WO2016162045A1

WO2016162045A1 - Controlling multi connectivity

Info

Publication number: WO2016162045A1
Application number: PCT/EP2015/057448
Authority: WO
Inventors: Mikko Saily; Per Henrik Michaelsen; Simone BARBERA
Original assignee: Nokia Solutions and Networks Oy
Current assignee: Nokia Solutions and Networks Oy
Priority date: 2015-04-07
Filing date: 2015-04-07
Publication date: 2016-10-13
Anticipated expiration: 2017-10-07

Abstract

Apparatuses and methods for controlling multi connectivity are disclosed. A primary connection to a user terminal configured to operate using connectivity with plurality of connections is maintained (200). A set of base stations to which the user terminal may establish at least one secondary connection is determined (202) on the basis of one or more given criteria. Information on the set is transmitted (204) to the user terminal. The set of base stations is prepared (206) for the possible one or more secondary connections.

Description

DESCRIPTION

TITLE CONTROLLING MULTI CONNECTIVITY

Technical Field

The invention relates to communications.

Background

The following description of background art may include insights, discover- ies, understandings or disclosures, or associations together with disclosures not known to the relevant art prior to the present invention but provided by the invention. Some of such contributions of the invention may be specifically pointed out below, whereas other such contributions of the invention will be apparent from their context.

In recent years, the phenomenal growth of mobile Internet services and proliferation of smart phones and tablets has increased a demand for mobile broadband services, and hence more data transmission capacity is required. One possibility to increase a data transmission rate of a user apparatus is dual or multi connectivity. The basic principle of the dual connectivity is that the user apparatus may consume radio resources provided by at least two different network nodes, each network node controlling one or more cells. One of the network nodes has a primary connection to the user apparatus and it is called a master network node which controls radio resources for the user apparatus.

Brief description

The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the invention. It is not intended to identify key/critical elements of the invention or to delineate the scope of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to a more detailed description that is presented later.

According to an aspect of the present invention, 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: maintain a primary connection to a user terminal configured to operate using connectivity with plurality of connections; determine on the basis of one or more given criteria a set of base stations to which the user terminal may establish at least one secondary connection; transmit information on the set to the user terminal; and control the preparation of the set of base stations for the possible one or more secondary connections.

According to an aspect of the present invention, 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: maintain a primary connection to a first base station serving a cell; receive from the first base station information on a set of base stations with which the apparatus may establish at least one secondary connection; measure signal strengths of base stations belonging to the set; and request establishing a secondary connection with at least one base station of the set if the signal strength of the base station is stronger than a given threshold.

According to an aspect of the present invention, there is provided a method comprising: maintaining a primary connection to a user terminal configured to operate using connectivity with plurality of connections; determining on the basis of one or more given criteria a set of base stations to which the user terminal may establish at least one secondary connection; transmitting information on the set to the user terminal; and controlling the preparation of the set of base stations for the possible one or more secondary connections.

According to an aspect of the present invention, there is provided a method comprising: maintaining a primary connection to a first base station serving a cell; receiving from the first base station information on a set of base stations with which the apparatus may establish at least one secondary connection; measuring signal strengths of base stations belonging to the set; and requesting establishing a secondary connection with at least one base station of the set if the signal strength of the base station is stronger than a given threshold.

One or more examples of implementations are set forth in more detail in the accompanying drawings and the description below. Other features will be apparent from the description and drawings, and from the claims.

Brief description of drawings

In the following the invention will be described in greater detail by means of preferred embodiments with reference to the accompanying drawings, in which

Figure 1 illustrates a simplified example of a communication environment; Figures 2 and 3 are flowcharts illustrating example embodiments of the invention;

Figure 4A, 4B and 5 illustrate simplified examples of apparatuses applying some embodiments of the invention. Detailed description of some embodiments

Embodiments are applicable to any base station, user equipment (UE), server, corresponding component, and/or to any communication system or any combination of different communication systems that support dual or multi connectivity and required functionalities.

The protocols used, the specifications of communication systems, servers and user terminals, especially in wireless communication, develop rapidly. Such development may require extra changes to an embodiment. Therefore, all words and expressions should be interpreted broadly and they are intended to illustrate, not to restrict, embodiments.

Many different radio protocols to be used in communications systems exist. Some examples of different communication systems are the universal mobile telecommunications system (UMTS) radio access network (UTRAN or E-UTRAN), long term evolution (LTE, known also as E-UTRA), long term evolution advanced (LTE-A), Wireless Local Area Network (WLAN) or Wi-Fi based on IEEE 802.1 I stardard, world- wide interoperability for microwave ac-cess (WiMAX), Bluetooth®, personal communications services (PCS) and systems using ultra-wideband (UWB) technology. IEEE refers to the Institute of Electrical and Electronics Engineers.

Figure 1 illustrates a simplified view of a communication environment only showing some elements and functional entities, all being logical units whose imple- mentation may differ from what is shown. The connections shown in Figure 1 are logical connections; the actual physical connections may be different. It is apparent to a person skilled in the art that the systems also comprise other functions and structures. It should be appreciated that the functions, structures, elements and the protocols used in or for communication are irrelevant to the actual invention. Therefore, they need not to be discussed in more detail here.

