Abstract:
An access node in the boundary area of a single frequency network service area transmits a boundary indication to the mobile terminals in the boundary area to notify the mobile terminals that they are approaching the boundaries of the single frequency network. The mobile terminals receiving the boundary indications may then take steps to maintain continuity of broadcast services when the mobile terminals leave the service area of the single frequency network.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates generally to broadcast and multicast services in mobile communication networks and, more particularly, to a method of maintaining continuity of a broadcast service when a mobile terminal leaves a single frequency network service area. 
       BACKGROUND 
       [0002]    The 3 rd  Generation Partnership Project (3GPP) includes a work item known as Long term Evolution (LTE) to improve the Universal Mobile Telecommunications System (UMTS) standard in response to increased demand for mobile data services. Multimedia Broadcast and Multicast service (MBMS) is one service offered in the UMTS standard. MBMS is a broadcast service similar to conventional television and radio broadcast systems in which the same content is transmitted to multiple users located in a specific service area, referred to herein as the broadcast service area. Mobile TV, a service that enables mobile terminals to receive ordinary television programs, is one example of a MBMS. 
         [0003]    Two different scenarios are contemplated for providing MBMS: single cell transmission and MBMS Single Frequency Network (MBSFN). With single cell transmission, the broadcast content is transmitted to the user from a single cell and a user may be handed over as it moves from one cell to another. In MBSFN, multiple transmitters in different cells transmit the same broadcast service synchronously using the same radio resources. There is no need for the network to handover the user as the user moves between cells within the MBSFN service area. Outside the MBSFN service area, the mobile terminal receives the broadcast service from a single cell. 
         [0004]    In general, MBSFN may be preferable in service areas where there are many users. MBSFN provides an efficient way of providing services to a large user population that is dispersed over a wide area. On the other hand, single cell transmission may be preferred in service areas where there are a small number of users or the user density is low. Consequently, a broadcast service area may include both MBSFN service areas and non-MBSFN cells. 
         [0005]    When a mobile terminal is operating within an MBSFN service area, the mobility of the mobile terminal will typically not be an issue because the broadcast service will be provided by all cells within the MBSFN service area. There is no need for the network to track the mobility of the mobile terminal in order to provide the broadcast service. However, when the mobile terminal moves from an MBSFN service area to a non-MBSFN cell, the broadcast service may be interrupted or lost unless the mobile terminal can receive the broadcast service in the non-MBSFN cell. If the mobile terminal is operating in a receive-only mode, the network may not know the exact location of the mobile terminal and thus may not be aware when the mobile terminal is approaching a boundary of the MBSFN service area. Therefore, some mechanism is needed in order to maintain continuity of broadcast services as the mobile terminal moves between an MBSFN service area and a non-MBSFN cell. 
       SUMMARY 
       [0006]    The present invention provides a method and apparatus for indicating to a mobile terminal when it is approaching a boundary of an MBSFN service area. According to one exemplary embodiment, border cells in an MBSFN service area transmit an explicit or implicit boundary indication to mobile terminals in the boundary region of the MBSFN service area. When a mobile station receives the boundary indication, it may perform a service continuity action in order to maintain continuity of the broadcast service as the mobile terminal moves from the MBSFN service area to a single cell transmission in a non-MBSFN cell. In one exemplary embodiment, the boundary indication comprises an explicit boundary indicator transmitted on a broadcast control channel in the boarder cells of the MBSFN service area. In another exemplary embodiment, the formatting of type of information transmitted on the broadcast control channel in the border cells serves as an implicit indication. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  shows an exemplary mobile communication network. 
           [0008]      FIG. 2  shows logical entities for transmitting broadcast services. 
           [0009]      FIG. 3  shows an exemplary broadcast service area. 
           [0010]      FIG. 4  shows an exemplary service area that provides service continuity. 
           [0011]      FIG. 5  shows an exemplary procedure for maintaining service continuity according to the present invention. 
           [0012]      FIG. 6  shows another exemplary procedure for maintaining service continuity according to the present invention. 
