WO2008039124A1 - A method, a serving cell controller and a system for detecting support for packet-switched handover - Google Patents

A method, a serving cell controller and a system for detecting support for packet-switched handover Download PDF

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Publication number
WO2008039124A1
WO2008039124A1 PCT/SE2006/050348 SE2006050348W WO2008039124A1 WO 2008039124 A1 WO2008039124 A1 WO 2008039124A1 SE 2006050348 W SE2006050348 W SE 2006050348W WO 2008039124 A1 WO2008039124 A1 WO 2008039124A1
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WIPO (PCT)
Prior art keywords
packet
target cell
switched handover
candidate neighbour
neighbour target
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PCT/SE2006/050348
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French (fr)
Inventor
Jari Vikberg
Peter ÖSTRUP
Jens Bergqvist
John Diachina
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Telefonaktiebolaget Lm Ericsson (Publ)
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Priority to PCT/SE2006/050348 priority Critical patent/WO2008039124A1/en
Publication of WO2008039124A1 publication Critical patent/WO2008039124A1/en

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    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04WWIRELESS COMMUNICATION NETWORKS
    • H04W36/00Hand-off or reselection arrangements
    • H04W36/0005Control or signalling for completing the hand-off
    • H04W36/0083Determination of parameters used for hand-off, e.g. generation or modification of neighbour cell lists
    • H04W36/00835Determination of the neighbour cell list
    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04WWIRELESS COMMUNICATION NETWORKS
    • H04W36/00Hand-off or reselection arrangements
    • H04W36/14Reselecting a network or an air interface
    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04WWIRELESS COMMUNICATION NETWORKS
    • H04W84/00Network topologies
    • H04W84/02Hierarchically pre-organised networks, e.g. paging networks, cellular networks, WLAN [Wireless Local Area Network] or WLL [Wireless Local Loop]
    • H04W84/10Small scale networks; Flat hierarchical networks
    • H04W84/12WLAN [Wireless Local Area Networks]

Abstract

The present invention relates to a method for dynamically and automatically detecting at least one target cell without support for packet-switched handover from a serving cell controller (600, 510, 730) of a first radio access technology (720) towards a cell supported by a second radio access technology (740). In the method, the serving cell controller (600, 510, 730) selects a candidate neighbour target cell (506) for packet-switched handover, initiates (508) a packet-switched handover attempt towards the selected cell and dynamically and automatically stores (509, 541 ) in a list (511 , 631) maintained in a database (630) of the serving cell controller (600, 510, 730) a result of an unsuccessful packet-switched handover attempt made towards the target cell, wherein the result comprises the identity (512) of the target cell and a timestamp (513) for the target cell indicating when the target cell was most recently added to the list (51 1 , 631). The timestamp is further used by the serving cell controller (600, 510, 730) to decide when to enable a new packet-switched handover attempt towards the target cell. The present invention also relates to a serving cell controller (600, 510, 730) and to a mobile telecommunication system (700) including such serving cell controller (500, 510, 730).

Description

A METHOD, A SERVING CELL CONTROLLER AND A SYSTEM FOR DETECTING SUPPORT FOR

PACKET-SWITCHED HANDOVER

TECHNICAL FIELD

The present invention relates generally to the field of wireless communications, and, more specifically, to a method, a serving cell controller and a system for detecting packet-switched handover support in other serving cell controllers.

BACKGROUND

Wireless access networks have become a key element of a variety of telecommunications network environments. As to enterprise network environments, they provide convenient wireless access to network resources for employers or customers carrying laptops and/or mobile handheld devices. In addition, wireless access points operable with diverse communication devices, such as laptops, mobile phones, etc., are broadly used in public environment such as e.g., hotels, train stations, airports, restaurants, schools, universities and homes, and are mostly used to offer high-speed internet access.

The telecommunication industries are therefore focusing on the integration of cellular access networks with the capability to interface with wireless access networks such as Wireless Local Area Networks (WLAN) and are focusing on developing inter-working mechanisms in such multi-network environment so as to facilitate efficient handover of services from one type of access network to the other. Some examples of cellular access networks are: the Global System for Mobile telecommunications (GSM) network providing circuit-switched services; the General Packet Radio Service (GPRS) network providing packet-switched services and utilizing the infrastructure of a GSM system; and the Universal Mobile Telecommunication Systems (UMTS) network providing circuit-switched and packet- switched services. Two further examples of cellular access networks are EDGE and EGPRS which are further enhancements to GSM and GPRS. EDGE refers to enhanced Data rates for GSM Evolution, and EGPRS refers to EDGE for GPRS. In order for the different access networks to work seamlessly and to support high speed data in addition to the traditional circuit-switched services, packet-switched handover mechanisms between the different access networks must be defined, which is usually agreed on through standardization.

Packet-switched handover is introduced to support real time packet-switched services with more demanding quality of service (QoS) requirements for latency and packet loss. An advantage realized with packet-switched handover is minimizing the service interruption times by allowing continuous data transfer between a mobile station and a cellular access network, when the mobile station is moving from one cell towards another cell where the respective radio access technologies can be of the same access type or of different access types. For example, packet-switched handover has been included into the release 6 of GERAN specifications, which allow for packet-switched handover between GERAN cells and packet-switched handover between GERAN and UTRAN cells. GERAN is the name given to the third Generation Partnership Project (3GPP) standards for GSM/EDGE radio access network, and UTRAN is the UMTS Terrestrial Radio Access Network. Packet- switched handover principles between GERAN cells and between GERAN cells and UTRAN cells are currently defined in 3GPP TS 43.129.

Figure 1 illustrates the reference architecture for packet-switched handover in GERAN AJGb mode as defined in 3GPP TS 43.129 V6.8.0.

A core network 100 (CN) comprises a gateway GPRS support node (GGSN) 102, a mobile switching centre (MSC) 104, a serving GPRS support node (SGSN) 106, and a SGSN 108. A BSS (base station system)/GERAN 110 includes a base station controller (BSC) 112, and a plurality of base transceiver stations (BTS), such as BTS 1 14. A radio network system (RNS) or a base station system (BSS) 118 includes a radio network controller (RNC) or a base station controller (BSC) 120 and a plurality of Node Bs such as Node B 122. Also illustrated is a mobile station (MS) 1 16 which may connect to the core network 100 via the BSS/GERAN 1 10 or the RNS (or BSS) 1 18. The packet-switched handover procedure is used to handover an MS with one or more packet flows from a source cell to a target cell. The source and target cells can be allocated within either the same BSS, also known as Intra BSS handover, different BSSs connected to the same SGSN, also known as Intra SGSN handover, or belonging to different BSSs not connected to the same SGSNs, also known as Inter SGSN handover, or systems with different radio access types, known as Inter RAT (radio access technology) or Inter mode handover (i.e. GERAN A/Gb mode to GERAN Iu mode handover).