In the example of Figure 1 , a radio system based on LTE/SAE (Long Term Evolution/System Architecture Evolution) network elements is shown. However, the embodiments described in these examples are not limited to the LTE/SAE radio systems but can also be implemented in other radio systems.

The simplified example of a network of Figure 1 comprises a SAE Gate- way 100 and an MME 102. The SAE Gateway 100 provides a connection to Internet 104. Figure 1 shows a network done or a base station or an eNodeB (denoted as MeNB) 106 serving a macro cell 108 and another base station or eNodeB (denoted as MeNB) 1 10 serving an adjacent macro cell 1 12. The MeNBs are connected to both MME 102 and SAE GW 100. The MeNBs may further have an X2 interface connection 1 14 with each other.

The mobility management entity (MME) represents a mobility anchor entity in a core network that is involved in the bearer activation/deactivation processes, for example. The mobility management entity may be configured to support dual or multi connectivity. The MME may be configured to recognize signaling relating to dual connectivity, and act upon it. The serving gateway (S-GW) routes and forwards user data packets further.

The user terminal UT ( or user apparatus, user equipment) 1 16 illustrates one type of an apparatus to which resources on the air interface are allocated and assigned, and thus any feature described herein with user terminal (user equipment) may be implemented with a corresponding apparatus. The user terminal 1 16 refers to a portable computing device that includes wireless mobile communication devices operating with or without a subscriber identification module (SIM), including, but not limited to, the following types of devices: mobile phone, smart-phone, personal digital assistant (PDA), laptop computer, e-reading device, and tablet. The user terminal 1 16 may be configured to support also dual connectivity.

In the example of Figure 1 , there are a set of base stations (denoted as SeNB) in the service area of both MeNB 106 and MeNB 1 10. These base stations SeNB1 - SeNB9 are base stations serving so called small cells (SC), which have considerably smaller coverage area compared to the macro cells. The SeNBs may have the same connections as the MeNBs, but for clarity the connections are not illustrated in Figure 1 .

In the example of Figure 1 , the network nodes 106, 1 10, and SeNBs depict an apparatus controlling one or more cells via which access is provided to the network the user apparatuses and the network nodes are connected to. In an LTE-A system, such a network node is an evolved node B (eNB, eNodeB). The evolved node B 106 or any corresponding network apparatus controlling one or more cells, is a computing device configured to control the radio resources, and connected to the evolved packet core network, thereby providing the user equipment 1 16 a connection to the communication system. Typically, but not necessarily, the evolved node B comprises all radio-related functionalities of the communication whereby the evolved node B, for example, schedules transmissions by assigning certain uplink resources for the user equipment and informing the user equipment about transmission formats to be used. The nodes 106, 1 10, and SeNBs may be configured to perform one or more of evolved node B functionalities described below with an embodiment, and to perform functionalities from different embodiments.

The evolved node B also provides the cells but the exemplary embodiments may be implemented with a solution having a separate controlling apparatus, and separate cell providing apparatuses controlled by a controlling apparatus. Further, the cells may be macro cells, and/or small cells.

In dual connectivity, a user terminal is connected to a master evolved node B (MeNB) controlling a primary cell (PCell) and comprising a radio resource controlling unit for the user apparatus, and to a secondary evolved node B (SeNB) controlling a secondary cell (SCell). If the cell sizes are different, the evolved node B controlling the macro cell is typically selected to be the master evolved node B, since then fewer MeNB handovers are needed when the user apparatus moves compared to the situation in which the evolved node B controlling the small cell would have been selected to be MeNB. Dual connectivity is below used an example of multi connectivity. In multi connectivity, user terminal may have a plurality of primary and secondary connections. Therefore, wherever dual connectivity is mentioned it serves only as an example embodiment. In other embodiments, user terminal may be connected through multiple eNBs, access points or other network elements, such as baseband pool controllers, remote radio heads, could-RAN and include multi-connectivity intended for future 5G systems.

In addition, embodiments of the invention are applicable to situations where control and user plane are separated, such that user terminal control plane is connected to an eNB or plurality of eNBs, typically an eNB with large coverage, and the user plane is connected through small cells that need to be prepared by initial configuration for accepting the possible later access by the user terminal. Therefore, wherever multi connectivity is mentioned it serves as an example embodiment where the control plane and Radio Resource Control (RRC) protocol can be operated independently for each connection in multi connectivity or operated as a primary RRC for MeNB or master access point and secondary RRC for SeNB or slave access point.

In the example of Figure 1 , the user terminal 1 16 has a primary connection 1 18 with the MeNB 106. The user terminal may also have a secondary connec- tion to a SeNB not shown in the Figure. The user terminal may be on the move in the direction illustrated by the arrow 120. As the user terminal moves the primary connec- tion may stay the same but the base station with which the secondary is established changes as the coverage areas of the SeNBs is small. As a result there is a lot of RRC signalling related to dual connectivity connection.

