Technical Field

Examples of the present disclosure relate to reporting or receiving channel state information (CSI), for example from a User Equipment (UE) configured with carrier aggregation (CA) using a first component carrier and at least one second component carrier.

Background

To meet demand for data centric applications, the 3rd Generation Partnership Project (3GPP) is developing the 5th Generation (5G) wireless communication standard. The following are the requirements for 5G networks:

• Data rates of several tens of megabits per second should be supported for tens of thousands of users

• 1 gigabit per second to be offered simultaneously to tens of workers on the same office floor

• Several hundreds of thousands of simultaneous connections to be supported for massive sensor deployments

• Spectral efficiency should be significantly enhanced compared to 4G

• Coverage should be improved

• Signaling efficiency should be enhanced

• Latency should be reduced significantly compared to LTE

Multiple-input multiple-output (MIMO) systems can significantly increase the data carrying capacity of wireless systems. For these reasons, MIMO is an integral part of the 3rd and 4th generation wireless communication standards. 5G systems will also employ MIMO systems, also called massive MIMO systems, which may employ hundreds of antennas at the transmitter side and the receiver side. Typically, where N _t denotes the number of transmit antennas and N _r denotes the receive antennas, the peak data rate multiplies with a factor of N _t over single antenna systems in rich scattering environment.

Figure 1 shows a typical message sequence chart 100 for downlink data transfer in 5G systems. From the pilot or reference signals (cell specific / UE specific reference signals 102 from gNB 104), the UE 106 computes the channel estimates then computes the parameters needed for channel state information (CSI) reporting, shown in box 108. The CSI report consists of for example channel quality indicator (CQI), precoding matrix index (PMI), rank information (Rl) CSI-RS Resource Indicator (CRI the same as beam indicator), etc.

The CSI report is sent in step 110 to the network via a feedback channel either on request from the network aperiodically or configured to report periodically. The network scheduler uses this information in choosing the parameters for scheduling of this particular UE. For example, the gNB determines parameters for downlink (DL) transmission, such as modulation and coding scheme (MCS), power, physical resource blocks (PRBs) etc., based on the CSI, shown in box 112. The network sends the scheduling parameters to the UE in the downlink control channel 114. After that, actual data transfer takes place from network to the UE on data traffic channel 116.

Downlink reference signals are predefined signals occupying specific resource elements within the downlink time-frequency grid. There are several types of downlink reference signals that are transmitted in different ways and used for different purposes by the receiving terminal:

• CSI reference signals (CSI-RS): These reference signals are specifically intended to be used by terminals to acquire channel-state information (CSI) and beam specific information (beam RSRP). In 5G, CSI-RS is UE specific so it can have a significantly lower time/frequency density.

• Demodulation reference signals (DM-RS): These reference signals also sometimes referred to as UE-specific reference signals, are specifically intended to be used by terminals for channel estimation for data channel. The label “UE-specific” relates to the fact that each demodulation reference signal is intended for channel estimation by a single terminal. That specific reference signal is then only transmitted within the resource blocks assigned for data traffic channel transmission to that terminal.

Other than these reference signals, there are other reference signals, such as Phase Tracking Reference Signal (PTRS) and Tracking Reference Signals (TRS), that may be used for various other purposes.

The uplink control channel carries HARQ-ACK information corresponding to the downlink data transmission, and channel state information. The channel state information typically consists of CSI-RS resource indicator (CRI), rank indicator (Rl), channel quality indicator (CQI), precoding matrix indicator (PMI) and Layer Indicator, etc. The CSI can be divided into two categories, one for subband and the other for wideband. The configuration of subband or wideband CSI reporting is done through RRC signalling as part of CSI reporting configuration. The table below shows the contents of CSI report for PMI format indicator = Wideband, CQI format indicator = wideband and for PMI format indicator = subband, CQI format indicator = subband.

Note that for New Radio (NR), the subband is defined according to the bandwidth part of the OFDM in terms of PRBs as shown in the table below. The subband configuration is also done through RRC signalling.

