Abstract:
A method of controlling communication between a first network node and a second network node in a multi-carrier wireless communications system, network nodes and a computer program product are disclosed. The method of controlling communication between a first network node and a second network node in a multicarrier wireless communications system in which a predetermined set of a plurality of carriers are utilised to support communication between the first network node and the second network node, the predetermined set of the plurality of carriers comprising a primary carrier and at least one secondary carrier, comprises the steps of: determining an occurrence of an event requiring a change in the primary carrier; indicating to a third network node that a physical layer instruction is to be transmitted between said first network node and the second network node; and transmitting a physical layer instruction between the first network node and the second network node, the physical layer instruction encoding reselection information to cause the second network node to select a different carrier from the plurality of carriers as a new primary carrier. By sending the instruction using the layer one or physical layer, the speed at which the instruction can be transmitted between the network nodes is vastly increased, which enables fast switching to occur, thereby preventing a loss of communications between the network nodes. By indicating to a third network node that a physical layer instruction is to be transmitted between the first network node and the second network node, maintenance of synchronisation is simplified and unpredictable network behaviour is minimised.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a method of controlling communication between a first network node and a second network node in a multi-carrier wireless communications system, network nodes and a computer program product. 
       BACKGROUND 
       [0002]    Single carrier wireless telecommunications systems are known. In those known systems, radio coverage is provided to user equipment, for example, mobile telephones, by geographical area. A base station is located in each geographical area to provide the required radio coverage. User equipment in the area served by a base station receives information and data from the base station and transmits information and data to the base station. In a high-speed downlink packet access (HSDPA) telecommunications network, data and information is sent between user equipment and a base station in data packets on a radio frequency carrier. 
         [0003]    Information and data transmitted by the base station to the user equipment occurs on radio frequency carriers known as downlink carriers. Information and data transmitted by user equipment to the base station occurs on radio frequency carriers known as uplink carriers. 
         [0004]    In known wireless telecommunication systems operating in single carrier mode, user equipment can move between geographical base station coverage areas. Service provided to user equipment is overseen by a radio network controller (RNC). The radio network controller communicates with user equipment and base stations and determines which base station each user equipment is primarily connected to. Furthermore, a radio network controller acts to control and communicate with a base station and user equipment when user equipment moves from the geographical area served by one base station to a geographical area served by another base station. 
         [0005]    It has been proposed to allow base stations and user equipment to each transmit simultaneously on more than one carrier. Furthermore it has been proposed to allow user equipment and base stations to receive simultaneously on more than one carrier frequency. Each carrier, both uplink and downlink, is typically independently power controlled by a base station. Provision of more than one downlink carrier, for example, on four frequency carriers, allows for an increase in data throughput to the user equipment. Networks having more than two carriers may be referred to as “multi cell high speed downlink packet access” (MC-HSDPA) networks. The term “multi-carrier” network used herein is envisaged to cover the case where two, three, four, or more downlink (or uplink) carriers are provided for in a network. 
         [0006]    Provision of multi-carrier functionality may have associated problems. Accordingly, it is desired to improve the operation of a wireless telecommunications network having multi-carrier functionality. 
       SUMMARY 
       [0007]    According to a first aspect, there is provided a method of controlling communication between a first network node and a second network node in a multicarrier wireless communications system in which a predetermined set of a plurality of carriers are utilised to support communication between the first network node and the second network node, the predetermined set of the plurality of carriers comprising a primary carrier and at least one secondary carrier, the method comprising the steps of: determining an occurrence of an event requiring a change in the primary carrier; indicating to a third network node that a physical layer instruction is to be transmitted between the first network node and the second network node; and transmitting a physical layer instruction between the first network node and the second network node, the physical layer instruction encoding reselection information to cause the second network node to select a different carrier from the plurality of carriers as a new primary carrier. 
         [0008]    The first aspect recognises that a problem with existing techniques utilised in multi-carrier systems is that the primary carrier carries essential control channels and the network node will declare a radio link failure if the primary carrier fails. Those existing techniques enable the primary carrier to be changed using a radio bearer reconfiguration or other radio resource control (RRC) signalling from a radio network controller. However, the first aspect also recognises that this process is slow and may not be sufficient in the event that user equipment loses its radio link connection quickly or if a fast primary carrier change is required to, for example, quickly manage loading between the carriers, in which case communication between the network nodes may be lost. 
         [0009]    Accordingly, a method of controlling communication between a first network node and a second network node in a multi-carrier wireless communication system in which predetermined sets of carriers are used to support communications between the nodes is provided. The set of carriers include a primary carrier and one or more secondary carriers. An event requiring a change of primary carrier is detected. It will be appreciated that such an event may occur due to a wide variety of circumstances. When the need to change primary carrier has been detected, a physical layer or layer 1 instruction is transmitted between the network nodes. The physical layer instruction encodes information which instructs the second network node to change to a new primary carrier. By sending the instruction using the layer 1 or physical layer, the speed at which the instruction can be transmitted between the network nodes is vastly increased, which enables fast switching to occur, thereby preventing a loss of communications between the network nodes. 
         [0010]    The first aspect recognises that by indicating to a third network node that a physical layer instruction is to be transmitted between the first network node and the second network node, a network can be readily informed about changes made in communication regime between the first and second network node. By transmitting information relating to the physical layer instruction more widely, unpredictability within the network can be minimised. It may be, for example, that the third network node may itself be operable to cause the second network node to select a different carrier from the plurality of carriers as a new primary carrier. If the third network node is unaware of an instruction sent by the first network node, it may itself instruct a change and the second network node may receive conflicting information. In particular, the first and third network nodes may lose synchronisation on the mobility status of the second network node, and the second network node may receive conflicting instructions. Use of signalling more widely between network nodes allows an increase in robustness of the overall procedure and helps to avoid misalignment regarding the mobility status of the second network node. 
         [0011]    In one embodiment, the first network node is a base station, the second network node is an item of user equipment and the third network node is an RNC (remote network controller) or similar network co-ordination node, for example, a mobility management entity (MME). 
         [0012]    In one embodiment, a MAC message is used in place of a physical layer message. Accordingly, any message which is decodable by the first network node can be utilised successfully and may offer improved speed of primary carrier change over use of RRC signalling from an RNC. In such embodiments, the first network node is effectively informed of an occurrence of an event requiring a change in the primary carrier. 
         [0013]    In one embodiment, the step of determining comprises receiving a notification of the event from the second or third network node. Accordingly, a base station, for example, may receive notification of a triggering event from either user equipment or from an RNC. 
         [0014]    In one embodiment, the step of indicating occurs prior to the step of transmitting. Accordingly, notification of a possible change may propagate through a network before that change is made. 
         [0015]    In one embodiment, the step of indicating comprises requesting to perform the transmitting step. Accordingly, rather than unilaterally making a change, the first network node may request permission, or a clear instruction from a third network node, thereby mitigating the chances of inducing misalignment in a network. 
         [0016]    In one embodiment, the method further comprises the step of awaiting an instruction to perform the transmitting step prior to performing the transmitting step. Accordingly, a node elsewhere in the network may retain control and visibility over the process. 
         [0017]    In one embodiment the method further comprises the step of monitoring for an indication of safe receipt of the physical layer instruction. Accordingly, by monitoring for an indication of safe receipt of the physical layer instruction, it will be understood that visibility of the process of selection of a different carrier can be maintained and that the status of the second network node can be more fully understood by the first network node. 
         [0018]    In one embodiment the method further comprises the step of indicating to the third network node that an indication of safe receipt has been received. Accordingly, a network may be kept informed of the progress of the selection of a different carrier by the second network node. 
         [0019]    In one embodiment, the physical layer instruction encodes reselection information to cause the second network node to select a different carrier from the set of the plurality of carriers as the new primary carrier. Accordingly, the instruction instructs the second network node to select one of the carriers within the set of carriers being utilised by the second network node as the new primary carrier. 
         [0020]    In one embodiment, the physical layer instructions encodes reselection information to cause the second network node to select a predetermined carrier from the set of the plurality of carriers as the new primary carrier. Accordingly, the instruction may encode information which instructs the second network node to select a particular carrier from the set of carriers as the new primary carrier. It will be appreciated that this provides a particularly efficient and convenient technique for identifying the new primary carrier to be selected. 