To reduce the RRC signalling, it has been proposed that control of the user terminal cell management could partly be transferred to the user terminal. Currently all handovers and cell changes related to PCell and SCell changes are fully network controlled. For example, if the user terminal could autonomously perform SCell changes without active network control the amount of RRC signalling could be reduced. In this kind of solution, SCell base stations would have to be prepared to handle autonomous user terminal changes.

The preparation of a set of SCells (eNBs), typically Small Cells (SC), is much similar to handover preparation, such that each eNB is prepared to and agrees to accept becoming the SCell/SeNB for a given user terminal, should this choose to access the cell. This operation may be performed by the currently serving eNB, typi- cally a macro cell eNB, and each SC/eNB provides a preparation reply indicating the SeNB configuration (SCG) that it accepts to provide to the user terminal. The serving eNB may configure the user terminal, by an extended RRCConnectionReconfiguration message, and provide the set of SeNB configuration from all eNBs that accepted the preparation.

Thus, when the user terminal measures a given small cell eNB and finds it sufficiently strong for becoming a SCell it simply performs random access in this cell. If dual connectivity is not yet active, this will trigger a SeNB add operation, alternatively it will trigger a SeNB change operation. In either case the SC eNB becomes a SCell of the user terminal, without the user terminal having to send a measurement report, and receiving an RRCConnectionReconfiguration message, so RRC signaling is reduced.

It has been estimated that up to 2/3 of RRC signaling may be avoided using the above procedure. Since the solution is also called Autonomous Dual Connectivity, it may also be referred to as AutoDC.

There is, however, a cost of Auto DC in terms of increased X2 signaling, i.e. a core network transport overhead, since the preparation of M cells requires the transmission of 2M X2 messages, as one request and one reply per cell being prepared is required. Also, when the user terminal releases the last SeNB, the prepared cells must be informed that they can now release resources that were reserved as part of the preparation, which requires another M X2 messages.

The M equals the number of small cell eNBs in the cluster where AutoDC is applied. So whenever a user terminal enters the area, the M cells are prepared. Also, when a user terminal gets out a range of current SeNB without having a substitute, it releases this, hence releases DC, implying that AutoDC must be reconfigured when adding the next small cell eNB as SeNB. The latter becomes less likely the higher the cell density, since stretches of always having a small cell eNB suitable for becoming a SeNB within range are longer. User terminals at higher speed will cross/leave the area quickly, so connect to less SeNBs.

It may be noted that there is also a gain in X2 signaling by AutoDC, since with AutoDC less X2 messages are required for each add/change/release operation than for ordinary dual connectivity SeNB operations. So there is an overall increase in X2 signaling when the number of prepared cells is high as compared to the number of SeNB operations performed until AutoDC is deconfigured, i.e. the many X2 messages used for AutoDC configuration are dominating over saving one or two X2 messages in each SeNB operation.

It has been noted that the increase in X2 signaling depends on the number of cells being prepared, i.e. M. It also depends on the UE speed. So far, it has been the assumption that all M cells in a cluster of small cells are prepared for SCell operation. However, if the number of cells that are prepared for SCell operation is limited, the required X2 signaling for AutoDC configurations may become so low that we achieve a gain, in particular when the cell density, hence the number of SeNB operations, is high.

In an embodiment, the number of prepared cells when configuring AutoDC is limited. A method is proposed to proactively select for preparation a limited number of cells, which are most likely used for autonomous operations. The target is to choose a limited number of prepared cells, which is less than the number of small cells within the macro footprint. Criteria are chosen so that the AutoDC gain is maximized while the number of prepared but unused cells is minimized.

Figure 2 is a flowchart illustrating an example embodiment. The example illustrates the operation of the eNB acting as PCell for a user terminal.

In step 200, the eNB maintains a primary connection to the user terminal which is configured to operate using dual connectivity or multi connectivity.

In step 202, the eNB is configured to determine on the basis of one or more given criteria a set of base stations to which the user terminal may establish at least one secondary connection. Examples of criteria are given below.

In step 204, the eNB is configured to transmit information on the set to the user terminal. In step 206, the eNB is configured to control the preparation of the set of base stations for the possible one or more secondary connections.

The limiting of the number of prepared cells when configuring AutoDC is not straightforward, since the eNB acting as PCell that configures AutoDC needs to prepare a set of cells selected in such a way that the user terminal is very likely to initiate SeNB operations for (some of) these cells. If the set is too small, the user terminal will soon get out of range, and will have to release, i.e. deconfigure AutoDC soon followed by a re-configuration of AutoDC, and expected gains may turn into losses.

In an embodiment, the selection of the set of base stations may be based on the measurement report from the user terminal. The report may contain information on measured signal power of neighbour cells. Optionally the user terminal may report the list of visited cells or location and movement as described later. This information, along with a signalling "neighbour candidates" between cells and potential candidate cells, will make it very likely for the user terminal to be located within the coverage area of the prepared cells and potentially move towards another prepared cell among the cluster of small cells.

