HK1246021B - Csi reporting for a set of csi-rs resources - Google Patents

Description

CSI reporting for a set of CSI-RS resources

This application is a divisional application of the invention patent application with an international application date of October 31, 2012, international application number PCT/EP2012/071531, Chinese national application number 201280054528.6, and invention name “CSI reporting for a set of CSI-RS resources”.

Priority claim

This application claims priority to U.S. Provisional Patent Application No. 61/557,509, filed November 9, 2011, which is hereby incorporated by reference in its entirety.

Technical Field

The present invention generally relates to channel state information reporting and transmission rank adaptation based on such reporting, and more particularly to performing channel state information reporting for a set of Channel State Information Reference Symbol (CSI-RS) resources according to a common transmission rank determined based on a subset of these resources.

Background Art

LTE uses OFDM in the downlink and DFT-spread OFDM in the uplink. Figures 1-3 provide an overview of LTE downlink transmission. With specific reference to Figure 1 , the basic LTE physical resources can be viewed as a time-frequency grid, where each resource element corresponds to a subcarrier during an OFDM symbol interval (on a specific antenna port).

In the time domain, LTE downlink transmissions are organized into 10 ms radio frames as shown in Figure 1, each consisting of ten equally sized 1 ms subframes as shown in Figure 2. A subframe is divided into two slots, each of 0.5 ms duration.

As shown in Figure 2, resource allocation in LTE is described in terms of resource blocks, where a resource block corresponds to one time slot in the time domain and to 12 consecutive 15kHz subcarriers in the frequency domain. Two resource blocks that are consecutive in time represent a resource block pair and correspond to a time interval of scheduling operation.

Transmissions in LTE are dynamically scheduled in each subframe, where the base station transmits downlink assignments/uplink grants to certain UEs via the Physical Downlink Control Channel (PDCCH). The PDCCH is transmitted in the first (first few) OFDM symbols in each subframe and spans (more or less) the entire system bandwidth. A UE that has decoded the downlink assignment carried by the PDCCH knows which resource elements in the subframe contain data targeted for the UE. Similarly, after receiving an uplink grant, the UE knows on which time/frequency resources it should transmit. In the LTE downlink, data is carried by the Physical Downlink Shared Channel (PDSCH), and in the uplink the corresponding channel is called the Physical Uplink Shared Channel (PUSCH). For additional information about the physical layer in LTE, see, for example, 3GPP TS 36.213, “Physical layer procedures”.

Demodulating the transmitted data requires estimating the radio channel, which is done by using transmitted reference symbols (RS), symbols known to the receiver. In LTE, cell-specific reference symbols (CRS) are transmitted in all downlink subframes, and in addition to assisting downlink channel estimation, they are also used for mobility measurements performed by the UE. LTE also supports UE-specific RS that are targeted only at assisting channel estimation for demodulation purposes. Figure 3 illustrates how the mapping of physical control/data channels and signals can be completed on resource elements within a downlink subframe. In this example, the PDCCH occupies the first OFDM symbol among three possible OFDM symbols, so in this specific case, data mapping can already start in the second OFDM symbol. Since CRS is common to all UEs in the cell, it is not easy to adapt CRS transmission to suit the needs of a specific UE. This is in contrast to UE-specific RS, which means that each UE has its own RS placed in the data area of Figure 3 as part of the PDSCH.

3, the length of the control region, which can vary based on the subframe, is conveyed in the Physical Control Format Indicator Channel (PCFICH). The PCFICH is transmitted within the control region at a location known to the terminal. After the terminal has decoded the PCFICH, it knows the size of the control region and in which OFDM symbol data transmission begins.

A Physical Hybrid ARQ Indicator Channel is also transmitted in the control region. This channel carries the ACK/NACK response to the terminal to inform whether the uplink data transmission in the previous subframe was successfully decoded by the base station.

Precoding

A core component of LTE is support for MIMO antenna deployments and MIMO-related technologies. LTE Rel-10 supports up to eight layers of spatial multiplexing, with the possibility of channel-dependent precoding. The goal is high data rates in favorable channel conditions. Figure 4 provides an illustration of spatial multiplexing.