         [0021]    In one embodiment, the physical layer instruction encodes reselection information to cause the second network node to deselect the primary carrier. Accordingly, the second network node may be instructed to deselect the current primary carrier in order cause a change in primary carrier. 
         [0022]    In one embodiment, the physical layer instruction is transmitted using the new primary carrier. Accordingly, the instruction itself may not need to explicitly identify the new primary carrier at all. Instead, the designation of the new primary carrier may be inferred by receiving the instruction on the carrier which is to be used as the new primary carrier. It will be appreciated that this avoids having to create a new physical layer or layer one instruction. 
         [0023]    In one embodiment, the method comprises the step of: transmitting preconfiguration information to the second network node to enable the second network node to support communication using at least one other primary carrier. Accordingly, the second network node may be provisioned with information to enable that network node to utilise different carriers. In this way, not only can a primary carrier be selected from an existing set of carriers, the primary carrier can even be selected from a different set of carriers which may be provided on different frequencies. 
         [0024]    In one embodiment, the physical layer instruction comprises a High Speed Shared Control Channel (HS-SCCH) order encoding the reselection information. It will be appreciated that an HS-SCCH order provides a particularly convenient mechanism for the transmission of the instruction. 
         [0025]    In one embodiment, the step of determining an occurrence of an event comprises detecting a radio link failure. Accordingly, the occurrence of an actual or impending radio link failure may be detected which initiates a change in primary carrier, thereby enabling communications between the first and second network nodes to continue. 
         [0026]    In one embodiment, the step of determining an occurrence of radio link failure comprises receiving an indication from the second network node of a radio link failure. 
         [0027]    Accordingly, the detection of a potential radio link failure may be determined based on information provided by the second network node. It will be appreciated that the detection of potential radio link failure may be derived either explicitly or implicitly from information provided by the second network node. 
         [0028]    In one embodiment, the indication is encoded in at least one of a radio relocation control message and an Enhanced Dedicated Channel Transport Format Combination Identifier. Accordingly, the indication from the second network node may come either via a radio relocation control message or via an enhanced dedicated channel transport format combination identifier. It will be appreciated that the detection via a radio relocation control message may need to occur earlier since this occurs via a higher layer, which may not be fast enough if the detection is not performed early enough. However, the enhanced dedicated channel transport format combination identifier occurs at the physical or layer one level and so can be transmitted much more quickly. Analogously, an Uplink Order Request message, sent at layer 1 or MAC, may be used, which also can be transmitted much more quickly to identify possible radio link failure. 
         [0029]    In one embodiment, the indication is encoded in at least one of a Radio Relocation Control message and an Enhanced Dedicated Channel Transport Format Combination Identifier. The measurement reports may be received via the radio relocation control message and so may need to be transmitted early enough to enable the switching to occur. However, the channel quality information, which indicates the throughput that the network node can support based on the quality of the radio link on each carrier is sent at the physical or layer one level by the high speed dedicated physical control channel for each carrier. The channel quality information is sent periodically and usually at a higher rate than that of the measurement reports. Since channel quality information may be interpreted at the first network node, the network node can make a decision on whether to change the primary carrier. In embodiments, the second network node can implicitly warn the first network node of a possible radio link failure by sending a predetermined value of carrier control quality information (such as, for example, “0” or another predetermined value for the primary carrier even if the channel quality information is above 0 or a different value). Again, it will be appreciated that this provides a particularly fast and efficient technique for changing primary carrier. In embodiments, the channel quality information indicators can be used to offer an indication of ranking of suitability of available secondary carriers for use as a replacement primary carrier. Provision of such information may allow an intelligent choice of new primary carrier candidate to be made. 
         [0030]    In one embodiment, the method further comprises the steps of determining that an event requiring a change in the primary carrier is no longer occurring; and not transmitting the physical layer instruction between the first network node and the second network node. Accordingly, if normal operation resumes before a physical layer instruction to change primary carrier is sent, the proposed change can be cancelled, or the change order never sent. 
         [0031]    In one embodiment, the first network node comprises a base station and the second network node comprises user equipment. 
         [0032]    According to a second aspect, there is provided a network node operable to control communication with a second network node in a multicarrier wireless communications system in which a predetermined set of a plurality of carriers are utilised to support communication between the network node and the second network node, the predetermined set of the plurality of carriers comprising a primary carrier and at least one secondary carrier, the network node comprising: determination logic operable to detect an occurrence of an event requiring a change in the primary carrier; indication logic operable to indicate to a third network node that a physical layer instruction is to be transmitted to the second network node; and transmission logic operable to transmit a physical layer instruction between the network node and the second network node, the physical layer instruction encoding reselection information to cause the second network node to select a different carrier from the plurality of carriers as a new primary carrier. 
         [0033]    In one embodiment, the determination logic is operable to receive a notification of the event from the second or third network node. 
         [0034]    In one embodiment, the indication logic is operable to perform the step of indicating prior to the step of transmitting. 
         [0035]    In one embodiment, the indication logic is operable to request to perform said transmitting step. 
         [0036]    In one embodiment, the transmission logic is operable to await an instruction to transmit, prior to transmitting a physical layer instruction. 
         [0037]    In one embodiment, the network node further comprises monitoring logic operable to monitor for an indication of safe receipt of the physical layer instruction. 
         [0038]    In one embodiment, the monitoring logic is further operable to indicate to the third network node that an indication of safe receipt has been received. 
         [0039]    In one embodiment, the physical layer instruction encodes reselection information to cause the second network node to select a different carrier from the set of the plurality of carriers as the new primary carrier. 
         [0040]    In one embodiment, the physical layer instructions encodes reselection information to cause the second network node to select a predetermined carrier from the set of the plurality of carriers as the new primary carrier. 
         [0041]    In one embodiment, the physical layer instruction encodes reselection information to cause the second network node to deselect the primary carrier. 
         [0042]    In one embodiment, the transmission logic is operable to transmit the physical layer instruction using the new primary carrier. 
         [0043]    In one embodiment, the transmission logic is operable to transmit preconfiguration information to the second network node to enable the second network node to support communication using at least one other primary carrier. 
         [0044]    In one embodiment, the physical layer instruction comprises a High Speed Shared Control Channel order encoding the reselection information. 
         [0045]    In one embodiment, the determination logic is operable to detecting a radio link failure. 
         [0046]    In one embodiment, the determination logic is operable to detect a radio link failure from an indication received from the second network node. 
         [0047]    In one embodiment, the indication is encoded in at least one of a Radio Relocation Control message and an Enhanced Dedicated Channel Transport Format Combination Identifier. 
         [0048]    In one embodiment, the indication comprises at least one of a measurement report and a channel quality indicator received from the second network node and from which radio link failure is determined. 
         [0049]    According to a third aspect, there is provided a network node operable to communicate with a first network node in a multicarrier wireless communications system in which a predetermined set of a plurality of carriers are utilised to support communication between the network node and the first network node, the predetermined set of the plurality of carriers comprising a primary carrier and at least one secondary carrier, the network node comprising: reception logic operable to receive a physical layer instruction encoding reselection information from the first network node; and reselection logic operable in response to the reselection information to select a different carrier from the plurality of carriers as a new primary carrier. 
         [0050]    In one embodiment, the network node further comprises notification logic operable to provide a notification of an occurrence of an event requiring a change in the primary carrier. 
         [0051]    In one embodiment, the network node further comprises safe receipt logic operable to provide an indication of safe receipt of the physical layer instruction to the first network node if it has been safely received. 
         [0052]    In one embodiment, the reselection logic is operable, in response to the reselection information, to select a different carrier from the set of the plurality of carriers as the new primary carrier. 
         [0053]    In one embodiment, the reselection logic is operable, in response to the reselection information, to select a predetermined carrier from the set of the plurality of carriers as the new primary carrier. 
         [0054]    In one embodiment, the reselection logic is operable, in response to the reselection information, to deselect the primary carrier. 
         [0055]    In one embodiment, the reselection logic is operable to select that carrier on which the physical layer instruction is received as the new primary carrier. 
         [0056]    In one embodiment, the reception logic is operable to receive preconfiguration information to enable the second network node to support communication using at least one other primary carrier. 
