Mobile telecommunications system with soft handover from a GSM network to a UMTS network

Abstract: A method of processing a handover request from a Base Station Controller (BSC) of a Global System for Mobile Communication (GSM) -type network. The method includes the steps of passing a handover request with GSM-type parameters from a Base Station Controller (BSC), through a Master Switching Centre (MSC) of the GSM-type network, to a Radio Network Controller (RNC) of a Universal Mobile Telecommunications System (UMTS) network and to a UMTS Core Network (CN), translating the GSM-type parameters to UMTS Terrestrial Radio Access Network (UTRAN) parameters in the RNC, and allocating UTRAN resources in response to the translated parameters.

Description: Background of the Invention The present invention relates to a method of performing soft handovers from a GSM-type network to a UMTS-type network, and to a corresponding Radio Network Controller. It has been generally proposed that it would be desirable to provide handover between a GSM network and a UMTS network. However, this is not a straightforward task to accomplish and there are a number of problems to implement before this can be successfully achieved; the present invention is concerned with these problems. It is, however, more generally applicable to interoperability between networks using different protocols; in such a case, the terms used herein are to be construed as applying to the relevant equivalent components of the networks to which the invention is applied. Although aspects of the invention provide independent solutions to various aspects of the problems concerned with GSM to UMTS handover, all are concerned with this common problem and in particular are concerned with a particular novel solution in which an RNC of the UMTS network controls handover to a large extent. EP-A-0 898 438 describes a method for performing handover, eg. of a dual-mode terminal which can serve either as a GSM terminal or as a UMTS terminal, wherein a list of candidate base stations is transmitted to the terminal prior to handover. The invention is described in the context of GSM and UMTS networks for ease of understanding but is applicable to handover between other networks with similar characteristics. Accordingly, all references to GSM and UMTS and all terms of art used herein are to be construed as encompassing equivalent features of other networks. In particular, the present Japanese PDC network is of similar architecture to GSM and the term GSM (or GSM-type) as used in the specification and claims is intended to encompass such a network. Similarly, the term UMTS (and related terms) is intended to encompass a UMTS system in accordance with present standards and proposals and any derivatives or equivalents thereof. A method of processing a handover request from a base station controller (BSC) of a GSM network may include the steps of: passing a handover request with GSM parameters from a base station controller (BSC) through a Master Switching Centre (MSC) of the GSM network to a UMTS core network (CN) and to a Radio Network Controller (RNC) of the UMTS network; in the radio network controller, translating the GSM parameters to UTRAN parameters; and allocating UTRAN resources in response to the translated parameters. Thus, rather than translating the GSM parameters on entry to the UMTS network, the GSM parameters pass transparently through at least a portion of the UMTS network to the RNC. The parameters may include one or more of data rate, call type (voice, data, fax, other, e.g. video, IP), Quality of Service etc. This reduces processing load elsewhere in the network and enables the RNC to allocate the optimum UTRAN resources for the call. The RNC may be arranged to switch directly from a GSM connection to a soft handover UMTS mode. This is advantageous compared to simply establishing an individual connection from the User Equipment (UE) to a single UMTS access node. This feature may be provided in a method of switching from a GSM network to a UMTS network that includes switching directly from a mode in which User Equipment (UE) is in communication with a GSM base station to a UMTS diversity mode in which the User Equipment is in communication with a plurality of UMTS access nodes, and may include configuring the UTRAN so that a plurality of access nodes are prepared to communicate with the UE and/or supplying a list of potential access nodes to the UE. The Radio Network Controller (RNC) may be arranged, following translation of the GSM format handover request firstly to perform radio link setup and then, following a response from the radio link, to perform data transport setup for communication between the Radio Network Controller and the Core Network. This allows communication with the Core Network to be established following successful radio setup rather than independently, thus avoiding unnecessary use of CN to RNC (Iu