GB2445987A

GB2445987A - Relocation in a cellular communication system

Info

Publication number: GB2445987A
Application number: GB0701060A
Authority: GB
Inventors: David Padfield; Gerard Foster; Luis Lopes
Original assignee: Motorola Inc
Current assignee: Motorola Solutions Inc
Priority date: 2007-01-19
Filing date: 2007-01-19
Publication date: 2008-07-30
Anticipated expiration: 2027-01-19
Also published as: GB2445987B; GB0701060D0; WO2008088592A1

Abstract

A network element (115) for a cellular communication system supports base stations (111,113) with a shared pilot signal scrambling code. The network element (115) receives a relocation request message for a remote station (117) supported by a base station (101). The relocation request comprises an identification of the shared scrambling code. A group processor (205) determines a group of potential target base stations (111, 113) for the relocation request in response to the identification of the shared pilot signal scrambling code. A request processor (207) transmits a request message to the target base stations (111, 113). The request message comprises a parameter indication for an uplink transmission from the remote station (117) to the base station (101) and requests the target base stations (111, 113) to measure the uplink transmission. A measurement processor (209) receives measurement reports for the uplink transmission from the target base stations and a selection processor (211) selects a handover target base station (111) from the target base stations (111, 113) based on the measurement reports.

Description

RELOCATION IN A CELLULAR COMMUNICATION SYSTEM

Field of the invention

The invention relates to relocation in a cellular communication system and in particular, but not exclusively, to handover from a macro-cell to a pico-cell in a 3' Generation cellular communication system.

Background of the Invention

A method which has been used to increase the capacity of cellular communication systems is the concept of hierarchical cells wherein a macro-cell layer is underlayed by a layer of typically smaller cells having coverage areas within the coverage area of the macro-cell. In this way, smaller cells, known as micro-cells or pico-cells (or even femto-cells), are located within larger macro-cells. The micro-cells and pico-cells have much smaller coverage thereby allowing a much closer reuse of resources.

Frequently, the macro-cells are used to provide coverage over a large area, and micro-cells and pico-cells are used to provide additional capacity in e.g. densely populated areas and hotspots. Furthermore, pico-cells can also be used to provide coverage in specific locations such as within a residential home or office.

In order to efficiently exploit the additional resource, it is important that handover performance between the macro-cell layer and the underlying layer is optimized. The process of handover can be separated into three phases.

Firstly, identifying that a handover might be required, secondly, identifying a suitable handover candidate and finally, switching the mobile user from one base station to another.

The current trend is towards introducing a large number of pico-cells to 3G systems. For example, it is envisaged that residential access points may be deployed having a target coverage area of only a single residential dwelling or house. A widespread introduction of such systems would result in a very large number of small underlay cells within a single macro-cell.

However, underlaying a macro-layer of a 3G network with a pico-cell (or micro-cell) layer creates several issues. For example, the introduction of a large number of underlay cells creates a number of issues related to the identification of individual underlay cells when e.g. handing over to an underlay cell. In particular, 35 communication systems are developed based on each cell having a relatively low number of neighbours and extending the current approach to scenarios wherein the mobile station may need to consider large numbers of potential neighbour cells is not practical.

One problem of extending current approaches to scenarios where there are many underlaying pico-cells is how to uniquely and efficiently identify a pico-cell (or micro-cell) . Specifically, it is not practically feasible to assign individual pilot signal scrambling codes or frequency/base station identity combinations to each underlay cell and to identify all potential handover underlay cells as neighbours of the macro-cell as this would require very large neighbour lists. These large neighbour lists would e.g. result in the neighbour list exceeding the maximum allowable number of neighbours in the list, slow mobile station measurement performance as a large number of measurements would need to be made, increased resource usage etc. It would furthermore require significant operations and management resource in order to configure each macro-cell with the large number of neighbours and would complicate network management, planning and optimisation. However, sharing scrambling codes for the pilot signals of the pico-cells results in a target ambiguity and prevents the mobile station from uniquely identifying a potential handover target. For example, if a group of base stations supporting different underlay cells underlaying a given macro-cell use an identical shared pilot signal scrambling code, a mobile station detecting the presence of this shared scrambling code will be aware that a potential handover target has been detected but will not be able to uniquely identify which of the group of underlay cells has been detected.

Hence, an improved cellular communication system would be advantageous and in particular a system allowing increased flexibility, improved suitability for large numbers of potential handover targets/neighbour cells, improved suitability for overlay/underlay handovers, reduced neighbour lists, increased practicality, reduced measurement requirements, facilitated and/or improved handover target detection/identification and/or improved performance would be advantageous.

Sununary of the Invention Accordingly, the Invention seeks to preferably mitigate, alleviate or e'iminate one or more of the above mentioned disadvantages singly or in any combination.

