WO2010125151A1

WO2010125151A1 - Serving network element and method for establishing a neighbour cell link

Info

Publication number: WO2010125151A1
Application number: PCT/EP2010/055834
Authority: WO
Inventors: Christopher Edward John Kilgour
Original assignee: IP Access Ltd
Current assignee: IP Access Ltd
Priority date: 2009-04-29
Filing date: 2010-04-29
Publication date: 2010-11-04
Anticipated expiration: 2011-10-29
Also published as: WO2010125151A4; GB2469857B; GB2469857A; GB0907480D0

Abstract

A serving network element for a cellular communication network comprises a transceiver for transmitting and receiving radio frequency (RF) signals to and from at least one wireless communication unit, and a signal processing logic module. The signal processing logic module is arranged to receive transmit power information relating to at least one further serving network element from the at least one wireless communication unit, and to determine from the received transmit power information a likely class of the at least one further serving network element to which the transmit power information relates. If it is determined that the likely class of the at least one further serving network element to which the transmit power information relates is appropriate for establishing a link therewith, the signal processing logic module is arranged to initiate the establishment of a neighbour cell link with the at least one further serving network element.

Description

SERVING NETWORK ELEMENT AND METHOD FOR ESTABLISHING A NEIGHBOUR

CELL LINK

Field of the invention The field of the invention relates to a serving network element and a method for establishing a neighbour cell link. In particular, the field of the invention relates to a 3GPP serving network element establishing a neighbour cell link with a neighbouring macro cell base station in a combined macro cell/femto cell cellular communication system.

Background of the Invention

Wireless communication systems, such as th e 2^nd Generation (2G) (otherwise referred to as Global System for Mobile (GSM) communications) and the 3^rd Generation (3G) of mobile telephone standards and technology, are well known. An example of such 3G standards and technology is the Universal Mobile Telecommunications System (UMTS), developed by the 3^rd Generation Partnership Project (3GPP) (www.3qpp.org).

Typically, wireless communication units, or User Equipment (UE) 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 radio network subsystems, each radio network subsystem comprising one or more communication cells to which UEs may attach, and thereby connect to the network.

The 3^rd generation of wireless communications has been developed for macro-cell mobile phone communications. Such macro cells utilise high power base stations (NodeBs in 3GPP parlance) to communicate with UEs operating within a relatively large coverage area. Lower power (and therefore smaller coverage area) femto-cells or pico-cells are a recent development within the field of wireless cellular communication systems. Femto-cells or pico-cells (with the term femto-cell being used hereafter to encompass pico-cell or similar) are classified under local area base stations in the 3GPP standard specifications.

Femto cells are effectively communication coverage areas supported by low power base stations (otherwise referred to as Access Points (APs)). These cells are 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-cell APs include, by way of example, residential and commercial (e.g. office) locations, '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, since the quality of services (voice/data) suffer due to massive attenuation of macro cell transmissions going through concrete walls or metallised glass planes in order to reach the user in-building. In a femto cell network it is known that there may be a very large number of femto cells compared to the number of macro cells; with femto cells often residing within, or overlapping, macro cells in the same geographic area.

Typically, in a cellular network each cell is assigned a list of neighbouring cells that may be used as potential target cells by UEs within that cell during a handoff procedure.

Furthermore, the 3^rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) of the

Universal Mobile Telecommunications System (UMTS) proposes the use of neighbour cell links in the form of X2 interface links between neighbouring cells for purposes such as power control and mobility handling. Due to the large number of cells present within 3G cellular networks, and the complexity of the relations between the cells, automatic neighbour relation

(ANR) systems are proposed to determine appropriate neighbour cell links for each cell.

Referring now to FIG. 1 , there is illustrated an example of a known automatic neighbour relation (ANR) procedure as proposed in the 3^rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) of the Universal Mobile Telecommunications System (UMTS). For the example illustrated in FIG. 1 , the ANR procedure relies on a UE 110 attached to a serving cell 120 to report details of other cells, such as cell 130, that it has detected communications from, but that are not within the defined neighbour list for that serving cell 120. In this regard, the UE 1 10 measures and reports signal level/quality criteria for cells that it detects. Detected cells may comprise cells within the neighbour list and cells not within the neighbour list, including in some instances cells belonging to a different radio access technology to that supported by the serving cell 120. Thus, for the illustrated example, the UE 1 10 measures 132 the signal level/quality for a detected cell 130, and then sends a measurement report 122 for the detected cell 130 to the serving cell 120. The NodeB for the serving cell 120 receives the measurement report from the UE 110 and, upon determining that the measurement report relates to a cell not within its neighbour cell list, instructs 124 the UE 1 10 to report the Global Cell identifier (ID) for the detected cell. The UE 110 reads 134 the Global Cell ID for the detected cell 130, which is broadcast within the Broadcast Control CHannel (BCCH) of the detected cell 130, and reports 126 the Global cell ID back to the serving cell 120. The NodeB of the serving cell 120 may then update 128 its neighbour cell list to take into account the detected cell 130. The serving cell 120 sends 142 the updated neighbour cell list to its Operation And Management (OAM) sub-system 140, and receives 144 the IP address for the detected cell 130 from the OAM sub-system 140. The OAM subsystem 140 also determines whether an X2 interface link is to be established between the two cells, based on a neighbour relation scheme. If required, the NodeB of the serving cell 120 may then establish 150 an X2 interface link with the NodeB of the detected cell 130.