As shown in Figure 4, the information carrying symbol vector s is multiplied by an _NT × r precoder matrix. This precoder matrix is used to distribute the transmission energy in a subspace of the _NT (corresponding to the _NT antenna ports)-dimensional vector space. The precoder matrix is typically selected from a codebook of possible precoder matrices and is typically indicated by a precoder matrix indicator (PMI), which specifies the unique precoder matrix in the codebook. If the precoder matrix is restricted to having orthogonal columns, the design of the codebook of precoder matrices corresponds to the Grassmann subspace layout problem. The r symbols in s each correspond to a layer, and r is called the transmission rank. In this way, spatial multiplexing is achieved because multiple symbols can be transmitted simultaneously using the same time-frequency resource element (TFRE). The number of symbols r is typically adapted to suit the current channel properties.

LTE uses OFDM in the downlink (and DFT-precoded OFDM in the uplink), so the received _NR ×1 vector _yn for a certain TFRE on subcarrier n (or alternatively data TFRE number n) assuming no inter-cell interference is therefore modeled by:

where _en is the noise vector obtained as a realization of a random process. The precoder can be a wideband precoder that is frequency-constant 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. This is also often referred to as closed-loop precoding and essentially aims to focus the transmission energy into a subspace that is strong in the sense of delivering a large amount of transmission energy to the UE. In addition, the precoder matrix can also be chosen to aim 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 eNodeB for an appropriate precoder to use based on channel measurements on the forward link (downlink). Feedback can include a single precoder that is assumed to cover a large bandwidth (wideband precoding). It can also be beneficial to adapt to the frequency variations of the channel and instead feed back frequency-selective precoding reports, for example, for several precoders, one per subband. This is an example of a more general case of channel state information (CSI) feedback, which also encompasses feeding back other entities besides the precoder to assist the eNodeB in subsequent transmissions to the UE. Such other information can include a channel quality indicator (CQI) and a transmission rank indicator (RI).

Coordinating Node/Point-CoMP

In a classic cellular deployment, a given service area is covered by several sites at different geographical locations. Each site has an antenna that serves the area surrounding it. Sites are often further subdivided into multiple sectors, with the most common scenario perhaps using three 120-degree-wide sectors. Figure 5 illustrates such a sector scenario. A sector corresponds to a cell, and the base station associated with a cell controls and communicates with the UEs within that cell. Scheduling, as well as transmissions to and receptions from UEs, are largely independent from one cell to another.

Simultaneous transmissions on the same frequency will naturally interfere with each other, thus degrading the quality of reception. Interference is a major obstacle in cellular networks and in such classic deployment scenarios is mainly controlled by careful network planning, placing sites at appropriate locations, tilting antennas, etc.

Performing independent scheduling between different cells has the advantage of being simple and requiring relatively modest communication capabilities between different sites. On the other hand, cells influence each other, as signals originating from one cell can be seen as interference in nearby cells. This indicates that there are potential benefits in coordinating transmissions from nearby cells. Frequency, time, and space can be exploited in the coordination to mitigate interference. Such coordination has recently received a lot of attention in both the academic literature and standards for new wireless technologies. In fact, so-called coordinated multi-point transmission/reception (CoMP) is considered one of the key technical components for the upcoming Release 11 of LTE (see, for example, 3GPP TR 36.819, V1.2.0, "Coordinated Multi-Point Operation for LTE").

The concept of a point deserves elaboration. A point corresponds to a collection of antennas that are intended to cover substantially the same geographic area in a substantially similar manner. Thus, a point may correspond to one of the sectors of a site, but it may also correspond to a site with one or more antennas that are all intended to cover a similar geographic area in a similar manner. Different points often represent different sites. Antennas correspond to different points when they are sufficiently geographically separate and/or have antenna patterns pointing in sufficiently different directions, but generally not when they belong to the same sector. The techniques used for CoMP require that dependencies be introduced when scheduling or transmitting/receiving among different points, in contrast to conventional cellular systems, where points operate more or less independently of other points from a scheduling perspective.

Downlink CoMP can be divided into coordinated scheduling and joint scheduling. In the former, transmissions to the UE come from a single point at a time, while in the latter, multiple points participate simultaneously. Figure 5 illustrates the use of CoMP for a group of seven points (see the area surrounded by the dashed line), a so-called CoMP cluster. In this specific case, each point has a one-to-one correspondence with a cell.

Obviously, coordination between sites requires some kind of communication between the sites. This can take many forms, and the requirements on data rate and latency depend heavily on the exact coordination scheme used.