The signalling messages used during packet-switched handover within GERAN are divided into three groups depending on the utilized interface as illustrated in Figure 1 :

- Packet-switched handover signalling messages on the Um interface are RLC/MAC (radio link control/mobile allocation control) signalling blocks.

- Packet-switched handover signalling messages on the Gb interface are BSS GPRS protocol (BSSGP) signalling messages. - Packet-switched handover signalling messages on the Gn interface are GPRS tunnelling protocol (GTP) signalling messages

- For circuit-switched services, the A interface is used.

In Figure 1 , bold lines illustrate interfaces supporting user traffic, and thin lines illustrate interfaces supporting signalling.

The Generic Access Network (GAN), which was formerly known as Unlicensed Mobile Access (UMA), until it was adopted by the 3GPP in April 2005, describes a telecommunication system allowing seamless roaming and handover between wireless local area networks and cellular access networks using a mobile phone that supports multiple radio access technologies. The wireless local area network could be based on private unlicensed spectrum technologies like Bluetooth or IEEE 802.1 1. The cellular access network may be any of the GSM/GPRS/EDGE/UTRAN radio access networks described above.

The goal of using the GAN network as a wireless local area network is that it lets mobile operators deliver voice, data and IMS/SIP (IP Multimedia Subsystem/Session Initiation Protocol) applications to mobile devices on wireless local access networks that offer higher data rates than can be expected on cellular access networks. Its ultimate goal is the convergence of mobile, fixed and Internet telephony (Fixed Mobile Convergence). On the cellular access network, the mobile handset is communicating over the air with a base station, through a base station controller with servers in the core network. When the handset or mobile device detects the availability of a WLAN (Wireless Local Area Network) while still being served by the cellular access network, it establishes a secure IP connection through a security gateway to a serving cell controller within the WLAN also called a GAN Controller (GANC). The GANC translates the signals coming from the handset or mobile device to make it appear to be coming from just another base station within the cellular access network. Thus, when a mobile moves from a GSM to a WLAN network such as a Bluetooth or IEEE 802.1 1 WiFi network, it appears to the core network as if it is simply on a different GSM base station. Current GAN specifications are, as mentioned above, part of the 3GPP standards and defined in 3GPP TS 43.318, and 3GPP TS 44.318.

Figure 2 illustrates the known Generic Access Network (GAN) functional architecture 200 as defined in 3GPP TS 43.318.

According to Figure 2, a core cellular access network 240 is accessible to an MS 210 through a GAN which consists of the Generic IP Access Network 220 and associated GAN controller (GANC) 230. The Generic IP access network 220 is operable to co-exist with the GERAN/UTRAN infrastructure described earlier, and it interconnects to the core cellular access network 240 via the same AJGb interfaces used by a standard GERAN BSS network clement. In operation, the GANC 230 appears to the core cellular access network (PLMN) 240 as just another GERAN BSS network element by mimicking the role of a Base Station Controller (BSC) in the GERAN architecture as seen from the perspective of the AJGb interfaces. Similarly to the GERAN architecture described earlier, the ^-interface defines the interface for GSM-based circuit-switched services and is deployed between GANC 230 and an MSC 241 of the core cellular access network 240. The Gό-interface defines the interface for packet-switched services and is deployed between the GANC 230 and a SGSN 242 of the core cellular access network 240. The Up interface is defined between the GANC 230 and the MS 210. The GAN functional architecture illustrated in Figure 2 comprises other network elements and interfaces which are known to those skilled in the art.

As mentioned above, packet-switched handover between GERAN cells and between GERAN cells and UTRAN cells are already defined in the current specifications 3GPP TS 43.129. In the current solution for packet-switched handover between GERAN/UTRAN cells, the information regarding whether or not a neighbour GERAN or UTRAN cell controlled by a different Base Station Controller/Radio Network Controller (BSC/RNC) (an external cell) supports the packet-switched handover feature is stored in each Base Station System (BSS) of each cell, wherein operation and maintenance configuration procedures are commonly used to provide the BSS with this feature. Thus, the packet-switched handover feature of each neighbouring cell in the GERAN/UTRAN case is manually configured using the operation and maintenance configuration procedures. An advantage with this solution is that a BSS will know not to initiate packet-switched handover towards an external GERAN/UTRAN cell if that cell does not support the packet-switched handover feature and if this information regarding packet-switched handover support by that cell is contained in the BSS.

Figure 3 schematically illustrates the known reference architecture for a packet-switched handover preparation phase between GERAN A/Gb mode and UTRAN mode, also known as Inter-RAT ( Inter-Radio Access Technology).

In this known scenario, a packet-switched handover decision 301 is first made by either the mobile station MS 300 or the source BSS 310. However, the BSS 310 is in the preparation phase responsible in initiating a packet-switched handover after that MS 300 has sent measurement reports to the source BSS 310. After that the packet-switched handover decision is made, the source BSS 310 sends a packet-switched handover message, "PS handover Required" message 302 to the SGSN 330. This message 302 includes the source cell identifier, the target RNC identifier, the target cell identifier and other parameters as defined in the technical specifications 3GPP TS 43.129. Next, the third generation/second generation (3G/2G) SGSN 330 determines from the target RNC identifier that the type of handover is an Inter-RAT (i.e. in this case, from GERAN A/Gb mode to UTRAN mode). The 3G/2G SGSN 330 constructs a "Relocation Request" message 303 and sends it to the target RNC/BSS 320. Message 303 includes a request for radio access bearer to be established by the target RNC/BSS 320 and also other parameters which are outside the scope of the present invention, and known to those skilled in the art. The target RNC/BSS 320 then acknowledges the "Relocation Request" message 303 using the "Relocation Request Acknowledge" message 304 and sends message 304 to the 3G/2G SGSN 330 and gets prepared to receive packet data units from the 3G/2G SGSN 230 for the accepted radio access bearers. When the 3G/2G SGSN 330 receives the "Relocation Request Acknowledge" message 304, the packet switched handover preparation phase is finished and the execution phase will follow as defined in existing standards and known to those skilled in the art.