         [0057]    In one embodiment, the physical layer instruction comprises a High Speed Shared Control Channel order encoding the reselection information. 
         [0058]    In one embodiment, the network node comprises failure logic operable to provide an indication of an event requiring a change in the primary carrier. 
         [0059]    In one embodiment, the failure logic is operable to detect a radio link failure. 
         [0060]    In one embodiment, the failure logic is operable to encode the indication in at least one of a Radio Relocation Control message and an Enhanced Dedicated Channel Transport Format Combination Identifier. 
         [0061]    In one embodiment, the indication comprises at least one of a measurement report and a channel quality indicator. 
         [0062]    According to a fourth aspect, there is provided a computer program product operable, when executed on a computer, to perform the method steps of the first aspect. 
         [0063]    According to a further aspect, there is provided a method of encoding carrier change information between a first network node and a second network node in a multicarrier wireless communications system in which a predetermined set of a plurality of carriers are utilised to support communication between the first network node and the second network node, the predetermined set of the plurality of carriers comprising a primary carrier and at least one secondary carrier, the method comprising the steps of: determining an occurrence of an event requiring a change in the primary carrier; identifying a carrier from said plurality of carriers as a candidate for a new primary carrier; and transmitting an encoded message including an indication of an occurrence of said event and a candidate for a new primary carrier. 
         [0064]    A yet further aspect provides a network node operable to communicate with a first network node in a in a multicarrier wireless communications system in which a predetermined set of a plurality of carriers are utilised to support communication between the first network node and the second network node, the predetermined set of the plurality of carriers comprising a primary carrier and at least one secondary carrier, the network node comprising: event determination logic operable to determine an occurrence of an event requiring a change in the primary carrier; carrier candidate logic operable to identify a carrier from said plurality of carriers as a candidate for a new primary carrier; and transmission logic operable to transmit an encoded message including an indication of an occurrence of said event and a candidate for a new primary carrier. 
         [0065]    Features of previous aspects are provided in combination with these further aspects. 
         [0066]    Further particular and preferred aspects are set out in the accompanying independent and dependent claims. Features of the dependent claims may be combined with features of the independent claims as appropriate, and in combinations other than those explicitly set out in the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0067]    Embodiments of the present invention will now be described further, with reference to the accompanying drawings, in which: 
           [0068]      FIG. 1  illustrates a wireless telecommunications system  10  according to one embodiment; 
           [0069]      FIG. 2  illustrates a radio link failure evaluation performed by the user equipment according to one embodiment; 
           [0070]      FIG. 3  illustrates an explicit radio link failure evaluation performed by the user equipment according to one embodiment; 
           [0071]      FIG. 4  is a signalling diagram of showing explicit radio link failure according to one embodiment; 
           [0072]      FIG. 5  is a signalling diagram of showing implicit radio link failure according to one embodiment; 
           [0073]      FIG. 6  illustrates fast primary carrier change according to one embodiment; 
           [0074]      FIG. 7   a  is a schematic signalling diagram illustrating a signalling method for use in a potential radio link failure scenario according to one embodiment; 
           [0075]      FIG. 7   b  schematic signalling diagram illustrating a signalling method for use in a potential radio link failure scenario according to one embodiment; 
           [0076]      FIG. 7   c  schematic signalling diagram illustrating a signalling method for use in a potential radio link failure scenario according to one embodiment; 
           [0077]      FIG. 8  is a schematic illustration of an implicit radio link failure message according to one embodiment; 
           [0078]      FIG. 9  is a schematic signalling diagram illustrating radio link failure according to one embodiment; 
           [0079]      FIG. 10  is a schematic signalling diagram illustrating radio link failure according to one embodiment; 
           [0080]      FIG. 11  is a schematic signalling diagram illustrating radio link failure according to one embodiment; 
           [0081]      FIG. 12  is a schematic signalling diagram illustrating radio link failure according to one embodiment; 
           [0082]      FIG. 13  is a schematic signalling diagram illustrating radio link failure according to one embodiment; 
           [0083]      FIG. 14   a  illustrates primary carrier change according to one embodiment; 
           [0084]      FIG. 14   b  illustrates primary carrier change according to one embodiment; and 
           [0085]      FIG. 15  is a schematic signalling diagram illustrating radio link failure according to one embodiment. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0086]      FIG. 1  illustrates a wireless telecommunications system  10  according to one embodiment. User equipment  50  roam through the wireless telecommunications system. Base stations  20  are provided which support areas of radio coverage  30 . A number of such base stations  20  are provided and are distributed geographically in order to provide a wide area of coverage to user equipment  50 . When user equipment is within an area served by a base station  20 , communications may be established between the user equipment and the base station over associated radio links. Each base station typically supports a number of sectors within the geographical area of service  30 . 
         [0087]    Typically a different antenna within a base station supports each associated sector. Accordingly, each base station  20  has multiple antennas and signals sent through the different antennas are electronically weighted to provide a sectorised approach. Of course, it will be appreciated that  FIG. 1  illustrates a small subset of the total number of user equipment and base stations that may be present in a typical communications system. 
         [0088]    The radio access network of the wireless communications system is managed by a radio network controller (RNC)  40 . The radio network controller  40  controls operation of the wireless communications system by communicating with a plurality of base stations over a backhaul communications link  60 . The network controller also communicates with user equipment  50  via each base station. 
         [0089]    A radio network controller  40  maintains a neighbour list which includes information about geographical relationships between sectors supported by base stations  20 . In addition, the radio network controller  40  maintains location information which provides information on the location of user equipment  50  within the wireless communication system  10 . The radio network controller is operable to route traffic via circuit switched and packet switched networks. Hence, a mobile switching centre is provided with which the radio network controller may communicate. The mobile switching centre can communicate with a circuit switched network such as a public switched telephone network (PSTN)  70 . Similarly, a network controller can communicate with serving general package radio service support nodes (SGSNs) and a gateway general packet radio service support node (GGSN). The GGSN can communicate with a packet switched core such as for example, the Internet. 
         [0090]    User equipment  50  typically transmits information and data to a base station  20  so that it can be re-routed within a wireless telecommunications network. User equipment may, for example, need to transmit data to the base station in order to relay text messages, voice information when a user Is using the equipment to make a telephone call, or other data. The base station  20 , in combination with parameters set by the radio network controller  40 , allocates resources to user equipment in a manner that aims to optimise operation of the wireless telecommunications network  10 . 
         [0091]    In Universal Mobile Telecommunications System (UMTS), a Multi-Cell High Speed Downlink Packet Access (MC-HSDPA) arrangement is provided. In MC-HSDPA, a sector is defined as the geographical coverage area of a base station or Node B. A sector can consist of several cells, where each cell aims to cover the same geographical coverage as the sector and uses a separate frequency carrier for its transmission. The frequency carrier can be within the same frequency band or distributed over two frequency bands. MC-HSDPA is an extension to Dual Cell High Speed Downlink Packet Access (DC-HSDPA). In MC-HSDPA, user equipment can receive up to four simultaneous downlink transmissions from four different cells. Hence, MC-HSDPA can potentially double and quadruple the downlink throughput of DC-HSDPA and (Single Cell) HSDPA respectively. MC-HSDPA is also sometimes referred to as 4C-HSDPA (four Cell HSDPA) or 3C-HSDPA when the user equipment receives simultaneous transmissions from four or three cells respectively. 
         [0092]    In a multi-carrier system, each carrier will have independent downlink radio links from a base station to user equipment. Those downlink radio links are managed independently since each carrier will likely have different radio propagation paths to user equipment. For HSDPA systems capable of operating in multi-carrier mode, more than two downlink carriers may be provided. It will be appreciated that in a multi-carrier network, the number of downlink carriers may not match the number of uplink carriers. Furthermore, the number of downlink carriers provided may not be exactly double the number of uplink carriers provided. In HSDPA multi-carrier mode, each sector served by a base station can have several carrier frequencies or “carriers” associated therewith. A carrier, or cell supported by a carrier, covers the same geographical region as a sector. Each cell is served by a different carrier frequency. It will therefore be understood that in a single carrier system, a cell is equivalent to a sector since a sector has only one cell or carrier frequency. Nonetheless, in a multi-carrier network each sector may comprise several cells each cell being served simultaneously by a different carrier frequency. 