Aside from potential issues with site-to-site communication capabilities, utilizing time and frequency coordination is readily implemented for OFDM systems such as LTE using the normal dynamic resource allocation feature that can transmit PDSCH to a specific UE on a selected RB pair and in a certain subframe. Spatial coordination involves using multiple antennas for transmission. By modeling the signal as a vector-valued signal and applying appropriate complex-valued matrix weights, the transmission can be focused in the direction of the UE (in physical space or in a more abstract vector space) while minimizing interference to other UEs, thereby increasing SINR and ultimately increasing system performance.

Classic cell deployment vs. single cell deployment

The classic way to deploy a network is to have different transmission/reception points form separate cells. That is, the signals transmitted from or received at a point are associated with a cell ID that is different from the cell IDs used for other nearby points. Conventionally, each point transmits its own unique signals for broadcast (PBCH) and synchronization channels (PSS, SSS).

For heterogeneous deployments, the classical strategy of one cell ID per point is depicted in Figure 6, where multiple low-power (pico) points are placed within the coverage area of a higher-power macro point. Note that similar principles obviously also apply to classical macrocellular deployments, where all points have similar output power and may be placed in a more regular manner than in the case of heterogeneous deployments.

An alternative to classic deployment is to instead serve all UEs within the geographic area encompassed by a high-power macro site with signals associated with the same cell ID. In other words, from the UE's perspective, the received signal appears to originate from a single cell. This is illustrated in Figure 7. Note that only one macro site is shown; other macro sites will typically use different cell IDs (corresponding to different cells) unless they are co-located at the same site (corresponding to different sectors of the macro site). In this latter case, where several macro sites are co-located, the same cell ID can be shared across the co-located macro site and the pico sites corresponding to the union of the macro site's coverage areas. Synchronization, BCH, and control channels are all transmitted from the high-power site, while data can also be transmitted to the UE from the low-power site using a shared data transmission (PDSCH) that relies on a UE-specific RS. This approach benefits UEs capable of PDSCH based on UE-specific RS, while UEs that only support CRS for PDSCH (potentially including at least all Release 8/9 for FDD) must account for transmissions from the high-power site and therefore do not benefit from the deployment of additional low-power sites on the downlink.

The single-cell ID approach is suitable for scenarios where there is fast backhaul communication between points associated with the same cell. A typical case would be a base station serving one or more sectors at the macro level and having fast fiber connections to remote radio units (RRUs), which act as other points sharing the same cell ID. Those RRUs could represent low-power points, each with one or more antennas. Another example is where all points have similar power classes, with no single point having more importance than the others. The base station would then process signals from all RRUs in a similar manner.

A significant advantage of the shared cell approach over the classic approach is that the handover procedures typically involved between cells need only be invoked on a macro basis. Another important advantage is a significant reduction in interference from CRSs, as CRSs do not need to be transmitted from every point. There is also much greater flexibility in coordination and scheduling among points, meaning the network can avoid the inflexible concept of "low-interference" subframes that rely on semi-static configurations, as in Rel-10e ICIC. The shared cell approach also allows for decoupling of the downlink from the uplink, enabling, for example, path loss-based reception point selection in the uplink without causing significant interference issues in the downlink, where a UE can be served by a different transmission point than the one used for uplink reception.

CSI-RS

As previously indicated, CRS is not the only reference symbol available in LTE. As of LTE Release 10, a new RS concept was introduced with a separate UE-specific RS for demodulating the PDSCH and an RS for measuring the channel for the purpose of channel state information (CSI) feedback from the UE. The RS used to measure the channel is called CSI-RS. CSI-RS are not transmitted in every subframe, and they are generally more sparse in time and frequency than the RS used for demodulation. CSI-RS transmissions can occur in every 5th, 10th, 20th, 40th, or 80th subframe depending on the RRC-configured periodicity parameter and the RRC-configured subframe offset.