A neighbour cell link is a logical connection between two cells deemed to be neighbours from a network topology perspective, which will typically also include some physical proximity and that allows for the exchange of information between cells. There does not need to be any physical direct connection between the cells for such a link to exist. There are many purposes of the neighbour cell link. The key benefits of such links include: potential support of continuation between eNodeBs of services offered by the Core Network over the S1 interface; the support of radio interface mobility between eNodeBs of UEs having a connection to the enhanced UMTS terrestrial radio access network (E-UTRAN); and supporting the interconnection of eNodeBs by different manufacturers.

Support of radio interface mobility and maintaining network performance can also include functionality, such as interference coordination, mitigation and power control optimisation between cells, load management and handover support. In the case of the X2 interface, as specified by 3GPP LTE, the fundamentals of the X2 interface are specified in TS

36.420 version 8.0.0 "Evolved Universal Terrestrial Radio Access Network (E-UTRAN); X2 general aspects and principles". In this instantiation, a pre-operational mechanism can be used for a cell to start establishing neighbour cell links after it has powered up and been assigned an IP address. It can then contact part of the Operations and Maintenance System and supply its configuration, and in turn be allocated some neighbour cell links to make before entering operational mode. During operation, a cell may choose to attempt a neighbour cell link if the ANR function, used in conjunction with a UE, detects a new cell within proximity.

The use of such X2 interface links was targeted at the macro-cell network. However, with the recent development of femto cells within cellular communication networks, current

ANR systems are insufficient in terms of coping with scalability issues arising from the large number of femto cells that may be deployed within the vicinity of a macro cell. In particular, the introduction of femto cells into a cellular network could cause current ANR systems to create signalling overload within the network due to the potentially large number of cells that a UE may detect in any single location.

Thus, there exists a need for a method and apparatus for establishing a neighbour cell link, particularly within a 3GPP combined femto cell/macro cell communication network, which aims to address at least some of the shortcomings of past and present techniques and/or mechanisms.

Summary of the Invention

Accordingly, the invention seeks to mitigate, alleviate or eliminate one or more of the abovementioned disadvantages singly or in any combination.

According to a first aspect of the invention, there is provided a serving network element for a cellular communication network that comprises a transceiver for transmitting and receiving radio frequency (RF) signals to and from at least one wireless communication unit, and a signal processing logic module. The signal processing logic module is arranged to receive transmit power information relating to at least one further serving network element from the at least one wireless communication unit, and to determine from the received - A -

transmit power information a likely class of the at least one further serving network element to which the transmit power information relates. If it is determined that the likely class of the at least one further serving network element to which the transmit power information relates is appropriate for establishing a link therewith, the signal processing logic module is arranged to initiate the establishment of a neighbour cell link with the at least one further serving network element.

In this manner, the signal processing logic module is able to differentiate between different classes of serving network elements based on the transmit power information. As a result, the signal processing logic module is able to preclude initiating the establishment of neighbour cell links with other serving network elements that do not appear to be suitable based on their transmit power information. As a result, the amount of signalling between the serving network element, neighbour serving network elements and core network elements required in order to initiate the establishment of links with neighbouring serving network elements may be significantly reduced, in particular where large numbers of lower power cells such as femto cells are present. As a result, the likelihood of signalling overload within the network due to the potentially large number of cells that a UE may detect may be significantly reduced.

In one optional embodiment of the invention, the signal processing logic module may be arranged, upon receipt of a cell measurement report for a detected cell from a wireless communication unit, to instruct the wireless communication unit to report transmit power information relating to a further serving network element for that detected cell. In this manner, the signal processing logic module may be kept up to date with the presence of neighbouring cells, and to determine whether to establish neighbour cell links with neighbouring cells.

In one optional embodiment of the invention, the transmit power information may comprise information relating to transmit power for a common pilot channel (CPICH) broadcast by the at least one further serving network element. In this manner, no additional signalling is required by neighbouring cells in order to enable the signal processing logic module to determine a likely class of the neighbouring cell base stations.

In one optional embodiment of the invention, the signal processing logic module may be arranged to determine from the received transmit power information a likely class of the at least one further serving network element to which the transmit power information relates by comparing the received transmit power information to one or more threshold value(s), and if the received transmit power information comprises a value greater than the threshold, it may be determined that the likely class of the at least one further serving network element to which the transmit power information relates is appropriate for establishing a neighbour cell link therewith. The threshold value may be configurable.

