GB2508604A - Inter-cell interference coordination in a cellular communication system - Google Patents

Abstract

A method and apparatus for enhancing intercell interference co-ordination and management, particularly in LTE heterogeneous networks, enables a small cell eNodeB (104) to feed back to a potential interfering macrocell eNodeB (103) a calculated RNTP threshold value that is desired for its own interference scheduling operations if an initial RNTP threshold value received from the macrocell eNodeB in a Load Information message is too high. The macrocell eNodeB subsequently uses the calculated RNTP threshold value in its next Load Information message. The calculated RNTP threshold value is determined in the small cell eNodeB from path loss estimates and knowledge of its own maximum transmit power and is transmitted to the macrocell eNodeB as a modified Load Information message over the X2 interface 112.

Description

NETWORK ELEMENTS, WIRELESS COMMUNICATION SYSTEM AND METHODS THEREFOR

Field of the invention

The field of this invention relates to network elements, a wireless communication system and methods for operating a wireless communication system.

Background of the Invention

Wireless communication systems, such as the 3 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 (UMTSTM), developed by the 3 Generation Partnership Project (3GPPTM) (www.3ppp.org). The 3 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 (UE5) 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 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 second generation wireless communication system (2G), also known as GSM, is a well-established cellular, wireless communications technology whereby "base transceiver stations" (equivalent to the Node B's of the 3G system) and mobile stations" (user equipment) can transmit and receive voice and packet data. Several base transceiver stations are controlled by a Base Station Controller (BSC), equivalent to the RNC of 3G systems.

Communications systems and networks are developing towards a broadband and mobile system. The 3rd Generation Partnership Project has proposed a Long Term Evolution (LTE) solution, namely, an Evolved Universal Mobile Telecommunication System Territorial Radio Access Network, (E-UTRAN), for a mobile access network, and a System Architecture Evolution (SAE) solution, namely, an Evolved Packet Core ( EPC), for a mobile core network. An evolved packet system (EPS) network provides only packet switching (PS) domain data access so voice services are provided by a 2G or 3G Radio Access Network (RAN) and circuit switched (CS) domain network or via packet voice solutions such as Voice over Internet Protocol (V0IP). User Equipment( UE) can access a CS domain core network through a 2G/3GRAN such as the (Enhanced Data Rate for GSM Evolution, EDGE) Radio Access Network (GERAN) or a Universal Mobile Telecommunication System Terrestrial Radio Access Network ( UTRAN), and access the EPC through the E-UTRAN.

Lower power (and therefore smaller coverage area) cells are a recent development within the field of wireless cellular communication systems. Such small cells are effectively communication coverage areas supported by low power base stations. The terms "picocell" and "femtocell" are often used to mean a cell with a small coverage area, with the term femtocell being more commonly used with reference to residential small cells. Small cells are often deployed with minimum RF (radio frequency) planning and those operating in consumers' homes are often installed in an ad hoc fashion.

The low power base stations which support small cells are referred to as Access Points (AP's) with the term Home Node B (HNB's) or Evolved Home Node B (HeNB) identifying femtocell Access Points used in 3G and LTE respectively. Each small-cell is supported by a single Access Point. These small cells are intended to augment the wide area macro network and support communications to multiple User Equipment devices in a restricted, for example, indoor environment. An additional benefit of small cells is that they can offload traffic from the macro network, thereby freeing up valuable macro network resources An HNB is an Access Point that provides a wireless interface for user equipment connectivity. It provides a radio access network connectivity to a user equipment (UE) using the so- called luh interface to a network Access Controller, also known as a Home Node B Gateway (HNB-GW). One Access Controller (AC) can provide network connectivity of several HNB's to a core network.

Typical applications for such Access Points include, by way of example, residential and commercial locations, communication hotspots', etc., whereby Access Points can be connected to a core network via, for example, the Internet using a broadband connection or the like. In this manner, small cells can be provided in a simple, scalable deployment in specific in-building locations where, for example, network congestion or poor coverage at the macro-cell level may be problematic.