According to the current packet-switched handover solutions as described above, the information regarding whether or not an external neighbouring cell belonging to a UTRAN system (or a neighbouring cell belonging to a GERAN cell) supports the packet-switched handover feature is already stored in each (BSS) or each RNS, wherein operation and maintenance manual configuration procedures are commonly used to provide the BSS or the RNS with this information.

The procedures for enabling packet-switched handover between GERAN/UTRAN cells and GAN cells are in the process of being standardised. The working assumption for packet- switched handover between GERAN/UTRAN mode and GAN mode has been that requirements on packet-switched handover between GERAN cells or between GERAN cells and UTRAN cells should also apply for GAN mode. For example, when considering the case of a mobile station having one or more active packet data sessions while operating in GAN mode, a GANC could, similarly to the GERAN/UTRAN case, have all GERAN/UTRAN cells of, for example, an entire country (or more) as its neighbouring cells stored, using operation and maintenance configuration procedures.

A disadvantage with this solution is that the GANC must rely on manual operation and maintenance procedures to acquire knowledge of which neighbour GERAN/UTRAN cells that provide support for packet switched handover. Furthermore, in case a neighbouring

GERAN/UTRAN cell does not have support for packet-switched handover and the GANC (or a GERAN/UTRAN controller for packet-switched handover from a GERAN/UTRAN cell) has no prior knowledge regarding the neighbouring cell's support of packet-switched handover, a packet-switched handover attempt towards this cell may lead to an unnecessary waste of radio resources and also waste of CPU processing power in the various network nodes.

SUMMARY

An object of the present invention is thus to obviate at least some of the above disadvantages and provide an improved packet-switched handover solution that eliminates or at least mitigates the need for manual configuration of neighbouring cell packet-switched handover capability information in the network.

According to a first aspect of the invention, the above stated problem is solved by means of a method of dynamically and automatically detecting, by a serving cell controller belonging to a first radio access technology, candidate neighbour target cells without support for packet- switched handover belonging to a second radio access technology. The method comprises the steps of: maintaining in the serving cell controller a list arranged to contain candidate neighbour target cells without support for packet-switched handover; selecting by the serving cell controller of a candidate neighbour target cell for packet-switched handover; initiating a packet-switched handover attempt by the serving cell controller towards the selected candidate neighbour target cell; and dynamically storing in the list the result of an unsuccessful packet-switched handover attempt made towards the selected candidate neighbour target cell and a corresponding timestamp for the selected cell indicating when the cell was most recently added to the list and using the timestamp by the serving cell controller to decide when to enable a new packet-switched handover attempt towards that candidate neighbour target cell.

According to a second aspect of the present invention, the above stated problem is solved by means of a serving cell controller of a first radio access technology for dynamically and automatically detecting candidate neighbour target cells without support for packet-switched handover belonging to a second radio access technology. The serving cell controller according to the second aspect of the present invention comprises: selection means for selecting a candidate neighbour target cell for packet-switched handover; initiation means for initiating a packet-switched handover towards the selected candidate neighbour target cell; and a local database for dynamically and automatically storing in a list the result of an unsuccessful packet-switched handover attempt made towards the selected candidate neighbour target cell. The result comprises the identity of the candidate neighbour target cell and a corresponding timestamp indicating when the selected candidate neighbour target cell was most recently added to the list, and using the timestamp by the serving cell controller to decide when to enable a new packet-switched handover attempt towards the candidate neighbour target cell.

According to a third aspect of the present invention, the above stated problem is solved by means of a mobile telecommunication system comprising a serving cell controller for dynamically and automatically detecting candidate neighbour target cells without support for packet-switched handover.

In the present invention, because the serving cell controller dynamically and automatically detects candidate neighbour target cells without support for packet-switched handover, the serving cell controller does not need to have prior knowledge regarding external neighbour cell's support for packet-switched handover, i.e. all external neighbour cells could at first be considered as potential candidates for packet-switched handover.

Furthermore, the first radio access technology and the second radio access technology could be of the same type or could be of different type.

An advantage with the present invention is that neighbouring cell information regarding packet-switched handover support in external neighbouring cells will no longer have to be manually configured in a serving cell controller. Another advantage with the present invention is that unnecessary packet-switched handover signalling within the wireless access network and the corresponding CPU processing is kept to a minimum.

The present invention will now be described in more details by means of preferred embodiments and with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 schematically illustrates the known reference architecture for a packet-switched handover in GERAN mode.

Figure 2 schematically illustrates the known GAN functional architecture.

Figure 3 schematically illustrates the known reference architecture for a packet-switched handover, and more specifically the preparation phase signalling events, between GERAN mode and UTRAN mode.

Figure 4 schematically illustrates the known signalling in the circuit-switched handover between GAN mode and GERAN/UTRAN mode.

Figure 5a illustrates signalling messages according to a first embodiment of the present invention.

Figure 5b illustrates a data structure or a list and its corresponding elements according to the present invention.

Figure 5c illustrates signalling messages according to a second embodiment of the present invention. Figure 5d illustrates signalling messages according to a third embodiment of the present invention.

Figure 6 illustrates a block diagram of an exemplary embodiment of a serving cell controller according to the present invention.

Figure 7 illustrates a block diagram of an exemplary embodiment of a mobile telecommunication system according to the present invention.

DETAILED DESCRIPTION

The present invention is described herein by way of reference to particular example scenarios. In particular the invention is described in a general context in relation to a GAN radio access network and to a GERAN/UTRAN radio access network.

Referring to Figure 4, there is illustrated the signalling in the circuit-switched handover procedure between GAN mode and GERAN/UTRAN mode, as defined in 3GPP TS 43.318 V6.7.0. The signalling messages of Figure 4 are known in the art, and only those aspects of the signalling are discussed in detail herein as are necessary to understand how preparation phase signalling events are carried in a circuit-switched handover between GAN mode and GERAN/UTRAN mode.

According to Figure 4, a mobile station MS 400 has an ongoing circuit-switched connection to the GAN and is perceived as beginning to leave GAN coverage by the GANC 410 based on information 401 (local measurements, received real time control protocol (RTCP) reports, as well as any uplink quality indications) received by the GANC 410 from the MS 400.

