WO2012075809A1

WO2012075809A1 - Method and device of communication

Info

Publication number: WO2012075809A1
Application number: PCT/CN2011/076928
Authority: WO
Inventors: Fredrik Berggren; Yuhua Chen; Jianghua Liu
Original assignee: Huawei Technologies Co Ltd
Current assignee: Huawei Technologies Co Ltd
Priority date: 2010-12-08
Filing date: 2011-07-07
Publication date: 2012-06-14
Anticipated expiration: 2013-06-08

Abstract

The invention concerns a method for triggering aperiodic channel state information reports in a cellular communications system employing carrier aggregation. The method is distinguished by: - transmitting at least one triggering bit in a downlink control channel, said at least one triggering bit corresponding to a plurality of trigger states, - associating at least one of the trigger states of said at least one triggering bit with at least one bitmap comprising a number of bits corresponding to bitmap states, - determining from said bitmap, when said at least one associated trigger state is valid for said bitmap, a set of carriers for which aperiodic channel state information should be reported, - arranging transmission of channel state information reports of said determined set of carriers. The invention also concerns a mobile terminal and a base station.

Description

METHOD AND DEVICE OF COMMUNICATION

TECHNICAL FIELD

The invention relates to a method for triggering aperiodic channel state information reports in a cellular communications system employing carrier aggregation. It further relates to a mobile terminal and a base station.

RELATED ART AND BACKGROUND OF THE INVENTION

In cellular systems, Channel State Information (CSI) (or alternatively, Channel Quality Information, (CQI)) reports are sent from the mobile terminal to the base station, including information of the downlink channel quality useful in the scheduling of data packets. In the cellular communications system 3GPP LTE, one type of reports contains aperiodic CSI, and such reports are scheduled by the radio base station

(eNodeB) . An aperiodic report is triggered by a specific CQI request bit in the uplink (UL) grant transmitted in the

Physical Downlink Control Channel, PDCCH. If this ^ACQI request bit' is positive (i.e., ^Λ1'), an aperiodic CSI report is transmitted from the user equipment (UE) , i.e. the mobile terminal, to the eNodeB on the Physical Uplink Shared Channel (PUSCH) , the uplink channel shared among UEs, according to the format given by the UL grant. In LTE-Advanced, carrier

aggregation is supported, wherein transmission may occur on multiple downlink carriers (also referred to as cells) which necessitate CSI reports from multiple downlink carriers.

Carrier aggregation introduces issues for the aperiodic CSI reporting. As a side note, a cell is a combination of downlink and optionally uplink resources. The linking between the carrier frequency of the downlink resources and the carrier frequency of the uplink resources is indicated in the system information transmitted on the downlink resources.

A first issue is that aperiodic CSI report triggering requires the transmission of an UL grant in the Physical Downlink

Control Channel (PDCCH) . If each carrier's aperiodic CSI report is triggered separately, multiple PDCCH transmissions will be needed which increases the control-overhead in the system. Since the capacity of the PDCCH is limited, an

increased amount of UL grants may cause that other PDCCH transmissions will be blocked. Hence, it may be desirable if aperiodic CSI reports for multiple carriers could be triggered from a single UL grant, thereby decreasing the PDCCH usage. A second issue is for cases where the number of downlink (DL) carriers is larger than the number of uplink (UL) carriers, i.e., asymmetric carrier aggregation. In LTE-Advanced, UL grants cannot be transmitted on a DL carrier that is not paired to an UL carrier. Hence, it would not be possible to trigger aperiodic reports for unpaired DL carriers.

In LTE, the information about the carrier pairing (e.g., frequency duplex distance) between an UL carrier and a DL carrier is broadcast in the cell in a so called SIB-2 message. Fig. 1, shows one example of the SIB-2 linkage for symmetric carrier aggregation and asymmetric carrier aggregation.

A third issue is a potential ambiguity of the carriers

comprising the aggregation. The eNodeB configures a UE with a set of carriers to be used for carrier aggregation.

Reconfiguration of this set may be needed, e.g., addition or removal of carriers and during the reconfiguration period, the eNodeB does not know whether the UE has switched to the new configuration, or if it is still assuming the old configuration. This may incur ambiguity concerning which carriers that have been triggered for aperiodic CSI, which in turn could lead to losses of CSI reports and jeopardize the data scheduling.

