GB2482920A

GB2482920A - Calculating a timing offset between a source cell and a target cell following receipt of system frame numbers of the source cell and the target cell

Info

Publication number: GB2482920A
Application number: GB1014047.3A
Authority: GB
Inventors: Michael Eric Morrin
Original assignee: IP Access Ltd
Current assignee: IP Access Ltd
Priority date: 2010-08-20
Filing date: 2010-08-20
Publication date: 2012-02-22
Also published as: GB201014047D0; WO2012022800A1

Abstract

A network element, e.g. gateway controller 142, base station 130, target access point 131 or Node B 124, within a cellular communication network 100 receives, from a wireless subscriber unit 114, a message comprising an indication of an observed time difference between a source cell 150, supporting communication with the wireless subscriber unit 114, and a target cell 154. A timing offset between the source cell 150 and the target cell 154 is calculated based on the message indication, following receipt of a system frame number (SFN) of the source cell 150 at a point in time and a SFN of the target cell 154 at a point in time. A timing of the communication of the wireless subscriber unit 114 is reconfigured for the target cell 154 and handover of the communication is initiated from the source cell 150 to the target cell 154.

Description

Title: NETWORK ELEMENT, CELLULAR COMMUNICATION SYSTEM AND METHOD

THEREFOR

Description

Field of the invention

The field of the invention relates to a network element, a cellular communication system and method therefor. The invention is applicable to, but not limited to, a network element for supporting communication within at least one cell of a cellular communication system, and in particular a method for handover between communication cells within a cellular communication system.

Background of the Invention

Wireless communication systems, such as the 3rd Generation (30) of mobile telephone standards and technology, are well known. An example of such 30 standards and technology is the Universal Mobile Telecommunications System (UMTSTM), developed by the 3d Generation Partnership Project (3GPPTM) (Www.3arDp.org). The 3rd generation of wireless communications has generally been developed to support macro-cell mobile phone communications. Such macro cells utilise high power base stations (NodeBs in 3GPP parlance) to communicate with wireless communication units within a relatively large geographical coverage area. Typically, wireless * ..*** * communication units, or User Equipment (UEs) as they are often referred to in 3G parlance, communicate with a Core Network (CN) of the 3G wireless communication system via a Radio Network Subsystem (RNS). A wireless communication system typically comprises a plurality of S..

radio network subsystems, each radio network subsystem comprising one or more cells to which UEs may attach, and thereby connect to the network. Each macro-cellular RNS further comprises a controller, in a form of a Radio Network Controller (RNC), operably coupled to the one or more Node Bs, via a so-called lub interface.

The use of lower power (and therefore smaller coverage area) femto cells (or pico-cells) is a recent development within the field of wireless cellular communication systems. Femto cells or pico-cells (with the term femto cells being used hereafter to encompass pico-cells or similar) are effectively communication coverage areas supported by low power base stations (otherwise referred to as Access Points (APS) of Home Node B's). These femto cells are intended to be able to be piggy-backed onto the more widely used macro-cellular network and support communications to UEs in a restricted, for example in-building', environment.

Typical applications for such femto APs include, by way of example, residential and commercial (e.g. office) locations, communication hotspots', etc., whereby an AP can be connected to a core network via, for example, the Internet using a broadband connection or the like. In this manner, femto cells can be provided in a simple, scalable deployment in specific in-building locations where, for example, UEs may come into close proximity to a femto AP.

Femto APs are intended to enhance the coverage of a UMTSTM Radio Access Network (RAN) within residential and/or private commercial environments, and it is planned that the number of femto APs in a macro cell may number in the thousands. Accordingly, it is not likely to be feasible to completely control the deployment of femto cells within the UMTS' RAN.

In a call on a cellular network the UE typically transmits and receives a stream of user plane data frames that are implicitly sequenced by the frame timing structure of the associated air interface. When the user plane data frames are transported internally between the various nodes of the cellular network, the sequencing of the data frames must be re-generated to ensure that they reach their destination, and as such the data frames require sufficient information to reconstruct the originally transmifted sequence. This is typically achieved by tagging each user plane data frame with one or more of: a Frame Number (FN), a timestamp or a Sequence Number (hereinafter collectively referred to as an FN).

It is possible for calls to be handed over between cells. In some network topologies the user-plane data streams are re-initialised at the point of handover and the receiver expects some form of discontinuity of FN. In other topologies, for example two femto cells sharing a common gateway to the core network, it is possible to have an internal handover that does not involve the Core Network (CN). In this scenario, the CN does not expect a discontinuity in the sequencing of data frames. However, in this scenario, a problem exists with initialising the FN sequence at the target cell, so that the sequence seamlessly continues from that initiated by the source cell. To S...

date, particularly in cases where the cells are connected to a gateway via asynchronous links with * .* . .* * significant and variable propagation delays, such as the links typically used for femto cells, there is :. no known mechanism to accurately initialise the sequence numbers at the target cell.

