WO2013131250A1

WO2013131250A1 - Apparatus, method and computer program product for enabling reliable communication using an unlicensed frequency band

Info

Publication number: WO2013131250A1
Application number: PCT/CN2012/072001
Authority: WO
Inventors: Na WEI; Wei Bai; Erlin Zeng; Haiming Wang; Wei Hong; Pengfei Sun
Original assignee: Renesas Mobile Corporation
Priority date: 2012-03-06
Filing date: 2012-03-06
Publication date: 2013-09-12

Abstract

The present invention addresses a method, apparatus and computer program product for systems operated on shared bands, which enhance reliability of channels. The present invention includes determining candidate carriers among carriers in unlicensed frequency bands, which enable provision of communication control function, and aggregating, for communication in carrier aggregation mode, at least two primary carriers among the determined candidate carriers, and at least one secondary carrier.

Description

APPARATUS, METHOD AND COMPUTER PROGRAM PRODUCT FOR ENABLING RELIABLE COMMUNICATION USING AN UNLICENSED

FREQUENCY BAND

Field of the invention

The present invention generally relates to enhancements on channel reliability in scenarios operating on unlicensed frequency bands. More particularly, the present invention addresses an apparatus, method and computer program product for systems operated on unlicensed frequency bands, which enhance reliability of channels.

Background

Prior art which is related to this technical field can e.g. be found in technical specifications according to 3GPP TR 36.300 (version 10.3.0).

Mobile data transmission and data services are constantly making progress. With the increasing penetration of such services, a need for increased bandwidth for conveying the data is emerging. In conventional scenarios, networks operated on reserved bands (licensed bands) within the available spectrum, which were reserved for the particular network. As licensed band operation has been increasingly utilized, portions of the radio spectrum that remain available have become limited. Thus, operators, service providers, communication device manufacturers, and communication system manufacturers, are all seeking efficient solutions to utilize unlicensed shared band operation. Communication on an unlicensed shared band is generally based on sharing an available channel between different communication devices.

Currently, a system known as Long Term Evolution LTE is being further developed. When the LTE system concept is further extended in a way that it can be deployed also on unlicensed bands, the devices and local access points have potentially more spectrum available. That spectrum is to be used opportunistically as explained above. This setting can be considered as a kind of non-contiguous carrier aggregation, in which unlicensed spectrum is used as resources for secondary carriers/cells for the licensed spectrum primary and secondary carriers/cells, controlled by the network transceiver station (or access node) known as Evolved Node_B, eNB. One step further would be to deploy an eNB totally on some shared band, like in television white space TVWS or in the industrial, scientific and medical, ISM band without any anchor in licensed spectrum.

As a future LTE-A system may be deployed on unlicensed bands (e.g. TVWS or ISM bands), for example via carrier aggregation methods as mentioned above, the environment of the spectrum sets further requirements/challenges for the system to operate appropriately.

However, if a primary cell/carrier (PCell) is formed in an unlicensed band scenario, such as in an ISM band, television white space TVWS, etc., the reliability of the PCell may not be possible to maintain anymore. For standalone case, when all involved carriers are on the unlicensed band, each carrier can experience interference or need to switch off from time to time to allow co-existing system's transmission, or just to follow regulatory requirements. For hybrid case (e.g. both licensed and unlicensed bands are available), in case the PCell is formed in an unlicensed band, the similar issue will arise.

Thus, there is still a need to further improve strategies for more efficiently handling reliability of PCell in an unlicensed band.

Summary of the Invention

It is an object of the present invention to address the above problems. In particular, it is an object of the present invention to provide an apparatus, a method and a computer program product for enabling reliable communication using an unlicensed frequency band. According to a first aspect of the present invention, there is provided an apparatus, which comprises at least one processor and at least one memory including computer program code, wherein the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus at least to perform determining candidate carriers among carriers in unlicensed frequency bands, which candidate carriers enable provision of communication control function, and aggregating, for communication in carrier aggregation mode, at least two primary carriers among the determined candidate carriers, and at least one secondary carrier.