The first two issues could be handled by adding triggering bits in the UL grants. Thereby, one PDCCH transmission (i.e., one UL grant) could trigger aperiodic CSI reports also for multiple carriers. This gives desirable flexibility. However, it is undesirable to increase the amount of bits in the UL grant since it increases control overhead and thereby reduces the coverage and capacity of the system. This DL PDCCH control overhead would be unavoidable and remain even if no aperiodic CSI reporting is triggered. An alternative is to not increase the number of triggering bits and instead assume that upon a positive CSI trigger, aperiodic reports should always be given for all DL carriers. However, this would imply unavoidable large overhead in the UL, since it would not be possible to report aperiodic CSI for a single carrier only. The ability to report aperiodic CSI for a single carrier only would be useful for power limited UEs, where the large control overhead of reporting multiple carriers cannot easily be accommodated. Thus mandating reporting for all carriers would not offer sufficient flexibility for aperiodic CSI reporting.

It is generally desirable with an aperiodic CSI triggering solution that can enable both triggering of a single carrier as well as triggering of multiple carriers. In LTE-Advanced, a hybrid solution is adopted in the sense that one extra CSI trigger bit has been added and the interpretation of certain trigger states is configured by Radio Resource Control (RRC) signalling, i.e. signalling of control messages between a UE and an eNodeB. Hence, it is not a fully dynamic solution

(using only trigger bits in the UL grant) , nor a static

solution (always triggering all carriers) . In that sense, the solution can provide a certain degree of flexibility

facilitating triggering of single as well as multiple carriers.

However, the amount of triggering flexibility, overhead

signalling and the ability to address the ambiguity issue depends on how the interpretation of the trigger states should be done. In this invention, methods are disclosed that allow efficient triggering.

As mentioned, it is desirable that aperiodic CSI reports

should be provided for carrier aggregation such that

· reports can be triggered for a single carrier

• reports can be triggered for multiple carriers

while assuring that there is no ambiguity for the triggering during reconfiguration periods of the carriers. In LTE Rel-8/9, carrier aggregation is not supported and the User Equipment (UE) can only communicate on one downlink (DL) carrier and one uplink (UL) carrier. Hence, the aperiodic CSI report applies to the DL carrier being SIB-2 linked to the UL carrier transmitting the report.

Aperiodic CSI reporting is enabled by a single CSI trigger bit included in the UL grant which is transmitted by the PDCCH.

Before introducing the concept of carrier aggregation in 3GPP LTE Rel-10, a sidenote on search spaces may be helpful. In LTE, a mobile terminal (user equipment, UE) may search for channels where it can receive instructions from a base station, e.g., the physical downlink control channel (PDCCH) . The mobile terminal may search for such channels in two different search spaces, namely a common search space and a UE-specific search space. These search spaces constitutes different downlink time-frequency resources and search spaces may or may not be overlapping. In the common search space, all mobile terminals would search for information that would be common to all terminals. In addition, UE-specific control information could also be transmitted in the common search space. In a UE- specific search space, the detected control information only applies to the given mobile terminal. Information that is communicated in the UE-specific search space is such

information that is only relevant for that single mobile terminal, such as specific uplink frequency-time resources where the terminal may transmit on a shared uplink channel.

For LTE Rel-10 (LTE-Advanced) , for UEs configured with carrier aggregation, aperiodic CSI triggering is done according to 3GPP Rl-106540, "Way forward on aperiodic CSI triggering", Jacksonville, USA, Nov. 15-19, 2010, with the exception that the interpretation of the trigger bit ^Λ1' in the common search space is defined as having the same function as when carrier aggregation is not used, i.e. it is triggering by SIB-2 linkage. In the common search space, all UEs, even those not configured with carrier aggregation, should be able to receive the Physical Downlink Control Channel (PDCCH) . That would not be possible if the sizes of the uplink grants are different. Hence, only the single CSI trigger bit is present. If the CSI trigger bit is ^Λ0', no reporting should be done.

In the UE-specific search space, two CSI request bits are present. Trigger ^Λ00' means that no reporting should be done, trigger ^Λ01' indicates the downlink component carrier (DL CC) that is SIB2-linked to the uplink component carrier (UL CC) transmitting the CSI report and the meanings of trigger ^Λ10' and ^Λ11' are configurable by Radio Resource Control (RRC) .

Hence, by trigger ^Λ01' the same behaviour of triggering a single carrier as in Rel-8/9 is maintained, while triggers ^Λ10' and ^Λ11' could enable triggering of multiple carriers.

In LTE-Advanced, configured carriers can be either activated or deactivated. The UE is only transmitting the aperiodic CSI report for activated carriers among the carriers that have been triggered.

