HK1206881B - Methods and arrangements for csi reporting - Google Patents

Description

Method and apparatus for CSI reporting

Technical Field

The present invention relates to a method and apparatus for reporting channel state information.

Background Art

The 3rd Generation Partnership Project (3GPP) is responsible for the standardization of the Universal Mobile Telecommunications System (UMTS) and Long Term Evolution (LTE). The 3GPP work on LTE is also known as the Evolved Universal Terrestrial Access Network (E-UTRAN). Long Term Evolution is a technology for enabling high-speed packet-based communications that is capable of high data rates in both the downlink and uplink, and is considered to be the next generation of mobile communications systems relative to UMTS. To support high data rates, LTE allows for system bandwidths of 20 MHz or up to 100 Hz when carrier aggregation is employed. LTE is also capable of operating in different frequency bands and in at least frequency division duplex (FDD) and time division duplex (TDD) modes.

LTE uses orthogonal frequency division multiplexing (OFDM) in the downlink and discrete Fourier transform spread (DFT spread) OFDM in the uplink. The basic LTE resource resources can be viewed as a time-frequency grid, as shown in Figure 1, where each time-frequency resource element (TFRE) corresponds to a subcarrier during an OFDM symbol interval on a specific antenna port. There is one resource grid per antenna port. Resource allocation in LTE is described in terms of resource blocks, where a resource block corresponds to a time slot in the time domain and 12 consecutive 15 kHz subcarriers in the frequency domain. Two time-contiguous resource blocks represent a resource block pair, which corresponds to the time interval over which scheduling operates.

An antenna port is a "virtual" antenna defined by an antenna port-specific reference number (RS). An antenna port is defined so that the channel over which a symbol on the same antenna port is transmitted can be inferred from the channel over which another symbol on the same antenna port is transmitted. The signal corresponding to an antenna port may be transmitted by multiple physical antennas, which may also be geographically distributed. In other words, an antenna port may be transmitted from one or more transmission points. Conversely, a single transmission point may transmit one or more antenna ports. Antenna ports are interchangeably referred to as "RS ports."

Multiple antenna technology can significantly increase the data rate and reliability of wireless communication systems. Performance can be particularly improved if both the transmitter and receiver are equipped with multiple antennas, resulting in a multiple-input, multiple-output (MIMO) communication channel. Such systems and/or related technologies are generally referred to as MIMO.

The LTE standard is currently evolving with enhanced MIMO support. A core component in LTE is support for MIMO antenna deployment and MIMO-related technologies. LTE Release 10 and above (also known as Advanced LTE) enable support for eight layers of spatial multiplexing, potentially utilizing channel-dependent precoding. This type of spatial multiplexing is intended for high data rates under good channel conditions. An illustration of precoded spatial multiplexing is provided in Figure 2.

As can be seen, the information-carrying symbol vector s is multiplied by an _NT × r precoder matrix, which is used to distribute the transmission energy among subspaces of the _NT- dimensional vector space, where _NT corresponds to the number of antenna ports. Each of the r symbols in s is part of a symbol stream (a so-called layer), and r is called the transmission rank. This enables spatial multiplexing, as multiple symbols can be transmitted simultaneously on the same TFRE. The number of layers, r, is typically adapted to the current channel characteristics.

Furthermore, the precoder matrix is often selected from a codebook of possible precoder matrices and is typically indicated by a precoder matrix indicator (PMI), which specifies a unique precoder matrix in the codebook for a given rank. If the precoder matrix is restricted to have orthogonal columns, the design of the precoder matrix codebook corresponds to the Grassmann subspace packing problem.

The received N _R ×1 vector y _n on the data TFRE with index n can be modeled by

where _en is the noise plus interference vector modeled as a realization of a random process. The precoder for rank r can be a wideband precoder that is constant with respect to frequency or frequency selective.

The precoder matrix is often chosen to match the characteristics of the _NR × _NT MIMO channel H, resulting in so-called channel-dependent precoding. When based on UE feedback, this is generally called closed-loop precoding and essentially strives to focus the transmission energy into a subspace that is robust in the sense of delivering a large amount of transmission energy to the UE. In addition, the precoder matrix can also be chosen to strive to orthogonalize the channel, which means that after appropriate linear equalization at the UE, inter-layer interference is reduced.

In closed-loop precoding, the UE transmits a recommendation to the base station, called an evolved NodeB (eNodeB) in LTE, of the appropriate precoder to use based on channel measurements in the forward link or downlink. A single precoder that is assumed to cover a large bandwidth (wideband precoding) can be fed back. It may also be beneficial to match the frequency variation of the channel and instead feed back frequency-selective precoding reports (e.g., multiple precoders), one per subband. This is an example of a more general case of channel state information (CSI) feedback, which also covers feeding back other entities besides the precoder to assist the eNodeB in subsequent transmissions to the UE. Thus, the channel state information may include one or more of a PMI, a channel quality indicator (CQI), or a rank indicator (RI).

Signal and channel quality estimation is an essential part of modern wireless systems. Noise and interference estimation is not only used in the demodulator, but is also an important quantity when estimating, for example, the Channel Quality Indicator (CQI), which is typically used for link adaptation and scheduling decisions on the eNodeB side.

The term _en in (1) represents the noise and interference in the TFRE and is typically characterized in terms of second-order statistics such as variance and correlation. Interference can be estimated in a variety of ways, including from cell-specific reference symbols (RS) present in the time-frequency grid of LTE. Such RS may correspond to the Rel-8 cell-specific RS, CRS (antenna ports 0-3) shown in Figure 3, and the new CSI RS available in Rel-10, which will be described in more detail below. CRS is sometimes also called a common reference signal.

Estimates of interference and noise can be formed in various ways. This estimate can be easily formed based on the TFRE containing the cell-specific RS, since sn and _Hn are then known and _Hn is given by the channel estimator. It should also be noted that the interference with the TFRE and the data scheduled for the UE in question can also be estimated as soon as the data symbol _sn is detected, since it can then be considered a known symbol. The latter interference can also alternatively be estimated based on second-order statistics of the received signal and the signal intended for the UE of interest, thus potentially avoiding the need to decode the transmission before estimating the interference term. Alternatively, the interference can be measured on a TFRE where the desired signal is muted, since the received signal corresponds only to the interference. This has the advantage that the interference measurement can be more accurate and UE processing becomes trivial since no decoding or desired signal subtraction needs to be performed.

Channel State Information Reference Signal (CSI-RS)

In LTE Release 10, a new reference symbol sequence, the CSI-RS, was introduced for the purpose of estimating channel state information. CSI-RS has several advantages over basing CSI feedback on the cell-specific reference symbols (CRS) used for this purpose in previous releases. First, CSI-RS is not used for demodulation of data signals and therefore does not require the same density. In other words, the overhead of CSI-RS is significantly smaller. Second, CSI-RS provides a more flexible means for configuring CSI feedback measurements. For example, it is possible to configure which CSI-RS resource to measure on in a UE-specific manner. In addition, support for antenna configurations greater than 4 antennas must rely on CSI-RS, as CRS is only defined for up to 4 antennas.