In an embodiment, the eNB acting as PCell may be configured to obtain information on the location and movement direction of the user terminal and utilise the information when determining the set of base stations. The set may include base stations within certain distance of the location of the UE, for example. This may be based on actual distance between the base stations and the user terminal (when precise location information is available), distance between the base stations and the current SeNB of the user terminal, the required area to be covered around the user terminal (based on predetermined base station coverage maps), the base stations included in current user terminal measurement reports, or any other similar method of determining that there is an area of certain size around the user terminal that is covered by the small cell eNBs in question.

The set may include base stations situated in the direction of movement of the user terminal, based on direct location and speed information, on trail of visited SeNBs, or similar.

Location and movement related data may be partly or fully provided as part of network configuration, or derived from processing of current network statistics, such as user terminal measurements, mobility history information or user terminal history information, for example.

The proposed solution provides many advantages. Signalling loss associ- ated with cells which are prepared for AutoDC but never used is minimised.

AutoDC signalling load over X2 interface is reduced without sacrificing the operation of AutoDC. The embodiments of the invention can adapt to changes in the deployment, such as addition/change of small cells, new pedestrian routes and capac- ity increase in shopping malls etc. In addition, proposed solution can be integrated into Self Organising Network (SON) solutions and Automatic Neighbour Relations (ANR) functionality.

Figure 3 is a flowchart illustrating an example embodiment. The example illustrates the operation of user terminal which utilizes Autonomous Dual Connectivity, AutoDC or autonomous multi connectivity.

In step 302, the user terminal is configured to maintain a primary connection to a first base station serving a macro cell.

In step 304, the user terminal is configured to receive from the first base station information on a set of base stations with which the user terminal may estab- lish at least one secondary connection.

In step 306, the user terminal is configured to measure signal strengths of base stations belonging to the set.

In step 308, the user terminal is configured to request establishing a secondary connection with one or more base stations of the set if the signal strength of the base station is stronger than a given threshold. For example, the user terminal may perform random access in this cell operated by the base station. If dual connectivity is not yet active, this will trigger a SeNB add operation, alternatively it will trigger a SeNB change operation.

Let us study an example referring to Figure 1. Base stations or eNBs 106, 1 10 serve macro cells 108, 1 12 which cover areas with deployment of clusters of small cells SeNB1 - SeNB9. The sizes of the clusters are assumed to be large enough to justify the use of AutoDC. Dense deployment of small cells is required for AutoDC to be efficiently applicable throughout a macro area. However, embodiments of the invention do not depend on clustering, so they are also applicable when the small cells are deployed throughout a macro area, only the most likely scenario is to have clustering.

In the following it is assumed that, when user terminal is in Dual Connectivity, a large macro base station takes on the role of MeNB, and a small base station takes on the role as SeNB. This is a realistic, but arbitrary, assumption on a given real network setup. As one skilled in the art is aware, any eNB can act as MeNB and SeNB, not simultaneously for the same user terminal, but it may simultaneously take on both roles towards two different user terminals. Embodiments of the invention are not limited to the specific network setup. Embodiments of the invention may be utilised basically whenever AutoDC is applicable.

For simplicity in the following it is assumed that a macro cell acts as MeNB, as it has wide coverage, whereas a small cell (SC) only acts as SeNB, as it has lower coverage.

The MeNB acting as a PCell cells may hold information about the base stations serving small cells within its coverage area. This information may consist of Physical Cell Identities (PCI) of cluster of small cells and the RRC configuration, for example.

The user terminals may be configured to perform measurements and send a measurement report to the MeNB acting as a PCell. The MeNB acting as a PCell receives from the user terminal measurement report regarding measured signal strengths of nearby base stations and utilises the measurement report when determin- ing the set of base stations of possible SeNB candidates.

The MeNB acting as a PCell may send the user terminal information on the potential nearby SeNB candidates. These nearby cells are typically some of the SeNB small cells located within the coverage of the MeNB macro cell acting as PCell. Further, the MeNB macro cell acting as PCell may control the preparation of the se- lected set of base stations for the possible secondary connection.

In the example of Figure 1 , the set of base stations might include SeNB1 , SeNB2 and SeNB3, but not base stations SeNB5 and SeNB6.

In an embodiment, the set of base stations selected as potential nearby SeNB candidates may not be explicitly located within coverage of the same macro cell but are deployed as independent eNB for coverage reasons. In the example of Figure 1 the set might include base stations SeNB1 , SeNB2 and SeNB3 belonging to the coverage area of MeNB 106 and base stations SeNB7, SeNB8 and SeNB9 belonging to the coverage area of MeNB 1 10. The MeNB 106 and MeNB2 1 10 may exchange information about the potential small cell cluster utilising interface X2 1 14. Such clus- ter of small cells may be prepared for the user terminal 1 16 when user terminal 1 16 reports one of the cells belonging to the said cluster. The proposed deployment supports the case where a user terminal changes the MeNB and will keep the configuration of SeNB and configuration of candidate SeNBs for AutoDC.