A UE in connected mode may be requested by the base station to perform channel state information (CSI) reporting, such as reporting an appropriate rank indicator (RI), one or more precoding matrix indices (PMIs), and a channel quality indicator (CQI). Other types of CSI are also conceivable, including explicit channel feedback and interference covariance feedback. CSI feedback assists network scheduling, including deciding the subframes and RBs to use for transmission, which transmission scheme/precoder to use, and providing information about the appropriate user bit rate for transmission (link adaptation). In LTE, both periodic and aperiodic CSI reporting are supported. In the case of periodic CSI reporting, the terminal reports CSI measurements on a configured periodic time basis on the physical uplink control channel (PUCCH), while with aperiodic reporting, CSI feedback is transmitted on the physical uplink channel (PUSCH) at a pre-specified time after receiving a CSI grant from the base station. With aperiodic CSI reporting, the base station can therefore request CSI in a specific subframe that reflects the downlink radio conditions.

Figures 8A-8C provide detailed illustrations of which resource elements within a resource block pair can potentially be occupied by the new UE-specific RS and CSI-RS. CSI-RS utilizes an orthogonal cover code of length two to overlay two antenna ports on two consecutive REs. As can be seen, many different CSI-RS patterns are available. For the case of two CSI-RS antenna ports, there are twenty different patterns within a subframe. The corresponding number of patterns is 10 and 5 for four and eight CSI-RS antenna ports, respectively. For TDD, several additional CSI-RS patterns are available. Patterns can correspond to one, two, four, or eight CSI-RS antenna ports in LTE Rel-10.

8A-8C , the resource elements corresponding to the CSI-RS resources share the same shading. In such cases, the resource corresponds to a specific pattern present in a particular subframe. Thus, in both cases, two different patterns in the same subframe or the same CSI-RS pattern in different subframes constitute two separate CSI-RS resources. In LTE Rel-10, the CSI-RS resource can alternatively be considered to be indicated by a combination of "resourceConfig" and "subframeConfig" configured by higher layers.

The CSI-RS pattern can also correspond to so-called zero-power CSI-RS, also known as muted REs. Zero-power CSI-RS corresponds to a CSI-RS pattern where REs are muted, meaning no signal is transmitted on those REs. This muting pattern is configured with a resolution corresponding to the CSI-RS pattern for four antenna ports. Therefore, the minimum muting unit corresponds to four REs.

The purpose of zero-power CSI-RS is to improve the SINR for CSI-RS in a cell by configuring zero-power CSI-RS in the interfering cell, thereby silencing the REs that originally caused the interference. Therefore, the CSI-RS pattern in a certain cell matches the corresponding zero-power CSI-RS pattern in the interfering cell. Improving the SINR level for CSI-RS measurement is particularly important in applications such as coordinated multi-point (CoMP) or in heterogeneous deployments. In CoMP, the UE may need to measure channels from non-serving points, and in this case the interference from the much stronger serving point is destructive. Zero-power CSI-RS is also needed in heterogeneous deployments, where the zero-power CSI-RS in the macro layer is configured so that it is transmitted simultaneously with the CSI-RS in the micro layer. This avoids interference from the macro node when the UE measures the channel with the micro node.

PDSCH is mapped around the REs occupied by CSI-RS and zero-power CSI-RS, so it is important that both the network and the UE use the same CSI-RS/zero-power CSI-RS configuration, otherwise the UE cannot decode PDSCH in subframes containing CSI-RS or their zero-power CSI-RS.

CSI feedback for CoMP

To assist with scheduling and link adaptation when performing CoMP, it is useful for the UE to provide the network with CSI corresponding to the channels at multiple points. This feedback allows the network to assess the impact of scheduling the UE on certain resources and with a certain precoder on system performance (i.e., considering multiple points). This can then be used to design efficient scheduling strategies across multiple points.

CSI feedback for CoMP can take many different forms, but a common approach is to have each UE report CSI feedback for each CSI-RS resource in a set of relevant CSI-RS resources used for feedback reporting, the so-called (CoMP) reporting set. The relevant CSI-RS resources typically correspond to transmissions of CSI-RS patterns that can be heard clearly enough by the UE. Such transmissions will often be from a specific point, which means that feedback per CSI-RS resource can be considered per-point CSI feedback.

Figure 9 illustrates an example prior art CSI reporting configuration, in which the CSI report transmitted by the UE includes separately determined feedback for each CSI-RS resource (i.e., per-CSI-RS resource CSI feedback), and each of the multiple precoders uses a separate feedback value from the feedback values. Furthermore, in Figure 9 , the CQI and precoder W _k are determined separately for each CSI-RS resource (implicit from the PMI and RI). Thus, the CQI, rank, and precoder are determined separately for each CSI-RS resource reported. Similar concepts have been employed for carrier aggregation, where CSI is determined separately for each carrier (cell). The transmission format and procedures used for carrier aggregation can therefore be reused for CoMP feedback, greatly simplifying the introduction of new feedback into the specification. Per-CSI-RS resource feedback also has the benefit of limiting UE complexity, as determining CSI separately for each CSI-RS resource is less complex than determining CSI jointly for all CSI-RS resources at once.

