CHANNEL STATE INFORMATION REPORTING ON LICENSED AND UNLICENSED CARRIERS

Abstract

The present disclosure relates to methods for reporting channel state information. The present disclosure also provides mobile stations for performing these methods, and computer readable media the instructions of which cause the mobile station to perform the methods described herein. For this purpose, the mobile station receives a trigger message that triggers the reporting of channel state information for at least one of the plurality of downlink component carriers, the trigger message being received in a subframe n.sub.Trigger, and reports the triggered channel state information for the at least one of the plurality of downlink component carriers based on reference signals present on the at least one of the plurality of downlink component carriers, in a subframe n.sub.Report later than n.sub.Trigger. The received trigger message indicates that the reference signals are present in a subframe n.sub.RS on the at least one of the plurality of downlink component carriers, where n.sub.Trigger≦n.sub.RS<n.sub.Report.

Claims

1. A method for reporting channel state information, CSI, from a mobile station to a base station in a mobile communication system in which a plurality of downlink component carriers and at least one uplink component carrier are configured for communication between the base station and the mobile station, the method comprising, by the mobile station: receiving from the base station a trigger message that triggers the reporting of channel state information for at least one of the plurality of downlink component carriers, the trigger message being received in a subframe n.sub.Trigger; and reporting to the base station, the triggered channel state information for the at least one of the plurality of downlink component carriers based on reference signals, RS, present on the at least one of the plurality of downlink component carriers, in a subframe n.sub.Report later than n.sub.Trigger, wherein the received trigger message indicates that the reference signals, RS, on the basis of which the channel state information is to be reported, are present in a subframe n.sub.RS on the at least one of the plurality of downlink component carriers, where n.sub.Trigger≦n.sub.RS<n.sub.Report.

2. The method according to claim 1, wherein the trigger message is received in form of a downlink control information, DCI, format, preferably carrying an uplink resource assignment.

3. The method according to claim 1, wherein the triggered channel state information is reported aperiodically by the mobile station, thereby defining an aperiodic channel state information, CSI, report, and, preferably, the aperiodic channel state information report is reported in the physical uplink shared channel, PUSCH, on the at least one uplink component carrier.

4. The method according to claim 1, wherein the trigger message is received in the subframe n.sub.Trigger on another one of the plurality of downlink component carriers which is different from the at least one of the plurality of downlink component carriers for which the reporting is triggered, and, preferably, the at least one of the plurality of downlink component carriers is an unlicensed component carrier and the other one of the plurality of downlink component carriers is a licensed component carrier.

5. The method according to claim 1, wherein the received trigger message indicates that the reference signals, on the basis of which the channel state information is to be reported, are present in a set of sub-bands, S, which spans the downlink system bandwidth of the at least one of the plurality of downlink component carriers.

6. The method according to claim 5, wherein the reporting further includes: evaluating reference signals of contiguous or distributed physical resource blocks in the set of sub-bands, S, on the at least one of the plurality of downlink component carriers; and reporting the triggered channel state information on an uplink component carrier based on the evaluated reference signals for the at least one of the plurality of downlink component carriers.

7. The method according to claim 1, wherein the triggered channel state information is reported in form of one or two channel quality indicator, CQI, value(s) for a set of sub-bands, S, which spans the downlink system bandwidth of the at least one of the plurality of downlink component carriers.

8. The method according to claim 1, wherein the triggered channel state information is reported in form of: a single channel quality indicator, CQI, value, corresponding to a codeword, if the mobile station is not configured to report rank indicator, RI, feedback, or if the to be reported RI=1; and two channel quality indicator values, corresponding to different codewords, if the to be reported RI>1.

9. The method according to claim 1, wherein the triggered channel state information is reported in the form of: a wideband channel quality indicator, CQI, value per codeword which is calculated assuming downlink transmission using a single precoding matrix in all sub-bands and that the reference signals are present on a set of sub-bands, S; and a selected precoding matrix indicator, PMI, or a first and second precoding matrix indicator corresponding to the selected single precoding matrix, wherein the single precoding matrix is selected from the codebook subset assuming downlink transmission on the set of sub-bands, S, and wherein the to be reported precoding matrix indicator, PMI, and the to be reported channel quality indicator, CQI, values are calculated conditioned on the reported rank indicator, RI, or are calculated conditioned on RI=1.

10. The method according to claim 1, wherein, in case a physical downlink shared channel, PDSCH, utilizes the same subframe n.sub.RS in which the reference signals are present, the mobile station assumes a punctured PDSCH transmission.

11. The method according to claim 1, wherein the reference signals, indicated by the received trigger message, are at least one of: cell-specific reference signals, CRS; and channel state information reference signals, CSI-RS.

12. The method according to claim 11, wherein the mobile station is configured with at least one channel state information, CSI, reference signal including at least one of: a non-zero-power CSI-RS configuration for which the mobile station assumes non-zero transmission power in a subframe n.sub.NZP-CSI-RS=n.sub.RS; and a zero-power CSI-RS configuration for which the mobile station assumes zero transmission power in a subframe n.sub.ZP-CSI-RS≠n.sub.RS, and, preferably, wherein the subframe n.sub.ZP-CSI-RS is earlier than the subframe n.sub.NZP-CSI-RS, where n.sub.Trigger≦n.sub.ZP-CSI-RS<n.sub.NZP-CSI-RS<n.sub.Report.

13. The method according to claim 11, wherein the mobile station is configured with at least one channel state information, CSI, reference signal, wherein the CSI reference signal configuration includes a zero-power CSI-RS configuration for which the mobile station assumes zero transmission power indicating at least one resource element, RE, in the subframe n.sub.RS corresponding to a resource element prescribed for cell specific reference signal, CRS, transmission, and/or wherein the CSI reference signal configuration is indicated in the trigger message received from the base station, and preferably, the trigger message is received in form of a downlink control information, DCI, format, preferably carrying an uplink resource assignment.