By measuring on the CSI-RS, the UE can estimate the effective channel on which the CSI-RS is traveling, including the radio propagation channel, antenna gain, and any possible antenna virtualization. The CSI-RS port can be precoded so that it is virtualized compared to multiple physical antenna ports; that is, the CSI-RS port can be transmitted on multiple physical antenna ports with different gains and phases. More strictly speaking, this means that if a known CSI-RS signal _xn is transmitted, the UE can estimate the coupling between the transmitted signal and the received signal, that is, the effective channel. Therefore, if no virtualization is performed during transmission, then:

_yn ＝ _Hnxn + _{e n}

The UE can measure the effective channel as _Heff = _Hn . Similarly, if the precoder is used to virtualize the CSI-RS to

Then the UE can estimate the effective channel

Related to CSI-RS is the concept of zero-power CSI-RS resources (also known as silent CSI-RS), which are configured like regular CSI-RS resources so that the UE knows that data transmission is mapped around those resources. The intention of zero-power CSI-RS resources is to enable the network to silence transmissions on the corresponding resources, thereby increasing the SINR of the corresponding non-zero-power CSI-RS that may be transmitted in neighboring cells/transmission points. For LTE Rel-11, special zero-power CSI-RS that will be forced on the UE to measure interference plus noise is under discussion. As the name indicates, the UE can assume that the TP of interest is not transmitting on the silent CSI-RS resources and can therefore use the received power as a measure of the interference plus noise level.

Based on the designated CSI-RS resources and interference measurement configuration, e.g., muted CSI-RS resources, the UE can estimate the effective channel and noise plus interference, and therefore also determine which rank, precoder, and transport format to recommend that best matches the specific channel.

Power measurement offset

As mentioned above, in LTE, the terminal provides the network with channel state information, such as a combination of PMI, RI, and CQI, by recommending a specific transmission for the effective channel being measured. To enable this recommendation, the UE needs to know the relative power offset between the reference signal (used to measure the effective channel) and the assumed upcoming data transmission. In the following, we refer to this power offset as the power measurement offset (PMO). This power offset is constrained to a specific reference signal, for example, it involves the parameter Pc as part of the configuration message used to set up the CSI-RS measurement or the parameter nomPDSCH-RS-EPRE-Offset for CRS.

In practice, CQI is rarely perfect and may contain significant errors, which means that the estimated channel quality does not correspond to the actual channel quality experienced for the link over which the transmission occurs. The eNodeB can to some extent reduce the adverse effects of erroneous CQI reports by means of outer loop adjustment of the CQI values. By monitoring the ACK/NACK signaling of the hybrid ARQ, the eNodeB can detect whether the block error rate (BLER) or a related metric is below or above a target value. Using this information, the eNodeB can decide to use a more aggressive (or defensive) MCS than the one recommended by the UE. However, outer loop control is a crude tool for improving link adaptation and the convergence of the loop may be slow.

Furthermore, it is more difficult for the eNodeB to deviate from the recommended rank because the CQI report is directly related to the rank. Variations in rank therefore make the information provided by the CQI report difficult or impossible to utilize—that is, if the eNodeB were to ignore the rank recommended by the UE, the eNodeB would otherwise face severe difficulties in deciding which MCS to use for different data streams.

The network can improve rank reporting by adjusting the PMO in the UE. For example, if the power measurement offset is reduced (forcing the terminal to assume that the power used for the transmitted data channel is lower), the terminal will tend to recommend a lower rank because the "best" rank increases with SINR.

Coordinated Multi-Point (CoMP)

CoMP transmission and reception refers to a system in which transmission and/or reception at multiple geographically separated antenna sites are coordinated to improve system performance. More specifically, CoMP refers to the coordination of antenna arrays with different geographic coverage areas. In the following discussion, we refer to a group of antennas that cover essentially the same geographic area in the same manner as a point, or specifically a transmission point (TP). Thus, a point might correspond to one of multiple sectors at a site, but it can also correspond to a site with one or more antennas that all aim to cover the same geographic area. Different points often represent different sites. Antennas correspond to different points when they are sufficiently geographically separated and/or have antenna patterns pointing in sufficiently different directions. While this disclosure primarily focuses on downlink CoMP transmission, it should be appreciated that, in general, transmission points can also serve as reception points. Coordination between points can be distributed, using direct communication between different sites or using a central coordination node. Another coordination possibility is a "floating cluster," in which each transmission point is connected to and coordinates with a set of neighboring points (or two neighboring points). A group of points performing coordinated transmission and/or transmission is hereinafter referred to as a CoMP coordination cluster, a coordination cluster, or simply a cluster.

FIG5 illustrates an exemplary wireless network having a CoMP coordinated cluster including three transmission points indicated as TP1, TP2, and TP3.

CoMP is a tool introduced in LTE to improve coverage, cell-edge throughput, and/or increase system throughput at high data rates. In particular, the goal is to achieve a more even distribution of user-perceived performance across the network by controlling interference in the system, by reducing interference, and/or by better predicting interference.

CoMP operation targets many different deployments, including coordination between sites and sectors in cellular macro deployments and different configurations of heterogeneous deployments, where, for example, a macro node coordinates the transmissions of pico nodes within a macro coverage area.

There are many different CoMP transmission schemes that are considered; for example,

Dynamic point blanking, where multiple transmission points coordinate transmissions so that neighboring transmission points can silence transmissions on time-frequency resources (TFREs) allocated to a UE experiencing significant interference.

Coordinated beamforming, where TPs coordinate transmission in the spatial domain by beamforming the transmit power in a way that suppresses interference to UEs served by neighboring TPs.

Dynamic point selection, where data transmission to the UE can be switched dynamically (in terms of time and frequency) between different transmission points so that the transmission points are fully utilized.

Joint transmission, where signals to a UE are transmitted simultaneously from multiple TPs on the same time/frequency resources. The purpose of joint transmission is to increase received signal power and/or reduce received interference if the cooperating TPs would otherwise serve some other UE without considering our JT UE.

CoMP Feedback

A common feature of CoMP transmission schemes is that the network requires CSI information for not only the serving TP but also for the channels linking neighboring TPs to the terminal. By configuring a unique CSI-RS resource for each TP, for example, the UE can distinguish the effective channel for each TP by measuring on the corresponding CSI-RS. Note that the UE is likely unaware of the physical existence of a specific TP; it is simply configured to measure on a specific CSI-RS resource and is unaware of any association between the CSI-RS resource and the TP.