Problems with existing solutions

MIMO with rank adaptation-based spatial multiplexing is commonly employed in LTE to match transmission to the channel properties, thereby improving performance and providing high peak rates under good channel conditions. However, existing solutions do not clearly envision how to perform efficient rank determination for CoMP CSI feedback.

Summary of the Invention

One or more embodiments herein recognize that with separate CSI feedback for each CSI-RS resource, the reported rank will typically be different. This creates problems for the network to determine channel quality and appropriate precoder weights. Additionally, this results in reduced system performance and/or additional complexity in the network.

Thus, one or more embodiments relate to improvements in providing CSI feedback for multiple channel state information reference symbol (CSI-RS) resources. For a given set of CSI-RS resources, a common transmission rank determined based on the set of CSI-RS is used for CSI reporting. In some embodiments, this is advantageously used to reduce signaling overhead between a wireless terminal transmitting CSI feedback and a network node receiving the CSI feedback and using it to perform rank adaptation.

With this in mind, an exemplary method implemented by a wireless terminal for reporting channel state information (CSI) to a wireless communication network according to one or more embodiments is disclosed. The wireless terminal receives reference symbols on a set of CSI-RS resources and determines a common transmission rank for the set of CSI-RS resources based on the reference symbols received on a subset of those CSI-RS resources. The terminal generates CSI feedback based on the common transmission rank and transmits the CSI feedback to the communication network.

In one or more embodiments, CSI feedback is generated to include the determined common transmission rank. In one such embodiment, the transmission rank for each CSI-RS resource in the set is determined to be the common transmission rank, and the common transmission rank is correspondingly included one or more times in the CSI feedback.

In another example, generating the CSI feedback includes determining different transmission ranks for different CSI-RS resources within the set based on a common transmission rank and including the different transmission ranks in the CSI feedback. In this latter embodiment, determining different transmission ranks for different CSI-RS resources may include determining, for each CSI-RS resource within the set, a transmission rank for the CSI-RS resource as a minimum value between the common transmission rank and a maximum possible transmission rank for the CSI-RS resource.

In one example, different transmission ranks are determined for different CSI-RS resources within a set according to a common transmission rank, and a precoder and a channel quality indicator (CQI) are determined for each CSI-RS resource within the set based on the transmission rank determined for that CSI-RS resource. In this example, CSI feedback is generated to include the precoder and CQI determined for each CSI-RS resource within the set.

In one example, the subset includes only a single CSI-RS resource from the CSI-RS resources in the set.In the same or another embodiment, the wireless terminal determines which CSI-RS resources to include in the subset based on signaling received from the wireless communication network.

According to one or more additional embodiments, the step of determining a common transmission rank is performed by a network node rather than a wireless terminal. Thus, in these embodiments, the wireless terminal receives reference symbols on a set of CSI-RS resources and generates CSI feedback based on the common transmission rank, which is common to all CSI-RS resources. The wireless terminal then transmits the CSI feedback to the communication network.

Also disclosed is a corresponding wireless terminal operable to transmit channel state information (CSI) feedback to a wireless communication network in accordance with one or more of the various embodiments discussed above.

Also disclosed is a method, implemented by a network node, for adapting a transmission rank of transmissions between a wireless device and a wireless communication network. The network node transmits reference symbols on a set of CSI-RS resources and receives CSI feedback including a common transmission rank for the set of CSI-RS resources, wherein the common transmission rank in the CSI feedback is common to the set of CSI-RS resources. The network node also performs rank adaptation for the set of CSI-RS resources based on the common transmission rank.

In one or more embodiments, the received feedback includes a common transmission rank. In one such embodiment, the network node determines that the transmission rank for each CSI-RS resource in the set is the common transmission rank based on the only transmission rank included in the CSI feedback for the set being the common transmission rank.