In one optional embodiment of the invention, if it may be determined that the likely class of the at least one further serving network element to which the transmit power information relates is appropriate for establishing a neighbour cell link therewith, the signal process logic module is further arranged to add a cell supported by the at least one further serving network element to a neighbour cell list.

In one optional embodiment of the invention, the serving network element may supply its own class - whether it is a femtocell, or whether it is access controlled or if it is designated to be part of the macro network layer within the radio access network to the at least one further serving network element when it attempts to create a neighbour cell link. In this manner, the at least one further serving network element may choose to reject the creation of the neighbour cell link based on signal processing logic instructions associated with the at least one further serving network element, thereby reducing the potential signalling load. In one optional embodiment of the invention, the at least one further serving network element may supply its own class - whether it is a femtocell, or whether it is access controlled or if it is designated to be part of the macro network layer within the radio access network to the serving network element when it responds to an attempt to create a neighbour cell link. In this manner, the serving network element may choose not to complete the creation of the neighbour cell link based on signal processing logic instructions associated with the serving network element, thereby reducing the potential signalling load.

In one optional embodiment of the invention, the signal processing logic module may be arranged to initiate the establishment of a neighbour cell link with the at least one further serving network element by requesting an address lookup for the at least one further serving network element.

In one optional embodiment of the invention, the serving network element may comprise a NodeB within a Universal Mobile Telecommunications System (UMTS) cellular communication network. In one optional embodiment of the invention, the serving network element comprises an Enhanced NodeB (eNodeB) within a E-UTRAN cellular communication network.

In one optional embodiment of the invention, if it is determined that the likely class of the at least one further serving network element to which the transmit power information relates is appropriate for establishing a neighbour cell link therewith, to initiate the establishment of an X2 interface link with the at least one further serving network element. According to a second aspect of the invention, there is provided a wireless communication system adapted to support the serving network element of the first aspect of the invention.

According to a third aspect of the invention, there is provided a method for establishing a neighbour cell link with a detected neighbour cell within a cellular communication network. The method comprising, at a network element, at least one further receiving transmit power information relating to a serving network element from a wireless communication unit, and determining from the received transmit power information a likely class of the at least one further serving network element to which the transmit power information relates. If it is determined that the likely class of the at least one further serving network element to which the transmit power information relates is appropriate for establishing a neighbour cell link therewith, the method further comprises initiating the establishment of a neighbour cell link with the at least one further serving network element.

According to a fourth aspect of the invention, there is provided a computer-readable storage element having computer-readable code stored thereon for a programming signal processing logic module to perform a method for establishing a neighbour cell link with a detected neighbour cell within a cellular communication network according to the third aspect of the invention.

These and other aspects, features and advantages of the invention will be apparent from, and elucidated with reference to, the embodiments described hereinafter.

Brief Description of the Drawings

Embodiments of the invention will be described, by way of example only, with reference to the accompanying drawings, in which: FIG. 1 illustrates an example of a known automatic neighbour relation procedure

FIG. 2 illustrates an example of part of a 3GPP network adapted to support embodiments of the invention.

FIG. 3 illustrates a simplified example of part of a cellular communication network adapted in accordance with some embodiments of the invention. FIG. 4 illustrates a simplified example of a message flow between the various elements of FIG. 3 in accordance with some embodiments of the invention.

FIG. 5 illustrates a simplified flowchart of an example of a method for establishing a neighbour cell link with a detected neighbour cell within a cellular communication network according to some embodiments of the invention. FIG. 6 illustrates a typical computing system that may be employed to implement processing functionality in embodiments of the invention.

Detailed Description of Embodiments of the Invention

Embodiments of the invention find particular applicability in a cellular communication system that supports a number of overlapping communication coverage areas, for example a communication system that comprises a combination of femto cells and macro cells.

Embodiments of the invention may be used by any communication unit forming part of, say, an automatic neighbour relation (ANR) system. For example, embodiments of the invention may be used by a base station comprising a wideband code division multiple access (WCDMA) transceiver. Embodiments of the invention may be employed particularly by a base-station transceiver operating in a cellular communication system comprising both femto cell base-stations (termed access points (APs) hereafter) and macro-cell base-stations

(termed NodeBs hereafter). In particular, embodiments of the invention enable signal processing logic of a cellular base station to establish a connection with a neighbouring macro cell base station in a combined macro cell/femto cell cellular communication system.

Those skilled in the art, however, will recognize and appreciate that the specifics of this example are merely illustrative of some embodiments, and that the teachings set forth herein are applicable in a variety of alternative settings. For example, since the teachings described hereafter do not depend on a particular cellular communication network conforming to any specific standard, it is envisaged that the teachings and inventive concept described herein can be applied to any type of cellular communication network, although a 3^rd generation partnership project (3GPP) network is shown in this embodiment. As such, other alternative implementations within cellular communication networks conforming to different standards and radio access technologies are contemplated and are considered as being within the scope of the various teachings described.