A detailed example is provided in Figure 4, which shows which resource elements within a resource block pair could potentially be occupied by UE-specific RS and CSI-RS. In this example, the CSI-RS utilizes an orthogonal cover code of length two to cover two antenna ports over two consecutive REs. As can be seen, many different CSI-RS patterns can be used. For example, for the case of 2 CSI-RS antenna ports, there are 20 different patterns within a subframe. For 4 and 8 CSI-RS antenna ports, the corresponding number of patterns is 10 and 5, respectively.

CSI-RS can be described as a pattern of resource elements over which a specific CSI-RS configuration is transmitted. One way to determine the CSI-RS resources is a combination of parameters "resourceConfig", "subframeConfig" and "antennaPortsCount" that can be configured by RRC signaling.

There can be multiple different types of CoMP feedback. Most alternatives are based on per-CSI-RS resource feedback, possibly utilizing CQI aggregation across multiple CSI-RS resources and possibly also utilizing some common phase information between CSI-RS resources. The following is a non-exclusive list of relevant alternatives (note that any combination of these alternatives is also possible):

Per-CSI-RS resource feedback corresponds to a separate report of channel state information (CSI) for each of a set of CSI-RS resources. Such CSI reports may, for example, include one or more of a precoder matrix indicator (PMI), a rank indicator (RI), and/or a channel quality indicator (CQI), which represents the recommended configuration for a hypothetical downlink transmission on the same antenna used for the associated CSI-RS or RS used for channel measurement. More generally, the recommended transmission should be mapped to the physical antenna in the same manner as the reference symbols used for CSI channel measurement.

Typically, there is a one-to-one mapping between CSI-RS and TP, in which case per-CSI-RS resource feedback corresponds to per-TP feedback; that is, a separate PMI/RI/CQI is reported for each TP. Note that there can be interdependencies between CSI reports; for example, they can be constrained to have the same RI. Interdependencies between CSI reports have many advantages, such as a reduced search space when the UE calculates the feedback, reduced feedback overhead, and a reduced need to perform rank disregard at the eNodeB in the case of RI reuse.

The CSI-RS resources considered are configured by the eNodeB as CoMP measurement groups. In the example shown in FIG5 , different measurement groups may be configured for wireless devices 540 and 550. For example, the measurement group for wireless device 540 may consist of CSI-RS resources transmitted by TP1 and TP2, as these points may be suitable for transmission to device 540. The measurement group for wireless device 550 is instead configured to consist of CSI-RS resources transmitted by TP2 and TP3. The wireless devices will report CSI information for the transmission points corresponding to their respective measurement groups, enabling the network to select the most appropriate transmission point for each device, for example.

Aggregate feedback corresponds to CSI reporting for channels corresponding to the aggregation of multiple CSI-RSs. For example, a joint PMI/RI/CQI may be recommended for joint transmission on all antennas associated with multiple CSI-RSs.

However, joint search can be computationally expensive for the UE, and a simplified form of aggregation is to evaluate an aggregate CQI combined with a per-CSI-RS resource PMI, which typically all have the same rank corresponding to the aggregated CQI or CQIs. This approach also has the advantage that the aggregate feedback can share more information with the per-CSI-RS resource feedback. This is beneficial because many CoMP transmission schemes require per-CSI-RS resource feedback, and to enable eNodeB flexibility in dynamically selecting CoMP schemes, aggregate feedback is typically transmitted in parallel with per-CSI-RS resource feedback. To support coherent joint transmission, such per-CSI-RS resource PMIs can be supplemented with common phase information to enable the eNodeB to rotate the per-CSI-RS resource PMIs so that the signals combine coherently at the receiver.

Interference Measurement for CoMP

For efficient CoMP operation, capturing appropriate interference assumptions when determining CSI is as important as capturing appropriate received desired signals.

For the purposes of this disclosure, a CSI process is defined as a reporting process for CSI (e.g., CQI and potentially associated PMI/RI) for a specific effective channel and interference measurement resource. Optionally, a CSI process may also be associated with one or more interference simulation configurations, as explained below. An effective channel is defined by a reference signal resource comprising one or more associated reference sequences. The interference measurement resource is a set of resource elements where one or more signals are received that are assumed to interfere with a desired signal. An IMR may correspond to a specific CQI reference resource, such as a CRS resource. Alternatively, an IMR may be a resource specifically configured for measuring interference.

In an uncoordinated system, a UE can effectively measure the interference observed from all other TPs (or all other cells), which will be the relevant interference level in the upcoming data transmission. Such interference measurements are typically performed by analyzing the residual interference on the CRS resource after the UE subtracts the effect of the CRS signal. In a coordinated system implementing CoMP, such interference measurements become increasingly less relevant. Most specifically, within a coordinated cluster, the eNodeB has a significant degree of control over which TPs interfere with the UE in any particular TFRE. Consequently, there will be multiple interference hypotheses under which the TP transmits data to other terminals.

In addition to improving interference measurement, new functionality was introduced in LTE Release 11, where the network was able to configure which specific TFREs would be used for interference measurements for a specific UE; this was defined as the Interference Measurement Resource (IMR). The network could thus control the interference experienced on the IMRs by, for example, silencing all TPs within the coordinated cluster on the associated TFREs. In this case, the terminal would effectively measure inter-CoMP cluster interference. In the example shown in Figure 5, this would correspond to silencing TP1, TP2, and TP3 in the TFREs associated with the IMRs.

Consider, for example, a dynamic point blanking scheme, where there are at least two related interference hypotheses for a particular UE; in one interference hypothesis, the UE does not experience interference from the coordinated transmission point; while in the other hypothesis, the UE experiences interference from a neighboring point. In order to enable the network to effectively determine whether the TP should be silenced, the network may configure the UE to report two or more CSIs corresponding to different interference hypotheses in general—that is, there may be two CSI processes corresponding to different interference situations. Continuing with the example of FIG. 5 , assume that the wireless device 550 is configured to measure CSI from TP3. However, TP2 may potentially interfere with the transmission from TP2, depending on how the network schedules the transmission. Therefore, the network may configure the device 550 with two CSI processes for TP3 (or more specifically, for measuring the CSI-RS transmitted by TP3). One CSI process is associated with the interference hypothesis that TP2 is silent, while the other CSI process corresponds to the hypothesis that TP3 is transmitting an interfering signal.

To facilitate this approach, it has been proposed to configure multiple IMRs, where the network is responsible for implementing each relevant interference hypothesis in the corresponding IMR. Therefore, by associating a specific IMR with a specific CSI process, relevant CSI information, such as CQI, can be used by the network for efficient scheduling. In the example of Figure 5 , the network can, for example, configure one IMR in which only TP2 transmits and another IMR in which both TP2 and TP3 are silent. Each CSI process can then be associated with a different IMR.