         [0093]    User equipment can be in Idle mode or in Radio Resource Control (RRC) Connected Mode. Cell_DCH state is one of the states within RRC Connected Mode where user equipment can transmit and receive high data throughput. The MC-HSDPA feature operates in Cell_DCH state, where in this state user equipment and Node B maintain physical layer synchronisations in the uplink and downlink. A Radio Link Failure (RLF) occurs when the user equipment loses its radio link connection due to loss of downlink physical layer synchronisation with the serving Node B. Radio link failure will cause the user equipment to exit the Cell_DCH state and thereby loose its high data throughput capability. In MC-HSDPA, a radio link failure will cause the user equipment to operate in a single cell with low throughput. Hence, the loss in throughput due to radio link failure may be much higher in MC-HSDPA compared to that in a single cell HSDPA operation. Radio link failure can have a significant impact on user experience, for example, 4C-HSDPA is typically used when a user is utilising a high bandwidth service, such as high quality video streaming. 
         [0094]    In MC-HSDPA, the primary carrier is the cell that carries essential control channels and it cannot be deactivated. There is only one primary carrier and the other cells are called secondary carriers (e.g. Secondary Carrier  1 , Secondary Carrier  2  and Secondary Carrier  3 ). The user equipment will declare radio link failure if the primary carrier radio link fails, even if the secondary carriers are working perfectly (e.g. if they are lightly loaded or if they have a better radio channel). The secondary carriers provide natural radio link redundancies but these redundancies cannot be utilised. 
         [0095]    Although the primary carrier can be changed using Radio Bearer Reconfiguration or other RRC signalling from the radio network controller  40  this process is usually triggered using event 2× (and event 2× is one of a multitude of inter-frequency reporting events defined in 3GPP TS 25.331) and is usually slow. The trigger may not be sufficient in the event where user equipment is losing its radio link connection or if a fast primary carrier change is required (e.g. to quickly manage loading among the carriers). Since the process is slow, it is possible that UE will fall into radio link failure before it is able to trigger event 2×. Furthermore, use of Radio Bearer Reconfiguration or other RRC signalling requires UE to operate in Compress Mode (CM) which degrades UE reception. 
         [0096]    Embodiments provide a technique to perform fast primary carrier change and use this to utilise the natural radio link redundancies provided by the secondary carriers in MC-HSDPA to give high radio link reliability for the user equipment thereby increases its robustness. 
       Primary Carrier Change—Overview 
       [0097]    Upon detecting an event requiring a change in primary carrier, a primary carrier change is sent using a HS-SCCH order, which is layer one or physical layer instruction. Because the HS-SCCH order is a layer one or physical layer instruction, the change in primary carrier can be effected very quickly to enable a fast change to occur when, for example, rapid degradation in the primary carrier occurs or when load balancing between carriers is required. To enable such switching to occur, the user equipment is preconfigured for operation with any of the secondary carriers acting as a primary carrier. This is required since the HS-SCCH order bypasses the radio network controller  40  and therefore the radio access bearer (RAB) information required to configure the user equipment to operate in another primary carrier is not sent. In such RAB preconfiguration, the required RAB information is sent to the user equipment during setup (for example, when user equipment moves to a new serving Node B) for use if the relevant HS-SCCH order is sent. The HS-SCCH order may encode the change in carrier information using suitable bit fields of this order. 
         [0098]    The HS-SCCH order will typically be sent on the secondary carrier that is selected by the Node B to be the potential primary carrier and the HS-SCCH order indicates that the existing primary carrier is deactivated. The advantages of this approach are that it avoids having to define a new HS-SCCH order (i.e. no extra signalling message required); the existing primary carrier is likely to be in a poor radio condition (hence the need to change it) and may not be received if transmitted on the primary carrier—however, since the HS-SCCH order comes from the potential primary carrier (i.e. one of the secondary carriers) that is likely to have a better radio condition, it has a higher chance of reaching the user equipment; in the same HS-SCCH order, in addition to changing the primary carrier, the Node B is able to activate/deactivate other secondary carrier(s); and fast primary carrier change is possible since the radio network controller  40  is bypassed. 
         [0099]    Upon receiving the HS-SCCH order, the user equipment will send an acknowledgement to the Node B and proceed to change the primary carrier to the frequency carrier (i.e. one of the existing secondary carriers) specified by the Node B within a specified amount of time. When the primary carrier change is complete the user equipment will send a Primary Change confirmation message to the radio network controller  40 . The Node B can also inform the radio network controller  40  of the change in primary carrier for that user equipment once it receives an acknowledgement from the user equipment. This can optionally save the user equipment from having to send a confirmation message to the radio network controller  40 . 
         [0100]    As will be mentioned in more detail below, in embodiments, the user equipment may defer changing the primary carrier for a predetermined period of time to avoid making unnecessary changes in primary carrier due to transient events. Should no change be necessary, then the user equipment will inform the Node B that no change to primary carrier occurred. 
       Event Detection 
       [0101]    User equipment either explicitly or implicitly provides indicators which warn the network that a primary carrier change is required. Typically, the user equipment needs to warn the network of a possible need to change primary carrier before the event which could lead to loss or disruption in communications occurring. For example, the user equipment needs to warn of a possible radio link failure so that a change in primary carrier can be executed before radio link failure occurs. 
       Radio Link Failure 
       [0102]    The radio link failure evaluation performed by the user equipment summarized in  FIG. 2 . At step S 10 , The user equipment in Cell_DCH continuously checks for downlink physical layer (layer 1) synchronisation by determining the quality of the downlink DPCCH (Dedicated Physical Control Channel) or F-DPCH (Fractional Dedicated Channel). If the user equipment is in sync, it will send an in-sync primitive, CPHY-Syn-IND and if the user equipment is out of sync, it will send an out-of-sync primitive, CPHY-Out-of-Sync-IND to the RRC layer. 
         [0103]    At the RRC layer, the user equipment counts the number of consecutives CPHY-Out-of-Sync-IND primitive at step S 20  and, if it is above a threshold N 313 , the timer T 313  is started at step S 30 . 
         [0104]    At step S 40 , timer T 313  is stopped and reset if the RRC layer receives N 315  consecutive CPHY-Syn-IND primitives from the physical layer. 
         [0105]    At step S 50  a determination is made of whether T 313  has expired (typically after 3 seconds) and, if so, at step S 60 , the user equipment declares radio link failure. 
         [0106]    If the user equipment is performing DRx (Discontinuous Reception) it does not send CPHY-Sync-IND or CPHY-Out-of-Sync-IND primitives. The counting leading up to N 315  consecutive CPHY-Syn-IND or N 313  consecutive CPHY-Out-of-Sync-IND primitives ignores missing sync primitives. 
       Explicit Radio Link Failure Warning 
       [0107]      FIG. 3  shows an embodiment where an explicit warning can be sent to the network. The warning is sent at step S 90  and occurs as soon as N WARN  CPHY-Out-of-Sync-IND consecutive primitives have been determined at step S 70  to be received from the physical layer. If N WARN  is set smaller than N 313 , an early warning will be provided allowing the network to change the user equipment&#39;s primary carrier faster. 
         [0108]    This can lead to less service interruption since user equipment do not need to wait for timer T 313  to expire. However, this may led to premature warnings or trigger a high number of unnecessary primary carrier changes. To avoid this, N WARN  can be set greater than N 313 , but this may cause the timer T 313  to expire before N WARN  consecutive CPHY-Out-of-Sync-IND are reached. This is more likely to happen when user equipment is performing DRx where it does not evaluate the sync quality and therefore does not send any synchronization primitives. 
         [0109]    Alternatively, the radio link failure warning can be sent when the timer T 313  exceeds a value T WARN . T WARN  is another timer that is started when T 313  starts and has an earlier expiry time to that in timer T 313 . Both T WARN  and T 313  stop and reset at the same time. Since the T 313  timer can be stopped and reset if N 315  consecutive CPHY-Syn-IND primitives are received, using T WARN  criteria, the radio link failure warning to the network can be cancelled if the radio link of the existing primary carrier recovers. In contrast, using the N WARN  criteria does not allow the radio link failure warning to be cancelled. 
