WO2007092686A2

WO2007092686A2 - Soft handover in a cellular communication system

Info

Publication number: WO2007092686A2
Application number: PCT/US2007/061105
Authority: WO
Inventors: Fernando De La Cruz
Original assignee: Motorola, Inc.
Priority date: 2006-02-08
Filing date: 2007-01-26
Publication date: 2007-08-16
Also published as: GB2435151B; GB2435151A; WO2007092686A3; GB0602527D0

Abstract

A handover apparatus comprises functionality for adding a soft handover connection to an active set of a remote station served by at least a first base station in a cellular communication system. A soft handover processor determines that the soft handover connection should be added to the active set. In response a resource processor sets up resources in support of a communication link between a base station supporting the soft handover connection and the remote station. In parallel, an instructing processor transmits a handover addition message to the remote station instructing the remote station to include the communication link in the active set. The instructing processor is arranged to transmit the handover addition message before the setting up of resources by the resource processor is completed. The invention is particularly suitable for a UMTS communication system and may provide reduced delay and improved soft handover performance.

Description

SOFT HANDOVER IN A CELLUI-AR COMMUNICATION SYSTEM

Field of the invention

The invention relates to addition of a soft handover connection to an active set for a remote station of a cellular communication system.

Background of the Invention

Currently, the most ubiquitous cellular communication system is the 2nd generation communication system known as the Global System for Mobile communication (GSM) .

Further description of the GSM TDMA communication system can be found in ^λThe GSM System for Mobile

Communications' by Michel Mouly and Marie Bernadette Pautet, Bay Foreign Language Books, 1992, ISBN

2950719007.

3rd generation systems have recently been rolled out in many areas to further enhance the communication services provided to mobile users. One such system is the

Universal Mobile Telecommunication System (UMTS) , which is currently being deployed. Further description of CDMA and specifically of the Wideband CDMA (WCDMA) mode of UMTS can be found in ^λWCDMA for UMTS', Harri Holma (editor), Antti Toskala (Editor), Wiley & Sons, 2001,

ISBN 0471486876. The core network of UMTS is built on the use of SGSNs and GGSNs thereby providing commonality with GPRS. In a cellular communication system, a geographical region is divided into a number of cells each of which is served by a base station. The base stations are interconnected by a fixed network which can communicate data between the base stations. A mobile station is served via a radio communication link by the base station of the cell within which the mobile station is situated.

As a mobile station moves, it may move from the coverage of one base station to the coverage of another, i.e. from one cell to another. As the mobile station moves towards a base station, it enters a region of overlapping coverage of two base stations and within this overlap region it changes to be supported by the new base station. As the mobile station moves further into the new cell, it continues to be supported by the new base station. This is known as a handover or handoff of a mobile station between cells.

Some cellular communication systems, such as CDMA 3^rd Generation cellular communication systems, support soft handovers wherein transmissions between a mobile station and a plurality of base stations are combined during a handover interval. Thus, during the handover interval, the mobile station is simultaneously supported by a plurality of communication links and (typically) by a plurality of base stations.

In order to achieve efficient and reliable soft handover operation, the number of communication links or legs supporting a given mobile station may be dynamically varied depending on the experienced conditions. Specifically, in UMTS the efficient addition of an extra leg to an active set of one or more base stations supporting the mobile station is critical to ensure a low call drop rate in fast changing radio environments, such as dense urban, highway or high speed train scenarios. Also, the lower the delay is for adding new cells to a soft handover, the sooner the macro-diversity benefits are obtained. Accordingly, many network operators consider soft handover processing time as a key performance parameter that characterizes robustness and reliability of a network.

The addition of a new UMTS soft handover leg to an active set requires a number of tasks to be performed and specifically, the UMTS Technical Specifications describe that the following steps are performed sequentially:

1. Timing evaluation The Radio Network Controller (RNC) evaluates Frame offset and Chip offset parameters required for the new communication link. In addition the DPCH Frame Offset parameter is determined.

2. Radio link setup procedure Resources at the new base station are reserved for the new leg to be added to the soft handover.