According to a first aspect of the invention there is provided a network element for a cellular communication system, the network element being arranged to support at least a plurality of base stations having a shared pilot signal scrambling code; the network element comprising: means for receiving a relocation request message for a remote station supported by a first base station, the relocation request comprising an identification of the shared pilot signal scrambling code; determining means for determining a group of base stations as potential target base stations for the relocation request in response to the identification of the shared pilot signal scrambling code; request means for transmitting a request message to the group of base stations, the request message comprising a parameter indication for an uplink transmission from the remote station to the first base station and requesting the group of base stations to measure the uplink transmission; means for receiving measurement reports for the uplink transmission from the group of base stations; and selection means for selecting a handover target base station from the group of base stations in response to the measurement reports.

The invention may allow improved and/or facilitated operation in a cellular communication system and may in particular allow improved identification of a handover target from a group of cells sharing a pilot signal scrambling code thereby improving e.g. handover performance.

In particular, the invention may allow improved identification of an underlay target handover cell for a remote station currently served by a macro-cell. The invention may allow highly robust underlay cell identification while using a reduced amount of resources. In particular, the invention may require fewer scrambling codes while still allowing a given number of underlay cells to be identified. The invention may in some embodiments facilitate haridover. The invention may in some embodiments facilitate or enable support of large numbers of underlay cells.

The invention may e.g. allow improved handover in a cellular communication system. In particular, the invention may facilitate or improve handovers in systems wherein a remote station may have a large number of possible handover targets. In particular, the invention may allow a reduced number of measurements being required by a remote station to determine a suitable handover target, may allow reduced neighbour lists and/or may reduce the required number of scrambling codes.

The cells supported by the group of base stations may e.g. be micro-cells, pico-cells and/or femto-cells. The cellular communication system may be a Code Division Multiple Access cellular communication system such as a Universal Mobile Telecommunication System (UMTS) . The remote station may for example be a User Equipment or a mobile communication unit, e.g. of a 3Id generation cellular communication system.

The network element may in some embodiments appear to the network as a virtual Radio Network Controller (RNC) which supports a plurality of base stations sharing a pilot signal scrambling code. Following identification of the handover target base station, the routing/handover setup may switch to the cictual RNC supporting the bandover target base station and a conventional handover process may e.g. be used between this RNC and the RNC supporting the remote station prior to the handover. Specifically, the identification of the handover target base station may result in a message being transmitted to the RNC serving the handover target base station in response to which the RNC may proceed in setting up the relocation process.

A relocation of a remote station may be any process or activity wherein the remote station moves from being supported by one cell to being supported by another cell.

Specifically, the relocation request message may be any indication that a movement or switch of the remote station from the first base station to the handover target base station may be desired. This movement/switch may for example be a handover of the remote station to the handover target base station from the first cell. The relocation may be associated with a modification of a routing path in the fixed network for the remote terminal (e.g. from a core network element to the handover target access point) The relocation request message may be generated internally in the network element, e.g. it may be a flag or other indication generated by a handover evaluation functionality comprised in the network element.

According to an aspect of the invention there is provided a Code Division Multiple Access, CDMA, cellular communication system comprising a network element as previously described and further comprising: a source radio network controller for the first base station, the radio network controller being arranged to generate the relocation reguest message in response to receiving an indication of detection of the shared pilot signal scrambling code from the first remote station; and address means for determining an network element address of the network element in response to the shared pidot signal scrambling code; means for transmitting the relocation request message to the network element using the network element address The generated relocation request message may be represented by different relocation messages at different locations/stages of the communication and may be modified by intervening network elements. The address means and/or the means for transmitting may be located at any suitable physical or logical location in the network including in a Mobile Switching Centre (MSC) and/or in the source radio network controller.

For example, the source radJo network controller may generate a relocation request message in the form of a 3GPP Relocation Required message which does not specifically address any destination. A serving MSC may convert this message into a relocation request message in the form of a 3GPF Relocation Request message and may further comprise the address means arranged to address the 3GPP Relocation Request message to the network element.

The term relocation request message may be considered a common generic term comprising all specific messages used to communicate a relocation request from the source radio network controller to the network element and/or a radio network controller supporting the handover target base station. As such it encompasses different messages used at different stages of the path and/or may include a plurality of parallel messages used to indicate the request for a relocation.

According to another aspect of the invention, there is provided a method of operation for a network element of a cellular communication system, the network element being arranged to support at least a plurality of base stations having a shared pilot signal scrambling code; the method comprising the network element performing the steps of: receiving a relocation request message for a remote station supported by a first base station, the relocation request comprising an identification of the shared pilot signal scrambling code; determining the group of base stations as potential target base stations for the relocation request in response to the identification of the shared pilot signal scrambling code; transmitting a request message to the group of base stations, the request message comprising a parameter indication for an uplink transmission from the remote station to the first base station and requesting the group of base stations to measure the uplink transmission; receiving measurement reports for the uplink transmission from the group of base stations; and selecting a handover target base station from the group of base stations in response to the measurement reports.

These and other aspects, features and advantages of the invention wiil be apparent from and elucidated with reference to the embodiment(s) described hereinafter.