Referring now to the drawings, and in particular FIG. 2, an example of part of a 3GPP network, adapted in accordance with embodiments of the invention, is illustrated and indicated generally at 200. In FIG. 2, there is illustrated an example of a communication system combining macro cells 285 and femto cells 250, in accordance with one embodiment of the invention. In a femto cell network it is known that there may be a very large number of femto cells per macro cell. Thus, the coverage area of a single macro cell will inevitably encompass a coverage area of a large number of femto cells. For the embodiment illustrated in FIG. 2, a radio network sub-system (RNS) comprises two distinct architectures to handle the respective macro cell and femto cell communications. In the macro cell scenario, the RNS comprises one or more serving network elements 224, which for the illustrated embodiment is/are in a form of a macro cell base station, or NodeB in 3GPP parlance, and a radio network controller 236 to which the NodeB(s) is/are operably coupled. The radio network controller (RNC) 236 is operably coupled to a core network (CN) element 242, such as a serving general packet radio system (GPRS) support node (SGSN)/mobile switching centre (MSC), as known. The core network element 242 in turn is operably coupled to an Operation And Management (OAM) sub-system 238. It is advantageous for neighbour cell links to be established between neighbouring macro cellular base stations for purposes such as power control and mobility handling.

In a femto cell scenario, an RNS 210 comprises a network element in a form of an Access Point (AP) 230, and a controller in a form of an AP controller 240. As will be appreciated by a skilled artisan, an Access Point (AP) 230 is a communication element that facilitates access to a communication network via a communication cell, such as a femto-cell. One application that has resulted in the interest in femto cell technology is that an AP 230 may be purchased by a member of the public and installed in their home. The AP 230 may then be connected to an AP controller 240 over the owner's broadband internet connection 265. Thus, an AP 230 may be considered, in one embodiment of the invention, as being a scalable, multi-channel, two-way communication device that may be provided within, say, residential and commercial (e.g. office) locations, 'hotspots' etc, to extend or improve upon network coverage within those locations. Although there are no standard criteria for the functional components of an AP, an example of a typical AP for use within a 3GPP system may comprise some Node-B functionality and some aspects of radio network controller (RNC) 236 functionality. The AP communicates with UEs, such as UE 214, via a wireless interface (Uu) 222.

The AP controller 240 may be coupled to the core network (CN) element 242 via an Iu interface, as shown. In this manner, the AP 230 is able to provide voice and data services to a cellular handset, such as UE 214, in a femto cell 250 in contrast to the macro cell 285, in the same way as a conventional Node-B, but with the deployment simplicity of, for example, a Wireless Local Area Network (WLAN) access point. Due to the lower power and the generally restricted access nature of femto cells, neighbour cell links are generally not deemed necessary at the femto cellular level.

In accordance with some embodiments of the invention, a serving network element for a cellular communication network, such as NodeB 224, comprises a receiver, as illustrated at 226 in the case of NodeB 224, for receiving radio frequency (RF) signals from at least one wireless communication unit, for example UE 214. The serving network element further comprises a signal processing module, as illustrated at 228 in the case of NodeB 224, arranged to receive transmit power information relating to at least one further serving network element from the at least one wireless communication unit. The signal processing module is arranged to determine from the received transmit power information a likely class of the at least one further serving network element to which the transmit power information relates. If it is determined that the likely class of the at least one further serving network element to which the transmit power information relates is appropriate for establishing a neighbour cell link therewith, the signal processing logic module is further arranged to initiate the establishment of a neighbour cell link with the at least one further serving network element.

In this manner, the signal processing logic module is able to differentiate between different classes of serving network elements based on the transmit power information. As a result, the signal processing logic module is able to preclude initiating the establishment of neighbour cell links with other serving network elements that do not appear to be suitable based on their transmit power information. For example, if a Network Operator set up a macro cell specifically to broadcast at e.g. a sporting event, for which subscription would be required to receive the broadcast signal. The Network Operator would have access control applied and, thus, may not want it to appear as part of the macro network.

As a result, the amount of signalling between the serving network element and core network elements required in order to initiate the establishment of links with neighbouring serving network elements may be significantly reduced, in particular where large numbers of lower power cells such as femto cells are present. As a result, the likelihood of signalling overload within the network due to the potentially large number of cells that a UE may detect may be significantly reduced.

Referring now to FIG. 3, there is illustrated a simplified example of part of a cellular communication network 300 adapted in accordance with some embodiments of the invention. The cellular communication network 300 comprises a first serving network element in a form of NodeB 310. NodeB 310 is arranged to support macro cell area A 320, and comprises signal processing logic module 330. The cellular communication network 300 further comprises a second serving network element in a form of NodeB 360. NodeB 360 is arranged to support macro cell area C 350, and comprises signal processing logic module 365. The cellular communication network 300 further comprises a third serving network element in a form of femto access point (AP) 380. Femto AP 380 is arranged to support femto cell area B 370, and comprises signal processing logic module 385. Each of the serving network elements 310, 360, 380 is operably coupled to a core network, and in particular to OAM sub-system 390. For the illustrated embodiment, a wireless communication unit 340 is attached to macro cell area A 320, and as such establishes communication with NodeB 310. The operation of the various architecture elements will now be described further with respect to FIG. 4 and FIG. 5.