3. ALCAP (Access Link Control Application Protocol) establishment for SRB (Signalling Radio Bearer) and existing RABs (Radio Access Bearers) . New AAL2 (ATM Adaptation Layer 2) connections are required for the interface between the RNC and the base station and these are accordingly set up. Specifically, one connection is set up for an SRB and an additional connection is set up per RAB.

4. FP (Frame Protocol) synchronization on new AAL2 connections . FP synchronization is performed for the new AAL2 connections between the RNC and the new base station.

5. Active set update procedure.

The RNC then commands the mobile station to add the reported cell in the active set so that it is included in the handover. Specifically an RRC ACTIVE SET UPDATE message is transmitted to the mobile station and in response the mobile station starts a physical layer synchronization for the new leg. Finally, as specified in 3GPP Technical

Specifications Release 5 TS 25.331 section 8.3.4.3, the remote station sends an RRC ACTIVE SET UPDATE COMPLETE message to confirm that the new leg has been added to the active set.

However, although the specified procedure provides reliable performance in many scenarios, it is suboptimal and has a number of disadvantages. Specifically, the method tends to result in a significant delay which reduces performance and may result in dropped calls thereby leading to a degraded user experience .

Hence, an improved approved approach for adding an additional soft handover connection to an active set would be advantageous and in particular an approach allowing increased flexibility, reduced delay, a reduced number of dropped calls, facilitated operation or implementation and/or improved performance would be advantageous .

Summary of the Invention

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

According to a first aspect of the invention there is provided an apparatus for adding a soft handover connection to an active set of a remote station served by at least a first base station in a cellular communication system; the apparatus comprising: determining means for determining that the soft handover connection should be added to the active set; initiating means for setting up resources in support of a communication link between a base station supporting the soft handover connection and the remote station; instructing means for transmitting a handover addition message to the remote station instructing the remote station to include the communication link in the active set; and wherein the instructing means is arranged to transmit the handover addition message before the setting up of resources by the initiating means is completed.

The invention may allow improved handover performance in a cellular communication system. In particular, a faster addition of a soft handover connection can be achieved and/or a reduced number of dropped calls, an improved reliability and/or improved quality of service is possible . The Inventor of the current invention has realised that in contrast to the prior art and the explicit recommendations of the UMTS Technical Specifications, it is feasible and advantageous to instruct a remote station to include a new communication link in an active set before the resources supporting that communication link have been set up.

The invention may allow improved and/or facilitated operation and/or implementation.

The initiating means and the instructing means may be arranged to set up resources and instruct the remote station in parallel and/or in overlapping time intervals. An active set is the set of connections/cells/base stations and/or cell sectors which support the remote station e.g. during a soft handover.

According to an optional feature of the invention, the initiating means is arranged to set up an air interface resource for the communication link.

This may allow improved performance. The invention may in particular allow a delay associated with the addition of a new soft handover connection to be less dependent on the delay associated with setting up of the air interface resource for the new communication link.

According to an optional feature of the invention, the initiating means is arranged to set up a fixed network communication link resource. This may allow improved performance. The invention may in particular allow a delay associated with addition of a new soft handover connection to be less dependent on the delay associated with setting up of fixed network communication link resources supporting the new connection .

The fixed network communication link may for example be a connection on an Iub interface between a base station and an RNC of a UMTS cellular communication system.

According to an optional feature of the invention, the initiating means is arranged to set up a network element processing resource in support of the soft handover connection.

This may allow improved performance. The invention may in particular allow a delay associated with addition of a new soft handover connection to be less dependent on a delay associated with setting up of a network element processing resource for supporting the new connection. The resource may for example be a processing resource of an RNC.

According to an optional feature of the invention, the determining means is arranged to determine that the soft handover connection should be added to the active set in response to a measurement report from the remote station.

This may allow improved handover performance in a cellular communication system. According to an optional feature of the invention, the apparatus further comprises means for determining if resources are available for the communication link prior to the initiating means initiating the setting up of resources and wherein the transmitting means is arranged to only transmit the handover addition message if resources are available.