14. A mobile station for reporting channel state information, CSI, to a base station in a mobile communication system in which a plurality of downlink component carriers and at least one uplink component carrier are configured for communication between the base station and the mobile station, comprising: a receiving unit that receives from the base station a trigger message that triggers the reporting of channel state information for at least one of the plurality of downlink component carriers, the trigger message being received in a subframe n.sub.Trigger, and a transmitting unit that reports to the base station, the triggered channel state information for the at least one of the plurality of downlink component carriers based on reference signals, RS, present on the at least one of the plurality of downlink component carriers, in a subframe n.sub.Report later than n.sub.Trigger, wherein the received trigger message indicates that the reference signals, RS, on the basis of which the channel state information is to be reported, are present in a subframe n.sub.RS on the at least one of the plurality of downlink component carriers, where n.sub.Trigger≦n.sub.RS<n.sub.Report.

15. A non-transitory computer-readable medium storing instructions that when executed by a mobile station cause the mobile station to report channel state information, CSI, to a base station in a mobile communication system in which a plurality of downlink component carriers and at least one uplink component carrier are configured for communication between the base station and the mobile station, comprising: receiving from the base station a trigger message that triggers the reporting of channel state information for at least one of the plurality of downlink component carriers, the trigger message being received in a subframe n.sub.Trigger; and reporting to the base station, the triggered channel state information for the at least one of the plurality of downlink component carriers based on reference signals, RS, present on the at least one of the plurality of downlink component carriers, in a subframe n.sub.Report later than n.sub.Trigger, wherein the received trigger message indicates that the reference signals, RS, on the basis of which the channel state information is to be reported, are present in a subframe n.sub.RS on the at least one of the plurality of downlink component carriers, where n.sub.Trigger≦n.sub.RS<n.sub.Report.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0141] FIG. 1 shows an exemplary architecture of a 3GPP LTE system;

[0142] FIG. 2 shows an exemplary overview of the overall E-UTRAN architecture of 3GPP LTE;

[0143] FIG. 3 shows exemplary subframe boundaries on a downlink component carrier as defined for 3GPP LTE (Release 8/9);

[0144] FIG. 4 shows an exemplary downlink resource grid of a downlink slot as defined for 3GPP LTE (Release 8/9);

[0145] FIG. 5 shows the 3GPP LTE-A (Release 10) Layer 2 structure with activated carrier aggregation for the downlink;

[0146] FIG. 6 shows the 3GPP LTE-A (Release 10) Layer 2 structure with activated carrier aggregation for the uplink;

[0147] FIG. 7 exemplifies a mapping of resource signals onto a PRB indicating Cell-Specific RSs—CRSs indicated as Px, of Demodulation RSs—DM-RSs indicted as Dx, and Channel State Information RSs—CSI-RSs indicated as RS in the downlink of LTE-A, respectively;

[0148] FIG. 8 illustrates an exemplary licensed-assisted access scenario, with various licensed and unlicensed cells;

[0149] FIG. 9 shows a sequence diagram of the improved channel state information reporting mechanism according to one embodiment;

[0150] FIG. 10 illustrates a radio communication employing the improved channel state information reporting mechanism according to an implementation of the embodiment;

[0151] FIG. 11 shows a first variant of a mapping of CRS and CSI-RS onto a PRB for use with 8 CSI-RS ports in connection with the improved channel state information reporting mechanism according to another embodiment;

[0152] FIG. 12 shows a second variant of a mapping of CRS and CSI-RS onto a PRB for use with 2 CSI-RS ports in connection with the improved channel state information reporting mechanism according to a further embodiment;

[0153] FIG. 13 shows a third variant of a mapping of CRS and CSI-RS onto a PRB, wherein FIG. 13 is for 2 CSI-RS ports according to yet another embodiment;

[0154] FIG. 14 shows a third variant of a mapping of CRS and CSI-RS onto a PRB, wherein FIG. 14 is for 2 CSI-RS ports according to yet another embodiment;

[0155] FIG. 15 shows a third variant of a mapping of CRS and CSI-RS onto a PRB, wherein FIG. 15 is for 4 CSI-RS ports according to yet another embodiment;

[0156] FIG. 16 shows a third variant of a mapping of CRS and CSI-RS onto a PRB, wherein FIG. 16 is for 4 CSI-RS ports according to yet another embodiment;

[0157] FIG. 17 shows a third variant of a mapping of CRS and CSI-RS onto a PRB, wherein FIG. 17 is for 8 CSI-RS ports according to yet another embodiment;

[0158] FIG. 18 shows a fourth variant of a mapping of CRS and CSI-RS onto a PRB for use with 8 CSI-RS ports in connection with the improved channel state information reporting mechanism according to an even further embodiment; and

[0159] FIG. 19 shows a fifth variant of a mapping of CRS and CSI-RS onto a PRB in set 1 and another PRB in set 2 for use with 8 CSI-RS ports in connection with the improved channel state information reporting mechanism according to another embodiment.

DETAILED DESCRIPTION

[0160] It should be noted that the embodiments may be advantageously used, for example, in a mobile communication system such as 3GPP LTE-A (Release 10/11/12) communication systems as described in the Technical Background section above, but the embodiments are not limited to its use in this particular exemplary communication networks.

[0161] A mobile station or mobile node or user terminal or user equipment is a physical entity within a communication network. One node may have several functional entities. A functional entity refers to a software or hardware module that implements and/or offers a predetermined set of functions to other functional entities of a node or the network

[0162] Nodes may have one or more interfaces that attach the node to a communication facility or medium over which nodes can communicate. Similarly, a network entity may have a logical interface attaching the functional entity to a communication facility or medium over it may communicate with other functional entities or correspondent nodes.

[0163] The term “radio resources” as used herein has to be broadly understood as referring to physical radio resources, such as time-frequency resources.

[0164] The term “unlicensed carrier”, and conversely “licensed carrier” are to be understood in connection with the new LTE work item licensed-assisted access (LAA). Correspondingly, “licensed carriers” is the term for the situation where a carrier is licensed for exclusive use to an operator, usually by a regulatory body that has authority over the radio frequency usage for a defined geographical region. “Unlicensed carriers” would be the term used for carrier(s) which cover(s) frequencies which are at the moment not licensed for LTE, and are in particular open for any usage that complies with certain regulations, or are otherwise shared non-exclusively. As described in the background section there are several differences between licensed carriers and unlicensed carriers, as regards, e.g., reliability, power level and QoS.

[0165] The term “higher layer signaling” as used herein has to be understood broadly referring to layers above the PHY layer (according to the OSI model), comprising the MAC layer (e.g., MAC CE), RRC layer, and further layers above, and their corresponding signals and messages.

