLF to transmit a signal that re-activates the dedicated bearer for the UE  112 . In a still further example, the processor  205  may execute a VoLTE handover application  250 . As will be described in further detail below, the VoLTE handover application  250  may provide a mechanism for the UE  110  to determine capabilities of the roamed access point and transmit a signal to transfer the VoLTE call with a further established dedicated bearer. 
     It should be noted that the above noted applications each being an application (e.g., a program) executed by the processor  205  is only exemplary. The functionality associated with the applications may also be represented as a separate incorporated component of the UE  200  or may be a modular component coupled to the UE  200 , e.g., an integrated circuit with or without firmware. In addition, in some UEs, the functionality described for the processor  205  is split among two processors, a baseband processor and an applications processor. The exemplary embodiments may be implemented in any of these or other configurations of a UE. 
     The memory  210  may be a hardware component configured to store data related to operations performed by the UE  200 . Specifically, the memory  210  may store data related to the various applications  235 - 250 . For example, the VoLTE call application  235  may utilize a phone book functionality that stores contact information for other users and UEs. The display device  215  may be a hardware component configured to show data to a user while the I/O device  220  may be a hardware component that enables the user to enter inputs. It should be noted that the display device  215  and the I/O device  220  may be separate components or integrated together such as a touchscreen. 
     The transceiver  225  may be a hardware component configured to transmit and/or receive data. That is, the transceiver  225  may enable communication with other electronic devices directly or indirectly through a network based upon an operating frequency of the network. The transceiver  225  may operate on a variety of different frequencies or channels (e.g., set of consecutive frequencies) that are related to the VoLTE call functionality. Thus, an antenna (not shown) coupled with the transceiver  225  may enable the transceiver  225  to operate on the LTE frequency band. 
       FIG. 3  shows a first exemplary signaling diagram  300  for re-activating a dedicated bearer. Specifically, the signaling diagram  300  relates to when the UE  110  executes the reestablishment application  240 . Accordingly, the signaling diagram  300  further relates to when the UE  110  experiences a RLF that results in the dedicated bearer being lost. It may be assumed that the other UE  112  does not experience any conditions that result in a loss of its dedicated bearer on its end such that the reestablishment application  240  is capable of maintaining the call continuity. 
     The signaling diagram  300  illustrates a generic procedure to first connect to the LTE-RAN  122  by both the UE  110  and the UE  112 . The signaling diagram  300  also assumes that the UE  110  and the UE  112  are located in different operating areas of the LTE-RAN  122  such that the UE  110  connects to the LTE-RAN  122  via an eNB  122 A while the UE  112  connects to the LTE-RAN  122  via an eNB  122 B. Therefore, the UE  110  may detect the presence of the LTE-RAN  122  and transmit a connection request  305  to the eNB  122 A. Upon performing an association procedure, the UE  110  may establish a connection  310  to the LTE-RAN  122  via the eNB  122 A. Furthermore, upon establishing the connection the LTE-RAN  122 , the eNB  122 A may assign a default bearer for the UE  110  and establish the default bearer. A substantially similar operation may be performed for the UE  112  with the eNB  122 B, (e.g., connection request  315  and connection establishment  320  with a default bearer). 
     With both the UE  110  and the UE  112  connected to the LTE-RAN  122 , the UE  110  may execute the VoLTE call application  235 . When initiating the VoLTE call, the VoLTE call application  235  of the UE  110  may be used where contact information for the client station  350  is used to generate an invite. Those skilled in the art will understand that the VoLTE call functionality is an operation supported by the IMS  150 . Therefore, the IMS  150  is also shown in the signaling diagram  300 . Accordingly, the invite  325  is transmitted from the UE  110  to the IMS  150  (e.g., via the eNB  122 A and the LTE-RAN  122 ) and the invite  330  is then transmitted from the IMS  150  to the UE  112 . It should be noted that the invite  325 ,  330  is transmitted from the UE  110  to the IMS  150  and ultimately to the UE  112  through various networks and network components. However, for illustrative purposes, only the end points of the transmission of the invite  325 ,  330  are shown in the signaling diagram  300 , particularly from the UE  110  to the IMS  150  and from the IMS  150  to the UE  112 . It should be noted that the UE  110  being the MO UE and the UE  112  being the MT UE is only exemplary and an opposite configuration may also be used. 
