Patent Description:
It has been decided, as part of the 3GPP standardisation process, that downlink operation for system bandwidths beyond <NUM> will be based on the aggregation of a plurality of component carriers at different frequencies. Such carrier aggregation can be used to support operation in a system both with and without a contiguous spectrum (for example, a non-contiguous system may comprise component carriers at <NUM>, <NUM>, and <NUM>). Whilst a legacy mobile device may only be able to communicate using a single, backward compatible, component carrier, a more advanced multi-carrier capable terminal would be able to simultaneously use the multiple component carriers.

As mobile (cellular) communication technology has developed there have been proposals to provide enhanced communication in relatively small geographic regions by having small cells (e.g. 'pico' or 'femto' cells) that coexist with a larger ('macro') cell and provide enhanced communication capabilities in the localised geographic region that the small cell covers. These small cells can be provided on the same carrier as the macro cell or can be provided on a different (e.g. higher frequency) dedicated carrier.

More recently, it has been proposed to allow user data for a particular user device such as a mobile telephone or other mobile communication device (also referred to as 'user equipment' or a 'UE') to be communicated via a different cell to the cell via which control data for that user device is communicated Specifically, it has been proposed to allow the user plane (U-plane) and control plane (C-plane) for a particular user device to be split between the small cell and the macro cell such that U-plane data is communicated via the small cell and C-plane data is communicated via the macro cell.

The small cell of this proposal is, effectively a 'pseudo' cell or 'phantom' cell because it does not provide conventional cell-specific signals and/or channels such as carrier reference signals, master information/system information broadcasts, primary/secondary synchronisation signals, etc..

In theory, the C-plane/U-plane split of this proposal provides an optimisation of: the benefits of the better connectivity typically offered by a macro cell for critical control signalling; and the benefits of higher throughput and more flexible, energy efficient, and cost effective communication offered by a small cell using a higher and/or wider frequency band for higher volume user data.

However, the C-plane/U-plane split proposal presents a number of challenges that need to be addressed if such a proposal is to be implemented practically in the global communication network.

One such challenge is the provision of appropriate communication security where different base stations are responsible for U-plane signalling and C-planc signalling respectively whilst ensuring that the user device is able to encipher/decipher user data and control data correctly. This has the potential to add significant unwanted complexity to signalling between the core network, the base station, and the user device.

Moreover, in order to ensure appropriate security it is beneficial to be able to, from time to time, regenerate the security keys used for encryption and integrity protection ('re-keying' or 'key-refresh'). Such dynamic key changing can be the result of explicit re-keying or implicit key-refresh procedures. To ensure that the security parameters used for ciphering and integrity protection remain unique, for example, key refresh is typically required when the Packet Data Convergence Protocol (PDCP) counter ('PDCP COUNT'), which is used as a ciphering input, reaches its limit and 'wraps around' or 'rolls over* back to its starting value. Re-keying / key-refresh avoids the risk that previously used PDCP COUNT values are re-used, in combination with the same security key, as inputs for ciphering thereby avoiding the cyclic re-use of earlier security parameters.

However, currently, such dynamic key refreshing is not possible when the U-plane and C-plane are split because the PDCP count is maintained in the U-plane whilst the control signalling required for re-keying occurs in the C-plane.

<CIT> discloses a cellular communication system including a macro base station and a micro base station within coverage of the macro base station. The control plane of the user served by the micro base station can be separated from the data plane of the user, so that the resources of the micro base station can be used for data communication.

<CIT> discloses a method in which a femtocell monitors user interface uplink and downlink packet data convergence protocol (PDCP) COUNT values of each user equipment (UE) connected with the femtocell, and sends information on the user interface uplink and downlink PDCP COUNT values to a macro base station.

The invention therefore aims to provide a mobile communication system, a mobile communication device, a communication node and associated methods which overcomes or at least mitigates the above issues.

The invention is set out in the independent claims appended hereto, and preferred embodiments are indicated in the dependent claims that follow.

Embodiments of the invention will now be described by way of example only with reference to the attached figures in which:.

<FIG> schematically illustrates a mobile (cellular) telecommunication system <NUM> in which a user of any of a plurality of mobile communication devices <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM> can communicate with other users via one or more of a plurality of base stations <NUM>-<NUM>, <NUM>-<NUM> and <NUM>-<NUM> and a core network <NUM>. In the system illustrated in <FIG>, each base station <NUM> shown is an Evolved Universal Terrestrial Radio Access Network (E-UTRAN) base station (or 'eNB') capable of operating in a multi-carrier environment.

The core network <NUM><NUM> comprises a plurality of functional/logical entities including a Mobility Management Entity (MME) <NUM>, a Home Subscriber Server (HSS) <NUM>, and an Authentication Centre (AuC) <NUM>.

The MME <NUM> is a key control-node for the LTE access-network. It is responsible for, among other things, authenticating the user (by interacting with the HSS <NUM>). Non Access Stratum (NAS) signalling terminates at the MME <NUM>. The MME <NUM> is also the termination point in the network for ciphering/integrity protection for NAS signalling and handles the security key management.

The HSS <NUM> comprises a central database that contains user-related and subscription-related information. The functions of the HSS <NUM> include functionalities such as mobility management, call and session establishment support, user authentication and access authorization. The HSS <NUM>, in this exemplary embodiment, includes the functionality of the AuC <NUM> (although this could be provided separately). The AuC <NUM> function provides authentication of each mobile communication device <NUM> (or more specifically the associated subscriber identity module (SIM) card) that attempts to connect to the core network <NUM> (e.g. when the mobile communication device <NUM> is powered on). Once the authentication is successful, the HSS <NUM> manages the SIM and services as described above. As described in more detail below, an encryption key is also generated by the AuC <NUM> function that is subsequently used to encrypt all wireless communications (voice, SMS, etc.) between the mobile communication devices <NUM> and the core network <NUM>.

In <FIG>, the base station labelled <NUM>-<NUM> comprises a so called 'macro' base station operating a relatively geographically large 'macro' cell <NUM>-<NUM> using an associated component carrier having a first frequency (F1). The other base stations <NUM>-<NUM>, <NUM>-<NUM> shown in <FIG>, each comprises a so called 'pico' base station operating a respective 'pico' cell <NUM>-<NUM>, <NUM>-<NUM>. Each pico cell <NUM>-<NUM>, <NUM>-<NUM> is operated on a respective component carrier having a corresponding frequency band (F2). The power used to provide pico cells <NUM>-<NUM>, <NUM>-<NUM> is low relative to the power used for the macro cell <NUM>-<NUM> and the pico cells <NUM>-<NUM>, <NUM>-<NUM> are therefore small relative to the macro cell <NUM>-<NUM>.

The macro base station <NUM>-<NUM> provides control signalling <NUM>-<NUM> in a control plane ('C-plane') to mobile communication devices, such as a mobile communication device <NUM>-<NUM>, that are located in the macro cell <NUM>-<NUM> that it operates. The macro base station <NUM>-<NUM> also communicates user data <NUM>-<NUM> in a user plane ('U-plane') to and from mobile communication devices, such as a mobile communication device <NUM>-<NUM>, that are located in the macro cell that it operates.

In the case of the pico cells <NUM>-<NUM>, <NUM>-<NUM>, however, the provision of the U-plane and the C-plane is split between the macro base station <NUM>-<NUM> and the pico base station <NUM>-<NUM> or <NUM>-<NUM> that operates the pico cell <NUM>-<NUM>, <NUM>-<NUM>. Specifically, the macro base station <NUM>-<NUM> provides control signalling <NUM>-<NUM>, <NUM>-<NUM>, in the control plane ('C-plane'), to mobile communication devices, such as a mobile communication device <NUM>-<NUM> and <NUM>-<NUM>, that are located in the pico cells <NUM>-<NUM> and <NUM>-<NUM> that are operated by the pico base stations <NUM>-<NUM>, <NUM>-<NUM>. Contrastingly, each pico base station <NUM>-<NUM>, <NUM>-<NUM> communicates user data <NUM>-<NUM>, <NUM>-<NUM>, in the U-plane, with a respective mobile communication device <NUM>-<NUM>, <NUM>-<NUM>, in the pico cell <NUM>-<NUM>, <NUM>-<NUM>, which that pico base station <NUM>-<NUM>, <NUM>-<NUM> operates.

