Abstract:
A mechanism is provided by a method and an arrangement for performing Circuit Switched fallback from a packet switched domain to a circuit switched domain, in order to speed up the time it takes for a User Equipment to access an 1×RTT system before initiating services. Thus, the teachings herein provide a method of Circuit Switched fallback (i.e. for handling Circuit Switched Services) for a User Equipment residing in a Packet Switched domain, comprising of the eNB, located within the packet switched domain, transmitting an indication that Circuit Switched Services are supported to a User Equipment and registering to a Circuit Switched domain by signalling Circuit Switched Registration information, via the eNB, enabling the User Equipment to register to a Circuit 
     Switched domain, associated to the Circuit Switched Services and furthermore receiving information comprising Circuit Switched specific parameter setting, to preparing for Circuit Switched fallback and applying the Circuit Switched specific parameter setting received and switching the Packet Switched domain to the Circuit Switched domain.

Description:
TECHNICAL FIELD 
       [0001]    The described embodiments of present invention relates to a method and an arrangement, in a communication network system, and more particularly, to a method and an arrangement for facilitating fallback from a packet switched environment to a circuit switched environment. 
       BACKGROUND 
       [0002]    The 3rd Generation Partnership Project (3GPP) standardization body is currently working on the specification of the evolved 3G mobile system called Long Term Evolution (LTE) or E-UTRAN. This system will support inter-working with CDMA2000 networks (HRPD) and (1×RTT). The inter-working will be in the form of packet switched (PS) handovers for ongoing IP real-time services between LTE and HRPD as well as handover for ongoing Voice over IP calls from LTE to circuit switched (CS) 1×RTT networks (the latter mechanism is called Voice Call Continuity (VCC)). 
         [0003]    The solutions above are described in 3GPP TS36.300 section 10.3.2, and which form part of the prior art. However recently it has also been considered important to introduce support for circuit switched (CS) fallback from LTE to legacy networks, such as Global System for Mobile communications (GSM) and 1×RTT. In short, by Circuit Switched Fallback mechanisms, the CS-domain services are realized by reuse of Circuit Switched infrastructure (radio and core network) and by Circuit Switched fallback is meant that the user equipment (UE) camps on a Packet Switched only system (e.g. E-UTRAN) but switch over to Circuit Switched system (e.g. GERAN/UTRAN) to establish originating Circuit Switched call. 
         [0004]    For incoming terminating calls the user equipment is paged in the Packet Switched only system (e.g. E-UTRAN) but respond to the page in the Circuit Switched system. The Circuit Switched fallback mechanism differs from VCC in that the VCC mechanism is applied to on-going services while Circuit Switched fallback is intended to switch directly to the Circuit Switched domain, before setting up any Circuit Switched services. 
         [0005]    The principle for the Circuit Switched fallback feature is that the mobile terminal performs registration, while in LTE, to the Circuit Switched domain. Once the terminal then wants to make a Circuit Switched voice call, or receives an incoming paging for a Circuit Switched voice call, it will leave the LTE domain and switch over to the Circuit Switched radio access network (e.g. 1×RTT, GSM) and initiate the call setup in the Circuit Switched network. 
         [0006]    A drawback with the above described is that the access delay for the mobile to access the target Circuit Switched system will be quite long, which will have a negative impact on the service performance, i.e. the service interruption time will be long, in the order of seconds depending on the implementation. The reasons for this is that the user equipment need to perform cell search on the 1×RTT carriers, acquire synchronization, read information from broadcast channel etc. before it is able to respond to the incoming page or setup up the mobile originated calls. 
         [0007]    It is of course possible to re-use concepts for Packet Switched to Circuit Switched handovers from LTE to 1×RTT in order to speed up the time it takes for the user equipment to access the 1×RTT system. The principle for LTE to 1×RTT handover is based on that resources and signaling are performed towards the 1×RTT system before the user equipment leaves LTE. The concept is also based on that they user equipment are performing measurements on 1×RTT cells prior to leaving LTE. These concepts not only rather complicated and have large impact on both the networks (3GPP and 3GPP2) and terminals, but they are also meant for ongoing services, not for redirection before initiating services. 
         [0008]    Then there is also general concept defined for reducing service interruption time at cell change for ongoing Packet Switched based services (e.g. in GERAN/UTRAN), which are well known are used both for handover (when target cell is prepared) and for network assisted or controlled cell re-selection (when the target cell is not prepared but the access delay is still reduced since the user equipment does not need to read broadcast information), again these concepts are not designed for re-directing before initiating services and they are also not directly applicable to the Circuit Switched domain or to 1×RTT which uses other access methods and synch procedures. 
       SUMMARY 