While the possibility of associating a CSI process with one or more IMRs enables the network to obtain a better basis for making link adaptation and scheduling decisions, there is still room for further improvement in determining channel state information. In particular, there is a need for improved mechanisms for estimating the interference for a specific CSI process.

Summary of the Invention

It is an object of some embodiments to provide an improved mechanism for CSI reporting.It is another object of some embodiments to enable improved link adaptation.

It is another object of some embodiments to improve the estimation of interference for CSI processes, especially in CoMP scenarios.

Some embodiments provide a method in a wireless device for reporting CSI for a channel state information (CSI) process. The CSI process corresponds to a reference signal resource and an interference measurement resource. The wireless device obtains an adjustment value associated with the CSI process. The wireless device then estimates an effective channel based on one or more reference signals received in the reference signal resource and applies the adjustment value to the estimated effective channel, thereby obtaining an adjusted effective channel. The wireless device then determines channel state information based on the adjusted effective channel and based on interference estimated based on the interference measurement resource. Finally, the wireless device transmits the channel state information to a network node.

Some embodiments provide a method in a network node for receiving CSI for a channel state information (CSI) process from a wireless device. The network node is associated with a cluster for coordinated multi-point transmission. The network node transmits an indication of an adjustment value associated with the CSI process to the wireless device. The wireless device then receives channel state information associated with the CSI process from the wireless device.

Some embodiments provide a wireless device for reporting CSI for a channel state information (CSI) process. The wireless device includes processing circuitry and radio circuitry. The processing circuitry is configured to obtain an adjustment value associated with the CSI process, estimate an effective channel based on one or more reference signals received in reference signal resources via the radio circuitry, apply the adjustment value to the estimated effective channel, obtain an adjusted effective channel, determine channel state information based on the adjusted effective channel and based on interference estimated based on an interference hypothesis, and transmit the channel state information to a network node via the radio circuitry.

Some embodiments provide a network node for receiving CSI for a channel state information (CSI) process from a wireless device. The network node includes processing circuitry and is connectable to radio circuitry. The processing circuitry is configured to transmit, via the radio circuitry, to the wireless device an indication of an adjustment value associated with the CSI process. The processing circuitry is further configured to receive, via the radio circuitry, channel state information related to the CSI process from the wireless device.

Some embodiments provide improved power measurement offset configuration, resulting in improved link adaptation. This in turn translates into reduced retransmissions in hybrid ARQ and increased performance in terms of spectral efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a schematic diagram illustrating an LTE time-frequency resource grid.

FIG2 is a schematic block diagram illustrating a transmission structure of a precoded spatial multiplexing mode in LTE.

FIG3 is a schematic diagram illustrating a cell-specific reference signal.

FIG4 is a diagram illustrating an exemplary layout of reference signals.

FIG5 is a schematic diagram illustrating a CoMP coordinated cluster in a wireless network.

FIG6 is a schematic diagram illustrating a CoMP coordinated cluster in a wireless network.

FIG7 is a schematic diagram illustrating a CoMP coordinated cluster in a wireless network.

8-11 are flow charts illustrating methods according to some embodiments.

Figure 12a is a block diagram illustrating a network node according to some embodiments.

Figure 12b is a block diagram showing details of a network node according to some embodiments.

Figure 13a is a block diagram illustrating a wireless device according to some embodiments.

Figure 13b is a block diagram showing details of a wireless device according to some embodiments.

DETAILED DESCRIPTION

A particular problem with implementing interference measurements for CoMP is that, even within a single CoMP coordinated cluster, CoMP measurements for different TPs within the cluster will be configured with different UEs; that is, each UE may be configured with a separate CoMP measurement group that does not span all nodes in the coordinated cluster. Consequently, each such UE will experience a different set of TPs, such as residual interference or uncoordinated interference.

Especially for larger CoMP clusters, it may become prohibitive to configure a different IMR for each such residual interference combination. Thus, for some configurations of CoMP measurement groups, the UE will measure residual interference that lacks contributions from one or more interfering TPs and/or actually includes one or more TPs that should not interfere.

This mismatch between the interference measured for CSI reporting and the actual interference experienced in downlink transmissions will degrade the network's link adaptation and reduce the network's overall performance and spectral efficiency. A particularly challenging problem is when incorrectly measured interference levels cause the UE to report a mismatched transmission rank, which is difficult for the eNodeB to ignore due to the tight coupling with one or more CQI and PMI.

Furthermore, the interference levels experienced for different CSI reports may be significantly different, which may make it challenging to make the power measurement offset have the desired effect for all different operating points.

Some embodiments address these issues by providing a CSI process-specific adjustment value, which may be a power measurement offset or scaling factor, and applying it to the effective channel estimated based on the reference signal configuration of the CSI process. Channel state information is then determined based on the adjusted effective channel. The adjustment value is determined so that it fully or partially compensates for the incorrectly measured or estimated interference level. Certain embodiments enable different power measurement offset characteristics for different CSI reports. Therefore, the power measurement offset should have a specific component for each CSI process, in contrast to the prior art where the power measurement offset is always constrained to a specific reference signal.

By configuring the power measurement offset separately for different CSI processes, the impact of incorrect interference measurements, which typically affect different CSI processes differently, can be compensated in the UE, thereby improving the recommended transmission rank and corresponding CQI. In addition, different operating points for power measurement caused by different interference levels can be accommodated, thereby achieving desired characteristics regarding, for example, the rank reported for each CSI process.

FIG5 illustrates an exemplary wireless communication system 500 in which various embodiments of the present invention may be implemented. Three transmission points 510, 520, and 530 form a CoMP coordinated cluster. Hereinafter, for purposes of illustration and not limitation, it will be assumed that communication system 500 is an LTE system. Transmission points 510, 520, and 530 are remote radio units (RRUs) controlled by an eNodeB 560. In an alternative scenario (not shown), the transmission points may be controlled by their respective eNodeBs. It should be appreciated that, in general, each network node, such as an eNodeB, may control one or more transmission points, which may be physically co-located with the network node or geographically distributed. In the scenario shown in FIG5 , transmission points 510, 520, and 530 are assumed to be connected to eNodeB 560, for example, using fiber optic cables or point-to-point microwave connections. In the event that some or all of the transmission points forming the cluster are controlled by different eNodeBs, those eNodeBs will be assumed to be connected to one another, for example, via a transport network, to enable exchange of information for possible coordination of transmission and reception.

It should be appreciated that although the examples herein refer to an eNodeB for illustrative purposes, the present invention is applicable to any network node. The term "network node" as used in this disclosure is intended to encompass any radio base station, such as an eNodeB, NodeB, Home eNodeB or Home NodeB, or any other type of network node that controls all or part of a CoMP cluster.