Thus, MeNB 106 may by user terminal measurement reports observe small cells in the vicinity of the user terminal but outside its coverage area and include them in an AutoDC configuration (in the set of base stations as potential candidates). If the neighbouring base station MeNB 1 10 has indicated that these small cells are considered part of a cluster considered covered by the MeNB 1 10, the MeNB 106 may decide to include the whole of this cluster in the AutoDC configuration.

In an embodiment, the MeNB acting as a PCell may be configured to col- lect information from signalling messages between user terminals and/or between eNBs and utilise the information when determining the set of base stations. These messages depend on the system. The MeNB may request user terminal to transfer some information back to the MeNB.

In 3GPP based systems, U El nformation Response message is used to send mobility history Information as a list denoted as VisitedCelllnfoList-r12. The list comprises information about the cells the user terminal has visited and time spent in each cell. The cells may be PCells in RRC Connected mode or serving cells in RRC Idle mode.

Another example in 3GPP based systems is Historylnformation informa- tion element, which is utilized between eNBs over S1 and X2 interfaces respectively to collect information of the cells a user terminal has visited and thereafter identify the potential candidates for Auto DC preparation.

Yet another example in 3GPP based systems is Neighbour Candidate Table, which may be maintained by the Automatic Neighbour Relations (ANR) function- ality of each eNB. This table provides information on all identified neighbours of each eNB.

In an embodiment, the base stations to be included in the set of potential candidates for Auto DC may be the ones within certain range of measurements, timing advance and signal strength.

In an embodiment, the MeNB acting as a PCell may be configured to configure AutoDC for a user terminal entering area served by the MeNB if given criteria are met. The user terminal must support given requirements (such as support Dual Connectivity, for example). The user terminal is not entering from certain directions (certain neighbouring cells). The AutoDC may be limited to user terminals having mo- bility below a given threshold.

In an embodiment, the MeNBs acting as a PCell may be configured maintain a database for each base station, the database comprising information on where the user terminals connected to the base station have changed their connection to; and utilise the information when determining the set of base stations.

In general, base station which may act as a PCell may be configured in certain geographical area to build information and criteria on how to choose a limited number of prepared small cell candidates for autonomous Dual Connectivity operations. Table 1 illustrates an example of a possible database or a part of a database.

Table 1

The table may comprise information received from on user terminals located in the macro area MeNB 106 and MeNB 1 10. Neighbour candidates with SeNB change probability are collected from user terminal measurements and exchanged between cells (over X2 interface, for example). In an embodiment, the numbers in the Source/Target SeNB table represent the number of reported neighbour cells when connected to MeNB 106. The numeric values are illustrative only.

When selecting the base stations to the set of base stations as possible AutoDC candidates, various kind of information may be taken into account.

In an embodiment, small cell reports sent by any user terminal, regardless of whether the user terminal utilises Dual Connectivity or not, are taken into account. Counters may be maintained in the database for each reported small cell base station.

The reported cells may be in separate measurement reports or several cells in a single measurement report may be reported. When given cell is reported one or more times by same user terminal, the counter of the cell (or base station) is incremented as if it would have been a separate report.

Probability of the change to target cell can interpreted as a counter values normalized to unity sum. Values may be represented as a vector per cell. In an em- bodiment, the set of cells, for which this measure exceeds a given threshold, is the set of cells reported with some certainty by user terminals while connected to the macro cell. When this set is used as the set of cells for which AutoDC is configured, it is equal for all cells served by the macro cell base station. Any reported small cell may be included. Cells which lie in a neighbouring macro coverage can provide coverage also in the current macro area.

In an embodiment, small cells which are reported by any user terminal that utilised Dual Connectivity and has particular small cell a SeNB, are taken into ac- count. In this case, a value per cell pair (current SeNB, reported small cell) is maintained in the database. When normalizing a row in the collected source/target table a value is obtained that illustrates probability a small cell is indicated in a report from user terminals that are in Dual Connectivity with the macro as MeNB and a given (other) small cell as SeNB. The statistics may be collected independent for the SeNB that the user terminal is connected to at the time of reporting. The statistics may comprise a probability that a user terminal reports a given other small cell CC2, while being connected to a small cell SC1 as SeNB. In this example, the AutoDC configuration or the set of base stations may comprise the set of SC2 for which reporting probability exceed a given threshold.

As an example, when SeNB changes of a user terminal, the MeNB of the user terminal may compare the current set of cells for which AutoDC is configured with the set of cells determined from the statistics for the new SeNB.

In another example, reconfiguration (set update) of AutoDC is only done when there are significant changes, possibly determined by number of cells in new set and not in current set.

In another example, when configuring AutoDC the set of cells is determined by joining the set of cells determined by the statistics collected for the small cell SC that becomes SeNB, and for neighbours of this cell. The higher the number of neighbours included, the more cells will be included, so there is a trade-off between likelihood of having to reconfigure AutoDC soon and the size of the set of cells for which AutoDC is configured.

In an embodiment, selection of the base stations to the set of base stations as possible AutoDC candidates may take into account SeNB changes of all user terminals applying Dual Connectivity, regardless whether the user terminals are using AutoDC or not. In this example, the probability that a small cell change will go from a given small cell as SeNB to another given small cell as SeNB may be calculated. The MeNB may be configured to collect statistics on what SeNB changes actually happen and include the cells that are commonly targets for a SeNB change.