Thus, an AP is 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 3G system may comprise Node-B functionality and some aspects of Radio Network Controller (RNC) functionality as specified in 3GPP TS 25.467. These small cells are intended to be able to be deployed alongside the more widely used macro-cellular network and support communications to UEs in a restricted, for example in-building', environment. In the case of LTE, the functionalities of a HeNB and a macro eNodeB are essentially the same, but a eNodeB may support and control more than one cell underneath it and communicate with the core network and neighbours, whilst a HeNB is limited to supporting and controlling a single cell and carrying out the communication with the core network and neighbours.

In some applications, small cell NodeB"s or eNode B's or HNB/HeNB's are provided with a network listen device which listens for broadcasts of neighbouring cells.

Herein, the term small cell" means any cell having a small coverage area and includes "picocells" and "lemlocells." Initial deployments of LTE networks are based on so-called homogeneous networks consisting of high power eNode B base stations providing coverage via a plurality of macrocells. A recent proposal has been the concept of heterogeneous networks where a high powered niacrocell is overlaid (or underlaid) with lower power small cells, for example femto cells served by HeNB's which might be present in buildings. Typically, the macrocell network is planned but the small cells are deployed in a less well-planned or even and co-ordinated manner. Such heterogeneous deployments can achieve improved overall capacity and cell edge performance. For example, the deployment of low-power eNodeB.s inside buildings can fill coverage holes that typically occur due to the losses imposed by the walls. LTE is designed to have a 1:1 frequency reuse therefore LTE macrocell deployment can experience heavy interference at the boundaries of a cell. Furthermore, when considering closed subscriber groups which are allowed access to a femtocell, this creates another problem for interference management if UE's are in the coverage area of the femtocell yet not allowed access to it. In the downlink (transmissions from an eNode B to the UE) a UE is exposed to interference from the HeNB serving the femtocell, whereas in the uplink, (from the UE to an eNodel B) the UE affects transmissions from other UE's to the HeNB. Therefore some form of intercell interference coordination along with signalling to support such coordination is required for heterogeneous networks.

The so-called "X 2 interface" for LIE is an interface between eNodeB's and is used (amongst other things) to enable intercell interference management and load management. The LIE X2 Load Information message as specified in 3GPP TS 36.423 currently contains RNTP (Relative Narrowband Transmit Power) Information to be sent from one eNodeB to its neighbours which use the same frequencies. In a heterogeneous network, a macrocell may overlap several small cells and therefore the nacro eNodeB will be able support UE's present in any one of several cells that it controls (as opposed to a HeNB that supports only one (small) cell), and so a Load Information message transmitted from the macro eNode B lists all the underlying cells and their RNTP profiles (including RNIF Threshold values).

The RNTP part of a Load Information message provides information about which Physical Resource Blocks (specifying a number of allocated subcarriers) the eNode B guarantees to transmit below a specified power level threshold (denoted RNTPthmshcld). The purpose of this is to allow interference aware (frequency) scheduling (particularly for UE's near a cell edge) by a neighbouring eNode B which is in receipt of this message. Scheduling decisions can then be taken immediately rather than relying on measurement reports received from UE's.

However, current arrangements do not allow an eNodeB receiving a Load Information message to feedback to the eNode B which transmitted the Load Information message any confirmation or otherwise that this threshold is useful for any type of intercell interference co-ordination or scheduling. For example, the transmitting eNode B may be so close to the receiving eNode B that only a lower threshold would be useful for scheduling transmissions. This is a particular issue when one cell (eg. a femtocell served by a HeNB) is completely within the coverage of another cell (eg. a macrocell) so it is likely that the RNTP threshold value transmitted by the macro eNode B is above the transmission capabilities of the HeNB. Say, for example, that a first set of Physical Resource Blocks will be transmitted by a macro eNode B at less than a first power threshold and a second set will be transmitted at a second power threshold which is higher than the first and the macro eNode B is configured to transmit the second power threshold value in its RNTP part of a Load Information message. Say that an HeNB, whose area of coverage is overlaid by part of the macro eNode B's coverage area, has a transmit power capability between the first and second power thresholds mentioned above. This HeNB will not find the RNTP message useful for scheduling its own transmissions. However, if the macro node B had transmitted the first power threshold value in RNTP part of the Load Information message then the HeNB would know that certain physical resource blocks could be available for its own use.