     In a substantially similar manner, the reply signal  335  may be transmitted from the UE  112  to the IMS  150  and the reply  340  is then transmitted from the IMS  150  to the UE  110 . Subsequently, the VoLTE call  345  may be established between the UE  110  and the UE  112 . Once the VoLTE call has been established, the eNB  122 A may assign and establish a dedicated bearer  350  for the UE  110  for use during the VoLTE call while the eNB  122 B may assign and establish a dedicated bearer  355  for the UE  112  for use during the VoLTE call. 
     As discussed above, the UE  110  may experience a loss of the dedicated bearer from a variety of conditions such as from a RLF  360 . The RLF  360  may be triggered due to any number of reasons including an indication from the radio link control (RLC) that a maximum number of retransmissions has been reached, upon expiry of a timer, a random access issue from the media access control (MAC), etc. The UE  110  may be configured to detect the RLF  360  and perform subsequent procedures. 
     According to the exemplary embodiments, one such procedure performed by the reestablishment application  240  is to generate a preparation signal  365  that is transmitted to the eNB  122 A. Therefore, upon detection of the RLF  360 , the reestablishment application  240  generates the preparation signal  365 . More specifically, the reestablishment application  240  may operate with the baseband layer of the UE  110  to notify the eNB  122 A that the RLF  360  occurred. When the eNB  122 A receives the preparation signal  365  from the UE  110 , the eNB  122 A may be notified that the RLF  360  occurred but also be notified of a desire of the UE  110  to keep or trigger the network to reestablish/reactivate the dedicated bearer. Thus, the preparation signal  365  may also serve to prepare the eNB  122 A for subsequent steps that are to be performed. 
     Further procedures that are performed upon detecting the RLF  360  may include performing RRC reestablishment procedures  370 . The RRC reestablishment procedures  370  are performed to reestablish signaling radio bearer operations and reactive security algorithms. Specifically, the RRC reestablishment procedure  370  entails performing a plurality of operations (e.g., starting/stopping/suspending timers, resetting the MAC, etc.) on the UE side and transmitting a RRC reestablishment message to the eNB  122 A. The RRC reestablishment message may include UE context information such as physical cell identification, a reason for the request, etc. The RRC reestablishment procedure  370  is only successful when the eNB  122 A has a valid UE context. Thus, when the UE  110  receives a response indicating a successful RRC reestablishment, the UE  110  may perform subsequent operations (e.g., stopping a timer, reestablishing PDCP, reestablishing RLC, etc.). 
     As discussed above, regardless of whether the RRC reestablishment procedure  370  is successful, the dedicated bearer  350  that is in use for the VoLTE call may still be lost either from a UE side or from a network side. Specifically, when the RRC reestablishment procedure  370  fails, the UE  110  enters the RRC idle state and loses the dedicated bearer  350 . When the RRC reestablishment procedure  370  succeeds, the network may release the dedicated bearer  350  upon the RLF  360  occurring. However, the reestablishment application  240  of the exemplary embodiments has already transmitted the preparation signal  365  so that the dedicated bearer  350  is not released by the LTE-RAN  122 . 
     The reestablishment application  240  may perform a common operation to re-activate the dedicated bearer under two circumstances having different states. When the RRC reestablishment procedure  370  fails, the UE  110  enters the RRC idle state. There may be a timeout (TO) period  375  (shown as dotted period in the signaling diagram  300 ) in which the dedicated bearer  350  becomes lost. Prior to this TO period  375  expiring, the reestablishment application  240  may transmit an invite message  380  to the IMS  150 . The invite message  380  may be a SIP message invite. Those skilled in the art will understand that the SIP message invite may trigger the RRC connection request. However, with the dedicated bearer  350  not being released, the UE  110  may have the dedicated bearer re-activated  385  prior to the TO period  375  for the VoLTE call to continue. When the RRC reestablishment procedure  370  succeeds, the UE  110  may remain in the RRC connected state. There may be a respective TO period  375  for this process as well. Prior to this TO period  375  expiring, the reestablishment application  240  may again transmit a SIP message invite  380  that triggers the RRC connection request to re-activate  385  the dedicated bearer and continue the VoLTE call. 