Thus, and typically, the target cell will re-start the numbering sequences at an arbitrary

S

value, without being able to take into account the point in the number sequence that was reached **,, 25 by the source cell for that call. Such a discontinuous sequencing of the user plane data frames, i.e. resulting in a jump forwards or backwards in the numbering sequence, has been found to be disruptive to the correct operation of a receiver in the Core Network (CN), such as a media gateway. For example, the media gateway may be unable to reconstruct the relative timing of the data stream across the handover between femto cells. In particular, if the media gateway is using the sequence number for detecting lost or duplicate data packets, the unexpected jump in numbering may cause undesirable behaviour, such as erasure of data frames or insertion of unnecessary null data frames.

3GPP TS 25402 describes the standardization of 3GPP air-interlace synchronization, a portion of which provides for UE received signal measurements to initiate handover between communication cells.

Thus, a need exists for an improved method and apparatus for handover within a cellular communication system.

Summary of the invention

Accordingly, the invention seeks to mitigate, alleviate or eliminate one or more of the above mentioned disadvantages, singly or in any combination. Aspects of the invention provide a network element, a wireless subscriber communication unit such as a UE, integrated circuits therefor, a cellular communication system, and a method and tangible computer program product therefor, as described in the appended claims, These and other aspects of the invention will be apparent from, and elucidated with reference to, the embodiments described hereinafter.

Brief Description of the Drawings

Further details, aspects and embodiments of the invention will be described, by way of example only, with reference to the drawings. Elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. Like reference numerals have been included in the respective drawings to ease understanding.

FIG. 1 illustrates an example of part of a cellular communication system, adapted in accordance with an example embodiment.

FIG. 2 illustrates an example of a simplified block diagram of a network element adapted in accordance with an example embodiment.

FIG. 3 illustrates a simplified message sequence chart of an example method for handover within a cellular communication system.

* .*.** * FIG. 4 illustrates a typical computing system that may be employed to implement signal processing functionality in example embodiments. *

* Detailed Description

** 25 Examples of the invention will be described in terms of a 3rd generation (3G) Radio Network Sub-system (RNS) for supporting one or more cells within a Universal Mobile Telecommunications System (UMTSTM) cellular communication network. However, it will be appreciated by a skilled artisan that the inventive concept herein described may be embodied in any type of network element for supporting communications within a cellular communication network. In particular, it is contemplated that the inventive concept is not limited to being implemented within a network element for supporting one or more cells within a UMTSTM cellular communication network, but may be equally applied within one or more network element(s) adapted in accordance with alternative cellular communication technologies.

In a number of applications, the adaptation of a network element in accordance with examples of the invention may effectively perform a method for handover within a cellular communication system. The method comprises determining when a wireless communication unit within the coverage of at least one cell wishes to handover to another cell, for example by receipt of a handover trigger measurement event from the wireless communication unit. Upon the wireless communication unit determining that a handover is desirable, the wireless communication unit performs (if it has not already done so) a measurement (such as, in one example, SFN-SFN Observed Time Difference) in order to obtain relative timing (SFN) information between a source cell and a target cell. In one example embodiment, the UE calculates an SFN offset to the target cell and relays this information to its source cell. In one example, a network element such as an AP in the source cell may combine that information with internal data relating the timing offset between the SFN of the cell and the FN of the call, in order to produce an value for the required target cell SFN to FN offset, to be used by the target network element such as a target AP, to use when continuing the call/communication with the UE following handover.

In one example embodiment, a gateway controller, or the AP associated with the target cell, receives the target SFN to FN offset information, from the UE or the source AP, and adapts the on-going communication within the target cell to align the FN used on the target cell with the offset calculated in the source cell, in order to complete the handover of the UE communication, as described in further detail below.

Referring now to the drawings, and in particular FIG. 1, an example of part of a cellular communication system, adapted in accordance with an example embodiment of the invention, is illustrated and indicated generally at 100. In FIG. 1, there is illustrated an example of a communication system in a form of a 3GPPTM UMTSTM network IOU that comprises a combination of a macro cell 185 and a plurality of femto cells 150, 152, 154. For the example embodiment s.

illustrated in FIG. 1, radio network sub-systems (RNSs) comprise two distinct architectures to * ** * .* * handle the respective macro cell and femto cell communications.

In the macro cell scenario, the RNS 110 comprises a controller in a form of a Radio Network Controller (RNC) 136 having, inter alia, one or more signal processing module(s) 138. S..