Thus there is a need for a method and apparatus which aims to address at some of the shortcomings of the current techniques for inter-cell interference co-ordination and management.

Summary of the invention

Accordingly, the present invention seeks to mitigate one or more of the above-mentioned disadvantages.

Aspects of the invention provide network elements, a wireless communication system and methods therefor as described in the appended claims.

According to a first aspect of invention there is provided a method for operating a wireless communication system the method including; at a first base station, receiving from a neighbouring base station, a message containing a transmit power threshold value, comparing the received transmit power threshold value with a calculated transmit power threshold value and if the received transmit power threshold value exceeds a calculated transmit power threshold value, transmitting from the first base station, a message which contains the calculated transmit power threshold value to the neighbouring base station.

The invention facilitates self-organisation (of small cells for example) and optimisation in a wireless communication network without specific operator or OA&M (Operations, Administration and Maintenance) intervention.

In some embodiments, the invention may enable deployment of LTE cells supporting the X2 interface and where placement of eNode B's is not controlled by the Operator. The invention can facilitate this by auto-configuration of the RNTP interference thresholds.

The method may further include, at the neighbouring base station, receiving, from the first base station, the message containing the calculated transmit power threshold value and adjusting a subsequent transmission based on the calculated transmit power threshold value.

The message received from the neighbouring base station may comprise a Load information message transmitted over an X2 interface and containing a Relative Narrowband Transmit Power part indicating which Physical Resource Blocks of one or more cells of the neighbouring base station will be transmitted by the neighbouring base station below a defined threshold value.

The message which contains the calculated transmit power threshold value may be transmitted over an X2 interface as a modified Load Information message or by amending some alternative message or by defining a new message. Alternatively the message which contains the calculated transmit power threshold value may be transmitted via a core network as a Radio Access Network Information Management (RIM) message. In this latter case, a specific container could be used to carry the details of the message so that both the first base station and the receiving neighbouring base station would correctly interpret the contents. As a further alternative, 0MM methods may be used for transmitting the calculated transmit power threshold.

In one embodiment, the method further includes receiving from the neighbouring base station a reference signal and a message relating to a transmit power of the reference signal, estimating a path loss between the first base station and the neighbouring base station and determining the calculated transmit power threshold value based on an estimated path loss and a transmit power capability of the first base station.

Estimating a path loss may use measurement reports received from a user equipment located in a coverage area of the first base station.

In one example, determining the calculated transmit power threshold value may be based on a total expected interference at at least one point in a cell served by the first base station, which total expected interference may be based on at least one estimated path loss from one or more neighbouring base stations.

According to a second aspect of the invention, there is provided a network element comprising a signal processor arranged to compare a transmit power threshold value received from a remote network element with a calculated transmit power threshold value and to generate a message including the calculated transmit power threshold value for transmission to the remote network if the received transmit power threshold value exceeds the calculated transmit power threshold value.

The network element may comprise or be a part of an Access Point serving a small cell or a HeNB serving a femtocell or a eNodeB serving a macrocell, for example.

The network element may include a receiver for receiving the transmit power threshold value from a remote network element and a transmitter for transmitting the calculated transmit power threshold value to the remote network element. The network element may also include a network listen module.

The signal processor may be further arranged to estimate a path loss and to determine the calculated transmit power threshold value based on an estimated path loss and a transmit power capability of the network element. The estimated path loss may be an estimated path loss to one or more neighbouring base stations.

The signal processor of the network element may be implemented in one or more integrated circuits.