[0166] The term “wideband” in the narrow sense can be understood as spanning the system bandwidth of, for example, a component carrier. Nevertheless, the term “wideband” as used herein shall not be construed as only referring to configuration where the system bandwidth is covered in its entirety, namely, of a set of contiguous subbands making up the entire system bandwidth; rather, the term “wideband” shall also be understood as representing a set of adjacent and/or distributed physical resources (such as subbands, resource blocks, or subcarriers).

[0167] As explained in the background section, for unlicensed carriers it is not finally decided how channel state information, CSI, reporting by the mobile station is implemented, i.e., how the reporting is carried out in a mobile communication system in which a plurality of downlink (component) carriers and at least one uplink (component) carrier are configured for communication between the base station and the mobile station, where at least one component carrier is available for downlink transmission. It should be noted that even though in a time domain duplexing scheme a single frequency carrier is used for uplink and downlink (though not simultaneously), for simplicity of the description, this case should also be understood as having one component carrier for uplink and having one component carrier for downlink.

[0168] Specifically, in the background section it has been established that present implementations of channel state information, CSI, reporting are mechanisms that are inadequate considering the regulatory requirements that have to be taken into account when designing LAA procedures. For operation on unlicensed bands, changes are necessary, particularly to the current CSI reporting implementation.

[0169] When operating an unlicensed carrier as downlink (component) carrier, the presence of continuous, non-interfered reference signals, RS, namely, cell specific reference signals CRS and/or channel state information—reference signals CSI-RS, can no longer be ensured. The unlicensed carrier access is limited to, for instance, at most 10 ms continuous usage in Europe.

[0170] Further, the unlicensed carrier is supposed to be shared between various operators and/or radio access technologies, including, for instance, WIFI. However, coexistence with WIFI nodes on an unlicensed carrier is difficult as a WIFI node would occupy the unlicensed carrier in its entirety, assuming full 20 MHz (or even a plurality of 20 MHz carriers) for active transmissions.

[0171] Unlicensed carrier access is generally targeting burst transmissions, i.e., a scenario where the unlicensed carrier is occupied for a short period of time for a bursty downlink transmission between a base station and a mobile station. However, even in this case, the channel state information, and hence the reporting thereof, is crucial for an efficient adaptation to the channel.

[0172] The CSI reporting mechanism relies on the presence of reference signals on which the CSI reporting is based. A reference signal is a signal which is known to the receiver, and which is inserted into a transmitted signal at defined positions in order to facilitate channel estimation for coherent demodulation and measurements.

[0173] In the LTE downlink, cell-specific RSs are provided which are available to all UEs in a cell; UE-specific RSs may be embedded in the data for specific UEs for purposes of estimating the channel for data demodulation, but not for channel state information reports. In LTE Release 10 support for the transmission of channel state information reference signals CSI-RS was introduced, with the main goal of obtaining channel state feedback for up to eight transmit antenna ports to assist the base station in the precoding operations, and potentially different resources for measuring the signal strength and the noise+interference strength. The configuration of CSI-RS is established by RRC signaling.

[0174] Presently, the CSI reporting allows configuration of periodic as well as aperiodic CSI reporting schemes. The CSI reporting is configured by an RRC message for a (i.e., downlink) component carrier. The configuration assumes that the component carrier includes the reference signals on the basis of which the CSI reporting is effected. In this respect, CSI reporting can in theory be configured for licensed and unlicensed carriers.

[0175] For periodic CSI reporting, the configuration by RRC is sufficient to determine and initiate the periodic CSI report transmissions. An aperiodic CSI reporting needs to be triggered on PHY layer, for example, by use of a DCI format 0 message specifying in the “CSI request field” that the transmission of a CSI report is requested. In other words, a message indicating that a CSI report is requested, may trigger at a mobile station the transmission of an aperiodic CSI report.

[0176] In this respect, in the direct comparison between the periodic and the aperiodic CSI reporting, the aperiodic CSI reporting appears better suited for utilization in connection with unlicensed carriers. Moreover, the transmission of an aperiodic CSI report can be triggered by a base station which, thereby, can ensure that the regulatory requirements are satisfied.

[0177] Nevertheless, even the aperiodic CSI reporting scheme has disadvantages when configured for an unlicensed carrier, namely, for the following reasons: [0178] The aperiodic CSI reports equally rely on periodic CSI-RS transmissions on the unlicensed carrier. The configuration of CSI-RS presently assumes a same re-occurring mapping to resource elements on a downlink component carrier, and the configuration is facilitated exclusively by RRC message(s). [0179] The aperiodic CSI reports presently combine wideband and frequency-selective feedback. However, the frequency-selective feedback is conceptually unsuitable for a shared radio resource, e.g., due to inaccuracies, provided on the unlicensed carrier and only results in unnecessary signaling overhead. [0180] The aperiodic CSI report may be based on preceding CRS or CSI-RS which no longer reflects the channel conditions, in particular for LTE-U burst scenario, where a considerable time-span may elapse between the transmission of a periodic CSI-RS and the subsequent report instance, leading to inaccuracies due to fluctuations of the channel conditions due to, e.g., fading or mobility effects.

[0181] The following exemplary embodiments are conceived by the inventors to mitigate the problems explained and to provide a reliable and efficient CSI reporting mechanism, particularly for unlicensed carriers (although it is equally applicable to licensed carriers) that are at least partially used for downlink communication.

[0182] In the following, several exemplary embodiments will be explained in detail. Some of these are supposed to be implemented in the specification as given by the 3GPP standards and explained partly in the present background section, with the particular key features as explained in the following pertaining to the various embodiments.

[0183] It should be noted that the embodiments may be advantageously used, for example, in a mobile communication system such as 3GPP LTE-A (Release 10/11/12) communication systems as described in the Technical Background section above, but the embodiments are not limited to its use in this particular exemplary communication networks.

[0184] The explanations should not be understood as limiting the scope of the disclosure, but as a mere example of embodiments to better understand the present disclosure. A skilled person should be aware that the general principles of the present disclosure as laid out in the claims can be applied to different scenarios and in ways that are not explicitly described herein. Correspondingly, the following scenarios assumed for explanatory purposes of the various embodiments shall not limit the present disclosure as such.

[0185] In the following a set of embodiments will be explained. To simplify the illustration of the underlying principles, several assumptions are made; however, it should be noted that these assumptions should not be interpreted as limiting the scope of the present application, as broadly defined by the claims.