According to the current standards, the handover from GAN to GERAN/UTRAN procedure is essentially triggered by the MS 400 as a result of this information 401 sent to the GANC 410.

In Figure 4. dotted arrows are used to indicate the handover specific signalling messages towards the UTRAN system. Signalling messages that are common for both GERAN and

UTRAN handover are illustrated using bold line arrows. The MS 400 sends the "GA-CSR HANDOVER INFORMATION" message 401 to its serving GANC 410 including a list of candidate neighbour target GERAN and/or UTRAN cells, in order of preference for circuit-switched handover, and includes the received signal strength for each identified neighbour cell. If GANC selects a target GERAN cell or a target UTRAN cell, the serving GANC 410 starts the handover procedure by signalling to the core network (CN) 430 the need for handover using the "Handover Required" message 402 and includes the UTRAN cell list or GERAN cell list or subsets of the cell lists provided by MS 400.

In case a UTRAN cell is the target cell, the CN 430 then starts the relocation resource allocation procedure towards the target RNC 420 and requests from the RNS of the UTRAN network to allocate the necessary resources using the "Relocation Request" message 403. The RNC 420 of the target UTRAN acknowledges the request for resource allocation and sends a "Relocation Request Acknowledge"' message 404 to the CN 430. Thereafter, the CN 430 signals the serving GANC 410 to handover the MS 400 to the UTRAN cell using "Handover Command" message 405, starting the execution phase of the handover procedure.

In case a GERAN cell is selected as the target cell, which in this example is a GERAN A/Gb cell, the CN 430 requests the BSC 420 of the GERAN network to allocate the necessary resources for handover using "Handover Request" message 403'. The BSC 420 of the target

GERAN acknowledges the request for resource allocation and sends a "Handover Request

Acknowledge" message 404' to the CN 430. Thereafter, the CN 430 signals the GANC 410 to handover MS 400 to the target GERAN cell in a "Handover Command" message 405, starting the execution phase of the handover procedure.

The steps following 405 are part of the handover execution phase and are outside the scope of the present invention, and known to those skilled in the art.

It is worth repeating that the above described handover procedure between GAN mode and GERAN/UTRAN mode as illustrated in Figure 4, is restricted to circuit-switched handover. In addition, the packet-switched handover procedure described in Figure 3 between GERAN mode and UTRAN mode relies on operation and maintenance procedures for the source BSS to acquire knowledge of neighbour cells support for packet-switched handover.

The embodiments of the present invention will now be described in more details based on a GAN mode to GERAN/UTRAN mode scenario. Although, the present invention may also be used for other scenarios, such as packet-switched handover/circuit-switched handover between: GERAN/UTRAN mode and WiMax mode; GERAN/UTRAN mode to GAN mode, GERAN mode to UTRAN mode; UTRAN mode to GERAN mode; dual-transfer mode (DTM) to GAN mode, or any other mode where it is desirable to dynamically and automatically detect if a target cell supports handover and also where it is desirable to avoid the need for manual configuration of a target cell handover capability. Note that in DTM, the DTM handover consists of the distinct procedures for packet-switched handover and circuit- switched handover running in parallel but resulting in the transmission of a single handover command to a mobile station in the source cell. Thus, when applying the embodiments of the present invention to the DTM handover scenario, the source serving cell controller, e.g. BSS/BSC/RNC/GANC is still arranged to maintain a list of those candidate neighbour target cells without support for DTM handover along with a corresponding timestamp in accordance with the present invention.

Referring to Figure 5a, there are illustrated signalling messages according to a first aspect of the present invention. As shown, a mobile station MS 500 is registered to a serving cell controller GANC 510 and is transmitting and/or receiving data packets in GAN packet transfer mode as illustrated by signalling 501 in Figure 5a.

According to the present invention, the GANC 510 maintains a data structure or a list denoted here Target_cells_without_PSHO_support list (PSHO for packet-switched handover), that is arranged to contain candidate neighbour target GERAN or UTRAN cells which do not have support for packet-switched handover. The data structure or the list is maintained in a local database of the GANC 510. However, the database could also be placed centrally with access from different GANC nodes. The elements of the list 511 are illustrated in Figure 5b and are further described below. For better understanding the present invention, it is here assumed that the GANC 510 has no prior knowledge regarding candidate neighbour target GERAN or UTRAN cells or if these cells support packet-switched handover, and therefore the list 51 1 or data structure 51 1 is initially empty. As mentioned earlier, this list is dynamically and automatically updated depending on the outcome of the packet-switched handover procedures initiated from GANC 510 towards a specific candidate neighbour target GERAN or UTRAN cell.

As illustrated in Figure 5b, the target cells without support for packet-switched handover are after detection identified using a candidate neighbour target cell identification (Cell ID) 512 and a timestamp (TS) 513 indicating when the candidate neighbour target cell was most recently added to the list 51 1. If the candidate neighbour target cell is a GERAN cell, then Cell ID 512 would include the combination of a Routing Area Identity (RAI) and a Cell Identity (CI). If the candidate neighbour target is a UTRAN cell, then Cell ID 512 would include the combination of a RAI, a Radio Network Controller Identification (RNC-ID) and a third generation Cell Identity (3G-CI). The RAI consists of a Location Area Identity (LAI) and a Routing Area Code (RAC). Further the LAI consists of a Mobile Country Code (MCC), a Mobile Network Code (MNC) and a Location Area Code (LAC).

According to preferred embodiments of the present invention, the timestamp TS 513 can be used by the GANC 510 to determine if sufficient time or a predetermined time has elapsed from the last time an unsuccessful packet-switched handover was attempted towards a specific candidate neighbour target cell for the GANC 510 to consider enabling a new packet- switched handover attempt to that candidate neighbour target cell.

Furthermore and in accordance with preferred embodiments of the present invention, the amount of time that GANC 510 may wait until enabling a new packet-switched handover attempt towards a given candidate neighbour target cell could depend on the number of unsuccessful packet-switched handover attempts towards that candidate neighbour target cell.