This may allow improved handover performance in a cellular communication system. In particular, it may reduce processing at the remote station if the communication link cannot be supported.

According to an optional feature of the invention, the apparatus further comprises means for determining a timing offset for the communication link prior to transmitting the handover addition message.

This may allow improved handover performance in a cellular communication system and may in particular facilitate handover synchronisation. The timing offset may for example be a spreading code timing offset.

According to an optional feature of the invention, the instructing means is arranged to include the timing offset in the handover addition message.

This may allow improved handover performance in a cellular communication system and may in particular facilitate handover synchronisation.

According to an optional feature of the invention, the apparatus further comprises means for transmitting a handover deletion message to the remote station if the setting up of resources is unsuccessful, the handover deletion message instructing the remote station to remove the communication link from the active set.

This may allow improved handover performance in a cellular communication system and may in particular provide an efficient error recovery if the communication link cannot be supported.

According to an optional feature of the invention, the soft handover connection is for a different base station than the first base station.

This may allow improved handover performance in a cellular communication system. The soft handover connection may be to a different cell.

According to an optional feature of the invention, the soft handover connection is for the first base station.

This may allow improved handover performance in a cellular communication system. The soft handover connection may be for a softer handover for the same cell and may specifically be a communication link to a different cell sector of the same cell as another soft handover connection.

According to an optional feature of the invention, the cellular communication system is a UMTS cellular communication system. The invention may allow particularly advantageous and improved handover performance in a UMTS cellular communication system.

According to an optional feature of the invention, the handover addition message is an RRC ACTIVE SET UPDATE message.

This may allow particularly advantageous and improved handover performance in a UMTS cellular communication system and may in particular allow improved performance and increased compatibility with the UMTS Technical Specifications .

According to an optional feature of the invention, the apparatus further comprises means for determining a DPCH Frame Offset for the communication link prior to transmitting the handover addition message.

This may allow particularly advantageous and improved handover performance in a UMTS cellular communication system and may in particular allow improved performance and increased compatibility with the UMTS Technical Specifications. The DPCH Frame Offset may be included in the handover addition message.

According to an optional feature of the invention, the initiating means is arranged to setup UMTS Radio Access Bearer resources or Signalling Radio Bearer resources using an Access Link Control Application Protocol (ALCAP) procedure . This may allow particularly advantageous and improved handover performance in a UMTS cellular communication system and may in particular allow improved performance and increased compatibility with the UMTS Technical Specifications.

According to another aspect of the invention, there is provided a method for adding a soft handover connection to an active set of a remote station served by at least a first base station in a cellular communication system; the method comprising: determining that the soft handover connection should be added to the active set; setting up resources in support of a communication link between a base station supporting the soft handover connection and the remote station; transmitting a handover addition message to the remote station instructing the remote station to include the communication link in the active set; and wherein the handover addition message is transmitted before the setting up of resources is completed.

According to another aspect of the invention, there is provided a cellular communication system including at least a remote station served by at least a first base station; the cellular communication system comprising: determining means for determining that a soft handover connection should be added to an active set of the remote station; initiating means for setting up resources in support of a communication link between a base station supporting the soft handover connection and the remote station; instructing means for transmitting a handover addition message to the remote station instructing the remote station to include the communication link in the active set; and wherein the instructing means is arranged to transmit the handover addition message before the setting up of resources by the initiating means is completed.

These and other aspects, features and advantages of the invention will 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 UMTS cellular communication system in accordance with some embodiments of the invention;

FIG. 2 illustrates an example for a handover apparatus for adding an additional soft handover connection to a an active set of a remote station in accordance with some embodiments of the invention;

FIG. 3 illustrates an example of signal conditions during a soft handover in accordance with UMTS Technical Specifications 3GPP Release 5 TR.25.931;

FIG. 4 illustrates an example of signal conditions during a soft handover in accordance with some embodiments of the invention; FIG. 5 illustrates an example of a signalling flow in accordance with some embodiments of the invention_/ and

FIG. 6 illustrates an example of a signalling flow 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 UMTS cellular communication 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 including for example a GSM cellular communication system.