[0186] According to a first embodiment illustrated in FIG. 9 the improved channel state information, CSI, reporting mechanism is provided for reporting CSI by a mobile station to a base station. For the great part of the following description of this first embodiment, it is assumed that the CSI reporting is performed for an unlicensed carrier. However, the improved CSI reporting mechanism is equally applicable for reporting CSI of a licensed carrier.

[0187] It is of fundamental importance to distinguish a “CSI report for a carrier” from a “CSI report on a carrier”; the former denotes the carrier for which the channel state is reported, while the latter denotes the carrier on which the channel state information (i.e., the feedback message) is transmitted. Therefore, the former refers to a downlink carrier (or the time-span when a carrier is usable for downlink), while the latter refers to an uplink carrier (or the time-span when a carrier is usable for uplink).

[0188] The improved CSI reporting mechanism is preferably carried out in a mobile communication system in which a plurality of downlink component carriers and at least one uplink component carrier are configured between the base station and the mobile station, even though it can also be carried out where only one downlink and one uplink carrier are configured. Referring to the terminology of LTE Release 10, the component carriers may equally be referred to as serving cells.

[0189] In such a mobile communication system, the mobile station receives (step S01—FIG. 9) from the base station a trigger message that triggers reporting of channel state information, CSI. By this trigger message, a CSI report is triggered for one or plural of the configured downlink component carriers. For the following, it shall be assumed that the mobile station receives the trigger message in a subframe with index n.sub.Trigger.

[0190] Notably, the one or plural downlink component carriers, for which the CSI report is triggered, is not necessarily the same downlink component carrier on which the trigger message is received. Rather, in one exemplary scenario the trigger message may be received on a downlink component carrier corresponding to a licensed carrier and the CSI report may be triggered for one or plural downlink component carriers corresponding to unlicensed carriers.

[0191] In an exemplary implementation, the trigger message is in form of a downlink control information, DCI, format, wherein a CSI request field indicates that CSI is to be reported by the mobile station. For example, a CSI request field is specified in the DCI format 0 and in the DCI format 4. In this respect, the triggered channel state information, CSI, is reported aperiodically by the mobile station, namely, as aperiodic CSI report. Aperiodic CSI reporting formats are defined in LTE release 10 as PUSCH CSI reporting modes.

[0192] In response to the received trigger message, the mobile station determines (step S02—FIG. 9) for the one or plural downlink component carriers a CSI report based on reference signals present on this downlink component carrier(s). In other words, the trigger message references (or indicates) the one or plural downlink component carriers on the basis of which the CSI report is to be established.

[0193] The CSI report is based on the indicated reference signals, RS, present on the at least one of the downlink component carriers. In other words, the mobile station evaluates the indicated reference signals with which it is configured and based thereon reports the triggered CSI to the base station. The RSs used for this purpose are preferably CRS or CSI-RS.

[0194] In another exemplary implementation, for the CSI reporting the mobile station evaluates reference signals of contiguous or distributed physical resource blocks, PRBs, in the set of sub-bands, S, on the at least one of the plurality of downlink component carriers for which the CSI is to be reported. The set of sub-bands S is a system parameter with which the mobile station is pre-configured.

[0195] Even though LTE specifications use the term “sub-band” as a plurality of physical resource blocks, the usage here should be construed as not being restricted to such a definition. Rather, a sub-band as described herein can also be an individual physical resource block, or even a part of a physical resource block such as one or a plurality of subcarriers.

[0196] In this respect, as the set S may only refer some of the resource blocks of the cell, it is necessary to pay attention to interpreting the term wideband (or set S) used in connection with the embodiments broader than only “wideband” (or “set S”) as such. For example, “wideband” shall not be construed to mean exclusively the whole system bandwidth, but rather the plurality of resource blocks contained in set S, which may furthermore be non-adjacent in the frequency domain.

[0197] Commonly, the set of sub-bands S is configured such that it spans the (e.g., entire) downlink system bandwidth of the one or plural component carriers. In this respect, in this exemplary implementation, the CSI is reported as a wideband CSI report for the set of sub-bands S spanning the (e.g., entire) downlink system bandwidth of the one.

[0198] In any case, the configured set of sub-bands S, for which the wideband CSI is reported in this exemplary implementation, differs from a frequency selective CSI report which may be configured in addition or alternative to the wideband CSI report. Moreover, a selective reporting of CSI for specific subbands contradicts the approach of reporting wideband CSI for the (e.g., entire) downlink system bandwidth of the one or plural component carriers.

[0199] Further, in response to the receipt of the trigger message the mobile station reports (step S03—FIG. 9) the triggered channel state information, CSI, report for the one or plural downlink component carriers. For the following, it shall be assumed that the mobile station transmits the CSI report in a subframe with index n.sub.Report later than n.sub.Trigger.

[0200] Further to the exemplary implementation, the trigger message in the DCI format carries an uplink resource assignment. Accordingly, the uplink resource in which the CSI report shall be transmitted is determined by the L1/L2 control signal carried in the trigger message. In a more specific example, the trigger message is in DCI format 0 or DCI format 4.

[0201] Due to the determined uplink resource in the trigger message, the CSI reported on the uplink resource may not be based on reference signals which are more recent than the CSI report. In other words, the reference signals must have been received prior to the subframe n.sub.Report in which the CSI report is to be transmitted. Accordingly, the subframe n.sub.Report may be understood as an upper/latest limit to the subframe where the RSs serving as the reference for the triggered CSI report are transmitted.

[0202] In another exemplary implementation, which can be combined with the above, the CSI report is reported on the physical uplink shared channel, PUSCH, on the at least one uplink component carrier. In particular, a pre-configured CSI reporting mode is used for conveying the CSI report to the base station.

[0203] As already described before, the reference signals on which the CSI report is based are transmitted in the one or plural downlink component carriers for which the mobile station reports the CSI. Specifically, the one or plural downlink component carriers on which the reference signals are present may be different from the downlink component carrier on which the trigger message is transmitted/received.

[0204] Advantageously, the trigger message indicates a subframe with index n.sub.RS on which the reference signals, RS, are present. The reference signals are present on the one or plural downlink component carriers based on which the CSI is to be reported.

[0205] In this respect, the trigger message does not leave open on the basis on which reference signals the CSI is to be reported. The indicated subframe with index n.sub.RS is sometimes also termed “reference resource” referencing the resources for the CSI report.