SUMMARY OF THE INVENTION

It is an object of the present invention to propose a solution for, or a reduction of, the problems of prior art. A main object is consequently to provide for an efficient triggering of channel state information reports in a carrier aggregated cellular communications system. The object is attained with a method comprising aperiodic CSI triggering for carrier aggregation comprising:

- transmission of at least one CSI trigger bit in a downlink control channel, where each instance of the CSI trigger bits forms a trigger state

- associating at least one of the trigger states with a bitmap

- reporting aperiodic CSI for carriers as indicated by the bitmap . In this way, a method for triggering of CSI reports is enabled that is very flexible and still frugal on communication resources . In one aspect of the invention, the bitmap size is equal to the maximum number of carriers that can be indexed, M. This enables an unambiguous correspondence between the bitmap and carriers in the system such that during, for instance, a

change (e.g. by reconfiguration) of carriers in an aggregation, there is no confusion as for which carrier an aperiodic CSI report should be provided. In a further aspect of the invention, the bitmap size is equal to the maximum number of carriers that can be aggregated, N. This setup will minimize the resources needed to do the

triggering of reports according to the invention. In a further aspect of the invention, it is possible to use states of the bitmap to indicate triggering by SIB-2 linkage. In this way, not only the direct indication of for which

carriers an aperiodic CSI report should be provided is enabled, but also an indirect mechanism is possible with the bitmap.

In a further aspect of the invention, it is possible to use CSI trigger bits to form sub-bitmaps from a given bitmap. Thus, a bitmap can serve as several bitmaps (sub-bitmaps) and

therefore signaling of the bitmap itself can be limited,

saving signaling resources.

The invention also comprises a terminal and a base station having advantages corresponding to those of the method of the invention . Further advantageous embodiments are disclosed in the

remaining dependent claims . BRIEF DESCRIPTION OF THE DRAWING

Embodiments exemplifying the invention will now be described in conjunction with the appended drawing, on which

Fig. 1 discloses SIB-2 linkage for symmetric carrier

aggregation (left) and asymmetric carrier aggregation (right) .

DETAILED DESCRIPTION

Consider a cellular system for carrier aggregation where a mobile terminal can be configured with up to N carriers.

Further we assume that there are M≥N carrier indices where each index uniquely corresponds to a downlink carrier. For example, m=0 , 1 , M-l could define a set of unique carrier indices. Aperiodic Channel State Information (CSI) triggering is assumed to be done by K bits in a downlink control channel, particularly K<N. The value of K may be different depending on the time-frequency position where the downlink control channel is transmitted, e.g., for different search spaces. The K trigger bits correspond to 2^K trigger states, where each trigger state should determine one or several of the

configured carriers that could be reported for aperiodic CSI.

It is further assumed that a configured carrier can be in two states, activated or deactivated. CSI reports will not be transmitted corresponding to deactivated carriers.

Given N carriers, the number of combinations of carriers can be triggered is

One method to represent all such combinations is to use a bitmap, wherein the position of a positive bit indicates the carrier that is triggered. At most, there would be 2^K bitmaps corresponding to 2^Ktrigger states in a downlink control channel, each bitmap being associated with one trigger state. If some of the 2 ^K trigger states may correspond to no aperiodic CSI triggering, the number of bitmaps may be less than 2 ^K.

Similarly, some of the 2^K trigger states may correspond to some predefined triggering rules (such as triggering by SIB-2 linkage) and not be associated with a bitmap. Thus the bitmap is only used to determine a set of triggered carriers when the associated trigger state is valid for the given bitmap. That is to say, that any state of the trigger bit(s) in the

downlink control channel that is not associated with a bitmap is denoted as not valid for using a bitmap for the triggering of channel state information reports. If there for instance are four states and state ^Λ00' represents no transmission of a report, state ^Λ01' represents transmission of a report for the downlink carrier that is linked to the uplink carrier on which the report should be sent and state ^Λ10' and ^Λ11' represents transmission according to a bitmap, then states ^Λ00' and ^Λ01' are not valid for the bitmap in the sense of that they do not trigger reports according to such a bitmap. Correspondingly, states ^Λ10' and ^Λ11' are valid for the bitmap since they would trigger a report according to their respectively associated bitmap. The association of a set of carriers to be triggered with a trigger state can be semi-statically configured, e.g. by Radio Resource Control (RRC) signalling.

