WO2015067308A1

WO2015067308A1 - Methods, apparatus and computer programs for registering a user equipment in a heterogeneous network

Info

Publication number: WO2015067308A1
Application number: PCT/EP2013/073149
Authority: WO
Inventors: Srinivasan Selvaganapathy; Dario Serafino Tonesi; Xiang Xu
Original assignee: Nokia Solutions And Networks Oy
Priority date: 2013-11-06
Filing date: 2013-11-06
Publication date: 2015-05-14

Abstract

A method comprising: sending to a user equipment, from a first apparatus controlling a cell, an identifier for identifying a group of cells comprising said cell, wherein said cell comprises a small cell and said group of cells comprises at least one large cell; sending to said user equipment information that said cell supports a registration procedure for registration of said user equipment; using said registration procedure for connection of said user equipment to said first apparatus; and sending a request for registration of said user equipment to a second apparatus, said request comprising said identifier for identifying a group of cells and an identifier of said user equipment.

Description

Title

METHODS, APPARATUS AND COMPUTER PROGRAMS FOR REGISTERING A USER EQUIPMENT

IN A HETEROGENEOUS NETWORK The present invention relates to heterogeneous networks, and more particularly to registration of a user equipment in a heterogeneous network.

A communication system can be seen as a facility that enables communication sessions between two or more nodes such as fixed or mobile devices, machine-type terminals, access nodes such as base stations, servers and so on. A communication system and compatible communicating entities typically operate in accordance with a given standard or specification which sets out what the various entities associated with the system are permitted to do and how that should be achieved. For example, the standards, specifications and related protocols can define the manner how devices shall communicate, how various aspects of communications shall be implemented and how devices for use in the system shall be configured.

A user can access the communication system by means of an appropriate communication device. A communication device of a user is often referred to as user equipment (UE) or terminal. A communication device is provided with an appropriate signal receiving and transmitting arrangement for enabling communications with other parties. Typically a device such as a user equipment is used for enabling receiving and transmission of communications such as speech and content data.

Communications can be carried on wireless carriers. Examples of wireless systems include public land mobile networks (PLMN) such as cellular networks, satellite based communication systems and different wireless local networks, for example wireless local area networks (WLAN). In wireless systems a communication device provides a transceiver station that can communicate with another communication device such as e.g. a base station of an access network and/or another user equipment. The two directions of communications between a base station and communication devices of users have been conventionally referred to as downlink and uplink. Downlink (DL) can be understood as the direction from the base station to the communication device and uplink (UL) the direction from the communication device to the base station. Some systems may have a number of small-cells overlying larger or macro-cells. The small- cells may share the same carrier with the macro-cell or use different carriers.

A user equipment may selectively connect to one or more of the small-cells and macro-cells.

In a first aspect there is provided a method comprising: sending to a user equipment, from a first apparatus controlling a cell, an identifier for identifying a group of cells comprising said cell, wherein said cell comprises a small cell and said group of cells comprises at least one large cell; sending to said user equipment information that said cell supports a registration procedure for registration of said user equipment; using said registration procedure for connection of said user equipment to said first apparatus; and sending a request for registration of said user equipment to a second apparatus, said request comprising said identifier for identifying a group of cells and an identifier of said user equipment.

Preferably said method comprises broadcasting said identifier for identifying a group of cells. Preferably said method comprises distributing a received paging message to said identified group of cells.

Preferably said paging message is received from said second apparatus

Preferably said method comprises using a security-token during said registration procedure.

Preferably said identifier for identifying a group of cells comprises an E-UTRAN Registration Area code.

Preferably said method comprises receiving a radio resource connection request from said user equipment.

Preferably said method comprises sending to said user equipment a connection release message for enabling said user equipment to enter an idle state. Preferably said identifier of said user equipment comprises a temporary identifier.

Preferably said temporary identifier comprises a temporary mobile subscriber identity.

Preferably said registration procedure comprises a pre-defined registration procedure.

Preferably said first apparatus comprises a base station.

Preferably said second apparatus comprises one of a gateway node and a mobility management entity. In a second aspect there is provided a computer program comprising computer executable instructions which when run on one or more processors perform the method of the first aspect. In a third aspect there is provided an apparatus comprising: at least one processor; and at least one memory including computer program code; the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus at least to: send to a user equipment an identifier for identifying a group of cells comprising a cell controlled by said apparatus, wherein said cell comprises a small cell and said group of cells comprises at least one large cell; send to said user equipment information that said cell supports a registration procedure for registration of said user equipment; use said registration procedure for connection of said user equipment to said apparatus; and send a request for registration of said user equipment to a second apparatus, said request comprising said identifier for identifying a group of cells and an identifier of said user equipment.

Preferably said apparatus is configured to broadcast said identifier for identifying a group of cells.

Preferably said apparatus is configured to distribute a received paging message to said identified group of cells. Preferably said apparatus is configured to receive said paging message from said second apparatus.

Preferably said apparatus is configured to use a security-token during said registration procedure.

Preferably said apparatus is configured to receive a radio resource connection request from said user equipment.

Preferably said apparatus is configured to send to said user equipment a connection release message for enabling said user equipment to enter an idle state.

Preferably said identifier of said user equipment comprises a temporary identifier. Preferably said temporary identifier comprises a temporary mobile subscriber identity.

Preferably said apparatus comprises a base station. Preferably said second apparatus comprises one of a gateway node and a mobility management entity.

In a fourth aspect there is provided an apparatus comprising means for sending to a user equipment an identifier for identifying a group of cells comprising a cell controlled by said apparatus, wherein said cell comprises a small cell and said group of cells comprises at least one large cell; means for sending to said user equipment information that said cell supports a registration procedure for registration of said user equipment; means for using said registration procedure for connection of said user equipment to said apparatus; and means for sending a request for registration of said user equipment to a second apparatus, said request comprising said identifier for identifying a group of cells and an identifier of said user equipment.

Preferably said apparatus comprises means for using a security-token during said registration procedure.

Preferably said apparatus is configured to send to said user equipment a connection release message for enabling said user equipment to enter an idle state. Preferably said identifier of said user equipment comprises a temporary identifier. Preferably said temporary identifier comprises a temporary mobile subscriber identity.

In a fifth aspect there is provided a method comprising: receiving information from at least one cell, said information comprising an identifier of a group of cells comprising said at least one cell, wherein said at least one cell comprises a small cell and said group of cells comprises at least one large cell, and said information comprises an identifier of a user equipment connecting with said at least one cell; storing an association between said identifier of said group of cells and said identifier of said user equipment; and using said association to determine whether to send at least one message to said at least one cell.