         [0110]    In addition to provide a warning of a possible radio link failure, the radio link failure warning message should also indicate to the network which secondary carrier the user equipment believes would be the best replacement for the existing (failing) primary carrier since the user equipment performs regular measurements (e.g. on the pilot) of all secondary carriers. 
         [0111]    The explicit radio link failure warning can be sent in one of two ways. The first is using an RRC message. Since RRC is at the higher layer, T WARN  needs to be set such that it has sufficient time margin between T WARN  and T 313  for this message to reach the network and react upon it. The second is using a Reserved E-TFCI (E-DCH Transport Format Combination Indicator) using the E-DPCCH (E-DCH Dedicated Physical Control Channel). This is sent at the physical layer, which is faster than the RRC level allowing a fast reaction from the Node B. E-TFCI has a length of 7 bits and contains information required to decode uplink HSUPA data packets contained in the E-DPDCH (E-DCH Dedicated Physical Data Channel) but some E-TFCI are not used (i.e. reserved for future use) and so may be utilised to encode the radio link failure warning. 
         [0112]      FIG. 4  is a signalling diagram of the radio link failure avoidance scheme with explicit radio link failure warning using the T WARN  criteria. The user equipment is configured with the following frequency carrier usage:
       Frequency Carrier F 1 =Primary Carrier   Frequency Carrier F 2 =Secondary Carrier  1     Frequency Carrier F 3 =Secondary Carrier  2     Frequency Carrier F 4 =Not activated       
 
         [0117]    Here, the user equipment receives N 313  consecutive CPHY-Out-of-Sync-IND primitives and starts the T 313  timer. When T 313  is greater than T WARN , the user equipment sends a radio link failure warning via RRC signalling to the radio network controller. In this message the user equipment indicates that frequency carrier F 3  is best suited as the new primary carrier based on its internal measurements. Upon receiving the radio link failure warning, the radio network controller informs the serving Node B for this user equipment. The Node B determines that frequency carrier F 3  should be the new primary carrier and that frequency carrier F 4  needs to be activated to sustain the downlink throughput. The Node B sends a HS-SCCH order via Secondary Carrier  2  (frequency carrier F 3 ) to the user equipment with the following settings encoded in the order:
       Frequency Carrier F 1 =Deactivate   Frequency Carrier F 2 =Secondary Carrier  1     Frequency Carrier F 3 =Primary Carrier   Frequency Carrier F 4 =Secondary Carrier  2         
 
         [0122]    The user equipment receives the HS-SCCH order and sends an acknowledgement to the Node B. The user equipment then proceeds to configure its carriers to conform to that in the HS-SCCH order. The Node B, after successfully receiving the user equipment acknowledgement, will inform the radio network controller of the new carrier settings. When the user equipment successfully makes the changes to those in the HS-SCCH order (change primary carrier and activate frequency carrier F 4  as a secondary carrier), it stops timer T 313  and starts to monitor for possible radio link failure on frequency carrier F 3  (the new primary carrier). The user equipment has, hence, avoided a radio link failure. 
         [0123]    If the user equipment uses explicit radio link failure warning, the user equipment can wait for timer T 313  to expire before executing the primary carrier change in the hope that the radio link of the existing primary carrier will improve thereby avoiding the need to change the primary carrier. This is shown in  FIG. 3  where the user equipment executes the primary carrier change (if it has received a HS-SCCH order to do so) after timer T 313  has expired. The user equipment needs to send a confirmation message to the radio network controller after the primary carrier change and in this case the Node B should not inform the radio network controller of any primary carrier change when it receives an acknowledgement from the user equipment. If the user equipment managed to restore its existing primary carrier, it needs to inform the radio network controller that it is sticking to the existing primary carrier. This method may reduce the number of primary carrier change in the user equipment but it may cause unpredictability in the network. 
       Implicit Radio Link Failure Warning 
       [0124]    Although the network does not know whether user equipment is approaching radio link failure, the network receives measurements on each carrier from the user equipment. The network therefore is able to implicitly determine whether the primary carrier is deteriorating beyond a threshold and also is able to determine a suitable secondary carrier (if any) to replace the existing primary carrier. The measurements that can be used are as follows (either alone or in combination). 
         [0125]    User equipment measurement reports may be used which indicate the quality of the CPICH of each carrier. This is at the RRC level (i.e. interpreted at the radio network controller) and the network may not respond fast enough. Furthermore, measurements on secondary carriers may need to be configured as inter-frequency measurements, which has less frequent updates and in some situation may not even be measured. 
         [0126]    Channel Quality Information (CQI) may be used which indicate the throughput (transport block size) that the user equipment can support based on the quality of the radio link on each carrier. This is sent at the physical layer by the HS-DPCCH (High Speed Dedicated Physical Control Channel) for each carrier. The CQI is sent periodically and usually at a higher rate than that of the user equipment measurement reports. Since CQI is interpreted at the Node B, the Node B needs to make the decision whether to change the primary carrier. The user equipment can implicitly warn the Node B of a possible radio link failure by encoding that the CQI=0 for the primary carrier and sending this to the Node B even if the CQI for primary carrier is above zero. This can be sent when either N WARN  or T WARN  criteria is met (as described above in relation to explicit radio link failure). A CQI of zero usually indicates “Out of Range” except when the user equipment is configured with MIMO—Multiple-In Multiple-Out (where it indicates a support for an actual transport block size). However, it is unlikely that user equipment experiencing possible radio link failure will be in MIMO mode since MIMO requires very high quality radio link and the Node B knows whether the user equipment is using MIMO. An example CQI criteria is that The Node B counts the number of consecutive CQI=0 and, if it is above a threshold NcQb the Node B will execute a primary carrier change using an HS-SCCH order. 
         [0127]    An implicit radio link failure warning (e.g. using CQI=0) is dependent upon the network&#39;s (e.g. Node B) ability to estimate that the user equipment is experiencing radio link failure. If the Node B is too slow in detecting a possible radio link failure (e.g. due to having a large N CQI  threshold), the user equipment may declare radio link failure before a change of primary carrier is made. On the other hand, a Node B that executes a change in primary carrier too early may cause frequent unnecessary primary carrier changes. However, this prevents a potential radio link failure or even prevents the user equipment from having to start the T 313  timer, which can potentially improve the overall throughput of the user equipment since the user equipment will always select a frequency carrier that is strong enough to be the primary carrier. 
         [0128]      FIG. 5  is a signalling diagram for an implicit radio link failure warning scheme using the T WARN  criteria. The user equipment is configured with the following frequency carrier usage:
       Frequency Carrier F 1 =Primary Carrier   Frequency Carrier F 2 =Secondary Carrier  1     Frequency Carrier F 3 =Secondary Carrier  2     Frequency Carrier F 4 =Secondary Carrier  3         
 
         [0133]    The user equipment receives N 313  consecutive CPHY-Out-of-Sync-IND primitives and starts the T 313  timer. When T 313  is greater than T WARN , the user equipment keeps reporting CQI=0 for the primary carrier. Here MIMO is disabled. The Node B keeps count of the number of consecutive CQIs that are equal to 0 on the primary carrier. When this number reaches a threshold N CQI , the Node B, based on CQIs on the secondary carriers, decides that frequency carrier F 2  (Secondary Carrier  1 ) is best suited to be the next primary carrier. The Node B also decides that it needs to deactivate frequency carrier F 4  (Secondary Carrier  3 ) due to load balancing. The Node B signals these changes via an HS-SCCH order using Secondary Carrier  1  to the user equipment by encoding the following:
       Frequency Carrier F 1 =Deactivate   Frequency Carrier F 2 =Primary Carrier   Frequency Carrier F 3 =Secondary Carrier  1     Frequency Carrier F 4 =Deactivate       
 
         [0138]    The user equipment after successfully receiving the HS-SCCH order will send an acknowledgement to the Node B and proceed to make the necessary changes using pre-configuration information. The Node B upon receiving the acknowledgement from the user equipment will inform the radio network controller of these changes. The user equipment will then stop timer T 313 , thereby avoiding a radio link failure, and continues to monitor for possible radio link failure on frequency carrier F 2 , the new primary carrier. 