[0206] In a more detailed implementation, the mobile station is adapted to interpret the received trigger message for triggering a CSI report such that it indicates a singular occasion for reference signals, RS, to be present in the subframe n.sub.RS on the one or plural downlink component carriers for which the CSI report is triggered (i.e., requested). In other words, the mobile station is adapted to utilize the received trigger message for one-shot CSI reporting only.

[0207] Exemplarily, the mobile station is adapted to interpret the received trigger message for triggering a CSI report such that it indicates singular occasions of reference signals, RS, are offset by a predefined or signalled subframe number I.sub.1 with respect to the trigger message in n.sub.Trigger. In this respect, the received trigger message indicates that the singular occasions of reference signals are present in subframe with index: n.sub.RS=n.sub.Trigger on the one or plural downlink component carriers for which the CSI report is triggered.

[0208] Alternatively, the mobile station is exemplarily adapted to interpret the received trigger message for triggering a CSI report such that it indicates plural occasions of reference signals, RS; which are offset by a predefined or signalled subframe number I.sub.1 with respect to the trigger message in n.sub.Trigger and are spaced at a further predefined or signalled subframe number I.sub.2.

[0209] In this respect, the received trigger message indicates that the plural occasions of reference signals are present in subframes with index: n.sub.RS=n.sub.Trigger+I.sub.1, n.sub.Trigger+I.sub.1+I.sub.2, n.sub.Trigger+I.sub.1+2*I.sub.2, . . . on the one or plural downlink component carriers for which the CSI report is triggered. In other words, the mobile station is adapted to utilize the received trigger message for plural CSI reports each for a respective one of the plural reference signals in the indicated subframes n.sub.RS.

[0210] Specifically, the indication of subframe with index n.sub.RS is advantageous for the CSI reporting of unlicensed carriers. In view of the indication of one or plural specific subframe(s) n.sub.RS that the CSI report is to be based on, the base station can reduce the number of subframes which have to be evaluated by the mobile station for the CSI reporting, and can at the same time ensure that only the reference signals of a relevant subframe, namely, of the subframe with index n.sub.RS, are evaluated for the CSI reporting.

[0211] For best adaptation to the shared nature of an unlicensed carrier, it is advantageous to indicate n.sub.RS in the DCI message triggering the report. However, since this may require an undesirable overhead, another solution is to indicate it by means of higher layer signalling such as RRC messages, preferably within the same message that configures other CSI report or CSI-RS parameters.

[0212] Nevertheless, it shall be emphasized that the trigger message (or as just mentioned, a higher layer configuration) not necessarily include a field for directly indicating the subframe on which the reference signals, RS, are present. Instead the trigger message may also indirectly reference a subframe based on a prescribed offset between n.sub.RS and n.sub.Trigger or between n.sub.RS and n.sub.Report, i.e., relative to the subframe with index n.sub.RS or n.sub.Trigger.

[0213] According to an exemplary implementation, a predefined CSI reporting mode may foresee that the CSI report for one or plural downlink component carriers is based on reference signals which are present in the same subframe as the trigger message but not necessarily on the same downlink component carrier of the trigger message.

[0214] In any case, the received trigger message indicates that the reference signals, RS, for the CSI report, are in subframe with index n.sub.RS, where n.sub.Trigger≦n.sub.RS<n.sub.Report. Thereby, not only the amount of buffering for reference signals reduces but also it can be ensured that CSI is reported only on the basis of reference signals, RS, from most recent subframes.

[0215] This keeps the influence of fading effects on the reposted CSI small, so that inaccuracies between the channel state at the time of measurement and at the time of reporting can be largely avoided; even more so in case only a single CQI value is reported for the set S instead of a CQI value per element (e.g., subband) within the set S. In other words, as result of the restrictions on the indicated subframe index n.sub.RS, it is not (i.e., no longer) possible that a CSI report is based on reference signals present in a subframe with an index n.sub.RS before n.sub.Trigger, and, hence, is outdated and only inaccurately reflects the channel state.

Implementation in LTE

[0216] Now, a more detailed implementation of the above embodiment shall be discussed in connection with FIG. 10. As shown therein, the improved CSI reporting mechanism is illustrated for a mobile communication system in which two downlink component carriers termed “DL Cell 1” and “DL Cell 2” and one uplink component carrier termed “UL Cell 1” are configured for communication between the mobile station and the base station.

[0217] This detailed implementation utilizes at least for “DL cell 2” the 2 CSI-RS port configuration for the transmission of CSI reference signals. Specifically, the mobile station is configured at least for “DL cell 2” with a CSI-RS mapping to resource elements according to CSI RS configuration 0 as introduced in the background section, namely, indicating (k′, l′) as (11, 4) as CSI reference signals on the basis of which the CSI reporting is to be carried out.

[0218] Further, the detailed implementation assumes the utilization of CSI reporting mode which prescribes an n.sub.RS to n.sub.Trigger offset equal to zero. In other words, for purposes of CSI reporting the mobile station is configured to refer to CSI reference signals in the same subframe as the subframe in which the CSI request (i.e., the trigger message) was received.

[0219] In the detailed implementation, the mobile station receives on the PDCCH of “DL Cell 1” in subframe n.sub.Trigger a DCI format 0 as trigger message including the “CSI request field” indicating (by a value of ‘1’, ‘01’, ‘10’, or ‘11’, depending on the length of the CSI request field and corresponding higher layer configuration; see 3GPP TS 36.213 v12.3.0 clause 7.2.1) that an aperiodic CSI report is triggered. At the same time the trigger message indicates that the CSI is to be reported for “DL Cell 2”. Further, the trigger message in DCI format 0 indicates an uplink resource assignment for subframe n.sub.Report (which is generally n.sub.Trigger+k, where k>=4) in “UL Cell 1”.

[0220] Applying the configuration to refer to the presence of CSI reference signals in the same subframe, i.e., assuming n.sub.RS=n.sub.Trigger, the mobile station refers to CSI reference signals according to CSI RS configuration 0 of “DL Cell 2” within the same subframe as the trigger message, namely, within same subframe n.sub.Trigger, for measuring/determining the CSI report. On the basis of this CSI reference signals in subframe n.sub.RS on “DL Cell 2”, the mobile station determines the triggered aperiodic CSI report.