As an example, GANC 510 may wait a shorter period of time if a single packet-switched handover failure has occurred and may wait a longer period of time if multiple packet- switched handover failures have occurred for any given candidate neighbour target cell. In addition, the list 51 1 also contains a reason field or cause field 514 for each target cell. This reason field or cause field 514 indicates the reason why a candidate neighbour target cell was initially added to the list 51 1. Different cause values arc for example "No Response Received" or "Negative Response Received" or any other cause value indicating why the candidate neighbour target cell was added to the list 51 1. Further, the reason field or cause field 514 can be used in order to differentiate when a new packet-switched handover attempt is made again. For example if it is due to "No Response Received" a new attempt should be made earlier than if a specific reject message stating that the message was not supported was received.

Referring back to Figure 5a, a method describing how a candidate neighbour target cell is automatically and dynamically detected as having no support for packet-switched handover, is illustrated and presented below.

When the MS 500 begins to leave the GAN coverage, the MS 500 or the GANC 510 decides 502 that a packet-switched handover from the GAN cell to a neighboring target cell hosted by a GERAN or UTRAN target system 520 is needed.

If the MS 500 decides that a packet-switched handover is required it would inform the GANC 510 of this decision as part of 502 using e.g. a GAN control plane message and include in this message the identity of one or more preferred candidate neighbour target GERAN/UTRAN cells therein. If multiple candidate neighbour target cells are identified in 502 the GANC 510 selects one as the target cell for which handover will be attempted. Whether or not the MS 500 is allowed to trigger a packet-switched handover could be indicated as part of the GAN system information acquired by the MS 500 when performing registration in the GANC 510. On the other hand, if the GANC 510 decides that a packet-switched handover is required for the MS 500, the GANC 510 could select a target GERAN/UTRAN cell based on measurement information received in 503 from the MS 500 while operating in GAN mode.

For the case where the MS 500 decides that a packet-switched handover is required, the MS 500 could send a specific GAN control plane message e.g. "GA-PSR HANDOVER

INFORMATION" message 503 to the GANC 510 as illustrated in Figure 5a. This message 503 includes measurement reports for one or more candidate neighbour target GERAN/UTRAN cells. In the example and according to Figure 5a, MS 500 includes information about two GERAN cells which are shown as CeIl-I (RAI-I, CI-I, RXLev-1) and Cell-2 (RAI-2, CI-2 and RXLev-2). RXLev-1 and RXLev-2 correspond to the received signal strength of CeIl-I and Cell-2 respectively as reported by the MS 500, and CI-I and CI-2 are the cell identities of CeIl-I and Cell-2 respectively.

It should be noted that if the MS 500 includes multiple candidate neighbour target GERAN or UTRAN cells, the GANC 510 selects one of the cells as the target cell for the packet-switched handover attempted.

Let us assume that in this example CeIl-I identified by (RAI-I and CI-I) is selected in 506. Since we are considering the case of packet-switched handover this takes place when the GANC 510 knows that the MS 500 is in idle mode in the circuit-switched domain and that the MS 500 is in active mode in the packet-switched domain i.e. actively sending and/or receiving data via the packet-switched domain of the GANC 510 while in the GAN mode packet- switched active state.

Next, the GANC 510 checks that CeIl-I is not in the Target_cells_without_PSHO_support list 51 1 and in this example the list is assumed to be empty. In case the CeIl-I was included in the list 51 1 and the timestamp 513 for that cell indicates that a new retry of packet-switched handover attempt is not yet allowed towards CeIl-I , then another cell would have been selected, as for example Cell-2 if that cell was also not included in the list.

According to embodiments of the present invention, the GANC 510 in 507 starts a timer which is here denoted PS_HO_capabiIity_detection timer to supervise the packet-switched handover preparation procedure. The timer is associated with a specific packet-switched handover attempt to a specific target cell (i.e. a specific instance of the packet-switched handover required procedure) and is in this embodiment associated with the CeIl-I identified by (RAI-I and Cl-I). Next, the GANC 510 initiates a packet-switched handover attempt (as the PS handover preparation phase) by sending a message 508 towards the target system 520 hosting the target cell, which in this case is the GERAN system wherein the candidate neighbour target cell CeIl-I is located. This message is denoted here "PS-HANDOVER-REQUIRED" message 508 and is first sent towards the SGSN where the GANC is currently connected to. The SGSN forwards the request towards the target system (i.e. for the case where the target cell is associated with a different SGSN). The source cell (located in the GAN) and the target cell (CeIl-I ) are also defined in this message 508.

According to the first embodiment of the present invention, if no response is received in 509 from the target system 520 in which the candidate neighbour target cell CeIl-I is located (i.e. the PS_HO_capability detection timer expires indicating that the target cell CeIl-I does not support packet-switched handover), the GANC 510 stores information about the target cell, i.e. the cell identity of CeIl-I , i.e. (RAI-I and CI-I) and a timestamp TS 513 for CeIl-I in the Target_cells_without_PSHO_support list 511. In addition, the reason field or cause field 514 for CeIl-I is set to "No Response Received" as described earlier.

As mentioned above, the timestamp TS 513 in Figure 5b indicates when the target cell, e.g. CeIl-I was most recently added to list 51 1 or updated in the list 51 1. The TS 513 is also used by the GANC 510 to determine if sufficient time or a predetermined time has elapsed from the last time an unsuccessful handover attempt was attempted for Cell- 1. For example, the GANC 510 could set the sufficient time or the predetermined time be equal to a value lying between 1 hour and 24 hours, or the GANC 510 could let the sufficient time or the predetermined time depend on the number of unsuccessful packet-switched attempts made toward CeIl-I , i.e. the GANC 510 could wait a shorter period of time before enabling a new attempt if a single unsuccessful packet-switched handover attempt has occurred and wait a longer period of time if multiple unsuccessful attempts have occurred for the target cell. In addition, the GANC 510 may use the reason field 514, which in this case is set to "No Response Received", together with the timestamp 513 in order to differentiate when a new attempt is made towards the target cell. As an example, since in this first embodiment, the GANC 510 did not receive a response from the target cell or target system, a new packet- switched handover attempt should be made earlier than for the case where multiple handover attempts have already failed for that target cell (e.g. the reason field can indicate:"77//»eøwf - first packet-switched handover failure", or could indicate "Timeout - two consecutive packet-switched handover failures" etc.).