     Since the LTE-RAN  122  has not yet cleared or released the dedicated bearer context, the SIP message invite  380  transmitted to the IMS  150  by the reestablishment application  240  may lead the LTE-RAN  122  to keep the context further. Despite the LTE-RAN  122  clearing the context, the SIP message invite  380  triggers the LTE-RAN  122  to re-activate the dedicated bearer for the UE once again. Accordingly, the UE  110  may continue the VoLTE call with the UE  112  after the RLF has happened. 
       FIG. 4  shows a second exemplary signaling diagram  400  for re-activating a dedicated bearer. Specifically, the signaling diagram  400  relates to when the UE  110  executes the bearer loss application  245 . Accordingly, the signaling diagram  400  further relates to when the UE  112  experiences a RLF that results in the dedicated bearer being lost. It may be assumed that the UE  110  does not experience any conditions that result in a loss of its dedicated bearer on its end such that the bearer loss application  245  is capable of maintaining the call continuity. 
     Initially, the signaling diagram  400  also illustrates signals and messages  405 - 455  transmitted by the UEs  110  and  112  to establish a connection to the LTE-RAN  122  via the respective eNB  122 A and eNB  122 B, respectively, establish a respective default bearer, establish the VoLTE call, and establish a respective dedicated bearer for the VoLTE call. 
     As discussed above, the UE  112  may experience a loss  460  of the dedicated bearer  455  from a variety of conditions such as from a RLF. According to the exemplary embodiments, the bearer loss application  245  of the UE  110  may monitor when RTP packets are being received for the VoLTE call. Those skilled in the art will understand that RTP defines a standardized packet format for delivering voice data for the VoLTE call. Therefore, there may be a RTP timer  465  associated with the VoLTE call for both UEs  110  and  112  (in the signaling diagram  400  of  FIG. 4 , the RTP timer  465  is only shown for UE  112 , but it should be considered that UE  110  may also implement an RTP timer). For example, the RTP timer  465  may be between 6 to 10 seconds. The RTP timer  465  may be selected by the bearer loss application  245  to provide a base time in which a further predetermined timer  470  is used. Again, the further predetermined timer  470  is only shown for UE  110  in the signaling diagram  400  of  FIG. 4 , but it should be considered that UE  112  may also implement a further predetermined timer. The further predetermined timer  470  may be a value less than the selected RTP timer  465 . Thus, when the bearer loss application  245  sets the RTP timer  465  to 10 seconds, the further predetermined timer  470  may be set to 8 seconds. 
     Using the above noted example of 10 seconds set for the RTP timer  465  and 8 seconds for the predetermined timer  470 , the bearer loss application  245  may determine a likelihood that the UE  112  is experiencing a dedicated bearer loss issue. Specifically, if the UE  110  does not receive downlink RTP packets within 8 seconds (the predetermined timer  470 ), the bearer loss application  245  may assume that the UE  112  is experiencing the dedicated bearer loss issue. Upon expiry of the predetermined timer  470 , the bearer loss application  245  may be configured to notify the IMS  150  that the UE  112  is experiencing the dedicated bearer loss issue by transmitting a re-invite message  475 . Specifically, the re-invite message  475  may be a SIP message re-invite. In a substantially similar manner discussed above with regard to the SIP message invite  380  relating to the signaling diagram  300 , the SIP message re-invite  475  may include the proper parameters. This SIP message re-invite  475  may trigger the dedicated bearer reestablishment of the UE  112  side. Therefore, the UE  110  transmits the re-invite message  475  to the IMS  150 . The IMS  150  in turn transmits the dedicated bearer reestablishment signal  480  to the eNB  122 B such that the dedicated bearer is reactivated  485  for the UE  112 . That is, the dedicated bearer of the UE  112  is not released and the call continuity may be maintained so long as the process is performed prior to the expiry of the RTP timer  465  that may also represent a TO period. Accordingly, the UE  110  may detect possible dedicated bearer loss problems of the other UE  112  and help to notify the LTE-RAN  122 , the corresponding eNB, and the IMS  150  so that the VoLTE call may be continued. 