Communication system 500 also includes two wireless devices 540 and 550. Within the context of this disclosure, the term "wireless device" encompasses any type of wireless node that is capable of communicating with a network node, such as a base station, or with another wireless device by transmitting and/or receiving wireless signals. Thus, the term "wireless device" encompasses, but is not limited to: user equipment, mobile terminals, fixed or mobile wireless devices for machine-to-machine communication, integrated or embedded wireless cards, external plugs in wireless cards, dongles, etc. A wireless device may also be a network node, such as a base station. Throughout this disclosure, whenever the term "user equipment" is used, it should not be construed as limiting, but rather as encompassing any wireless device as defined above.

As mentioned before, the model for the received data vector on the TFRE carrying the data symbols can be written as

Here, the subscripts are omitted for simplicity of notation. To perform feedback calculations, the UE needs to adopt a similar model for receiving the hypothetical transmission.

In one embodiment, the UE estimates a channel matrix based on a reference signal such as a Rel-8 cell-specific RS or a Rel-10 CSI RS, thereby generating a measurement channel matrix H _m . This channel is scaled with a CSI process-specific PMO factor α _CQI to generate a model for the data channel matrix H , which in turn is used to form a measurement model for feedback determination as

Note that α _CQI is not necessarily independently configurable for each CQI process. For example, some CQI processes can be grouped to use the same PMO configuration. In addition, the CSI process-specific PMO can be configured with the help of radio resource control or as part of the CSI report allocation in aperiodic CSI reporting. Alternatively, the PMO is specified as a predetermined value in the standard.

The PMO factor can take many equivalent forms, including being specified in units of dB or a linear scale, being reparameterized as a power offset rather than a scaling factor, etc.

The measurement model with CQI process specific scaling/PMO of the channel matrix part is used by the UE to determine the CSI to report: eg for selecting which ranks, PMI and CQI to report.

More specifically, some embodiments provide a method for reporting CSI for a CSI process in a wireless device, as will now be described with reference to the flowcharts of Figures 5 and 8. As described above, the CSI process corresponds to reference signal resources and interference measurement resources. The reference signal resources include a set of resource elements in which one or more reference signals corresponding to a desired signal are received. In this context, "desired signal" means a signal intended for reception by the wireless device. The interference measurement resources include a set of resource elements in which one or more signals assumed to interfere with the desired signal are received.

In certain embodiments, the reference signal resource is a CSI-RS resource. However, the reference signal resource may be any other type of RS resource that can be used to estimate the desired signal, such as a CRS resource.

The wireless device obtains 810 an adjustment value associated with the CSI process. The adjustment value may be obtained from a network node, such as a serving eNodeB. Alternatively, an indication of the adjustment value is obtained from the network node, such as in the form of an index into a lookup table, and the corresponding adjustment value is retrieved from a storage device, such as a memory of the wireless device.

In step 820, the wireless device estimates an effective channel based on one or more reference signals received in the reference signal resources, such as one or more CSI-RS. The wireless device then applies 830 the adjustment value to the estimated effective channel. As a result, the wireless device obtains an adjusted effective channel.

Applying the adjustment value can be accomplished in a variety of different ways, depending on the form of the adjustment value. In some variations, the adjustment value is an additive power measurement offset, and the wireless device applies the adjustment value by adding it to the channel estimate. In other variations, the adjustment value is a scaling factor, and the wireless device multiplies the channel estimate by the adjustment value. Furthermore, the adjustment value can be specified in dB or on a linear scale.

The wireless device then determines 840 channel state information based on the adjusted effective channel and based on interference estimated based on interference measurement resources. In some variants, the IMR may be a resource specifically configured to measure interference. For example, the IMR may consist of resource elements in which all transmission points within a CoMP cluster are silent, thereby enabling the wireless device to measure inter-cluster interference and noise. In other variants, the IMR may be a reference signal resource, such as a CRS resource. The wireless device may estimate interference in the CRS resource by analyzing a residual signal after subtracting the decoded CRS signal. Methods for determining CSI based on channel estimation and measured interference are known in the art and will not be described in detail here.

Finally, the wireless device transmits 850 the channel state information to the network node.

The effect of applying the adjustment value is to compensate for errors or mismatches in the measured interference. As described above, such errors may be caused, for example, by measuring an IMR that does not match the interference assumption that the network intends to apply to this CSI process. By associating the adjustment value with the CSI process, different adjustment values can be applied to each CSI process, even to CSI processes corresponding to the same reference signal resources.

In another embodiment, there is a component of power adjustment offset that is specific to the CQI process. For example, there may be a power measurement offset P _CQI (typically defined in dB scale) associated with a particular CQI process. This offset may then be applied in addition to other power measurement offsets associated with, for example:

Specific reference signals (such as Pc for CSI-RS),

Specific recommended launch rank,

The combined power measurement offset is thus obtained as

α _CQI =P _CQI +P _{CQI_agnostic} [dB]

Here, _{PCQI_agnostic} is a combined power measurement offset that is agnostic to a specific CQI process.

One such example corresponds to when measuring the desired signal effective channel on a specific CSI-RS that has an associated power measurement offset Pc that is agnostic to the specific CQI process. Two different CQI processes sharing the same desired effective channel will then result in two different power measurement offsets.

The flowchart in FIG9 illustrates a method for reporting CSI for a CSI process in a wireless device according to some embodiments. In these embodiments, a combination of CSI process-specific and CQI-agnostic power offsets is applied, similar to that described above. Note that a "CSI process" is defined in the same manner as described above in conjunction with FIG8 .

In a specific variant, the reference signal resource is a CSI-RS resource. However, as mentioned above, the reference signal resource may be any other type of RS resource that may be used to estimate the desired signal, such as a CRS resource.

The wireless device obtains 910 a power measurement offset associated with a CSI process. The power measurement offset may be obtained from a network node, such as a serving eNodeB. Alternatively, an indication of the power measurement offset is obtained from the network node, such as in the form of an index into a lookup table, and the corresponding power measurement offset is retrieved from a storage device, such as a memory of the wireless device.

In step 920, the wireless device estimates an effective channel based on one or more reference signals received in the reference signal resources, such as one or more CSI-RSs. The wireless device then applies 930 an adjustment value to the estimated effective channel. As a result, the wireless device obtains an adjusted effective channel.

In this embodiment, the wireless device also applies an additional non-CSI process-specific power offset to the estimated effective channel. This offset may also be referred to as a "CSI-agnostic offset." As a specific example, the reference signal resource is a CSI-RS, and the additional power offset is an offset _PC associated with the CSI-RS. As explained above, the offset _PC may have been previously signaled, for example, in downlink control information (DCI).

Another possibility is to apply multiple non-CSI process specific offsets in addition to the CSI process specific offset (e.g. _PC for CSI RS) and one or more offsets associated with a specific recommended transmission rank.

The CSI process specific offset and the additional offset (or offsets) may be added together to form a combined offset, which is then applied to the estimated effective channel.