The downlink control channel (PDCCH) carries information about scheduling grants. Typically, this consists of the number of MIMO layers scheduled, transport block sizes, modulation for each codeword, parameters related to HARQ, sub band locations and PM I corresponding to that sub bands. Note that downlink control information (DCI) formats may not transmit all the information as shown above, and in general the contents of PDCCH information depends on transmission mode and DCI format.

Note that with multiple antennas at the transmitter the channel characteristics such as delay spread, multipath profile etc. might vary between the antenna ports. However, some antenna ports might have the same profile. When the antenna ports have the same channel characteristic, these are called quasi collocated (QCL).

Carrier Aggregation is a technique for increasing bandwidth without any modifications the baseband. In the case of carrier aggregation, multiple NR carriers can be transmitted in parallel to or from the same terminal or User Equipment (UE), thereby allowing for an overall wider bandwidth and correspondingly higher per-link data rates. In the context of carrier aggregation, each carrier is referred to as a component carrier (CC) as, from an RF point of view, the entire set of aggregated carriers be a single (RF) carrier.

A terminal capable of carrier aggregation may receive or transmit simultaneously on multiple component carriers. It should be noted that aggregated component carriers do not need to be contiguous in the frequency domain. Rather, with respect to the frequency location of the different component carriers, three different cases can be identified

• Intra-band aggregation with frequency-contiguous component carriers

• Intra-band aggregation with non-contiguous component carriers

• Inter-band aggregation with non-contiguous component carriers.

A terminal capable of carrier aggregation has one downlink primary component carrier and an associated uplink primary component carrier. In addition, it may have one or several secondary component carriers in each direction. Different terminals may have different carriers as their primary component carrier; that is, the configuration of the primary component carrier is terminal specific.

Summary

Example embodiments of this disclosure may improve downlink throughput, while not significantly impacting uplink capacity as in conventional methods.

One aspect of the present disclosure provides a method of reporting Channel State Information (CSI) in a User Equipment (UE) configured with carrier aggregation using a first component carrier and at least one second component carrier. The method comprises reporting, to a first network node, first channel state information for the first component carrier, wherein the first channel state information comprises values for a plurality of parameters for the first component carrier. The method also comprises reporting, to a second network node, for each of the at least one second component carrier, second channel state information for the second component carrier, wherein the second channel state information comprises values for only a first subset of the parameters for the second component carrier.

Another aspect of the present disclosure provides a method of receiving Channel State Information (CSI) in a network node from a User Equipment (UE) configured with carrier aggregation using a first component carrier and at least one second component carrier. The method comprises receiving, from the UE, first channel state information for the first component carrier, wherein the first channel state information comprises values for a plurality of parameters for the first component carrier. The method also comprises receiving, from the UE, for each of the at least one second component carrier, second channel state information for the second component carrier, wherein the second channel state information comprises values for only a first subset of the parameters for the second component carrier.

A further aspect of the present disclosure provides apparatus for reporting Channel State Information (CSI) in a User Equipment (UE) configured with carrier aggregation using a first component carrier and at least one second component carrier. The apparatus comprises a processor and a memory. The memory contains instructions executable by the processor such that the apparatus is operable to report, to a first network node, first channel state information for the first component carrier, wherein the first channel state information comprises values for a plurality of parameters for the first component carrier, and report, to a second network node, for each of the at least one second component carrier, second channel state information for the second component carrier, wherein the second channel state information comprises values for only a first subset of the parameters for the second component carrier.

A still further aspect of the present disclosure provides apparatus for receiving Channel State Information (CSI) in a network node from a User Equipment (UE) configured with carrier aggregation using a first component carrier and at least one second component carrier. The apparatus comprises a processor and a memory. The memory contains instructions executable by the processor such that the apparatus is operable to receive, from the UE, first channel state information for the first component carrier, wherein the first channel state information comprises values for a plurality of parameters for the first component carrier, and receive, from the UE, for each of the at least one second component carrier, second channel state information for the second component carrier, wherein the second channel state information comprises values for only a first subset of the parameters for the second component carrier.