The RNC 136 is operably coupled to at least one NodeB 124 for supporting communications within ** 25 the macro cell 185. The NodeB 124 comprises signal processing module 126 and transceiver * . circuitry 128 arranged to enable communication with one or more wireless communication units located within the general vicinity of the macro communication cell 185, such as User Equipment (UE) 114. The RNC 136 is further operably coupled to a core network element 142, such as a serving general packet radio system (GPRS) support node (SGSN)/mobile switching centre (MSC), as known, which acts as a gateway controller 142 to other networks or parts of the cellular communication system, such as media gateway 143.

In a femto cell scenario, an RNS 112 comprises an Access Point (AP) 130, 131 (also known as a Home NodeB) that is arranged to perform a number of functions generally associated with a cellular communication base station, and a controller in a form of an Access controller (3G AC) 140, 141. As will be appreciated by a skilled artisan, an Access Point is a communication element that supports communications within a communication cell, such as a femto cell 150, 152, 154, and as such provides access to a cellular communication network via the femto cell 150, 152, 154. One envisaged application is that an AP 130, 131 may be purchased by a member of the -5-.-public and installed in their home. The AP 130, 131 may then be connected to an AC 140, 141 over, say, the owner's broadband internet connection 160, 161.

Thus, an AP 130, 131 may be considered as encompassing a scalable, multi-channel, two-way communication device that may be provided within, say, residential and commercial (e.g. oftice) locations, communication hotspots' etc., to extend or improve upon network coverage within those locations. An example of a typical 3G AP for use within a 3GPPTM system may comprise some NodeB functionality and some aspects of radio network controller (RNC) 136 functionality.

For the illustrated example embodiment, the APs 130, 131 comprise respective signal processing modules 165, 166 and transceiver circuitry 155, 156 arranged to enable communication with one or more wireless communication units located within the general vicinity of the femto communication cell 150 154, such as User Equipment (UE) 114, via a wireless interface (Uu).

The 3G Access Controller 140 may be coupled to the core network (CN) via an lu interface, as shown. In this manner, the AP 130 is able to provide voice and data services to a cellular handset, such as UE 114, in a femto cell, in the same way as a conventional NodeB would in a macro cell, but with the deployment simplicity of, for example, a Wireless Local Area Network (WLAN) access point.

In accordance with the illustrated example embodiment, the 3GPPTM UMTSTM (access) network 100 comprises a number of APs, with only two 3G APs 130, 131 shown to illustrate the ***, handover mechanism. The two 3G APs 130, 131 are connected to the CN via a common gateway 0�** 142. A UE, such as UE 114, is communicating in a call to one of the APs (for example the source * ..* * AP 130 supporting communications within source femto cell 150) and triggers a handover to another AP (for example the target AP 131 associated with target cell 154). The UE 114 sends user plane data frames that are received by the source AP 130, forwarded to the gateway *** * controller 142 and thereafter to the CN, for example a media gateway 143. In one example embodiment, when the UE roams close to an edge of the source cell 150, sufficient to enable the UE 114 to consider the possibility of handing over the communication/call to target cell 154, the UE 114 measures received signal levels in both the source cell 150 and the target cell 154, as well as measuring the relative system timing of the two cells. In such a case, when the communication/call is ongoing on a source cell 150 via a source AP 130, the UE 114 may trigger a handover to a target AP 131 that shares the same Gateway Controller 142 to the Core Network (CN), as described in more detail in relation to FIG. 3.

Referring now to FIG. 2, an example of a simplified block diagram of the femto AP 130, 131 is shown. The example femto AP 130 contains an antenna 202 coupled to the transceiver circuitry 155. More specifically for the illustrated example, the antenna 202 is preferably coupled to a duplex filter or antenna switch 204 that provides isolation between receive and transmit chains withinthefemtoAP 130, 131.

The receiver chain, as known in the art, includes receiver front-end circuitry 206 (effectively providing reception, filtering and intermediate or base-band frequency conversion). The front-end circuitry 206 is serially coupled to the signal processing module 165, 166. An output from the signal processing module 165,166 is provided to a transmit element of a network connection 210, for example operably coupling the signal processing module 165, 166 to the access controller 140, 141 of FIG. 1 via, say, the Internet 160, 161. The controller 214 is also coupled to the receiver front-end circuitry 206 and the signal processing module 165, 166 (generally realised by a digital signal processor (DSP)). The controller 214 and signal processing module 165, 166 are also coupled to at least one memory device 216 that selectively stores operating regimes, such as decoding/encoding functions, synchronisation patterns, code sequences, event measurement report data and the like.

