Patent Publication Number: US-2005143107-A1

Title: Method and apparatus for notifying unavailability of broadcast/multicast services

Description:
CROSS REFERENCE TO RELATED APPLICATION  
      This application claims the benefit of U.S. Provisional Application No. 60/525,966 filed Dec. 1, 2003, which is incorporated by reference as if fully set forth. 
    
    
     FIELD OF INVENTION  
      The present invention is related to a wireless communication system. More particularly, the present invention is related to a method for notifying a broadcast/multicast control (BMC) entity of service unavailability in a wireless transmit/receive unit (WTRU).  
     BACKGROUND  
      Broadcast/multicast control (BMC) is a sub-layer of Layer 2 (L2) in a user-plane of the universal mobile telecommunication system (UMTS) protocol stack. The BMC provides services necessary to deliver and receive cell broadcast messages. A user subscribes to certain types of broadcast or multicast services and selectively receives particular broadcast or multicast messages in accordance with a broadcast/multicast service (BMS) schedule.  
      As a WTRU moves across the boundary of a cell, the WTRU performs a cell selection and reselection process. During the cell selection and reselection process, the WTRU may enter an area where cell broadcast services (CBS) are not available or disrupted. Currently, there is no mechanism for the user to know that the CBS is currently not available.  
      In addition, due to a cell change, the discontinuous reception (DRX) schedule for the common traffic channel (CTCH) that was configured in the previous cell becomes invalid in the current cell. At this stage, the UMTS physical layer (PHY) has to be reconfigured in the new cell with a new DRX schedule by the BMC via radio resource control (RRC). Currently, there is no mechanism for BMC to know that the currently configured DRX schedule is invalid due to cell reselection. Until a valid new schedule is received, the PHY is unable to receive the BMS properly.  
       FIG. 1  is a signal flow diagram of a BMC process  100  in accordance with a prior art WTRU  150 , including non-access stratum (NAS), radio resource control (RRC), BMC, radio link control (RLC) and PHY entity. When a user activates BMS, an activation request is sent from the NAS to the BMC (step  102 ), which provides services to the NAS. If it is the first time the user activates the BMS, the BMC indicates to the RRC by transmitting a Start_CB message that cell broadcast (CB) message reception should be started (step  104 ). The RRC receives system information and configures the PHY in accordance with the CB configuration (step  106 ). The PHY receives all BMS messages over radio interface, (not shown), including a BMS schedule message (step  108 ) and forwards the BMS messages to the RLC, which in turn forwards them to the BMC (step  110 ).  
      The BMS schedule message informs which CB messages will be sent in the next DRX schedule. The BMC retrieves the schedule for BMS for the user and transmits the Start_DRX message to the RRC to indicate the BMC messages that should be received by the WTRU (step  112 ). The RRC then configures the PHY at which time intervals the PHY should receive messages (step  114 ). As a consequence, only the CB messages of interest are received by the PHY and delivered to the BMC and to the user (steps  116 ,  118 ,  120 ).  
      When a user moves across the cell, a cell reselection procedure is initiated (step  122 ). If CBS is not supported or disrupted in the new cell, the user is not able to receive BMS properly. The user does not have any information regarding the non-availability of CBS in the new cell. Even if the new cell supports the CBS and a new common traffic channel (CTCH) is acquired, the PHY does not have the DRX schedule required to select appropriate CB messages. The BMC does not have any way to know that cell reselection has occurred. The PHY is not able to pick up any relevant block sets due to a lack of an updated and appropriate DRX schedule. If the PHY continues to apply the DRX schedule of the old cell in the new cell, then due to the possibility of missing the reception of the schedule message, the BMC may never synchronize to receive the relevant messages.  
     SUMMARY  
      The present invention is related to a method for notifying a BMC of service unavailability. When a user (i.e. a WTRU) activates BMS, the WTRU receives BMS messages in accordance with a BMS schedule message. An RRC constantly monitors whether BMS is available in a cell. If the RRC detects that the BMS service is not available, the RRC sends a message to the BMC to inform the BMC of the unavailability of the BMS.  
      In accordance with the present invention, BMC operations are more predictable, robust and dependable. The WTRU can be provided with timely indication of CBS unavailability. The potential deadlock situation that could occur between the PHY not receiving updated DRX schedules after cell reselection and the BMC not knowing that such cell reselection has occurred can be avoided and normal operations can occur smoothly without any confusion. The present invention may be applied to different wireless communication systems, such as UMTS and GSM. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a signal flow diagram of BMS operation in accordance with the prior art.  
       FIG. 2  is a signal flow diagram of BMS operation between entities comprising a WTRU in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      Hereafter, the terminology “WTRU” includes but is not limited to a user equipment, a mobile station, a fixed or mobile subscriber unit, a pager, or any other type of device capable of operating in a wireless environment. When referred to hereafter, the terminology “base station” includes but is not limited to a Node-B, a site controller, an access point or any other type of interfacing device in a wireless environment.  