Brief Description of the Drawings

Embodiments of the invention will be described, by way of example only, with reference to the drawings, in which FIG. 1 illustrates an example of a cellular communication system in accordance with some embodiments of the invention; FIG. 2 illustrates an example of a network element in accordance with some embodiments of the invention; and FIG. 3 illustrates an example of a method of operation for a network element in accordance with some embodiments of the invention.

Detailed Description of Some Embodiments of the Invention The following description focuses on embodiments of the invention applicable to a COMA cellular communication system and in particular to a 3Ld Generation Cellular communication system such as a UMTS System. However, it will be appreciated that the invention is not limited to this application but may be applied to many other cellular communication systems. Also, the description will focus on scenarios where a remote station is handing over from a macro--cell to an underlay cell such as a micro-cell or a pico-cell. However, it will be appreciated that the described principles apply equally to other scenarios including e.g. some scenarios where a handover is made to a macro--cell out of a group of macro-cells using a shared pilot signal scrambling code.

FIG. 1 illustrates an example of a cellular communication system which in the specific example is a UMTS cellular communication system. In the system, a macro-layer is formed by macro-cells supported by base stations. Furthermore, an underlay layer of pico-cells are supported by a large number of small base stations which henceforth will be referred to as access points. Specifically, each access point may have an intended coverage of a single house or dwelling, and for a typical macro-cell coverage area of 10 to 30 km there may be hundreds or even thousands of pico-cells each supported by an individual access point.

In the system, the macro base stations each have a cell separation code in the form of a scrambling code that is unique within a given region which e.g. may be a reuse area for the cell scrambling codes. Specifically the macro base stations have an assigned scrambling code which is unique within the reuse area such that a set of defined neighbours for each cell always have unique cell scrambling codes.

Furthermore, each macro-cell base station has a unique hierarchical network address given by a unique base station ID for a given serving RNC, which itself has a unique RNC ID for a given MSC. Furthermore, each MSC has a unique identity in the network.

Accordingly, the neighbour lists transmitted by the base stations comprise indications of macro-cells which all have different cell scrambling codes. Furthermore, for each macro neighbour cell, a unique network address of the base station supporting the macro-cell can be determined from the detection of a specific neighbour cell pilot signal.

Accordingly, a handover to a target macro-cell may be initiated with an explicit and unique identification of the handover target base station.

In contrast, the access points (which in the specific example are base stations supporting pico-cells) use a scrambling code which is shared between a plurality of access points within the reuse area and specifically a given neighbour list may comprise indications of shared pilot signal scrambling codes for a plurality of underlay cells that are all considered as neighbours/potential handover targets for the current cell. By sharing a pilot signal scrambling code between a plurality of access points, a much reduced number of scrambling codes are required by the system. Furthermore, by keeping the number of scrambling codes low, the number of scrambling codes that must be evaluated by the remote station for handover determination can be reduced substantially thereby reducing the measurement time, power consumption and/or complexity of the remote station.

However, the use of a shared pilot signal scrambling code means that the remote station (or supporting network nodes) cannot uniquely identify the access point which has been detected by the remote station simply from the detected scrambling code. Rather, a remote station detecting a scrambling code does not uniquely identify a given target access point for a handover but at best identifies only a group of access points which all use the same shared pilot signal scrambling code.

In some embodiments, all access points within a coverage area supported by a single macro-RNC may use the same scrambling code. However, it will be appreciated that in other embodiments, a plurality of shared scrambling codes may be available for the access points. Therefore, the access points may be divided into a number of groups with the access points of each group sharing a scrambling code but with different scrambling codes being used for different groups. In such embodiments, the scrambling codes may be allocated to the access points such that a reuse pattern is established with the interference between pico-cells having the same shared scrambling code being reduced or minimised.

In the specific example of FIG. 1, one macro-base station 1OJ which supports a macro-cell with a typical coverage area of 10-30 kilometres is illustrated. The macro base station 101 is coupled to a macro RNC 103 which is furthermore coupled to other macro base stations (not shown) . The macro RNC 103 is furthermore coupled to a core network 105 which interfaces to other radio access networks and RNCs. In the example, the macro RNC 103 is coupled to a first MSC 107 which is further coupled to a second MSC 109 serving a different set of RNCs than the first MSC 107.

The system furthermore comprises a large number of pico-cell base stations/access points 111, 113 (for clarity only three access points are illustrated in FIG. 1) . Each of the access points 111, 113 supports a pico-cell having a coverage area of typically 10 to 50 meters. The access points 111, 113 implement the required functionality of a UMTS base station in order to support UMTS communications within the pico-cell. However, in contrast to conventional UMTS base stations, the access points 109 use a common shared pilot signal scrambling code.