As regards the transmit chain, this essentially includes a receiving element of a network connection 210, coupled in series through transmitter/modulation circuitry 222 and a power amplifier 224 to the antenna 202. The transmitter/modulation circuitry 222 and the power amplifier 224 are operationally responsive to the controller 214, and as such are used in transmitting data to a wireless communication unit, such as UE 118.

The signal processor module 165, 166 in the transmit chain may be implemented as distinct from the processor function in the receive chain. Alternatively, a single processor may be used to implement processing of both transmit and receive signals, as shown in FIG. 2. Clearly, the various components within the femto AP 130, 131 can be realised in discrete or integrated ** component form, with an ultimate structure therefore being merely an application-specific or design selection.

* . In accordance with examples of the invention, the memory device 216 stores computer-readable code thereon for programming the signal processing module 165, 166 to perform a method for mitigating interference within a cellular communication system. The code is operable for processing a measurement event from a UE, and upon determining that better service for the ** 25 wireless communications unit would be obtained from a neighbouring cell, causing that wireless * * communication unit to connect to a neighbouring cell.

For example, and in accordance with some example embodiments of the present invention, the signal processing module (in a source cell femto AP) may be configured to determine a Frame number offset for itself and, together with measurement data from the UE, an optimum Frame Number to SFN offset pertaining to the target cell for an on-going communication with a UE.

In one example, the receiver of the AP receives the determined SFN to SFN offset from the UE requesting a handover, as part of a reporting of a measurement event. Upon receipt of the SFN to SFN for the respective target AP, the signal processing module in the source cell femto AP cell may be configured to combine the measured system timing offset and the local timing offset between SFN and the FN sequence for this call.

In this manner, example embodiments of the invention utilise the knowledge that is available to the UE, namely that only the UE has simultaneous access to accurate timing from both source cell and target cell. Such measurements made and reported by the UE, as well as information from the source cell if the implementation makes the relative timing at that source AP variable. In the case where the target cell adjusts the timing, to provide sufficient information to the target cell that it can accurately continue the frame numbering sequence that was established by the source cell. It is worth noting here that in such a backhaul (rather than air-interface) link scenario, that the APs may be coupled on high latency links, and the FN (which typically increments every 2omsec.) would be inaccurate by the time it got to the target cell. In one example, the UE measures the delta timing between the cells, as no entity knows the absolute timing of any other entity in the CN, due to propagation and backhaul delays. In this example, a source cell offset may be used in a scenario where there has previously been such a handover to the source cell, and consequently an FN offset is in force.

A gateway controller, or the target AP on the target cell, then adapts the FN to use for the communication/call after handover to match that of the source AP communication/call with the UE, such that the Core Network media gateway does not perceive a discontinuity in the time sequencing of data frame sequence numbers.

Additionally or alternatively, in one example embodiment, the signal processing module 165, 166 of the femto AP 130, 131 may define a relationship between the Frame Number sequence at the source cell and the System Frame Number (SFN) broadcast by the source cell. In this example embodiment, the gateway controller or target cell may use the UE measurements (for example re-use those measurements performed for the purpose of air-interface synchronisation) to adjust the FN post-handover to be used by the target cell.

In one alternative or additional example, the source femto AP 130 in the source cell, for ***S* * example radio resource management logic within the source fernto AP 130 may be configured to send to the gateway the raw value of the SFN-SFN offset and the internal value of source cell SFN-FN offset for this call. In this example, the target cell may perform the calculation of the required *** target cell 151 SFN-FN offset.

**** 25 In a further alternative or additional example, it is envisaged that the target cell may initialise the Frame Number sequence to have a defined relationship with the SFN of the target cell, but without using the measured time difference for the source cell 150. In this further alternative or additional example, the Access Gateway may use the SFN time difference measured by the UE 114 and the FN offset if provided by the source AP 130 in the source cell 150 and the FN offset, if provided by the target AP 131 in the target cell 151, in order to calculate a value to adjust the Frame Numbers of each transmitted frame from the target cell. In this manner, although each cell has an independent FN offset, the gateway controller 142 has the measured offset between the cells, and hence the gateway is able to manipulate the FN in each data packet as it is sent towards the media gateway 143. A skilled artisan will appreciate that the same general functional elements of the AP in FIG. 2 also reside in a UE 114, with regard to transmitting and processing functionality.

Referring now to FIG. 3, there is illustrated an example of a simplified message sequence chart 300 implementing one example for handover within a cellular communication system, such as may be implemented within the communication system of FIG. 1. The message sequence chart 300 comprises communications between a user equipment (UE) 114, a base station in a source cell 150 such as a source Node B/Access Point, a base station in a target cell 151, such as a target Node B/Access Point, a gateway controller 142, such as a HNB-GW, and a gateway network element such as a media gateway 143. The message sequence chart 300 starts at step 330 with a UE 114 communicating in an existing call. In this regard, the UE is transmitting and receiving 335 user plane (U-plane) data from the UE 114 to/from the source cell 1 50, which in turn is transmitting and receiving 340 user plane (U-plane) data to/from the gateway controller 142, which in turn is transmitting and receiving 345 user plane (U-plane) data to/from the media gateway 143.