       FIG. 5  shows an exemplary LTE network arrangement  122 . The LTE-RAN  122  may represent a specific embodiment of the network arrangement  100  in which the UE  110  is performing a VoLTE call with the UE  112 . As was used in the above description, the UE  110  may be associated with the eNB  122 A while the UE  112  may be associated with the eNB  122 B. However, in this exemplary embodiment, the UE  110  may move a distance d to a position UE  110 ′ which is outside the operating area of the eNB  122 A but within the operating area of the eNB  122 C. Accordingly, the UE  110  has performed a roam from the eNB  122 A to the eNB  122 C. 
       FIG. 6  shows an exemplary signaling diagram  600  for establishing a further dedicated bearer. Specifically, the signaling diagram  600  relates to when the UE  110  executes the VoLTE handover application  250 . Accordingly, the signaling diagram  600  further relates to when the UE  110  experiences a roam that may result in an original dedicated bearer linked to an original default bearer being lost dependent upon operating parameters of the original eNB and the roamed eNB. The signaling diagram  600  will be described with regard to the specific embodiment shown in  FIG. 5 . It may be assumed that the UE  112  does not experience any conditions that result in a loss of its dedicated bearer on its end such that the VoLTE handover application  250  is capable of maintaining the call continuity. 
     The signaling diagram  600  begins with the VoLTE call already being established  605 . The signaling diagram  600  also begins with the UE  110  being associated with the eNB  122 A while the UE  112  is associated with the eNB  122 B. Substantially similar operations may be performed as described above with regard to the signaling diagram  300  of  FIG. 3  and the signaling diagram  400  of  FIG. 4 . 
     At a subsequent time, the UE  110  may move  610  such as from the operating area of the eNB  122 A to the operating area of the eNB  122 C. During this transition, the UE  110  may perform a roam procedure in which a connection request  615  is transmitted to the eNB  122 C while a teardown procedure  620  is performed with the eNB  122 A. Upon the connection  625  to the eNB  122 C is accomplished, a new default bearer may be established for the UE  110 . The VoLTE handover application  250  may be configured to trigger the LTE-RAN  122  to setup a dedicated bearer at the roamed eNB  122 C when the dedicated bearer at the original eNB  122 A is lost after handover due to the roam. In this manner, the VoLTE call may be successfully transferred so that voice data of the VoLTE call may continue to be transmitted via the dedicated bearer. 
     When the handover is completed and the UE  110  is now associated with the eNB  122 C, the RRC connection with the signaling radio bearer (SRB) and the dedicated radio bearer (DRB) for the default bearer may be established with the roamed eNB  122 C. The VoLTE handover application  250  may detect whether the dedicated bearer for the eNB  122 C is configured  630 . If the dedicated bearer of the eNB  122 C is not configured (e.g., based upon the signaling), the dedicated bearer being used for the VoLTE call may be lost and the LTE-RAN  122  may release the dedicated bearer for the UE  110  such that the VoLTE call is dropped. However, the VoLTE handover application  250  may be configured to transmit a re-invite message  635  to the IMS  150  with the proper parameters. Specifically, the re-invite message  635  may be a SIP message re-invite. In a substantially similar manner discussed above, the SIP message re-invite  635  may trigger the LTE-RAN  122  (e.g., the LTE core network) to set up  640  the dedicated bearer at the roamed eNB  122 C. Once the dedicated bearer at the roamed eNB  122 C is established  645 , the UE  110  may switch the VoLTE call data to the newly established dedicated bearer. Therefore, the VoLTE call continuity  650  may be maintained after a handover is performed. 