Another aspect of the present disclosure provides apparatus for reporting Channel State Information (CSI) in a User Equipment (UE) configured with carrier aggregation using a first component carrier and at least one second component carrier. The apparatus is configured to report, to a first network node, first channel state information for the first component carrier, wherein the first channel state information comprises values for a plurality of parameters for the first component carrier, and report, to a second network node, for each of the at least one second component carrier, second channel state information for the second component carrier, wherein the second channel state information comprises values for only a first subset of the parameters for the second component carrier.

An additional aspect of the present disclosure provides apparatus for receiving Channel State Information (CSI) in a network node from a User Equipment (UE) configured with carrier aggregation using a first component carrier and at least one second component carrier. The apparatus is configured to receive, from the UE, first channel state information for the first component carrier, wherein the first channel state information comprises values for a plurality of parameters for the first component carrier, and receive, from the UE, for each of the at least one second component carrier, second channel state information for the second component carrier, wherein the second channel state information comprises values for only a first subset of the parameters for the second component carrier. Brief of the Drawinas

For a better understanding of examples of the present disclosure, and to show more clearly how the examples may be carried into effect, reference will now be made, by way of example only, to the following drawings in which:

Figure 1 shows a typical message sequence chart for downlink data transfer in 5G systems;

Figure 2 is a flow chart of an example of a method of reporting Channel State Information (CSI) in a User Equipment (UE) configured with carrier aggregation using a first component carrier and at least one second component carrier;

Figure 3 is a flow chart of an example of a method of receiving Channel State Information (CSI) in a network node from a User Equipment (UE) configured with carrier aggregation using a first component carrier and at least one second component carrier;

Figure 4 shows an example of a rank indicator (Rl) reported over time (index) for a 3.5GHz component carrier;

Figure 5 shows an example of a rank indicator (Rl) reported over time (index) for a 3.6GHz component carrier;

Figure 6 shows an example of a wideband precoding matrix indicator (X1) reported over time (index) for the 3.5 GHz carrier;

Figure 7 shows an example of a wideband precoding matrix indicator (X1) reported over time (index) for the 3.6 GHz carrier;

Figure 8 is a schematic of an example of an apparatus for reporting Channel State Information (CSI) in a User Equipment (UE) configured with carrier aggregation using a first component carrier and at least one second component carrier; and

Figure 9 is a schematic of an example of an apparatus for receiving Channel State Information (CSI) in a network node from a User Equipment (UE) configured with carrier aggregation using a first component carrier and at least one second component carrier. Detailed Description

The following sets forth specific details, such as particular embodiments or examples for purposes of explanation and not limitation. It will be appreciated by one skilled in the art that other examples may be employed apart from these specific details. In some instances, detailed descriptions of well-known methods, nodes, interfaces, circuits, and devices are omitted so as not obscure the description with unnecessary detail. Those skilled in the art will appreciate that the functions described may be implemented in one or more nodes using hardware circuitry (e.g. analog and/or discrete logic gates interconnected to perform a specialized function, Application Specific Integrated Circuits (ASICs), Programmable Logic Arrays (PLAs), etc.) and/or using software programs and data in conjunction with one or more digital microprocessors or general purpose computers. Nodes that communicate using the air interface also have suitable radio communications circuitry. Moreover, where appropriate the technology can additionally be considered to be embodied entirely within any form of computer-readable memory, such as solid-state memory, magnetic disk, or optical disk containing an appropriate set of computer instructions that would cause a processor to carry out the techniques described herein.

Hardware implementation may include or encompass, without limitation, digital signal processor (DSP) hardware, a reduced instruction set processor, hardware (e.g. digital or analogue) circuitry including but not limited to application specific integrated circuit(s) (ASIC) and/or field programmable gate array(s) (FPGA(s)), and (where appropriate) state machines capable of performing such functions.

For NR MIMO systems, the UE in some examples reports channel state information (CSI) either periodically or aperiodically, or by using semi persistent CSI reporting for each component carrier. For example, if the UE is configured with 4 component carriers, the UE reports feedback for each component carrier on the uplink carrier. The table below shows the number of bits in a CSI sent if the UE is configured with 2 ports for each component carrier.