In one example embodiment, when the UE roams close to an edge of the source cell sufficient to enable the UE to consider the possibility of handing over the communication/call to a target cell, the UE measures received signal levels in both the source cell 150 and the target cell 151, as shown in step 350. In such a case, when the communication/call is ongoing on a source cell 150 via a source AP/NodeB, the UE 114 may trigger a handover to another AP that shares the same Gateway Controller 142 to the Core Network (CN). If the UE 114 wishes to initiate (trigger) a handover in step 355, the UE performs a measurement of the SFN-SFN observed time difference between the source cell 150 and the target cell 151. The UE then transmits this measurement event, for example a value of the system frame number (SFN)-Offset, or the respective SFN values of the source cell and target cell, to the source cell 150 in step 360. Next, at step 365, the source cell may calculate a frame number (FN) offset for the target cell based on the measurement event, with a value of the system frame number (SFN)-FN Offset. The source Node B/Access Point in the source cell 150 may then transmit to the gateway controller 142 an indication of a relocation * .* .�* * S required with the SFN-FN Offset based on the calculated SFN to frame number (FN) offset of the target cell, as shown in step 370.

The relocation request, together with an indication of the required SFN-FN Offset, as *.S * determined by the source Node B/Access Point in the source cell 150, is then routed to the target Node B/Access Point in the target cell 151, as shown in step 375. The target Node B/Access Point in the target cell 151 processes the relocation request, together with an indication of the SFN-FN Offset, and sends a relocation request acknowledgement (ack) message back to the gateway controller 142, in step 380. The gateway controller 142 processes the relocation request acknowledgement (ack) message and transmits a relocation command 385 back to the source cell 150. The source cell then re-configures the UE communication for the target cell in step 390.

Once the reconfiguration is complete, in step 394, the UE communication is handed over to the target Node B/Access Point in the target cell 151. Thus, thereafter, the UE 114 communicates with the media gateway 143 via the target cell by transmitting and receiving user plane (U-plane) data 398 from the UE 114 to/from the target cell 151, which in turn is transmitting and receiving user plane (U-plane) data to/from the gateway controller 142, which in turn is transmitting and receiving user plane (U-plane) data to/from the media gateway 143.

In one example, the observed time difference, along with a knowledge of the relationship between SFN and user plane Frame Number (FN) for the source AP/Node B in the source cell 150 is sent to the gateway controller 142 or target AP/Node B in the target cell 151. The observed time difference, along with a knowledge of the relationship between SFN and user plane Frame Number (EN) may be used by either the target AP/Node B in the target cell 151, or the gateway controller 142 to produce a user plane data stream with FNs that match the values that would have been used by the source cell/source AP/Node B in the source cell 150 for the same user plane data frames had the handover not occurred.

In one example, it is not necessary for the original source cell to set any particular alignment between SEN and FN. In this example, the FN counter inherently runs at a rate that is derived from the SFN, irrespective of the offset between them. Thus, in this example, a measurement of this offset within the source cell at handover time is all that is required.

Thereafter, the target cell uses the offset that it is receives from the source cell to correctly set its frame alignment.

In this manner, the user plane FN sequence may be used continuously across an intra-gateway handover. Thus, and advantageously, the media gateway 143 may continue processing the sequence of user plane data frames as if the handover had not occurred. Any discontinuity in sequence numbers at the media gateway 143 will now be as a consequence of data frames that were actually not transmitted by the UE 114, or were lost in transmission, and the media gateway 143 can calculate how many such frames are missing and adapt its operations accordingly.

Thus, example embodiments have been described that can be applied to in any cellular *...

communications network where the sequence of User Plane FNs should be sequential and * incrementing at a constant rate per unit time, but the numbering sequence is not explicitly known to :. the target AP. Furthermore, it will be appreciated that embodiments may be implemented within existing network elements of a cellular communication system using existing messaging and *.* * communication protocols. Accordingly, in some examples, modifications are not required to be ***. 25 made to existing wireless communication units already in the field in order to benefit from the inventive concept herein described. * *

Referring now to FIG. 4, there is illustrated a typical computing system 400 that may be employed to implement signal processing functionality in embodiments of the invention.