Referring now to FIG. 4, there is illustrated a simplified example of a message flow between the various elements of FIG. 3 in accordance with some embodiments of the invention.

For the illustrated example, the message flow begins at 405 with the UE 340 attaching to macro cell area A 320. The UE 340 detects femto cell B 370, and measures the signal level/quality for that cell, at 410. In this regard, the UE may measure a signal-to-noise (S/N) ratio or a carrier signal-to-noise (Eb/No) or a signal-to-noise plus interference of signals received from the serving base station. The UE may also measure any form of qualitative data parameter, for example bit-error-rate (BER), frame error rate (FER), etc., as would be appreciated by a skilled person. In a further example, the nature of which signal levels may be measured for UEs attached to the E-UTRAN is specified in 3GPP TS 36.331 Evolved Universal Terrestrial Radio Access (E-UTRA) Radio Resource Control (RRC) specification. The UE 340 then sends a measurement report at 415 comprising the signal level/quality measurement for femto cell area B to NodeB 310. Upon receipt of the measurement report, NodeB 310 instructs the UE 340 to report the cell identifier (cell ID) and transmit power information for the detected cell at 420. Accordingly, the UE 340 then reads the cell ID and transmits power information broadcast by the femto AP 380 of the detected cell area B 370.

By way of example, 3GPP Technical Specification (TS) 25.104 identifies four classes of base station: Wide Area Base Stations (e.g. a macro cell base station); Medium Range Base Stations (e.g. a micro cell base station); Local Area Base Stations (e.g. a femto/pico cell base station within a commercial environment); and Home Base Stations (e.g. a femto/pico cell base station within a residential environment). 3GPP TS 25.104 also specifies a maximum output power for each class of base station. In particular, the maximum output power for a Medium Range Base Station is limited to +38dBm, whilst the maximum output power for a Local Area Base Station is limited to +24dBm and the maximum output power for a Home Base Station is limited to +2OdBm without transmit diversity or multiple input multiple output (MIMO) functionality and +17dBm with transmit diversity or MIMO functionality. In practice, Local Area and Home Base Stations such as femto cell APs are unlikely to exceed a maximum output power of about +14dBm maximum output power. No limit is set for the maximum output power for a Wide Area Base Station.

For the purpose of simplicity and clarity, references herein to NodeBs and macro cell base stations encompasses both Wide Area (macro) Base Stations and Medium Range (micro) Base Stations, unless otherwise expressed.

This classification of base stations, and more particularly the specification of maximum output power levels for the different classes of base stations, provides a means by which, for example, femto cells may be differentiated from macro cells. Accordingly, by identifying the class of base station by which a detected neighbouring cell is supported, the establishment of neighbour cell links may be substantially limited to neighbouring cells supported by base stations of an appropriate class, i.e. to those that support macro cells. It is contemplated that for the illustrated embodiment of the invention, signal processing logic of, for the illustrated embodiment, the UE 340 may be arranged to decode transmit power information for a common channel broadcast by serving network elements such as femto AP 380. For example, in Wideband Code Division Multiple Access (WCDMA) cellular systems such as UMTS, and some other Code Division Multiple Access (CDMA) communication systems, a Primary Common Pilot CHannel (P-CPICH) is broadcast by all base stations with a constant power and of a known bit sequence. Typically, the transmit power of the P-CPICH is around 10% (approximately 1OdB less than) that of the maximum output power for the respective base station.

Each base station in a UMTS network broadcasts the transmit power level for its primary CPICH within an information element (IE) labelled "Primary CPICH TX Power", which is broadcast within a system information message. More specifically, a UMTS network utilises a Radio Resource Control (RRC) protocol. The RRC protocol is defined in the Universal Mobile Telecommunications System (UMTS) Radio Resource Control (RRC) Protocol specification (3GPP TS 25.331 ), and forms part of the network layer between (typically) a UE and the UMTS Terrestrial Radio Access Network (UTRAN). The RRC protocol comprises connection management procedures, which, in turn, comprise the broadcasting of system information by the UTRAN.

In particular, system information elements, such as IE "Primary CPICH TX Power", are broadcast in system information blocks, which group together system information elements of the same type. Typically, the information element "Primary CPICH TX Power" is broadcast within System Information Block 5 (SIB 5), or SIB 5bis. A generic 'SYSTEM INFORMATION' message is used to convey the system information blocks on a Broadcast Control CHannel (BCCH) logical channel, which, in turn, may be mapped onto either a Broadcast CHannel (BCH) or Forward Access CHannel (FACH) transport channel. The BCH transport channel is, in turn mapped on the Primary Common Control Physical RF Channel (P-CCPCH).