Applying the adjustment value can be accomplished in a variety of different ways, depending on the form of the adjustment value. In some variations, the adjustment value is an additive power measurement offset, and the wireless device applies the adjustment value by adding it to the channel estimate. In other variations, the adjustment value is a scaling factor, and the wireless device multiplies the channel estimate by the adjustment value. Furthermore, the adjustment value can be specified in dB or on a linear scale.

The wireless device then determines 940 channel state information based on the adjusted effective channel in the same manner as step 840 above.

Finally, the wireless device transmits 950 the channel state information to the network node.

Another method for estimating interference that can be used in conjunction with interference measurement resource-based measurements is to have the terminal simulate interference from within the coordination point based on an interference hypothesis, for example by assuming isotropic transmission from each transmission point assumed to be interfering for the interference hypothesis. This has the advantage that it may be sufficient for the terminal to perform interference measurements on a single IMR, where there is no interference from the coordination transmission point from which each interference hypothesis was derived. For example, if this residual interference and noise is measured by the terminal and characterized as a complex-valued Gaussian random process

e _n ∈CN(0,Q _e ),

where _Qe is the correlation matrix and the elements of _en correspond to the interference realizations at each receive antenna. The terminal can then modify the residual interference to correspond to a specific CoMP interference hypothesis by simulating the intra-CoMP cluster interference from the transmission point for which it has measured the effective channel _Heff as

where _qn is an isotropic random signal of a certain nominal power. However, note that for a terminal to be able to simulate interference within a CoMP cluster, it needs to obtain reliable channel estimates for each point where it should add interference. In practice this means

- The terminal needs to know the existence of the node, or more specifically, the existence of the associated reference signal on which the terminal will measure the channel

- The SINR of the reference signal needs to be high enough to perform a sufficiently accurate estimation of the effective channel

- The UE's processing must be sized to be able to track each of these valid channel estimates.

In practice, this means that the UE may only be able to simulate interference from within the configured CoMP measurement group, which is limited in size. Typically, the size of the measurement group is up to two or possibly three TPs (i.e., CSI-RS resources). Therefore, for CoMP coordinating clusters with more than two nodes, which is a typical case (e.g., three intra-site sector macro coordination), the CoMP measurement group may not represent all nodes, and therefore means other than UE simulation of interference must be used to capture interference from outside the CoMP measurement group but within the CoMP coordinating cluster.

In another embodiment, the CQI process involves recommending CSI for a hypothetical channel, where the UE simulates interference from interference sources, as outlined above.

where β _CQI is the power measurement offset of the effective channel for the simulated interferer.

This embodiment has the advantage that the impact of the simulated interference on a specific CSI process may be individually configurable.

In one embodiment, the power measurement offset of the interfering effective channel is not specific (shared) to the CSI process; that is,

β _CQI = β

In another embodiment, β _CQI is determined at least in part by a CSI process specific power measurement offset configuration. An example corresponds to

β _CQI =P _β,CQI +P _{β,CQI_agnostic} [dB]

Where P _β,CQI is the power measurement offset specific to a particular CSI process, and P _{β,CQI_agnostic} is other related power measurement offsets that are agnostic to the CSI process (e.g., Pc of the CSI-RS associated with the interferer).

In another embodiment, P _β,CQI = _PCQI . This embodiment has the advantage that it reduces complexity and configuration overhead, but still allows configuration of the effect of the residual interference e on a particular CSI process. Note that the effective SINR of (3) can be expressed as

Where S and I _emulated are the desired signal power and emulated interference power, respectively, excluding the associated power offset, and I _e is the measured interference and noise power (corresponding to e). Note that the power offset is expressed in units of a linear scale in the equation (rather than in dB as above). As can be seen, the CSI process-specific configuration _PCQI translates into a configuration of how much the measured residual interference should affect the CSI report for the CSI process.

FIG10 illustrates a method for reporting CSI for a CSI process in a wireless device according to some embodiments, in a scenario where the wireless device simulates interference. The CSI process corresponds to reference signal resources and interference measurement resources, where the reference signal resources and IMRs are defined as described above in conjunction with FIG8 . The CSI process also corresponds to one or more interference simulation configurations. Each interference simulation configuration is associated with a reference signal received from a hypothetical interference source.

In a specific variant, the reference signal resource is a CSI-RS resource. However, as mentioned above, the reference signal resource may be any other type of RS resource that may be used to estimate the desired signal, such as a CRS resource.

The wireless device obtains 1010 an adjustment value associated with the CSI process. The adjustment value may be obtained in any manner described in conjunction with FIG. 8 .

In step 1020, the wireless device estimates the effective channel and applies 1030 an adjustment value to the estimated effective channel. These steps correspond to steps 820 and 830 described above. Applying the adjustment value can be accomplished in a variety of different ways, as described above in conjunction with FIG.

The wireless device then simulates interference according to one or more simulation configurations in steps 1040-1050. In step 1040, the wireless device estimates an effective channel for each interference simulation configuration based on an associated reference signal. The wireless device then simulates 1050 interference for each interference simulation configuration based on the estimated effective channel for that configuration. As explained above, one way to simulate interference is to multiply the channel estimate by an isotropic random signal.

In a variation of this embodiment, the wireless device applies an adjustment value to the simulated interference, for example, by multiplying the simulated interference for each simulated configuration by a scaling factor. This adjustment value can be the same value applied to the channel estimate, i.e., the CSI process-specific adjustment value obtained in step 1010, or it can be a second adjustment value. The second adjustment value can be obtained, for example, via signaling from a network node, such as RRC signaling, or can be retrieved from a memory of the wireless device, for example, based on an index received from the network node.

The second adjustment value can be common to all CSI processes, i.e., non-CSI process-specific or CS-agnostic. Alternatively, the second adjustment value can be common to a group of CSI processes, or it can be specific to this particular CSI process. In the latter case, two CSI process-specific adjustment values are thus obtained in step 1010: one applied to the channel estimate corresponding to the desired signal, and one applied to one or more simulated interfering signals.

In other variations, the second adjustment value includes CSI-process-specific and non-CSI-process-specific components. For example, the second adjustment value may be a combination of a CSI-RS specific offset _PC and a CSI-process-specific value.

The wireless device then determines 1060 channel state information based on the adjusted effective channel, based on the estimated interference estimated based on the interference measurement resources, and based on the simulated interference. In certain variations, the wireless device adds the interference measured based on the IMR for each configuration and the simulated interference to form a combined interference estimate.

Finally, the wireless device transmits 1070 the channel state information to the network node.

In another embodiment, the CQI process involves recommending aggregate CSI for joint transmission on multiple hypothetical channels corresponding to different CSI-RS resources.

where index i corresponds to a different CSI-RS resource associated with the joint transmission, and where α _CQI,i is a CQI process-specific set of power measurement offsets for the channel H _m,i of the resource in question.