Preferably said information is comprised in a user equipment registration request. Preferably said identifier of a user equipment comprises a temporary identifier.

Preferably said identifier of a group of cells comprises an E-UTRAN Registration Area code.

Preferably said using said association to determine whether to send at least one message to said at least one cell comprises filtering out messages addressed to said large cell.

Preferably said message comprises a paging message. In a sixth aspect there is provided a computer program comprising computer executable instructions which when run on one or more processors perform the method of the fifth aspect.

In a seventh aspect there is provided an apparatus comprising: at least one processor; and at least one memory including computer program code; the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus at least to: receive information from at least one cell, said information comprising an identifier of a group of cells comprising said at least one cell, wherein said at least one cell comprises a small cell and said group of cells comprises at least one large cell, and said information comprises an identifier of a user equipment connecting with said at least one cell; store an association between said identifier of said group of cells and said identifier of said user equipment; and use said association to determine whether to send at least one message to said at least one cell.

Preferably said apparatus is configured to receive said information in a user equipment registration request. Preferably said identifier of a user equipment comprises a temporary identifier.

Preferably said use said association to determine whether to send at least one message to said at least one cell comprises filtering out messages addressed to said large cell.

Preferably said at least one message comprises a paging message. In an eighth aspect there is provided an apparatus comprising means for receiving information from at least one cell, said information comprising an identifier of a group of cells comprising said at least one cell, wherein said at least one cell comprises a small cell and said group of cells comprises at least one large cell, and said information comprises an identifier of a user equipment connecting with said at least one cell; means for storing an association between said identifier of said group of cells and said identifier of said user equipment; and means for using said association to determine whether to send at least one message to said at least one cell.

Preferably said at least one message comprises a paging message. In a ninth aspect there is provided a method comprising: receiving at an apparatus, from a cell, an identifier of a group of cells comprising said cell, wherein said cell comprises a small cell and said group of cells comprises at least one large cell; receiving information that said cell supports a registration procedure for registration of said user equipment; determining whether to send a registration request to said cell in dependence on said identifier of a group of cells; and using said registration procedure for connecting with said cell when it is determined to send said registration request.

Preferably said determining whether to send a registration request comprises determining whether said apparatus is already registered with said group of cells. Preferably said method comprises entering an idle mode in response to receiving a message from said cell.

Preferably said message from said cell comprises a radio resource connection release message.

Preferably said registration request comprises a radio resource control connection request. Preferably said method comprises sending a temporary identifier of said apparatus to said cell.

Preferably said apparatus comprises a user equipment.

In a tenth aspect there is provided a computer program comprising computer executable instructions which when run on one or more processors perform the method of the ninth aspect.

In an eleventh aspect there is provided an apparatus comprising: at least one processor; and at least one memory including computer program code; the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus at least to: receive, from a cell, an identifier of a group of cells comprising said cell, wherein said cell comprises a small cell and said group of cells comprises at least one large cell; receive information that said cell supports a registration procedure for registration of said apparatus; determine whether to send a registration request to said cell in dependence on said identifier of a group of cells; and use said registration procedure for connecting with said cell when it is determined to send said registration request. Preferably said apparatus is configured to determine whether said apparatus is already registered with said group of cells when determining whether to send a registration request.

Preferably said apparatus is configured to enter an idle mode in response to receiving a message from said cell.

Preferably said message from said cell comprises a radio resource connection release message. Preferably said registration request comprises a radio resource control connection request.

Preferably the apparatus is configured to send a temporary identifier of said apparatus to said cell.

Preferably said apparatus comprises a user equipment. In a twelfth aspect there is provided an apparatus comprising means for receiving, from a cell, an identifier of a group of cells comprising said cell, wherein said cell comprises a small cell and said group of cells comprises at least one large cell; means for receiving information that said cell supports a registration procedure for registration of said apparatus; means for determining whether to send a registration request to said cell in dependence on said identifier of a group of cells; and means for using said registration procedure for connecting with said cell when it is determined to send said registration request.

Preferably said apparatus is configured to determine whether said apparatus is already registered with said group of cells when determining whether to send a registration request.

Preferably said registration request comprises a radio resource control connection request.

Preferably said apparatus is configured to send a temporary identifier of said apparatus to said cell.

Preferably said apparatus comprises a user equipment.

Some embodiments are described with respect to the accompanying Figures in which:

Figure 1 shows a schematic diagram of a communication system comprising a base station and a plurality of communication devices;

Figure 2 shows a schematic diagram of a mobile communication device according to some embodiments;

Figure 3 shows a schematic diagram of a control apparatus according to some embodiments;

Figure 4a schematically illustrates a high speed user equipment moving through a macro- cell; Figure 4b schematically illustrates a macro-cell and some small-cells in the same area; Figure 4c schematically illustrates a macro-cell in an area, some small-cells and two other areas.

Figure 5 is a signalling diagram relating to a legacy system;

Figure 6 is a signalling diagram according to an embodiment;

Figure 7 is a signalling diagram according to an embodiment comprising security enhancements;

Figures 8a and 8b provide a comparison between a legacy system and some embodiments.

In the following certain exemplifying embodiments are explained with reference to a wireless or mobile communication system serving mobile communication devices. Before explaining in detail the exemplifying embodiments, certain general principles of a wireless communication system and mobile communication devices are briefly explained with reference to Figures 1 to 3 to assist in understanding the technology underlying the described examples.

In a wireless communication system mobile communication devices or user equipment (UE) 102, 103, 105 are provided wireless access via at least one base station or similar wireless transmitting and/or receiving node or point. In Figure 1 an example of two overlapping access systems or radio service areas of a cellular system 100 and 1 10 and three smaller radio service areas 1 15, 1 17 and 1 19 provided by base stations 106, 107, 1 16, 1 18 and 120 are shown. Each mobile communication device and base station may have one or more radio channels open at the same time and may send signals to and/or receive signals from more than one source. It is noted that the radio service area borders or edges are schematically shown for illustration purposes only in Figure 1. It shall also be understood that the sizes and shapes of radio service areas may vary considerably from the shapes of Figure 1. A base station site can provide one or more cells. A base station can also provide a plurality of sectors, for example three radio sectors, each sector providing a cell or a subarea of a cell. All sectors within a cell may be served by the same base station.