       [0009]    It is the object to obviate at least some of the above described disadvantages and provide an improved method and arrangement for performing a Circuit Switched fallback mechanism from a packet switched domain to a circuit switched domain in order to speed up the time it takes for a user equipment to access a 1×RTT system before initiating services. Thus, in at least one embodiment, the teachings herein provide a method of Circuit Switched fallback (i.e. for handling Circuit Switched Services) in a user equipment, residing in a Packet Switched domain, comprising the steps of receiving, from an eNB located within the packet switched domain, an indication that Circuit Switched fallback (i.e. Circuit Switched Services) is supported and registering the user equipment to a Circuit Switched domain by signalling Circuit Switched Registration information via the eNB, thereby enabling registry to the Circuit Switched domain associated to the Circuit Switched Services, followed by receiving information comprising Circuit Switched specific parameter setting to preparing for Circuit Switched fallback and then applying the Circuit Switched specific parameter settings received and switching from the Packet Switched domain to the Circuit Switched domain, leaving LTE. 
         [0010]    In at least one embodiment, such Circuit Switched specific parameter settings are receivable in a release and re-direct indication comprising 3G×1 parameters. Further, in at least one embodiment, such 3G×1 parameters as specified in C.S0024-A_v3.0 may comprise either 1×RTT frequency band, 1×RTT carrier frequency, PN offset, CDMA system time or Long Code State or a combination thereof. 
         [0011]    Furthermore, in at least one embodiment, wherein the user equipment being in idle mode, the method further comprises transiting from idle mode to active mode before receiving the Circuit Switched specific parameter setting. Further, where the user equipment is in idle mode and receiving an incoming Circuit Switched service, the method in at least one embodiment, comprise receiving a paging message from the eNB. 
         [0012]    Also, in at least one embodiment, the teachings herein provide a method of Circuit Switched fallback (i.e. for handling Circuit Switched Services) in an eNB, residing in a Packet Switched domain, comprising transmitting an indication, that Circuit Switched Services are supported, to a user equipment located within the Packet Switched domain and receiving, either a request for Circuit Switched fallback from a user equipment located within the packet switched domain, or a Circuit Switched fallback indication from a second node, wherein the eNB further transmitting a release indication comprising Circuit Switched specific parameter setting to the user equipment. 
         [0013]    In at least one embodiment, such Circuit Switched specific parameter setting is transmittable in a release and re-direct indication and may comprise 3G×1 parameters. Further, in at least one embodiment, such 3G×1 parameters as specified in C.S0024-A_v3.0 may comprise 1×RTT frequency band, 1×RTT carrier frequency, PN offset, CDMA system time or Long Code State or a combination thereof. 
         [0014]    Also, in at least one embodiment, the teachings herein provide an arrangement in a user equipment for Circuit Switched fallback (i.e. enabling Circuit Switched Services), comprising a receiver or receiving means configured to receiving, from a eNB located within the packet switched domain, an indication that Circuit Switched fallback (i.e. Circuit Switched Services) is supported and a transmitter or a transceiver means configured to transmitting and registering (i.e. Circuit Switched. Registration signalling via the eNB, enabling registry to the Circuit Switched domain associated to the Circuit Switched Services) to a Circuit Switched domain further comprising a receiver or a receiving means configured to receiving information comprising Circuit Switched specific parameter setting to preparing for Circuit Switched fallback and a processing means for applying the Circuit Switched specific parameter setting received and switching to the Circuit Switched domain. 
         [0015]    Also, in at least one embodiment, the teachings herein provide an arrangement for Circuit Switched fallback in an eNB, residing in a Packet Switched domain, comprising a receiver configured to receiving either a request for Circuit Switched fallback from a user equipment located within the packet switched domain, or a Circuit Switched fallback indication from a second node, wherein the eNB further comprise a transmitter configured to transmitting a release indication comprising Circuit Switched specific parameter setting to the user equipment. In this manner Circuit Switched fallback, is achieved with minimum delay. 
         [0016]    An advantage with the above described is the performance improvement experienced by the end-user, using the Circuit Switched fallback feature, by reducing the access delay in the Circuit Switched domain such as the 1×RTT system for mobile terminated or originating calls. The reduction of the access delay is achieved, while introducing only minor additional complexities in the network and terminal. Still other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    In the drawings, wherein like reference characters denote similar elements throughout the several views: 
           [0018]      FIG. 1  illustrates registration to a Circuit Switched domain according to prior solutions; 
           [0019]      FIG. 2  illustrates a user equipment terminated Circuit Switched call according to prior solutions; 
           [0020]      FIG. 3  illustrates a user equipment originated Circuit Switched call according to prior solutions; 
           [0021]      FIG. 4  illustrates the Circuit Switched set up for a user equipment in active mode according to described embodiments of present invention; 
           [0022]      FIG. 5  illustrates the Circuit Switched set up for a user equipment in idle mode, with an incoming Circuit Switched call, according to described embodiments of present invention. 
           [0023]      FIG. 6  illustrates the Circuit Switched set up for a user equipment in idle mode, with an outgoing Circuit Switched call, according to described embodiments of present invention. 
           [0024]      FIG. 7  is a flowchart describing the steps of the described invention. 
           [0025]      FIG. 8  is an exemplary implementation of a user equipment. 
           [0026]      FIG. 9  illustrates exemplary components of a user equipment. 
           [0027]      FIG. 10  illustrates exemplary components of an eNB. 
           [0028]      FIG. 11  illustrates exemplary components of a Mobility Management Entity or Gateway (MME or GW). 
       