         [0139]    Accordingly, it can be seen that the user equipment either explicitly or implicitly warns the network that its radio link connection is failing and may lead to a radio link failure. The network performs a fast primary carrier change via a HS-SCCH order (physical layer indication). The user equipment performs change of primary carrier and updates the network. It will be appreciated that different user equipment can have different frequency carriers as their primary carrier. 
       Fast Primary Carrier Change 
       [0140]    Fast primary carrier change using HS-SCCH orders can also be used for other purposes apart from avoiding radio link failure. As shown in  FIG. 6 , user equipment is initially attached to base station (Node B) NB 1  with the following carrier settings:
       Frequency Carrier F 1 =Primary Carrier   Frequency Carrier F 2 =Secondary Carrier  1     Frequency Carrier F 3 =Secondary Carrier  2     Frequency Carrier F 4 =Secondary Carrier  3         
 
         [0145]    The user equipment then moves into coverage area of base station NB 2  and performs a handover to base station NB 2 . The user equipment receives the pre-configuration information required to operate on all of the other frequency carriers (F 1 , F 2 , F 3  &amp; F 4 ). As the user equipment moves around the coverage area of base station NB 2 , the load on frequency carrier F 1  increases. However, frequency carrier F 3  is lightly loaded. Base station NB 2  decides that the essential control channels are better sent via frequency carrier F 3 . Hence, base station NB 2  decides to change the user equipment&#39;s primary carrier from F 1  to F 3 . To do this, the base station NB 2  performs a fast primary carrier change via an HS-SCCH order using frequency carrier F 3  with the following settings encoded in the order:
       Frequency Carrier F 1 =Secondary Carrier  1     Frequency Carrier F 2 =Secondary Carrier  2     Frequency Carrier F 3 =Primary Carrier   Frequency Carrier F 4 =Secondary Carrier  3         
 
         [0150]    The user equipment upon receiving the HS-SCCH order will acknowledge it and change its primary carrier to F 3 . The base station NB 2  upon receiving the user equipment&#39;s acknowledgement will inform the radio network controller of the change. In this example, the original primary carrier on F 1  is not deactivated. An order from a secondary carrier is understood as a change in primary carrier to that secondary carrier. 
         [0151]    Hence, it can be seen that user equipment may send an explicit radio link failure warning to the network using either N WARN  or T WARN  criteria using either RRC signalling or a reserved E-DPCCH. Alternatively, user equipment can send implicit radio link failure warnings using the CQI (by setting it to zero) using either N WARN  or T WARN  criteria. A fast primary carrier indication by the Node B can be effected using existing HS-SCCH orders. Base stations inform the radio network controller of a change in primary carrier at the user equipment (or user equipment can inform the RNC). User equipment is preconfigured to operate the primary carrier in any of the configured frequency carriers (i.e. existing secondary carrier). In this way, a rapid change in primary carrier can occur without risking a call being dropped or an unacceptable degradation in service occurring. 
         [0152]      FIG. 7   a  is a schematic signalling diagram illustrating radio link failure according to one embodiment. In this embodiment, when an RNC  40  determines, or is informed, that a UE  50  requires a change in primary carrier it is operable, rather than using an RRC procedure, sent to the UE, to send a message to the Node B (base station)  20  to, in turn, instruct the base station to issue an HS-SCCH order to the UE  50 . Upon completion of the required reconfiguration to change the primary carrier, the UE  50  is operable to send a “RAB configuration complete” message to the RNC. 
         [0153]    In this embodiment, the RNC  40  is the instigator of the primary carrier change procedure. Such an arrangement removes the possibility that the base station autonomously issues an HS-SCCH order to the UE instructing a change in primary carrier, without the network generally being aware of such a change. Ensuring the network retains visibility of changes in primary carrier mitigates the chances of synchronisation loss and ensures each base station in an active set of a UE can be kept up to date regarding the chosen primary carrier. Further more, since the RNC retains control of the primary carrier changes, it will not send a conflicting configuration message to the UE until it receives a RAB configuration complete from the UE. The RNC &amp; Node B remain aligned regarding the mobility status of a given UE. 
         [0154]    As shown in  FIG. 7   a , RNC  40  determines that UE  50  needs to perform a Primary Carrier change based on RRC measurement information from the UE. Rather than initiating an RRC procedure, for example, Radio Bearer Reconfiguration procedure, which informs the UE to perform a Primary Carrier change, the RNC triggers a new NBAP message to the serving base station currently hosting the primary carrier for the UE. The NBAP message instructs the base station to send an HS-SCCH order to the UE. The HS-SCCH order informs the UE to perform a Primary Carrier change. Since the RNC instigates the procedure, it is operable to wait until it receives an indication from the UE, for example, an RAB configuration complete message, that reconfiguration of the UE is complete before it initiates any other RRC procedures to that UE. Thus both the base station  20  and RNC  40  remain synchronised with respect to the state of a given UE. 
         [0155]      FIG. 7   b  is a schematic signalling diagram illustrating radio link failure according to one embodiment. In this embodiment, the base station requests a primary carrier change from the RNC  40 . The base station  20  waits for a response from the RNC before sending an HS-SCCH order to the UE  50 . The HS-SCCH order instructs a change the primary carrier being used by UE  50 . This embodiment has an advantage over the embodiment shown in  FIG. 7   a  in that the base station  20  is typically in possession of up to date information in relation to the radio condition of the UE (for example, from CQI reports) and can thus make a quicker determination of when a primary carrier change may be required in comparison to the RNC. The RNC is typically reliant on receiving RRC measurement reports from the UE, and those measurements may take some time to be delivered to the RNC, by which time changing primary carrier may be too late. 
         [0156]    As shown in  FIG. 7   b , base station  20  decides that UE  50  is required to perform a primary carrier change. The base station  20  makes that decision based on a determination of the radio conditions experienced by the UE, as reported back by messages from the UE. The base station  20  sends an NBAP request to RNC  40 , requesting that the RNC instruct the performance by the UE of a primary carrier change. In the embodiment shown, the RNC grants the request of the base station and informs the base station accordingly. Base station  20  initiates the transmission of an HS-SCCH order to the UE. Since the Node B initiated a request to the RNC, and the RNC subsequently gets an indication from the UE that the reconfiguration is complete, both RNC and base station remain in synchronisation with respect to the state of the UE. 
         [0157]      FIG. 7   c  is a schematic signalling diagram illustrating radio link failure according to one embodiment. In a third embodiment, as shown in  FIG. 7   c , the base station  20  is operable to notify RNC  40  that it wishes to transmit an HS-SCCH order to UE  50 . In this embodiment, the base station does not wait for a response from RNC  40  before sending that order. Such an arrangement removes the delay whilst the base station waits for the RNC response (present in the embodiment shown in  FIG. 7   b ). Once base station  20  has notified RNC  40 , it proceeds to perform a primary carrier change via an HS-SCCH order. UE  50  acknowledges the HS-SCCH order to base station  20 . In turn, base station  20  indicates the acknowledgement to RNC  40 , thus allowing RNC  40  to propagate the change in primary carrier implemented by UE to the non-serving cells belonging to the active set of UE  50 . Upon completing reconfiguration to change the primary carrier, UE  50  sends a RAB configuration complete message to the RNC. Since the RNC has received a notification of primary carrier change, it is configured such that it does not send any conflicting configuration messages to the UE for a predetermined period, or until it receives a RAB configuration complete message from UE  50 . 
         [0158]    As shown in  FIG. 7   c , base station  20  decides that UE  50  needs to perform a primary carrier change based on its determination of the radio conditions from the UE. The base station sends an NBAP indication to RNC  40  that it is about to instruct UE to perform a primary carrier change. Base station  20  issues a HS-SCCH order to UE  50  and when the UE responds by acknowledging receipt of that order, base station  20  sends an additional indication to the RNC  40 . UE  50  also indicates to RNC  40  that it has performed a reconfiguration by sending an RRC RAB configuration complete to the RNC. Since the RNC is aware that the base station has initiated a HS-SCCH order it waits until it gets the indications from both the base station and UE that the reconfiguration is complete before it initiates any other RRC procedures to that UE. Hence both the base station and RNC remain synchronised with respect to the state of the UE. 