That is, the size of the CSI payload depends on the reported rank. Similarly, the table below shows the number of bits needed for a CSI for each component carrier when the UE is configured with 4 CSI-RS ports.

Similarly, the table below shows the number of bits needed for a CSI for each component carrier when the UE is configured with 8 CSI-RS ports.

Similarly, the table below shows the number of bits needed for a CSI for each component carrier when the UE is configured with 16 CSI-RS ports.

Similarly, the table below shows the number of bits needed for a CSI for each component carrier when the UE is configured with 32 CSI-RS ports.

Finally, Table shows the total number of bits for CSI payload when the UE is configured with up to 4 component carriers. It can be observed that when the number of component carriers increases, the payload size increase proportionally.

Therefore, in some examples, when the UE is configured with multiple component carriers for increasing the downlink throughput, the uplink channel capacity (either control channel or shared channel) is impacted as each carrier needs to send the channel state information for each component carrier. For example, if 4 component carriers are configured for a UE, with 32 port CSI-RS, a maximum of 64 bits may be needed for reporting CSI compared to 16 bits for the single component carrier. Hence, an efficient solution is needed to reduce the impact on uplink capacity when multiple component carriers are configured. Examples of this disclosure propose methods to significantly reduce the uplink channel overhead when a UE is configured with multiple component carriers (e.g. carrier aggregation, CA) without impacting or significantly impacting downlink throughput. Methods proposed herein may for example configure the UE to report complete channel state information for only one or a few component carriers, while for the rest of the component carriers only a subset of channel state information parameters is reported. In some examples, the UE may be configured to report “full” CSI only on one or a few component carriers, while for the other component carriers the UE may be configured to report only a subset of CSI parameters, thereby reducing uplink overhead significantly.

Figure 2 is a flow chart of an example of a method 200 of reporting Channel State Information (CSI) in a User Equipment (UE) configured with carrier aggregation using a first component carrier and at least one second component carrier. The method 200 comprises, in step 202, reporting, to a first network node, first channel state information (CSI) for the first component carrier, wherein the first channel state information comprises values for a plurality of parameters for the first component carrier. The plurality of parameters may comprise, for example, at least one of the following parameters for the first component carrier:

• a channel state information reference signal (CSI-RS) resource indicator (CRI);

• a rank indicator;

• a layer indicator;

• a wideband precoding matrix indicator (X1 );

• a wideband channel quality indicator (CQI);

• a plurality of subband channel quality indicators (CQIs) for a plurality of subbands; or

• a subband precoding matrix indicator (X2).

Step 204 of the method 200 comprises reporting, to a second network node, for each of the at least one second component carrier, second channel state information (CSI) for the second component carrier, wherein the second channel state information comprises values for only a first subset of the parameters for the second component carrier. Therefore, for example, the second CSI may omit values or information for one or more of the parameters that were included in the first CSI, such as for example one or more of the parameters listed above. Thus, the second CSI may in some examples contain less information than the first CSI and may be smaller in size and be represented by a smaller number of bits. In this way, the second CSI may have a smaller impact on uplink capacity or overhead than a “full” CSI that includes values for all of the parameters for the second component carrier (and this also applies in some examples for each second component carrier, where there are multiple second component carriers). The first component carrier may or may not be the primary component carrier in some examples. In some examples, the first subset of parameters may comprise at least one of the following parameters for the second component carrier:

• the wideband precoding matrix indicator (X1 );

• the wideband channel quality indicator (CQI);

• the plurality of subband channel quality indicators (CQIs) for the plurality of subbands; and

• the subband precoding matrix indicator (X2).

The first network node and the second network node may in some examples be the same network node, for example where the same network node provides or serves both the first and second component carriers. Alternatively, for example, particularly where the component carriers are provided or served by different network nodes, the first and second network nodes may be different network nodes. Each network node may be for example one or more base stations, base station control units (CUs), base station distributed units (Dlls) and/or core network nodes.

In some examples, the network node receiving the second CSI (e.g. the second network node) may complete the second CSI by adding the “missing” parameters from the first CSI. This may be the case where for example the first and second component carriers are close enough in frequency that parameters from the first CSI can be considered as a good approximation for the “missing” parameters in the second CSI.