Computing systems of this type may be used in access points, base transceiver stations and wireless communication units. Those skilled in the relevant art will also recognize how to implement the invention using other computer systems or architectures. Computing system 400 may represent, for example, a desktop, laptop or notebook computer, hand-held computing device (PDA, cell phone, palmtop, etc.), mainframe, server, client, or any other type of special or general purpose computing device as may be desirable or appropriate for a given application or environment. Computing system 400 can include one or more processors, such as a processor 404. Processor 404 can be implemented using a general or special-purpose processing engine such as, for example, a microprocessor, microcontroller or other control module. In this example, processor 404 is connected to a bus 402 or other communications medium.

Computing system 400 can also include a main memory 408, such as random access memory (RAM) or other dynamic memory, for storing information and instructions to be executed by processor 404. Main memory 408 also may be used for storing temporary variables or other intermediate information during execution of instructions to be executed by processor 404.

Computing system 400 may likewise include a read only memory (ROM) or other static storage device coupled to bus 402 for storing static information and instructions for processor 404.

The computing system 400 may also include information storage system 410, which may include, for example, a media drive 412 and a removable storage interface 420. The media drive 412 may include a drive or other mechanism to support fixed or removable storage media, such as a hard disk drive, a floppy disk drive, a magnetic tape drive, an optical disk drive, a compact disc (CD) or digital video drive (DVD) read or write drive (R or RW), or other removable or fixed media drive. Storage media 418 may include, for example, a hard disk, floppy disk, magnetic tape, optical disk, CD or DVD, or other fixed or removable medium that is read by and written to by media drive 412. As these examples illustrate, the storage media 418 may include a computer-readable storage medium having particular computer software or data stored therein.

In alternative embodiments, information storage system 410 may include other similar components for allowing computer programs or other instructions or data to be loaded into computing system 400. Such components may include, for example, a removable storage unit 422 and an interface 420, such as a program cartridge and cartridge interface, a removable memory (for example, a flash memory or other removable memory module) and memory slot, and other * removable storage units 422 and interfaces 420 that allow software and data to be transferred from the removable storage unit 418 to computing system 400.

Computing system 400 can also include a communications interface 424. *.S

Communications interface 424 can be used to allow software and data to be transferred between computing system 400 and external devices. Examples of communications interface 424 can * * include a modem, a network interface (such as an Ethernet or other NIC card), a communications port (such as for example, a universal serial bus (USB) port), a PCMCIA slot and card, etc. Software and data transferred via communications interface 424 are in the form of signals which can be electronic, electromagnetic, and optical or other signals capable of being received by communications interface 424. These signals are provided to communications interface 424 via a channel 428. This channel 428 may carry signals and may be implemented using a wireless medium, wire or cable, fibre optics, or other communications medium. Some examples of a channel include a phone line, a cellular phone link, an RF link, a network interface, a local or wide area network, and other communications channels.

n this document, the terms computer program product' computer-readable medium' and the like may be used generally to refer to tangible media such as, for example, memory 408, storage device 418, or storage unit 422. These and other forms of computer-readable media may store one or more instructions for use by processor 404, to cause the processor to perform specified operations. Such instructions, generally referred to as computer program code' (which may be grouped in the form of computer programs or other groupings), when executed, enable the computing system 400 to perform functions of embodiments of the present invention. Note that the code may directly cause the processor to perform specified operations, be compiled to do so, and/or be combined with other software, hardware, and/or firmware elements (e.g., libraries for performing standard functions) to do so.

In an embodiment where the elements are implemented using software, the software may be stored in a computer-readable medium and loaded into computing system 400 using, for example, removable storage drive 422, drive 412 or communications interface 424. The control module (in this example, software instructions or executable computer program code), when executed by the processor 404, causes the processor 404 to perform the functions of the invention as described herein.

Furthermore, the inventive concept can be applied to any circuit for performing signal processing funct3onality within a network element. It is further envisaged that, for example, a semiconductor manufacturer may employ the inventive concept in a design of a stand-alone device, such as a microcontroller of a digital signal processor (DSP), or application-specific integrated circuit (ASIC) and/or any other sub-system element.

It will be appreciated that, for clarity purposes, the above description has described embodiments of the invention with reference to a single signal processing module. However, the * 20 inventive concept may equally be implemented by way of a plurality of different functional units and * processors to provide the signal processing functionality. Accordingly, it will be understood that the *:. term signal processing module' used herein is intended to encompass one or more signal processing functional units, circuits and/or processors. Thus, references to specific functional units are only to be seen as references to suitable means for providing the described functionality, rather 25 than indicative of a strict logical or physical structure or organization. * *

Aspects of the invention may 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 or configurable module components such as FPGA devices. Thus, 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.

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, for example, 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 performed 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. In addition, singular references do not exclude a plurality. Thus, references to a', an', first', second', etc. do not preclude a plurality.