The size of the prepared set of base stations may thus be reduced while keeping all the most likely target SeNB cells. Therefore the AutoDC signalling load over X2 interface is reduced without sacrificing the operation of AutoDC. For example, while a user terminal is connected to SC1 as SeNB it may report a given SC2. However, collected statistics may indicate that a SeNB change from SC1 to SC2 is a seldom event. Thus, it may be left out of the prepared set of base stations.

In an embodiment, successive operations of SeNB changes within the same AutoDC configuration are taken into account. For example, information related to given small cell SC1 may include the SCs towards which SeNB changes happen more often. As second step information related to each of these SCs include the next SCs towards which SeNB changes are often made. Also 3GPP information elements such as the VisitedCelllnfoList or Historylnformation, may be utilised and obtain infor- mation on multiple steps of SeNB changes. Thus, by collecting information on multiple SeNB change steps, this procedure leads to the set of cells that are most likely visited by a user terminal which was originally connected to SC1.

In an embodiment, selection of the base stations to the set of base stations as possible AutoDC candidates may take into account visited cell list information or history information. The network can collect for each user terminal in dual connectivity the number SeNB changes and/or the time from the SeNB add until SeNB release (or until call ends). The SeNB change count and/or DC duration may be attributed to the first SeNB, i.e. where addition happened. The visited cell list and history information include time of stay in each cell, which enables inclusion of the time dura- tion of connection to SCs as a metric. The rate of how the change of SeNB change happens may be relatively low in an area with generally slow moving user terminals. Visited cell list information or history information may be used to determine the likelihood of an average user terminal staying within range of configured SCs. This can be used to decide if reconfiguration of the set of base stations is needed.

As mentioned, location and movement information of user terminals may be taken into account. Obtaining the location and movement information may be performed in many ways as one skilled in the art is well aware. However, some examples of possible methods may be mentioned.

GPS information or similar satellite location information may be utilised to obtain location and movement information.

User terminal history information, i.e. information on the last handovers, which shows from what cell the user terminal entered the current serving cell, may be utilised. The history information also provides information on user terminal speed, rough estimate based on number of cells visited and time of stay in these cells.

Information on previously visited SeNBs, or at least current SeNB, may provide information on location, and with more visits also on movement. This is similar to applying user terminal history information. However, according to present standards Dual Connectivity operations are not necessarily recorded as part of the user terminal history information.

Measurement reporting on the small cell layer may be enabled, until the point where the user terminal reports a SC, in which case AutoDC is configured, as the UE entered. The reported SC provides location information.

Figure 4A illustrates an embodiment. The figure illustrates a simplified example of an apparatus applying embodiments of the invention. In some embodiments, the apparatus may be an eNodeB, eNB or a base station or a part of an eNB or a base station of a communications system, acting as a MeNB, for example.

It should be understood that the apparatus is depicted herein as an example illustrating some embodiments. It is apparent to a person skilled in the art that the apparatus may also comprise other functions and/or structures and not all described functions and structures are required. Although the apparatus has been depicted as one entity, different modules and memory may be implemented in one or more physical or logical entities.

The apparatus of the example includes a control circuitry 400 configured to control at least part of the operation of the apparatus.

The apparatus may comprise a memory 402 for storing data. Furthermore the memory may store software 404 executable by the control circuitry 400. The memory may be integrated in the control circuitry.

The apparatus comprises a transceiver 406. The transceiver is operationally connected to the control circuitry 400. It may be connected to an antenna arrangement (not shown). The transceiver may enable the apparatus to communicate with user terminals, for example.

The software 404 may comprise a computer program comprising program code means adapted to cause the control circuitry 400 of the apparatus at least to transmit at least one control signal unique to the apparatus within a given area and transmit a second control signal common to a set of apparatuses controlled by the same controller.

The apparatus may further comprise interface circuitry 408 configured to connect the apparatus to other devices and network elements of communication system, for example to core. The interface may provide a wired or wireless connection to the communication network. The apparatus may be in connection with core network elements, other eNodeB's, and with other respective apparatuses of communication systems. Figure 4B illustrates an embodiment. The figure illustrates a simplified example of an apparatus applying embodiments of the invention. In some embodiments, the apparatus may be a user terminal or a part of a user terminal of a communications system.

It should be understood that the apparatus is depicted herein as an example illustrating some embodiments. It is apparent to a person skilled in the art that the apparatus may also comprise other functions and/or structures and not all described functions and structures are required. Although the apparatus has been depicted as one entity, different modules and memory may be implemented in one or more physi- cal or logical entities.

The apparatus of the example includes a control circuitry 420 configured to control at least part of the operation of the apparatus.

The apparatus may comprise a memory 422 for storing data. Furthermore the memory may store software 424 executable by the control circuitry 400. The memory may be integrated in the control circuitry.