Base stations are typically controlled by at least one appropriate controller apparatus so as to enable operation thereof and management of mobile communication devices in communication with the base stations. In Figure 1 control apparatus 108 and 109 is shown to control the respective macro level base stations 106 and 107. The control apparatus can be part of, or can be separate from, those base stations. The control apparatus of a base station can be interconnected with other control entities. The control apparatus is typically provided with memory capacity and at least one data processor. The control apparatus and functions may be distributed between a plurality of control units. In some systems, the control apparatus may additionally or alternatively be provided in a radio network controller.

LTE systems may however be considered to have a so-called "flat" architecture, without the provision of RNCs; rather the (e)NB is coupled to a serving gateway (S-GW) and/or mobility management entity (MME), which entities may also be pooled meaning that a plurality of these nodes may serve a plurality (set) of (e)NBs. Each UE is served by only one MME and/or S-GW at a time and the (e)NB keeps track of current association.

In Figure 1 base stations 106 and 107 are shown as connected to a wider communications network 1 13 via gateway 1 12. A further gateway function may be provided to connect to another network.

The smaller base stations 1 16, 1 18 and 120 may also be connected to the network 1 13, for example by a separate gateway function and/or via the controllers of the macro level stations. In the example, stations 1 16 and 1 18 are connected via a gateway 1 1 1 whilst station 120 connects via the controller apparatus 108. In some embodiments, the smaller stations may not be provided. A possible mobile communication device will now be described in more detail with reference to Figure 2 showing a schematic, partially sectioned view of a communication device 200. Such a communication device is often referred to as user equipment (UE) or terminal. The communication device 200 may thus be equivalent to the user equipment 102, 103 and 105 as shown in Figure 1 . An appropriate mobile communication device may be provided by any device capable of sending and receiving radio signals. Non-limiting examples include a mobile station (MS) or mobile device such as a mobile phone or what is known as a 'smart phone', a computer provided with a wireless interface card or other wireless interface facility, personal data assistant (PDA) provided with wireless communication capabilities, or any combinations of these or the like. A mobile communication device may provide, for example, communication of data for carrying communications such as voice, electronic mail (email), text message, multimedia and so on. Users may thus be offered and provided numerous services via their communication devices. Non-limiting examples of these services include two-way or multi-way calls, data communication or multimedia services or simply an access to a data communications network system, such as the Internet. Users may also be provided broadcast or multicast data. Non-limiting examples of the content include downloads, television and radio programs, videos, advertisements, various alerts and other information. The mobile device 200 may receive signals over an air interface 207 via appropriate apparatus for receiving and may transmit signals via appropriate apparatus for transmitting radio signals. In Figure 2 transceiver apparatus is designated schematically by block 206. The transceiver apparatus 206 may be provided for example by means of a radio part and associated antenna arrangement. The antenna arrangement may be arranged internally or externally to the mobile device.

A wireless communication device can be provided with a Multiple Input / Multiple Output (MIMO) antenna system. MIMO arrangements as such are known. MIMO systems use multiple antennas at the transmitter and receiver along with advanced digital signal processing to improve link quality and capacity. Although not shown in Figures 1 and 2, multiple antennas can be provided, for example at base stations and mobile stations, and the transceiver apparatus 206 of Figure 2 can provide a plurality of antenna ports. More data can be received and/or sent where there are more antenna elements. A station may comprise an array of multiple antennas. Signalling and muting patterns can be associated with TX antenna numbers or port numbers of MIMO arrangements.

A mobile device is typically provided with at least one data processing entity 201 , at least one memory 202 and other possible components 203 for use in software and hardware aided execution of tasks it is designed to perform, including control of access to and communications with access systems and other communication devices. The data processing, storage and other relevant control apparatus can be provided on an appropriate circuit board and/or in chipsets. This feature is denoted by reference 204. The user may control the operation of the mobile device by means of a suitable user interface such as key pad 205, voice commands, touch sensitive screen or pad, combinations thereof or the like. A display 208, a speaker and a microphone can be also provided. Furthermore, a mobile communication device may comprise appropriate connectors (either wired or wireless) to other devices and/or for connecting external accessories, for example hands-free equipment, thereto.

Figure 3 shows an example of a control apparatus for a communication system, for example to be coupled to and/or for controlling a station of an access system, such as a base station. In some embodiments, base stations comprise a separate control apparatus. In other embodiments, the control apparatus can be another network element such as a radio network controller. In some embodiments, each base station may have such a control apparatus as well as a control apparatus being provided in a radio network controller. The control apparatus 300 can be arranged to provide control on communications in the service area of the system. In this respect the control apparatus may be considered equivalent to the control apparatus 108 and 109 of Figure 1 . The control apparatus 300 comprises at least one memory 301 , at least one data processing unit 302, 303 and an input/output interface 304. Via the interface the control apparatus can be coupled to a receiver and a transmitter of the base station. For example the control apparatus 300 can be configured to execute an appropriate software code to provide the control functions.

The communication devices 102, 103, 105 may access the communication system based on various access techniques, such as code division multiple access (CDMA), or wideband CDMA (WCDMA). Other non-limiting examples comprise time division multiple access (TDMA), frequency division multiple access (FDMA) and various schemes thereof such as the interleaved frequency division multiple access (IFDMA), single carrier frequency division multiple access (SC-FDMA) and orthogonal frequency division multiple access (OFDMA), space division multiple access (SDMA) and so on.

An example of wireless communication systems are architectures standardized by the 3rd Generation Partnership Project (3GPP). A latest 3GPP based development is often referred to as the long-term evolution (LTE) of the Universal Mobile Telecommunications System (UMTS) radio-access technology. The various development stages of the 3GPP LTE specifications are referred to as releases. More recent developments of the LTE are often referred to as LTE Advanced (LTE-A). The LTE employs a mobile architecture known as the Evolved Universal Terrestrial Radio Access Network (E-UTRAN). Base stations of such systems are known as evolved or enhanced Node-Bs (eNBs) and may provide E-UTRAN features such as user plane Radio Link Control/Medium Access Control/Physical layer protocol (RLC/MAC/PHY) and control plane Radio Resource Control (RRC) protocol terminations towards the communication devices. Other examples of radio access system include those provided by base stations of systems that are based on technologies such as wireless local area network (WLAN) and/or WiMax (Worldwide Interoperability for Microwave Access).