The C-plane signalling includes, amongst other control signalling, signalling related to U-plane communication such, for example, signalling controlling resources used for user plane communication, signalling for controlling establishment and release of the user plane communication bearers, and signalling for controlling mobility (e.g. handover) of user plane communication between cells.

In more detail, the C-plane signalling comprises control signalling, including radio resource control (RRC) signalling, for: broadcast of System Information; paging; establishment, maintenance and release of an RRC connection between the mobile communication device <NUM> and the network; security functions including key management; establishment, configuration, maintenance and release of point to point radio bearers; mobility functions (hand over and cell reselection); quality of service (QoS) management functions; measurement reporting and control of the reporting; and resource allocation for the U-plane communication.

Security information required for enciphering/deciphering (encrypting/decrypting) C-plane data (and for integrity protection in the C-plane) for each pico cell <NUM>-<NUM>, <NUM>-<NUM> is provided to the macro base station <NUM>-<NUM>. The macro base station <NUM>-<NUM> uses the security information to derive appropriate security keys for enciphering/deciphering (encrypting/decrypting) the control signalling for each mobile communication device <NUM>-<NUM>, <NUM>-<NUM> that is located in either of the pico cells <NUM>-<NUM>, <NUM>-<NUM>.

Security information required for enciphering/deciphering (encrypting/decrypting) the respective U-plane data (and for any integrity protection in the U-plane) for each pico cell <NUM>-<NUM>, <NUM>-<NUM> is provided to the pico base station <NUM>-<NUM>, <NUM>-<NUM> that operates that pico cell <NUM>-<NUM>, <NUM>-<NUM>. Each pico base station <NUM>-<NUM>, <NUM>-<NUM> determines appropriate security keys for enciphering/deciphering (encrypting/decrypting) the user data for each mobile communication device <NUM>-<NUM>, <NUM>-<NUM> that is located in the pico cell <NUM>-<NUM>, <NUM>-<NUM> operated by that base station <NUM>-<NUM>, <NUM>-<NUM>.

Security information required for enciphering/deciphering (encrypting/decrypting) C-plane data and U-plane data (and any integrity protection) is also provided to each mobile communication device <NUM>. Each mobile communication device <NUM> determines, from the security information, appropriate security keys for enciphering/deciphering (encrypting/decrypting) the user data and control data for that device.

Each mobile communication device <NUM>-<NUM>, <NUM>-<NUM> communicating via a pico cell <NUM>-<NUM>, <NUM>-<NUM> is also provided with an indication that the C-plane and U-plane are split between the macro and pico base stations. This beneficially helps to ensure that the mobile communication device <NUM>-<NUM>, <NUM>-<NUM> can keep track of which base station <NUM> is responsible for the C-plane and which base station <NUM> is responsible for U-plane. Accordingly, the mobile communication device <NUM>-<NUM>, <NUM>-<NUM> is able to derive the keys for enciphering/deciphering (encrypting/decrypting) the respective U-plane data (and for any integrity protection in the U-plane) correctly even though the base station handling U-plane communication is different to the base station providing C-plane communication.

<FIG> illustrate a Security Key Hierarchy and Key Derivation in the mobile telecommunication system of <FIG>.

Specifically, <FIG> illustrates a encryption / integrity key hierarchy used in the mobile telecommunication system of <FIG>. <FIG> illustrates a key derivation scheme used by a base station in the telecommunication system of <FIG> and <FIG> illustrates a key derivation scheme used by a mobile communication device in the telecommunication system of <FIG>.

<FIG> are based on a similar figure from 3GPP Technical Standard (TS) <NUM> v <NUM>. <NUM> which, as a skilled person would understand, includes further detail of the security mechanisms that are employed in the mobile telecommunication system of <FIG>.

Referring to <FIG>, the mobile telecommunication system <NUM> uses a number of security key parameters which, for the purposes of illustration, can be considered to be arranged in a hierarchy in which keys at a lower level in the hierarchy may be derived from keys higher up the hierarchy (possibly in combination with other parameters) using an appropriate key derivation function (KDF). In this exemplary embodiment, the KDF which is used to derive the security keys is the KDF described in 3GPP TS <NUM> v <NUM>. <NUM> (Annex B) with inputs as described in 3GPP TS <NUM> v <NUM>. <NUM> (Annex A).

As seen in <FIG>, the security information used in the mobile telecommunication system <NUM> includes the following security key parameters:.

A number of other notable parameters are also used in the security architecture of the mobile telecommunication network <NUM>. These include:.

When an MME <NUM> receives an attach request from a mobile communication device <NUM>, the MME <NUM> sends the authentication data request to the AuC/HSS <NUM>/<NUM>. After derivation of RAND, XRES, CK, IK, and AUTN the AuC <NUM> combines them into a so called authentication vector (AV = RAND ∥ XRES ∥ CK ∥ IK ∥ AUTN) which is sent to the MME <NUM>. The MME <NUM> can then retrieve the individual parameters from the AV for sending to the mobile communication device during an authentication and key generation process as described in more detail below.

In order to cipher / decipher user plane data a ciphering function is used that has, as its inputs: KUPenc; information identifying the radio bearer used for the communication ('BEARER'); a single bit indicator of the direction of the communication ('DIRECTION'); the length of the keystream required ('LENGTH') and a bearer specific, but time and direction dependent <NUM>-bit value of an incremental counter ('COUNT') which corresponds to the <NUM>-bit PDCP COUNT maintained in the PDCP layer for the mobile communication device <NUM> and the pico base station <NUM>-<NUM>, <NUM>-<NUM>.

<FIG> is a block diagram illustrating the main components of the mobile communication devices <NUM> shown in <FIG>. Each mobile communication device <NUM> comprises a mobile (or 'cell') telephone capable of operating in a multi-carrier environment. The mobile communication device <NUM> comprises a transceiver circuit <NUM> which is operable to transmit signals to, and to receive signals from, the base stations <NUM> via at least one antenna <NUM>. The mobile communication device <NUM> comprises a user interface <NUM> via which a user can interact with the device (e.g. a touchscreen, keypad, microphone, speaker and/or the like).

The mobile communication device includes a subscriber identity module (SIM) <NUM> in the form of a Universal SIM (USIM) running on a Universal Integrated Circuit Card (UICC). The SIM <NUM> comprises a USIM/UICC security module <NUM> for obtaining and storing the permanent key 'K' <NUM>-<NUM> which, in operation, is used for generating the other security parameters used for communication security. The USIM/UICC security module <NUM> is also operable to derive other security parameters <NUM>-<NUM> such as the cipher key (CK) and integrity key (IK) using K and a 'random' value (e.g. a value of RAND provided by the AuC <NUM> via the MME <NUM>). The SIM <NUM> has an identity <NUM> in the form of an international mobile subscriber identity (IMSI).

The operation of the transceiver circuit <NUM> is controlled by a controller <NUM> in accordance with software stored in memory <NUM><NUM>.

The software includes, among other things, an operating system <NUM>, a communication control module <NUM> and a security management module <NUM>.

The communication control module <NUM> is configured for managing communication with the macro and/or base stations <NUM> on the associated component carriers. The communication control module <NUM> is configured for managing NAS communication with the MME <NUM> (indirectly via the base station). The communication control module <NUM> includes a U-plane module <NUM> for handling user data and a C-plane module <NUM> for handling control signalling such as radio resource control messages.