According to this first embodiment, the GANC 510 will now after detection of a packet- switched handover failure, know that CeIl-I does not support packet-switched handover and will thus not initiate a new packet-switched handover towards the indicated cell when the MS 500 again requires that a packet-switched handover towards CeIl-I is needed. This is because the GANC 510 will use the timestamp TS 513 and the reason field 514 to control when a new packet-switched attempt is needed towards the CeIl-I . Consequently, the unnecessary waste of packet-switched handover signalling from the GANC towards that target cell and the associated CPU processing in the various network nodes is effectively kept to a minimum.

Figure 5c illustrates signalling messages according to a second embodiment of the present invention. This second embodiment differs from the first one in that a negative response message 540 is received from the target cell CeIl-I , indicating that packet-switched handover is not supported in CeIl-I. This message 540 is illustrated in Figure 5c and is here denoted "PS-Handover Not Supported". When the GANC 510 receives message 540, the PS_HO_capability_detection timer is stopped at 541 by the GANC 510 and the GANC 510 stores the cell information of CeIl-I, i.e. (RAI-I and CI-I) and a timestamp TS 513 for CeIl-I in the Target_cells_vvithout_PSHO_support list 51 1. In addition, the reason field or cause field 514 for CeIl-I is set to "Negative Response Received - first packet-switched handover failure". According to this second embodiment, the GANC 510 will now after detection know that CeIl-I does not support packet-switched handover and will thus not initiate a packet-switched handover towards the indicated cell even if the MS 500 decides again that a packet-switched handover towards CeIl-I is needed. Consequently, unnecessary waste of packet-switched handover signalling from GANC 510 towards that target cell and the associated CPU processing in the various network nodes is effectively avoided. However, similarly to the first embodiment of the present invention, the GANC 510 may use the timestamp TS 513 and the reason field 514 to decide when to trigger a new packet- switched handover attempt towards CeIl-I since CeIl-I may in the future support packet- switched handover. Therefore, the time that a certain Cell identity is stored in the Target cells without PSHO support list 51 1 of the GANC 510 is preferably limited through time supervision using the timestamp 513. Thus, after a certain time has elapsed from when a specific target cell identity was last added to the list 511, the timestamp 513 for that target cell can be checked to decide whether the packet-switched handover required procedure should again be initiated towards the indicated cell. This way, a change in packet-switched handover support in the target cell can be eventually detected by the GANC 510 when a packet-switched handover is triggered.

Furthermore, the timestamp 513 together with the reason field 514 can be used to determine if sufficient time or a predetermined time has elapsed from the last time an unsuccessful packet- switched handover was attempted towards the target cell.

Figure 5d illustrates signalling messages according to a third embodiment of the present invention. This embodiment differs from the first and second embodiments in that a positive response message 550 is received from the target cell CeIl-I, indicating that packet-switched handover is supported in CeIl-I . This message 550 is illustrated in Figure 5d and is denoted "PS-HANDOVER-REQUIRED-ACK". When the GANC 510 receives message 550, the PS_HO_capability_detection timer is stopped in 551 by the GANC 510. Since in this case CeIl-I (and intermediate nodes) supports packet-switched handover, CeIl-I is not added to the Target cells without PSHO support list 51 1 and the handover procedure continues as it would for the normal success case.

On the other hand, if the target cell information of CeIl-I was already included in the Targetcells without PSHO support list 511, then it is removed 551 from the list by the GANC 510. Thereafter, the packet-switched handover continues as it would for the normal success case. The reason why CeIl-I was already included in the list could either depend on the scenario described in the first embodiment of the present invention, i.e. no response received from CeIl-I, or could depend on the scenario described in the second embodiment of the present invention, i.e. a negative response is received from CeIl-I .

According to embodiments the present invention, the GANC 510 may also contain a garbage collection process that deletes cells from the Target_cells_\vithout_PSHO_support list 51 1 when the stored timestamp is older than a predefined limit. In this way, a cell/cells on this list obtains a neutral status. Thus, once removed from the list the GANC 510 will treat the cell/cells as candidate neighbour target cell/cells for which packet-switched handover never yet been attempted.

It is obvious from the discussion above that cell identities can be manually added to the Target_cells_without_PSHO_support list 511 by means of operator intervention. For these cells, it is preferable to set the timestamp to "infinite" indicating that the packet-switched handover should not be automatically triggered towards these cells. For example, these specific cells could be cells where the packet-switched handover feature will never be enabled and consequently unnecessary packet-switched handover attempts from the serving cell controller towards these cells are completely avoided.

Furthermore, it is also possible to remove all or some cell identities belonging to a certain GERAN system or UTRAN system from the Target cells without PSHO support list 51 1 by means of operator intervention. This removal allows the GANC 510 to potentially become aware of new target cells for packet-switched handover.

Figure 6 illustrates a block diagram of a serving cell controller 600 according to a second aspect of the present invention. The serving cell controller is for example a Generic Access Network controller 600 operable in a Generic Access Network, for dynamically and automatically detecting target cells without support for packet switched handover. The serving cell controller could also be a base station controller (BSC), a base station system (BSS) or a radio network controller (RNC). In Figure 6, elements that are not necessary for understanding the present invention have been omitted, such as for instance interfaces towards other nodes or towards mobile stations or other controller components. In addition, interconnections between the different components of the serving cell controller are omitted.

According to Figure 6, the serving cell controller 600 comprises selection means 610 for selecting a candidate neighbour target cell for packet-switched handover; initiation means 620 for initiating a packet-switched handover attempt towards the selected candidate neighbour target cell and a local database 630 arranged for storing in a list or data structure 631 the result of an unsuccessful packet-switched handover attempt made towards the selected candidate neighbour target cell, wherein the result comprises the candidate neighbour target cell identity and a timestamp for the candidate neighbour target cell, indicating when the target cell was most recently added to the list. The timestamp is also used by the serving cell controller 600 to decide when to enable a new packet-switched handover attempt towards that candidate neighbour target cell.

A illustrated in Figure 6, the serving cell controller 600 also comprises a garbage collection process 640 arranged to delete the identity of candidate neighbour target cells included in the list 631 when the timestamp for each candidate neighbour target cell is older that a predefined time limit.

Figure 7 illustrates a block diagram of a mobile telecommunications system 700 according to a third aspect of the present invention. The mobile system 700 comprises a serving cell controller 730 adapted for dynamically and automatically detecting candidate neighbour target cells without support for packet-switched handover.

In Figure 7, elements that are not necessary for understanding the present invention have been omitted.