Some embodiments may be used with LTE HetNet where a number of small-cells are deployed with macro-cells in an overlaid way. The small-cells may share the same carrier with the macro-cell, or use different carriers. Some embodiments are used where a relatively fast moving user equipment passes through a HetNet environment where a number of small-cells are deployed within the coverage of a macro-cell, as shown in Figure 4. In Figure 4a, a macro-cell 400 is served by a macro base station 401 . Overlying the macro- cell 400 are, in this example, four small-cells. The first small-cell 402 is served by a base station 403. The second small-cell 404 is served by a base station 405. Likewise the third small-cell 406 is served by base station 407 and the fourth small-cell 408 is served by base station 409. As can be seen, the four small-cells are within the coverage area of the macro- cell 400. One or more of the small-cells 402, 404, 406 and 408 may be only partially in the coverage area of the macro-cell 400, and may be also in the coverage area of another macro-cell (not shown). One or more other small cells (not shown), may be outside the coverage area of the macro-cell 400. A user equipment 410 is in a vehicle which travels through the macro-cell 400 relatively fast. It should be appreciated that the number of small- cells shown in Figure 4 is by way of example only and in other embodiments fewer or more than four small-cells may be provided which at least partially overlap the macro-cell.

It will be appreciated that the term "macro-cell" may also be used interchangeably with the term "large-cell" or "larger-cell". Likewise the term "small-cell" may be used interchangeably with the terms "micro-cell", or "home-cell", or "pico-ceH", or "femto-cell" or "smaller-cell". It will be understood that a large cell is larger than a small cell, and that a small cell is smaller than a large cell.

LTE hetnet deployments may comprise open, closed, and hybrid HeNBs. The various HeNB types may co-exist. If an HeNB is "closed" then only user equipment in a specified group can access and utilise the resources of the closed HeNB. On the other hand an "open" HeNB is accessible to all EUTRAN users. A hybrid HeNB may only allow a certain group of users access to its resources under certain conditions, yet allow open access if sufficient resources are available (e.g. non-subscribers can be admitted if their requirements do not adversely affect those of the subscriber group). Some embodiments are related to LTE hetnet deployments in which macro eNBs and open/hybrid HeNBs coexist.

Another hetnet deployment comprises small-cell Access Points (APs), which are controlled by a small cell controller. This deployment is similar to an open HeNB with HeNB-GW, although the small cell controller may have additional radio resource management functionality compared to a HeNB-GW. It will accordingly be appreciated that the embodiments and/or concepts described below comprising a HeNB connected to a HeNB- GW may also be applicable to a system comprising a small-cell AP connected to a small-cell controller. It will also be appreciated that concepts described in the context of LTE hetnet scenarios may also be applicable in other hetnet scenarios/technologies.

The macro eNBs and HeNBs can be grouped into "tracking areas" (TAs). Each tracking area may comprise one or more node-Bs. This enables identification of a location of a UE connecting to a Node-B in a tracking area. Each tracking area may be identified by a tracking area code (TAC). Accordingly, when a UE moves between tracking areas which do not belong to its currently assigned tracking area list it may need to send a TA update.

Two possible options for tracking area code assignment for HeNBs are discussed below.

In a first option the macro and the HeNBs are assigned with the same TAC or they belong to the same TAC list.

An example is shown in Figure 4b, in which a macro eNB is depicted by macro-cell 400', HeNBs are depicted by small-cells 402', 404', 406', 408', and they all belong to the same tracking area depicted by area 412'. This allows a UE to move between macro and HeNB without any additional signalling (provided the macro and HeNB within question share the same TAC). However, as a consequence of sharing the TAC with the macro-eNB, the HeNB will receive all paging messages for the UE in the tracking area identified by that TAC/TAC list, regardless of whether the UE is actually in the coverage area of the HeNB. This may create a large number of paging messages to be handled by the HeNBs.

For example, assuming the number of macro eNBs per TAC is 500, and the average number of camped users under each eNB is around 100, then the total number of camped subscribers under TAC will be 50,000. If there are 2 to 3 page messages (for example) per camped UE, including downlink data and SMS, and a repeat page factor of 20%, then the expected paging load will be roughly 180,000 messages (3 x 1 .2 x 50,000). Accordingly, when HeNBs are deployed under the same TAC, then all the HeNBs need to handle around 180,000 paging messages per busy hour. Assuming a uniform distribution of page messages over time, this would result in an average of 50 pages per second to be handled by each HeNB. However, the paging load which can be handled by an HeNB without a performance impact is around 5 to 6 pages per second due to the low processing capability of the HeNB platform. Accordingly the HeNBs may not be able to cope in this scenario. Therefore this option may provide simple management of TAC/TAC lists, although it may also result in a high number of paging messages, especially towards the HeNBs. This is also applicable for small-cell AP deployments.

To try and avoid the above situation, in a second option the macro and HeNBs may be allocated different TACs so that the paging load towards HeNBs is restricted to only the UEs camped in the HeNBs coverage area. An example is shown in Figure 4c, in which the macro eNB is depicted by macro-cell 400", the HeNBs are depicted by small-cells 402", 404", 406", and 408". While the macro-cell 400" belongs to an area 412", small-cells 402" and 404" belong to area 414" and small-cells 406" and 408" belong to area 416". If a HeNB- GW (gateway) is deployed, then the HeNB-GW may memorise the UEs last connected HeNB to optimise the implementation. Using this information, when required, the HENB-GW can first send the paging message to the last registered HeNB and if there is no response it can then send the page message to all the HeNBs of the same TAC. This may reduce the paging load towards a HeNB.

Although this may reduce the paging load, it may also increase the tracking area update (TAU) signalling load to the mobility management entity (MME) (at least in comparison with the first option), because the TAU procedure between the UE and MME may need to be executed for every UE movement between macro and HeNB areas. This load may increase linearly with the number of HeNBs deployed under the macro layer and also the number of UE mobility events. Accordingly this may also impact the UEs battery life. Therefore the second option may limit the number of paging messages, but may also result in a relatively high number of TAU signalling messages towards the MME, with consequent impact on the UE battery life.

Figure 5 shows an example of a message exchange sequence between a UE 502, a HeNB 504, a HeNB-GW 506, and a MME 508 in one example.

At step S1 the UE attempts to connect to the HeNB 504. Accordingly the UE 502 and HeNB 504 exchange RRC connection setup messages. Step S1 may comprise exchange of three messages.

At step S2 the HeNB 504 transmits a S1-initial-UE-message (tracking area update request) message to the MME 508. That is the HeNB 504 sends a message to the MME 508 to update the tracking area for the UE 502. At step S3 the MME 508 responds to the HeNB 504 with an S1-downlink-NAS (tracking area update accept) message. NAS (Non Access Stratum) is a protocol layer between UE and the Core-network element (MME) in LTE network. In this step the MME 508 sends a message to UE 502 via eNB 504. That is the MME 508 informs the HeNB 504 that the tracking area update has been accepted.