According to a second aspect of the present invention, there is provided a method, comprising determining candidate carriers among carriers in unlicensed frequency bands, which candidate carriers enable provision of communication control function, and aggregating, for communication in carrier aggregation mode, at least two primary carriers among the determined candidate carriers, and at least one secondary carrier. According to a third aspect of the present invention, there is provided a computer program product comprising computer-executable components which, when the program is run on a computer, are configured to determine candidate carriers among carriers in unlicensed frequency bands, which candidate carriers enable provision of communication control function, and to aggregate, for communication in carrier aggregation mode, at least two primary carriers among the determined candidate carriers, and at least one secondary carrier.

According to a fourth aspect of the present invention, there is provided an apparatus, which comprises determination means for determining candidate carriers among carriers in unlicensed frequency bands, which candidate carriers enable provision of communication control function, and aggregating means for aggregating, for communication in carrier aggregation mode, at least two primary carriers among the determined candidate carriers, and at least one secondary carrier.

Advantageous further developments or modifications of the aforementioned exemplary aspects of the present invention are set out in the dependent claims.

The present invention as defined in the claims provides, among others, the following advantages.

- The PCell function can be more reliability performed in unreliable unlicensed band scenario.

- Better reliability of critical information can be achieved with multiple carriers within the PCell group. Reduce unnecessary RLF or handover to other el\IB etc.

- The modification required in specification/implementation is minor.

Brief description of drawings

For a more complete understanding of example embodiments of the present invention, reference is now made to the following descriptions taken in connection with the accompanying drawings in which : Fig. 1 shows a principle configuration of an example for an apparatus according to certain embodiments of the present invention;

Fig. 2 shows a principle flowchart of an example for a method according to certain embodiments of the present invention; and

Fig. 3 illustrates an embodiment of the present invention, wherein the present invention is deployed in white space LTE time division duplex. Description of exemplary embodiments

Exemplary aspects of the present invention will be described herein below. More specifically, exemplary aspects of the present are described hereinafter with reference to particular non-limiting examples and to what are presently considered to be conceivable embodiments of the present invention. A person skilled in the art will appreciate that the invention is by no means limited to these examples, and may be more broadly applied. It is to be noted that the following description of the present invention and its embodiments mainly refers to specifications being used as non-limiting examples for certain exemplary network configurations and deployments. Namely, the present invention and its embodiments are mainly described in relation to 3GPP specifications being used as non-limiting examples for certain exemplary network configurations and deployments. In particular, a LTE/LTE-Advanced communication system is used as a non-limiting example for the applicability of thus described exemplary embodiments. As such, the description of exemplary embodiments given herein specifically refers to terminology which is directly related thereto. Such terminology is only used in the context of the presented non-limiting examples, and does naturally not limit the invention in any way. Rather, any other network configuration or system deployment, etc. may also be utilized as long as compliant with the features described herein. Hereinafter, various embodiments and implementations of the present invention and its aspects or embodiments are described using several alternatives. It is generally noted that, according to certain needs and constraints, all of the described alternatives may be provided alone or in any conceivable combination (also including combinations of individual features of the various alternatives).

Generally, carrier aggregation serves to enhance the bandwidth and/or transmission rate and is primarily deployed on downlink carriers in the frequency division duplex system (FDD), but can also be deployed in relation to a phase of a carrier if a TDD system is concerned (time division duplex). In carrier aggregation, individual carriers are "joined" to form the aggregated carrier so as to benefit from the combined bandwidth thereof. That is, carrier aggregation (CA) e.g. in LTE-Advanced extends the maximum bandwidth in the uplink (UL) or downlink (DL) directions by aggregating multiple carriers within a frequency band (intra-band CA) or across frequency bands (inter-band CA).

It has been proposed that a primary cell carrier (PCC) e.g. using LTE technology is configured on the licensed band for Primary Access providing e.g. at least one of mobility, security and state management for user terminals while secondary cell carrier (SCC) is opportunistically configured/activated on the unlicensed band for secondary access to provide additional data plane transport.

However, the licensed spectrum, supervised by the operators, is a scarce resource of which amount along with the current policy of using the licensed spectrum (lack of wide usage of flexible spectrum usage) may not be enough to support huge number of cellular devices and higher-QoS (Quality of Service) traffic in the near future. So traffic off-loading to unlicensed band e.g. ISM (industrial, scientific and medical) band and TV White Space (TVWS) may be an attractive solution for cellular operators.