According to Figure 7, a second radio access technology 740 is connected to a first radio access technology 720 and its associated serving cell controller 730. A mobile station 710 is also illustrated in Figure 7. The first radio access technology 720 is preferably a generic access network (GAN), and the second radio access technology 740 is preferably a GERAN and/or UTRAN network.

When the mobile station 710 is to be handed over to a candidate neighbour target cell of the second radio access technology 740, the serving cell controller 730 dynamically and automatically detects if the candidate neighbour target cell has support for packet-switched handover as described earlier.

Note that first and second radio access technologies 720 and 740 could be of the same type. In addition, the first radio access technology 720 could be a UTRAN/GERAN network, and the second radio access technology 740 could be a GAN network

The present invention, in its various embodiments, effectively eliminates or at least mitigates the need for neighbouring cell packet-switched handover capability information to be manually configured in a serving cell controller. The present invention also successfully ensures that unnecessary packet-switched handover signalling and the associated CPU processing in the various network nodes is either kept to a minimum or effectively avoided.

It is noted that whilst embodiments of the present invention have been described in relation to a handover between a GAN mode and a GERAN/UTRAN mode, embodiments of the proposed solution can be used in any communication system wherein advantages may be obtained by means of the embodiments of the present invention.

What is required is that a handover capability of a candidate neighbour target cell of a given radio access technology is dynamically and automatically detected by a serving cell controller associated with the same or a different radio access technology.

Therefore, the present invention is also applicable in a system where a dual-transfer mode

(DTM) handover capability of a cell is to be dynamically and automatically detected by a serving cell controller as described earlier.

While the invention has been described in terms of several preferred embodiments, it is contemplated that alternatives, modifications, peπnutations and equivalents thereof will become apparent to those skilled in the art upon reading of the specifications and study of the drawings. It is therefore intended that the following appended claims include such alternatives, modifications, permutations and equivalents as fall within the scope of the present invention