The security management module <NUM> is configured for managing communication security including the performance of authentication procedures, key and related security parameter generation and utilisation, and authentication and key agreement (AKA) to the extent that they are performed at the mobile communication device <NUM>. The security management module <NUM> is able to handle retrieval/generation of appropriate parameters <NUM> for use in authentication / key generation procedures. These parameters include: UICC/USIM parameters <NUM>-<NUM> retrieved from the SIM <NUM> (e.g. parameters <NUM>-<NUM> such as CK and IK derived by the SIM <NUM>); parameters <NUM>-<NUM> received from other sources (e.g. parameters such as AUTN and RAND received from the MME <NUM> in Non-Access Stratum (NAS) signalling); and parameters <NUM>-<NUM> that may be derived at the mobile communication device (e.g. KASME, KNASint, KNASenc, KeNB, KeNB*, NH, KUPenc, KRRCint, KRRCenc, etc.). The security management module <NUM> also includes an AKA module <NUM> for managing AKA procedures to the extent performed by the mobile communication device <NUM>.

<FIG> is a block diagram illustrating the main components of the macro base station <NUM>-<NUM> shown in <FIG>. The macro base station <NUM>-<NUM> comprises an E-UTRAN multi-carrier capable base station comprising a transceiver circuit <NUM> which is operable to transmit signals to, and to receive signals from, the mobile communication devices <NUM> via at least one antenna <NUM>. The base station <NUM>-<NUM> is also operable to transmit signals to and to receive signals from: the MME <NUM> of the core network <NUM> via an MME (S1) interface <NUM>; and other base stations <NUM> via an eNB (X2) interface <NUM>.

The operation of the transceiver circuit <NUM> is controlled by a controller <NUM> in accordance with software stored in memory <NUM>.

The communication control module <NUM> is configured for managing communication between the macro base station <NUM>-<NUM> and the mobile communication devices <NUM> operating within the geographic area covered by the macro cell <NUM>-<NUM>. The communication control module <NUM> is also configured to manage S1-AP signalling between the macro base station <NUM>-<NUM> and the MME <NUM> and X2-AP signalling between macro base station <NUM>-<NUM> and other base stations.

The communication control module <NUM> includes a U-plane module <NUM> for handling user data for the mobile communication device <NUM>-<NUM> communicating via the macro cell <NUM>-<NUM>. The communication control module <NUM> also includes a C-plane module <NUM> for generating control signalling, such as radio resource control (RRC) messages, for transmission to the mobile communication device <NUM>-<NUM> communicating via the macro cell <NUM>-<NUM> and for the mobile communication devices <NUM>-<NUM> and <NUM>-<NUM> that communicate user data via respective pico cells <NUM>-<NUM>, <NUM>-<NUM>.

The security management module <NUM> is configured for managing communication security including the performance of authentication procedures, key and related security parameter generation and utilisation, and authentication and key agreement (AKA) procedures to the extent that they are performed at the macro base station <NUM>-<NUM>.

The security management module <NUM> is able to handle receipt/generation of appropriate parameters <NUM> for use in authentication / key generation procedures. These parameters <NUM> include parameters <NUM>-<NUM> received from other sources (e.g. KeNB or NH received from the MME <NUM>, or KeNB* received from a source base station during handover). The parameters <NUM> also include parameters <NUM>-<NUM> that may be derived at the macro base station <NUM>-<NUM> during normal operation (e.g. KUPenc, KRRCint, KRRCenc) or during handover (e.g. KeNB* when operating as a source node or KeNB (= KeNB*) when operating as a target node etc.). The security management module <NUM> also includes an AKA module <NUM> for managing AKA procedures to the extent performed by the macro base station <NUM>-<NUM>.

<FIG> is a block diagram illustrating the main components of a pico base station <NUM>-<NUM>, <NUM>-<NUM> shown in <FIG>. The pico base station <NUM>-<NUM>, <NUM>-<NUM> comprises an E-UTRAN multi-carrier capable base station comprising a transceiver circuit <NUM> which is operable to transmit signals to, and to receive signals from, the mobile communication devices <NUM> via at least one antenna <NUM>. The pico base station <NUM>-<NUM>, <NUM>-<NUM> is also operable to transmit signals to and to receive signals from: the MME <NUM> of the core network <NUM> via an MME (S1) interface <NUM>; and other base stations via an eNB (X2) interface <NUM>.

The communication control module <NUM> is configured for managing communication between the pico base station <NUM>-<NUM>, <NUM>-<NUM> and the mobile communication devices <NUM>-<NUM>, <NUM>-<NUM> communicating via the pico cell <NUM>-<NUM>, <NUM>-<NUM>. The communication control module <NUM> is also configured for managing Sl-AP signalling between the pico base station <NUM>-<NUM>, <NUM>-<NUM> and the MME <NUM> and X2-AP signalling between pico base station <NUM>-<NUM>, <NUM>-<NUM> and other base stations.

The communication control module <NUM> includes a U-plane module <NUM> for handling user data for a mobile communication device <NUM>-<NUM>, <NUM>-<NUM> communicating via the pico cell <NUM>-<NUM>, <NUM>-<NUM>.

The security management module <NUM> is configured for managing communication security including the performance of authentication procedures, key and related security parameter generation and utilisation, and authentication and key agreement (AKA) procedures to the extent that they are performed at the pico base station <NUM>-<NUM>, <NUM>-<NUM>.

The security management module <NUM> is able to handle receipt/generation of appropriate parameters <NUM> for use in authentication / key generation procedures. These parameters <NUM> include parameters <NUM>-<NUM> received from other sources (e.g. KeNB in this embodiment). The parameters <NUM> also include parameters <NUM>-<NUM> that may be derived at the pico base station <NUM>-<NUM>, <NUM>-<NUM> (e.g. KUPenc). The security management module <NUM> also includes an AKA module <NUM> for managing AKA procedures to the extent performed by the pico base station <NUM>-<NUM>, <NUM>-<NUM>.

<FIG> is a block diagram illustrating the main components of the mobility management entity (MME) <NUM> shown in <FIG>. The MME <NUM> comprises a transceiver circuit <NUM> which is operable to transmit signals to, and to receive signals from other network devices (such as the HSS) via an associated network entity interface <NUM>. The transceiver circuit <NUM> is also operable to transmit signals to, and to receive signals from, abase station <NUM> via an eNB (S1) interface <NUM> including S1-AP signalling for the base station <NUM>, and NAS signalling, which is transparent to the base station, for the mobile communication device <NUM>.

The communication control module <NUM> is configured for managing NAS signalling between the MME <NUM> and the mobile communication devices <NUM> and S1-AP signalling between the MME <NUM> and the base station <NUM>.

The security management module <NUM> is configured for managing communication' security including the performance of authentication procedures, key and related security parameter generation and utilisation, and authentication and key agreement (AKA) procedures to the extent that they are performed at the MME <NUM>.

The security management module <NUM> is able to handle receipt/generation of appropriate parameters <NUM> for use in authentication / key generation procedures. These parameters <NUM> include parameters <NUM>-<NUM> received from other sources (e.g. CK, IK, AUTN, KASME, RAND, XRES retrieved from an AV received from the HSS/AuC <NUM>/<NUM> etc.). The parameters <NUM> also include parameters <NUM>-<NUM> that may be derived at the MME <NUM> (e.g. KNASint, KNASenc, KeNB, NH etc.). The security management module <NUM> also includes an AKA module <NUM> for managing AKA procedures to the extent performed by the MME <NUM>.

<FIG> show simplified timing diagrams each illustrating operation of the telecommunication system of <FIG> in the performance of a respective variation of a security procedure. As those skilled in the art will appreciate, the timing diagrams only show signalling that is particularly relevant to the security. Other signalling will generally occur but has, for reasons of clarity, been omitted from the simplified timing diagrams.

As seen in <FIG>, each security procedure illustrated uses a different respective mechanism for ensuring that appropriate security parameters (in particular appropriate values of KUPenc) are consistently used for U-plane protection both at the mobile communication device <NUM> and at the base station.