In one example embodiment, the network node determines different transmission ranks for different CSI-RS resources within a set based on a common transmission rank. The different transmission rank for a given CSI-RS resource may be determined as the minimum of the common transmission rank and a maximum possible transmission rank for the CSI-RS resource.

In one embodiment, the subset includes a single CSI-RS resource from among the CSI-RS resources in the set, and the network node dynamically switches between a coordinated multipoint (CoMP) transmission scheme and a non-CoMP transmission scheme based on CSI feedback for the single CSI-RS resource in the set (e.g., to non-CoMP if CSI feedback is provided only for the single CSI-RS resource). The network node may transmit an indication of the CSI-RS resource from the set to be included in the subset.

Also disclosed is a corresponding network node operable to adapt a transmission rank of transmissions between a wireless device and a wireless communication network according to one or more of the above-discussed embodiments.

An exemplary embodiment includes a method for reporting channel conditions implemented by a wireless terminal in a communication network. According to the method, reference symbols are received on two or more channel state information resources in a reporting set, each channel state information resource associated with one or more network nodes. A common rank impact parameter is determined for the channel state information resources in the reporting set based on the reference symbols received on a subset of the channel state information resources in the reporting set. A channel state report including the common rank impact parameter for the reporting set is sent to one or more network nodes associated with the channel state information resources in the reporting set.

Also disclosed is a corresponding wireless terminal operable to implement a method for reporting channel conditions. The wireless terminal includes a transceiver configured to receive reference symbols on two or more channel state information resources in a reporting set, each channel state information resource associated with one or more network nodes. The wireless terminal also includes a processor operatively coupled to the transceiver. The processor is configured to determine a common rank impact parameter for the channel state information resources in the reporting set based on the reference symbols received on a subset of the channel state information resources in the reporting set. The processor is further configured to send a channel state report including the common rank impact parameter for the reporting set to one or more network nodes associated with the channel state information resources in the reporting set.

Another exemplary embodiment of the present invention includes a channel state reporting method implemented in a network node in a wireless communication network. According to this method, the network node configures the channel state reporting of a wireless terminal by sending configuration information to the wireless terminal, the configuration information including: an indication of two or more channel state information resources forming a reporting set for the wireless terminal and an indication of a subset of the channel state information resources in the reporting set, the subset to be used by the wireless terminal to determine a common rank impact parameter for the channel state information resources in the reporting set. The network node also receives a channel state report from the wireless terminal, the channel state report including the common rank impact parameter for the channel state information resources in the reporting set.

Also disclosed is a corresponding network node operable to implement this method according to one or more embodiments. The network node includes a transceiver configured to send messages to and receive messages from a wireless terminal in a wireless communication network, and a processor operatively coupled to the transceiver. The processor is operable to configure a channel state report of the wireless terminal by sending configuration information to the wireless terminal, the configuration information including an indication of two or more channel state information resources forming a report set for the wireless terminal and an indication of a subset of the channel state information resources in the report set, the subset to be used by the wireless terminal to determine a common rank impact parameter for the channel state information resources in the report set. The processor is also operable to receive a channel state report from the wireless terminal via the transceiver, the channel state report including the common rank impact parameter for the channel state information resources in the report set.

In one or more embodiments, the indication of two or more channel state information resources corresponds to one or more channel state information reference symbols (CSI-RS), and the network node determines a multiple-input/multiple-output (MIMO) precoder rank for each of the multiple CSI-RS resources in response to the received rank impact parameter.

In one example, the public rank impact parameter includes a public rank. In one example, each of the reference symbols corresponds to a channel state information reference symbol (CSI-RS). The two or more channel information resources may, for example, include two, four, or eight resource elements. The number of resource elements within each of the multiple channel information resources and the channel information resources may be determined by the number of ports utilized by the network node transmitting the reference symbols to the wireless terminal.

In one or more embodiments, the wireless terminal further determines a separate precoder matrix index and a separate channel quality indicator for each channel state information resource.The channel state report may also include the determined precoder matrix index and channel quality indicator for a subset of the channel state information resources.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 illustrates the LTE downlink physical resource time-frequency grid.

FIG2 illustrates the LTE time domain structure.

FIG3 illustrates an LTE downlink subframe and its associated mapping of physical control channels, data channels and cell-specific reference signals.

FIG4 illustrates a transmission arrangement for a precoded spatial multiplexing mode in LTE.

FIG5 illustrates a cellular network including three sector sites and seven points.