The cell ID within a UMTS system is typically also broadcast within a system information element within the system information message broadcast on the BCCH. In this manner, and in accordance with some embodiments of the invention, a wireless communication unit, such as UE 340 may receive an RF signal comprising a system information message from a serving network element, for example femto AP 380 or NodeB 360, and decode the cell ID and Primary CPICH TX Power within the system information message, in order to retrieve the cell ID and a transmit power value for the primary CPICH of that serving network element.

Referring back to FIG. 4, once the UE 340 has read the cell ID and transmit power information for the detected femto cell area B 370 at 425, it sends a report message back to NodeB 310 containing the read cell ID and transmit power information at 430. Upon receipt of the report, the NodeB 310 ascertains that the detected cell is not present within its neighbour cell list, and determines from the received transmit power information a likely class of the base station supporting the detected cell to which the transmit power information relates. For example, the signal processing module 330 of NodeB 310 may be arranged to compare the received transmit power information to a threshold value.

As previously mentioned, 3GPP TS 25.104 identifies the four classes of base station, and specifies a maximum output power for each class of base station. In particular, the maximum output power for a Medium Range Base Station is limited to +38dBm, whilst the maximum output power for a Local Area Base Station is limited to +24dBm. No maximum output power is specified for a wide area base station. Thus, it is envisaged in one embodiment of the invention the threshold value may be set to, say, +24dBm. In this manner, if the received transmit power information indicates that the transmit power (e.g. the Primary CPICH TX Power) of the detected cell exceeds this threshold, the supporting base station is likely to be a micro or macro cell base station. Conversely, if the received transmit power information indicates that the transmit power of the detected cell is less than this threshold, then the supporting base station is likely to be a femto or pico cell base station. Alternatively, the threshold value may be set to, say, +38dBm, such that only Wide Area Base Stations may be identified, and as such considered appropriate for establishing a neighbour cell link with.

In accordance with some embodiments of the invention, it is envisaged that the threshold value with which the decoded transmit power information is compared may be dynamically configurable. In this manner, the properties of the transmit power information that determine whether a serving network element is suitable for establishing a link with may be configured, for example by a core network component of the wireless communication network, such as OAM sub-system 390 of FIG. 3. Accordingly, signal processing logic module 330 of the NodeB 310 may be arranged to configure the threshold value according to information received from, for example, the OAM sub-system 390.

Referring back to FIG. 4, since the received transmit power information relates to femto cell area B, the transmit power indicated within the received transmit power information will be less than the threshold value. Accordingly, the signal processing logic 330 of NodeB 310 determines that no link should be established with the femto AP 380 for femto cell area B, at 435. In one example, the NodeB 310 may record the cell id in the neighbour cell list with an indicator that it had decided not to use it, so that it did not request more detailed measurements from another UE when the UE turned up and operated in a similar manner.

The UE 340 subsequently detects macro Cell area C 350, and measures the signal level/quality for that cell, at 440. The UE 340 then sends a measurement report at 445 comprising the signal level/quality measurement for macro cell area C to NodeB 310. Upon receipt of the measurement report, NodeB 310 instructs the UE 340 to report the cell ID and transmit power information for the detected cell at 450. Accordingly, the UE 340 then reads the cell ID and transmit power information broadcast by the NodeB 360 of the detected cell area C 350. Once the UE 340 has read the cell ID and transmit power information for the detected macro cell 350 at step 455, it sends a report message back to NodeB 310 containing the read cell ID and transmit power information at 460. In practice, in accordance with the 3GPP standard, a portion of the cell identity (rather than a full identity) may be read before further SIB information, such as the full cell identity is read or the configured maximum transmit power may be requested by the network. In this manner, the work of the UE is reduced by only requesting the more detailed measurements when needed, and thus the SIB5 may be transmitted less frequently than the earlier SIBs and the Master Information Block. Upon receipt of the report, the NodeB 310 determines that the detected cell is not present within its neighbour cell list, and therefore determines from the received transmit power information a likely class of the base station supporting the detected cell to which the transmit power information relates. For this latter scenario, since the received transmit power information relates to macro cell area C, the transmit power indicated within the received transmit power information will be greater than the threshold value. Accordingly, the signal processing logic module 330 of NodeB 310 determines that the establishment of a neighbour cell link between NodeB 310 and the base station supporting the detected cell, namely NodeB 360 is appropriate at 465. The signal processing logic module 330 then adds cell C to its neighbour cell list and sends an updated neighbour cell list to the OAM sub-system 390, along with a request for an address lookup for the detected cell, at 470. The OAM sub-system 390 updates the neighbour cell list and performs the requested address lookup at 475, and sends the address, for example an internet protocol (IP) address, back to the signal processing logic module 330 of NodeB 310, at 480. Upon receipt of the address, the signal processing logic module 330 is then able to send a request to the signal processing logic 365 or NodeB 360 to establish a link, at 485.