    
    
     DETAILED DESCRIPTION 
       [0029]    The following detailed description refers to the accompanying drawings. The same reference numbers in different drawings may identify the same or similar elements. Also, the following detailed description does not limit the invention. 
       Abbreviations/Definitions 
       [0000]    
       
         UE User Equipment (mobile terminal) 
         1×RTT A CDMA2000 system supporting both Circuit Switched and Packet Switched traffic 
         HRPD or EVDO A CDMA2000 system only supporting Packet Switched traffic 
         MSC Mobile Switching Center (used for Circuit Switched services) 
         MME Mobility Management Entity 
         eNB or eNode B Long Term Evolution (LTE) base station 
       
     
         [0036]    The described concept is different from traditional handover concepts since it is applies the principles to Circuit Switched fallback from one system in a first environment to another system in a second environment and it further deals with Circuit Switched specific parameters, and CDMA2000 specific parameters in particular, like CDMA system time and PN offset that are not used in GERAN/UTRAN. 
         [0037]    The basic concept of the embodiments described herein is to, when the mobile terminal/user equipment (UE)  110  in LTE wants to setup a Circuit Switched call or receives a Circuit Switched page, let the LTE network provide the user equipment  110  with information, which will speed up the access towards the 1×RTT system. This information will include parameters such as 1×RTT carrier frequency, 1×RTT system time (Global Positioning System (GPS) time used for synchronization), target cell physical cell identity (PN offset) as well as any other information needed by the user equipment  110  in order to access the system. 
         [0038]    This information is either static and can be stored in the LTE network or can easily be generated by the LTE network (e.g. the system time), thus avoiding the need to perform any signaling with the 1×RTT network before the 1×RTT transition. 
         [0039]    The 1×RTT system time can be expressed as a reference to by the user equipment  110  and network known time in LTE (e.g. a specific system frame number). Which cell the user equipment  110  should go to, can also be statically configured, in order to avoid additional measurements on the 1×RTT cells from LTE (typically the 1×RTT and LTE cells can be co-sited). If these parameters are provided to the user equipment  110  the access delay can be significantly reduced. 
         [0040]    FIG.  1 —Illustrates a prior solution to registration to a Circuit Switched domain, the user equipment  110  receives an indication that Circuit Switched fallback is supported, and therefore performs a registration with the Circuit Switched domain. The registration signaling is tunneled transparently through the LTE system via the Mobility Management Entity (MME)  120  towards the CDMA2000 network  420 . The detailed signaling path on the CDMA2000 network  420  side is tunnelled transparently through the LTE system and, in  FIG. 1  is shown the solution when the signaling is piggybacked on registration signaling for HRPD but also other solutions are possible (e.g. where the signaling is performed over the S 102  interface between the MME  120  an 1×CS inter-working function). Once the registration is completed the user equipment  110  is known in the 1×RTT Mobile Switching Centre (MSC)  150  and can from there on receive incoming Circuit Switched calls. 
         [0041]    FIG.  2 —Illustrates the principle for an incoming Circuit Switched call, wherein a variant of Circuit Switched fallback feature is present, wherein the user equipment  110  performs registration, while in LTE (i.e. it camps in LTE or has ongoing services in LTE and is registered to the Circuit Switched domain), to the Circuit Switched domain. A Mobile Switching Centre (MSC)  150  receives an incoming Circuit Switched voice call and therefore generates a page message which is tunneled to the LTE network Once the terminal then receives an incoming paging for a Circuit Switched voice call, it will leave the LTE domain and switch over to the Circuit Switched radio access network (e.g. 1×RTT, GSM) and perform the page response procedure and initiate the call setup in the Circuit Switched network. 
         [0042]    FIG.  3 —Illustrates the principle for a user equipment  110  originated Circuit Switched call, wherein a variant of Circuit Switched fallback feature is present, wherein the user equipment  110  performs registration, while in LTE, to the Circuit Switched domain. Once the user equipment  110  then wants to make a Circuit Switched voice call, it will leave the LTE domain and switch over to the Circuit Switched radio access network  420  (e.g. 1×RTT, GSM) and initiate the call setup in the Circuit Switched network 
         [0043]    FIG.  4 —Enhancements are needed in the current Circuit Switched fallback method, i.e. there is a need for making the eNB  410  aware of the need for a user equipment  110  to perform Circuit Switched fallback. The basic principles are the following; the eNB (E-UTRAN)  410  receives knowledge about that the user equipment  110  is to perform a transition to 1×RTT and setup a Circuit Switched call. How the eNB  410  gets this knowledge is dependent on the mode that the user equipment  110  is currently in (i.e. Idle or Active) and if it is an incoming Circuit Switched call or an originating Circuit Switched call. When the user equipment  110  is active in LTE (e.g. has ongoing data transmission) the eNB can either receive a message from the core network (i.e. MME)  120 , or it can receive a message from the user equipment  110 , that the user equipment  110  should perform transition to 1 ×RTT (Circuit Switched). 