interface) resources if radio link setup is not possible. This feature may be provided in a method of establishing UMTS communication with User Equipment following receipt by a Radio Network Controller of a GSM handover request, the method including, in the Radio Network Controller: setting up a radio link to a Node B and, following acknowledgement of the radio link setup, in the Radio Network Controller, setting up data transport between the Radio Network Controller and the Core Network. The Radio Network Controller may be arranged (preferably following radio link setup) to pass a handover message containing UTRAN parameters through the GSM network (for example as a message), preferably through the Core Network to the GSM Master Switching Centre and preferably at least through the Base Station Controller of the GSM network, to the User Equipment; the method further including: interpreting the UTRAN parameters in the User Equipment and, in response to the parameters, initiating communication between the User Equipment and the UMTS network. This may be provided in a method of establishing UMTS communication between User Equipment and a UMTS network, wherein the User Equipment is in communication with a GSM network, the method including forwarding UTRAN parameter information, preferably including a list of potential UMTS Access Nodes via the GSM network, preferably from a Radio Network Controller of the UMTS network, to the User Equipment and, in the User Equipment, interpreting the UTRAN parameter information and initiating communication with the UMTS network. Thus, in contrast to the initial step, where GSM information tunnels into the UMTS network and is interpreted in the RNC, the UTRAN information tunnels through the GSM network without interpretation and is interpreted by the UE. This means that the GSM network need not include intelligence to process the information, simplifying interconnection with existing GSM networks. Following initiation of communication, the UE may be arranged to establish a link through the RNC of the UMTS network to the MSC of the GSM network (where the GSM call originated). This then completes establishing a link from the MSC to the UE via the UMTS network. At least during handover, the User Equipment (UE) may be arranged to communicate over both the GSM network and the UMTS network simultaneously or quasi-simultaneously. This feature may be provided in a method of communicating between User Equipment and GSM and UMTS networks that includes communicating information simultaneously or quasi-simultaneously via both networks during handover from one network to the other. All of the above aspects may be provided independently, but are advantageously combined in a system. Thus, there may be provided a method of handing over a GSM call to a UMTS network that includes: passing a handover request from the GSM network (preferably from a BSC, preferably containing GSM call parameters) to a RNC of the UMTS network; interpreting the request in the RNC and initiating call setup (preferably performing radio link setup and preferably thereafter, in response to a successful setup, performing data link to core network setup); forwarding handover information containing UTRAN parameters (preferably including a list of potential access nodes) to UE (preferably passing the UTRAN information transparently through the GSM network, for example as a message, to be interpreted by the UE); in the UE, setting up a UMTS call based on the UTRAN parameters. Summary of the Invention The invention provides a method of performing soft handover from a GSM network to a UMTS network, including the steps of: supplying a list of potential UTRAN access nodes to User Equipment; establishing communication between the UE and at least one UTRAN access node; and, deleting, from said list, potential access nodes with which satisfactory communication is not possible. In the invention, the UTRAN may be configured so that a plurality of access nodes are prepared to communicate with the UE. The invention further extends to a Radio Network Controller of a UMTS network, including means for carrying out the steps of the method of the invention. Brief Description of the Drawing An embodiment of the invention will now be described, with reference to the accompanying drawing in which Figure 1 depicts the sequence of events between GSM to UMTS handover via a MAP/E interface. This will also be applicable via the IWU. 1. The BSC sends Handover Required message to the GSM MSC. This includes the GSM information elements {service information rate, service type, etc.} and the UMTS cell/Node-B information on which good radio quality is achievable. 2. The GSM MSC sends MAP/E message Prepare Handover to the UMTS CN forwarding the UMTS cell-ids transparently. 3. The CN sends RANAP message Handover Request to the Target RNC. This message will contain information elements indicating from which network type this handover is taking place e.g. 