      The present invention provides a mechanism for the BMC to be notified of cell reselection, service unavailability, service resumption, and reconfiguration of the PHY with a new DRX schedule after cell reselection. The present invention also provides a mechanism to inform the NAS of service disruption and resumption.  
       FIG. 2  is a signal flow diagram of a BMS process  200  between entities in a WTRU  250  in accordance with one embodiment of the present invention. The WTRU  250  comprises a PHY  252 , an RLC  254 , a BMC  256 , an RRC  258 , and an NAS  260 . Although these entities are well known to those of skilled in the art, the following provides a brief overview of the function of each entity. The PHY  252  interfaces with the hardware and radio transmitter and receiver of the WRTU  250  to configure, receive and transmit data.  
      The RLC  254  provides the link layer functionality by supporting transparent, unacknowledged and acknowledged data transfer. The RLC  254  interfaces with the media access control (MAC) protocol layer, which interfaces with the PHY  252  to provide transport channels for data transport to the RLC  254 .  
      The BMC  256  is the control entity governing the reception of broadcast and multicast messages. It keeps track of the message identifiers of the BMC messages that the user is interested in receiving; or has subscribed to receive. It also generates a DRX schedule to configure the PHY  252  to receive only certain CTCH block sets which contain the messages in which the user is interested. Thus, by employing discontinuous reception, the BMC  256  helps in power conservation.  
      The RRC  258  is the radio resource control protocol layer, which manages the radio resources and configures other layers. It also performs measurements and receives the system information and paging information.  
      The NAS  260  is the non-access stratum, which keeps track of the mobility and session management. It also configures the BMC  256  and interfaces with the BMC  256  to receive the broadcast messages.  
      The interplay between all of the elements  252 - 260  of the WTRU  250  will be explained hereinafter with reference to the signal flow diagram of  FIG. 2  for the BMS process  200 . When a user activates BMS, an activation request is sent from the NAS  260  to the BMC  256  (step  202 ). If it is the first time the user activates the BMS, the BMC  256  indicates to the RRC  258  by transmitting a Start_CB message that cell broadcast (CB) message reception should be started (step  204 ). The RRC  258  configures the PHY  252  in accordance with the CB configuration (step  206 ). The PHY  252  receives all BMS messages, including a BMC schedule message from a base station via a CTCH (step  207 ) in accordance with the configuration specified by the RRC  258 , and forwards all the CTCH block sets containing the BMC messages to the RLC  254  (step  208 ), which in turn forwards them to the BMC  256  (step  210 ).  
      The BMC schedule message informs the BMC  256  which CB messages will be sent when in the next DRX schedule. The BMC  256  retrieves the schedule for BMS for the user and transmits a Start_DRX message to the RRC  258  to indicate the BMS messages that should be received by the WTRU  250  by providing a set of CTCH block set indexes (step  212 ). The RRC  258  then configures the PHY  252  at which time intervals the PHY  252  should receive messages (step  214 ). As a consequence, only the CB messages of interest are received by the PHY  252  and delivered to the BMC  256  and to the user as will be explained hereinafter.  
      After the PHY  252  receives the DRX schedule, the PHY  252  selectively receives the CTCH block sets by turning on the receiver only for the scheduled block sets and forwards them to the RLC  254  as indicated in step  216 . The RLC  254  then forwards the received information to the BMC  256  (step  218 ). If the received message contains a cell broadcast message (CBS), the BMC  256  then forwards the message to the NAS  260  (step  220 ). If the received message contains a schedule message then the BMC  256  generates a schedule message for the next schedule period and sends a Start_DRX message to the RRC  258  (step  212 ).  
      When a user moves across the cell, a cell reselection procedure is initiated (step  222 ). The RRC  258  constantly monitors messages broadcast from nearby cells, for example, via a broadcast control channel (BCCH). The RRC  258  monitors the BCCH of the cell to receive the system information. The system information Type 5 and 6 contains the information indicating the availability of CTCH and CBS related information in the current cell. System information Type 5 message contains a Boolean value indicating the availability of CTCH in the current cell and contains CBS related information if CTCH is available in the cell. If the RRC  258  fails to receive any messages from the new cell via the BCCH, the RRC  258  recognizes that CBS is not available in the new cell.  
      The present invention introduces a new signal, (CBMC_CBS_UNAVAILABLE), from the RRC  258  to the BMC  256  to indicate the unavailability or disruption of CBS. If the RRC  258  detects the unavailability of the BMS, the RRC  258  transmits a message CBMC_CBS_UNAVAILABLE to the BMC  256  to inform the BMC  256  of the disruption or unavailability of BMS (step  224 ). Other notifications may optionally be transmitted from the RRC  258  to the BMC  256  that could be conveyed to the NAS  260  including, but not limited to, the addition of enumerations or cause types to the existing signals. The disruption or unavailability of CBS service can be indicated by including a cause type in the CBMS_CBS_UNAVAILABLE message or by sending a separate message carrying the cause type. The disruption or unavailability can be due to many reasons, such as, but not limited to, the network not supporting it in current cell, the reception of CTCH being very poor, the WRTU not having enough resources, such as processing bandwidth or memory to currently handle CBS. Each of these causes can be enumerated and the enumeration identifier can be used to convey the cause of disruption.  