Whilst the different security procedures illustrated in <FIG> are shown separately, it will be appreciated that key features of the security procedures may be combined, where appropriate, or provided as alternative implementation options in a deployed system.

<FIG> shows a simplified timing diagram illustrating operation of the telecommunication system of <FIG> in the performance of a first security procedure in which appropriate security parameters, and in particular appropriate values of KUPenc, are generated at the pico base station <NUM>-<NUM>, <NUM>-<NUM> in response to signalling from the MME <NUM>.

At the start of the security procedure illustrated, a mobile communication device <NUM> wishing to initiate communication in the pico cell <NUM>-<NUM>, <NUM>-<NUM> sends a non access stratum (NAS) message requesting attachment (e.g. an 'NAS ATTACH REQUEST' message) to the MME <NUM> (transparently via the macro base stations <NUM>-<NUM>) at S910 including information identifying the SIM card <NUM> of the mobile communication device <NUM> (e.g. the 'international mobile subscriber identity (IMSI)').

The MME <NUM> responds to this request, at S912, by sending a message requesting authentication and including information identifying the SIM card <NUM> to the HSS <NUM> (e.g. and 'AUTHENTICATION DATA REQUEST' message). The AuC function <NUM> of the HSS <NUM> derives RAND, XRES, CK, IK, AUTN and combines them to form an authentication vector for the SIM card <NUM> (AV = RAND ∥ XRES ∥ CK ∥ IK ∥ AUTN) at S914 and sends the generated AV to the MME <NUM> at S916 (e.g. in a 'AUTHENTICATION DATA RESPONSE' message).

The MME <NUM> retrieves IK, CK, XRES, RAND and AUTN from the AV at S918 and sends the AUTN and RAND parameters to the mobile communication device <NUM> using NAS signalling at S920 (e.g. in an 'NAS AUTHENTICATION REQUEST' message).

The mobile communication device <NUM> responds, at S922, by authenticating the network using the received AUTN, and by deriving appropriate security related parameters (IK, CK, RES etc.) using the stored permanent security key 'K' and the received AUTN and RAND parameters (and any other parameters where necessary- e.g. AMF for the determination of XMAC). Assuming the authentication is successful, the mobile communication device <NUM> sends the calculated value of RES to the MME <NUM> at S924 (e.g. in an 'NAS AUTHENTICATION RESPONSE' message).

The MME <NUM> checks the received RES value against XRES at S926, resets the downlink NAS count, and derives values of KASME, KeNB, KNASint and KNASenc. The MME <NUM> then initiates NAS signalling security between the MME <NUM> and the mobile communication device <NUM>, at S928, by sending an NAS SECURITY MODE COMMAND message informing the mobile communication device <NUM> of the respective algorithms to use for integrity protection and (de)ciphering.

The mobile communication device <NUM> responds, at S930, by deriving values of KASME, KeNB, KNASint and KNASenc and then, at S932, by sending a response message informing the MME <NUM> that NAS signalling security initialisation is complete.

The method then proceeds by initiating security context setup at both the pico base station <NUM>-<NUM>, <NUM>-<NUM>, and at the macro base station <NUM>-<NUM>, by sending substantially duplicate S1 application (S1-AP) messages (e.g. `S I-AP INITIAL CONTEXT SETUP REQUEST' messages) to the pico base station <NUM>-<NUM>, <NUM>-<NUM> at S934, and to the macro base station <NUM>-<NUM> at S936. The S1-AP messages each include the derived value of KeNB and details of the security capabilities for the mobile communication device <NUM>.

The pico base station <NUM>-<NUM>, <NUM>-<NUM> then derives, at S938, the security parameters) required for U-plane enciphering/deciphering (e.g. KUPenc) from the received KeNB. Similarly, the macro base station <NUM>-<NUM> derives, at S940, the security parameter(s) required for C-plane enciphering/deciphering (e.g. KRRCint and KRRCenc) from the received KeNB.

At S942, assuming security context setup at the pico base station <NUM>-<NUM>, <NUM>-<NUM> is successful, the pico base station <NUM>-<NUM>. <NUM>-<NUM> confirms this to the MME <NUM> in an appropriate S1-AP message (e.g. an 'S1-AP INITIAL CONTEXT SETUP RESPONSE' message).

The macro base station <NUM>-<NUM> then initiates, at S944, an RRC (and user plane) security context setup at the mobile communication device <NUM> using RRC signalling (e.g. an 'RRC SECURITY MODE COMMAND' message) which includes information identifying the algorithms used for integrity protection and/or ciphering and an information indicating that the U-plane and C-plane are split (e.g. in the form of a dedicated information element (IE), a modified IE, or re-use of an existing IE).

The mobile communication device <NUM> responds, at S946, by initialising the RRC security context for communication with the macro base station <NUM>-<NUM> by deriving the values of KRRCint, KRRCenc from the previously calculated value of KeNB for use with control signalling from the macro base station <NUM>-<NUM>. The mobile communication device <NUM> also initialises the U-plane security context for communication with the pico base station <NUM>-<NUM>, <NUM>-<NUM> by deriving the value of KUPene from KeNB for use with user plane signalling to/from the pico base station <NUM>-<NUM>, <NUM>-<NUM>.

Assuming the security context setup is successful the mobile communication device <NUM> confirms this, at S932, by sending an appropriate response message to the macro base station <NUM>-<NUM> (e.g. an 'RRC SECURITY MODE COMPLETE' message) at S950.

The macro base station <NUM>-<NUM> confirms, at S952, successful security context setup to the MME <NUM> in an appropriate S1-AP message (e.g. an 'Sl-AP INITIAL CONTEXT SETUP REQUEST' message).

Once the various security contexts (NAS and AS) have been initialised successfully on the various devices, the control and user signalling connections can be setup at S954 and the mobile communication device <NUM> can commence communication in which control plane signalling (S956) is provided by the macro base station <NUM>-<NUM> and U-plane signalling is provided via the pico base station <NUM>-<NUM>, <NUM>-<NUM> (S958).

Advantageously, therefore, this method provides an efficient way of providing appropriate communication security where different base stations are responsible for U-plane signalling and C-plane signalling respectively. The user device is able to maintain an appropriate security context for both the U-Plane and the C-plane thereby allowing it to encipher/decipher user data and control data correctly and to keep track of the security parameters (keys) used in the different base stations.

This approach has the benefit over the other methods described herein that it avoids the need for the modification of base station to base station signalling (over the X2 or possibly a new interface) and the associated increase X2-AP the complexity. However, other methods described herein have the benefit that S1 signalling duplication is avoided and hence S1 signalling overhead is reduced.

<FIG> shows a simplified timing diagram illustrating operation of the telecommunication system of <FIG> in the performance of a second security procedure in which appropriate security parameters, and in particular appropriate values of KUPenc, are generated at the pico base station <NUM>-<NUM>, <NUM>-<NUM> in response to signalling from the macro base station <NUM>-<NUM>.

At the start of the security procedure illustrated in <FIG>, a mobile communication device <NUM> wishing to initiate communication in the pico cell <NUM>-<NUM>, <NUM>-<NUM> sends a non access stratum (NAS) message requesting attachment (eg. an 'NAS ATTACH REQUEST' message) to the MME <NUM> (transparently via the macro base stations <NUM>-<NUM>) at S1010 including information identifying the SIM card <NUM> of the mobile communication device <NUM> (e.g. the 'international mobile subscriber identity (IMSI)').

The MME <NUM> responds to this request, at S1012, by sending a message requesting authentication and including information identifying the SIM card <NUM> to the HSS <NUM> (e.g. and 'AUTHENTICATION DATA REQUEST' message). The AuC function <NUM> of the HSS <NUM> derives RAND, XRES, CK, IK, AUTN and combines them to form an authentication vector for the SIM card <NUM> (AV = RAND ∥ XRES ∥ CK ∥ IK ∥ AUTN) at S1014 and sends the generated AV to the MME <NUM> at S1016 (e.g. in a 'AUTHENTICATION DATA RESPONSE' message).