Claims

1. A method for a serving cell controller (510, 600, 730), of dynamically and automatically detecting at least one candidate neighbour target cell without support for packet-switched handover from said serving cell controller (510, 600, 730) of a first radio access technology towards a cell supported by a second radio access technology, c h a ra cterized in t h a t the method comprises the steps of:
- maintaining a list arranged (51 1, 631) to contain candidate neighbour target cells without support for packet-switched handover; - selecting (506) a candidate neighbour target cell for packet-switched handover;
- initiating (508) a packet-switched handover attempt towards said selected candidate neighbour target cell; and
- dynamically storing (509, 541 ) in said list (51 1 , 631 ), the result of an unsuccessful packet-switched handover attempt made towards the selected candidate neighbour target cell, wherein said result comprises the identity (512) of the candidate neighbour target cell and a corresponding tirnestamp (513) for the candidate neighbour target cell, indicating when said candidate neighbour target cell was most recently added to the list (51 1 , 631), and using said timestamp (513) to decide when to enable a new packet-switched handover attempt towards said candidate neighbour target cell.
2. The method according to claim 1 wherein said selecting step (506) comprises receiving from a mobile station (500, 710) registered in said serving cell controller (510, 600, 730), a message (503) comprising the identity of at least one preferred candidate neighbour target cell and selecting (506) by said serving cell controller (510, 600, 730) a candidate neighbour target cell for a packet-switched handover attempt.
3. The method according to claims 2 wherein said message (503) further includes measurements reports for at least one preferred candidate neighbour target cell and wherein said measurements are performed by said mobile station (500, 710).
4. The method according to any of claims 1-3 further comprises storing (509, 541) in a reason field (514) in said list (51 1 , 631), indicating the reason why said candidate neighbour target cell was initially stored in the list (51 1 , 631) and wherein said reason field (514) is used together with said timestamp (513) to enable a new packet-switched handover attempt towards said candidate neighbour target cell.
5. The method according to any of claims 1-4 further comprises the step of starting (507) a packet-switched handover detection timer by said serving cell controller (510, 600, 730), wherein said packet-switched handover detection timer is associated with a packet- switched handover attempt towards said candidate neighbour target cell and is used to supervise said packet-switched handover attempt.
6. The method according to claims 1-5 wherein said packet-switched handover detection timer expires (509) when no response is received from said candidate neighbour target cell, indicating that said candidate neighbour target cell does not support packet-switched handover, and wherein said reason field (514) indicates that no response is received from said candidate neighbour target cell.
7. The method according to claims 1 -5 wherein said packet-switched detection timer is stopped (541) by said serving cell controller (510, 600, 730) when a negative response of the packet-switched handover attempt is received (540) from said candidate neighbour target cell, indicating that said candidate neighbour target cell does not support packet- switched handover and wherein said reason field (514) indicates that a negative response is received (540) from said candidate neighbour target cell.
8. The method according to claims 1 -5 wherein said packet-switched detection timer is stopped (551 ) by said serving cell controller (510, 600, 730) when a positive response of the packet-switched handover attempt is received (550) from said candidate neighbour target cell, indicating that said candidate neighbour target cell does support packet- switched handover and completing a packet-switched handover towards said candidate neighbour target cell.
9. The method according to claim 8 wherein if a positive response of the packet-switched handover attempt is received (550) from the candidate neighbour target cell, the identity of said candidate neighbour target cell and the timestamp for said candidate neighbour target cell are removed (551) from said list (51 1 , 631) if said candidate neighbour target cell was previously included in said list (51 1 , 631 ).
10. The method according to any of claims 1 -9 wherein said timestamp (512) is used to determine if a predetermined time has elapsed from the last time said unsuccessful packet- switched handover was attempted for said candidate neighbour target cell.
1 1. The method according to 10 wherein once said predetermined time has elapsed, a new packet-switched handover attempt is made towards said candidate neighbour target cell if measurement reports received from said mobile station indicate that said candidate neighbour target cell is a candidate for packet-switched handover.
12. The method according to any of claims 10-11 wherein said predetermined time depends on the number of unsuccessful packet-switched handover attempts made by the serving cell controller (510, 600, 730) towards said candidate neighbour target cell.
13. The method according to any of claims 2-12 wherein said reason field (514) together with said timestamp (513) are used to determine if sufficient time has elapsed from the last time said unsuccessful packet-switched handover was attempted towards said candidate neighbour target cell.
14. The method according to any of claims 1-13 wherein said candidate neighbour target cell is a GSM/EDGE radio access network, GERAN, cell identified with the combination of a routing area identity, RAI, and a cell identity, CI.
15. The method according to any of claims 1 -13 wherein said candidate neighbour target cell is a UMTS terrestrial radio access network, UTRAN, cell identified with the combination of a routing area identity, RAI, a radio network controller identification, RNC-ID, and a third generation cell identity, 3G-C1.
16. The method according to any of claims 1-15 wherein said serving cell controller (510, 600, 730) is a generic access network controller, GANC, and wherein said first radio access technology is a generic access network, GAN.
17. The method according to any of claims 1-15 wherein said serving cell controller (510, 600, 730) is a base station controller, BSC, a base station system, BSS, or a radio network controller, RNC, and wherein said first radio access technology is a GERAN/UTRAN radio access network.
18. The method according to any of claims 1-17 wherein said second radio access technology is a GERAN/UTRAN radio access network.
19. The method according to any of claims 1-17 wherein said second radio access technology is a generic access network, GAN.
20. The method according to any of any of claims 1-19 wherein said serving cell controller (510, 600, 730) further comprises a garbage collection process (640) arranged to delete the identity of candidate neighbour target cells included in said list (51 1, 631), when said timestamp (513) for each candidate neighbour target cell included in said list (51 1 , 631) is older than a predefined time limit.
21. The method according to any of claims 1-20 wherein said packet-switched handover is part of a dual transfer mode handover, DTM handover, and the packet-switched handover is running in parallel with a circuit-switched handover in said DTM handover.
22. A serving network controller (600, 510, 730) operable in a first radio access technology (710) for dynamically and automatically detecting at least one candidate neighbour target cell without support for packet-switched handover from said serving cell controller (600, 510, 730) towards a cell supported by a second radio access (740) technology ch a ra c terized in th at said serving cell controller (600, 510, 730) comprises:
- selection means (610) for selecting a candidate neighbour target cell for packet- switched handover; - initiation means (620) for initiating a packet-switched handover attempt towards said selected candidate neighbour target cell; and
- a local database (630) arranged to dynamically store in a list (631, 51 1 ), the result of an unsuccessful packet-switched handover attempt made to said selected candidate neighbour target cell, wherein said result comprises the identity of the candidate neighbour target cell (512) and a corresponding timestamp (513) for the candidate neighbour target cell, indicating when said candidate neighbour target cell was most recently added to the list (631 , 51 1), and using said timestamp (513) to decide when to enable a new packet-switched handover attempt towards said candidate neighbour target cell.
23. The serving cell controller according to claim 22 wherein said serving cell controller (600, 510, 730) is a generic access network controller, GANC
24. The serving cell controller according claims 22 wherein said serving cell controller (510, 600, 730) is a base station controller, BSC, a base station system, BSS, or a radio network controller, RNC.
25. The serving cell controller according to any of claims 22-24 wherein said list (631, 51 1) further comprises a reason field (514) arranged to indicate the reason why said candidate neighbour target cell was initially stored in the list (631, 51 1) and wherein said reason field (514) is used together with said timestamp (513) to enable a new packet-switched handover attempt towards said candidate neighbour target cell.
26. The serving cell controller according to any of claims 22-25, is further configured to start a packet-switched handover detection timer wherein said timer is associated with a packet-switched handover attempt towards said candidate neighbour target cell and is used to supervise said packet-switched handover attempt.
27. The serving cell controller according to any of claims 22-26 wherein said packet-switched handover detection timer expires when no response is received from said candidate neighbour target cell, indicating that said candidate neighbour target cell does not support packet-switched handover, and wherein said reason field (514) indicates that no response is received from said candidate neighbour target cell.
28. The serving cell controller according to any of claims 22-27 is further configured to stop said packet-switched handover detection timer when a negative response of the packet- switched handover is received from said candidate neighbour target cell, indicating that said candidate neighbour target cell does not support packet-switched handover and wherein said reason field (514) indicates that a negative response is received from said at least candidate neighbour target cell.
29. The serving cell controller according to any of claims 22-28 is further configured to stop said packet-switched handover detection timer when a positive response of the packet- switched handover attempt is received from said candidate neighbour target cell, indicating that said candidate neighbour target cell does support packet-switched handover and wherein said serving cell controller (600, 510, 730) is configured to complete said packet-switched handover towards the candidate neighbour target cell.
30. The serving cell controller according to claim 29 is further configured to remove the identity (512) of the candidate target cell and the timestamp (513) for the candidate neighbour target cell in case said positive response of the packet-switched handover attempt is received and said candidate neighbour target cell was previously stored in said list (511, 631).
31. The serving cell controller according to any of claims 22-30 is further configured to use said timestamp (514) to determine if a predetermined time has elapsed from the last time said unsuccessful packet-switched handover was attempted for said candidate neighbour target cell.
32. The serving cell controller according to any of claims 22-32 wherein said selection means (610) is arranged to receive from a mobile station (500, 710) registered in said serving cell controller (600, 510, 730) a message comprising the identity of at least one preferred candidate neighbour target cell and wherein said serving cell controller (600, 510, 730) is arranged to select a candidate neighbour target cell for a packet-switched handover attempt.
33. The serving cell controller according to claim 32 wherein said received message further includes measurements reports for at least one preferred candidate neighbour target cell and wherein said measurements are performed by said mobile station (500, 710).
34. The serving cell controller according to any of claims 22 wherein said first radio access technology (720) is a generic access network, GAN, and wherein said second access technology (740) is a GSM/EDGE radio access network, GERAN/UTRAN.
35. The method according to any of claims 22 wherein said first radio access technology (720) is a GERAN/UTRAN network and said second radio access technology (740) is a GAN network.
36. The serving cell controller according to any of claims 22-35 further comprises a garbage collection process (640) configured to delete the identity of candidate neighbour target cells (512) included in said list (631 , 51 1), when said timcstamp (513) for each candidate neighbour target cell is older than a predefined time limit.
37. A mobile telecommunications system (700) comprising a serving cell controller (730, 600 510) according to any of claims 22-36, for dynamically and automatically detecting at least one candidate neighbour target cell without support for packet-switched handover.
PCT/SE2006/050348 2006-09-25 2006-09-25 A method, a serving cell controller and a system for detecting support for packet-switched handover WO2008039124A1 (en)

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