According to a third aspect of the invention there is provided a network device including a receiver for receiving a calculated transmit power threshold value from a network element, a processing module for generating a modified Load Information message based on the calculated transmit power threshold value, and a transmitter for transmitting the modified a Load Information message to the element.

According to a fourth aspect of the invention there is provided a wireless communication system arranged to support the method and network element of the above aspects. In one example embodiment, the network element is a first base station having a first coverage area and the network device is a second base station having a second coverage area which is greater than and overlays the first coverage area.

According to a fifth aspect of the invention, there is provided tangible computer program product having an executable computer program code stored thereon for execution by a processor to perform a method in accordance with the first aspect.

The tangible computer program product may comprise 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, a Programmable Read Only Memory, an Erasable Programmable Read Only Memory, an Electrically Erasable Programmable Read Only Memory and a Flash memory.

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

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 a part of a cellular communication system operating in accordance with an example embodiment, and FIG. 2 is a flow chart of an example of a method of operation of the system of FIG. 1.

Detailed Description

The inventive concept finds 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 small cells and macro cells.

Although the following description is directed to the Long Term Evolution (LIE) system, the present invention can be applied to other wireless communication systems supporting message exchange among the base stations.

Those skilled in the art will recognize and appreciate that the specifics of the specific examples described are merely illustrative of some embodiments and that the teachings set forth herein are applicable in a variety of alternative settings As such, other alternative implementations within cellular communication systems conforming to different standards are contemplated and are within the scope of the various teachings described.

Referring now to FIG.1, an example of part of a wireless communication system operating in accordance with embodiments of the invention is illustrated and indicated generally at 100 and comprises a LTE macrocell 101 which overlaps a plurality of LTE small cells, only one 104 of which being shown for the sake of clarity. The macrocell 101 is served by a high power eNodeB (base station) 103 and the small cell 102 is served by a low power eNodeB (base station) 104. The low power eNodeB 104 is provided with a transmitter 105, a receiver 106, a network listen module 107 and a signal processor 108. The high power eNodeB 103 is provided with a transmitter 109, a receiver 110 andaprocessing module 111.

The two eNodeBs 103, 104 can communicate with one another over an X2 interface 112. The network listen module 107 can detect broadcast transmissions from the high power eNodeB 103, eg.

reference signals such as the Reference Signal Power PDSCH (Physical Downlink Shared Channel) Communications configuration broadcast and its transmission level encoded in SIB2 (System Information Broadcast).

A User Equipment (UE) 114 located close to the edge of the small cell 102 can receive communication services from either the high power eNodeB 103 or the low power eNodeB 104, send measurement reports to either eNodeB 103, 104 when receiving services from the cell and requested to do so and also detect broadcast transmissions from either eNodeB 103, 104.

Referring now to FIG.2, at 201, the signal processor 108 of the lower power eNodeB 104 calculates a desired RNTP threshold value. In one exemplary mode of operation, the signal processor 108 uses inputs from the network listen module 107 and reported measurements form the UE 114, to assess path loss from the eNode B 103 by conventional methods such as comparing received signal strength of a reference signal broadcast by the eNodeB 103 with the actual received strength of the same reference signal at the network listen module 107. It also computes a path loss between the UE 114 and the eNodeB 103 and a path loss to the UE 114. Combined with knowledge of the lower power eNodeB's 104 maximum transmit power or potentially desired range and the possible interference level of other interference sources, the signal processor 108 estimates a desirable interference power threshold at which it would like to be notified for RNTP purposes The transmitter 109 of the higher power eNodeB 103 sends out a Load Information message over the X2 interface 112 which includes an RNTP threshold value and is received by the receiver 106 in the eNodeB 104 at step 202.