Furthermore, in the example, each of the access points 111, 113 comprises some RNC functionality such that the network interface to the access points 111, 113 is the same as to an RNC. In other words, each access point 111, 113 appears as an RNC to the network and each access point 111, 113 has an individual RNC identity (RNC ID) . In the specific example, the access points/pica base stations 111, 113 are accordingly coupled directly to a serving MSC which in this case is the second MSC 109.

The system of FIG. 1 furthermore comprises a network element which may operate as a virtual RNC 115 for the access points 111, 113 during the initial phases of a handover.

Specifically, when a remote station detects a shared pilot signal scrambling code, a relocation request message may be transmitted to the virtual RNC 115 which then may resolve the ambiguity and identify the detected access point 111, 113 as a suitable handover target.

For example, in the system of FIG. 1, a remote station 117 may initially be served by the macro base station 101. When monitoring the pilot signals (CPIOHs) of the neighbour list, the remote station 117 may detect the pilot signal of a first access point 111 of the access points 111, 113. The shared pilot signal scrambling code may he decoded to provide identification data for the first access point 111.

However, as the scrambling code is shared by a large number of access points 111, 113, it is not possible for the source system to uniquely determine the identity of the first access point 111.

However, in the example, the shared scrambling code may he associated with the address (RNC ID) of the virtual RNC 115.

In this case, the macro RNC 103 may determine the preference for a handover (based on the reported pilot signal measurements from the remote station 117) and may accordingly transmit a handover request message addressed to the virtual RNC 115. When this handover request message is received at the virtual RNC 115, it proceeds to determine which of the access points 111, 113 is the detected target handover access point 111.

Specifically, the virtual RNC 115 first uses the indication of the shared pilot signal scrambling code to determine a group of base stations/ access points which are potential targets base stations/access points. For example, the group may be determined as all the access points 111, 113 using the shared pilot signal scrambling code. It will be appreciated that this may be most practical in scenarios where a relatively low number of access points share a given shared pilot signal scrambling code and that in other embodiments further measures may be taken to reduce the group of potential targets as will be described later.

The virtual RNC 115 proceeds to instruct all access points 111, 113 in the potential target group to make measurements of the uplink transmissions from the remote station 117 to the macro-base station 101. Specifically, the measurements may comprise a signal level measurement determined as an amplitude/power indication of a correlation of uplink pilot bits unscrambled by the remote station's uplink scrambling code. This may further be combined with the observed time difference and chip offset information which wilJ result in a very high probability that one access point will achieve a strong correlation while all others will achieve no correlation.

The access points 111, 113 make these measurements and return the resulting measurement reports to the virtual RNC 115. If the pico-cells are reasonably spaced apart, it is likely that the detected access point 111 is the closest access point and that this will measure the highest signal level for the uplink transmission of the remote station 117.

Accordingly, the virtual RNC 115 can select the access point measuring the highest signal level as the target handover access point 111.

The virtual RNC 115 can then transmit a message to the selected target handover access point 111 identifying the source of the relocation request. The target access point 111 accordingly initiates a handover procedure by directly communicating with the macro-RNC currently serving the remote station 117. Such a handover process may follow a conventional handover procedure between a serving and target RNC in a UMTS system.

An exemplary handover will in the following be described in more detail with reference to FIG. 2 which illustrates the virtual RNC in more detail and FIG. 3 which illustrates an example of a method of operation for the virtual RNC 115.

Initially, the remote station 117 is served by the macro base station 101 and performs user data uplink transmissions supported by the macro base station 101. At the same time, the remote station 117 scans for the pilot signals of the cells indicated in the received neighbour list. This includes the shared pilot signal scrambling code. At some stage, the remote station 117 detects the presence of the shared pilot signal scrambling code and transmits a measurement report. For example, if the macro-cell level drops below a threshold and the remote station 117 measures the shared pilot signal scrambling code, the remote station 117 transmits a message which is an indication of the detection of the shared pilot signal scrambling code.

The message is received by the macro base station 101 and forwarded to the macro RNC 103. In response, the macro RNC 103 generates a relocation request message which is specifically implemented in the form of a UMTS Relocation Required message which is transmitted from the macro RNC 103 to the first MSC 107.

The Relocation Required message may not comprise any specific addressing of a target RNC (such as the virtual RNC 115) but may comprise various characterising data that allows the address of an RNC to be determined. The message can specifically include a Radio Resource Control (RRC) transparent container which comprises information that may enable or assist the routing of the message and/or the identification of a suitable set of target access points and/or may assist or facilitate the measuring performed by these.

In the specific example, the RRC container comprises an identification of the detected pilot signal scrambling code, the identity of the first cell and the scrambling code used for the uplink user data transmissions to the first base station 101 from the remote station 117. It will be appreciated that the RRC container may comprise other or additional information in other embodiments. Specifically, the RRC container may comprise an indication of the frequency used for the uplink transmissions.

The relocation request message including the RRC transparent container is transmitted to the first MSC 107 which proceeds to determine the network element address of the virtual RNC 115 based on the information received in the RRC transparent container.