An advantage of this embodiment is that it allows the eNodeB to configure the UE to compensate for potential signal strength losses due to rapidly changing phase variations between transmission points when performing joint transmissions, resulting in non-coherent combining upon transmission.

In another embodiment, the power measurement offsets for different channels are all equal within a CQI process α _CQI,i =α _CQI or share a common component _PCQI (ie, are individually configurable).

α _CQI,i =P _CQI +P _c,i [dB],

where P _c,i is the effective channel-specific offset (eg, constrained to a specific reference signal).

Referring again to FIG. 8 , a method for reporting CSI for a CSI process in a wireless device according to some embodiments in a joint transmission scenario will now be described. The CSI process corresponds to at least two reference signal resources and one interference measurement resource. The CSI process optionally also corresponds to one or more interference simulation configurations as described above. In certain variations, the reference signal resources are CSI-RS resources. However, as described above, the reference signal resources may be any other type of RS resource that can be used to estimate the desired signal, such as a CRS resource.

The wireless device obtains 810 an adjustment value associated with each of the reference signal resources used for the CSI process. The adjustment value may be obtained in any manner described in conjunction with FIG.

In step 820, the wireless device estimates an effective channel for each reference signal resource used for the CSI process and applies an adjustment value associated with the reference signal resource to the estimated effective channel, thereby obtaining an adjusted effective channel. As described above, applying the adjustment value can be accomplished in various ways.

The wireless device then determines 840 channel state information based on the adjusted effective channel and based on the estimated interference estimated based on the measured resources. Optionally, the wireless device also bases CSI on the simulated interference, as described above.

Finally, the wireless device transmits 850 the channel state information to the network node.

FIG11 illustrates a method in a network node for receiving CSI information for a CSI process from a wireless device, according to some embodiments. This method corresponds to the wireless device method illustrated in FIG8-10. The network node is included in or controls a cluster for coordinated multipoint transmission, such as clusters TP1-TP3 illustrated in FIG5. More generally, the network node is associated with the cluster. As a specific example, the network node may be an eNodeB 560, which is a remote radio head, controlling TP1-TP3. In an alternative scenario, such as illustrated in FIG6, the network node may be an eNodeB having three sector antennas corresponding to transmission points TP1-TP3. In another scenario, as illustrated in FIG7, TP1-TP3 may form a CoMP cluster, and the network node may be an eNodeB controlling TP1 and TP3 or an eNodeB controlling TP2 and serving a pico cell 720.

As described above, a CSI process corresponds to reference signal resources and interference measurement resources, and optionally also corresponds to one or more interference simulation configurations.

According to the method, the network node determines 1120 an adjustment value associated with a CSI process based on an interference hypothesis associated with the CSI process. The interference hypothesis corresponds to a set of transmission points that are assumed to interfere with a signal intended to be received by the wireless device.

In some variants, the adjustment value is determined such that it compensates for interference that would be transmitted from a hypothetical interfering transmission point according to the interference hypothesis but would not be estimated by the wireless device. For example, the adjustment value may be determined to compensate for interference from one or more transmission points that are hypothesized to be interfering according to the interference hypothesis but are not included in the measurement set for the wireless device.

Some specific methods for determining the adjustment values will now be described. The CSI process-specific adjustment parameters can be determined, for example, by the eNodeB by monitoring hybrid ARQ feedback from the UE: if the portion of the received hybrid ARQ message associated with a transport block recommended for transmission according to a particular CSI process corresponds to a NACK (e.g., not successfully decoded by the UE) that exceeds (or is below) a target threshold, then the adjustment value for that CSI process can be configured more conservatively (or aggressively) to better meet the target threshold. Such a process is often collectively referred to as outer loop link adaptation (OLLA), where the above process corresponds to CSI process-specific OLLA and where the network configures the OLLA adjustment to be performed by the UE with the aid of the CSI process-specific adjustment parameters (as opposed to the case with eNodeB-side compensation, where the eNodeB adjusts the reported CQI when selecting the transport format for downlink transmission).

In an alternative/supplementary embodiment, the eNodeB also utilizes hybrid ARQ messages transmitted by other UEs configured with similar CSI processes, which may accelerate the convergence of CSI process-specific OLLA.

In another such embodiment, the eNodeB utilizes deployment-specific information that leads to predictable biases in CSI reporting, such as predictable underestimation of interference levels for a particular CSI process, caused by, for example, interfering transmission points being silent on associated interference measurement resources.

The network node also transmits 1110 configuration information for the CSI process to the wireless device.

In step 1130, the network node 1130 transmits an indication of the adjustment value to the wireless device. In one variation, the indication is transmitted as part of the CSI process configuration information. By indicating the adjustment value, the network node enables the wireless device to compensate for incorrect or incomplete interference measurements, as described above with reference to Figures 8-10.

The network node then receives 1140 channel state information associated with the CSI process from the wireless device.

Optionally, the network node performs 1150 link adaptation based on the received channel state information.

12-13 illustrate devices configured to perform the methods described in FIGs. 8-11.

FIG12a illustrates a network node 1200 for receiving CSI for a channel state information (CSI) process from a wireless device 1300. Network node 1200 includes processing circuitry 1220 and may be connected to radio circuitry 1210. In some variations, radio circuitry 1210 is included within network node 1200, while in other variations, radio circuitry 1210 is external thereto. For example, in the exemplary scenario of FIG5 , network node 560 corresponds to network node 1200. The radio circuitry in this example resides in distributed transmission points TP1-TP3, which are not physically co-located with network node 560. However, in the example shown in FIG6 , the transmission points correspond to sector antennas at a network node, such as an eNodeB, and in this case, the radio circuitry may be included within the network node.

The processing circuit 1220 is configured to transmit an indication of an adjustment value associated with the CSI process to the wireless device 1300 via the radio circuit 1210 , and receive channel state information related to the CSI process from the wireless device 1300 via the radio circuit 1210 .

FIG. 12 a shows details of a possible implementation of the processing circuit 1220 .

FIG13a illustrates a wireless device 1300 for reporting CSI for a channel state information (CSI) process. The wireless device includes a radio circuit 1310 and a processing circuit 1320. The processing circuit 1320 is configured to obtain an adjustment value associated with the CSI process and estimate an effective channel based on one or more reference signals received in a reference signal resource via the radio circuit 1310. The processing circuit 1320 is further configured to apply the adjustment value to the estimated effective channel to obtain an adjusted effective channel, determine channel state information based on the adjusted effective channel and interference estimated based on an interference hypothesis, and transmit the channel state information to the network node 1200 via the radio circuit 1310.

FIG. 13 b shows details of a possible implementation of the processing circuit 1320 .