In a first embodiment, a bitmap of length N is used since it can be realized that this is the smallest bitmap that can encompass the 2^N -1 combinations. For example, a bitmap can be defined as where

1, carrier with ordinal number i is triggered

0, carrierwith ordinal number /^'is not triggered and the index i refers to an ordinal number of a carrier. An ordering could be arranged such that index i=0 , 1 , ..., Λ/-1 corresponds to the i+l:th configured carrier, where configured carriers are ordered by their carrier indices m. For example, if carrier m=0 , m=3 and m=l are configured, the relationship could be:

i = 0 -» m = 0

i - 1 -» m - 3

The bitmap would contain zeros for all remaining indices i. It would further be advantageous if the definition of the bitmap avoids any ambiguity with regards to the configured set of component carriers. In that way, aperiodic CSI reports could be triggered even during a period under which the set of carriers is reconfigured. That property would be an advantage for the system since it may allow scheduling of data and thus maintain system performance. This implies that the

interpretation of the bitmap should not depend on the set of configured carriers. Equivalently, the carrier associated with position i in the bitmap should not change depending on carrier configuration status. Furthermore, an advantage is that this also makes it possible to independently configure the bitmap and the set of configured carriers. Thereto, as long as the carriers indicated by the bitmap comprises a subset of the carriers that may be perceived by the UE as configured during the reconfiguration period (i.e., the union of the set of carriers in the old configuration and the set of carriers in the new configuration) there is no ambiguity in triggering according to the bitmap. This is an advantage to the system since it would reduce the need for reconfiguring the bitmaps. In a second embodiment, it is realized that this can be facilitated by defining a bitmap of length M, wherein index i refers to carrier i, such that the bitmap is defined as

.. , 6 _i] where 1=0, 1, ..., -1,

and ^_jb_i < N . Since the indexing of the carriers is fixed, i.e., index i in the bitmap always refers to a carrier with index i, regardless if that carrier is configured or not, there is no ambiguity in interpreting the bitmap.

A person skilled in the art could construct other bitmaps where the association between the carrier index and the position follows some arbitrary but known rule. One example is where position i in the bit map corresponds to carrier index M-l-i.

In a third embodiment, it is realized that a bitmap of length N, could represent 2^N combinations of carriers for triggering. However, according to equation (1) above, the number of used states is given by 2^N -1. There is thus one unused state, namely the one with 6_; = 0, Vz . This state encodes that no carrier is triggered for aperiodic CSI. It is realized in this

invention that the same information (i.e., no triggering) can be conveyed by one of the K bit combinations in the UL grant (e.g., trigger ^Λ0' or trigger ^Λ00') . Therefore this spare state in the bitmap can be reused to encode additional

information .

Similarly, it is realized that a bitmap of length M could represent 2^M combinations of carriers for triggering. When M>N there are 2^M-2^W +1 unused states, including the one with 6_; =0,Vz , which can be used for encoding additional information.

In prior art solution of LTE-Advanced of 3GPP Rl-106540

mentioned above, in the UE-specific search space both single and multiple carriers can be triggered since there are 2 CSI trigger bits in the UL grant. Trigger ^Λ01' facilitates

aperiodic CSI reporting for a single carrier, i.e., the SIB-2 linked carrier. However, in the common search space, only one CSI trigger bit is available. Hence it is a problem to

accommodate triggering of single and multiple carriers. This may imply restrictions in obtaining aperiodic CSI reports and therefore limit the ability to schedule data. By the above observations, the unused states in the bitmap can be utilized to represent triggering by SIB-2 linkage. For example, a

bitmap with 6_; =0,Vz could together with trigger ^Λ1' indicate that the DL carrier being SIB-2 linked to the UL carrier

should be reported. By proper configuration of the bitmap, it is therefore possible to enable either SIB-2 linkage

triggering, or triggering according to any combination

according to the bitmap. This gives large flexibility for the base station to select the appropriate triggering mechanism from the common search space. A generic case of the previous example would be where one state of said bitmap implies that the downlink carrier being linked to the uplink carrier

transmitting said report, should be reported. Such a linkage may be assigned by a broadcast transmission (SIB-2 is an

example of assigning such a linkage) . Said one state of said bitmap may be the all-zero state of said bitmap, as was also the case of the previous SIB-2 linkage example above, and can be triggered by one triggering bit being sent in a common search space. In a fourth embodiment, the control signalling overhead can be reduced. Each bitmap needs to be signalled to the user

equipment, which causes extra signalling overhead in the system. It is therefore possible to reduce this signalling overhead by associating one bitmap to several trigger states in the uplink grant. For example, suppose one bitmap has been defined B=[b₀,b_l,...,b_Ml] . Different trigger states could imply that different sub-bitmaps of B are used. Such different sub- bitmaps may include: all even indexed bits of said bitmap, all odd indexed bits of said bitmap or a contiguous set of bits of said bitmap