Specifically, in some embodiments, the virtual RNC 115 may support all access points 111, 113 that share a given shared pilot signal scrambling code and the first MSC 107 may simply comprise a list uniquely linking a given shared pilot signal scrambling code to a specific network element address for the corresponding virtual RNC 115. Furthermore, the network element address of the virtual RNC 115 is in the example unique within the network such that the routing of the relocation request message to the virtual RNC 115 may be performed based only on this address.

In a more typical scenario, the same shared pilot signal scrarnb]Jng code may be re-used in the cellular communication system such that different virtual RNC5 may support different access points using the same shared pilot signal scrambling code. However, the virtual RNCs will typically be distributed geographically such that all access points within a (potentially large) geographic area using the same shared pilot signal scrambling code will be served by the same virtual RNC. Specifically, all access points that can be accessed from a given macro-cell will typically be served by the same virtual RNC 115 and accordingly, the MSC 107 may determine the virtual RNC 115 address by a unique link from the combination of the identity of the first macro-cell and the detected shared pilot signal scrambling code. The linking information may be provided manually by the network operator as part of the operations and maintenance process.

Once the network element address of the virtual RNC 115 has been identified, the relocation request message is transmitted to the virtual RNC 115 using this address.

Specifically, the first MSC 107 generates a UMTS Relocation Request message addressed to the virtual RNC 115 and comprising the RRC transparent container received from the macro RNC 103. The first MSC 107 may in some embodiments modify the transparent container and may specifically add or delete specific information elements.

The Relocation Request message is routed to the virtual RNC 115 via the second MSC 109. The virtual RNC 115 comprises an MSC interface 201 which is arranged to exchange messages with the MSO 201. The MSO interface 201 is coupled to a relocation processor 203 which executes step 301 wherein the Relocation Request message is received. Furthermore, the relocation processor 203 extracts the information contained in the RRC transparentcontainer. In particular, it extracts the shared pilot signal scrambling code.

The relocation processor 203 is coupled to a group processor 205 which executes step 303 wherein a group of base stations/access points 111, 113 is determined as potential target base stations/access points for the relocation request. The group of access points are the potential target access points for the relocation request which will be requested to monitor the remote station 117's uplink transmissions.

The group processor 205 determines the potential target access points in response to the shared pilot signal scrambling code. In some embodiments, only relatively few access points share the same pilot signal scrambling code and the group processor 205 may determine the group of access points as all access point using the identified shared pilot signal scrambling code.

However, it will be appreciated that in most practical embodiments, a large number of access points using the same shared pilot signal scrambling code will be served by the virtual RNC 115. In such embodiments, the group processor 205 may use further information comprised in the received relocation request message to determine a reduced group of access points to instruct to perform measurements. This will reduce the amount of measurements thereby reducing the communication overhead and the measurement resource overhead.

Specifically, the identification of the serving macro-cell comprised in the RRC transparent container can be used to determine a reduced group of access points using the shared pilot signal scrambling code. For example, in many scenarios, the pico-cells supported by the access points using the same scrambling code may be distributed within the coverage area of a relatively large number of macro-cells.

In this case, a remote station within one macro-cell will only be able to detect pilot signals from pico-cells within that macro-cell (or potentially within neighbouring macro-cells it the remote station is close to the border of the macro-cell) . Accordingly, the group processor 205 can determine the group of access points which are potential targets as the access points which are located within the currently serving macro-cell (and possibly within the neighbouring macros cells) . However, any access points located further away can be discarded as they currently cannot be potential handover targets for the remote station.

Thus, the group processor 205 uses the indication of the currently serving cell to determine a reduced group of potential target access points.

The group processor 205 is coupled to a request processor 207 which is arranged to execute step 305 wherein the virtual RNC 115 transmits a request message to the identified group of access points 111, 113. The request message instructs the group of access points 111, 113 to make measurements of the uplink transmission from the remote station 117 to the first base station 101. The request message includes a parameter indication for the uplink transmission which may be used by the access points 111, 113 to adapt the measurements to the uplink transmission.

Specifically, the request processor 207 may include the uplink scrambling code used by the remote station 117 for the communication to the first base station 101.

Specifically, the information of the uplink scrambling code received in the RRC transparent container may be forwarded to the access points 111, 113 and used to set the chip code used by a RAKE measurement receiver.

The request processor 207 is coupled to the MSC interface 201 and in the specific example the measurement request message is transmitted to the access points 111, 113 via the second MSC 109. It will be appreciated that in other embodiments, the virtual RNC 115 may be physically, logically or structurally located elsewhere. For example, the virtual RNC 115 may be located in the path between the second MSC 109 and the access points 111, 113 and the request message may be sent directly to the access point 111, 113. It will also be appreciated, that the measurement request message may be transmitted as a broadcast message which is received by all the relevant access points 111, 113 or may e.g. be transmitted by a plurality of messages each of which may be individually addressed to a specific access point 111, 113.