           [0013]      FIG. 7  shows an exemplary access node in communication with an exemplary mobile terminal. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    Referring now to the drawings,  FIG. 1  illustrates an exemplary mobile communication network indicated generally by the numeral  10 . The exemplary mobile communication network  10  is based on the Long-Term Evolution (LTE) architecture currently being developed by the 3 rd  Generation Partnership Project (3GPP). Those skilled in the art will appreciate, however, that the present invention is also applicable to other network standards, such as the Universal Mobile Telecommunications System (UMTS) Radio Access Network (UTRAN). 
         [0015]    Mobile communication network  10  comprises a radio access network (RAN)  12  and a core network  14 . RAN  12  comprises a single type of node referred to in the standard as an evolved node B (eNB). In this application, the eNB is referred to generically as an access node (AN)  16 . AN  16  is a base station that communicates over the air interface with mobile terminals  100  (MT)  100 . There are typically many ANs  16  in the RAN  12 . Each AN  16  serves one or more cells  18 . The cells  18  served by the AN  16  may use a single antenna site or multiple antenna sites. The antennas sites contain the radio equipment for communicating with the mobile terminals  100 . The control circuits for the AN  16  may be located at one of the antenna sites or may be remotely located from the antenna sites. 
         [0016]    The main elements of the core network  14  include the serving gateway (SGW)  20 , mobility management entity (MME)  22 , and packet data network gateway (PDN GW)  24 . The ANs  16  connect to an SGW  20 . The SGW  20  routes and forwards user data packets and acts as a mobility anchor for the mobile terminal  100  during handovers. For an idle state mobile terminal  100 , the SGW  20  terminates the downlink data path and triggers paging when data arrives for the mobile terminal  100 . The MME  22  is the entity responsible for mobility management. The MME  22  tracks the location of the mobile terminal  100  and pages the mobile terminal  100  when data arrives for an idle mode mobile terminal  100 . The PDN GW  24  provides connection to external packet data networks (PDNs)  30 . A mobile terminal  100  may simultaneously connect with more than one PDN GW  24  to access multiple PDNs  30 . The PDN GW  24  is responsible for policy enforcement and charging. 
         [0017]    The mobile communication network  10  supports Multimedia Broadcasts and Multicast Services (MBMS). MBMS enables multimedia content, such as television programs, movies, and other audio or video programs, to be transmitted to a mobile terminal  100  over the mobile communication network  10 .  FIG. 2  illustrates the logical entities involved in transmitting broadcast services to a mobile terminal  100 . The multimedia content originates at a Broadcast Multicast Source (BMSC)  32 . The BMSC  32  is responsible for scheduling and announcing broadcast services and is the data source for the broadcast service. The MBMS Gateway (MBMS GW)  34  is a logical entity in the core network  14  that forwards broadcast packets to each AN  16  transmitting the broadcast service. The MBMS GW  32  may use the IP multicast to forward the broadcast packets to the AN  16 . The MCE  36  is a logical entity that allocates radio resources used by the AN  16  to transmit broadcast services. The MBMS Coordination Entity (MCE)  36  may also determine the modulation and coding schemes used for the broadcast service. 
         [0018]    The transmission of broadcast services from the ANs  16  to the mobile terminal  100  may be by single cell transmission or MBSFN. In single cell transmission, the broadcast service is transmitted independently in each cell and there is no combining of transmissions from multiple cells by the mobile terminal  100 . Scheduling is handled by the AN  16  responsible for the cell. MBSFN is characterized by coordinated transmission of the broadcast service from multiple transmitters within a common service area. A broadcast service area may comprise a mixture of MBSFN service areas and single cell service areas. 
         [0019]      FIG. 3  illustrates a broadcast service area  50  comprising an MBSFN service area  52  and a plurality of individual cells  18  outside the MBSFN service area  52 . The MBSFN service area  52  comprises multiple cells  18  shown with diagonal fill whose transmissions are coordinated by the MCE  36 . Outside the MBSFN service area  52 , each individual cell  18  provides only single cell transmission of the broadcast service. Typically, a control channel for MBMS, referred to as the MBMS control channel (MCCH), carries information that indicates, among other things, whether the cell  18  is part of the MBSFN and whether the MBMS is transmitted as a single-cell transmission. The MCCH may also be used to advertise broadcast services that are available in the cell  18 . 
