Multicast transmission in a cellular network

Description of equivalent WO 2004028179 A1

MULTICAST TRANSMISSION IN A CELLULAR NETWORK

0002Field of the Invention

0003The invention relates generally to a multicast service in a telecommunications system. More specifically, the invention relates to a method and a system for performing multicast transmission in a cellular network, and to a mobile station for a cellular network.

0004Background of the Invention In the present-day telecommunication environments, so-called point- to-multipoint services, which allow data to be transmitted from a single source point to multiple endpoints, are commonly known. In addition to the fixed networks, these services are expected to be used extensively in mobile and wireless networks. This has been attended to in the standardization work of the 3rd

0005Generation Partnership Project (3GPP) by defining a service called the Multimedia Broadcast/Multicast Service (MBMS), which is a unidirectional point-to-multipoint bearer service in which data is transmitted from a single source entity to multiple recipients. The MBMS has two modes of operation: a broadcast mode and a multicast mode. When using MBMS, data is transmitted over a common radio channel in order to efficiently use radio/network resources. In the broadcast mode the data is transmitted to all users in one or more broadcast areas, whereas in the multicast mode the data is transmitted to a multicast group in a multicast area. The multicast group is a group of users who have activated the service in the multicast mode and are therefore capable of receiving the data.

0006In the on-going standardization process of studying the architecture of the Multimedia Broadcast/Multicast Service, one of the main problems is how to initiate the service efficiently, without wasting network resources. In the 3GPP specification TS 23.846, for example, it is required that "in the multicast mode, MBMS should support multicast resource allocation whereby data transmission to a multicast group is carried out in a certain cell only if multicast group members are to be found in that cell". However, a problem related to this is that the current cell of a mobile station that has joined a multicast group is not always known, i.e. if the mobile station is in a state where its current cell is unknown to the network.

0007The standardization process has brought about several proposals for initiating multicast data transfer and establishing a radio access bearer for the service. Some of the proposals suggest the use of traditional paging to find out the current cells of the mobile stations which have joined the multicast group. However, traditional paging mechanisms are not preferred in finding out the whereabouts of the mobile stations, since paging may cause a plurality of mobile stations to reply simultaneously or within a short period, which in turn may cause congestion on the uplink signaling channel. Version 1.1.1 of the above-mentioned specification TS 23.846 suggests that in a multicast case, the SGSN (Serving GPRS Support Node) sets up the radio resources on a Routing Area basis, i.e. for the Routing Areas where members of the multicast group have been detected. Thus in this case the mobile stations are not required to perform any uplink signaling in order to indicate their presence and/or willingness to receive the service, but the radio resources are set up immediately in the above-described manner. However, a major drawback of this method is that it wastes network resources, since the resources are set up for all RNCs (Radio Network Controllers) serving the Routing Area, even though there would be no MBMS capable users under the RNCs coverage area.

0008It is an objective of the invention to bring about a solution whereby it is possible to avoid the above-mentioned drawbacks.

0009Summary of the Invention The objective of the invention is to accomplish a mechanism whereby a multicast service can be provided in a cellular network so that the resources allocated for the service correspond to the real need in the network and so that excessive signaling on the uplink channels can be avoided in the start-up phase of the service. These and other objectives of the invention are accomplished in accordance with the principles of the present invention by providing a multicast transmission scheme which utilizes service notifications sent to the members of the multicast group in order to inform them of an upcoming multicast session. Upon receiving the service notification, a mobile station belonging to the multicast group determines the moment for sending a response to the notification. The response moments of the mobile stations can be spread over a certain period so that only one or a few of said mobile stations give an immediate response to the network. Radio resources are established in cells when the responses received meet desired criteria, and the service is delivered through the resources established. Thus one aspect of the invention is providing a method for performing multicast transmission in a cellular network which includes mobile stations, the method comprising the steps of

0010- transmitting a multicast service notification to a certain multicast group, said notification to inform of an upcoming multicast session, - receiving the multicast service notification in mobile stations belonging to said multicast group,

0011- in response to said receiving step, selecting a moment for a response to said multicast service notification in said mobile stations,

0012- sending a presence report from at least one of said mobile stations at the response moment of said at least one mobile station,

0013- receiving the presence report(s) in a radio access network,

0014- setting up radio resources for multicast transmission in a cell of the cellular network, when the presence report(s) received in said cell meet(s) predetermined criteria, and - transmitting the multicast data using the radio resources set up for the multicast transmission.