When the access points 111, 113 receive the measurement request message, they proceed to scan for the uplink transmissions. Specifically, the access points 111, 113 begin to determine a received signal level for the uplink signal of the remote station 117 using the specified uplink scrambling reported in the request message. The measurement request message may in some embodiments comprise an indication of a frequency of the uplink transmission and the access points 111, 113 can accordingly tune to this frequency when making the measurements. The measured signal level is then reported to the virtual RNC 115 by the transmission of one or more measurement reports from each of the access points 111, 113.

Thus, each of the identified group of potential target access points 111, 113 will measure a received signal level for a received signal which has parameters corresponding to the uplink transmission as indicated by the parameter indication. The exact timing of the uplink scrambling code may not be known and the access point 111, 113 may accordingly perform a search to synchroriise the measurements to the uplink transmissions.

Typically when a remote station detects the presence of a pilot signal from a pico-cell it will be located very close to that pico-cell and substantially further away from any other pico-cell using the same shared pilot signal scrambling code (e.g. a reuse pattern for a plurality of shared pilot signal scrambling codes may be applied) Accordingly, the uplink transmission from the remote station is likely to be received at a high signal level at only one of the pico-cells. Furthermore, even if the uplink transmission was detected with a high signal level at more than one access point, this is likely to be due to the remote station being located close to more than one access point with all of these access points being likely to be acceptable handover targets.

The virtual RNC 115 specifically comprises a measurement processor 209 coupled to the MSC interface 201. The measurement processor 209 executes step 307 wherein the measurement reports are received from the access points 111, 113.

The measurement processor 209 is coupled to a selection processor 211 which executes step 309 wherein a handover target access point 111 is selected from the group of potential target access points 111, 113 in response to the measurement reports. Specifically, the selection processor 211 compares the received signal levels and selects the access point 111 having the highest measured receive level as the handover target base station.

In the example, the selection processor 211 then proceeds to transmit a relocation message to the radio network controller which supports the handover procedures for the selected target access point 111. In the specific example, each of the access points 111, 113 comprise RNC functionality and the relocation message may therefore be transmitted directly to the target access point 111. The relocation message is in the specific example the LIMTS Relocation Request message which was received by the virtual RNC 115 from the first MSC 107.

When the target RNC (i.e. the RNC functionality of the access point 111 or an RNC supporting the selected access point) receives the Relocation Request, it proceeds to respond as if the message had been received directly from the first MSC 107. Specifically, the target RNC can transmit a relocation acknowledge message (e.g. a Relocation Request Acknowledge) to the macro-RNC thereby initiating a handover procedure generally following a conventional approach.

However, as the macro RNC 103 and/or the first MSC 107 has previously used the network element address of the virtual RNC 115, the routing path may be changed to use the network address of the target RNC.

As a specific example, when receiving the Relocation Request message, the target RNC allocates resources in the target pico-cell for the incoming hard handover and returns the matching configuration in e.g. a Physical Channel Reconfiguration message in an RRC transparent container to the macro RNC 103 (possibly via the virtual RNC 115) The macro RNC 103 then passes the reconfiguration to the remote station 117 which then attempts to access the target access point using the specified configuration. Thus, the source RNC (i.e. the macro RNC 103) transmits an access characteristic for the handover target base station to the remote station 117 when receiving the relocation acknowledge message from the target RNC. The remote station 117 then initiates an attachment to the target access point 111 using the indicated configuration/access characteristic.

The single selected access point 111 then receives an access from the remote station 117 and specifically the access point 111 detects uplink synchronisation at layer 1 and then receives the RRC reconfiguration confirm message from the remote station 117. A relocation detect and relocation complete is then signalled to the core network.

Furthermore, a second rapid relocation (without the remote station 117 being involved) can be executed to relocate the lu signalling connection from the virtual RNC 115 to the target RNC.

It will be appreciated that in some embodiments, the virtual RNC 115 may instead of (or as well as) transmitting the relocation message to the target RNC, transmit a relocation message back to a network element supporting the macro base station 101 with an identification of at least one network element supporting communications in the target cell.

Specifically, the selection processor 111 may generate a message which comprises the network address of the target RNC and transmit this message to the first MSC 107. In response, the first MSC 107 may initiate a retransmission of the original UMTS Relocation Request message but this time directly addressing the appropriate target RNC. A conventional haridover process between the source and the target network elements may then proceed.

In the above description, the serving base station identity and the uplink scrambling code was used to assist the relocation. However, it will be appreciated that in other embodiments, only one or none of these may be used. Also, it will be appreciated that other information may alternatively or additionally be used.

For example, the relocation request message may comprise an indication of a timing characteristic for the uplink transmission by the remote station 117. The indication may for example be included in the RRC transparent container.

The timing characteristic may specifically be a timing offset such as a frame timing offset and specifically a System Frame Number offset. E.g. the RRC transparent container can comprise an SFN-SFN observed time difference between the SFNs of the macro-cell and the detected pica-cell.