         [0159]    Embodiments shown in  FIGS. 7   a ,  7   b  and  7   c  allow the RNC and base station to maintain synchronisation on the mobility status of the UE when the network needs to instruct the UE to perform a primary carrier change. Without such a procedure there is the risk that the RNC and base station lose synchronisation which can lead to Radio Link failure on the base station, resulting in a significant drop in the potential downlink data throughput to the UE. A loss of synchronization between RNC and base station can result in the issuance of contradictory instructions which can in turn result in complete radio link failure. 
       Radio Link Failure Signalling 
       [0160]    In some multi-carrier systems, for example, 4C-HSDPA systems, a base station may be unable to order UE to make a change to its primary carrier. In such systems, only the RNC is operable to configure UE to change its primary carrier. Consequently, any radio link failure (RLF) warning message sent by UE needs to reach RNC  40 . One method is for UE to send a RRC message to the RNC. However, in such a signalling method, base station  20  remains unaware of the RLF Warning, acting simply to route the message from UE to RNC, rather than decode any part of it. It can be beneficial for a base station to know if possible radio like failure of a primary carrier, since base station  20  performs the scheduling for the multi carrier network and the RRC message from the RNC to change the primary carrier may be sent to UE via the primary carrier which is experiencing RLF. If the primary carrier change instruction is sent on a carrier experiencing radio link failure UE may not receive the primary carrier change instruction and is hence unable to avoid the radio link failure. A problem to solve is to design signalling methods to avoid this issue. 
         [0161]    In some embodiments, part of a radio link failure warning signaling process transits, or terminates at, a base station  20 . UE may only evaluate possible radio link failure for the primary carrier since it is on that carrier that control and mobility commands are based and failure of such a carrier is critical to the operation of the UE. Accordingly, if a radio link failure warning is sent by UE  50  then the primary carrier is likely to be in very poor radio condition. If base station  20  is made aware that the UE may go into radio link failure it can operate to stop routing packets to UE  50  via the primary carrier, with the result that RRC messages from RNC  40  are not sent via the primary carrier. Signaling associated with implementations of embodiments depend upon whether a radio link failure warning generated by UE  50  is sent via a RRC or Layer 1/MAC message (both of which can be decoded by a base station). 
         [0162]    RRC messages are usually slow since they are typically long and have to reach the RNC. It can therefore be advantageous, as described previously, to send a radio link failure message via Layer 1 or using a MAC message. Such messages terminate at a base station, enabling the base station to be aware of an impending radio link failure. 
         [0163]    Whether it is a base station or RNC which is operable to instruct a change to the UE primary carrier, it is important that the relevant network node is aware of which secondary carriers could be candidates for use as a new primary carrier. The UE itself is typically in the best position to evaluate the signal quality of all its downlink carriers. Accordingly, it is best placed to provide a recommendation to the RNC or base station regarding which secondary carrier to use as a new primary carrier and the signal quality (eg CQI) of this secondary carrier or carriers. 
         [0164]    During normal operation, a base station receives frequent Channel Quality Indicator (CQI) updates in relation to each carrier from the UE. However, UE typically stops sending CQI feedbacks if it moves into the radio link failure evaluation process illustrated in  FIG. 3 . 
         [0165]    In one embodiment, a base station is operable to use the last CQI feedback received from user equipment  50  to determine autonomously which secondary carrier to route the primary carrier change order comprising either an RRC message or an HS-SCCH order. The last CQI feedbacks may, however, not be representative of the radio condition currently being experienced by user equipment since there is a delay between the UE shutting off its transmitter and a base station routing a primary carrier change command. In some embodiments, the UE is also operable to provide a secondary carrier recommendation to a base station or RNC as part of a radio link failure warning. That radio link failure warning message may be sent using a Layer 1 or MAC message. A radio link failure message transmitted using Layer 1 can be explicit or implicit as described above. 
         [0166]    In the case of explicit signalling described above, in some embodiments, a reserved E-TFCI scheme cannot contain sufficient bits to carry a secondary carrier recommendation and 
         [0000]    an Uplink Order Request may be used instead in some embodiments. 
         [0167]    In the case of implicit signalling described above, in some embodiments it is proposed to transmit a CQI=0 repeatedly to the base station. In order to signal a recommended secondary carrier, UE sends a feedback CQI=0 for all carriers other than the recommended secondary carrier. In some embodiments, a recommended secondary carrier is indicated by sending, in respect of the recommended secondary carrier, a CQI=31 (highest CQI value). 
         [0168]      FIG. 8  is a schematic illustration of an implicit radio link failure message according to one embodiment. As illustrated, frequency F 1  is operating as secondary carrier  1 , F 2  is operating as primary carrier, F 3  is operating as secondary carrier  2  and F 4  is operating as secondary carrier  3 . The implicit radio link failure message shown in  FIG. 8  indicates that the recommended carrier is secondary carrier  2  which has a CQI=31 whilst the other carriers have been allocated a CQI=0. The illustration of  FIG. 8  is a logical representation of the implicit radio link failure message including CQI feedback. The actual feedback carried by the HS-DPCCH (High Speed Dedicated Physical Control Channel) has a different bit format. 
         [0169]    It will be appreciated that in an alternative embodiment, it is possible to indicate, using appropriate values of CQI, a ranking indicating the suitability of each of the secondary carriers as a replacement primary carrier. 
         [0170]    In embodiments, if there is no secondary carrier that can act as the primary carrier, the radio link failure warning is still sent since it is possible that the RNC can instruct handover of the UE to another cell. A base station according to one embodiment can decide that an implicit radio link failure warning is sent if there is a lack of feedback from the UE to the base station after a predetermined time period. Silence for a predetermined time period may be indicative that a UE is experiencing possible radio link failure, since a UE experiencing radio link failure typically shuts off its transmitter when it enters radio link failure evaluation. This type of implicit warning may, however, result in incorrect messaging since an absence of feedback may not indicate an impending radio link failure. Furthermore, in such an embodiment, it is not possible for the UE to recommend a secondary carrier to be used as the new primary carrier. 
         [0171]    Depending on the settings of T WARN and N WARN  criteria in the radio link failure evaluation method shown in  FIG. 3 , UE  50  may recover from a loss of synchronization after sending a radio link failure warning. In such a case, UE  50  is then operable to feedback a valid HS-DPCCH (one without CQI=0) to a base station  20 . 
         [0172]    Reception of a normal HS-DPCCH message from UE  50  at a base station after that UE has sent a radio link failure warning message indicates that UE  50  has recovered from imminent radio link failure. If recovery occurs before the RNC has sent a primary carrier change order, the base station becomes aware that the UE has recovered and is able to receive on the primary carrier. In embodiments, a base station sends a HS-SCCH order to change the primary carrier, the reception of a valid HS-DPCCH prior to the sending of an HS-SCCH order results in the base station  20  cancelling, for example, by never sending, the HS-SCCH order. 
         [0173]      FIGS. 9 to 15  illustrate various signaling methods used when a UE sends a radio link failure warning message directly to an RNC via RRC message and/or to a base station via a Layer 1/MAC message. 
       Radio Link Failure Using RRC Message 
       [0174]    According to one embodiment, user equipment  50  sends a radio link failure warning as an RRC message. That message is sent directly to RNC  40  bypassing the base station  20 . That is to say, the base station does not decode the RRC message which passes through it from user equipment to the RNC. In such an embodiment, the RNC operates to inform the base station of the radio link failure warning. The signaling methods for RNC initiated primary carrier change and base station initiated primary carrier change are described. 
         [0175]      FIG. 9  illustrates schematically the signaling for a scenario in which an RNC  40  initiates a primary carrier change. Upon receiving a radio link failure warning from the UE, the RNC informs the base station of a radio link failure warning for this UE via an NBAP message. The NBAP message also contains an indication of the UE recommended secondary carrier. When the base station receives the NBAP message from the RNC, it stops scheduling packets on the primary carrier unless it receives (from the UE) a HS-DPCCH message indicating good CQI on the primary carrier. The RNC then proceeds to send a RRC message to the UE to change its primary carrier. This message can be routed via a secondary carrier (for example, as recommended by user equipment  50  as a replacement primary carrier) instead of the current primary carrier. In some embodiments, the RNC and base station are operable to route a primary carrier change message via a carrier other than that recommended as a primary carrier by the UE. 