FIG. 1 illustrates an example of a UMTS cellular communication system 100 in which embodiments of the invention may be employed.

In the example of FIG. 1, a first remote station 101 and a second remote station 103 are in a first cell supported by a first base station 105.

The first base station 105 is coupled to a first RNC 107. An RNC performs many of the control functions related to the air interface including radio resource management and routing of data to and from appropriate base stations. In the example, the first RNC 107 is also coupled to a second base station 109. The first RNC 107 is coupled to a core network 111. A core network interconnects RNCs and is operable to route data between any two RNCs, thereby enabling a remote station in a cell to communicate with a remote station in any other cell. In addition, a core network comprises gateway functions for interconnecting to external networks such as the Public Switched Telephone Network (PSTN) , thereby allowing remote stations to communicate with landline telephones and other communication terminals connected by a landline. Furthermore, the core network comprises much of the functionality required for managing a conventional cellular communication network including functionality for routing data, admission control, resource allocation, subscriber billing, remote station authentication etc.

The core network 111 is further coupled to a second RNC 113 which is coupled to a third base station 115. The third base station 115 supports a third remote station 117.

In the example, the first remote station 101 is initially in the cell served by the first base station 105 (Cell A) but is moving towards the cell of the second base station 109 (Cell B) . Accordingly, the first remote station 101 is initially served only by the first base station 105 but as the radio conditions for this remote station 101 deteriorate and the conditions for the second base station 109 improve, the first remote station 101 at some point reaches a handover point where it enters a soft handover state by setting up a second connection to the second base station 109. Thus, the first remote station 101 enters a soft handover mode wherein it is simultaneously supported by both the first and second base station 105, 109.

Generally, as a mobile station moves around in the cellular communication system it dynamically changes from being simultaneously supported by one, two or more base stations. The set of connections supporting the remote station is referred to as the active set and may e.g. include the base stations, cells and/or cell sectors currently supporting the remote station.

In order to achieve efficient performance and to exploit Macro diversity it is important that the addition of new soft handover legs can be achieved with high efficiency and low delay.

In the example of FIG. 1 the addition of a new soft handover leg to the active set is controlled by functionality predominantly located in the RNCs. Specifically, the first RNC 107 comprises functionality for adding a new soft handover connection to an active set of the first remote station 101 (including the case where the remote station enters a soft handover configuration by the addition of the new soft handover connection) .

FIG. 2 illustrates an example of a handover apparatus 200 for adding an additional soft handover connection to an active set of a remote station in accordance with some embodiments of the invention.

The handover apparatus 200 comprises a base station interface 201 which interfaces with the first and second base stations 105, 109 over the UMTS interface known as the Iub interface.

The base station interface 201 is coupled to a handover processor 203 which controls the handover operation for the first remote station 101. The handover processor 203 is furthermore coupled to the core network interface 205 which is operable to interface to the core network 111 through a connection on the UMTS interface known as the Iu interface.

Specifically, the handover processor 203 can determine when a change is needed to the handover configuration of the first remote station 101. The handover processor 203 determines this in response to measurement reports received from the first remote station 101. The first remote station 101 monitors the received signal levels or qualities from the serving base station as well as for the neighbouring base stations and reports these back to the handover apparatus 200. As a specific example, if the received measurement reports indicate that the signal strength from the serving first base station 105 is falling towards a critical level whereas the signal strength from the second base station 109 is increasing above this, the handover processor 203 can decide to include a soft handover connection to the active set of the second base station 109 thereby entering the first remote station 101 into a soft handover mode.

When the handover processor 203 determines that a new soft handover connection should be added to the active set of the first remote station 101 (including the situation where the first remote station 101 previously has only one connection in the active set and is accordingly not in a soft handover configuration) , it proceeds to allocate resources for this new soft handover connection and in parallel, and before the resource allocation has completed, it instructs the first remote station 101 to include the second base station 109 in the active set.