0015Another aspect of the invention is that of providing a system for performing multicast transmission in a cellular network, the system comprising

0016- a radio access network adapted to transmit a multicast service notification to mobile stations belonging to a multicast group,

0017- in said mobile stations, first means for selecting a moment for a response to said multicast service notification and for sending a presence report to the radio access network when the response moment arrives, whereby the radio access network is adapted to receive the presence reports, - second means for establishing radio resources for the multicast transmission in individual cells of the cellular network, the second means being adapted to establish the radio resources for a cell when the presence report(s) received in the cell meet(s) predetermined criteria.

0018In a further aspect the invention provides a mobile station for a cellular network, the mobile station comprising - reception means for receiving a multicast service notification informing of an upcoming multicast session,

0019- response means for selecting a moment for a response to said multicast service notification and for sending a presence report at the response moment, the presence report indicating the willingness of the mobile station to receive the multicast service, whereby the reception means are adapted to receive a notification informing of a presence report sent by another mobile station.

0020The invention provides a mechanism which enables the network to be aware of whether there are any recipients of a multicast service in a particular cell of a cellular network. The mechanism therefore also enables resource allocation only in cells containing a required amount of recipients for the service, i.e. allocation of resources can be avoided in cells with too few or no recipients at all. Furthermore, the mechanism according to the invention prevents the uplink signaling channels from becoming congested, even if a lot of mobile stations within the same cell are awaiting the same multicast service and many of these are in a state, such as an idle state, which normally requires signaling with the network prior to any other actions. Congestion can thereby be avoided, since a major number of the mobile stations can immediately change over to a reception mode when the service is about to start, without a need to signal in the uplink direction.

0021Other features and advantages of the invention will become apparent by reference to the following detailed description and accompanying drawings.

0022Brief Description of the Drawings

0023In the following, the invention and its preferred embodiments are described more closely with reference to the examples shown in FIG. 1 to 4 in the appended drawings, wherein: FIG. 1 illustrates network architecture to which the present invention can be applied,

0024FIG. 2 illustrates an example of message transfer between the elements of the system of FIG. 1,

0025FIG. 3 is a flow diagram illustrating the basic operation of a mobile station according to the invention, and FIG. 4 is a schematic presentation of a mobile station according to the invention.

0026Detailed Description of the Invention Below, the invention and its preferred embodiments are described using the terms and concepts commonly used in association with a UMTS environment. However, it is to be noted that the present invention is not tied to a particular technology, such as UMTS, but can be applied to any radio access network where multicast services are provided. FIG. 1 illustrates a general UMTS (Universal Mobile

0027Telecommunication System) architecture to which the present invention can be applied. As is commonly known, a UMTS network consists of three interacting domains: User Equipment (UE), Radio Access Network (RAN), and a Core Network (CN). In the figure, the Core Network is denoted by reference numeral 120, the Radio Access Network (such as UTRAN, Universal Mobile Telecommunication System Terrestial Radio Access Network) by reference numeral 110, and the User Equipment is shown as a plurality of mobile stations MS. In this context, the term "mobile station" refers to any terminal device (mobile equipment plus a subscriber identity module) which the user can control and which can communicate with the Radio Access Network. The mobile stations are connected via the Uu radio interface to node B elements 111 , which are the physical units for radio transmission/reception in the cellular network. Depending on the sectoring of its antennas, a node B can serve one or more cells. In addition to the node B elements, the Radio Access Network further comprises Radio Network Controllers (RNC) 112, each of which is connected through the lub interface to a set of node B elements. Each Radio Network Controller is responsible for the control of the radio resources within its domain, i.e. in the set of node B elements connected to it. The Radio Network Controllers 112 form the service access points for the services the RAN provides to the Core Network 120.

0028The Core Network is divided into circuit-switched and packet-switched domains, the former being responsible for the traditional circuit-switched services and the latter for packet-switched services. The circuit-switched domain is connected via a Mobile services Switching Centre (MSC) 121 and the packet-switched domain via a Serving GPRS Support Node (SGSN) 123 to the Radio Access Network.