However, to find a totally free channel in unlicensed band becomes more and more difficult, since there are many other systems which might utilize those frequencies too. For example, the potential co-exisiting systems could be: IEEE 802.11 b/a/g/n/ac, Bluetooth, Zigbee, etc. In case the LTE also wish to be utilize the unlicensed band, it may face the challenges of more agile spectrum sharing or time sharing with those systems. Therefore, it is important to study the impact of different types of interference from other possible co-existing systems in the unlicensed band.

By the way of utilization a shared band, the technologies can be divided into two categories: - Standalone utilization : The whole LTE system is operating in shared band

- Hybrid utilization : In this scenario, both licensed band and unlicensed band are available for LTE.

When a Carrier Aggregation (CA) is configured, the User Equipment (UE) only has one Radio Resource Control (RRC) connection with the network. At RRC connection establishment/re-establishment/handover, one serving cell provides the Non-Access-Stratum (NAS) mobility information (e.g. TAI), and at RRC connection re-establishment/handover, one serving cell provides the security input. This cell is referred to as the primary cell (PCell). In the downlink, the carrier corresponding to the PCell is the Downlink Primary Component Carrier (DL PCC) while in the uplink it is the Uplink Primary Component Carrier (UL PCC).

The configured set of serving cells for a UE therefore always consists of one PCell and one or more Secondary Cells (SCells) : - PCell can only be changed with handover procedure (i.e. with security key change and RACH procedure);

- PCell is used for transmission of PUCCH;

- Unlike SCells, PCell cannot be de-activated;

- Re-establishment is triggered when PCell experiences RLF, not when SCells experience RLF;

- IMAS information is taken from PCell.

And due to reliable of PCell, some features are considered only happen in PCell, such as:

- When CA is configured, the contention based random access procedures occur on the PCell while contention resolution can be cross-scheduled by the PCell; - Cross-carrier scheduling does not apply to PCell; i.e. PCell is always scheduled via its PDCCH ;

- When CA is configured, semi-persistent uplink/downlink resources can only be configured for the PCell and only PDCCH allocations for the PCell can override the semi-persistent allocation.

Fig. 1 shows a principle configuration of an example for an apparatus according to certain embodiments of the present invention. The apparatus 100 comprises at least one processor 101 and at least one memory 102 including computer program code, which are connected by a bus 104 or the like. As indicated with a dashed line in Fig. 1, an interface 103 may optionally be connected to the bus 104 or the like, which may enable communication e.g. to/from a network entity, a base station, user equipment UE, or the like. The at least one memory and the computer program code are arranged to, with the at least one processor, cause the apparatus at least to perform determining candidate carriers among carriers in unlicensed frequency bands, which candidate carriers enable provision of communication control function, and aggregating, for communication in carrier aggregation mode, at least two primary carriers among the determined candidate carriers, and at least one secondary carrier.

Fig. 2 shows a principle flowchart of an example for a method according to certain embodiments of the present invention. In Step S21, candidate carriers among carriers in unlicensed frequency bands are determined, wherein the candidate carriers enable provision of communication control function. According to certain embodiments, those carriers of the first frequency band (unlicensed band) are determined, which would be able to fulfill the requirements of a primary cell/carrier as e.g. defined in 3GPP TR 36.300 (version 10.3.0), i.e. which enable the provision of at least one of mobility, security and state management.

In Step S22, at least two primary carriers among the determined candidate carriers and at least one secondary carrier are aggregated for communication in carrier aggregation mode. According to certain embodiments, the aggregation is carried out according to well known aggregation methods according to carrier aggregation as defined e.g. in 3GPP TR 36.300 (e.g. version 10.3.0). Thereby, as one scenario, it is possible to aggregate carriers in uplink communication and downlink communication in a first frequency band, such as an unlicensed frequency band, or as hybrid scenario in a first frequency band, such as an unlicensed band, and in a second frequency band, such as a licensed band. According to certain embodiments, by assigning the function of a primary cell to at least two candidate carriers, a PCe!l group is formed, wherein each of the actually assigned PCells within the PCell group are located within the unlicensed band. In the PCell group thus formed, at one time, at least one primary cell of the group of primary cells is an active carrier. That is, according to certain embodiments, since two or more PCells are assigned, in case any of these PCells may break down, another one or plural PCells of the PCell group may still be operable. Thereby, the PCell function can be more reliably performed in unreliable unlicensed band scenario, better reliability of critical information can be achieved with multiple carriers within the PCell group, and unnecessary RLF or handover to other eNB may be reduced, etc, wherein the required modification in specification/implementation is minor.