Thus, for example, reporting, to the second network node, for each of the at least one second component carrier, second channel state information for the second component carrier, wherein the second channel state information comprises values for only the first subset of the parameters for the second component carrier, in step 204 may be performed in response to a difference between a frequency of the first component carrier and a frequency of each of the at least one second component carrier being below a first threshold.

In some examples, if the difference between the frequency of the first component carrier and the frequency of each of the at least one second component carrier is above the first threshold and below a second threshold, the method 200 may further comprise reporting, to the second network node, for each of the at least one second component carrier, third channel state information for the second component carrier. The third channel state information comprises values for only a second subset of the parameters for the second component carrier, and wherein the second subset of the parameters includes more parameters than the first subset of parameters. In addition, in some examples, if the difference between the frequency of the first component carrier and the frequency of each of the at least one second component carrier is above the second threshold, the method 200 may further comprise reporting, to the network node, for each of the at least one second component carrier, fourth channel state information for the second component carrier. The fourth channel state information comprises values for all of the parameters for the second component carrier. Thus, for example, as the difference between the first and second component carrier frequencies widens, the values of the parameters for the first CSI that are “missing” from the second CSI may be regarded as less accurate approximations for the second CSI and thus fewer such parameters from the first CSI are used as the difference widens.

Alternatively, however, in some examples there may only be one threshold, and hence if the difference between the frequency of the first component carrier and the frequency of each of the at least one second component carrier is above the second threshold, the method 200 may comprise reporting, to the second network node, for each of the at least one second component carrier, fourth channel state information for the second component carrier, wherein the fourth channel state information comprises values for all of the parameters for the second component carrier.

In some examples, the first or second network node may configure the UE to report the second CSI (e.g. a CSI with reduced parameters compared to a “full” CSI). Thus, for example, the method 200 may comprise receiving, from the first network node or the second network node, a configuration indicating that the second channel state information for the second component carrier comprises only the first subset of the parameters. In some examples, the configuration may identify the first subset of parameters that are to be in the second CSI. Alternatively, for example, the parameters in the second CSI may be preconfigured or specified in a communication standard. The configuration may be received for example via Radio Resource Control (RRC) signalling.

The method 200 may in some examples comprise receiving, from the first network node or the second network node, an instruction to report, to the network node, for each of the at least one second component carrier, the second channel state information for the second component carrier, wherein the second channel state information comprises values for only the first subset of the parameters for the second component carrier. The method 200 may comprise receiving, from the first network node or the second network node, an indication identifying the first subset of the parameters and/or an indication identifying the first component carrier. Thus for example one or more of these may be configurable by the network. The parameters in the first subset and/or the first component carrier may also be configurable for other UE(s), e.g. these may be the same or different for other UEs.

In some examples, the method may comprise reporting the first channel state information and/or the second channel state information to the first network node and/or the second network node on a physical uplink control channel (PUCCH) or physical uplink shared channel (PUSCH).

The values for first subset of the parameters for each of the at least one second component carrier may in some examples be based on a rank indicator for the first component carrier. That is, for example, the values for the parameters in the second CSI may be different depending on which Rl is chosen for the second CSI. However, if the first subset of parameters does not include the Rl for the second component carrier, then the values for the first subset of parameters may be chosen based on (e.g. assuming that) the Rl is the same as the Rl in the first CSI. Thus, for example, the first subset of parameters may not include a rank indicator.

In a particular illustrative example, it is noted that 5G systems and future 6G systems may use wide bandwidths, typically in the range of 50-100 MHz, and that rank information and some other parameters related to CSI reporting such as wideband precoding matrix indicator (X1 ) in some examples may not differ significantly when two component carriers are adjacent or in the same band, or are sufficiently close together in frequency. For example, if one component carrier is at 3.5 GHz and another is at 3.6 GHz, then the rank information reported by the UE with each component carrier bandwidth of 100 MHz at these two component carriers will be similar over time. Figure 4 shows an example of a rank indicator (Rl) reported over time (index) for the 3.5GHz component carrier, and Figure 5 shows an example of a rank indicator (Rl) reported over time (index) for the 3.6GHz component carrier. It can be seen that in most cases, the rank indicator is the same, and differs only by a small amount in other cases. In the following example, the 3.5GHz carrier is referred to as a reference component carrier, and may be for example the first component carrier referred to herein, for which the “full” CSI is reported; and the 3.6 GHz carrier is referred to as the secondary component carrier, and may be for example the second component carrier referred to herein, for which the first subset of CSI parameters is reported.