0029The MSC comprises a Visitor Location Register (VLR), which is a database holding a copy of a visiting user's service profile and information on the location of the mobile station. The MSC VLR is connected through a Gateway MSC 122 to external circuit-switched networks 130, such as Public

0030Switched Telephone Networks (PSTNs).

0031The SGSN is connected to a Gateway GPRS Support Node (GGSN) 124, which connects the Core Network to external packet-switched networks 140, such as the Internet. The SGSN and the GGSN have functionalities similar to those of the MSCΛ LR and GMSC, respectively, but in relation to packet-switched services. Some network elements of the Core Network, such as the Home Location Register (HLR) 125, are shared by both domains.

0032A further network element belonging to the system of the invention is the source of the multicast service data to be transmitted to the mobile stations that have joined the multicast service group. The multicast data source can be the Broadcast Multicast - Service Centre (BM-SC) 150 or some other entity internal or external to the UMTS system. Internal data sources may provide their data directly, while data delivery from external sources is controlled by Border Gateways (BG), which may allow, for example, data from single addresses and ports to pass into the PLMN for delivery by an MBMS service. Depending on the security features implemented in it, the system according to the invention may further utilize the services of a Key Distribution Center (KDC) 160, as described later.

0033FIG. 2 is a diagram illustrating the procedure according to one embodiment of the present invention for initiating data transfer and RAB (Radio Access Bearer) establishment for the multicast service. It is assumed here that the two preceding phases related to a multicast service provision, i.e. the registration phase and the joining phase, have been performed in advance, and that the MBMS context has been stored in the network. In the registration phase, the users who possess the mobile stations agree with the service provider or operator on the conditions regarding the provision of the multicast service. In a subsequent joining phase the users indicate willingness or readiness to receive the multicast service. As a result of these two phases, an MBMS context is defined and stored in the network. The MBMS context here refers to an information entity including data required for the provision of the multicast service, such as address and subscriber information. An MBMS context can be service-specific or there can be a context for each subscriber of a service, for example. Below, the users who have performed the above two phases are termed members of the multicast group. When the service session is about to begin, the BM-SC first sends the

0034MBMS data, i.e. the actual service data, to at least one GGSN (step 210). Each receiving GGSN then sends the data to all the bearers (e.g. Gn/Gp GTP tunnels) which have been established between the GGSN and the relevant SGSNs for this multicast address (step 211). Thus in terms of the present invention, the GGSN only has routing functionality. The SGSNs buffer the multicast data.

0035In order to establish radio resources for the multicast transmission, each SGSN receiving the multicast data initiates an establishment process by sending an MBMS RAB assignment request (step 212) to the RNCs controlling the Routing Areas (RA) where there are mobile stations which have joined the multicast service group, i.e. which have joined the MBMS context. The MBMS RAB assignment request indicates the QoS parameters, for example. In the UMTS, a mobile station is tracked by the SGSN on the Routing Area level if no RRC connection has been established for the mobile, whereas the UTRAN tracks the mobile on the cell level when the RRC connection has been established.

0036In the basic embodiment of the invention, each of the RNCs receiving the MBMS RAB assignment request then sends an MBMS notification to all cells controlled by the RNC (step 213), notifying the mobile stations of incoming multicast data. A second embodiment, where the sending of the MBMS notification can be omitted in some of the cells, is discussed below after the basic embodiment has been examined.

0037The MBMS notification contains a membership query inquiring whether there are any recipients for the multicast data in the cell, i.e. any subscribers that have joined the multicast group and are thus members of the group.

0038When a mobile station receives this notification, it initiates a response process if it is a member of the group. In this process, the mobile station defines a moment for its response to the membership query. This is preferably implemented so that the mobile station sets a delay timer to a value which is specific to each of the mobile stations. In this case the above-mentioned membership query preferably includes a value for the maximum response delay and each mobile station selects a random value between zero and the said maximum response delay. In this way the response moments of the mobile stations are spread randomly over a period whose length corresponds to said maximum delay.