         [0044]      FIG. 5  illustrates the user equipment  110  in idle mode in LTE, where it will, for incoming Circuit Switched call, first receive a page message on the paging channel and then perform a transition to LTE active (using the service request procedure). Once the user equipment  110  is LTE active mode the eNB can either receive a message from the core network (i.e. MME), or it can receive a message from the user equipment, that the user equipment  110  should perform transition to 1×RTT (Circuit Switched). 
         [0045]      FIG. 6  illustrates the user equipment  110  in idle in LTE, wherein it wants to perform a Circuit Switched call, then it will first perform a transition to LTE active (using the service request procedure). Once the user equipment  110  is LTE active the eNB  410  can either receive a message from the core network (i.e. MME)  120  or it can receive a message from the user equipment  110  the user equipment  110  should perform transition to 1×RTT (Circuit Switched). 
         [0046]      FIG. 7  is a combined signalling scheme and flowchart of an exemplary process relating to embodiments of the present solution which will be described in detail below. 
         [0047]      FIG. 8  is a diagram of an exemplary implementation of user equipment  110 . In the example illustrated in  FIG. 8 , user equipment  110  is implemented as a cell phone. User equipment  110  may include a microphone  810 , a speaker  820 , a group of input elements  830 , and a display  840 . 
         [0048]    Microphone  810  may receive audible information from a user of user equipment  110 . Speaker  820  may provide audible information to a user of user equipment  110 . Input elements  830  may include control buttons and/or a keypad. The control buttons may permit a user to interact with user equipment  110  to cause user equipment  110  to perform one or more operations. For example, the control buttons may be used to cause user equipment  110  to transmit information. The keypad may include a standard telephone keypad. Display  840  may provide visual information to a user. For example, display  840  may display text input into user equipment  110 , text and/or graphics received from another device, and/or information regarding incoming or outgoing calls or text messages, media, games, phone books, address books, the current time, etc. 
         [0049]    Although  FIG. 8  shows exemplary components of user equipment  110 , in other implementations, user equipment  110  may contain fewer, different, or additional components than depicted in  FIG. 8 . In still other implementations, one or more components of user equipment  110  may perform the tasks described as being performed by one or more other components of user equipment  110 . 
         [0050]    In  FIG. 8 , the user equipment  110  is connected to the wireless communication network  420  via a wireless connection to a network node  410 . In some embodiments, the user equipment  110  is “RRC connected”, where RRC refers to the Radio Resource Control (RRC) protocol that in UTRAN or E-UTRAN, among other things, configures and controls the connection between the base station and the user equipment. I.e. the user equipment  110  is in ACTIVE MODE. 
         [0051]      FIG. 9  is a diagram of exemplary components of user equipment  110  of  FIG. 8 . As illustrated, user equipment  110  may include an antenna  930 , a transceiver  905 , processing logic  910 , a memory  915 , an input device(s)  920 , an output device(s)  925 , and a bus  930 . 
         [0052]    Antenna  930  may include one or more antennas to transmit and/or receive radio frequency (RF) signals over the air. Antenna  930  may, for example, receive RF signals from transceiver  905  and transmit the RF signals over the air to an eNB and receive RF signals over the air from said eNB and provide the RF signals to transceiver  905 . 
         [0053]    Transceiver  905  may include, for example, a transmitter that may convert baseband signals from processing logic  910  to RF signals and/or a receiver that may convert RF signals to baseband signals. Alternatively, transceiver  905  may include a transceiver to perform functions of both a transmitter and a receiver. Transceiver  905  may connect to antenna  930  for transmission and/or reception of the RF signals. 
         [0054]    Processing logic  910  may include a processor, microprocessor, an application specific integrated circuit (ASIC), field programmable gate array (FPGA), or the like. Processing logic  910  may control operation of user equipment  110  and its components. 
         [0055]    Memory  915  may include a random access memory (RAM), a read only memory (ROM), and/or another type of memory to store data and instructions that may be used by processing logic  910 . Input device(s)  920  may include mechanisms for entry of data into user equipment  110 . For example, input device(s)  920  may include input mechanisms, such as microphone  810 , input elements  830 , display  840 , etc. Output device(s)  925  may include mechanisms for outputting data in audio, video and/or hard copy format: For example, output device(s)  925  may include speaker  820 , display  840 , etc. Bus  930  may interconnect the various components of user equipment  110  to permit the components to communicate with one another. 
         [0056]    Although  FIG. 9  shows exemplary components of user equipment  110 , in other implementations, user equipment  110  may contain fewer, different, or additional components than depicted in  FIG. 9 . In still other implementations, one or more components of user equipment  110  may perform the tasks described as being performed by one or more other components of user equipment  110 . 
         [0057]      FIG. 10  is a diagram of exemplary components of eNB  410 . eNB  410  may be similarly configured. As illustrated, eNB  410  may include antennas  1010 , transceivers  1020 , a processing system  1030 , and an interface  1040 . 