At step S4 the HeNB 504 sends an RRC downlink-info transfer message informing the UE 502 of the acceptance of the tracking area update.

At step S5 an RRC uplink info transfer message is sent from the UE 502 to the HeNB 504 confirming completion of the tracking area update.

At step S6 the HeNB 504 sends an S1-uplink-NAS (tracking area update complete) message to the MME 508, confirming completion of the tracking area update.

At step S7 the MME 508 sends a S 1 -U E-context-release-command/response message to the HeNB 504. Then at step S8 the HeNB 504 sends an RRC connection release message to the UE 502.

At this stage i.e. upon receipt of the RRC Connection Release message, the UE can then enter the RRCJDLE state.

Accordingly, from the RRC perspective, during the procedure depicted in Figure 5 the UE needs to execute:

• RRC connection setup procedure

• RRC-NAS-message transfer

• RRC connection release procedures.

This may result in six RRC messages being sent/received as part of a single TAU procedure. If there is a different tracking area assigned for macro and HeNB, then there will be one TAU procedure for the UE entering the HeNB coverage area and another TAU procedure when the UE moves back to the macro coverage area.

From an S1 signalling perspective, at the MME there is a requirement for TAU- request/accept/complete and UE context release procedures as part of the TAU procedure. This may require five S1-AP messages to be handled at the MME. As discussed above there are separate area update procedures to be handled for UE entry and exit from HeNB coverage.

From the perspective of the HeNB it needs to handle the RRC connection setup as well as S1-NAS message transfer and UE context release procedures. The macro eNB also needs to handle similar procedures when the UE moves from the HeNB to the macro HeNB. Figure 6 is a signalling diagram in accordance with an embodiment.

The flow chart of Figure 6 shows the signalling between a UE 602, a HeNB 604, a HeNB- GW 606, and a MME 608. It will of course be understood that this is a simplified network architecture, and that in other embodiments there may be more than one of each of the UE 602, the HeNB 604, the HeNB-GW 606, and the MME 608.

According to this embodiment each HeNB 604 is assigned an E-UTRAN Registration Area Code (ERAC). This code may allow UE mobility between HeNBs of the same ERA without additional signalling. Accordingly, in this embodiment each node (e.g. UE, HeNB, HeNB- GW, and MME) may make one or more of the other nodes aware that it is capable of executing a reduced or "lighter" RRC level registration procedure in light of the ERAC.

Accordingly, at step S1 on a downlink shared channel (DL- SCH) the HeNB 604 sends the UE 602 an RRC system information block (SIB). Within this RRC SIB there may be comprised the ERAC, and the HeNB 604 may also indicate to the UE 602 that it supports a simplified RRC registration procedure. Accordingly, at step S2 the simplified RRC registration procedure continues with random access and random access response messages between the HeNB 604 and the UE 602. As part of this step the HeNB 604 may provide uplink allocation to the UE 602. The UE 602 may also be provided with a temporary cell-ID.

At step S3 the UE 602 sends to the HeNB 604 a temporary mobile subscriber identity (S- TMSI) in the RRC Connection Request message. The S-TMSI may be allocated by the MME 608.

This information may be sent on the physical uplink shared channel (PUSCH) as part of the RRC connection request. Step S3 may be initiated at the UE in response to receiving an ERA update from the HeNB 604. In some embodiments the UE initiates the sending of this information on detecting change of ERA when the UE moves from macro to HeNB (Establishment Cause: ERA Update).

At step S4 the HeNB 604 sends to the UE 602 an RRC connection release message, which may be sent on the physical downlink share channel (PDSCH). This may be sent in response to having received the ERA update (Establishment Cause: ERA-Update- Received).

At step S5 the HeNB 604 updates the HeNB-GW 606 with an S1-AP message, requesting the HeNB-GW to register the status of the UE 602. In step S5 the HeNB 604 may provide the HeNB-GW 606 with the ERAC and the S-TMSI.

At step S6 the HeNB-GW can then store the S-TMSI for the UE 602 and the identity of the HeNB 604 in a memory of the HeNB-GW 606. That is the HeNB-GW 606 "binds" (or creates an association or link between) the S-TMSI and the HeNB for the UE 602. At step S7 the HeNB-GW 606 sends to the HeNB 604 an S1 AP: UE Registration Accept message. This confirms successful registration of the updated status of the UE 602.

When the UE 602 moves out from a HeNB it checks its TA against the macro TA. If the TA is the same the UE will not send TAU.

Therefore when the macro and HeNB are configured with the same TA or TA-list then the TAU is avoided.

In some embodiments the HeNB provides the timer value for which the mapping is maintained at HeNB-GW to UE as part of RRC-Connection Release. In such a case the UE is not required to send ERA-Update when it comes back to the same ERA after moving to the macro-cell (if this is completed before the timer times out). This may avoid the additional signalling (even at RRC level) when UE moves between macro and small-cells within this configurable time-out.

It will be appreciated that in the embodiment of Figure 6 the MME 608 is not involved in signalling, and therefore experiences a relatively reduced signalling load.

Thus some embodiments enable macro-eNBs and HeNBs to share the same TACs. This may enable simple management of TACs and TAC lists, and reduced TAU signalling link load at the MME. This may also prevent UE battery drain.

As will be appreciated from Figure 6 the signalling reduction is at least in part achieved by the simplified TAU procedure (i.e. no need to update if moving within the same ERA), and because no TAU is triggered when the UE moves out to macro HeNB coverage. Embodiments may also allow a reduced paging signalling load at the HeNB, because the paging messages will be sent only to HeNBs belonging to certain EHAs (and not to the HeNBs belonging to certain TAs).

Figure 7 shows an embodiment with additional security.

Steps S1 to S3 are the same as steps S1 to S3 in Figure 6. At step S4 the HeNB 704 sends the HeNB-GW 706 a UE registration request that comprises aUE-ID, ERA-ID, a security token generated from the S-TMSI, and the S-TMSI.

At step S5 this information is forwarded from the HeNB-GW 706 to the MME 708.

At step S6, in an S1 AP::UE Registration Accept message comprising the UE-ID and the S- TMSI is sent to the HeNB-GW 706. This may be in response to a successful update of the ERA.