[0221] Subsequently, the mobile station transmits the CSI report in the subframe n.sub.Report n.sub.Trigger+k) on the uplink resources indicated in the uplink resource assignment of the DCI format 0 trigger message. Moreover, the mobile station is reporting the aperiodic CSI on the basis of CSI reference signals that are present in the subframe n indicated through the received DCI format 0 trigger message.

Aperiodic CSI Reporting Mode

[0222] Now, reference is made to a specific implementation of the aperiodic CSI report which shall be understood as a new aperiodic CSI reporting mode which differs from the modes for CSI reporting using PUSCH disclosed in 3GPP TS 36.213 V12.3.0, section 7.2.1. This CSI reporting mode is preferable for an unlicensed carrier. However, it shall be understood that this aperiodic CSI reporting mode is equally applicable for licensed carriers and, hence, shall not be limited in this respect.

[0223] This aperiodic CSI reporting mode assumes the reporting of wideband CSI. In other words, reference signals, RS, on the basis of which the aperiodic CSI reporting is performed, are present in a set of sub-bands, S, which is a subset of or spans the downlink system bandwidth of the one or plural downlink component carriers for which the aperiodic CSI report is triggered. It should be noted that the set S can be specific to each downlink component carrier.

[0224] More specifically, the aperiodic CSI reporting is carried out in form of one or two channel quality indicator, CQI, value(s) for the set of sub-bands, S, which is a subset of or spans the downlink system bandwidth of the at least one of the plurality of downlink component carriers. Whether one or two CQI value(s) are reported for the set of sub-bands, S, depends on the rank indicator, RI, which is configured for the mobile station.

[0225] It is also possible that a first set of subbands S1 of resources where RS are present is different from the second set S2 of resources for which a CQI value is reported. For example, it may be preferable to transmit RS only on a first subset of the downlink system bandwidth of a downlink component carrier, while the CQI value is determined assuming that the measured channel state is applicable to the whole downlink bandwidth of a downlink component carrier and that transmission would occur on the whole downlink bandwidth of the downlink component carrier.

[0226] This is similar to obtaining a limited number of samples that is representative of the ensemble or its average. In general, the first and second sets of subbands are configurable independently, where preferably the second set S2 of resources where transmission is assumed is a superset of, or equal to, the first set S1 of resources where RS are transmitted/received.

[0227] If the mobile station is not configured to report rank indicator, RI, feedback, or if the to be reported rank indicator equals 1 (RI=1), a single channel quality indicator, CQI, value (corresponding to a codeword) is reported. Further, if the to be reported rank indicator is larger than one (RI>1) two channel quality indicator values (corresponding to different codewords) are reported.

[0228] In this respect, the channel state information CSI reporting mode is reported in the form of: [0229] a wideband channel quality indicator, CQI, value per codeword which is calculated assuming downlink transmission using a single precoding matrix in a first set of sub-bands and that the reference signals are present on a second set of sub-bands; and [0230] a selected precoding matrix indicator, PMI, or a first and second precoding matrix indicator corresponding to the selected single precoding matrix.

[0231] The single precoding matrix is selected from the codebook subset assuming downlink transmission on the set of sub-bands, S; and the to be reported precoding matrix indicator, PMI, and the to be reported channel quality indicator, CQI, values are calculated conditioned on the reported rank indicator, RI, or are calculated conditioned on RI=1.

[0232] In other words, for aperiodic CSI Reports for an unlicensed carrier, a new aperiodic reporting mode is defined, as follows:

[0233] A single precoding matrix is selected from the codebook subset assuming transmission on a set S of subbands. A UE shall report a wideband CQI value per codeword which is calculated assuming the use of the single precoding matrix in all subbands and transmission on the set S of subbands. The UE shall report the selected single precoding matrix indicator except with 8 CSI-RS ports configured for transmission modes 9 and 10 or with alternativeCodeBookEnabledFor4TX-r12=TRUE configured for transmission modes 8, 9, and 10, in which case a first and second precoding matrix indicator are reported corresponding to the selected single precoding matrix. For transmission modes 4, 8, 9, and 10, the reported PMI and CQI values are calculated conditioned on the reported RI. For other transmission modes they are reported conditioned on rank 1.

Mapping of CRS and CSI-RS onto Subframe n.sub.RS

[0234] Now, reference is made to a specific implementation of the subframe on which the reference signals, RS, are present. Reference signals shall be understood as at least one of the cell-specific reference signals, CRS, and channel state information reference signals, CSI-RS defined in 3GPP TS 36.211 V12.3.0, section 6.10.1 for CSR and 6.10.5 for CSI-RS.

[0235] For the cell-specific reference signals, CRS, the base station configures a cell with a number of so-called CRS ports, which—amongst other purposes—determines the number and location of resource elements where CRS are transmitted in a subframe. The resource element location is further a function of the physical cell ID.

[0236] Further, for the channel state information reference signals, CSI-RS, a mobile station is configured with one or plural sets of CSI reference signals. The mapping of CSI reference signal transmissions is pre-configured on a per-subframe basis. Specifically, CSI-RS are used for downlink transmission mode 10.

[0237] In more detail, a mobile station may presently be configured with multiple sets of CSI reference signals, namely, up to three configurations for which the mobile station shall assume non-zero transmission power for the CSI-RS (commonly also referred to as NZP-CSI-RS), and zero or more configurations for which the mobile station shall assume zero transmission power (commonly referred to as ZP-CSI-RS) as defined in TS 36.211 under section 6.10.5.2.

[0238] In one exemplary implementation, the CRS or CSI-RS transmissions are punctured into potential PDSCH resource elements within the same downlink subframe. This implementation contradicts the general approach that only those resource elements, REs, can be utilized for PDSCH transmission which is not reserved for other purposes (i.e., RSs, synchronization signals, PBCH, and control signaling). Hence, puncturing of the PDSCH would contradict this general approach in that the mobile station would assume at the REs could be reserved for PDSCH but instead carry the CRS or CSI-RS.

[0239] In other words, in case a physical downlink shared channel, PDSCH, utilizes the same subframe n.sub.RS in which the CRS or CSI-RS are present, the mobile station assumes a punctured PDSCH transmission. However, this implementation is advantageous in that the PDSCH can be decoded irrespective of whether the trigger message for triggering a CSI report on the basis of the CRS or CSI-RS presence was received or not.