Thereby, a transmission strategy using Carrier Aggregation is provided, wherein two or more primary cells/carriers located in an unlicensed band are assigned and operable. Hence, a system is provided which comprises two or more PCells, and which may comprise one or more SCells. The SCells may be located in the licensed frequency band and/or the unlicensed frequency band.

Furthermore, as advantageous modification, the determination function is configured to determine a candidate carrier from the aggregated carriers when the reliability of the candidate carrier exceeds a preset reliability threshold. This enables to further improve the reliability of PCell function in unreliable unlicensed band scenario.

The PCell group according to one of embodiments of the invention consists of a group of carriers, and can together share the responsibilities of one original PCefl. According to certain embodiments, some exemplary rules on its formation are listed below.

- Only carriers which can carry system information for itself, and carry DL control region are candidate carriers, e.g., Release 8 compatible carriers could be candidate carriers.

- Relatively reliable carriers are candidate carriers.

- Any carrier within the PCell group may be turned off or experience break down due to interference or regulatory requirements; at one time, there is at least one active carrier in the PCell group.

- At least one carrier within the PCell group is not cross-scheduled.

- PCell group change can be updated with new defined signaling such as new MAC CE or RRC signaling.

- The determination of UL PCell group is based on DL PCell group's SIB2 linkage. The update of DL PCell group implicitly implies update on UL PCell group.

According to an exemplary embodiment, the PCell group of the present invention may have the following properties: - NAS and Security related information could be transmitted in one or some carriers within the PCell group.

- Re-establishment is triggered only when all carriers within PCell group experience RLF, not when SCells experience RLF.

- System info is to be transmitted on some or all carriers within the PCell group. The dedicated RRC for some PCell carriers and/or ail

Scells could be delivered from one or some carriers within the PCell group. - EACH : UE may send simultaneous RACH on one or multiple carriers within the UL PCell group, and feedback could be on any carrier within the DL PCell group.

- SPS could be on one or more carriers within the PCell group.

- Mobility: Triggered measurement events related to PCell are to consider all carriers within the PCell group.

As an exemplary embodiment, the present invention can be exemplified in white space LTE TDD standalone deployment, wherein an overview of channels for connection of an UE with an e B is shown in Fig. 3.

Fig. 3 illustrates, as an example, a VHF band (low) comprising channels 2 to 6, a VHF band comprising the channels 7 to 13, and a UHF band comprising channels 14 to 51. In the UHF band, TV channels are broadcasted. Not allowed channels are crossed out, and channels in which Fixed to Fixed communication is allowed are shaded.

As one exemplary embodiment, during initial access, the UE establishes a connection with an eNB via Channel 27, and Channel 27 is marked as Cell index 0 for this UE, and defined as the index 0 for the PCell group for this UE. Later, the UE with CA capability is configured with more carriers, and one example is: PCell group (Channel 27 (cell index 0) and 28 (eel! index 2)), Sce!ls (Channel 21(cell index 1), 35(celf index 4)). Channel 21 may be a new carrier type which does not have control channel, and thus can not be included in the PCell group. Since the PCell group contains both Channel 27 and 28, in case Channel 27 is broken due to interference, the UE will not trigger RLF procedure, and the eNB is still connected with UE via Channel 28.

The determination of UL (uplink) PCell group is based on DL (downlink) PCell group's SIB2 linkage. The update of DL PCell group implicitly implies update on UL PCell group. In LTE Rel-10 the PUCCH region is only configured on the only UL PCell. According to the UL PCell group concept of the present invention, it is supported that UE can be configured PUCCH region on one or some of the UL CCs within the PCell group already before any of the cell in the UL PCell group is broken. With some pre-configurations, it is supported that UE switches from a first UL CC to a second UL CC quickly after the first UL CC is broken.

One possible configuration according to an embodiment of the present invention is that there are two carriers assigned in a PCell group. However all PCell functions are only carried out in one carrier in this PCell group. For example, the PCell group consists of Channel 27 and 28 (cf. Fig. 3), while all PCell functions are carried out in Channel 27 only while it is active. But when Channel 27 breaks down, by default all PCell functions are performed in Channel 28 instead now.