         [0176]      FIG. 10  illustrates a signaling scheme in which a base station  20  is operable to Itself initiates a primary carrier change, for example, by use of a HS-SCCH order (or a layer 1 order). In the embodiment shown, UE  50  is pre-configured to operate the primary carrier in any configured carriers. Upon receiving a radio link failure warning from UE in relation to the current primary carrier, the RNC informs the base station  20  of an occurrence of a radio link failure warning for the UE via a NBAP message. This NBAP message also contains an indication of the UE recommended secondary carrier. The base station  20  then sends a HS-SCCH order to the UE instructing a change to the primary carrier. The order is sent via a secondary carrier (for example, the secondary carrier recommended by the UE). The UE acknowledges receipt of the HS-SCCH order and also informs the RNC that the primary carrier configuration change is complete. 
       Radio Link Failure Warning Via Layer 1 or MAC Layer 
       [0177]    In some embodiments, a radio link failure warning is sent to a base station using a Layer 1 or MAC message, which terminates at a base station. Signaling methods for RNC and base station initiated primary carrier changes are described. 
         [0178]      FIG. 11  illustrates a signaling method in which the RNC initiates the primary carrier change. In the illustrated embodiment, the base station  20  receives a radio link failure warning from user equipment  50  and informs the RNC  40  via a NBAP message. In the NBAP message, the base station also recommends the best secondary carrier that can be used as the new primary carrier, based upon UE recommendation. The recommendation need not match the UE recommendation and can, for example, be a combination of the information provided by user equipment  50  in conjunction with an algorithm implemented by a base station. 
         [0179]    In embodiments in which base station initiates the primary carrier change, the base stafion  20  informs the RNC  40  of the radio link failure warning upon receiving it from the UE  50 . As described previously, the base station  20  can be operable either to wait for a response from RNC  40  before changing the UE primary carrier (as illustrated in  FIG. 12 ) or proceed to change the UE primary carrier without waiting (as illustrated in  FIG. 13 ). 
         [0180]    In accordance with the embodiment shown in  FIG. 12 , the base station  20  informs RNC  40  of a radio link failure warning and a recommended secondary carrier. The base station waits for the RNC  40  to decide whether a primary carrier change is required. According to such an embodiment, the RNC retains control over the UE  50  and it is possible that RNC  40  can decide to handover the UE to another cell rather than change its primary carrier, in which case the handover command (to another cell) may be sent via the recommended secondary carrier. In  FIG. 12 , the RNC orders the base station to instruct a change the UE primary carrier and also indicates the secondary carrier that it wishes to be the new primary carrier. The base station sends an appropriate order in the form of an HS-SCCH order to the UE. The UE acknowledges the HS-SCCH order and confirms the primary carrier configuration to the RNC. 
         [0181]    If the base station  20  waits for confirmation from the RNC  40  as shown in  FIG. 12 , the primary carrier change may be slow and the delay in waiting for a confirmation may cause the UE to declare radio link failure in the interim, thus disrupting the user service. 
         [0182]    In accordance with the embodiment shown in  FIG. 13 , base station  20  proceeds to change the UE primary carrier after informing the RNC without wailing for a RNC feedback. Since the base station  20  is in control, it can cancel the primary carrier change if it subsequently discovers that the UE has recovered its primary carrier, for example on receipt of a valid HS-DPCCH from a UE. If the base station wishes to cancel the primary carrier change, it also informs the RNC, thereby ensuring the RNC retains visibility of primary carrier changes. 
         [0183]    In  FIG. 14   a , a UE is attached to a base station, NB 2 . The UE  50  is located in an overlapping region of coverage between base stations NB 1  and NB 2 . NB 1  operates only in frequency F 1  and F 2  whilst NB 2  operates in 4C-HSDPA with F 1 , F 2 , F 3  and F 4 . The UE primary carrier is F 1  and Secondary Carrier  1  (SC 1 ) is F 2 , Secondary Carrier  2  (SC 2 ) is F 3  and Secondary Carrier  3  (SC 3 ) is F 4 . Since NB 1  also operates in F 1  and F 2 , the UE experiences higher interference in F 1  and F 2  and to a point where the UE loses sync on its primary carrier in F 1 . The UE moves into RLF evaluation and sends a RLF Warning using a Layer 1 or MAC message to the NB with SC 2  as the recommended secondary carrier best suited as the new primary carrier. In this illustrated example, only the RNC is able to change the primary carrier of the UE and signalling as shown in  FIG. 11  is executed to instruct the UE to change its primary carrier from F 1  to F 3 . 
         [0184]    In  FIG. 14   b  a UE is attached to NB 2  at the overlapping region of coverage between NB 1  and NB 2 . As in  FIG. 14   a , NB 1  operates in F 1  and F 2  whilst NB 2  operates in 4C-HSDPA with F 1 , F 2 , F 3  and F 4 . F 1  is the primary carrier. Similar  FIG. 14   a , the UE primary carrier deteriorates and the UE moves into RLF evaluation. The UE sends a RLF Warning using Layer 1 or MAC message with F 3  (Secondary Carrier  2 ) as the recommended secondary carrier best suited to be the new primary carrier. 
         [0185]    According to this example, the NB is able to initiate the primary carrier change. The NB informs the RNC of the change in primary carrier as described in  FIG. 15 . The UE moves further into NB 2  and this causes the interference from NB 1  on the primary carrier (F 1 ) to reduce and the UE recovers its primary carrier. The UE then sends HS-DPCCH feedback as in normal operation. The NB receiving the valid HS-DPCCH decides to cancel its primary carrier change. It then informs the RNC of this cancellation. The UE continues to use F 1  as its primary carrier. 
         [0186]    A person of skill in the art would readily recognize that steps of various above-described methods can be performed by programmed computers. Herein, some embodiments are also intended to cover program storage devices, e.g., digital data storage media, which are machine or computer readable and encode machine-executable or computer-executable programs of instructions, wherein said instructions perform some or all of the steps of said above-described methods. The program storage devices may be, e.g., digital memories, magnetic storage media such as a magnetic disks and magnetic tapes, hard drives, or optically readable digital data storage media. The embodiments are also intended to cover computers programmed to perform said steps of the above-described methods. 
         [0187]    The functions of the various elements shown in the Figures, including any functional blocks labelled as “processors” or “logic”, may be provided through the use of dedicated hardware as well as hardware capable of executing software in association with appropriate software. When provided by a processor, the functions may be provided by a single dedicated processor, by a single shared processor, or by a plurality of individual processors, some of which may be shared. Moreover, explicit use of the term “processor” or “controller” or “logic” should not be construed to refer exclusively to hardware capable of executing software, and may implicitly include, without limitation, digital signal processor (DSP) hardware, network processor, application specific integrated circuit (ASIC), field programmable gate array (FPGA), read only memory (ROM) for storing software, random access memory (RAM), and non volatile storage. Other hardware, conventional and/or custom, may also be included. Similarly, any switches shown in the Figures are conceptual only. Their function may be carried out through the operation of program logic, through dedicated logic, through the interaction of program control and dedicated logic, or even manually, the particular technique being selectable by the implementer as more specifically understood from the context. 
         [0188]    It should be appreciated by those skilled in the art that any block diagrams herein represent conceptual views of illustrative circuitry embodying the principles of the invention. Similarly, it will be appreciated that any flow charts, flow diagrams, state transition diagrams, pseudo code, and the like represent various processes which may be substantially represented in computer readable medium and so executed by a computer or processor, whether or not such computer or processor is explicitly shown. 
         [0189]    The description and drawings merely illustrate the principles of the invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements that, although not explicitly described or shown herein, embody the principles of the invention and are included within its spirit and scope. Furthermore, all examples recited herein are principally intended expressly to be only for pedagogical purposes to aid the reader in understanding the principles of the invention and the concepts contributed by the inventor(s) to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions. Moreover, all statements herein reciting principles, aspects, and embodiments of the invention, as well as specific examples thereof, are intended to encompass equivalents thereof.