     It should be noted that the above examples of  FIGS. 5 and 6 , it was considered that the UE  110  performed a roaming procedure to associate with a different eNB. However, those skilled in the art will understand that the UE  110  may associate with a different eNB via a handover that does not include performing a roaming procedure. Thus, the mechanism described above for maintaining VoLTE call continuity may also be used in situations where a handover occurs without any roaming procedure being used, i.e., the above procedure may be used for any handover scenario. 
     The above exemplary embodiments illustrate the various mechanisms that the UE  110  may be configured with which to maintain VoLTE call continuity. The mechanisms enable the LTE-RAN  122  and its components such as the eNBs  122 A,  122 B,  122 C to be prepared. Accordingly, dedicated bearers that are being used for the VoLTE call may be kept and not released for a reestablishment procedure to be performed and dedicated bearers that will be used for the VoLTE call may be triggered for use in a preparatory manner. It should be noted that the mechanisms described herein are only exemplary. The exemplary embodiments may incorporate other mechanisms that ensure or increase a probability that the call continuity is maintained. Furthermore, the exemplary embodiments relating to the VoLTE call is only exemplary as the call continuity being maintained may be applied to any real-time communication. 
       FIG. 7  shows a first exemplary method  700  for re-activating a dedicated bearer when experiencing a RLF. Specifically, the method  700  relates to when the UE  110  executes the reestablishment application  240  when the UE  110  experiences the RLF that results in the dedicated bearer to be lost. Again, it may be assumed that the UE  112 , which is the other party to the VoLTE call, is not experiencing any conditions that potentially result in its dedicated bearer to be lost. The method  700  will be described with regard to the UE  110 . The method  700  will be described with reference to the network arrangement  100  of  FIG. 1 , the UE  200  of  FIG. 2 , and the signaling diagram  300  of  FIG. 3 . 
     In step  705 , the UE  110  establishes a connection to the LTE-RAN  122 . As discussed above, the UE  110  may perform an association procedure with the eNB  122 A when in the operation area thereof. The association procedure may include an exchange of signals that authorizes the UE  110 . Once the connection to the eNB  122 A and the LTE-RAN  122  is established, the eNB  122 A may assign at least one default bearer having a unique IP address to the UE  110  which is used for the exchange of data. One such functionality includes a VoLTE call. In step  710 , the UE  110  performs the VoLTE call and in step  715 , the VoLTE call is established with the UE  112  (which is also connected to the LTE-RAN  122  having its own default bearer assigned by eNB  122 B). When the VoLTE call is established, the eNB  122 A assigns a dedicated bearer that is linked to the default bearer for use by the UE  110  to exchange VoLTE call data such as voice. 
     During the VoLTE call, in step  720 , the UE  110  determines whether a RLF is detected. As discussed above, the UE  110  may be configured to detect the RLF that may occur under any number of conditions or reasons. If no RLF is detected, the UE  110  continues the method  700  to step  725 . In step  725 , the VoLTE call is continued. In step  730 , the UE  110  determines whether the VoLTE call has ended. If not ended, the UE  110  returns the method to step  720 . 
     If the UE  110  determines that the RLF is detected, the UE  110  continues the method to step  735 . In step  735 , the reestablishment application  240  of the UE  110  generates a preparation signal to be transmitted to the eNB  122 A. As discussed above, the preparation signal indicates to the eNB  122 A that the dedicated bearer for the UE  110  is to be kept and not released. The preparation signal may be transmitted at any time prior to the expiry of a timeout period that would otherwise result in the dedicated bearer being released and/or the VoLTE call from being dropped. 
     In step  740 , the UE  110  performs RRC reestablishment procedures upon detecting the RLF. As discussed above, the RRC reestablishment procedures may entail a variety of operations to resolve the UE context with the eNB  122 A. In step  745 , the UE  110  determines whether the RRC reestablishment procedures are successful. When the UE context has been restored, the RRC reestablishment procedures are successful. 