FIG6 illustrates an example heterogeneous network using a separate cell id for each point.

7 illustrates an example heterogeneous network using the same cell id for a macro point and multiple associated pico points.

8A-8C illustrate multiple example resource subframes and the locations of CSI-RS within those subframes.

FIG9 illustrates a prior art MIMO precoding arrangement.

10 illustrates an example wireless communication network.

FIG11 illustrates a method for reporting channel state information implemented by a wireless terminal in a network.

FIG12 illustrates an example CSI reporting configuration.

FIG13 illustrates another example CSI reporting configuration.

FIG14 illustrates yet another example CSI reporting configuration.

FIG15 illustrates another method implemented by a wireless terminal for reporting channel state information.

FIG16 illustrates a method implemented by a network node for adapting a transmission rank of transmissions between a wireless device and a wireless communication network.

FIG. 17 illustrates an example user terminal operating to implement the methods of FIGs. 11 and 15 .

FIG. 18 illustrates an example network node operable to implement the method of FIG. 16 .

Of course, the present invention is not limited to the above features and advantages. In fact, those skilled in the art will recognize additional features and advantages upon reading the following detailed description and upon viewing the accompanying drawings.

DETAILED DESCRIPTION

10 illustrates a wireless communication network 30 in which a wireless terminal (WT) 100 communicates wirelessly with one or more network nodes 200 in a radio access network (RAN) 31 portion of a larger network 30. The WT 100 may include a cellular telephone, user equipment (UE), smartphone, machine type communication (MTC) device, laptop computer, etc. The one or more network nodes 200, in turn, communicate with a core network (CN) 32 portion of the network 30. The core network 32 communicates with one or more external networks, such as a packet data network (PDN) 34 (e.g., the Internet) and a public switched telephone network (PSTN) 36.

According to one or more embodiments, the wireless terminal 100 of FIG10 implements the process shown in FIG11 for reporting channel state information (CSI) to the wireless communication network 30. Specifically, the process at the wireless terminal 100 includes receiving reference symbols on a set 40 of K CSI reference symbol (CSI-RS) resources 42 (block 302). The CSI-RS resources 42 are a selection of resource elements corresponding to channel state information reference symbols (e.g., see FIG8A-8C).

In at least some embodiments, wireless terminal 100 receives reference symbols from one or more network nodes 200. In one embodiment, for example, terminal 100 receives reference symbols from a single network node 200. In other embodiments, wireless terminal 100 receives reference symbols from multiple network nodes 200. In one such embodiment, for example, the wireless terminal receives reference symbols from a first one of network nodes 200 on a first portion of CSI-RS resources 42 in set 40, and receives reference symbols from a second one of network nodes 200 on a second portion of CSI-RS resources 42 in set 40.

Regardless of whether the wireless terminal 100 receives reference symbols from one or more network nodes 200, the CSI-RS resources 42 correspond to different CoMP points in the CoMP configuration in some embodiments. In this regard, a single network node 200 can act as multiple points in the CoMP configuration. Additionally or alternatively, different network nodes 200 can correspond to different points in the CoMP configuration. In one embodiment, for example, the wireless terminal 100 receives reference symbols from a first one of the network nodes 20 (acting as the first point in the CoMP configuration) on a first portion of the CSI-RS resources 42, and receives reference symbols from a second one of the network nodes 200 (acting as the second point in the CoMP configuration) on a second portion of the CSI-RS resources 42. In an extension of this embodiment, each CSI-RS resource 42 originates from a different CoMP point in the CoMP configuration.

In any case, processing at the wireless terminal 100 also requires determining a common transmission rank ( _rcommon ) (block 304). As used herein, a common transmission rank is a transmission rank that collectively affects the transmission rank determined for each CSI-RS resource 42 in the set 40 of K CSI-RS resources in a rank adaptation process (performed by one or more network nodes in the network node 200). In a sense, _rcommon is then considered a "common rank influencing parameter." Note that the wireless terminal 100 determines this common transmission rank based on a subset 41 of the set 40 of CSI-RS resources 42. When the terminal 100 has determined this common transmission rank in this manner, processing also includes generating CSI feedback 50 based on the common transmission rank (block 306) and transmitting the feedback 50 to the network node 200 (block 308). In one or more embodiments, this arrangement can be equivalently considered to be that the CSI feedback associated with one CSI-RS resource 42 inherits the rank value (or a function thereof) from the CSI feedback associated with another CSI-RS resource 42. In other words, the CSI feedback acts as a rank reference.