Then at step S7 the HeNB-GW can store in its memory the S-TMSI for the UE 702 and the identity of the HeNB 704. That is the HeNB-GW creates a "binding" or association between the S-TMSI and HeNB. At step S8 the HeNB-GW 706 sends to the HeNB 704 an S1 AP:UE Registration Accept message which also contains the UE-ID and the S-TMSI. This may be in response to successful update of the ERA.

At step S9 the HeNB 704 sends to the UE 702 an RRC Connection Release message on the PDSCH. This is equivalent to step S4 in Figure 6. The UE can then enter RRCJDLE state. The additional security aspects may be provided by way of the UE sending a short MAC value (16 bit) calculated by using S-TMSI value as the input message and using a NAS integrity protection algorithm in the RRC connection request message along with the S- TMSI. In order to route the message to the correct MME, the RRC connection request may also contain a registered MME parameter. The HeNB then forwards this short MAC value along with S-TMSI to the MME/HeNB- GW in the S1-UE- registration request message. This is equivalent to steps S4 and S5 of Figure 7. On successful validation of the S-TMSI the MME sends S 1 -U E-registration-accept message with the S-TMSI value. This is equivalent to steps S8 and S9 of Figure 7.

The HeNB-GW can then create binding of the information based on the S1-UE-registration- accept message. If MME sends S 1 -U E-registration-reject, then no binding is carried out at the HeNB-GW.

If the integrity check fails based on the received short MAC value, the MME will send UE- registration-failure to HeNB. On reception of the failure message the HeNB will send RRC connection release with cause value "TAU required". The UE may then trigger regular TAU on reception of this cause via a new RRC connection. The above additional security steps increase the number of inter-node messages between MME and HeNB-GW by two, compared to the approach of Figure 6. However, this is still fewer than the total number of signalling messages for the TAU procedure shown in Figure 5. For a deployment without HeNB-GW there is no additional messaging. In some embodiments the additional security is controlled via a system information message from HeNB, or as part of attach/TAU between UE and MME.

The nodes and interfaces may in some embodiments require changes and/or new functionalities. The HeNB, for example, may be configured to broadcast the proposed E- UTRAN HeNB area code (ERAC) to one or more user equipment. As discussed above the HeNBs belonging to the same HeNB mobility cluster (for example all HeNBs of an enterprise or owner) may be assigned with the same E-UTRAN HeNB area code. As discussed with respect to Figure 6, the HeNB may indicate its support for the simplified RRC registration procedure in a system information message. In some embodiments the presence of the ERA may be used as an indication the HeNB supports the "simplified" RRC-Registration. In terms of UE behaviour, when a UE is reselecting towards an open/hybrid HeNB in the same tracking area list, then the UE reselects from a macro eNB or a HeNB to an HeNB supporting the simplified RRC registration which belongs to the same tracking areas/tracking area list. If the new cell's ERAC is different from the previous cell, then the UE triggers an RRC connection request. The RRC connection request will have the new establishment cause, as shown at step 4 of Figure 6: ERA-Update-Received.

This message may also contain a remaining TAU timer value. The HeNB may then respond with the RRC connection release on CCCH so that the UE can release the RRC connection. This is shown in step S4 of Figure 6.

The HeNB can then inform the HeNB-GW and/or the MME of the arrival of the S-TMSI via the S1 message: UE registration request. This is shown in step S5 of Figure 6. In some embodiments this message may be terminated at the HeNB-GW. In such an embodiment the MME is not aware of the UE entry into HeNB.

As shown at step S6 of Figure 6 the HeNB-GW creates a binding between the UEs S-TMSI and the ERAC on reception of the new S1 message (step S5). That is the HeNB-GW stores in its memory a link, or association, between the S-TMSI and the identity of the HeNB.

In some embodiments, to reduce the impact to standard interfaces, instead of a new S1 procedure the S1— initial-UE message and the UE-context-release procedure may be enhanced for this purpose. The cause value in the initial UE message can be used by HeNB-GW to terminate the message at the HeNB-GW.

The "binding" or "linking" referred to above (i.e. step S6 of Figure 6 and step S8 of Figure 7) may in embodiments maintain the mapping between the initial S-TMSI and the latest HeNB. That is, in some embodiments, once a UE connects to a new HeNB then the binding between the S-TMSI and the previous HeNB may be removed from the memory of the HeNB-GW. That is, if the UE connects via a new HeNB, then the "binding" should be replaced with new HeNB identity.

If the S-TMSI changes as part of further signalling via the HeNB, then the mapping may be updated to reflect the latest S-TMSI of the UE. This may be achieved by the following procedure between MME/HeNB-GW. If the UE context release is initiated by the MME, then the S1 message carrying the UE context release may also contain the latest S-TMSI. This enables the HeNB-GW to maintain the latest binding of S-TMSI and HeNB.

In some embodiments the binding between S-TMSI and HeNB identity at the HeNB -GW is maintained for a duration of a configured TAU timer value. This timer may be started whenever the binding is created. The binding may be created in the following cases.

1. Whenever HNB-GW receives an indication from MME that S-TMSI is valid based on short-MAC value ,the binding is created..

2. Whenever HNB-GW receives First S1 message for S-TMSI for which there is no binding existing already,the binding is created.

3. Whenever the HNB-GW establishes S1 link for the same S-TMSI, the timer is stopped if running.

4. The Timer is restarted again or started whenever HNB-GW receives the UE context release corresponding to the S-TMSI. In a case of this message carrying new S- TMSI ,then binding is created for the new S-TMSI and old binding value is removed.

5. The HeNB-GW may then delete the UE context upon TAU timer expiration.

Whenever binding is updated as part of UE context release, the binding of the information may be kept for a time corresponding to TAU timer value. This value may be configurable at the HeNB-GW. Accordingly this may enable the HeNB-GW to maintain the S-TMSI/HeNB identity binding.

As discussed above, one or more macro eNBs and one or more HeNBs may share the same tracking area/tracking area list. Embodiments may enable the paging load to be optimised with the help of RRC level registration information at MME/ HeNB-GW.

Where the macro UE and HeNB-GW share the same tracking area, the MME may not be aware of the UE presence in HeNB-GW because the UE registration may be terminated at the HeNB-GW itself. This is shown in and described with respect to Figure 6, which shows that the MME is not involved in signalling when registering the UE. Accordingly the MME may send all paging messages corresponding to the relevant tracking area towards the HeNB-GW and other eNBs in that tracking area. The HeNB-GW can then filter the paging message using the mapping created from the RRC level registration message (i.e. the mapping between the S-TMSI and HeNB identity). Accordingly the HeNBs will not receive paging messages corresponding to the macro UE (for example when the UE is camped onto macro cells), since those paging messages have been filtered out at the HeNB-GW.