[0240] In more detail, when a mobile station receives the trigger message for a CSI report then reference signals are indicated for a subframe on the basis of which the CSI is to be reported. Accordingly, the mobile station assumes whether or not corresponding REs are carrying CRS or CSI-RS in the indicated subframe, depending on whether or not the mobile station has received the CSI report indicating the CRS or CSI-RS are present in the subframe.

[0241] In this respect, should the mobile station have misconceived or missed the receipt of the CSI trigger indicating that the CRS or CSI-RS are present in a specific subframe, the mobile station can validly assume that the REs are not reserved for other purposes and hence include the punctured PDSCH.

[0242] Even if the REs are conversely carrying the CRS or CSI-RS instead of the PDSCH, the mobile station correctly receives, due to the puncturing, the remainder of the allocated PDSCH. The puncturing prevents from downlink buffer corruptions, so that even if CRS or CSI-RS symbols are erroneously included in the decoding of PDSCH, the remaining redundancy provided by forward error correction of the PDSCH can be sufficient to compensate for such an error and therefore still result in a successful decoding of the PDSCH codeword(s).

[0243] Further to the CRS and CSI-RS mapping, the mobile station may be configured to use the same or different CRS or CSI-RS to measure the signal strength, S, and/or the interference plus noise strength, I+N. More particularly, the NZP-CSI-RS are well suited for measurement of the signal component within the SINR, and the ZP-CSI-RS are well suited for measurement of the interference plus noise component within the SINR.

[0244] Nevertheless, presently the ZP-CSI-RS are configured for channel state information-interference measurement CSI-IM via Radio Resource Control, RRC, layer signalling (also, presently the NZP-CSI-RS are configured by RRC). Instead, in one implementation the improved CSI reporting mechanism indicates reference signals on which the CSI is to be reported utilizing DCI signalling via the PHY layer, by indicating at least one of NZP and ZP RS.

[0245] In this respect, in a further exemplary implementation, it is proposed that not only the location (i.e., subframe) of the non-zero-power (NZP) reference signals is indicated in CSI trigger message in form of DCI signalling, but also the location (i.e., subframe) of the zero-power (ZP) reference signals, namely, CRS and/or CSI-RS, is indicated by the same CSI triggering message in form of DCI signalling. As described above, both indications of a subframe may be direct or indirect, for example, based on a prescribed offset relative to CSI trigger message.

[0246] The NZP reference signals and the ZP reference do not necessarily have to be in a same subframe. Accordingly, in another exemplary implementation, the mobile station is configured with at least one reference signal configuration including at least one of: a non-zero-power CSI-RS configuration for which the mobile station assumes non-zero transmission power in a subframe n.sub.NZP-CSI-RS=n.sub.RS; and a zero-power CSI-RS configuration for which the mobile station assumes zero transmission power in a subframe n.sub.ZP-CSI-RS≠n.sub.RS, and, preferably, the subframe n.sub.ZP-CSI-RS is earlier than the subframe n.sub.NZP-CSI-RS, where n.sub.Trigger≦n.sub.ZP-CSI-RS<n.sub.NZP-CSI-RS<n.sub.Report.

[0247] Consequently, each of the CSI-RS follow independently the equation defined with respect to the above described embodiments. The non-zero-power and the zero-power reference signals are not in the same subframe of the one or plural component carriers for which the CSI reporting is triggered but can be carried in different subframes of the one or plural component carriers.

[0248] In this respect, burst downlink transmission can reserve more REs in a subframe where only the non-zero-power reference signals are mapped whereas the non-zero-power reference signals are mapped to a preceding, hence, different silent subframe. In this respect, silent period as specified in the regulatory requirements can be optimized, and more resources can be made available for data transmission during an active period.

[0249] In a further exemplary implementation, downlink component carrier utilization for unlicensed carriers implementing Listen-Before-Talk, LBT, shall be optimized. In case a CSI report is triggered for such a LBT downlink component carrier, the non-zero-power and the zero-power reference signals are indicated in the trigger message as discussed before. However, a distinction is made with respect to how the base station gains access to such a LBT downlink component carrier.

[0250] The base station has to transmit on the LBT downlink component carrier as soon as its availability is detected (e.g., the base station listened and detected that it is free) a signal to reserve its usage for “useful” signal transmissions. Nevertheless, in case of plural downlink, namely, licensed and unlicensed, carriers transmissions must generally be aligned between the carriers. Hence, the necessity for carrier alignment could prevent the base station from immediately starting “useful” signal transmissions on the LBT downlink component carrier.

[0251] In this respect, the base station will transmit a “reservation signal” for reserving the LBT downlink component carrier prior to a competing noted blocking the LBT downlink component carrier through its transmission. In this exemplary embodiment it is proposed that the “reservation signal” includes zero-power resource elements which can be used for interference plus noise measurements by the mobile station, and may furthermore also include non-zero-power resource elements which can be used for signal strength measurements by the mobile station.

[0252] In other words, as the misalignment prevents “useful” transmissions as part of the “reservation signal” on the LBT downlink component carrier, the base station is forced to postpone all signal transmissions to the next radio frame boundary. Nevertheless, the base station can trigger CSI reporting via a different component carrier and indicate the subframe of the “reservation signal” comprising the zero-power and non-zero-power resource elements for channel state measurements.

[0253] According to a further exemplary implementation, the density of the CRS and/or CSI-RS is reduced in the frequency domain. For example, the mapping of CRS and/or CSI-RS is adapted that every second configured CRS and/or CSI-RS (e.g., the even/odd numbered reference signals) are transmitted in form of non-zero-power CRS and/or non-zero-power CSI-RS; whereas the other configured CRS and/or CSI-RS (e.g., the odd/even numbered reference signals) are transmitted in form of zero-power CRS and/or zero-power CSI-RS. This mapping can be configured as part of a CSI reporting mode, or beneficially may also be included in the CIS trigger message in form of the DCI.

[0254] As another example, some sub-bands might contain zero-power CRS or zero-power CSI-RS, while other sub-bands might contain non-zero-power CRS or non-zero-power CSI-RS, while yet other sub-bands might contain no CRS or CSI-RS. This serves to keep the resource and power overhead due to RS transmissions low, increasing the efficiency for data transmission, and/or increasing the available transmit power for other resources in the subframe.