As one example, trigger ^Λ10' could imply that carriers should be reported according to B, while trigger ^Λ11' could imply that carriers should be reported according to B' , where B' is a bitmap of length M obtained from B according to some pre^¬ defined rule. Such a rule may, e.g., be that;

1. B' includes all odd indices i of b_j

2. B' includes all even indices i of b_j

3. B' includes the first H indices i of b_j

4. B' includes the last H indices i of b_j

where H is a positive integer. Assuming B= [ 1 1 0 0 1], the above rules result in the following sub-bitmaps (for H=2) :

1. B'=[0 1 0 0 0]

2. B'=[l 0 0 0 1]

3. B'=[l 1 0 0 0]

4. B'=[0 0 0 0 1]

The skilled person in the art can arrange the number of bitmaps and the number of triggering bits K to optimize the performance. For example: • K=p triggering bits in a common search space correspond to p bitmap (s)

• K=p' triggering bits in a UE-specific search space correspond to p' bitmap (s)

or

• one bitmap is used for all triggering bits.

The skilled person in the art can also combine the embodiments without any contradiction. For example a bitmap of size M could be used where the all-zero state refers to triggering by SIB-2 linkage when the trigger state is ^Λ1' .

Mobile terminal and base station according to the invention

The above description of the invention has been concentrated on the method for triggering aperiodic channel state information reports in a cellular communications system. The method has been described in conjunction with a base station and a mobile terminal (also known as User Equipment, UE) . Even though the mobile terminal and the base station have not been described in detail, it is implicit that they, in order to be able to perform their respective steps of the method of the invention, are equipped with means for executing such steps.

Thus, the invention also comprises a mobile terminal for a cellular communications system employing carrier aggregation, wherein the mobile terminal can be configured with a set of up to N carriers, said set of up to N carriers being associated with at least one out of M carriers. The mobile terminal is distinguished in that it has means to:

- receive at least one triggering bit in a downlink control channel, said at least one triggering bit corresponding to a plurality of trigger states, - associating at least one of the trigger states of said at least one triggering bit with at least one bitmap comprising a number of bits corresponding to bitmap states,

- determining from said bitmap, when said at least one associated trigger state is valid for said bitmap, a set of carriers for which aperiodic channel state information should be reported,

- arranging transmission of channel state information reports of said determined set of carriers.

Further, the invention also comprises a base station for a cellular communications system employing carrier aggregation, wherein in the cellular communications system a mobile

terminal can be configured with a set of up to N carriers, said set of up to N carriers being associated with at least one out of M carriers. The base station is distinguished in that it has means to:

- transmit at least one triggering bit in a downlink control channel, said at least one triggering bit corresponding to a plurality of trigger states,

wherein at least one of the trigger states of said at least one triggering bit is associated with at least one bitmap, wherein said bitmap represents a set of carriers for which aperiodic channel state information should be reported.

The base station may further comprise means to signal the at least one bitmap to a mobile terminal. List of exemplary embodiments of a method

As a compact exemplification of a method according to the invention a number of embodiments a) through r) are disclosed below . Embodiment a) , a method for triggering aperiodic channel state information reports in a system employing carrier aggregation, wherein a set of up to N carriers can be configured, said set being associated with at least one out of M carriers, characterized by;

- transmitting at least one triggering bit in a downlink control channel

- associating at least one of the trigger states of said at least one triggering bit with at least one bitmap

- determining from said bitmap when said at least one associated trigger state is valid, a set of carriers for which aperiodic channel state information should be reported

- arranging transmission of the reports of said set.

Embodiment b) , a method as in embodiment a, where said transmission of the reports of said set only includes the carriers which are activated.

Embodiment c) , a method as in embodiment a, where one state of said bitmap implies that the downlink carrier being linked to the uplink carrier transmitting said report, should be reported .

Embodiment d) , a method as in embodiment c, where said linkage is assigned by a broadcast transmission.

Embodiment e) , a method as in embodiment c, where said one state is the all-zero state of said bitmap.

Embodiment f ) , a method as in embodiment a, where the determination from said bitmap includes forming a sub-bitmap, where said sub-bitmap is dependent of said at least one triggering bit. Embodiment g) , a method as in embodiment f , where said sub- bitmap includes all even indexed bits of said bitmap.