That is, Channel 28 is a backup PCell for channel 27 only when Channel 27 breaks down. In this case, the cell on channel 28 is treated as a secondary PCell. The difference of such secondary PCell compared with a Re!-10 (Release 10) SCeii in LTE is that the necessary configurations of cell on channel 28 have not been ready even when channel 27 operates well, which means that the cell of channel 28 can function as PCell quickly after channel 27 is broken compared to the quite long RRC reconfiguration period.

In view of the above mentioned specifications defined in 3GPP TR 36.300 (e.g. version 10.3.0), there are the following three event triggered reports which are related with a PCell.

- Event A3 (Neighbour becomes offset better than PCell)

- Event A5 (PCell becomes worse than thresholdl and neighbour becomes better than threshoid2)

- Event B2 (PCell becomes worse than thresholdl and inter RAT neighbour becomes better than thresho!d2)

Upon implementing the present invention, that is, when assigning a PCell group in an unlicensed band, as an example, these events may be updated to : - Event A3 (Neighbour becomes offset better than all carriers within PCell group)

- Event A5 (all carriers within PCell group becomes worse than thresholdl and neighbour becomes better than threshold2)

- Event B2 (all carriers within PCell group becomes worse than thresholdl and inter RAT neighbour becomes better than threshold2)

- Besides, one new triggered event reports may be added for PCell group.

- Event A7 (Neighbour becomes offset better than one or some carriers within PCell group)

Based on Event A7, as an example, it could be useful for eNB to consider UE's suggestion on updating the PCell group.

In the foregoing exemplary description of the apparatus, only the units that are relevant for understanding the principles of the invention have been described using functional blocks. The apparatuses may comprise further units that are necessary for its respective function. However, a description of these units is omitted in this specification. The arrangement of the functional blocks of the apparatuses is not construed to limit the invention, and the functions may be performed by one block or further split into sub- blocks. According to exemplarily embodiments of the present invention, a system may comprise any conceivable combination of the thus depicted devices/apparatuses and other network elements, which are arranged to cooperate as described above. Embodiments of the present invention may be implemented in software, hardware, application logic or a combination of software, hardware and application logic. The software, application logic and/or hardware generally, but not exclusively, may reside on the devices' modem module. In an example embodiment, the application logic, software or an instruction set is maintained on any one of various conventional computer-readable media. In the context of this document, a "computer-readable medium" may be any media or means that can contain, store, communicate, propagate or transport the instructions for use by or in connection with an instruction execution system, apparatus, or device, such as a computer or smart phone, or user equipment.

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) to combinations of circuits and software (and/or firmware), such as (as applicable): (i) to a combination of processor(s) or (ii) to portions of processor(s)/software (including digital signal processor(s)), software, and memory(ies) that work together to cause an apparatus, such as a mobile phone or server, to perform various functions) and (c) to 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 application, including in any claims. 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 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 claim element, a baseband integrated circuit or applications processor integrated circuit for a mobile phone or a similar integrated circuit in server, a cellular network device, or other network device.

The present invention relates in particular but without limitation to mobile communications, for example to environments under LTE, WCDMA, WIMAX and WLAN and can advantageously be implemented in user equipments or smart phones, or personal computers connectable to such networks. That is, it can be implemented as/in chipsets to connected devices, and/or modems thereof. If desired, the different functions discussed herein may be performed in a different order and/or concurrently with each other. Furthermore, if desired, one or more of the above-described functions may be optional or may be combined.

Although various aspects of the invention are set out in the independent claims, other aspects of the invention comprise other combinations of features from the described embodiments and/or the dependent claims with the features of the independent claims, and not solely the combinations explicitly set out in the claims.

It is also noted herein that while the above describes example embodiments of the invention, these descriptions should not be viewed in a limiting sense. Rather, there are several variations and modifications which may be made without departing from the scope of the present invention as defined in the appended claims.