It is contemplated in other embodiments of the invention that the signal processing logic module 330 of the NodeB 310 may determine a likely class of serving network elements to which the transmit power information relates based on the transmit power information received in combination with any other information that is able to signify the class/type of transmitting base station, for example knowledge of a PCI split supplied by e.g. a macro-layer broadcast, to indicate the femto layer, or a CSG-bit or CSG identity read from the candidate serving cell.

It is possible that a network operator may configure one or more macro cell NodeBs to have a transmit power that is lower than the threshold used to determine the likely class of serving network elements as described above. As a result, although such a NodeB might be appropriate for establishing a link therewith, it may be treated by other NodeBs adapted in accordance with some embodiments of the invention as a femto cell AP, and therefore a link would not be established with it. However, such a low power macro cell NodeB, if adapted in accordance with the present invention, would be able to identify macro cells within its vicinity and seek to establish the links therewith itself.

Referring now to FIG. 5, there is illustrated a simplified flowchart 500 of an example of a method for establishing a neighbour cell link with a detected neighbour cell within a cellular communication network according to some embodiments of the invention. The method starts at step 510 with the receipt of a cell measurement report for a detected cell from a wireless communication unit, which for the illustrated example is in the form of a UE. Next, in step 520, the UE is instructed to report the cell ID and transmit power information for the detected cell. Upon receipt of the cell ID and transmit power information for the detected cell in step 530, a likely class of the serving network element of the detected cell to which the transmit power information relates is determined based on the received transmit power information. For the illustrated embodiment the likely class of the serving network element is determined by comparing the received transmit power information to a threshold, in step 540.

If the received transmit power information is less than the threshold value, it is determined that the detected cell is likely to be a femto or pico cell, and the method moves to step 590 and ends. However, if the received transmit power information is greater than (or equal to for the illustrated example) the threshold value, it is determined that the detected cell is likely to be a macro cell, and the method moves to step 550 where the detected cell is added to the neighbour cell list. Next, in step 560, the updated neighbour cell list is sent to an OAM sub-system, along with a request for an address lookup for the detected cell in order to initiate the establishment of a link with the base station of the detected cell.

Upon receipt of the address for the detected cell, in step 570, the method moves on to step 580, and a link with the base station of the detected cell is established. The method then ends at step 590.

FIG. 6 illustrates a typical computing system 600 that may be employed to implement processing functionality in embodiments of the invention. Computing systems of this type may be used in UEs or femto cell downlink receiver (in particular, processing logic module in UE or a femto cell modem that handles frequency synchronisation). Those skilled in the relevant art will also recognize how to implement the invention using other computer systems or architectures. Computing system 600 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 600 can include one or more processors, such as a processor 604. Processor 604 can be implemented using a general or special-purpose processing engine such as, for example, a microprocessor, microcontroller or other control logic module. In this example, processor 604 is connected to a bus 602 or other communications medium. Computing system 600 can also include a main memory 608, such as random access memory (RAM) or other dynamic memory, for storing information and instructions to be executed by processor 604. Main memory 608 also may be used for storing temporary variables or other intermediate information during execution of instructions to be executed by processor 604. Computing system 600 may likewise include a read only memory (ROM) or other static storage device coupled to bus 602 for storing static information and instructions for processor 604.

The computing system 600 may also include information storage system 610, which may include, for example, a media drive 612 and a removable storage interface 620. The media drive 612 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 618 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 612. As these examples illustrate, the storage media 618 may include a computer-readable storage medium having particular computer software or data stored therein.

In alternative embodiments, information storage system 610 may include other similar components for allowing computer programs or other instructions or data to be loaded into computing system 600. Such components may include, for example, a removable storage unit 622 and an interface 620, 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 622 and interfaces 620 that allow software and data to be transferred from the removable storage unit 618 to computing system 600. Computing system 600 can also include a communications interface 624.

Communications interface 624 can be used to allow software and data to be transferred between computing system 600 and external devices. Examples of communications interface 624 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 624 are in the form of signals which can be electronic, electromagnetic, and optical or other signals capable of being received by communications interface 624. These signals are provided to communications interface 624 via a channel 628. This channel 628 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.

In this document, the terms 'computer program product' 'computer-readable medium' and the like may be used generally to refer to media such as, for example, memory 608, storage device 618, or storage unit 622. These and other forms of computer-readable media may store one or more instructions for use by processor 604, 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 600 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 600 using, for example, removable storage drive 618, drive 612 or communications interface 624. The control logic module (in this example, software instructions or computer program code), when executed by the processor 604, causes the processor 604 to perform the functions of the invention as described herein.

It will be appreciated that, for clarity purposes, the above description has described embodiments of the invention with reference to different functional units or logic module elements or signal processors. However, it will be apparent that any suitable distribution of functionality between different functional units or logic module elements or signal processors, for example with respect to the UE, may be used without detracting from the invention. For example, it is envisaged that functionality illustrated to be performed by separate processors or controllers or logic module elements may be performed by the same processor or controller or logic module element. Hence, references to specific functional or logical 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. Some 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. 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 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 invention is limited only by the 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.