This information may be used to identify an appropriate virtual RNC. For example, if a given shared pilot signal scrambling code may be used by access points served by different virtual BNCs, the first MSC 107 may build up a database indicative of a current frame timing offset (SEN offset) for different access points (e.g. based on previous relocations) and may compare the current offset to the stored data to select the appropriate virtual RNC.

Alternatively or additionally, the SEN offset may be compared to stored values by the group processor 205 and used to select a subset of potential target access points.

Alternatively or additionally, the SFN offset may be fed to the potential target access points and used by these to perform suitable measurements. For example, the detection of the uplink transmissions may be verified by determining if the current measured frame synchronisation offset between the uplink signal and the access point corresponds to that of the SFN offset indicated in the relocation message.

It will be appreciated that a chip offset of the current macro-cell uplink transmissions by the remote station 117 may be used in a similar way.

As another example, in some embodiments, the relocation request message may comprise an indication of a propagation delay for the uplink transmission between the remote station 117 and the macro base station 101.

This propagation delay may for example be used to determine the group of target base stations. Specifically, the propagation delay will typically be indicative of a distance from the base station to the remote station. Typically, the location of the access points are known by the virtual RNC and the propagation delay that is likely to be experienced by a transmission to the macro base station for a remote station within a given pico-cell can thus be determined.

Accordingly, the virtual RNC can compare the received propagation delay to determine which pico-cells (and thus access points) are likely to be close to the remote station and thus are likely to be the access point currently detected.

Alternatively or additionally, the propagation delay may be fed to the group of potential target access points and used to synchronise the measurements e.g. by setting the initial timing for a search for the uplink scrambling code.

The above description has focussed on embodiments wherein the source MSC 107 processes the message from the macro RNC 103 in order to address the virtual RNC 115, and in some cases in order to provide additional information. However, it will be appreciated that the invention is not limited to this example and that in other embodiments the MSC 107 may e.g. be arranged to merely forward messages from the macro RNC 103 without processing these. Specifically, the macro RNC 103 may generate the message comprising all relevant information and directly address this to the virtual RNC 115. This data packet may be routed unmodified to the virtual RNC 115 via the macro MSC 107.

It will also be appreciated that although the above description focuses on a UMTS embodiment, it is equally applicable to other systems and is specifically applicable to hybrid communication systems using different radio access technologies. For example, the macro base station 101 may be a UMTS base station whereas the access point 111 may be a GSM base station capable of supporting GSM air interface communications but not UMTS air interface communications.

However, the access point 111 may still transmit a pilot signal using the shared pilot signal scrambling code which can be detected by the remote station 117. Furthermore, the access point 111 may be arranged to monitor for the UMTS uplink transmission to determine a signal level.

Thus, the same method of determining the handover target may be used. However, in the example, the remote terminal 117 is a dual mode remote station and the handover is an intersystem handover where the remote terminal 117 hands over from UMTS to GSM.

Also, although the above description has focussed on a macro RNC 103 remote from the virtual RNC 115, it will be appreciated that the described principles are equally applicable to a situation where these are integrated in the same physical or logical network element. For example, a virtual RNC may be built into a macro RNC. In such an example, the detection of the shared pilot signal scrambling code by a remote station may be evaluated by a subroutine which generates a relocation request message (e.g. by setting a flag) if the pilot signal is detected. In response thereto another routine can initiate the transmission of measurement requests to the access points and proceed to use these to identify the appropriate handover target. Once, the S handover target is identified the handover can be performed.

it will be appreciated that the above description for clarity has described embodiments of the invention with reference to different functional units and processors.

However, it will be apparent that any suitable distribution of functionality between different functional units or processors may be used without detracting from the invention. For example, functionality illustrated to be performed by separate processors or controllers may be performed by the same processor or controllers. Hence, references to specific functional units are only to be seen as references to suitable means for providing the described functionality rather than indicative of a strict logical or physical structure or organization.

The invention can be implemented in any suitable form including hardware, software, firmware or any combination of these. The invention may optionally be implemented at least partly as computer software running on one or more data processors and/or digital signal processors. The elements and components of an embodiment of the invention may be physically, functionally and logically implemented in any suitable way. Indeed the functionality may be implemented in a single unit, in a plurality of units or as part of other functional units. As such, the invention may be implemented in a single unit or may be physically and functionally distributed between different units and processors.

Although the present invention has been described in connection with some embodiments, it is not intended to he limiLed to the specfc form set forth herein. Rather, the scope of the present invention is limited only by the accompanying claims. Additionally, although a feature may appear to be described in connection with particular embodiments, one skilled in the art would recognize that various features of the described embodiments may be combined in accordance with the invention. In the claims, the term comprising does not exclude the presence of other elements or steps.