0039When the first timer expires and the relevant RNC receives the first response to the membership query (step 214), the said RNC can either initiate the setting up of radio resources immediately or it can wait for more responses to arrive. When the response(s) received within an individual cell meet(s) certain predetermined criteria, the RNC initiates the setting up of radio resources for that cell. As a result of this set-up procedure (step 215), appropriate radio bearers are established for the multicast service. Here the responses are also termed presence or membership reports, since they indicate that a member is present in the cell.

0040When the radio resources have been set up, the relevant RNCs send a radio bearer assignment notification to the mobile stations (step 216), informing them about the radio parameters for receiving the service. This sending takes place within the cells for which radio resources have been set up. If a mobile station belonging to the multicast group receives this notification before its delay timer expires, it stops the timer and cancels the transmission of the presence report in order to suppress the number of responses sent to the network. Thus a mobile station sends the presence report only if its delay timer expires before it receives the radio bearer assignment notification from the network or a report from another mobile station indicating that no further responses are needed.

0041In order to enable the mobile stations entering a cell to detect the upcoming multicast session, MBMS notification is preferably sent periodically until the maximum response time has expired. In the repeated MBMS notification, the value of the maximum response time is updated (i.e. decreased) accordingly.

0042In response to the setting up of the radio resources, the RNC also sends a reply (step 217) to the request received from the SGSN at step 212, the reply indicating the SGSN that the radio access bearer has established successfully. In response to said reply, the multicast data is transmitted from the SGSN to the mobile stations (step 218).

0043During the service session, the MBMS notification and the radio bearer assignment notification are preferably transmitted periodically in order for mobile stations which enter the cell to detect the on-going session. The two messages can be transmitted successively as separate messages, or they can be combined into a single message.

0044In the above-described scheme the moment for the response to the membership query is determined by setting a timer to a random value between zero and the maximum delay value. The idea here is to determine a member- specific response moment for each group member present in the cell. The response moment can be determined in many different ways. Instead of setting a timer to a random value, each mobile station can, for example, calculate its response moment by means of an algorithm having at least one input specific to each mobile station. Such a calculation can be based on the International Mobile Subscriber Identity (IMS!) and the current frame number, for example. As the majority of the mobile stations cancel the sending of the report before the response moment arrives, uplink congestion can be avoided.

0045The above-described scheme can be modified at various steps described above. As already mentioned briefly, according to one embodiment of the invention the sending of the MBMS notifications can be omitted for at least some of the cells. A prerequisite for this is that the network knows the current cells and states of at least some members of the multicast group so that it can immediately identify the cells containing enough members capable of receiving the service data. In this case the RNCs can communicate with the SGSN or another network element holding the Mobility Management (MM) context which includes the locations on a cell level. Alternatively, the SGSN can send this information in the RAB assignment request to the relevant RNCs. In UMTS, the serving RNCs perform the cell tracking, whereby they know upon receiving the RAB assignment request whether the sending of the MBMS notification can be omitted in any of the cells.

0046When the network knows that some of the cells contain enough multicast group members in the appropriate state(s) for receiving the service directly, a point-to-multipoint bearer can be set up for the service delivery and the process can jump from step 212 directly to step 214 with respect to these cells. Thus in this embodiment of the invention no membership queries are transmitted in these cells, but the radio bearer assignment notification is the first notification to the mobile stations indicating that a multicast service session is to be started. Thus the radio bearer assignment notification has a twofold meaning for the mobile stations. On the one hand, if there has been a prior multicast service notification, the mobile station stops the delay timer from running and cancels the sending of the membership report. On the other hand, a radio bearer assignment notification without a preceding multicast service notification indicates that the multicast transmission is about to begin, whereby no more signaling to the RAN is needed. Thus in the latter case a mobile station does not have to activate the transmitter part of the device.

0047The method for finding out the relevant RNCs and the relevant cells can vary depending on the nature of the service, such as the geographical coverage of the service, and on the contents of the MM and/or MBMS contexts stored in the network. In regard to service which is geographically wide, for example, the location of each subscriber can be stored in the MBMS context in connection with the joining phase, and the location can be tracked thereafter, whereby the locations of the subscribers, and also the relevant RNCs, are known when the service data arrives. Thus the method of the invention can include additional steps for updating the MM and/or MBMS contexts stored in the network, or the method can rely on the contexts already available in the network.