Similarly, Figure 6 shows an example of a wideband precoding matrix indicator (X1 ) reported over time (index) for the 3.5 GHz carrier, and Figure 7 shows an example of a wideband precoding matrix indicator (X1) reported over time (index) for the 3.6 GHz carrier. It can be seen that the value for X1 is the same or similar for both component carriers for each time index.

Thus, for example, the network node determines if the difference between the reference component carrier frequency where the full CSI is reported from the UE and a second component carrier where some CSI parameters are derived based on the parameters obtained from the first component carrier (e.g. the “missing” parameters referred to in examples above) is less than a pre-defined threshold or not. That is:

If the difference between the frequencies is less than a threshold (T), then:

8 _f < T

If so, the network node may inform the UE not to transmit a full CSI for the second component carrier, and instead transmit a subset of parameters. The network may then use the first component carrier as a reference in deriving the remaining CSI parameters of the second component carrier, thereby significantly reducing the uplink channel overhead for reporting CSI for the second component carrier.

In an example, the network node asks the UE to report X1 , X2 and CQI for the second component carrier based on the first reference carriers rank indicator (Rl). In another example, the network node asks the UE to report only X2 and CQI based on the first reference carrier’s Rl and X1 . In another example, the network node may determine two thresholds T 1 and T2, where T 1 <T2. Then, if the difference between the component carrier frequencies is less than T 1 , then both Rl and X1 for the second carrier is based on the first reference carrier. Also, for example, if the difference between the component carrier frequency is less than T2 and greater than T 1 , then only Rl for the second carrier is based on the first reference carrier (while X1 and the other parameters are included in the subset of parameters in the second carrier’s CSI). Thus, in some examples, only certain parameters are requested based on the difference between the carrier frequencies.

Figure 3 is a flow chart of an example of a method 300 of receiving Channel State Information (CSI) in a network node from a User Equipment (UE) configured with carrier aggregation using a first component carrier and at least one second component carrier. The network node may be for example the first and/or second network node referred to above with reference to the method 200 of Figure 2. In some examples, the UE may perform the method 200 of Figure 2. The network node may comprise for example one or more base stations, base station control units, base station distributed units and/or core network nodes.

The method 300 comprises, in step 302, receiving, from the UE, first channel state information for the first component carrier, wherein the first channel state information comprises values for a plurality of parameters for the first component carrier. The plurality of parameters may comprise, for example, at least one of the following parameters for the first component carrier:

• a channel state information reference signal (CSI-RS) resource indicator (CRI);

• a rank indicator;

• a layer indicator;

• a wideband precoding matrix indicator (X1 );

• a wideband channel quality indicator (CQI);

• a plurality of subband channel quality indicators (CQIs) for a plurality of subbands; or

• a subband precoding matrix indicator (X2).

The method 300 also comprises, in step 304, receiving, from the UE, for each of the at least one second component carrier, second channel state information for the second component carrier, wherein the second channel state information comprises values for only a first subset of the parameters for the second component carrier. Therefore, for example, the second CSI may omit values or information for one or more of the parameters that were included in the first CSI, such as for example one or more of the parameters listed above. Thus, the second CSI may in some examples contain less information than the first CSI and may be smaller in size and be represented by a smaller number of bits. In this way, the second CSI may have a smaller impact on uplink capacity or overhead than a “full” CSI that includes values for all of the parameters for the second component carrier (and this also applies in some examples for each second component carrier, where there are multiple second component carriers). The first component carrier may or may not be the primary component carrier in some examples. In some examples, the first subset of parameters may comprise at least one of the following parameters for the second component carrier:

• the wideband precoding matrix indicator (X1 );

• the wideband channel quality indicator (CQI);

• the plurality of subband channel quality indicators (CQIs) for the plurality of subbands; and

• the subband precoding matrix indicator (X2).