Specifically, the handover processor 203 is coupled to a resource processor 207 which is arranged to set up resources in support of a communication link between the second base station 109 and the first remote station 101. The communication link supports the additional soft handover connection to the second base station 109.

Also, the handover processor 203 is coupled to an instructing processor 209 which is arranged to generate and transmit a handover addition message to the user equipment instructing the user equipment to include the communication link in the active set. Specifically, for UMTS the instructing processor 209 generates an RRC ACTIVE SET UPDATE message and transmits this to the first remote station 101 via the base station interface 201 and the first and second base stations 105, 109.

The operations of the instructing processor 209 and the resource processor 207 are performed in parallel. Thus, when a handover processor 203 determines that a new handover connection should be added, it instructs both the instructing processor 209 and the resource processor 207 to proceed with the corresponding handover procedure without waiting for the other processor. Accordingly, the resource processor 207 proceeds to identify, reserve and allocate the resource required to support the communication link from the second base station 109 to the first remote station 101. This resource setup includes setting up of an air interface resource by allocating the Signalling Radio Bearers (SRB) and Radio Access Bearers (RAB) as appropriate. Furthermore, communication resource within the fixed network, such as resource on the interface between the first RNC 107 and the second base station 109, is allocated. Furthermore, the required processing resources are set up. For example, resources in the first RNC 107 required for supporting the new communication link are set up.

The setting up of resource is associated with a significant delay due to the complex procedures and interactions that are required in order to avoid e.g. conflicts with other services supported by the communication system. However, in the handover apparatus 200 the instructing processor 209 proceeds to transmit the handover addition message to the first remote station 101 as soon as the instruction is received from the handover processor 203. Thus the instructing processor 209 proceeds to instruct the first remote station 101 to include the new communication link in the active set before this communication link has been set up and even before the resources have been allocated for the communication link.

When the first remote station 101 receives the handover addition message from the handover apparatus 200, it proceeds to include the new communication link to the second base station 109 in the active set and specifically it proceeds to synchronise to the communication link and to transmit and receive on the communication link. Thus, when the communication link becomes available, the first remote station 101 exploits the benefits provided thereby without any additional delay.

Thus, the handover apparatus 200 may substantially reduce the delay associated with adding a new soft handover connection to an active set of the first remote station 101. This may substantially improve handover performance, improve the perceived quality, reduce the number of dropped calls and improve the performance of the communication system as a whole.

In the following, a specific example of the operation of the UMTS communication system of FIG. 1 will be described in more detail.

FIG. 3 illustrates an example of signal conditions during a soft handover in accordance with UMTS Technical Specifications 3GPP Release 5 TR.25.931. Specifically, the figure illustrates the signal to noise ratios (Ec/Io) for cell A 301 corresponding to the cell supported by the first base station 105 and cell B 303 supported by the second base station 109 for a conventional addition of a soft handover connection.

The following things can be observed in FIG. 3:

^■ When Cell B fulfils the event IA conditions (namely that the difference between the Ec/Io of cell B and cell A is less than the threshold IA) as reported to the RNC 107 by KRC MEASUREMENT REPORT messages, the RNC initiates the addition of cell B.

■ While the RNC sets up the new handover connection, the radio conditions in Cell A degrade and the radio conditions in Cell B improve significantly.

^■ By the time the RNC sends the RRC ACTIVE SET UPDATE message, the radio conditions on the only active set cell (Cell A) are unacceptable and prevent the message from being received. Therefore, the new cell cannot be added to the active set at the remote station.

^■ As the radio conditions in Cell A do not improve, the call is finally dropped despite the radio conditions in cell B being sufficient to support the remote station.

In current operational UMTS systems, it has been found that as much as half of the dropped calls in a fast changing radio environment are due to delays in adding new cells to the active set.