The MME <NUM> retrieves IK, CK, XRES, RAND and AUTN from the AV at S1018 and sends the AUTN and RAND parameters to the mobile communication device <NUM> using NAS signalling at S1020 (e.g. in an 'NAS AUTHENTICATION REQUEST' message).

The mobile communication device <NUM> responds, at S1022, by authenticating the network using the received AUTN, and by deriving appropriate security related parameters (IK, CK, RES etc.) using the stored permanent security key 'K' and the received AUTN and RAND parameters (and any other parameters where necessary - e.g. AMF for the determination of XMAC). Assuming the authentication is successful, the mobile communication device <NUM> sends the calculated value of RES to the MME <NUM> at S1024 (e.g. in an 'NAS AUTHENTICATION RESPONSE' message).

The MME <NUM> checks the received RES value against XRES at S1026, resets the downlink NAS count, and derives values of KASME, KeNB, KNASint and KNASenc. The MME <NUM> then initiates NAS signalling security between the MME <NUM> and the mobile communication device <NUM>, at S1028, by sending an NAS SECURITY MODE COMMAND message informing the mobile communication device <NUM> of the respective algorithms to use for integrity protection and (de)ciphering.

The mobile communication device <NUM> responds, at S1030, by deriving values of KASME, KeNB, KNASint and KNASenc and then, at S1032, by sending a response message informing the MME <NUM> that NAS signalling security initialisation is complete.

The method then proceeds by initiating security context setup at the macro base station <NUM>-<NUM>, by sending an S1 application (S1-AP) message (e.g. an 'S1-AP INITIAL CONTEXT SETUP REQUEST' message) to the macro base station <NUM>-<NUM> at S1034. The S1-AP message includes the derived value of KeNB and details of the security capabilities for the mobile communication device <NUM>.

The macro base station <NUM>-<NUM> initiates security context setup at the pico base station <NUM>-<NUM>, <NUM>-<NUM>, by sending an X2 application (X2-AP) message (e.g. a new 'X2-AP CONTEXT SETUP' message) to the pico base station <NUM>-<NUM>, <NUM>-<NUM> at S1036. The X2-AP message includes the derived value of KeNB and details of the security capabilities for the mobile communication device <NUM>.

The pico base station <NUM>-<NUM>, <NUM>-<NUM> then derives, at S1038, the security parameters) required for U-plane enciphering/deciphering (e.g. KUPenc) from the received KeNB received from the macro base station <NUM>-<NUM>. Similarly, the macro base station <NUM>-<NUM> derives, at S1040, the security parameter(s) required for C-plane enciphering/deciphering (e.g. KRRCint and YRRCenc) from the KeNB received from the MME <NUM>.

At S1042, assuming security context setup at the pico base station <NUM>-<NUM>, <NUM>-<NUM> is successful, the pico base station <NUM>-<NUM>, <NUM>-<NUM> confirms this to the macro base station <NUM>-<NUM> in an appropriate X2-AP message (e.g. an 'X2-AP CONTEXT SETUP RESPONSE' message).

The macro base station <NUM>-<NUM> then initiates, at S1044, an RRC (and user plane) security context setup at the mobile communication device <NUM> using RRC signalling (e.g. an 'RRC SECURITY MODE COMMAND' message) which includes information identifying the algorithms used for integrity protection and/or ciphering and an information indicating that the U-plane and C-plane are split (e.g. in the form of a dedicated information element (IE), a modified IE, or re-use of an existing IE).

The mobile communication device <NUM> responds, at S1046, by initialising the RRC security context for communication with the macro base station <NUM>-<NUM> by deriving the values of KRRCint, KRRCenc from the previously calculated value of KeNB for use with control signalling from the macro base station <NUM>-<NUM>. The mobile communication device <NUM> also initialises the U-plane security context for communication with the pico base station <NUM>-<NUM>, <NUM>-<NUM> by deriving the value of KUPenc from KeNB for use with user plane signalling to/from the pico base station <NUM>-<NUM>, <NUM>-<NUM>.

Assuming the security context setup is successful the mobile communication device <NUM> confirms this, at S1032, by sending an appropriate response message to the macro base station <NUM>-<NUM> (e.g. an 'RRC SECURITY MODE COMPLETE' message) at S <NUM>.

The macro base station <NUM>-<NUM> confirms, at S1052, successful security context setup to the MME <NUM> in an appropriate S1-AP message (e.g. an 'Sl-AP INITIAL CONTEXT SETUP REQUEST' message).

Once the various security contexts (NAS and AS) have been initialised successfully on the various devices, the control and user signalling connections can be setup at S1054 and the mobile communication device can commence communication in which control plane signalling (S1056) is provided by the macro base station <NUM>-<NUM> and U-plane signalling is provided via the pico base station <NUM>-<NUM>, <NUM>-<NUM> (S1058).

Advantageously, therefore, this method provides another efficient way of providing appropriate communication security where different base stations are responsible for U-plane signalling and C-plane signalling respectively. The user device is able to maintain an appropriate security context for both the U-Plane and the C-plane thereby allowing it to encipher/decipher user data and control data correctly and to keep track of the security parameters (keys) used in the different base stations.

Informing the mobile communication device of the C-plane/U-plane split in this way provides an efficient way of ensuring that the mobile communication device has the information required to establish that derivation of the user plane security parameter (KUPenc) is required for communication with the pico cell.

This approach has the benefit over the first method described herein that it avoids S1 signalling duplication and hence reduces S1 signalling overhead. The first method has the benefit, however, that it avoids the need for the modification of base station to base station signalling (over the X2 or possibly a new interface) and the associated increase X2-AP the complexity.

<FIG> shows a simplified timing diagram illustrating operation of the telecommunication system of <FIG> in the performance of a third security procedure in which appropriate security parameters, and in particular appropriate values of KUPenc, are generated at the macro base station <NUM>-<NUM> in response to signalling from the MME <NUM>, and are forwarded to the pico base station <NUM>-<NUM>, <NUM>-<NUM> over the X2 interface.

At the start of the security procedure illustrated in <FIG>, a mobile communication device <NUM> wishing to initiate communication in the pico cell <NUM>-<NUM>, <NUM>-<NUM> sends a non access stratum (NAS) message requesting attachment (e.g. an 'NAS ATTACH REQUEST' message) to the MME <NUM> (transparently via the macro base stations <NUM>-<NUM>) at S1110 including information identifying the SIM card <NUM> of the mobile communication device <NUM> (e.g. the 'international mobile subscriber identity (IMSI)').

The MME <NUM> responds to this request, at S1112, by sending a message requesting authentication and including information identifying the SIM card <NUM> to the HSS <NUM> (e.g. and 'AUTHENTICATION DATA REQUEST' message). The AuC function <NUM> of the HSS <NUM> derives RAND, XRES, CK, IK, AUTN and combines them to form an authentication vector for the SIM card <NUM> (AV = RAND ∥ XRES ∥ CK ∥ IK ∥ AUTN) at S1114 and sends the generated AV to the MME <NUM> at S1116 (e.g. in a 'AUTHENTICATION DATA RESPONSE' message).

The MME <NUM> retrieves IK, CK, XRES, RAND and AUTN from the AV at S1118 and sends the AUTN and RAND parameters to the mobile communication device <NUM> using NAS signalling at S1120 (e.g. in an 'NAS AUTHENTICATION REQUEST' message).

The mobile communication device <NUM> responds, at S1122, by authenticating the network using the received AUTN, and by deriving appropriate security related parameters (IK, CK, RES etc.) using the stored permanent security key 'K' and the received AUTN and RAND parameters (and any other parameters where necessary - e.g. AMF for the determination of XMAC). Assuming the authentication is successful, the mobile communication device <NUM> sends the calculated value of RES to the MME <NUM> at S1124 (e.g. in an 'NAS AUTHENTICATION RESPONSE' message).