In some examples, the network node may complete the second CSI by adding the “missing” parameters from the first CSI. This may be the case where for example the first and second component carriers are close enough in frequency that parameters from the first CSI can be considered as a good approximation for the “missing” parameters in the second CSI. Thus, in some examples, the method 300 may comprise using, for each of the at least one second channel state information, values for the parameters in the first channel state information other than the first subset of parameters. This may comprise for example selecting, for each of the at least one second channel state information, one or more communication parameters for wireless communication between the network node and the UE based on the values for the parameters in the first channel state information other than the first subset of parameters (and also for example using the first subset of parameters in the second CSI).

The step 304 of receiving, from the UE, for each of the at least one second component carrier, second channel state information for the second component carrier, wherein the second channel state information comprises values for only a first subset of the parameters for the second component carrier, may in some examples performed in response to a difference between a frequency of the first component carrier and a frequency of each of the at least one second component carrier being below a first threshold. This may be for example similar to certain examples of the method 200 as described above. In some examples, the method 300 comprises, if the difference between the frequency of the first component carrier and the frequency of each of the at least one second component carrier is below the first threshold, sending an instruction to the UE to send, for each of the at least one second component carrier, second channel state information for the second component carrier, wherein the second channel state information comprises values for only the first subset of the parameters for the second component carrier. The instruction may in some examples identify the first subset of the parameters. In some examples, if the difference between the frequency of the first component carrier and the frequency of each of the at least one second component carrier is above the first threshold and below a second threshold, the method 300 may further comprise receiving, from the UE, for each of the at least one second component carrier, third channel state information for the second component carrier, wherein the third channel state information comprises values for only a second subset of the parameters for the second component carrier, and wherein the second subset of the parameters includes more parameters than the first subset of parameters. The method 300 may also further comprise, if the difference between the frequency of the first component carrier and the frequency of each of the at least one second component carrier is above the first threshold and below the second threshold, sending an instruction to the UE to send, for each of the at least one second component carrier, the third channel state information for the second component carrier, wherein the third channel state information comprises values for only a second subset of the parameters for the second component carrier, and wherein the second subset of the parameters includes more parameters than the first subset of parameters. In some examples, the instruction identifies the second subset of parameters.

The method 300 may also comprise, in some examples, if the difference between the frequency of the first component carrier and the frequency of each of the at least one second component carrier is above the second threshold, receiving, from the UE, for each of the at least one second component carrier, fourth channel state information for the second component carrier, wherein the fourth channel state information comprises values for all of the parameters for the second component carrier.

Thus, as for examples of the method 200 described above, the larger the frequency gap between the component carriers, then the CSI sent for the second component carrier(s) may include values for more parameters, until the gap is large enough that the CSI includes the “full” set of parameters.

Alternatively, where there is only one threshold, for example, the method 300 may comprise, if the difference between the frequency of the first component carrier and the frequency of each of the at least one second component carrier is above the second threshold, receiving, from the UE, for each of the at least one second component carrier, fourth channel state information for the second component carrier, wherein the fourth channel state information comprises values for all of the parameters for the second component carrier. In some examples, the method 300 may comprise sending, to the UE, an instruction to report, to the network node, for each of the at least one second component carrier, the second channel state information for the second component carrier, wherein the second channel state information comprises values for only the first subset of the parameters for the second component carrier.

The network node may in some examples send, to the UE, an indication identifying the first subset of the parameters and/or an indication identifying the first component carrier.

As indicated above, in some examples, the values for first subset of the parameters for each of the at least one second component carrier are based on a rank indicator for the first component carrier. Thus the first subset of parameters may not include a rank indicator in some examples.

In some examples, where different component carriers are served or provided by different network nodes (such as for example the first and second network nodes referred to above with reference to the method 200 of Figure 2), or where the network node does not serve or provide any component carriers (e.g. where the network node is a core network node), one or more CSI reports may be received via other network node(s). Thus, for example, the first channel state information may be received via another network node, and/or each of the at least one second component carrier is received via a respective other network node.