The processing circuits 1220 and 1320 may include one or more microprocessors 1630, digital signal processors, and the like, as well as other digital hardware and memory. The memory stores program code for executing one or more telecommunication and/or data communication protocols and for executing one or more of the techniques described herein, and the memory may include one or more types of memory such as read-only memory (ROM), random access memory, cache memory, flash memory devices, optical storage devices, etc. The memory also stores program data and user data received from the wireless device.

Not all steps described herein must be performed in a single microprocessor or even in a single module.

It should be noted that although terminology from 3GPP LTE has been used in this disclosure to illustrate the invention, this should not be seen as limiting the scope of the invention to only that system. Other wireless systems including WCDMA, WiMax, UMB and GSM may also benefit from utilizing the concepts encompassed within this disclosure.

When the word "include" or "comprising" is used, it should be interpreted in a non-limiting sense, ie, meaning "consisting of.

The present invention is not limited to the preferred embodiments described above. Various replacements, modifications, and equivalents may be used. Therefore, the above embodiments should not be construed as limiting the scope of the present invention as defined by the appended claims.

Claims (22)

1. A method in a wireless device for reporting CSI for a Channel State Information (CSI) procedure, the CSI procedure corresponding to a reference signal resource and an interference measurement resource, wherein the reference signal resource includes a set of resource units in which one or more reference signals corresponding to a desired signal are received, and wherein the interference measurement resource includes a set of resource units in which one or more signals assumed to interfere with the desired signal are received, the method characterized in that: - Obtain (810) the adjustment value associated with the CSI process; - Estimate (820) the effective channel based on one or more reference signals received in the reference signal resources; - Apply the adjustment value (830) to the estimated effective channel to obtain the adjusted effective channel; - Based on the adjusted effective channel and based on interference to determine (840) channel state information, the interference being estimated based on the interference measurement resources; and - Transmit the channel state information (850) to the network node. 2. The method of claim 1, wherein the CSI process corresponds to at least two reference signal resources, and wherein the adjustment value is associated with each of the reference signal resources. 3. The method according to claim 2, further comprising: - For each reference signal resource in the CSI process, based on one or more reference signals received in the reference signal resource, an effective channel is estimated (820), and an adjustment value associated with the reference signal resource is applied to the estimated effective channel to obtain an adjusted effective channel; and - Based on the adjusted effective channel and the estimated interference, determine (840) channel state information. 4. The method according to any one of claims 1-3, wherein the adjustment value is a power measurement offset. 5. The method of claim 4, wherein the method further comprises applying (930) an additional non-CSI process-specific power offset to the estimated effective channel. 6. The method of claim 5, wherein the reference signal is a channel state information reference signal (CSI-RS), and wherein an additional power offset is associated with the CSI-RS. 7. The method according to any one of claims 1-3, wherein the adjustment value is a scaling factor. 8. The method according to any one of claims 1-3, further comprising determining channel state information for at least one other CSI procedure based on the adjustment value. 9. The method according to any one of claims 1-3, wherein the adjustment value is obtained from a network node. 10. The method according to any one of claims 1-3, further comprising receiving an index from a network node, and obtaining the adjustment value by retrieving the adjustment value corresponding to the index from a predefined lookup table. 11. The method according to any one of claims 1-3, wherein the channel state information includes one or more of the following: channel quality indicator, precoding matrix indicator, rank indicator, and precoding matrix type. 12. The method according to any one of claims 1-3, wherein the wireless device is configured with two CSI processes corresponding to the same reference signal resource and associated with different adjustment values. 13. The method according to any one of claims 1-3, wherein the reference signal resource is a CSI-RS resource. 14. The method according to any one of claims 1-3, wherein the interference measurement resource is a cell-specific reference signal resource, and wherein the interference is estimated by subtracting the decoded cell-specific reference signal from the signal received from the cell-specific reference signal resource. 15. A method in a network node for receiving CSI for a Channel State Information (CSI) procedure from a wireless device, the CSI procedure corresponding to reference signal resources and interference measurement resources, wherein the reference signal resources include a set of resource units in which one or more reference signals corresponding to a signal intended to be received by the wireless device are transmitted, and wherein the interference measurement resources include a set of resource units in which one or more signals assumed to interfere with a desired signal are received, the network node being included in a cluster for coordinated multipoint transmission, the method characterized in that: - Transmit (1130) an indication of the adjustment value associated with the CSI process to the wireless device for adjusting the estimated effective channel at the wireless device; - Receive (1140) channel state information associated with the CSI process from the wireless device. 16. The method of claim 15, further comprising determining (1120) the adjustment value based on an interference hypothesis associated with the CSI process, the interference hypothesis corresponding to a set of transmission points assumed to be interfering with signals intended to be received by the wireless device. 17. The method of claim 16, wherein the adjustment value is determined such that it compensates for interference from a hypothetical interference transmission point based on the interference hypothesis, the interference being not estimated by the wireless device. 18. The method according to any one of claims 15-16, wherein the adjustment value compensates for interference from one or more transmission points that are assumed to be interfering according to the interference assumption but are not included in the measurement group for the wireless device. 19. The method according to any one of claims 15-16, further comprising: - Transmit (1110) configuration information for the CSI process to the wireless device, wherein the indication of the adjustment value is included in the configuration information. 20. The method according to any one of claims 15-16, further comprising performing (1150) link adaptation based on received channel state information. 21. A wireless device (1300) for reporting CSI for a Channel State Information (CSI) procedure, the CSI procedure corresponding to a reference signal resource and an interference measurement resource, wherein the reference signal resource includes a set of resource units in which one or more reference signals corresponding to a desired signal are received, and wherein the interference measurement resource includes a set of resource units in which one or more signals assumed to interfere with the desired signal are received, the wireless device comprising a radio circuit (1310) and a processing circuit (1320), characterized in that the processing circuit (1320) is configured to: - Obtain the adjustment value associated with the CSI process; - Estimate the effective channel based on one or more reference signals received in the reference signal resource via the radio circuit (1310); - Apply the adjustment value to the estimated effective channel to obtain the adjusted effective channel; - Based on the adjusted effective channel and channel state information determined based on interference, which is estimated based on interference assumptions; and - The channel state information is transmitted to the network node (1200) via the radio circuit (1310). 22. A network node (1200) for receiving CSI for a Channel State Information (CSI) process from a wireless device (1300), the CSI process corresponding to reference signal resources and interference measurement resources, wherein the reference signal resources include a set of resource units in which one or more reference signals corresponding to a signal intended to be received by the wireless device are transmitted, and wherein the interference measurement resources include a set of resource units in which one or more signals assumed to interfere with a desired signal are received, the network node (1200) comprising radio circuitry (1210) and processing circuitry (1220), characterized in that the processing circuitry (1220) is configured to: - Transmit an indication of the adjustment value associated with the CSI process to the wireless device (1300) via the radio circuit (1210) for use in adjusting the estimated effective channel at the wireless device; - Receive channel state information related to the CSI process from the wireless device (1300) via the radio circuit (1210).