The handover apparatus 200 provides a faster implementation for active set additions based on a parallel execution of the UTRAN (UMTS Terrestrial Radio Access Network) resources setup and the active set update at the remote station. This solution significantly reduces soft handover addition processing time and provides a more robust implementation in fast changing radio propagation scenarios resulting in a substantially reduced call drop ratio. FIG. 4 illustrates an example of signal conditions during a soft handover in accordance with some embodiments of the invention. Specifically, the figure illustrates the signal to noise ratios (Ec/Io) for cell A 301 corresponding to the cell supported by the first base station 105 and cell B 303 supported by the second base station 109.

The following things can be observed in FIG. 4: ^■ When Cell B fulfils the event IA conditions

(namely that the difference between the Ec/Io of cell B and cell A is less than the threshold IA) as reported to the RNC 107 by RRC MEASUREMENT REPORT messages, the RNC initiates the addition of cell B.

^■ While the RNC sets up the new handover connection, the radio conditions in Cell A degrade and the radio conditions in Cell B improve significantly. ^■ The RNC sends the RRC ACTIVE SET UPDATE message with half the delay of the conventional approach of FIG. 3.

^■ When the message is transmitted, the radio conditions on the only active set cell (Cell A) are still acceptable and allow the message to be received. Therefore, the new cell can be added to the active set at the remote station.

^■ Although the conditions in Cell A deteriorate the remote station can synchronize to Cell B once UTRAN resources are fully established and can then begin to use the communication link to the second base station 109. ■ As the radio conditions of Cell A continue to degrade, the call continues to be supported by cell B.

The handover processor 203 may specifically perform the following method.

Step A. Validation and admission of reported cell

When the first RNC receives the RRC MEASUREMENT REPORT message that indicates that the second base station should be added, it initially proceeds to perform the following initial operations:

^■ Cell validation

Validation of the reported cell is performed. For example, it may be determined that the reported cell is a valid cell identity etc. ^■ Admission checking The RNC can in advance evaluate if the new cell can be added to the soft handover. Specifically, it can be verified that the following criteria are met:

• Resources are available at the first RNC 107

(processing capabilities, etc) • Resources are available at the second base station 109 (available communication channels, power, etc)

• The interference on downlink and uplink communications is acceptable. • Bandwidth is available on the Iub interface for the new required AAL2 (AAL2 (ATM Adaptation Layer 2) connections to the second base station 109.

It will be appreciated that this and the previous validation step can be performed when the handover leg addition is initiated or can be performed earlier, such as when the cell is first reported by the first remote station 101.

^■ Timing evaluation

The RNC can evaluate the Frame offset and Chip offset required for the second base station 109. These timing offset values can be determined from OFF and Tm parameters reported from first remote station 101. Moreover, the DPCH Frame Offset is calculated.

If the validation checks are positive, the handover apparatus 203 proceeds to initiate the handover connection addition. However, if the checks are negative, e.g. if the indication is that the required resources are not available for the communication, then the procedure is terminated and specifically no handover addition message is transmitted to the remote station.

B. Soft handover addition

The following steps B.I and B.2 are executed in parallel by the handover apparatus 203.

B .1 UTRJVN resources setup The resource processor 207 proceeds to set up the required resources in the UTRAN for the new communication link to the second base station 109. A similar process to that described in the UMTS Technical Specifications may be used. Specifically, the following operations may be performed:

^■ Radio link setup procedure.

The resources at the second base station 109 are reserved for the new communication link. The Frame offset and Chip offset timing offsets are transmitted to the second base station 109 for synchronization.

■ ALCAP (Access Link Control Application Protocol) establishment for SRB and existing RABs. New AAL2 connections are required between the first RNC 107 and the second base station 109 and these connections are accordingly set up. Once this operation has been completed, the second base station 109 starts downlink DPCCH transmission.

■ FP (Frame Protocol) synchronization on new AAL2 connections . FP synchronization of the new AAL2 connections between the first RNC 107 and the second base station 109 is performed.