'handover-type' = 'from GSM'. It will also contain information elements allowing transparent transfer of GSM Parameters to the RNC, e.g. GSM Bearer Capability, version number and other relevant parameters relating to the GSM call. Additionally, the relevant UMTS cell/Node-B information provided by the MS/UE is also transferred transparently to the RNC. 4. The RNC maps GSM appropriate call parameters to the relevant UMTS logical channel, transport channel and allocates radio resource parameters as appropriate (e.g. DL channelization code, UL spreading factor, UL scrambling code, Radio Frequency, Radio Link ID, Link Reference, S-RNTI, Transport Format Sets, Transport Format Combination Set, Initial DL Power). This function can only be done at this stage, since only the RNC knows the status of the radio resources. 5. Since dedicated transport channels are selected, the GSM call can go into immediate soft-handover. i) The RNC sends NBAP message Radio Link Setup to all relevant Node Bs. ii) All relevant Node Bs start transmission/reception. iii) All relevant Node Bs send NBAP message Radio Link Setup Response to the RNC. 6. The RNC sets up Iub transport bearer to all relevant Node Bs. The RNC is in a condition to set up the Transport Bearer across Iub since it knows if the UTRAN resources have been set up successfully. 7. The RNC sets up Iu transport bearer. The RNC is in a condition to set up the Transport Bearer across Iu since it knows if the UTRAN resources have been set up successfully. 8. The RNC sends Handover Request Acknowledge passing the appropriate UMTS parameters to the CN (e.g. DL channelization code, UL Spreading Factor, UL Scrambling Code, etc.) which are destined for the UE allowing the successful handover of the UE to UMTS. 9. The CN sends Prepare Handover Response to the GSM MSC (forwarding the UMTS parameters). 10. The GSM MSC sends Handover Command to the BSC (forwarding the UMTS parameters). 11. BSC sends the RR message Handover Command to the UE. This contains the necessary UMTS parameters. 12. The UE interprets the UMTS parameters within the RR message and is able to switch the call from a GSM call into a UMTS call with multiple or single diversity branches as indicated in the UMTS parameters. At this stage, the mobile unit is receiving information from the GSM network using its GSM capabilities and uses this information to switch communication to the UMTS network or simultaneously communicate with the UMTS network and GSM network using its UMTS and GSM capabilities. 13. The UE starts Tx/Rx. 14. On detection of synchronisation, the Node B sends NBAP message Handover Detect to the RNC. 15. If at least one of the Radio links has been successfully set up, then the RNC is able to establish the RLC link for the DTCH and the RRC for the DCCH to the UE. The RNC is then able to signal to the CN and onwards to the MSC that the handover is complete. The MSC is able to switch the call through on the new UTRAN leg. 16. In the case of having allocated multiple radio links on the DL and if UL synchronisation has not been detected, then the appropriate signalling may take place between Node B and RNC to release the unused radio resources. It will be appreciated that the invention can be applied to other network configurations. To assist in appreciating how the invention may be extended, a glossary of terms used in the art pertaining to UMTS systems is provided; any of the terms of art may be replaced by functionally equivalent components in a non-UMTS system. The following documents are incorporated herein by reference:- [1]Draft-ETR/SMG-50102, "Special Mobile Group (SMG) Vocabulary for the Universal Mobile Telecommunications System". [2]ETSI DTR/SMG-0225xxU, "Universal Mobile Telecommunications System (UMTS); Vocabulary for the UTRAN", UMTS 25.XX Version 0.1.0. Terms and definitions Active Set Set of radio links simultaneously involved in a specific communication service between a UE and a UTRAN. Cell A cell is a geographical area that can be identified by a UE from a (cell) identification that is broadcast from one UTRAN Access Point. Coded Composite Transport Channel (CCTrCH) A data stream resulting from encoding and multiplexing of one or several transport channels . The data stream of the CCTrCH is fed to a data splitter unit that splits the CCTrCH's data stream onto one or several Physical Channel Data Streams. Contention Resolution A functionality or procedure to solve the collision of identities on the initial random access messages from two (or more) UEs. Forward Handover A type of handover initiated by the UE. The UE sends the request for establishment of a new radio link in the new cell, i.e. it does not use the current radio link for performing handover but a radio link of the new cell. Gateway UE R /Seed A ODMA relay node that also communicates with the UTRAN using either TDD or FDD mode. Handover Handover is a family