         [0020]    When the mobile terminal  100  is within the MBSFN service area  52 , each of the cells  18  in the MBSFN service area  52  transmits the same service synchronously using the same radio resources. Because the same broadcast service is being transmitted in the cells in the MBSFN service area  52 , mobility is not an issue. The mobile terminal  100  does not need to take any action to maintain service as long as it remains within the MBSFN service area  52 . However, as the mobile terminal  100  moves beyond the boundary of the MBSFN service area  52 , the mobile terminal  100  will need to take action in order to maintain service continuity. For example, the mobile terminal  100  may establish a single cell transmission of the broadcast service in a specified target cell  18  outside of the MBSFN service area  52  as the mobile terminal  100  moves from the MBSFN service area  52  toward the target cell. However, the mobile terminal  100  may not have any knowledge of the boundaries of the MBSFN service area  52 . 
         [0021]    According to exemplary embodiments of the present invention, a mobile terminal  100  is notified when the mobile terminal  100  is approaching a boundary of an MBSFN service area  52 . The notification may be explicit or implicit. Upon receipt of such notification, the mobile terminal  100  may take action to maintain continuity of the broadcast service. In one exemplary embodiment, the mobile terminal  100  may immediately establish a single cell transmission of the broadcast service by a target cell, which may be received by the mobile terminal  100  in parallel with the MBSFN transmission in the MBSFN service area  52 . In other embodiments, the mobile terminal  100  may initiate quality measurements of the received signal in the MBSFN and trigger a single cell transmission only when the quality measurements indicate a need to do so. This approach would be more efficient from a resource utilization standpoint. 
         [0022]      FIG. 4  illustrates one exemplary method of providing notification to mobile terminal  100  when the mobile terminal  100  approaches a boundary of the MBSFN service area  52 . The broadcast service area  50  comprises an MBSFN service area  52  and a plurality of individual cells  18  outside the MBSFN service area  52 . The MBSFN service area  52  is divided into an inner region  54  and a boundary region  56  as shown by the dotted line in  FIG. 4 . Cells  18  located in the inner region  54  are shown with diagonal fill. The broadcast service is transmitted to the mobile terminal  100  by transmitters (e.g., base stations) located in both the interior region  54  and boundary region  56  of the MBSFN service area  52 . In one exemplary embodiment, a new broadcast information element, referred to herein as a boundary indicator, is introduced to allow the mobile terminal  100  to determine the region it is currently located in. The boundary indicator is an explicit indicator that is transmitted to indicate whether the transmitter is within a boundary region  56 . In one embodiment, the boundary indicator may comprise a single bit that is set to a first value (e.g., “1”) when the transmitter is located in a boundary region  56 , and is set to a second value (e.g., “0”) when the transmitter is located in an interior region  54 . The mobile terminal  100  monitors the boundary indicator transmitted in the MBSFN service area  52  to determine whether the mobile terminal  100  is approaching the boundary of the MBSFN service area  52 . 
         [0023]    In other embodiments, an implicit notification can be provided to the mobile terminal  100  to indicate when it is in a border region. As discussed above, control information for the MBMS is broadcast to the mobile terminals  100  on the MCCH. In one embodiment, a mobile terminal could differentiate a boundary region  56  from an interior region  54  by the formatting or the type of information that is broadcast on the MCCH. For example, a transmitter in the interior region  54  in the MBSFN service area  52  could advertise the availability of MBMSFN transmission, while the transmitters in the boundary region  56  contribute to the MBSFN transmission without advertising the availability of the MBSFN transmission. When the mobile terminal  100  enters the boundary region  56 , it may assume based on the lack of advertising that MBSFN transmission is no longer available and begin searching for a cell that offers the broadcast service as a single-cell transmission. However, the search for a single-cell transmission would not preclude the mobile terminal from continuing to receive the MBSFN transmission. 
         [0024]    The mobile terminal  100  may operate in a “reception only” mode (e.g., RRC_IDLE state) when the mobile terminal  100  is receiving the broadcast service in the MBSFN service area  52 , although it may be engaged in other concurrent communications. In the “reception only” mode, the mobile terminal  100  receives the broadcast service and may perform basic measurements on the signal quality from neighboring cells. Because no uplink is needed in the “reception only” mode, the mobile terminal  100  does not send the measurement reports to the network. As the mobile terminal  100  moves toward the boundary of the MBSFN service area  52 , it will first enter the boundary region  56 . The boundary indicators transmitted in the MBSFN service area notify the mobile terminal  100  when it has entered the boundary region  56  of the MBSFN service area  52 . The mobile terminal  100  may then transition from a “reception only” state to an “active” state (e.g., RRC_Connected state) and take appropriate action to maintain service continuity. 