      Once the BMC  256  is notified of the unavailability of BMS, the BMC  256  sends a message to the NAS  260  to inform the NAS  260  of the unavailability of BMS (step  226 ). The BMC  256  sends a BMC_ERROR_IND to indicate the existence of an error to the NAS  260 . The BMC  256  preferably includes a cause of the error, (CBS_UNAVAILABLE), in the BMC_ERROR_IND to indicate a disruption or unavailability of BMS as noted hereinbefore.  
      In an alternative embodiment, the RRC  258  may directly inform the NAS  260  of the disruption of BMS, however, it is preferable for the BMC  256  to inform the NAS  260  since the BMC  256  has an existing interface (BMC_ERROR_IND) defined to convey error information to the NAS  260 . The NAS  260 , in turn, notifies the user of the unavailability of BMS (step  228 ).  
      The BMC  256  can utilize the information of service unavailability to reset to a state from where it can commence generation of the new DRX schedule on reception of a schedule message when BMS resumes. It can release the resources (processing and memory) and stop verification of the received messages with the configured DRX schedule to enter a dormant state till the BMS is available again to increase efficiency and reduce battery power consumption.  
      As the WTRU  250  further moves to another cell, another cell reselection procedure is initiated (step  230 ). If the RRC  258  detects that BMS is available in the new cell, the RRC  258  retrieves system information and configures the PHY  252  in accordance with the CB configuration (step  232 ). Alternatively, it can convey the availability of the service to the BMC  256  which can wake up from its dormant state and send a Start_CB message to the RRC  258  to configure the PHY  252  to resume CTCH reception. The PHY  252  receives all BMS messages, including a BMC schedule message, and forwards the BMS schedule message to the RLC  254  (step  234 ), and normal operation for BMS is resumed.  
      The current mechanism for starting reception of cell broadcast messages by the PHY  252  is by the RRC  258  instructing the PHY  252  when the RRC  258  receives a CBMC_RX_IND from the BMC  256  with START_CB. CBMC_RX_IND is an interface primitive defined in the 3GPP specifications for the BMC  256 -RRC  258  interface. CBMC_RX_IND contains the instruction for the RRC  258  to initiate CB reception (Start_CB), to stop CB reception (Stop_CB), to initiate discontinuous reception (Start_DRX), or to stop discontinuous reception (Stop_DRX).  
      The RRC  258  forwards these instructions to the PHY  252  to act upon. CBMC_RX_IND may also contain the DRX schedule when Start_DRX instruction is issued. The PHY  252  applies the DRX schedule and receives only the indicated CTCH block sets. In a single cell scenario, (i.e., when the WRTU does not change a cell), the RRC  258  configures the PHY  252  to begin receiving all the CTCH block sets on reception of the Start_CB message from the BMC  256 . When the BMC  256  is ready with a DRX schedule for the next scheduling period, it sends the DRX schedule, indicating the CTCH block sets to be received, in CBMC_RX_IND primitive with Start_DRX to the RRC  258 . The RRC  258  forwards this to the PHY  252  to act upon.  
      When the BMC  256  derives a new DRX schedule, it sends it again in the CBMC_RX_IND to the RRC  258 . If a user alters a subscription or a misalignment of actually received and configured CTCH block sets is observed, the BMC  256  may send CBMC_RX_IND with Stop_DRX to the RRC  258 . The RRC  258  forwards it to the PHY  252  which begins receiving and forwarding all the CTCH block sets. To terminate cell broadcast reception, the BMC  256  can send CBMC_RX_IND to the RRC  258  with Stop_CB. When the WRTU changes a cell, the BMC  256  is not informed of the cell reselection and hence the problems of deadlock may occur.  
      In accordance with the present invention, as the BMC  256  has no knowledge of cell reselection, the RRC  258  maintains the state of BMS reception, (START_CB/STOP_CB sent by the BMC  256 ), and configures the PHY  252  with the same state after cell reselection. The state of CBS reception is configured by the BMC  256  by CBMC_RX_IND message with START_CB or STOP_CB as its parameters.  
      The RRC  258  maintains the last known CBS reception state until the WTRU  250  moves into a new cell where CBS is available. The RRC  258  recognizes the availability of CBS, and sends the CBS state information, START_CB or STOP_CB to the PHY  252  after cell reselection. The physical layer starts picking up all the block sets and forwarding them to the RLC  254 , which sends it to the BMC  256 . The BMC  256 , in turn, picks up the next schedule message, rebuilds the DRX selection list and passes it to the PHY  252  via the RRC  258 . This restores the normal BMC operation.  
      Although the features and elements of the present invention are described in the preferred embodiments in particular combinations, each feature or element can be used alone without the other features and elements of the preferred embodiments or in various combinations with or without other features and elements of the present invention.