         [0058]    Antennas  1010  may include one or more directional and/or omni-directional antennas. Transceivers  1020  may be associated with antennas  1010  and include transceiver circuitry for transmitting and/or receiving symbol sequences in a network, such as network  100 , via antennas  1010 . 
         [0059]    Processing system  1030  may control the operation of eNB  410 . Processing system  1030  may also process information received via transceivers  1020  and interface  1040 . As illustrated, processing system  1030  may include processing logic  1032  and a memory  1034 . It will be appreciated that processing system  1030  may include additional and/or different components than illustrated in  FIG. 10 . 
         [0060]    Processing logic  1032  may include a processor, microprocessor, an ASIC, FPGA, or the like. Processing logic  1032  may process information received via transceivers  1020  and interface  1040 . The processing may include, for example, data conversion, forward error correction (FEC), rate adaptation, and quadrature phase shift keying (QPSK) modulation, etc. In addition, processing logic  1032  may generate control messages and/or data messages and cause those control messages and/or data messages to be transmitted via transceivers  1020  and/or interface  1040 . Processing logic  1032  may also process control messages and/or data messages received from transceivers  1020  and/or interface  1040 . Memory  1034  may include a RAM, a ROM, and/or another type of memory to store data and instructions that may be used by processing logic  1032 . 
         [0061]    Interface  1040  may include one or more line cards that allow eNB  410  to transmit data to and receive data from other devices over wired and/or wireless connections. As illustrated, interface  1040  may include an S 1  interface  1042  that allows eNB  410  to communicate, for example, with a MME/GW  120 , and an X 2  interface  1044  that allows eNB  410  to communicate with another eNB. 
         [0062]    eNB  410  may perform certain operations in response to processing logic  1032  executing software instructions contained in a computer-readable medium, such as memory  1034 . A computer-readable medium may be defined as one or more physical and/or logical memory devices. The software instructions may be read into memory  1034  from another computer-readable medium or from another device via interface  1040 . The software instructions contained in memory  1034  may cause processing logic  1032  to perform processes described herein. Alternatively, hardwired circuitry may be used in place of or in combination with software instructions to implement processes described herein. Thus, embodiments described herein are not limited to any specific combination of hardware circuitry and software. 
         [0063]    Although  FIG. 10  shows exemplary components of eNB  410 , in other implementations, eNB  410  may contain fewer, different, or additional components than depicted in  FIG. 10 . In still other implementations, one or more components of eNB  410  may perform the tasks described as being performed by one or more other components of eNB  410 . 
         [0064]      FIG. 11  is a diagram of exemplary components of MME/GW  120 . MME/GW  120  may be similarly configured. As illustrated, MME/GW  120  may include a processing system  1110  and an interface  1120 . 
         [0065]    Processing system  1110  may control the operation of MME/GW  120 . Processing system  1110  may also process information received via interface  1120 . As illustrated, processing system  1110  may include processing logic  1112  and a memory  1114 . It will be appreciated that processing system  1110  may include additional and/or different components than illustrated in  FIG. 11 . 
         [0066]    Processing logic  1112  may include a processor, microprocessor, an ASIC, FPGA, or the like. Processing logic  1112  may process information received via interface  1120 . In addition, processing logic  1112  may generate control messages and/or data messages and cause those control messages and/or data messages to be transmitted via interface  1120 . Processing logic  1112  may also process control messages and/or data messages received from interface  1120 . Memory  1114  may include a RAM, a ROM, and/or another type of memory to store data and instructions that may be used by processing logic  1112 . 
         [0067]    Interface  1120  may include one or more line cards that allow MME/GW  120  to transmit data to and receive data from other devices over wired and/or wireless connections. As illustrated, interface  1120  may include an S 1  interface  1122  that allows MME/GW  120  to communicate, for example, with an eNB. It will be appreciated that interface  1120  may include additional interfaces than illustrated in  FIG. 11 . For example, interface  1120  may include an interface for communicating with another network, such as a PDN. 
         [0068]    MME/GW  120  may perform certain operations in response to processing logic  1112  executing software instructions contained in a computer-readable medium, such as memory  1114 . The software instructions may be read into memory  1114  from another computer-readable medium or from another device via interface  1120 . The software instructions contained in memory  1114  may cause processing logic  1112  to perform processes described herein. Alternatively, hardwired circuitry may be used in place of or in combination with software instructions to implement processes described herein. Thus, embodiments described herein are not limited to any specific combination of hardware circuitry and software. 
         [0069]    Although  FIG. 11  shows exemplary components of MME/GW  120 , in other implementations, MME/GW  120  may contain fewer, different, or additional components than depicted in  FIG. 11 . In still other implementations, one or more components of MME/GW  120  may perform the tasks described as being performed by one or more other components of MME/GW  120 . 