of procedures that adds or removes one or several radio links between one UE and UTRAN when a RRC connection exists and the position of the UE is known on cell level in the UTRAN. Hard Handover Hard handover is a category of handover procedures where all the old radio links in the UE are abandoned before the new radio links are established. Logical Channel A logical channel is an information stream dedicated to the transfer of a specific type of information over the radio interface. ODMA Relay Node A relay device, such as a UE R or Seed, that is capable of relaying using the ODMA protocol. Physical Channel In FDD mode, a physical channel is defined by code, frequency and, in the uplink, relative phase (I/O). In TDD mode, a physical channel is defined by code, frequency, and timeslot. Physical channel data stream In the uplink, a data stream that is transmitted on one physical channel. In the downlink, a data stream that is transmitted on one physical channel in each cell of the active set. Radio access bearer The service that the access stratum provides to the non-access stratum for transfer of user data between UE and CN. Radio frame A radio frame is a numbered time interval of 10 ms duration used for data transmission on the radio physical channel. A radio frame is divided into 16 time slots of 0.625 ms duration. The unit of data that is mapped to a radio frame (10 ms time interval) may also be referred to as radio frame. Radio link The set of (radio) physical channels comprised in a transmission path between a UE to one UTRAN access point . Radio link addition The procedure where a new radio link is added to the active set. Radio link removal The procedure where a radio link is removed from the active set. Radio Network Temporary Identifier (RNTI) A Radio Network Temporary Identifier is an identifier for a UE when an RRC connection exists. It is e.g. used by the MAC protocol on common Transport Channels (RACH, FACH, PCH). Relay A device capable of receiving and transmitting information for another user. Relaying The process of receiving and transmitting information for another user, such as carried out by a UE R . Relaylink Relaylink is the communications line between two ODMA relay nodes. Root Replay ODMA relay node where communications are either sourced or sunk. RRC connection A point-to-point bi-directional connection between RRC peer entities on the UE and the UTRAN sides, respectively. An UE has either zero or one RRC connection. Seed A ODMA relay node which is deployed by a network operator and is generally fixed, constantly powered, and has no display/keypad. Signalling connection An acknowledged-mode link between the user equipment and the core network to transfer higher layer information between peer entities in the non-access stratum. Signalling link Provides an acknowledged-mode link layer to transfer the UE-UTRAN signalling messages as well as UE - Core Network signalling messages (using the signalling connection ) . Soft Handover Soft handover is a category of handover procedures where the radio links are added and abandoned in such manner that the UE always keeps at least one radio link to the UTRAN. Transmission Time Interval Transmission Time Interval is defined as the inter-arrival time of Transport Block Sets, i.e. the time it should take to transmit a Transport Block Set. It is always a multiple of 10ms (the length of one Radio Frame ). Transport Block Transport Block is defined as the basic unit passed down to L1 from MAC, for L1 processing. An equivalent term for Transport Block is "MAC PDU". Transport Block Set Transport Block Set is defined as a set of Transport Blocks which is passed to L1 from MAC at the same time instance using the same transport channel. An equivalent term for Transport Block Set is "MAC PDU Set". Transport Block Set Size Transport Block Set Size is defined as the number of bits in a Transport Block Set. Transport Block Size Transport Block Size is defined as the size (number of bits) of a Transport Block Transport channel The channels offered by the physical layer to Layer 2 for data transport between peer L1 entities are denoted as Transport Channels. Different types of transport channels are defined by how and with which characteristics data is transferred on the physical layer, e.g. whether using dedicated or common physical channels are employed. Transport Format A Transport Format is defined as a format offered by L1 to MAC for the delivery of a Transport Block Set during a Transmission Time Interval on a Transport Channel. The Transport Format constitutes of two parts - one dynamic part and one semi-static part. Transport Format Combination A Transport Format Combination is defined as the combination of currently valid Transport Formats on all Transport Channels of a UE, i.e. containing one Transport Format from each Transport Channel. Transport Format Combination Set A Transport Format Combination Set is defined