The MME <NUM> checks the received RES value against XRES at S1126, resets the downlink NAS count, and derives values of KASME, KeNB, KNASint and KNASenc. The MME <NUM> then initiates NAS signalling security between the MME <NUM> and the mobile communication device <NUM>, at S1128, by sending an NAS SECURITY MODE COMMAND message informing the mobile communication device <NUM> of the respective algorithms to use for integrity protection and (de)ciphering.

The mobile communication device <NUM> responds, at S1130, by deriving values of KASME, KeNB, KNASint and KNAenc and then, at S1132, by sending a response message informing the MME <NUM> that NAS signalling security initialisation is complete.

The method then proceeds by initiating security context setup at the macro base station <NUM>-<NUM>, by sending an S1 application (S1-AP) message (e.g. an 'S1-AP INITIAL CONTEXT SETUP REQUEST' message) to the macro base station <NUM>-<NUM> at S1134. The S1-AP message includes the derived value of KeNB and details of the security capabilities for the mobile communication device <NUM>.

The macro base station <NUM>-<NUM> derives, at S1140, the security parameter(s) required for C-plane protection (e.g. KRRCint and KRRCenc) and for U-plane protection (e.g. KUPenc) from the KeNB received from the MME <NUM>.

The macro base station <NUM>-<NUM> initiates security context setup at the pico base station <NUM>-<NUM>, <NUM>-<NUM>, by sending an X2 application (X2-AP) message (e.g. a new 'X2-AP CONTEXT SETUP' message) to the pico base station <NUM>-<NUM>, <NUM>-<NUM> at S1136. The X2-AP message includes the derived value of KUPenc and details of the security capabilities for the mobile communication device <NUM>.

The pico base station <NUM>-<NUM>, <NUM>-<NUM>, on receipt of KUPenc from the macro base station <NUM>-<NUM>, and assuming security context setup at the pico base station <NUM>-<NUM>, <NUM>-<NUM> is successfol, confirms this to the macro base station <NUM>-<NUM> in an appropriate X2-AP message (e.g. an 'X2-AP CONTEXT SETUP RESPONSE' message) at S1142.

The macro base station <NUM>-<NUM> then initiates, at S1144, an RRC (and user plane) security context setup at the mobile communication device <NUM> using RRC signalling (e.g. an 'RRC SECURITY MODE COMMAND' message) which includes information identifying the algorithms used for integrity protection and/or ciphering and an information indicating that the U-plane and C-plane are split (e.g. in the form of a dedicated information element (IE), a modified IE, or re-use of an existing IE).

The mobile communication device <NUM> responds, at S1146, by initialising the RRC security context for communication with the macro base station <NUM>-<NUM> by deriving the values of KRRCint, KRRCenc from the previously calculated value of KeNB for use with control signalling from the macro base station <NUM>-<NUM>. The mobile communication device <NUM> also initialises the U-plane security context for communication with the pico base station <NUM>-<NUM>, <NUM>-<NUM> by deriving the value of KUPenc from KeNB for use with user plane signalling io/from the pico base station <NUM>-<NUM>, <NUM>-<NUM>.

Assuming the security context setup is successful the mobile communication device <NUM> confirms this, at S1132, by sending an appropriate response message to the macro base station <NUM>-<NUM> (e.g. an 'RRC SECURITY MODE COMPLETE' message) at S1150.

The macro base station <NUM>-<NUM> confirms, at S1152, successful security context setup to the MME <NUM> in an appropriate S1-AP message (e.g. an 'S1-AP INITIAL CONTEXT SETUP REQUEST' message).

Once the various security contexts (NAS and AS) have been initialised successfully on the various devices, the control and user signalling connections can be setup at S1154 and the mobile communication device can commence communication in which control plane signalling (SI <NUM>) is provided by the macro base station <NUM>-<NUM> and U-plane signalling is provided via the pico base station <NUM>-<NUM>, <NUM>-<NUM> (S1158).

This approach has the benefit over the other methods described herein that the pico base station does not have to derive KUPenc itself thereby simplifying it further which is in keeping with the general desire to keep its complexity to a minimum. However, other methods described herein have the security benefit, over this method, that the value of KUPenc (which is also used by the mobile telephone <NUM>) is not transmitted and therefore cannot be as easily compromised (e.g. by 'eavesdropping'), which can lead to user data security being compromised. If KeNB, which is transferred in other methods, is compromised it is not a trivial matter to derive KUPenc from it because of the need to know other security keys to do so.

It will be appreciated that, currently, the transfer of KeNB and KUPenc between base stations is not supported in any circumstances. <FIG> shows a simplified timing diagram illustrating operation of the telecommunication system of <FIG> in the performance of a fourth security procedure in which, rather than transfer a parameter for which between base stations transfer is not currently supported, a security parameter (KeNB*) is transferred for which transfer between base stations is currently supported, albeit in limited circumstances. Specifically, currently the transfer of KeNB* between base stations is supported during handover. Accordingly, this fourth security procedure extends the circumstances in which KeNB* is supported to security context setup in the case of a U-plane/C-plane split.

Specifically, KeNB* is generated at the macro base station <NUM>-<NUM> (as it would be during a handover) in response to signalling from the MME <NUM>, and is forwarded to the pico base station <NUM>-<NUM>, <NUM>-<NUM> over the X2 interface.

At the start of the security procedure illustrated in <FIG>, a mobile communication device <NUM> wishing to initiate communication in the pico cell <NUM>-<NUM>, <NUM>-<NUM> sends a non access stratum (NAS) message requesting attachment (e.g. an 'NAS ATTACH REQUEST' message) to the MME <NUM> (transparently via the macro base stations <NUM>-<NUM>) at S1210 including information identifying the SIM card <NUM> of the mobile communication device <NUM> (eg. the 'international mobile subscriber identity (IMSI)').

The MME <NUM> responds to this request, at S1212, by sending a message requesting authentication and including information identifying the SIM card <NUM> to the HSS <NUM> (e.g. and 'AUTHENTICATION DATA REQUEST' message). The AuC function <NUM> of the HSS <NUM> derives RAND, XRES, CK, IK, AUTN and combines them to form an authentication vector for the SIM card <NUM> (AV = RAND ∥ XRES ∥ CK ∥ IK ∥ AUTN) at S1214 and sends the generated AV to the MME <NUM> at S1216 (e.g. in a 'AUTHENTICATION DATA RESPONSE' message).

The MME <NUM> retrieves IK, CK, XRES, RAND and AUTN from the AV at S1218 and sends the AUTN and RAND parameters to the mobile communication device <NUM> using NAS signalling at S1220 (e.g. in an 'NAS AUTHENTICATION REQUEST' message).

The mobile communication device <NUM> responds, at S1222, by authenticating the network using the received AUTN, and by deriving appropriate security related parameters (IK, CK, RES etc.) using the stored permanent security key 'K' and the received AUTN and RAND parameters (and any other parameters where necessary - e.g. AMF for the determination of XM AC). Assuming the authentication is successful, the mobile communication device <NUM> sends the calculated value of RES to the MME <NUM> at S1214 (e.g. in an 'NAS AUTHENTICATION RESPONSE' message).

The MME <NUM> checks the received RES value against XRES at S1226, resets the downlink NAS count, and derives values of KASME, KeNB, KNASint and KNASenc. The MME <NUM> then initiates NAS signalling security between the MME <NUM> and the mobile communication device <NUM>, at S1228, by sending an NAS SECURITY MODE COMMAND message informing the mobile communication device <NUM> of the respective algorithms to use for integrity protection and (de)ciphering.

The mobile communication device <NUM> responds, at S1230, by deriving values of KASME, KeNB, KNASint and KNASenc and then, at S1232, by sending a response message informing the MME <NUM> that NAS signalling security initialisation is complete.

The method then proceeds by initiating security context setup at the macro base station <NUM>-<NUM>, by sending an S1 application (Sl-AP) message (e.g. an 'Sl-AP INITIAL CONTEXT SETUP REQUEST' message) to the macro base station <NUM>-<NUM> at S1234. The S1-AP message includes the derived value of KeNB and details of the security capabilities for the mobile communication device <NUM>.