         [0070]    Below the embodiments of the present solution which will be described in more detail, reference is made to  FIG. 7  which is a combined signalling scheme and flowchart of an exemplary process. 
         [0071]    Step  700  a Network Node  410 , servicing a user equipment  110  residing in a Packet Switched domain, transmits an indication to the User equipment  110 , the indication comprises information to the user equipment  110  that Circuit Switched Services are supported and that the Network Node  410  may enable a Circuit Switched fallback mechanism to a legacy network  420 . In some embodiments, the Circuit Switched fallback mechanism enables fallback from the E-UTRAN network towards the CDMA2000 network. 
         [0072]    Step  705  the user equipment  110 , which resides/camps in the Packet Switched domain, i.e. LTE or has ongoing services in the Packet Switched domain, i.e. LTE, accordingly receives the indication transmitted from the Network Node  410 , that Circuit Switched Services are supported. 
         [0073]    Step  710  the Network Node  410 , follows the transmitted indication by tunneling a signaling procedure, further described in step  715  below, enabling the User Equipment  110  to register to the Circuit Switched domain. 
         [0074]    Step  715 , in response to the received indication of step  705  the user equipment  110  performs one or more attempts to initiate a signaling procedure, via the Network Node  410  enabling registry to the Circuit Switched domain. 
         [0075]    The user equipment  110 , thus after receiving the indication that Circuit Switched services are supported therefore performs a registration with the Circuit Switched domain. The registration signaling is tunneled transparently through the Packet Switched system, i.e. the LTE system, via a Third Node  120 , i.e. the MME, towards the 1×RTT network  420 , i.e. the CDMA2000 network. 
         [0076]    The detailed signaling path on the CDMA2000 network side is tunnelled transparently through the LTE system and in  FIG. 1  is shown the solution where the signaling is piggybacked on registration signaling for HRPD, but other solutions are also possible e.g. where the signaling is performed over the S 102  interface between the MME  120  and an 1×CS inter-working function. Once the registration is completed, the user equipment  110  is known in the 1×RTT Mobile Switching Centre (CS-MSC)  150  and can from thereon receive incoming Circuit Switched services, i.e. Circuit Switched voice calls. 
         [0077]    Step  720 , should the user equipment  110  be in Idle Mode; i.e. no on-going Packet Switched Services and the Mobile Switching Centre of the Circuit Switched network (CS-MSC)  150  receives an incoming Circuit Switched service, i.e. an incoming Circuit Switched voice call, the CS-MSC  150  generates a page message, which is tunneled via the LTE network and received at the user equipment  110  on the paging channel. The user equipment  110  is thus aware of an incoming Circuit Switched service and the user equipment  110 , after having received the page message, then performs a transition from Idle Mode to LTE Active Mode (i.e. RRC connected). The CS-MSC  150  is not aware of which state the user equipment  110  is in, so it always sends a page message to the MME  120  regardless of state. The MME  120  is responsible for making sure this is message is delivered. And in case the user equipment  110  is in Idle mode it will first page the user equipment  110  with a normal LTE page and then deliver the 1×RTT Circuit Switched page as a tunnelled message to the user equipment  110   
         [0078]    Should the User Equipment  110  be in Idle Mode and about to initiate a Circuit Switched Service, the User Equipment  110  only has to perform a transition from Idle Mode to LTE Active Mode. 
         [0079]    Step  725  once the User Equipment  110  is in LTE Active Mode, either the User Equipment  110  can send a request for Circuit Switched Services to the Network Node  410  and inform the network that the User Equipment  110  is about to perform a transition from the Packet Switched domain to the Circuit Switched domain, enabling Circuit Switched Services, or in 
         [0080]    Step  730  the Network Node  410  can receive a request message from a Third Node  120 , i.e. a MME, that the User Equipment  110  is having incoming Circuit Switched Services and is about to perform a transition from the Packet Switched domain to the Circuit Switched domain, enabling the Circuit Switched Services. 
         [0081]    Step  740  once the Network Node  410  is aware that the User Equipment  110  is about to perform a Circuit Switched fallback, enabling Circuit Switched services, the Network Node  410  will send a message to the User Equipment  110  informing the User Equipment  110  to leave the Packet Switched domain (i.e. leave LTE) and also informing the User Equipment  110  of the specific Circuit Switched parameters needed for this transition. 
         [0082]    In some embodiments the message comprise the 3G1× parameters as specified in C.S0024-A; and in particular the parameters; 1×RTT frequency band, 1×RTT carrier frequency, PN offset (which in principle means the physical cell identity of the target cell), CDMA system time, Long Code State (needed by the User Equipment  110  to decode downlink channels in 1×RTT). Some of these parameters (e.g. CDMA system time) can, as an alternative, be provided on the LTE broadcast channel. 
         [0083]    In some embodiments the message can be an extension of an existing RRC connection Release with re-direction message, or a handover message. 
         [0084]    Step  745  the User Equipment  110  receives the Circuit Switched specific information, including the parameters as above in order to help the User Equipment  110  to access the target system/cell with minimum delay. In the listing below is illustrated 3G1× Parameters from C.S0024-A 
         [0000]    
       