At 203 the signal processor 108 compares the received RNTP threshold value with the calculated value. If the received value lies below the calculated one, then at 204, interference co-ordination and scheduling operations can proceed as is conventional. For example, in the case of LTE networks, although it is designed to (potentially) have 1-1 frequency re-use, a fractional frequency re-use can be employed whereby one subset of the frequency band is used near the small cell's centre and another subset is used near the cell's edge, where interference from the higher power eNodeB 103 is likely to be greater. The natural scheduling approach for an eNodeB receiving information that a neighbour was going to be transmitting at a low' power on a certain set of PRBs would be to try and schedule its own downlink transmissions on said PRBs since they would be less affected by interference from the neighbouring cell.

If, however, the received threshold value is greater than the desired, calculated threshold value, then at 205, the signal processor 108 generates a message containing the desired RNTP threshold value which is transmitted by the transmitter 105 to the receiver 103 of the higher power eNodeB 103 over the X2 interface 112 thereby notifying the latter eNodeB of a requested change to the RNTP threshold. The processing module in the eNodeB 103 subsequently uses this received, desired RNTP threshold value in the next Load Information message that it transmits (to the lower power eNodeB 104).

As mentioned above, certain reference signals at a known power (e.g. the Reference Signal Power PDSCH Common Configuration) are conventionally broadcast by a cell and their values encoded in SIB2 (System Information Block) broadcast by eNodeBs. Such a broadcast transmission from the eNodeB 103 can be read by the network listen module 107 associated with eNodeB 104.

This broadcast transmission is also automatically read by the UE 114 (which is in the higher power eNodeB's area of coverage 101 as well as that of the lower power eNodeB 104. The UE 114 can be commanded (by its serving eNodeB, in this case eNodeB 104) to report the measured value of the received Reference Signal Power. In this way a eNodeB with a network listen module and serving a UE may calculate (by subtracting the measured received power from the signalled transmit power), the radio signal path loss between another eNodeB and the UE, another eNodeB and itself, and also between itself and the UE. From a range of UE measurements the serving eNodeB 104 can estimate the path loss to the interfering eNodeB 103 at different points on the edge of the serving cell 102. The serving eNodeB 104 is thus in a position to estimate the received signal strength of the interfering eNodeB 103 on UE's in the serving cell 102. The serving eNodeB 104 also knows what the maximum noise value is that it can tolerate in order to deliver communications services. The serving eNodeB may also estimate the received signal strength from any other interfering eNode B's and include this in its estimation.

Hence, the signal processor 108 can estimate i) if a transmission by any neighbouring eNodeB (103 in the example of FIG. 1) at its signalled RNTP_Threshold will still adversely affect its (ie. eNodeB 104) ability to deliver its desired Quality of Service and also U) what the upper limit of that RNTF_threshold can be for the serving cell (eNodeB 104) to still be able to deliver that desired quality of service.

If UE measurements are not available then the path loss estimates can still be made from network listen module results.

The signal processing functionality of the embodiments of the invention, particularly the signal processing modules 108 and 111 may be achieved using computing systems or architectures known to those who are skilled in the relevant ad. Computing systems such as, 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 can be used. The computing system can include one or more processors which can be implemented using a general or special-purpose processing engine such as, for example, a microprocessor, microcontroller or other control module.

The computing system can also include a main memory, such as random access memory (RAM) or other dynamic memory, for storing information and instructions to be executed by a processor. Such a main memory also may be used for storing temporary variables or other intermediate information during execution of instructions to be executed by the processor. The computing system may likewise include a read only memory (ROM) or other static storage device for storing static information and instructions for a processor.

The computing system may also include an information storage system which may include, for example, a media drive and a removable storage interface. The media drive 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 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. The storage media may include a computer-readable storage medium having particular computer software or data stored therein.

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

The computing system can also include a communications interface. Such a communications interface can be used to allow software and data to be transferred between a computing system and external devices. Examples of communications interfaces 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 a communications interface are in the form of signals which can be electronic, electromagnetic, and optical or other signals capable of being received by a communications interface medium.