The following meanings for the abbreviations used in this specification apply:

CA Carrier Aggregation

CC Component Carrier

CCE Control Channel Elements

DL Downlink

eNB Evolved Node B

FDD Frequency Division Duplex

ISM Industrial, Scientific and Medical

PDCCH Physical Downlink Control Channel

PDSCH Physical Downlink Shared Channel

RAT Radio Access Technology

TVWS TV White Space

UE User Equipment

UL Uplink

LTE Long Term Evolution

EUTRAN Evolved Universal Terrestrial Radio Access Network WLAN Wireless Local Area Network

IEEE Institute of Electrical and Electronics Engineers

CQI Channel Quality Indication

RRC Radio Resource Control

RRM Radio Resource Management

MCS Modulation and Coding Scheme

SINR Stgnal-to-Interference-and-Noise-Ratio

DCI Downlink Control Information

PCC Primary Component Carrier

see Secondary Component Carrier

Claims

What is Claimed is:

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 perform determining candidate carriers among carriers in unlicensed frequency bands, which candidate carriers enable provision of communication control function; and

aggregating, for communication in carrier aggregation mode, at least two primary carriers among the determined candidate carriers, and at least one secondary carrier.

2. The apparatus according to claim 1, wherein during communication in carrier aggregation mode, at least one of the at least two primary carriers is an active carrier.

3. The apparatus according to claim 1 or 2, wherein the communication control function comprises at least one of mobility information, security information and system information.

4. The apparatus according to any of the preceding claims, wherein the determination further comprises assigning a candidate carrier among carriers in unlicensed frequency bands if the reliability thereof exceeds a preset reliability threshold.

5. The apparatus according to any of the preceding claims, wherein at least one carrier within the at least two primary carriers is not cross-scheduled.

6. The apparatus according to any of the preceding claims, wherein the determination of a group of uplink communication primary carriers is based on a system information block linkage of the group of downlink communication primary carriers.

7. The apparatus according to claim 1, wherein the at least one memory and the computer program code are configured to, with the at least one processor, cause the apparatus at least to further perform an update function configured to update the at least two primary carriers.

8. The apparatus according to claim 7, wherein the update of the at least two primary carriers is performed with at least one of a medium access control signaling and radio resource control signaling.

9. The apparatus according to any of the preceding claims, wherein non- access stratum, security related information, and system information are transmitted in at least one carrier within the at least two primary carriers.

10. The apparatus according to any of the preceding claims, wherein the at least one memory and the computer program code are configured to, with the at least one processor, cause the apparatus at least to further perform a communication re-establishment function configured to trigger a re- establishment of communication when all carriers within the at least two primary carriers experience a radio link failure.

11. A method, comprising

determining candidate carriers among carriers in unlicensed frequency bands, which candidate carriers enable provision of communication control function; and

12. The method according to claim 11, wherein during communication in carrier aggregation mode at least one of the at least two primary carriers is an active carrier.

13. The method according to claim 11 or 12, wherein the communication control function comprises at least one of mobility information, security information and system information.

14. The method according to any of claims 11 to 13, wherein the determination further comprises assigning a candidate carrier among carriers in unlicensed frequency bands if the reliability thereof exceeds a preset reliability threshold.

15. The method according to any of claims 11 to 14, wherein at least one carrier within the at least two primary carriers is not cross-scheduled.

16. The method according to any of claims 11 to 15, wherein the determination of a group of uplink communication primary carriers is based on a system information block linkage of the group of downlink communication primary carriers.

17. The method according to any of claims 11 to 15, further comprising updating the at least two primary carriers.

18. The method according to claim 17, wherein the update of the at least two primary carriers is performed with at least one of a medium access control signaling and radio resource control signaling.

19. The method according to any of claims 11 to 18, wherein non-access stratum, security related information, and system information are transmitted in at least one carrier within the at least two primary carriers.

20. The method according to any of claims 11 to 19, further comprising triggering a re-establishment of communication when all carriers within the at feast two primary carriers experience a radio link failure.

21. A computer program including a program for a processing device, comprising software code portions for performing the steps according to any of claim 10 to 18 when the program is run on the processing device.

22. The computer program product according to claim 21, wherein the computer program product comprises a computer-readable medium on which the software code portions are stored.

23. The computer program product according to claim 21, wherein the program is directly loadable into an internal memory of the processing device.

24. An apparatus, comprising

determination means for determining candidate carriers among carriers in unlicensed frequency bands, which candidate carriers enable provision of communication control function; and

aggregating means for aggregating, for communication in carrier aggregation mode, at least two primary carriers among the determined candidate carriers, and at least one secondary carrier.