         
               
               
               
               
             
           
               
                   
               
               
                   
                 ENCRYPT_MODE 
                   
                 IMSI_T_SUPPORTED 
               
               
                 MessageID 
                 Included 
                 MAX_ADD_SERV_INSTANCE 
                 Included 
               
               
                   
               
             
             
               
                 TransactionID 
                 ENCRYPT_MODE 
                 HOME_REGIncluded 
                 IMSI_T_SUPPORTED 
               
               
                 3G1XParameters 
                 ENC_SUPPORTEDIncluded 
                 HOME_REG 
                 RECONNECT_MSG_IND 
               
               
                 Signature 
                   
                   
                 Included 
               
               
                 SIDIncluded 
                 ENC_SUPPORTED 
                 FOR_SID_REGIncluded 
                 RECONNECT_MSG_IND 
               
               
                 SID 
                 SIG_ENCRYPT_SUP 
                 FOR_SID_REG 
                 RER_MODE_SUPPORTED 
               
               
                   
                 Included 
                   
                 Included 
               
               
                 NIDIncluded 
                 SIG_ENCRYPT_SUP 
                 FOR_NID_REGIncluded 
                 RER_MODE_SUPPORTED 
               
               
                   
                 MSG_INTEGRITY_SUP 
               
               
                   
                 Included 
               
               
                 NID 
                 MSG_INTEGRITY_SUP 
                 FOR_NID_REG 
                 TKZ_MODE_SUPPORTED 
               
               
                   
                   
                   
                 Included 
               
               
                 REG_ZONEIncluded 
                 SIG_INTEGRITY_SUP_INCLIncluded 
                 POWER_UP_REGIncluded 
                 TKZ_MODE_SUPPORTED 
               