         [0025]      FIG. 5  illustrates an exemplary procedure  200  implemented by a mobile terminal  100  for maintaining service continuity after detection of the boundary indicator. The procedure shown in  FIG. 5  begins with the mobile terminal  100  receiving the broadcast service. The mobile terminal  100  may be in a “reception only” state, but could also be engaged in other concurrent communications. The mobile terminal  100  monitors the transmission within the MBSFN service area  52  to detect the boundary indicator (block  202 ). When the boundary indicator is detected, the mobile terminal  100  transitions to an active state if it is not already in the active state (block  204 ). Once in the active state, the mobile terminal  100  may initiate a single cell transmission in a target cell  18  that borders the MBSFN service area  52  (block  206 ). The target cell  18  may be selected based on signal quality measurements made by the mobile terminal  100 . The single cell transmission may be received in parallel with the MBSFN transmission in the MBSFN service area  52  (block  208 ). Thus, the mobile terminal  100  will not lose service as the mobile terminal  100  moves from the MBSFN service area  52  into the target cell  18 . 
         [0026]      FIG. 6  illustrates an alternate procedure  250  that may be implemented by the mobile terminal  100  to maintain service continuity. The procedure begins with the mobile terminal  100  in a “reception only” mode. The mobile terminal  100  monitors the transmissions in the MBSFN service area  52  to determine whether it is within a boundary region  56  (block  252 ). When the mobile terminal  100  detects that it is within a boundary region  56 , the mobile terminal  100  transitions to an active state if it is not already in the active state (block  254 ). In the active state, the mobile terminal  100  may perform quality measurements of the received signal in the MBSFN service area  52  (block  256 ). The mobile terminal  100  may then determine when to hand over to a single cell transmission based on the quality measurements (block  158 ). When the handover decision is made, the mobile terminal  100  may initiate single cell transmission in a selected target cell  18 . Alternatively, the mobile terminal  100  may send measurement reports to the network and the handover decision may be made in the network. The network would then send a handover command to the mobile terminal  100  to initiate transition to a single cell transmission. One disadvantage of this approach is that signaling is required between the mobile terminal  100  and the network. 
         [0027]    An implicit notification that the mobile terminal has left the MBSFN service area  52  may occur when the mobile terminal recognizes that the MBSFN transmission is neither transmitted nor announced in the cell  18 . In this case, the mobile terminal  100  may initiate the MBMS service as a single cell transmission while still receiving the MBSFN transmission from MBSFN service area  52 . This may cause the mobile terminal to transition from “reception only” to “active state” upon leaving area  56  and  52  and entering a single cell  18  outside the MBSFN service area  52 . 
         [0028]      FIG. 7  illustrates an exemplary access node  16  and mobile terminal  100 . The access node  16  includes a control unit  16   a  and radio equipment  16   b ,  16   c  for communicating with mobile terminals  100 . The radio equipment, which may be located at an antenna site, includes reception equipment  16 b and transmission equipment  16   c  for communicating over the air interface with mobile terminals. The control unit  16   a  includes logic for performing the functions of the AN  16 , such as radio resource control and scheduling. The mobile terminal  100  comprises a radio transceiver  102  for communicating over the air interface with the AN  16  and a control unit  104  to control the operation of the mobile terminal  100  as hereinabove described. 
         [0029]    The present invention provides a method to maintain service continuity as the mobile terminal  100  moves from an MBSFN service area  52  into single cell transmission service area. By transmitting a boundary indicator in the boundary region  56  of the MBSFN service area  52 , the interruption of broadcast service during handover for MBSFN to single cell transmission is prevented. In some embodiments, the transition from MBSFN to single cell transmission is made without the need for an uplink channel or mobile terminal  100  feedback. 
         [0030]    The present invention may, of course, be carried out in other ways than those specifically set forth herein without departing from essential characteristics of the invention. The present embodiments are to be considered in all respects as illustrative and not restrictive, and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.