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 the feature may be 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. does not preclude a plurality. Thus, a method and apparatus for performing frequency synchronisation, for example by a femto AP in a cellular communication system that combines macro cells and femto cells, has been described that may alleviate at least some of the shortcomings of past and present techniques and/or mechanisms.

Claims

1. A serving network element for a cellular communication network comprises a transceiver for transmitting and receiving radio frequency (RF) signals to and from at least one wireless communication unit, and a signal processing logic module arranged to: receive transmit power information relating to at least one further serving network element from the at least one wireless communication unit; determine from the received transmit power information a likely class of the at least one further serving network element to which the transmit power information relates; and if it is determined that the likely class of the at least one further serving network element to which the transmit power information relates is appropriate for establishing a neighbour cell link therewith, to initiate the establishment of a neighbour cell link with the at least one further serving network element.

2. The serving network element of Claim 1 wherein the signal processing logic module is further arranged, upon receipt of a cell measurement report for a detected cell from a wireless communication unit, to instruct the wireless communication unit to report transmit power information relating to a further serving network element for that detected cell.

3. The serving network element of Claim 1 or Claim 2 wherein the transmit power information comprises information relating to transmit power for a common pilot channel (CPICH) broadcast by the at least one further serving network element.

4. The serving network element of any preceding Claim wherein the signal processing logic module is further arranged to determine from the received transmit power information a likely class of the at least one further serving network element to which the transmit power information relates by comparing the received transmit power information to a threshold value.

5. The serving network element of Claim 4 wherein, if the received transmit power information comprises a value greater than the threshold, it is determined that the likely class of the at least one further serving network element to which the transmit power information relates is appropriate for establishing a neighbour cell link therewith.

6. The serving network element of Claim 5 wherein the threshold value comprises one of: +24dBm and +38dBm.

7. The serving network element of any of Claims 4 to 6 wherein the threshold value may be configurable.

8. The serving network element of any preceding Claim wherein, if it is determined that the likely class of the at least one further serving network element to which the transmit power information relates is appropriate for establishing a neighbour cell link therewith, the signal process logic module is further arranged to add a cell supported by the at least one further serving network element to a neighbour cell list.

9. The serving network element of Claim 8 wherein, if it is determined that the likely class of the at least one further serving network element to which the transmit power information relates is appropriate for establishing a neighbour cell link therewith, to initiate the establishment of an X2 interface link with the at least one further serving network element.

10. The serving network element of any preceding Claim, wherein the signal processing logic module is further arranged to initiate the establishment of a neighbour cell link with the at least one further serving network element by requesting an address lookup for the at least one further serving network element.

11. The serving network element of any preceding Claim wherein the serving network element comprises a NodeB within a Universal Mobile Telecommunications System (UMTS) cellular communication network.

12. The serving network element of any of preceding Claims 8 to 11 wherein, if it is determined that the likely class of the at least one further serving network element to which the transmit power information relates is appropriate for establishing a neighbour cell link therewith, the signal processing logic module is arranged to initiate the establishment of an X2 interface link with the at least one further serving network element.

13. The serving network element of Claim 12 wherein the signal processing logic module is arranged to supply a network layer assignment with its initiating request.

14. The serving network element of any of preceding Claims 8 to 13 wherein, upon receiving an X2 interface link request from the at least one further serving network element the signal processing logic module is arranged to receive a network layer assignment and access control information and based thereon to decide whether to establish a neighbour cell link therewith.

15. The serving network element of any preceding Claim wherein the serving network element includes its own network layer assignment and/or access control information in a response.

16. The serving network element of any preceding Claim wherein the serving network element is arranged to decide not to complete X2 interface link establishment based on a response from the at least one further serving network element.

17. A wireless communication system adapted to support the serving network element of any preceding Claim.

18. A method for establishing a neighbour cell link with a detected neighbour cell within a cellular communication network, the method comprising, at a network element: receiving transmit power information relating to at least one further serving network element from a wireless communication unit; determining from the received transmit power information a likely class of the at least one further serving network element to which the transmit power information relates; and

if it is determined that the likely class of the at least one further serving network element to which the transmit power information relates is appropriate for establishing a neighbour cell link therewith, initiating the establishment of a neighbour cell link with the at least one further serving network element.

19. A computer-readable storage element having computer-readable code stored thereon for programming signal processing logic module to perform a method for frequency synchronising a wireless communication unit, the code operable for, at a network element: receiving transmit power information relating to at least one further serving network element from a wireless communication unit; determining from the received transmit power information a likely class of the at least one further serving network element to which the transmit power information relates; and if it is determined that the likely class of the at least one further serving network element to which the transmit power information relates is appropriate for establishing a neighbour cell link therewith, initiating the establishment of a neighbour cell link with the at least one further serving network element.

20. The computer-readable storage element of Claim 19, wherein the computer- readable storage medium comprises at least one 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, EPROM, an Electrically Erasable Programmable Read Only Memory, EEPROM, and a Flash memory.