as a set of Transport Format Combinations to be used by a UE. Transport Format Combination Indicator (TFCI) A Transport Format Combination Indicator is a representation of the current Transport Format Combination . Transport Format Indicator (TFI) A label for a specific Transport Format within a Transport Format Set. Transport Format Set A Transport Format Set is defined as the set of Transport Formats associated to a Transport Channel . URA updating URA updating is a family of procedures that updates the UTRAN registration area of a UE when a RRC connection exists and the position of the UE is known on URA level in the UTRAN. User Equipment/Relay enabled (UE R ) A UE with ODMA relay operation enabled. UTRAN Registration Area (URA) The UTRAN Registration Area is an area covered by a number of cells. The URA is only internally known in the UTRAN. UTRAN access point A conceptual point within the UTRAN performing radio transmission and reception. A UTRAN access point is associated with one specific cell, i.e. there exists one UTRAN access point for each cell. It is the UTRAN-side end point of a radio link . Abbreviations ARQ Automatic Repeat Request BCCH Broadcast Control Channel BCH Broadcast Channel BPSK Binary Phase Shift Keying BSS Base Station System BTS Base Transceiver Station C- Control- CC Call Control CCCH Common Control Channel CCH Control Channel CCTrCH Coded Composite Transport Channel CDMA Code Division Multiple Access CN Core Network CRC Cyclic Redundancy Check DC Dedicated Control (SAP) DCA Dynamic Channel Allocation DCCH Dedicated Control Channel DCH Dedicated Channel DHO Diversity Handover DL Downlink DRNC Drift Radio Network Controller DS-CDMA Direct-Sequence Code Division Multiple Access DSCH Downlink Shared Channel DTCH Dedicated Traffic Channel DTX Discontinuous Transmission FACH Forward Link Access Channel FAUSCH Fast Uplink Signalling Channel FCS Frame Check Sequence FDD Frequency Division Duplex GC General Control (SAP) HO Handover HHO Hard Handover ITU International Telecommunication Union kbps kilo-bits per second ksps kilo-symbols per second L1 Layer 1 (physical layer) L2 Layer 2 (data link layer) L3 Layer 3 (network layer) LAC Link Access Control MAC Medium Access Control MM Mobility Management Mcps Mega-chips per second Nt Notification (SAP) OCCCH ODMA Common Control Channel ODCCH ODMA Dedicated Control Channel ODCH ODMA Dedicated Channel ODMA Opportunity Driven Multiple Access ORACH ODMA Random Access Channel ODTCH ODMA Dedicated Traffic Channel PCCH Paging Control Channel PCH Paging Channel PDU Protocol Data Unit PHY Physical layer PhyCH . Physical Channel RACH Random Access Channel RLC Radio Link Control RNC Radio Network Controller RNS Radio Network Subsystem RNTI Radio Network Temporary Identity RRC Radio Resource Control SAP Service Access Point SCCH Synchronization Control Channel SCH Synchronization Channel SDU Service Data Unit SIR Signal-to-Interference Ratio SRNC Serving Radio Network Controller SRNS Serving Radio Network Subsystem TCH Traffic Channel TDD Time Division Duplex TFCI Transport Format Combination Indicator TFI Transport Format Indicator TN Termination Node TPC Transmit Power Control TRX Transmitter/Receiver U- User- UE User Equipment UE R User Equipment with ODMA relay operation enabled UL Uplink UMTS Universal Mobile Telecommunications System URA UTRAN Registration Area UTRA UMTS Terrestrial Radio Access UTRAN UMTS Terrestrial Radio Access Network Each feature disclosed in this specification (which term includes the claims) and/or shown in the drawings may be incorporated in the invention independently of other disclosed and/or illustrated features. Statements in this specification of the "objects of the invention" relate to preferred embodiments of the invention, but not necessarily to all embodiments of the invention falling within the claims. The description of the invention with reference to the drawings is by way of example only.

Claims: A method of performing soft handover from a Global System for Mobile Communications (GSM) -type network to a Universal Mobile Telecommunications System (UMTS) -type network, comprising the steps of: supplying a list of potential UMTS Terrestrial Radio Access Network (UTRAN) access nodes to User Equipment (UE); establishing communication between the UE and at least one UTRAN access node; and, deleting, from said list, potential access nodes with which satisfactory communication is not possible. A method according to Claim 1, comprising the further step of: configuring the UTRAN so that a plurality of access nodes are prepared to communicate with the UE. A Radio Network Controller of a UMTS network comprising means for carrying out the steps of the method according to any one of the preceding claims.

Inventors: PARMAR GINA, PATTERSON ROBERT, PARMAR, GINA, PATTERSON, ROBERT
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 2000
Main classification: H04Q7
IPC codes: H04W36