Furthermore, although individually listed, a plurality of means, elements or method steps may be implemented by e.g. a single unit or processor. Additionally, although individual features may be included in different claims, these may possibly be advantageously combined, and the inclusion in different claims does not imply that a combination of features is not feasible and/or advantageous. Also the inclusion of a feature in one category of claims does not imply a limitation to this category but rather indicates that the feature is equally applicable to other claim categories as appropriate. Furthermore, the order of features in the claims does not imply any specific order in which the features must be worked and in particular the order of individual steps in a method claim does not imply that the steps must be performed in this order. Rather, the steps may be performed in any suitable order.

Claims

1. A network element for a cellular communication system, the network element being arranged to support at least a plurality of base stations having a shared pilot signal scrambling code; the network element comprising: means for receiving a relocation reguest message for a remote station supported by a first base station, the relocation request comprising an identification of the shared pilot signal scrambling code; determining means for determining a group of base stations as potential target base stations for the relocation request in response to the identification of the shared pilot signal scrambling code; request means for transmitting a request message to the group of base stations, the request message comprising a parameter indication for an uplink transmission from the remote station to the first base station and requesting the group of base stations to measure the uplink transmission; means for receiving measurement reports for the uplink transmission from the group of base stations; and selection means for selecting a handover target base station from the group of base stations in response to the measurement reports.

2. The network element of claim 1 further comprising means for transmitting a relocation message to a radio network controller supporting relocation procedures for the handover target base station.

3. The network element of claim 1 further comprising means for transmitting a relocation message to a remote network element supporting the first base station, the relocation message comprising an identification of at least one network element supporting communications in a cell supported by the handover target base station.

4. The network element of claim 1 wherein the relocation request message comprises an indication of an uplink scrambling code used for the uplink transmission by the first remote station; and the request means is arranged to set the parameter indication to include an indication of the uplink scrambling code.

5. The network element of claim 1 wherein the relocation request message comprises an indication of a serving cell supported by the first base station; and the determining means is arranged to determine the group of base stations in response to the serving cell.

6. The network element of claim 1 wherein the relocation request message comprises an indication of a timing characteristic of the uplink transmission by the first remote station.

7. The network element of claim 6 wherein the timing characteristic is a frame timing offset.

8. The network element of claim 1 wherein the relocation request message comprises an indication of a propagation delay for the uplink transmission between the first remote station and the first base station.

9. The network element of claim 1 wherein measurement reports comprises signal level measurements for the uplink transmission; and the selection means is arranged to select the handover target base station as a base station measuring a highest signal level for the uplink transmission.

10. The network element of claim 1 wherein the first cell is a macro-cell and cells supported by the group of base stations are underlay cells of macro-cells.

11. The network element of claim 1 wherein the relocation request message comprises a 3d Generation Partnership Project, 3GPP, Radio Resource Control, RRC, transparent container.

12. A Code Division Multiple Access, CDMA, cellular communication system comprising a network element according to claim 1 and further comprising: a source radio network controller for the first base station, the source radio network controller being arranged to generate the relocation request message in response to receiving an indication of detection of the shared pilot signal scrambling code from the first remote station; address means for determining a network element address of the network element in response to the shared pilot signal scrambling code; and means for transmitting the relocation request message to the network element using the network element address

13. The CDMA cellular communication system of claim 12 wherein the address means is arranged to determine the network element address in response to a cell identity of a cell supported by the first base station.

14. The CDMA cellular communication system of claim 13 wherein there is a unique link from the shared pilot signal scrambling code and the cell identity to the network element address.

15. The CDMA cellular communication system of claim 12 wherein the network element address is a unique network address.

16. The CDMA cellular communication system of claim 12 further comprising the group of base stations arranged to measure, in response to receiving the request message, a received signal level for a received signal having parameters corresponding to the parameter indication.

17. The CDMA cellular communication system of claim 12 wherein the address means is comprised in a I4obile Switch Centre, MSC, supporting the source radio network controller.

18. The CDMA cellular communication system of claim 12 further comprising a target radio network controller supporting relocation procedures for the handover target base station and wherein the network element is arranged to transmit a relocation message to the target radio network controller and the target radio network controller is arranged to transmit a relocation acknowledge message to the source radio network controller in response to receiving the relocation message.

19. The ODMA cellular communication system of claim 18 wherein the source radio network controller is arranged to transmit an access characteristic for the handover target base station to the remote station in response to receiving the relocation message and the remote station is arranged to initiate an attachment to the handover target base station in accordance with the access characteristic.

20. A method of operation for a network element of a cellular communication system, the network element being arranged to support at least a plurality of base stations having a shared pilot signal scrambling code; the method comprising the network element performing the steps of: receiving a relocation request message for a remote station supported by a first base station, the relocation request comprising an identification of the shared pilot signal scrambling code; determining the group of base stations as potential target base stations for the relocation request in response to the identification of the shared pilot signal scrambling code; transmitting a request message to the group of base stations, the request message comprising a parameter indication for an uplink transmission from the remote station to the first base station and requesting the group of base stations to measure the uplink transmission; receiving measurement reports for the uplink transmission from the group of base stations; and selecting a handover target base station from the group of base stations in response to the measurement reports.