Signaling from the network node to the UE may in some examples be via higher layer signaling such as Radio Resource Control (RRC) signaling as part of CSI report setting. For example, the network node or base station determines which carrier(s) are the reference carrier(s) and which component carrier(s) should use one or more parameters from the reference carrier’s CSI parameters.

According to examples disclosed herein that use rank information of the first (e.g. reference) carrier for the second component carrier(s), the bit savings in the uplink channel are shown in the table below, which shows an example of the number of bits that may comprise the CSI payload for various numbers of CSI-RS ports, numbers of component carriers and Rl of the reference carrier. The number of bits according to examples of this disclosure is shown in brackets after the number of bits according to conventional methods and as indicated in tables above.

It can be observed that the uplink overhead is significantly reduced. For example, for the case of 4 carriers with rank 4 using an example method of this disclosure for 32 ports CSI- RS, the CSI payload size is 15 bits compared to 64 bits using the conventional approach. Hence there is a 75% reduction in uplink channel overhead for the CSI for the second component carrier according to this example.

Figure 8 is a schematic of an example of an apparatus 800 for reporting Channel State Information (CSI) in a User Equipment (UE) configured with carrier aggregation using a first component carrier and at least one second component carrier. The apparatus 800 comprises processing circuitry 802 (e.g. one or more processors) and a memory 804 in communication with the processing circuitry 802. The memory 804 contains instructions, such as computer program code 810, executable by the processing circuitry 802. The apparatus 800 also comprises an interface 806 in communication with the processing circuitry 802. Although the interface 806, processing circuitry 802 and memory 804 are shown connected in series, these may alternatively be interconnected in any other way, for example via a bus.

In one embodiment, the memory 804 contains instructions executable by the processing circuitry 802 such that the apparatus 800 is operable/configured to report, to a first network node, first channel state information for the first component carrier, wherein the first channel state information comprises values for a plurality of parameters for the first component carrier; and report, to a second network node, for each of the at least one second component carrier, second channel state information for the second component carrier, wherein the second channel state information comprises values for only a first subset of the parameters for the second component carrier. In some examples, the apparatus 800 is operable/configured to carry out the method 200 described above with reference to Figure 2.

Figure 9 is a schematic of an example of an apparatus 900 for receiving Channel State Information (CSI) in a network node from a User Equipment (UE) configured with carrier aggregation using a first component carrier and at least one second component carrier. The apparatus 900 comprises processing circuitry 902 (e.g. one or more processors) and a memory 904 in communication with the processing circuitry 902. The memory 904 contains instructions, such as computer program code 910, executable by the processing circuitry 902. The apparatus 900 also comprises an interface 906 in communication with the processing circuitry 902. Although the interface 906, processing circuitry 902 and memory 904 are shown connected in series, these may alternatively be interconnected in any other way, for example via a bus.

In one embodiment, the memory 904 contains instructions executable by the processing circuitry 902 such that the apparatus 900 is operable/configured to receive, from the UE, first channel state information for the first component carrier, wherein the first channel state information comprises values for a plurality of parameters for the first component carrier; and receive, from the UE, for each of the at least one second component carrier, second channel state information for the second component carrier, wherein the second channel state information comprises values for only a first subset of the parameters for the second component carrier. In some examples, the apparatus 900 is operable/configured to carry out the method 300 described above with reference to Figure 3.

It should be noted that the above-mentioned examples illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative examples without departing from the scope of the appended statements. The word “comprising” does not exclude the presence of elements or steps other than those listed in a claim, “a” or “an” does not exclude a plurality, and a single processor or other unit may fulfil the functions of several units recited in the statements below. Where the terms, “first”, “second” etc. are used they are to be understood merely as labels for the convenient identification of a particular feature. In particular, they are not to be interpreted as describing the first or the second feature of a plurality of such features (i.e. , the first or second of such features to occur in time or space) unless explicitly stated otherwise. Steps in the methods disclosed herein may be carried out in any order unless expressly otherwise stated. Any reference signs in the statements shall not be construed so as to limit their scope.