In some embodiments, if the S-TMSI is mapped to HeNB, or HeNB-registration area, the paging message will be forwarded to the HeNBs of the latest ERA in the binding. If the S- TMSI does not have any entry in the mapping table, then this may be an indication that the UE is not camped on any of the HeNBs.

In some embodiments where there is no HeNB-GW, then the MME may know by using the UE registration message whenever a UE enters the HeNB/EHA. In such an embodiment the MME may store a mapping of the S-TMSI against the HeNB/HRA that the UE most recently connected to (similar to the mapping at step S6 carried out at the HeNB-GW in Figures 6 and 7). This mapping table may be maintained for the remaining duration of the TAU timer. The remaining TAU timer value may be calculated as TAU timer value minus (last TAU time minus UE registration time via HeNB).

When forwarding paging to an S-TMSI which has a valid entry in the mapping table, the MME may first send the page to the ERA and if there is no response the MME can then send the page to all the eNBs in the tracking area. This may ensure that the page reaches the UE. If the paged S-TMSI is not in the mapping table, then in some embodiments no page is sent towards HeNBs. Accordingly the paging load towards the HeNBs may be reduced.

It is envisaged that both legacy UEs and UEs with the capability of operating with the modified RRC level registration may coexist within the system. Therefore in some embodiments macro-eNBs and HeNBs may be assigned different tracking areas based on the UE capability. This information may be stored in the MME. Accordingly in some embodiments the UEs may be segregated based upon their capability. Therefore in some embodiments the MME may assign the tracking area list including HeNB tracking area as a tracking area list for a UE having RRC level registration capability. The MME may then assign a unique or different tracking area for one or more legacy UEs, depending on the current location of those UEs. Such behaviour at the MME may facilitate legacy UEs carrying out the tracking area update when moving between macro and HeNB, which may reduce the paging load. As the tracking area list for UEs having the simplified RRC registration capability contains tracking area of macro-eNBs and HeNB, then these UEs may only do RRC level registration when reselecting from macro eNB to HeNB, and not when reselecting from HeNB to macro eNB. If the MME does not assign the tracking area list based on capability, then the legacy UE may receive the same tracking area for macro and HeNB, which may result in a relatively higher paging load to the HeNB.

Figures 8a and 8b provide a comparison of legacy mobility behaviour (Figure 8a), and behaviour in the proposed embodiments (Figure 8b). It will be appreciated that these figures have been simplified for the purposes of explanation.

In Figure 8a the UE is shown at 802 and the network is shown generally at 810. Step S1 is a summary of a UE reselection procedure from macro-eNB to HeNB. As shown the UE update procedure requires six messages.

Turning to Figure 8b the UE is shown at 802' and the network is shown generally at 810'. Step S1 ' shows generally a UE reselection procedure from macro eNB to HeNB. As shown this procedure comprises two messages between the UE 802' and the network 810'. This compares with six messages of the legacy procedure shown in Figure 8a.

Turning back to Figure 8a, step S2 shows generally a reselection procedure of a UE from HeNB to macro eNB. Again, this requires six messages between the UE 802 and the network 810.

In contrast, as shown at step S2' of Figure 8b, when the UE 802' reselects from HeNB to macro eNB there is no tracking area update required and accordingly no messages are required for signalling an update.

Accordingly it can be appreciated from the comparison of Figures 8a and 8b that some embodiments may reduce the signalling load in the system and the nodes comprised therein. Accordingly it will be appreciated from the above description that the RRC connection request itself may be used as an indication for arrival of a UE into a small-cell or HeNB area, and may trigger an update of the UE registration with the HeNB-GW, or other small-cell controller. This procedure may save signalling load at the UE (and consequently save battery power), and may also reduce the signalling load at the MME related to TAU procedures.

Embodiments may also enable more efficient idle mobility for a user equipment due to reduced signalling requirements to put the user equipment in its idle mode. Accordingly battery use at the user equipment may be reduced since it may spend more time in an idle state.

In deployments where there is no HeNB-GW or if the validation/integrity check of S-TMSI is needed (as set out in Figure 7) the procedure may be enhanced to include the validation/update of MME with the S-TMSI and additional integrity protection information. Even including the additional security enhancements the signalling procedure may still require fewer paging messages than the legacy tracking area update procedure.

In embodiments the latest S-TMSI may be informed to the HeNB-GW/HeNB via a UE context release message so that the paging table (which for example comprises the binding information) at the HeNB-GW remains up-to-date with the latest S-TMSI changes.

The use of the ERA may reduce the amount of registration updates required for a mobile user equipment, and the "binding" or association of ERA and S-TMSI at the gateway node may reduce the number of paging messages received at the small cell base stations (e.g. HeNB) for a given UE. The signalling load in the system may therefore be reduced.

Table 1 below provides a summary of UE/ HeNB, S1 interface, and HeNB-GW/MME changes that may be implemented according to some embodiments.

UE/HeNB changes S1-lnterface changes HeNB-GW/MME

changes

System Information changes : New procedure or Terminated at HeNB- existing S 1 -Initial UE GW.

Inform the support for new

message /S1 -context

procedure and EHA transmission. No MME changes.

release can be reused

with some changes

RRC-connection Request :

additional establishment cause

value.

UE to expect RRC Connection

Release also instead of RRC

connection Setup.

Whenever HNB-GW receives an

indication from MME that S-TMSI

is valid based on short-MAC value

,the binding may be created. This

may require an indication in the S1

message.

System Information changes : New procedure or MME validates the S- existing messages to be TMSI. If not valid will

Inform the support for new

enhanced to transfer inform via S1 procedure and EHA transmission

additional parameters response message. and integrity protection-needed for

new procedure RRC-connection Request

additional establishment cause

value and new parameters for

Integrity protection and MME

details.

UE-Contention resolution Id should

use only first 48 bits of CCCH

message instead of whole message.

RRC Connection Release will

contain additional info to ask UE

trigger regular TAU.

3 S 1 -U E-Context-Release- MME should include command message S-TMSI in Context should contain the latest release and handover S-TMSI value so that messages.

binding can be

maintained against

HeNB.

S-TMSI also needed in

handover messages to

inform the S-TMSI of the

UE.