0048The terminating of the response procedure can also be triggered by a message other than the radio bearer assignment notification. For example, the RNCs can echo the first presence report to all mobile stations in the same cell so that they can terminate the response procedure. In some networks, it can also be possible for mobile stations to receive directly the presence report sent on the uplink by a mobile station in the same cell. A separate terminating message can also be sent to the mobile stations before the establishment of the radio resources. Furthermore, different threshold criteria can be used for the setting up of the radio resources. For example, the number of reports required per cell may change, or the criteria can depend on the subscribers in the cell.

0049The maximum value of the response time may also be known to the mobile station in advance, so that it does not have to be inserted in the membership query. However, if the value is inserted in the membership query, the operator can control the number of the mobile stations replying to the query. For example, if the maximum value is set to zero, all the mobile stations will automatically send a reply. The operator may wish to do this for charging purposes, for example. Not every step from 210 to 218 shown in connection with FIG. 2 is necessarily comprised of only one message, but can comprise a process which includes more than one message between the relevant parties.

0050FIG. 3 is a flow diagram illustrating the basic operation of a mobile station belonging to the multicast group. As mentioned above, the two messages giving prior indication of an upcoming multicast session are the MBMS notification and the radio bearer assignment notification, whereby the mobile station constantly monitors whether one of them has arrived (steps 300 and 302).

0051If the mobile station receives the MBMS notification, it initializes the MBMS response procedure (step 301), i.e. determines the moment for its membership report, and waits for a possible radio bearer assignment notification and for the response moment to arrive (steps 303 and 305). If the radio bearer assignment notification arrives before the response moment, the mobile station terminates the MBMS response procedure, i.e. cancels the sending of the membership report (step 304). If no radio bearer assignment notification arrives before the response moment, the mobile station sends its group membership report to the network (step 306) and waits for the radio bearer assignment notification to arrive (step 307). When this notification arrives, the mobile station changes to the multicast reception mode, where it listens to the MBMS data channel in order to receive the service data (step 308) transmitted on that channel.

0052If the mobile station receives the radio bearer assignment notification as the first indication of the upcoming multicast session (step 302/yes), it changes immediately to the multicast reception mode in order to receive the service data on the MBMS data channel.

0053Due to mobility, the number of users in a cell changes constantly. As a result of this, a cell may become void of group members. In order to prevent service transmission in such a cell, the radio access network may perform a membership query periodically during an on-going service session (e.g. when there are no members in the PMM-connected mode in the cell). An alternative solution to this is that the mobile stations currently receiving multicast session data must perform cell updates.

0054Furthermore, the mobile station which was the last to report its presence in a cell can be given the status of a flag-holder, so that if that mobile station moves out of the cell and reports this (e.g. by explicitly reporting a "cell leave" or by performing a cell update in a new cell), a check whether there are still other mobile stations remaining in the old cell can be initiated immediately. In an alternative embodiment, every mobile station reporting its presence can be made a flag-holder, and the total number of flag-holders must drop below a predetermined number before a new check is performed. In a further alternative embodiment, the flag-holder's "cell leave" is forwarded to the other members in the cell, thereby prompting them to report their status.

0055Due to delays in the network, it may happen that more than one mobile station sends its membership report before the RAN finds time to respond to the first report. In this case, multiple mobile stations can be set as "flag-holders", or just one of the said mobile stations is given this status.

0056In one preferred embodiment of the invention, the RAN verifies that the users in a cell are authorized to receive the service data, thereby preventing accidental or intentional misuse of the network resources. If this verification indicates that none of the members in. a cell is authorized to receive the service data, no network resources within that cell need to be allocated for service delivery.

0057There are several known authentication techniques that can be used at steps 213 and 214 above to check whether a mobile station is authorized to receive a particular multicast service. The user's authorization can be verified through group authentication, i.e. by finding out whether a message was sent by one of the members of an authorized group, or through a source authentication, i.e. by checking if a message was sent by a particular member of an authorized group. Alternative techniques that support both group authentication and source authentication are the use of message authentication codes and digital signatures. The latter technique can be further improved by the use of digital certificates. The use of message authentication codes is discussed first.