The macro base station <NUM>-<NUM> initiates security context setup at the pico base station <NUM>-<NUM>, <NUM>-<NUM>. by sending an X2 application (X2-AP) message (e.g. a new 'X2-AP CONTEXT SETUP' message) to the pico base station <NUM>-<NUM>,<NUM>-<NUM> at S1236. The X2-AP message includes a value of KeNB*, derived from the value of KeNB from the MME <NUM> (and possibly a value of NH), and details of the security capabilities for the mobile communication device <NUM>. The value of KeNB* is effectively derived in the same way as for handover although it may be given another name (e.g. KeN8**) to allow it to be distinguished from the handover case.

The pico base station <NUM>-<NUM>, <NUM>-<NUM>, on receipt of KeNB* from the macro base station <NUM>-<NUM>, derives, at S1238, the security parameter(s) required for U-plane enciphering/deciphering. Specifically, the pico base station <NUM>-<NUM>, <NUM>-<NUM> derives a value of KeNB (effectively a 'pico' KeNB) from the received KeNB* and a value of KUPenc from the derived pico KeNB. Similarly, the macro base station <NUM>-<NUM> derives, at S1240, the security parameters) required forC-plane enciphering/deciphering (e.g. KRRCint and KRRCenc) from the KeNB received from the MME <NUM>.

Assuming that security context setup is successful, the pico base station <NUM>-<NUM>, <NUM>-<NUM> confirms this to the macro base station <NUM>-<NUM> in an appropriate X2-AP message (e.g. an 'X2-AP CONTEXT SETUP RESPONSE' message) at S1242.

The macro base station <NUM>-<NUM> then initiates, at S1244, an RRC (and user plane) security context setup at the mobile communication device <NUM> using RRC signalling (e.g. an `RRC SECURITY MODE COMMAND' message) which includes information identifying the algorithms used for integrity protection and/or ciphering and an information indicating that the U-plane and C-plane are split (e.g. in the form of a dedicated information element (IE), a modified IE, or re-use of an existing IE).

The mobile communication device <NUM> responds, at S1246, by initialising the RRC security context for communication with the macro base station <NUM>-<NUM> by deriving the values of KRRCint, KRRCenc from the previously calculated value of ('macro') KeNB for use with control signalling from the macro base station <NUM>-<NUM>. The mobile communication device <NUM> also initialises the U-plane security context for communication with the pico base station <NUM>-<NUM>,<NUM>-<NUM> by deriving the value of KeNB* and hence the 'pico' KeNB from which the correct KUPenc can be found for use with user plane signalling to/from the pico base station <NUM>-<NUM>, <NUM>-<NUM>.

Assuming the security context setup is successful the mobile communication device <NUM> confirms this, at S1232, by sending an appropriate response message to the macro base station <NUM>-<NUM> (e.g. an 'RRC SECURITY MODE COMPLETE' message) at S1250.

The macro base station <NUM>-<NUM> confirms, at S1252, successful security context setup to the MME <NUM> in an appropriate S1-AP message (e.g. an 'Sl-AP INITIAL CONTEXT SETUP REQUEST' message).

Once the various security contexts (NAS and AS) have been initialised successfully on the various devices, the control and user signalling connections can be setup at S1254 and the mobile communication device can commence communication in which control plane signalling (S1256) is provided by the macro base station <NUM>-<NUM> and U-plane signalling is provided via the pico base station <NUM>-<NUM>, <NUM>-<NUM> (S1258).

This approach has the benefit over the other methods described herein that it does not require security parameters to be transferred between base stations for which such transfer is not currently supported. However, other methods described herein have the benefit added complexity in order to properly derive KeNB* in non-handover scenarios.

<FIG> shows a simplified diagram illustrating operation of the telecommunication system of <FIG> in the performance of a further security procedure in which, rather than run a single AKA procedure for the macro base station <NUM>-<NUM> during which appropriate security parameters are passed to and/or derived at the pico base station <NUM>-<NUM>, <NUM>-<NUM> (e.g. as illustrated in each of <FIG>), separate AKA procedures are run for the macro base station <NUM>-<NUM> and pico base station <NUM>-<NUM>, <NUM>-<NUM>.

As seen in <FIG>, the procedure involves, at S1313, AS security procedures being run for communications between the macro base station <NUM>-<NUM> and the mobile communication device <NUM>. During this procedure the mobile communication device <NUM>, the macro base station <NUM>-<NUM> and the MME <NUM>, each generates and maintains its own respective security context S1312-<NUM>, S1312-<NUM>, S1312-<NUM> for the C-plane signalling between the macro base station <NUM>-<NUM> and the mobile communication device <NUM>. The generation of each security context includes the derivation of appropriate macro /C-plane specific security keys (e.g. KRRCint, KRRCenc etc. as described previously).

The procedure also involves, at S1314, AS security procedures being run for communications between the pico base station <NUM>-<NUM>, <NUM>-<NUM> and the mobile communication device <NUM>. During this procedure the mobile communication device <NUM>, the pico base station <NUM>-<NUM>, <NUM>-<NUM> and the MME <NUM>, each generates and maintains its own respective security context S1316-<NUM>, S1316-<NUM>, S1316-<NUM> for the U-plane signalling between the pico base station <NUM>-<NUM>,<NUM>-<NUM> and the mobile communication device <NUM>. The generation of each security context includes the derivation of appropriate pico / U-plane specific security keys (e.g. KUPenc etc. as described previously).

It will be appreciated that the procedures of S1313 and S1314 may be run sequentially in any appropriate order or in parallel.

It can be seen, therefore, that as a result of the procedure in <FIG>, the MME <NUM> and the mobile communication device <NUM> each maintain two active security contexts. In order to support the presence of two active security contexts, handover signalling (for handover from macro+pico to another macro+pico) in this example is modified to allow the exchange of two security contexts. For example, the signalling might be modified to allow two KeNB* (based on each KeNB) to be generated and transferred, different security algorithm if different algorithms are used) to be notified, and the signalling of other general information relating to the two different security contexts.

Further, in order to support the dual AKA procedure of <FIG>, RRC and NAS messages are modified appropriately. For example, the RRC SECURITY MODE COMMAND is modified to include information identifying the security algorithm for each AKA procedure and the NAS security messages are modified to include duplicates of the security parameters where necessary.

The mobile communication device <NUM> maintains two ciphering instances (e.g. in the PDCP layer) each with its own set of security keys - one for control plane ciphering and one for user plane ciphering.

It will be appreciated that, a similar procedure could be applied, if necessary, for generating separate NAS contexts for the pico and macro base stations AKA procedure being run. The procedure would be similar to that described with reference to <FIG> but, during this procedure, the mobile communication device <NUM> and the MME <NUM> would each generate and maintain an NAS security context for the macro base station <NUM>-<NUM> and a separate NAS security context for the pico base station <NUM>-<NUM>, <NUM>-<NUM>. The generation and transmission of the NAS security contexts might include the derivation and transmission of duplicate security parameters (one copy for each context) where appropriate.

Regardless of which of the above procedures is implemented, in order to avoid potential security issues associated with PDCP COUNT rollover, the pico base station <NUM>-<NUM>, <NUM>-<NUM>, is operable to inform the macro base station <NUM>-<NUM>, when PDCP COUNT rollover has occurred, or is about to occur, using a new X2-AP message including an information element indicating the KUPenc requires changing (e.g. a 'KUPenc key change" IE). In response to receiving this message the macro base station <NUM>-<NUM> initiates an inter-cell hand over which will, ultimately, result in communication continuing in the current pico/macro cell pair but using a different value of KUPenc for user plane ciphering.