Table 1

In general, the various embodiments may be implemented in hardware or special purpose circuits, software, logic or any combination thereof. Some aspects of the invention may be implemented in hardware, while other aspects may be implemented in firmware or software which may be executed by a controller, microprocessor or other computing device, although the invention is not limited thereto. While various aspects of the invention may be illustrated and described as block diagrams, flow charts, or using some other pictorial representation, it is well understood that these blocks, apparatus, systems, techniques or methods described herein may be implemented in, as non-limiting examples, hardware, software, firmware, special purpose circuits or logic, general purpose hardware or controller or other computing devices, or some combination thereof. The software may be stored on such physical media as memory chips, or memory blocks implemented within the processor, magnetic media such as hard disk or floppy disks, and optical media such as for example DVD and the data variants thereof, CD.

The foregoing description has provided by way of exemplary and non-limiting examples a full and informative description of the exemplary embodiment of this invention. However, various modifications and adaptations may become apparent to those skilled in the relevant arts in view of the foregoing description, when read in conjunction with the accompanying drawings and the appended claims. However, all such and similar modifications of the teachings of this invention will still fall within the scope of this invention as defined in the appended claims. Indeed there is a further embodiment comprising a combination of one or more of any of the other embodiments previously discussed.

Claims

1. A method comprising: sending to a user equipment, from a first apparatus controlling a cell, an identifier for identifying a group of cells comprising said cell, wherein said cell comprises a small cell and said group of cells comprises at least one large cell; sending to said user equipment information that said cell supports a registration procedure for registration of said user equipment; using said registration procedure for connection of said user equipment to said first apparatus; and sending a request for registration of said user equipment to a second apparatus, said request comprising said identifier for identifying a group of cells and an identifier of said user equipment.

2. A method as set forth in claim 1 comprising broadcasting said identifier for identifying a group of cells.

3. A method as set forth in any preceding claim, comprising distributing a received paging message to said identified group of cells.

4. A method as set forth in any preceding claim, wherein said method comprises using a security-token during said registration procedure.

5. A method as set forth in any preceding claim, wherein said identifier for identifying a group of cells comprises an E-UTRAN Registration Area code.

6. A computer program comprising computer executable instructions which when run on one or more processors perform the method of any of claims 1 to 5.

7. An apparatus comprising: at least one processor; and at least one memory including computer program code; the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus at least to: send to a user equipment an identifier for identifying a group of cells comprising a cell controlled by said apparatus, wherein said cell comprises a small cell and said group of cells comprises at least one large cell; send to said user equipment information that said cell supports a registration procedure for registration of said user equipment; use said registration procedure for connection of said user equipment to said apparatus; and send a request for registration of said user equipment to a second apparatus, said request comprising said identifier for identifying a group of cells and an identifier of said user equipment.

8. An apparatus as set forth in claim 7, wherein said apparatus is configured to broadcast said identifier for identifying a group of cells.

9. An apparatus as set forth in claim 7 or claim 8, wherein said apparatus is configured to distribute a received paging message to said identified group of cells.

10. An apparatus as set forth in any of claims 7 to 9, wherein said apparatus is configured to use a security-token during said registration procedure.

1 1 . An apparatus as set forth in any of claims 7 to 10, wherein said identifier for identifying a group of cells comprises an E-UTRAN Registration Area code.

12. A method comprising: receiving information from at least one cell, said information comprising an identifier of a group of cells comprising said at least one cell, wherein said at least one cell comprises a small cell and said group of cells comprises at least one large cell, and said information comprises an identifier of a user equipment connecting with said at least one cell; storing an association between said identifier of said group of cells and said identifier of said user equipment; and using said association to determine whether to send at least one message to said at least one cell.

13. A method as set forth in claim 12, wherein said information is comprised in a user equipment registration request.

14. A method as set forth in claim 12 or claim 13, wherein said identifier of a group of cells comprises an E-UTRAN Registration Area code.

15. A method as set forth in any of claims 12 to 14, wherein said using said association to determine whether to send at least one message to said at least one cell comprises filtering out messages addressed to said large cell.

16. A computer program comprising computer executable instructions which when run on one or more processors perform the method of any of claims 12 to 15.

17. An apparatus comprising: at least one processor; and at least one memory including computer program code; the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus at least to: receive information from at least one cell, said information comprising an identifier of a group of cells comprising said at least one cell, wherein said at least one cell comprises a small cell and said group of cells comprises at least one large cell, and said information comprises an identifier of a user equipment connecting with said at least one cell; store an association between said identifier of said group of cells and said identifier of said user equipment; and use said association to determine whether to send at least one message to said at least one cell.

18. An apparatus as set forth in claim 17, wherein said apparatus is configured to receive said information in a user equipment registration request.

19. An apparatus as set forth in claim 17 or claim 18, wherein said identifier of a group of cells comprises an E-UTRAN Registration Area code.

20. An apparatus as set forth in any of claims 17 to 19, wherein said use said association to determine whether to send at least one message to said at least one cell comprises filtering out messages addressed to said large cell.

21 . A method comprising: receiving at an apparatus, from a cell, an identifier of a group of cells comprising said cell, wherein said cell comprises a small cell and said group of cells comprises at least one large cell; receiving information that said cell supports a registration procedure for registration of said user equipment; determining whether to send a registration request to said cell in dependence on said identifier of a group of cells; and using said registration procedure for connecting with said cell when it is determined to send said registration request.

22. A method as set forth in claim 21 , wherein said determining whether to send a registration request comprises determining whether said apparatus is already registered with said group of cells.

23. A method as set forth in claim 21 or claim 22, comprising entering an idle mode in response to receiving a message from said cell.

24. A method as set forth in any of claims 21 to 23 comprising sending a temporary identifier of said apparatus to said cell.

25. A computer program comprising computer executable instructions which when run on one or more processors perform the method of any of claims 21 to 24.

26. An apparatus comprising: at least one processor; and at least one memory including computer program code; the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus at least to: receive, from a cell, an identifier of a group of cells comprising said cell, wherein said cell comprises a small cell and said group of cells comprises at least one large cell; receive information that said cell supports a registration procedure for registration of said apparatus; determine whether to send a registration request to said cell in dependence on said identifier of a group of cells; and use said registration procedure for connecting with said cell when it is determined to send said registration request.

27. An apparatus as set forth in claim 26, wherein said apparatus is configured to determine whether said apparatus is already registered with said group of cells when determining whether to send a registration request.

28. An apparatus as set forth in claim 26 or claim 27, wherein said apparatus is configured to enter an idle mode in response to receiving a message from said cell.

29. An apparatus as set forth in any of claims 26 to 28, wherein the apparatus is configured to send a temporary identifier of said apparatus to said cell.