B.2 Addition of the reported cell in the active set

In parallel with step B.I, and before resource setup is completed, the instructing processor 209 begins the RRC Active Set Update procedure which generates a handover addition message that commands the first remote station 101 to add the second base station 109 to the active set. Specifically, the instructing processor 209 generates an RRC ACTIVE SET UPDATE message comprising the DPCH Frame Offset and transmits this to the first remote station 101. As all the required information for this message is generated in step A, the instructing processor 209 need not wait for step Bl to complete before the RRC ACTIVE SET UPDATE message is transmitted to the first remote station 101.

Furthermore the readiness of the UTRAN resources is not required for completing RRC Active Set Update procedure. Specifically, the first remote station can transmit an RRC ACTIVE SET UPDATE COMPLETE message without waiting for the completion of the physical layer synchronization for the new connection. Hence, the new cell is considered as part of the active set even if no finger at the Rake receiver of the first remote station 101 is synchronized to the new cell. As soon as the UTRAN resources of the leg are available, the first remote station 101 automatically synchronizes to the second base station 109.

Although the availability of resources are confirmed in step A, it is possible that the setup of UTRAN resources fails (in step B.I) . In this case, the first RNC 107 can transmit a handover deletion message to the first remote station 101 indicating that the new connection should be removed from the active set. In response, the first remote station 101 removes the connection. The handover deletion message can specifically be an RRC Active Set Update message indicating that the new connection should be removed. The RRC Active Set Update message can be transmitted when an RRC Active Set Update Complete message is received from the first remote station 101.

An example of a signalling flow in accordance with the above description is provided in FIG. 5. In the figure, the UMTS specific terminology is used and thus the term Node B is used for base station and UE (User Equipment) is used for the remote station.

It will be appreciated that although the above description has focussed on a soft handover connection with a new cell, the described principles apply equally to a softer handover wherein an additional soft handover connection to a cell already serving the remote station may be added to the active set. For example, a new connection to a different sector of the first base station 105 may be added for the first remote station 101. An example of a signalling flow for a softer handover is illustrated in FIG. 6.

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 be limited to the specific 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. An apparatus for adding a soft handover connection to an active set of a remote station served by at least a first base station in a cellular communication system, the apparatus comprising: determining means for determining that the soft handover connection should be added to the active set; initiating means for setting up an air interface resource in support of a communication link between a base station supporting the soft handover connection and the remote station; timing means for determining a timing offset for the communication link prior to transmitting a handover addition message; instructing means for transmitting the handover addition message to the remote station instructing the remote station to include the communication link in the active set; and wherein the instructing means is arranged to transmit the handover addition message before the setting up of resources by the initiating means is completed.

2. The apparatus of claim 1 wherein the initiating means is arranged to set up a network element processing resource in support of the soft handover connection.

3. The apparatus of claim 1 wherein the determining means is arranged to determine that the soft handover connection should be added to the active set in response to a measurement report from the remote station.

4. The apparatus of claim 1 further comprising means for determining if resources are available for the communication link prior to the initiating means initiating the setting up of resources and wherein the transmitting means is arranged to only transmit the handover addition message if resources are available.

5. The apparatus of claim 1 wherein the timing offset for the communication link is included in the handover addition message.

6. The apparatus of claim 1 wherein the apparatus further comprises means for transmitting a handover deletion message to the remote station if the setting up of resources is unsuccessful, the handover deletion message instructing the remote station to remove the communication link from the active set.

7. The apparatus of claim 1 wherein the soft handover connection is for a different base station than the first base station.

8. The apparatus of claim 1 wherein the soft handover connection is for the first base station.

9. The apparatus of claim 1 wherein the handover addition message is an RRC ACTIVE SET UPDATE message, and further comprising means for determining a DPCH Frame Offset for the communication link prior to transmitting the handover addition message.

10. A method for adding a soft handover connection to an active set of a remote station served by at least a first base station in a cellular communication system, the method comprising: determining that the soft handover connection should be added to the active set; initiating the setting up of resources in support of a communication link between a base station supporting the soft handover connection and the remote station; determining a timing offset for the communication link prior to transmitting a handover addition message; transmitting the handover addition message to the remote station instructing the remote station to include the communication link in the active set; and wherein the handover addition message is transmitted before the setting up of resources is completed.