Similarly, where other security parameters, such as KeNB, are changed dynamically and provided by the MME <NUM> to the macro base station <NUM>-<NUM> (in accordance with current procedures) the macro. base station <NUM>-<NUM> is configured to forward the new KeNB to the pico base station <NUM>-<NUM>, <NUM>-<NUM> when the procedure shown in <FIG> is implemented. Where the procedure shown in <FIG> <FIG> is implemented the macro base station <NUM>-<NUM> is configured to forward a new KeNB* to the pico base station <NUM>-<NUM>, <NUM>-<NUM>. Where the procedure shown in <FIG> is implemented the macro base station <NUM>-<NUM> is configured to forward a new KUPenc to the pico base station <NUM>-<NUM>, <NUM>-<NUM>. Where the procedure shown in <FIG> is implemented the MME <NUM> is configured to forward the new KeNB to the pico base station <NUM>-<NUM>, <NUM>-<NUM> in a duplicate S <NUM> message.

A detailed embodiment and has been described above. As those skilled in the art will appreciate, a number of modifications and alternatives can be made to the above embodiment and variations whilst still benefiting from the inventions embodied therein.

In the above embodiments one macro cell <NUM> and two pico cells <NUM> are described; the pico cells are operated using component carriers having the same frequency band (F2) and the macro cell is operated using a component carrier having a different frequency band (F1) and. It will be appreciated that in a deployed system there may be any number of pico cells each of which may operate on a component carrier having a different respective frequency band and could potentially operate on a component carrier having the same frequency band as the macro cell.

In the above embodiments the macro, and pico base stations may have the same security capabilities. If different security capabilities are supported, however, the mobile communication device is informed (e.g. in the 'RRC SECURITY MODE COMMAND' or other similar message) of the appropriate capabilities for each base station thereby allowing the mobile communication device using the correct algorithms.

In the above examples the ciphering key may be transferred from the macro base station or , according to the invention, derived by the pico base station based on security information received from the macro base station. It will be appreciated that the other parameters required for U-plane ciphering/deciphering may be derived as follows: COUNT may be maintained by the pico base station at the PDCP level; BEARER identity may either be transferred from the macro base station or may be selected by the pico base station; and DIRECTION may be set either at the macro or the pico base station. Appropriate synchronisation may be provided between the macro and pico base stations to ensure, for example, that the correct bearer identity is known both at the macro and the pico base station. In the event that both base stations have the information and capability to decide a particular parameter (e.g. 'DIRECTION') only one will make the decision and this will be informed to the other base station.

The detailed description provided for the embodiments of <FIG> relate to procedures for initial connection establishment. It will be appreciated that a similar approach may be used when a decision to split the U-plane and C-plane is taken at a later stage. For example, where the mobile communication device is engaged in a type of user plane communication that is provided by the macro cell (say voice over IP 'VoIP) and then starts a different form of user plane communication which is provided by the pico cell (say a web browsing, session) a C-plane/U-plane split may need to be initiated. In this case an S1 UE CONTEXT MODIFICATION (with a corresponding response) may be used to provide the pico base with appropriate security information (analogous to the process of <FIG>). Similarly, a new X2 message (with a corresponding response) may be provided (e.g. an X2-AP CONTEXT MODIFICATION message) (analogous to the processes of any of <FIG>). Moreover, the RRC signalling used to provide the mobile communication device with appropriate security information (where the security parameters have changed) and/or an indication that a split has occurred may be a message such as an RRC Reconfiguration message (with a corresponding response).

Whilst specific new X2-AP messages (X2-AP CONTEXT SETUP and 'X2-AP CONTEXT SETUP RESPONSE) has been described it will be appreciated that any suitable X2-AP message may be used including the re-use of an existing message with the addition of appropriate information elements.

Further although information such as KeNB is described as being exchanged over an X2 interface between the pico and macro base stations, it will be appreciated that the interface between the base stations may be a new dedicated interface (e.g. an 'X3' interface).

Although the RRC SECURITY MODE COMMAND message has been described as including infonnation indicating that the U-plane and C-plane are split (e.g. in the form of a dedicated information element (IE), a modified IE, or re-use of an existing IE) an indication of the C-plane / U-plane split could (alternatively or additionally) be notified to the mobile communication device in an NAS message such as a NAS SECURITY MODE COMMAND message.

The information elements included in such a message will typically include, for example:.

Referring to the timing diagrams, it will be appreciated that in many cases the messaging may not need to follow the specific order shown but may follow any logical order.

Referring to <FIG>, by way of example, it will be appreciated by those skilled in the art that although the S1-AP messages sent to the pico and macro base stations to initiate AS security context setup (S934, S936), and the resulting U-plane and C-plane key derivation (S938, S940), are shown occurring in a particular order (for the purposes of clear illustration) they may occur in any appropriate order or, where appropriate, in parallel. Specifically, for example, initiation of security context initialisation and key derivation at the pico base station <NUM>-<NUM>, <NUM>-<NUM> (S934, S938) may occur wholly before, wholly after, or substantially in parallel with the corresponding initiation of security context initialisation and key derivation (S936, S940) at the macro base station <NUM>-<NUM>. Similarly, the associated S1-AP response messages may be sent at any appropriate juncture after successful security context initialisation.

Further whilst, in <FIG>, a duplicate of only one Sl-AP message (S1-AP INITIAL CONTEXT SETUP) is shown as being sent to the pico base station to provide appropriate security parameters. It will be appreciated that a duplicate of any suitable S1-AP message carrying security parameters may be provided to the pico base station including, for example, a UE CONTEXT MODIFICATION message or the like.

In relation to the dual AKA procedure described with reference to <FIG>, it will be appreciated that there may be a scenario in which the pico base station is not an E-UTRAN base station but instead connects to a non-EUTRAN or even a non-3GPP network. In this case the non-3GPP network may perform its own security procedure for the user plane whilst the macro base station still performs a 3GPP security procedure thereby resulting in a non-3GPP security context for the user plane and a 3GPP security context for the control plane.

It will be appreciated that although the communication system <NUM> is described in terms of base stations <NUM> operating as macro or pico base stations, the same principles may be applied to base stations operating as femto base stations, relay nodes providing elements of base station functionality, or other such communication nodes.

In the above embodiments, a mobile telephone based telecommunications system was described. As those skilled in the art will appreciate, the signalling techniques described in the present application can be employed in other communications system. Other communications nodes or devices may include user devices such as, for example, personal digital assistants, laptop computers, web browsers, etc. As those skilled in the art will appreciate, it is not essential that the above described relay system be used for mobile communications devices. The system can be used to extend the coverage of base stations in a network having one or more fixed computing devices as well as or instead of the mobile communicating devices.

In the embodiments described above, the base stations <NUM> and mobile communication devices <NUM> each include transceiver circuitry. Typically, this circuitry will be formed by dedicated hardware circuits. However, in some embodiments, part of the transceiver circuitry may be implemented as software run by the corresponding controller.

In the above embodiments, a number of software modules were described. As those skilled in the art will appreciate, the software modules may be provided in compiled or un-compiled form and may be supplied to the base station or the relay station as a signal over a computer network, or on a recording medium. Further, the functionality performed by part or all of this software may be performed using one or more dedicated hardware circuits.

Claim 1:
A user equipment (<NUM>) configured for connectivity involving simultaneous connection to a master communication apparatus (<NUM>-<NUM>) and to a secondary communication apparatus (<NUM>-<NUM>), the user equipment (<NUM>) comprising:
means for deriving a security key of the master communication apparatus (<NUM>-<NUM>);
means for deriving, using the derived security key of the master communication apparatus, at least one control plane security key for control plane communication between the user equipment (<NUM>) and the master communication apparatus (<NUM>-<NUM>);
means for deriving, using the derived security key of the master communication apparatus (<NUM>-<NUM>), a security key of the secondary communication apparatus (<NUM>-<NUM>), while establishing the connectivity;
means for deriving, using the derived security key of the secondary communication apparatus, a user plane security key; and
means for using the derived user plane security key for user plane communication between the user equipment (<NUM>) and the secondary communication apparatus (<NUM>-<NUM>).