Thus, an improved method and apparatus for handover within a cellular communication system have been described, wherein the aforementioned disadvantages with prior art arrangements have been substantially alleviated. *�., * a I..

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Claims

Claims 1. A method for handing over a wireless subscriber unit communication from a source cell (150) to a target cell (151) within a cellular communication system, the method comprising, at a network element within a cellular communication network: receiving (360), from the wireless subscriber communication unit, a message comprising an indication of an observed time difference between a source cell (150) supporting communication with the wireless subscriber communication unit and the target cell (151); calculating (365) a timing offset between the source cell and the target cell based on the message indication, following receipt of a system frame number (SFN) of the source cell 150 ata point in time and an SFN of the target cell at a point in time; re-configuring (390) a timing of the wireless subscriber communication unit communication for the target cell; and determining whether to initiate a handover of a communication from a source cell to a target cell in response to receiving said measured received signal levels.
2. The method of Claim 1 further comprising initiating handover of the wireless subscriber communication unit from the source cell (150) to the target cell (151) using the timing re-configured CO communication.
3. The method of Claim 1 or Claim 2 wherein the message comprising an indication of an observed time difference between the source cell (150) and the target cell (151) comprises a system frame number (SFN) of the source cell (150) identified at a first point in time and an SFN of the target cell (151) at a second point in time.
4. The method of Claim 3 wherein calculating (365) a timing offset between the source cell and the target cell based on the message indication comprises calculating a frame number (FN) offset for a target cell based on the message indication.
5. The method of Claim 4 wherein the network element has a knowledge of the relationship between SFN and user plane Frame Number (FN).
6. The method of any preceding Claim wherein re-configuring (390) the wireless subscriber communication unit communication for the target cell comprises performing frame sequence synchronisation based on the calculated timing offset
7. The method of any preceding Claim, further comprising producing a user plane data stream with frame numbers that match a value that would have been used by the source cell (150) for the same user plane data frames had the handover not occurred. -14-8. The method of any preceding Claim, wherein the network element is at least one from a group consisting of: a gateway controller, a base station, a target access point, a Node B. 9. A tangible computer program product having executable program code stored therein for handing over a wireless subscriber unit communication from a source cell (150) to a target cell (151) within a cellular communication system, the program code operable for, when executed at a network element within a cellular communication network, performing the method of any of the preceding Claims.10. A network element for handing over a wireless subscriber unit communication from a source cell (150) to a target cell (151) within a cellular communication system, the network element comprising: a receiver for receiving (360), from the wireless subscriber communication unit, a message comprising an indication of an observed time difference between a source cell (150) supporting communication with the wireless subscriber communication unit and the target cell (151); a signal processor arranged to: calculate (365) a timing offset between the source cell and the target cell based on CO the message indication, following receipt of a system frame number (SFN) of the source Q 20 cell 150 at a point in time and an SFN of the target cell at a point in time; re-configure (390) a timing of the wireless subscriber communication unit communication for the target cell; and initiate handover of the wireless subscriber communication unit from the source cell (150) to the target cell (151) using the timing re-configured communication.11. An integrated circuit for a network element for handing over a wireless subscriber unit communication from a source cell (150) to a target cell (151) within a cellular communication system, the integrated circuit comprising: a receiver for receiving (360), from the wireless subscriber communication unit, a message comprising an indication of an observed time difference between a source cell (150) supporting communication with the wireless subscriber communication unit and the target cell (151); a signal processor arranged to: calculate (365) a timing offset between the source cell and the target cell based on the message indication, following receipt of a system frame number (SFN) of the source cell 150 at a point in time and an SFN of the target cell at a point in time; re-configure (390) a timing of the wireless subscriber communication unit communication for the target cell; and initiate handover of the wireless subscriber communication unit from the source cell (150) to the target cell (151) using the timing re-configured communication.12. A method for hand over within a cellular communication system, the method comprising, at a wireless subscriber communication unit communicating within at least one communication cell of a cellular communication network: identifying a system frame number (SFN) of a source cell (150) supporting communication with the wireless subscriber communication unit at a point in time and identifying an SFN of a target cell (151) at a point in time; transmitting a message comprising an indication of an observed time difference between the source cell (150) and a target cell (151); and being handed over from the source cell (150) to the target cell (151) using a timing re-configured communication based on a calculated (365) timing offset between the source cell and the target cell based on the message indication.13. The method of Claim 12, wherein identifying SFNs comprises measurement of at least one received signal level in the source cell (150) and at least one received signal level in the target cell (151).14. The method of Claim 12 or Claim 13, wherein the indication