Embodiment h) , a method as in embodiment f , where said sub- bitmap includes all odd indexed bits of said bitmap.

Embodiment i) , a method as in embodiment f , where said sub- bitmap includes a contiguous set of bits of said bitmap.

Embodiment j), a method as in embodiment e, where one triggering bit is sent in a common search space.

Embodiment k) , a method as in embodiment a, where the bitmap is of length M.

Embodiment 1) , a method as in embodiment a, where the bitmap is of length N.

Embodiment m) , a method as in embodiment a, where the state of said associated bitmap is configured by RRC .

Embodiment n) , a method as in embodiment a, where the set of carriers that aperiodic channel state information should be reported is explicitly determined from said bitmap.

Embodiment o) , a method as in embodiment j, where in the bitmap

b_i is associated with carrier index i. Embodiment p) , a method as in embodiment j, where in the bitmap [6₀,6,,...,6 ,] , b_i is associated with carrier index M-l-i.

Embodiment q) , a method as in embodiment a, where one bitmap is associated with triggering bits of said downlink control channel transmitted through a common search space.

Embodiment r) , a method as in embodiment a, where two bitmaps are associated with triggering bits of said downlink control channel transmitted through a UE-specific search space. Applications and alternatives

The invention is applicable to carrier aggregation.

Claims

1. Method for triggering aperiodic channel state information reports in a system employing carrier aggregation, wherein a set of up to N carriers is configured for a mobile terminal, said set of up to N carriers being associated with at least one out of M carriers, characterized by:

- transmitting at least one triggering bit in a downlink control channel,

- associating at least one trigger state of said at least one triggering bit with at least one bitmap,

- determining from said bitmap, when said at least one associated trigger state is valid, a set of carriers for which aperiodic channel state information should be reported,

- arranging transmission of aperiodic channel state information reports of said determined set of carriers.

2. Method as in claim 1, where said transmission of the reports of said set only includes the carriers which are activated .

3. Method as in claim 1, where one state of said bitmap implies that a downlink carrier being linked to an uplink carrier transmitting said report, should be reported.

4. Method as in claim 3, where said linkage is assigned by a broadcast transmission.

5. Method as in claim 3, where said one state is the all- zero state of said bitmap.

6. Method as in claim 1, where the determination from said bitmap includes forming a sub-bitmap, where said sub- bitmap is dependent of said at least one triggering bit.

7. Method as in claim 6, where said sub-bitmap includes all even indexed bits of said bitmap.

8. Method as in claim 6, where said sub-bitmap includes all odd indexed bits of said bitmap.

9. Method as in claim 6, where said sub-bitmap includes a contiguous set of bits of said bitmap.

10. Method as in claim 5, where one triggering bit is sent in a common search space.

11. Method as in claim 1, where the bitmap is of length M.

12. Method as in claim 1, where the bitmap is of length N.

13. Method as in claim 1, where the state of said associated bitmap is configured by Radio Resource Control (RRC) .

14. Method as in claim 1, where the set of carriers that aperiodic channel state information should be reported for is explicitly determined from said bitmap.

15. Method as in claim 10 or 11, where in a bitmap

, b_i is associated with carrier index i.

16. Method as in claim 10 or 11, where in a bitmap

..,6 _,] , b_i is associated with carrier index M-l-i.

17. Method as in claim 1, where one bitmap is associated with triggering bits of said downlink control channel transmitted through a common search space.

18. Method as in claim 1, where two bitmaps are associated with triggering bits of said downlink control channel transmitted through a UE-specific search space.

M-\

19. Method as in claims 11, 15 or 16, where ^ b_t - N .

Mobile terminal for a system employing carrier aggregation, wherein a set of up to N carriers is configured for a mobile terminal, said set of up to N carriers being associated with at least one out of M carriers, characterized in that the mobile terminal has means to:

- receive at least one triggering bit in a downlink control channel,

21. Base station for a cellular communications system employing carrier aggregation, wherein in the cellular communications system a mobile terminal can be configured with a set of up to N carriers, said set of up to N carriers being associated with at least one out of M carriers, characterized in that the base station has means to: transmit at least one triggering bit in a downlink control channel, said at least one triggering bit corresponding to a plurality of trigger states, wherein at least one of the trigger states of said at least one triggering bit is associated with at least one bitmap, wherein said bitmap represents a set of carriers for which aperiodic channel state information should be reported . Base station as in claim 21, further comprising means signal the at least one bitmap to a mobile terminal.