[0255] According to a further exemplary implementation, the CSI reference signal configuration includes a zero-power CSI-RS configuration for which the mobile station assumes zero transmission power indicating at least one resource element, RE, in the subframe n.sub.RS corresponding to a resource element prescribed for cell specific reference signal, CRS, transmission.

[0256] This exemplary implementation is discussed in more detail in connection with the following examples of advantageous mapping of CRS and CSI-RS, as illustrated in FIGS. 11 to 19.

[0257] For this purpose, no further distinction is made between zero-power and non-zero-power reference signals since the configuration and mapping is equally applicable to both; likewise no further distinction is made between the CRS and CSI-RS for the purpose of estimating the channel state according to the embodiments and implementations. Consequently, only the term “reference signal” (RS) is utilized in the following discussion and corresponding figures.

[0258] According to FIG. 7, the state of the art supports time/frequency resources for RS predominantly resources I′=2 in a second slot of a subframe, as well as in resource elements (k′, l′)={(2, 5), (2, 6), (3, 5), (3, 6), (8, 5), (8, 6), (9, 5), (9, 6)} of each slot of a subframe. P0/P1/P2/P3 denote resource elements for the CRS transmission corresponding to ports 0/1/2/3, respectively, while D7-8 and D9-10 denote resource elements for the UE-specific RS (or DM-RS) transmission corresponding to ports 7-8 and 9-10, respectively. Where applicable, DM-RS for ports 11-12 are mapped to the same resources as for ports 7-8, and DM-RS for ports 13-14 are mapped to the same resources as for ports 9-10.

[0259] According to an implementation as exemplified in FIG. 11, the RS for measuring the CSI are mapped onto one or more resource elements that are candidates for CRS port 0/1/2/3 corresponding to I′=0, 1 in the first slot of a subframe. This is particularly reasonable if there is no data transmission in the same subframe, as it enables the earliest possible transmission time of the RS, and therefore a high amount of time for processing is available between the reception of the signal and the corresponding required CSI report, thereby enabling a relatively simple implementation of the measurement and reporting procedures.

[0260] It is also reasonable if a PDSCH transmission in the same subframe is done by a transmission scheme that does not rely on CRS for data demodulation, such as in the “Up to 8 layer transmission scheme” supported by transmission modes 9 and 10. It is furthermore beneficial for cases that the RS serve not only for obtaining the CSI, but also to occupy a shared carrier (such as an unlicensed carrier) to block other nodes from accessing the channel; in this case, the RS would serve as a kind of “reservation signal”, and is particularly relevant in the beginning of a bursty access to the unlicensed carrier.

[0261] Similarly to the notation found in Sesia S, Toufik I, Baker M, “LTE The UMTS Long Term Evolution—From Theory to Practice”, Second Edition, 2011, John Wiley & Sons, Ltd., chapter 29.4, in FIG. 29.4, the uppercase character A/B/C denote a given RS configuration, while the lowercase characters x/y/z/u denote corresponding antenna ports.

[0262] Consequently, FIG. 11 shows possible configurations for 8 RS ports. In case of configurations for 4 RS ports, corresponding labels Az/Au/Bz/Bu/Cz/Cu would be replaced by Dx/Dy/Ex/Ey/Fx/Fy, respectively, to allow more different RS configurations. Likewise, for configurations for 2 RS ports, additionally corresponding labels Ay/By/Cy/Dy/Ey/Fy would be replaced by Gx/Hx/Ix/Jx/Kx/Lx.

[0263] In another implementation, alternative or additional RS configurations can be supported by mapping RS to the second slot of a subframe. A corresponding example for additional RS configurations for the case of 2 RS ports is shown in FIG. 12, where a total of 24 different RS configurations are supported.

[0264] Further variants for different mappings are exemplarily shown for the cases of 2, 4, and 8 RS ports in FIGS. 13-17. The figures show the case that a single resource element carries a single antenna port RS, i.e., no further CDM multiplexing is necessary to support different ports on the same resource elements. For this purpose, each RS is represented by an uppercase letter (A/B/C/E) followed by a single number, where identical letters correspond to the same RS configuration and the number indicates the corresponding RS port.

[0265] Alternatives in the detailed arrangements are shown, for example, to map equal ports of different configurations to time-adjacent resource elements, or to map different ports of the same RS configuration to time-adjacent resource elements.

[0266] The former has the technical advantage that a better averaging effect can be obtained when combining different RS ports to obtain a channel state estimate, while the latter can expend more transmit power to each RS port in case, e.g., only one configuration is active per subframe and RB; as can be seen, e.g., for the 2 RS port case and configuration A, in symbol I′=0 of the first slot the full transmit power can be spent on symbol A1 in case of FIG. 14 while according to FIG. 13 the transmit power needs to be split among A1 and A2.

[0267] FIG. 18 shows another alternative implementation, where not only RS for CSI measurements are transmitted in the beginning and the end of a subframe, but furthermore DM-RS are transmitted in the beginning of the first slot of a subframe. Transmitting DM-RS in the beginning of a subframe can give the benefit that these signals not only serve to provide means to estimate the channel for data demodulation, but also as reservation signals to keep other nodes from assessing the channel as vacant at the beginning of a subframe. Therefore this DM-RS transmission method may be advantageous even in subframe where no RS for CSI estimation (such as CSI-RS or CRS) are transmitted.

[0268] In one implementation, the configurations shown exemplarily in FIGS. 11 to 18 are understood to be applicable for every resource block within the system bandwidth of a downlink component carrier. In another implementation, the configurations are applicable only to a subset of resource blocks within the system bandwidth of a downlink component carrier.

[0269] This is particularly applicable to reduce the overall overhead incurred by RS transmissions in a subframe. At the same time, when extending this principle to allowing a first RS configuration in a first set of resource blocks and a second RS configuration in a second set of resource blocks, it can be used to effectively support more simultaneous configurations in a subframe.

[0270] For example, FIG. 19 exemplarily shows two resource blocks of the same subframe supporting 8 RS ports based on FIG. 18. However, in this way a first UE can be configured with RS configuration A in resource block set 1, while a second UE can be configured with RS configuration B in resource block set 2.

[0271] Even though configurations A and B can be identical with respect to the resource element location within the resource block for the RS transmission, they can be used for different UEs due to the different detailed configuration for the UEs on which resource block(s) they should expect the RS.