     If the RRC reestablishment procedures are successful, the UE  110  continues the method  700  to step  750 . In step  750 , the RRC connected state is maintained. In the maintained RRC connected state, the reestablishment application  240  indicates to the UE  110  that a SIP message invite is to be transmitted to the IMS  150  which triggers the LTE-RAN  122  to continue the context for the UE  110 . In this manner, in step  760 , the dedicated bearer is re-activated. If the RRC reestablishment procedures fail, the UE  110  continues the method  700  from step  745  to step  765 . In step  765 , the UE  110  enters the RRC idle state. In the RRC idle state, the reestablishment application  240  indicates to the UE  110  that a SIP message invite is to be transmitted to the IMS  150  which triggers the LTE-RAN  122  to perform the RRC connection request. In this manner, in step  760 , the dedicated bearer is re-activated. With the dedicated bearer re-activated prior to the expiry of any timeout period, the VoLTE call may continue. 
       FIG. 8  shows a second exemplary method  800  for re-activating a dedicated bearer when experiencing a RLF. Specifically, the method  800  relates to when the UE  110  executes the bearer loss application  245  when the UE  110  detects that the UE  112  may be experiencing the RLF that results in the dedicated bearer to be lost. Again, it may be assumed that the UE  110  is not experiencing any conditions that potentially results in its dedicated bearer to be lost. The method  700  will be described with regard to the UE  110 . The method  800  will be described with reference to the network arrangement  100  of  FIG. 1 , the UE  200  of  FIG. 2 , and the signaling diagram  400  of  FIG. 4 . 
     Steps  805 - 815  relate to establishing a connection to the LTE-RAN  122 , performing a VoLTE call, and establishing the VoLTE call, respectively. Accordingly, these steps may be substantially similar to those described above with regard to step  705 - 715  of the method  700  of  FIG. 7 . In step  820 , the UE  110  determines whether the UE  112  has experienced a RLF based upon a predetermined timer that is less than a RTP timer. If no downlink RTP packets are received within the predetermined timer, the bearer loss application  245  of the UE  110  may assume that the UE  112  has experienced the RLF and has lost its dedicated bearer. If the bearer loss application  245  does not detect the RLF of the UE  112 , the UE  110  continues the method  800  to step  825  where the VoLTE call is continued. In step  830 , the UE  110  determines whether the VoLTE call has ended and if not ended, returns to step  820 . 
     If the bearer loss application  245  determines that the UE  112  has experienced the RLF from not receiving any downlink RTP packets within the predetermined timer, the UE  110  continues the method  800  to step  835 . In step  835 , the bearer loss application  245  indicates to the UE  110  that a SIP message re-invite is to be transmitted to the IMS  150 . This transmission may enable the IMS  150  to trigger a dedicated bearer reestablishment procedure for the UE  112 . Thus, in step  840 , the dedicated bearer reestablishment procedure is performed for the dedicated bearer for the UE  112  to be re-activated. With the dedicated bearer re-activated prior to the expiry of any timeout period, the VoLTE call may continue. 
       FIG. 9  shows an exemplary method  900  for transferring a VoLTE call upon experiencing a roam. Specifically, the method  900  relates to when the UE  110  executes the VoLTE handover application  250  when the UE  110  experiences a roam in which an original dedicated bearer is lost from the handover and a new dedicated bearer is established with the roamed eNB. Again, it may be assumed that the UE  112  which is the other party to the VoLTE call is not experiencing any conditions that potentially results in its dedicated bearer to be lost. The method  700  will be described with regard to the UE  110 . The method  900  will be described with reference to the network arrangement  100  of  FIG. 1 , the UE  200  of  FIG. 2 , the LTE-RAN  122  of  FIG. 5 , and the signaling diagram  600  of  FIG. 6 . 
     Steps  905 - 915  relate to establishing a connection to the LTE-RAN  122 , performing a VoLTE call, and establishing the VoLTE call, respectively. Accordingly, these steps may be substantially similar to those described above with regard to step  705 - 715  of the method  700  of  FIG. 7 . In step  920 , the UE  110  determines whether a roam is being performed. If no roam is performed, the UE  110  continues the method  900  to step  925  where the VoLTE call is continued. In step  930 , the UE  110  determines whether the VoLTE call has ended and if not ended, returns to step  920 . The UE  110  may determine a roam based upon a variety of factors such as positioning information and movement information. Thus, when the roam occurs, the UE  110  may move out of an operating area of the original eNB  122 A and into an operating area of the roamed eNB  122 C. As such, the default bearer from the original eNB  122 A may be lost including any linked dedicated bearers while a new default bearer is established with the roamed eNB  122 C. When the roam is determined, the UE  110  continues the method to step  935 . 