In this document, the terms computer program product', computer-readable medium' non-transitory computer-readable medium' and the like may be used generally to refer to tangible media such as, for example, a memory, storage device, or storage unit. These and other forms of computer-readable media may store one or more instructions for use by the processor comprising the computer system 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 to perform functions of embodiments of the present invention. Note that the code may directly cause a 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 using, for example, removable storage drive. A control module (in this example, software instructions or executable computer program code), when executed by the processor in the computer system, causes a processor to perform the functions of the invention as described herein.

Furthermore, the inventive concept can be applied to any circuit for performing signal processing functionality 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 processing logic. However, the inventive concept may equally be implemented by way of a plurality of different functional units and processors to provide the signal processing functionality. Thus, references to specific functional units are only to be seen as references to suitable means for providing the described functionality, rather than indicative of a strict logical or physical structure or organisation.

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. -12-

Claims (15)

1. Claims 1. A method for operating a wireless communication system the method including; at a first base station, receiving (202), from a neighbouring base station, a message containing a transmit power threshold value, comparing (203) the received transmit power threshold value with a calculated transmit power threshold value, and if the received transmit power threshold value exceeds the calculated transmit power threshold value, transmitting (205), from the first base station, a message which contains the calculated transmit power threshold value to the neighbouring base station.
2. 2. The method of claim 1 including, at the neighbouring base station, receiving, from the first base station, the message containing the calculated transmit power threshold value and adjusting a subsequent transmission based on the calculated transmit power threshold value.
3. 3. The method of either preceding claim wherein the message received from the neighbouring base station comprises a Load Information message transmitted over an X2 interface and containing a Relative Narrowband Transmit Power part indicating which Physical Resource Blocks of one or more cells of the neighbouring base station will be transmitted by the neighbouring base station below a defined threshold value.
4. 4. The method of any preceding claim wherein the message which contains the calculated transmit power threshold value is transmitted over an X2 interface as a modified Load Information message.
5. 5. The method of any preceding claim including, at the first base station, receiving from the neighbouring base station a reference signal and a message relating to a transmit power of the reference signal, estimating a path loss between the first base station and the neighbouring base station and determining (201) the calculated transmit power threshold value based on an estimated path loss and a transmit power capability of the first base station.
6. 6. The method of claim S wherein estimating a path loss is performed using measurement reports received from a user equipment located in a coverage area of the first base station.
7. 7. The method of claim 5 or claim 6 wherein determining (201) the calculated transmit power S threshold is based on a total expected interference at at least one point in a cell served by the first base station, which total expected interference is based on at least one estimated path loss from one or more neighbouring base stations.
8. 8. A network element (104) comprising a signal processor (108) arranged to compare a transmit power threshold value received from a remote network element (103) with a calculated transmit power threshold value and to generate a message including the calculated transmit power threshold value for transmission to the remote network if the received transmit power threshold value exceeds the calculated transmit power threshold value.
9. 9. The network element of claim 8 including a receiver (106) for receiving the transmit power threshold value from a remote network element and a transmitter (105) for transmitting the calculated transmit power threshold value to the remote network element.
10. 10. The network element of claim 9 including a network listen module (107).
11. 11. The network element of any of claims 8-10 wherein the signal processor is arranged to estimate a path loss and to determine the calculated transmit power threshold value based on an estimated path loss to one or more neighbouring base stations and a transmit power capability of the network element.
12. 12. A network device (103) including a receiver (110) for receiving a calculated transmit power threshold value from a network element, a processing module (111) for generating a modified Load Information message based on the calculated transmit power threshold value, and a transmitter (109) for transmitting the modified a Load Information message to the network element.
13. 13. A wireless communication system (100) arranged to support a method as claimed in any of claims 1 to 7 or a network element (104) as claimed in any of claims 6 to 10 or a network device (103) of claim 11 and wherein the network element is a first base station having a first coverage area (102) and the network device is a second base station having a second coverage area (101) which is greater than and overlays the first coverage area.. -14-
14. 14. The network element of any of claims 8-11 wherein the signal processor is incorporated in one or more integrated circuits.
15. 15. A tangible computer program product having an executable computer program code stored thereon for execution by a processor to perform a method in accordance with claim 1.