               
                 REG_ZONE 
                 SIG_INTEGRITY_SUP_INCL 
                 POWER_UP_REG 
                 TKZ_IDIncluded 
               
               
                 TOTAL_ZONES 
                 SIG_INTEGRITY_SUPIncluded 
                 POWER_DOWN_REGIncluded 
                 TKZ_ID 
               
               
                 Included 
               
               
                 TOTAL_ZONES 
                 SIG_INTEGRITY_SUP 
                 PARAMETER_REGIncluded 
                 PILOT_REPORTIncluded 
               
               
                 ZONE_TIMERIncluded 
                 AUTHIncluded 
                 PARAMETER_REG 
                 PILOT_REPORT 
               
               
                 ZONE_TIMER 
                 AUTH 
                 REG_PRDIncluded 
                 SDB_SUPPORTEDIncluded 
               
               
                 PACKET_ZONE —   
                 MAX_NUM_ALT_SOIncluded 
                 REG_PRD 
                 SDB_SUPPORTED 
               
               
                 IDIncluded 
               
               
                 PACKET_ZONE_ID 
                 MAX_NUM_ALT_SO 
                 REG_DISTIncluded 
                 AUTO_FCSO_ALLOWED 
               
               
                   
                   
                   
                 Included 
               
               
                 PZIDHystParameters 
                 USE_SYNC_IDIncluded 
                 REG_DIST 
                 AUTO_FCSO_ALLOWED 
               
               
                 Included 
               
               
                 PZ_HYST_ENABLED 
                 USE_SYNC_ID 
                 PREF_MSID_TYPEIncluded 
                 SDB_IN_RCNM_IND 
               
               
                   
                   
                   
                 Included 
               
               
                 PZ_HYST_INFO_INCL 
                 MS_INIT_POS_LOC_SUP_INDIncluded 
                 PREF_MSID_TYPE 
                 SDB_IN_RCNM_IND 
               
               
                 PZ_HYST_LIST_LEN 
                 MS_INIT_POS_LOC_SUP_IND 
                 EXT_PREF_MSID_TYPEIncluded 
                 FPC_FCH_Included 
               
               
                 PZ_HYST_ACT_TIMER 
                 MOB_QOSIncluded 
                 EXT_PREF_MSID_TYPE 
                 FPC_FCH_INIT_SETPT_RC3 
               
               
                 PZ_HYST_TIMER_MUL 
                 MOB_QOS 
                 MEID_REQDIncluded 
                 FPC_FCH_INIT_SETPT_RC4 
               
               
                 PZ_HYST_TIMER_EXP 
                 BAND_CLASS_INFO_REQIncluded 
                 MEID_REQD 
                 FPC_FCH_INIT_SETPT_RC5 
               
               
                 P_REVIncluded 
                 BAND_CLASS_INFO_REQ 
                 MCCIncluded 
                 T_ADD_Included 
               
               
                 P_REV 
                 ALT_BAND_CLASSIncluded 
                 MCC 
                 T_ADD 
               
               
                 NEG_SLOT_CYCLE —   
                 ALT_BAND_CLASS 
                 IMSI_11_12Included 
                 PILOT_INC_Included 
               
               
                 INDEX —   
               
               
                 SUPIncluded 
               
               
                 NEG_SLOT_CYCLE —   
                 MAX_ADD_SERV_INSTANCEIncluded 
                 IMSI_11_12 
                 PILOT_INC 
               
               
                 INDEX_SUP 
               
               
                   
               
             
          
         
       
     
         [0085]    Step  755  preparing for using Circuit Switched services. In the case with parameters provided on the LTE broadcast channel it is assumed that the User Equipment  110  at the time of Circuit Switched fallback already has acquired these parameters so that they can be used by the User Equipment  110  when accessing the 1×RTT system to reduce the delay. 
         [0086]    The 1×RTT frequency band, 1×RTT carrier frequency tells the User Equipment  110  which frequency the User Equipment  110  should access. The PN offset tells the User Equipment  110  which cell on that frequency and together with the CDMA system time the User Equipment  110  would obtain downlink synchronization with the target cell. The 3G1× Parameters are needed for the User Equipment  110  to be able to start signaling towards the 1×RTT network. 
         [0087]    Step  765  once the User Equipment  110  has received and prepared the parameter setting it continues with switching from Packet Switched to Circuit Switched domain (i.e. leaving LTE). 
         [0088]    Step  770  the eNB then starts transmitting a Circuit Switched service 
         [0089]    Step  775  the User Equipment  110  is then ready to receiving a Circuit Switched service