of an observed time difference CO between the source cell (150) and a target cell (151) comprises an indication of a relationship Q 20 between a SFN of the source cell (150) and an user plane Frame Number (FN) for a source access N... point/Node B in the source cell 150.15. The method of Claim 14 further comprising transmitting the indication of the relationship between the SFN of the source cell (150) and the user plane Frame Number (FN) to a gateway controller (142) or target access point/Node B in the target cell (151).16. The method of Claims 12 to 15, wherein the indication of an observed time difference between the source cell (150) and a target cell (151) comprises an indication of a communication handover requirement for the wireless subscriber communication unit.17. The method of any of Claims 12 to 16, wherein identifying a system frame number (SFN) of the source cell (150) and the target cell (151) comprises measuring (350) a timing parameter of at least one received signal in both the source cell (150) and the target cell (151), the method further comprising determining whether to initiate a handover of a communication from a source cell to a target cell in response to said measuring of said at least one received signal levels.18. The method of any of Claims 12 to 17, wherein determining whether to initiate a handover of a communication from a source cell to a target cell UE 114 comprises determining whether a target access point of the target cell (151) shares a same Gateway Controller (142) to the Core Network (CN) as a source access point of the source cell (150).19. A tangible computer program product having executable program code stored therein for hand over within a cellular communication system, the program code operable for, when executed at a wireless subscriber communication unit, performing the method of any of preceding Claims 12 to 18.20. A wireless subscriber communication unit communicating within at least one communication cell of a cellular communication network, the wireless subscriber communication unit comprising: a receiver for receiving at least one signal from a source cell (150) supporting communication with the wireless subscriber communication unit and receiving at least one signal from a target cell (151) a signal processor arranged to identify a system frame number (SFN) of the source cell (150) and identify an SFN of the target cell (151) at a point in time; a transmitter arranged to transmit a message comprising an indication of an observed time difference between the source cell (150) and a target cell (151); and CO such that the wireless subscriber communication unit is handed over from the source cell Q 20 (150) to the target cell (151) using a timing re-configured communication in response to a N... calculated (365) timing offset between the source cell and the target cell based on the message indication.21. An integrated circuit for a wireless subscriber communication unit communicating within at least one communication cell of a cellular communication network, the integrated circuit comprising: a receiver for receiving at least one signal from a source cell (150) supporting communication with the wireless subscriber communication unit and receiving at least one signal from a target cell (151) a signal processor arranged to identify a system frame number (SFN) of the source cell (150) and identify an SFN of the target cell (151) at a point in time; a transmitter arranged to transmit a message comprising an indication of an observed time difference between the source cell (150) and a target cell (151); and such that the wireless subscriber communication unit is handed over from the source cell (150) to the target cell (151) using a timing re-configured communication in response to a calculated (365) timing offset between the source cell and the target cell based on the message indication. -17-22. The tangible computer program product of Claim 9 or Claim 19 wherein the tangible computer program product comprises at least one from a group consisting of: a hard disk, a CD-ROM, an optical storage device, a magnetic storage device, a Read Only Memory, ROM, a Programmable Read Only Memory, PROM, an Erasable Programmable Read Only Memory, EPROM, an Electrically Erasable Programmable Read Only Memory, EEPROM, and a Flash memory.23. A cellular communication system comprising at least one network element for supporting communication within at least one communication cell of a cellular communication network and for handing over a wireless subscriber unit communication from a source cell (150) to a target cell (151) within a cellular communication system, wherein the network element comprises a signal processing module arranged to: a receiver for receiving (360), from the wireless subscriber communication unit, a message comprising an indication of an observed time difference between a source cell (150) supporting communication with the wireless subscriber communication unit and the target cell (151); a signal processor arranged to: calculate (365) a timing offset between the source cell and the target cell based on the message indication, following receipt of a system frame number (SFN) of the source CO cell 150 at a point in time and an SFN of the target cell at a point in time; Q 20 re-configure (390) a timing of the wireless subscriber communication unit communication for the target cell; and initiate handover of the wireless subscriber communication unit from the source cell (150) to the target cell (151) using the timing re-configured communication; and wherein the wireless subscriber communication unit comprises: a receiver for receiving at least one signal from the source cell (150) supporting communication with the wireless subscriber communication unit and receiving at least one signal from the target cell (151); a signal processor arranged to identify the system frame number (SFN) of the source cell (150) and identify the SFN of the target cell (151) at a point in time; a transmitter arranged to transmit a message comprising an indication of an observed time difference between the source cell (150) and the target cell (151); and such that the wireless subscriber communication unit is handed over from the source cell (150) to the target cell (151) using a timing re-configured communication in response to a calculated (365) timing offset between the source cell and the target cell based on the message indication.