0058As is commonly known, a Message Authentication Code (MAC) can be included in a message to provide authenticity without secrecy. The MAC is a value derived from the message by a key-dependent one-way function (such as the widely used HMAC). Only the party possessing the key can create or verify the MAC.

0059As to the present invention, the key can be shared either by all mobile stations authorized to access a particular multicast group (group authentication), or a different key can be assigned to each authorized mobile station (source authentication). In either case, both the mobile station and the RNC must have the same MAC key. Consequently, there is a need for a Key Distribution Center (KDC) which creates the MAC keys and delivers them securely (with authentication and secrecy) to the authorized mobile stations. The RNC also needs to request the MAC keys from the KDC, unless the keys are configured in the RNC in advance.

0060Just like MACs, digital signatures are used for providing authenticity without secrecy. However, a different key is used for signing the messages and verifying the signatures. A digital signature mechanism is typically implemented using public-key cryptography, utilizing the widely used RSA algorithm, for example.

0061In the present invention, either a different signing key can be provided for each authorized mobile station (source authentication) or the same signing key can be shared by all authorized mobile stations (group authentication). Similarly as in the case of the MAC keys, a secure delivery mechanism is needed for delivering the signing keys from the KDC to the mobile stations, and the RNC needs to be provided with the signature verification keys.

0062A digital certificate contains a public key, signed by a trusted third party usually known as the Certificate Authority (CA). Other information such as the validity period of the certificate is normally included and also protected by the signature.

0063As mentioned above, digital certificates can be used to improve the digital signature method described above. The mobile station first obtains a signing key and a certificate containing the corresponding signature verification key from a KDC. The KDC can be a network element internal (cf. FIG. 1) or external to the UMTS system. For example, an external KDC could be located at the content provider sending the multicast service data via the Internet, while an internal KDC could be co-located with the BM-SC. If the KDC exists within a UMTS system, it can be implemented either as a separate entity or as an element co-located at another UMTS network element. Even the security functionality of an existing UMTS network element, such as SGSN, can possibly be re-used for MBMS key generation and distribution.

0064When the mobile station replies to a multicast session notification, it signs the response and includes with it the certificate containing the signature verification key. Using the certificates, there is no need for the RNC to communicate with a KDC to verify if a mobile station is authorized to access a particular multicast group.

0065In order to be able to verify the signatures of the mobile stations, the RNC has to be provided with a signature verification key of one or more trusted CAs. In case more than one CA is used, their signature keys can either be configured separately to the RNC or certificate hierarchy can be utilized. For certificate hierarchy, just a single signature verification key of a top-level CA needs to be configured to the RNC: This key is then used to authenticate the certificates of lower-level CAs, each containing the signature verification key of that CA. With the hierarchical certificates, a mobile station has to include the certificate of a lower-level CA with each membership report.

0066In order to protect the system against replay attacks, where an attacker records valid group membership query/response messages and re- sends them at a later time, these messages can include information revealing such an attack. A suitable information field can carry a random number, a time stamp, or some other "number used once" (i.e. nonce). For example, the RNC could include a nonce in the group membership query as a challenge to the mobile station. The mobile station would then have to include the same value of the nonce in its response, protected by a signature or a MAC.

0067FIG. 4 illustrates the insertion of the MBMS functionalities according to the invention into the architecture of a mobile station. These MBMS functionalities, such as notification control and determination of the response moment, are preferably inserted in the RLC (Radio Link Control) layer (layer 2) of the device. These functionalities communicate with layer 1 (physical layer) in order to set the transmitter and receiver parts of the device to an appropriate state. One of the advantages of the invention is that the mobile station can immediately move to a reception state upon detecting a notification indicating that another mobile has already replied to the membership query. Thus the transmitter part can be maintained in an idle mode, as no signaling in the uplink direction is needed. In this way both the power consumption of the mobile and the uplink traffic can be kept low.

0068Although the invention was described above with reference to the examples shown in the appended drawings, it is obvious that the invention is not limited to these, but may be modified by those skilled in the art without departing from the scope and spirit of the invention. For example, the mobile stations do not necessarily need to stop their timers when the radio allocation is detected, instead they can report their status at random moments so that the network can detect their presence without the necessity that all reports arrive at the same time.