The software 424 may comprise a computer program comprising program code means adapted to cause the control circuitry 400 of the apparatus at least to control the operation of a set of base stations, receive information regarding a user terminal requesting a handover on the basis of a control signal transmitted by more than one base stations belonging to the set of base stations, determine the base station to which the user terminal is to be handed over, and control the handover of the user terminal to the determined base station.

The apparatus may further comprise a transceiver 426. The transceiver is operationally connected to the control circuitry 420. It may be connected to an anten- na arrangement (not shown). The transceiver may enable the apparatus to communicate with base stations, for example.

In an embodiment, as shown in Figure 5, at least some of the functionalities of the apparatus of Figure 4A may be shared between two physically separate devices, forming one operational entity. Therefore, the apparatus may be seen to de- pict the operational entity comprising one or more physically separate devices for executing at least some of the described processes. Thus, the apparatus of Figure 5, utilizing such shared architecture, may comprise a remote control unit RCU 500, such as a host computer or a server computer, operatively coupled (e.g. via a wireless or wired network) to a remote radio head RRH 502 located in the base station. In an em- bodiment, at least some of the described processes may be performed by the RCU 500. In an embodiment, the execution of at least some of the described processes may be shared among the RRH 502 and the RCU 500.

In an embodiment, the RCU 500 may generate a virtual network through which the RCU 500 communicates with the RRH 502. In general, virtual networking may involve a process of combining hardware and software network resources and network functionality into a single, software-based administrative entity, a virtual network. Network virtualization may involve platform virtualization, often combined with resource virtualization. Network virtualization may be categorized as external virtual networking which combines many networks, or parts of networks, into the server computer or the host computer (e.g. to the RCU). External network virtualization is targeted to optimized network sharing. Another category is internal virtual networking which provides network-like functionality to the software containers on a single system. Virtual networking may also be used for testing the terminal device.

In an embodiment, the virtual network may provide flexible distribution of operations between the RRH and the RCU. In practice, any digital signal processing task may be performed in either the RRH or the RCU and the boundary where the responsibility is shifted between the RRH and the RCU may be selected according to implementation.

The steps and related functions described in the above and attached fig- ures are in no absolute chronological order, and some of the steps may be performed simultaneously or in an order differing from the given one. Other functions can also be executed between the steps or within the steps. Some of the steps can also be left out or replaced with a corresponding step.

The apparatuses or controllers able to perform the above-described steps may be implemented as an electronic digital computer, which may comprise a working memory (RAM), a central processing unit (CPU), and a system clock. The CPU may comprise a set of registers, an arithmetic logic unit, and a controller. The controller is controlled by a sequence of program instructions transferred to the CPU from the RAM. The controller may contain a number of microinstructions for basic operations. The implementation of microinstructions may vary depending on the CPU design. The program instructions may be coded by a programming language, which may be a high-level programming language, such as C, Java, etc., or a low-level programming language, such as a machine language, or an assembler. The electronic digital computer may also have an operating system, which may provide system services to a computer program written with the program instructions. As used in this application, the term 'circuitry' refers to all of the following: (a) hardware-only circuit implementations, such as implementations in only analog and/or digital circuitry, and (b) combinations of circuits and software (and/or firmware), such as (as applicable): (i) a combination of processor(s) or (ii) portions of proces- sor(s)/software including digital signal processor(s), software, and memory(ies) that work together to cause an apparatus to perform various functions, and (c) circuits, such as a microprocessor(s) or a portion of a microprocessor(s), that require software or firmware for operation, even if the software or firmware is not physically present.

This definition of 'circuitry' applies to all uses of this term in this applica- tion. As a further example, as used in this application, the term 'circuitry' would also cover an implementation of merely a processor (or multiple processors) or a portion of a processor and its (or their) accompanying software and/or firmware. The term 'circuitry' would also cover, for example and if applicable to the particular element, a baseband integrated circuit or applications processor integrated circuit for a mobile phone or a similar integrated circuit in a server, a cellular network device, or another network device.

An embodiment provides a computer program embodied on a distribution medium, comprising program instructions which, when loaded into an electronic apparatus, are configured to control the apparatus to execute the embodiments described above.

The computer program may be in source code form, object code form, or in some intermediate form, and it may be stored in some sort of carrier, which may be any entity or device capable of carrying the program. Such carriers include a record medium, computer memory, read-only memory, and a software distribution package, for example. Depending on the processing power needed, the computer program may be executed in a single electronic digital computer or it may be distributed amongst a number of computers.

The apparatus may also be implemented as one or more integrated circuits, such as application-specific integrated circuits ASIC. Other hardware embodi- ments are also feasible, such as a circuit built of separate logic components. A hybrid of these different implementations is also feasible. When selecting the method of implementation, a person skilled in the art will consider the requirements set for the size and power consumption of the apparatus, the necessary processing capacity, production costs, and production volumes, for example.

It will be obvious to a person skilled in the art that, as the technology advances, the inventive concept can be implemented in various ways. The invention and its embodiments are not limited to the examples described above but may vary within the scope of the claims.