     In step  935 , the UE  110  establishes a connection to the LTE-RAN  122  via the roamed eNB  122 C. Again, upon associating with the roamed eNB  122 C, a new default bearer may be assigned by the eNB  122 C and established for the UE  110 . In step  940 , the VoLTE handover application  250  determines whether the eNB  122 C has a dedicated bearer configured based upon signaling information received during the association process. If the dedicated bearer is already configured, the UE  110  continues the method to step  925  as the VoLTE call may continue. 
     However, if the VoLTE handover application  250  determines that the eNB  122 C does not have the dedicated bearer configured, the UE  110  continues the method  900  to step  945 . In step  945 , the VoLTE handover application  250  indicates to the UE  110  that the SIP message re-invite is to be transmitted to the IMS  150 . The SIP message re-invite may indicate to the IMS  150  that a setup procedure is to be performed by the eNB  122 C. Thus, in step  950 , the setup procedure is performed. In step  955 , the new dedicated bearer for the UE  110  from the roamed eNB  122 C is established. In step  960 , the VoLTE call is transferred to the new dedicated bearer. With the dedicated bearer being established prior to the expiry of any timeout period, the VoLTE call may continue. 
     The exemplary embodiments provide a system and method of maintaining a VoLTE call continuity when a dedicated bearer has been determined to be lost or when a dedicated bearer has a high probability of being lost. In a first example, the UE has experienced a RLF so that the dedicated bearer is lost. The exemplary embodiments transmit a preparation signal to indicate to the LTE network to keep the dedicated bearer and not release it so that a subsequent action may re-activate the dedicated bearer for the VoLTE call to continue. In a second example, the UE may determine that no downlink RTP packets have been received from the other UE within a predetermined timer. The exemplary embodiments may provide a mechanism for the UE to assume that this lack of RTP packets within the predetermined timer is indicative of the other UE having experienced a RLF and lost its dedicated bearer. The exemplary embodiments transmit a SIP message re-invite to trigger a connection request and re-activate the dedicated bearer for the other UE. In a third example, the UE may roam and lose its original dedicated bearer. If the UE determines that the roamed eNB does not have a configured dedicated bearer, the exemplary embodiments are configured to transmit a SIP message re-invite for a setup procedure to be performed and establish a dedicated bearer for the UE and the roamed eNB. 
     Those skilled in the art will understand that the above-described exemplary embodiments may be implemented in any suitable software or hardware configuration or combination thereof. An exemplary hardware platform for implementing the exemplary embodiments may include, for example, an Intel x86 based platform with compatible operating system, a Mac platform and MAC OS, a mobile device having an operating system such as iOS, Android, etc. In a further example, the exemplary embodiments of the above described method may be embodied as a program containing lines of code stored on a non-transitory computer readable storage medium that, when compiled, may be executed on a processor or microprocessor. 
     It will be apparent to those skilled in the art that various modifications may be made in the present invention, without departing from the spirit or the scope of the invention. Thus, it is intended that the present invention cover modifications and variations of this invention provided they come within the scope of the appended claims and their equivalent.

Metadata:
Filing Date: 20150123
Publication Date: 20180417
Grant Date: 20180417
Priority Date: 20150123
Inventors: ZHAO, YINGJIE
ZHANG, Wanping
SU, LI
Assignee: APPLE INC
CPC Classifications: [{"code": "H04W76/12", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04W76/12", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04W76/19", "inventive": true, "first": true, "tree": "[]"}, {"code": "H04W76/19", "inventive": true, "first": true, "tree": "[]"}, {"code": "H04W76/022", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04W76/028", "inventive": true, "first": true, "tree": "[]"}]
Family ID: 56433571