Claims

1. 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:

maintain a primary connection to a user terminal configured to operate using connectivity with plurality of connections;

determine on the basis of one or more given criteria a set of base stations to which the user terminal may establish at least one secondary connection;

transmit information on the set to the user terminal; and

control the preparation of the set of base stations for the possible one or more secondary connections.

2. The apparatus of claim 1 , the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus, further to:

receive from the user terminal measurement report regarding measured signal strengths of nearby base stations; and

utilise the measurement report when determining the set of base stations.

3. The apparatus of claim 1 or 2, the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus, further to:

obtain information on the location and movement direction of the user terminal; and

utilise the information when determining the set of base stations.

4. The apparatus of any preceding claim, the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus, further to:

obtain information on the cells the user terminal has visited and time spent in the cells; and

utilise the information when determining the set of base stations.

5. The apparatus of claim 3, the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus, further to:

determine the set of base stations on the basis of the distance between each base station and the user terminal.

6. The apparatus of any preceding claim 1 to 4, the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus, further to:

determine the set of base stations on the basis of the distance between each base station and the base station currently having a secondary connection with the user terminal.

7. The apparatus of any preceding claim, the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus, further to:

receive measurements reports from user terminals, the reports comprising base stations detected by the user terminals;

maintain a database for each base station, the database comprising an indication of the probability of a user terminal connected to the base station to change to another base station; and

utilise the database when determining the set of base stations.

8. The apparatus of any preceding claim, the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus, further to:

maintain a database for each base station, the database comprising information on where the user terminals connected to the base station have changed their connection to; and

utilise the information when determining the set of base stations.

9. The apparatus of any preceding claim, wherein the apparatus is a macro cell base station and the base stations belonging to the set are a subset of base stations within the coverage area of the apparatus.

10. The apparatus of any preceding claim, wherein the apparatus is a macro cell base station and the set comprises base stations which are outside the coverage area of the apparatus.

1 1. The apparatus of any preceding claim, wherein the information on the set comprises Physical Cell Identities and Radio Resource Control Configurations of the base stations of the set.

12. An apparatus comprising:

at least one processor; and

at least one memory including computer program code,

maintain a primary connection to a first base station serving a cell;

receive from the first base station information on a set of base stations with which the apparatus may establish at least one secondary connection;

measure signal strengths of base stations belonging to the set; and request establishing a secondary connection with at least one base station of the set if the signal strength of the base station is stronger than a given threshold.

13. The apparatus of claim 12, wherein the information on the set comprises Physical Cell Identities and Radio Resource Control Configurations of the base stations of the set.

14. A method, comprising:

maintaining a primary connection to a user terminal configured to operate using connectivity with plurality of connections;

determining on the basis of one or more given criteria a set of base stations to which the user terminal may establish at least one secondary connection;

transmitting information on the set to the user terminal; and

controlling the preparation of the set of base stations for the possible one or more secondary connections.

15. The method of claim 14 further comprising:

receiving from the user terminal measurement report regarding measured signal strengths of nearby base stations; and

utilising the measurement report when determining the set of base stations.

16. The method of claim 14 further comprising:

obtaining information on the location and movement direction of the user terminal; and

utilising the information when determining the set of base stations.

17. The method of any preceding claim 14 to 16, further comprising:

obtaining information on the cells the user terminal has visited and time spent in the cells; and

utilising the information when determining the set of base stations.

18. The method of claim 16, further comprising:

determining the set of base stations on the basis of the distance between each base station and the user terminal.

19. The method of any preceding claim 14 to 17, further comprising:

determining the set of base stations on the basis of the distance between each base station and the base station currently having a secondary connection with the user terminal.

20. The method of any preceding claim 14 to 19, further comprising:

receiving measurements reports from user terminals, the reports comprising base stations detected by the user terminals;

maintaining a database for each base station, the database comprising an indication of the probability of a user terminal connected to the base station to change to another base station; and

utilising the database when determining the set of base stations.

21. The method of any preceding claim 14 to 20, further comprising:

maintaining a database for each base station, the database comprising information on where the user terminals connected to the base station have changed their connection to; and

utilising the information when determining the set of base stations.

22. The method of any preceding claim 14 to 21 , wherein the apparatus is a macro cell base station and the base stations belonging to the set are a subset of base stations within the coverage area of the apparatus.

23. The method of any preceding claim 14 to 22, wherein the apparatus is a macro cell base station and the set comprises base stations which are outside the coverage area of the apparatus.

24. The method of any preceding claim 14 to 23, wherein the information on the set comprises Physical Cell Identities and Radio Resource Control Configurations of the base stations of the set.

25. A method comprising:

maintaining a primary connection to a first base station serving a cell; receiving from the first base station information on a set of base stations with which the apparatus may establish at least one secondary connection;

measuring signal strengths of base stations belonging to the set; and requesting establishing a secondary connection with at least one base station of the set if the signal strength of the base station is stronger than a given threshold.

26. The method of claim 25, wherein the information on the set comprises

Physical Cell Identities and Radio Resource Control Configurations of the base stations of the set.

27. A computer program product embodied on a distribution medium readable by a computer and comprising program instructions which, when loaded into an apparatus, execute the method according to any of claims 14 to 26.