Of course, while set 40 is shown as including four CSI-RS resources 42 (i.e., K=4) and subset 41 is shown as including two CSI-RS resources 42, it is understood that these are merely examples and that other numbers of CSI-RS resources 42 may be included in set 40 and subset 41. In fact, in at least some embodiments, subset 41 includes a single CSI-RS resource 42. Such embodiments demonstrate that CoMP configuration is facilitated by assisting network 30 in supporting dynamic switching between CoMP and non-CoMP transmissions to wireless terminal 100. Non-CoMP feedback (i.e., including only a single CSI-RS resource in subset 41) may be used to enable network 30 to schedule according to more robust non-CoMP operation and dynamically switch between non-CoMP and CoMP when needed.

Figures 12-14 illustrate several example CSI reporting configurations in which wireless terminals 10 transmit CSI feedback 50 generated according to a common transmission rank ( _rcommon ). In each of these configurations, a dashed line is shown around the precoding matrix indicator (PMI) and rank value r to indicate that the PMI and r values are used to calculate the precoder w. Each CSI symbol (CSI ₁ , CSI ₂ , CSI ₃ , ..., CSI _K ) is used to show CSI data for a given CSI-RS resource. Although the CQI values CQI ₁ , CQI ₂ , etc. are not included within the dashed line, they are still considered part of the CSI data for their given CSI-RS resource. 12-14 describe the CSI information (e.g., CSI ₁ , CSI ₂ , CSI ₃ , . . . , CSI _K ) for a set 40 of K CSI-RS resources 42 as being reported collectively within a single CSI report 50 , those skilled in the art will understand that different terminology may be used to describe the CSI information for K different CSI-RS resources 42 as being reported separately within K different CSI reports.

With this in mind, FIG12 illustrates a novel CSI reporting configuration according to one embodiment of method 300, in which WT 100 reports the common transmission rank for CSI-RS resource set 40 in CSI report 50A. In this embodiment, CSI report 50A (formed collectively by CSI ₁ , CSI ₂ , ..., CSI _K ) is generated to actually include the determined common transmission rank rcommon based on the _common transmission rank _rcommon . In the embodiment of FIG12 , the common rank (e.g., in a dedicated field with _rcommon being the value used for that field) is transmitted once for each CSI-RS resource in the set. This embodiment advantageously provides backward compatibility because a receiving network node (e.g., node 200) receives the expected rank for each CSI-RS resource.

In the embodiment of FIG12 , a common transmission rank is used for all CSI-RS resources in the set 40. Thus, instead of determining CSI completely separately for each CSI-RS resource, a common transmission rank valid for more than one CSI-RS resource is reported. The common transmission rank is determined based on a subset 41 of the set 40 of CSI-RS resources used for CSI reporting (e.g., a CoMP report set). This effectively means that the rank is determined in the WT to match the characteristics of the channel corresponding only to the subset 41 of CSI-RS resources, rather than the full set 40 of CSI-RS resources covered by the CSI report.

In another embodiment of the method 300 shown in FIG13 , the wireless terminal 100 transmits a CSI report 50B that includes only a common transmission rank once in the entire report for a single CSI-RS resource (see CSI ₁ ) in the set. For the remaining CSI-RS resources, a null value may be used instead of a common transmission rank (see CSI ₂ , CSI ₃ , . . . , CSI _K ). In some embodiments, for example, the network node 200 intelligently extrapolates the common transmission rank to CSI-RS resources outside of the subset 41 of CSI-RS resources based on which the common transmission rank is determined. That is, the network node 20 determines that r _common , although received only for a single CSI-RS resource, is common to the other CSI-RS resources in the set 40. The configuration of FIG13 advantageously provides reduced control signaling overhead in CSI feedback because the rank report is compressed to report a common transmission rank that is common to all CSI-RS resources in the report set 40. Furthermore, in one example, for the embodiments of Figures 12 and 13, the task of determining feedback for the WT is simplified on the network side because there is no need to search over different rank hypotheses if the rank has already been determined based on a common transmission rank.

FIG14 illustrates yet another novel CSI reporting configuration according to another embodiment of method 300. In this embodiment, the CSI generated according to the common transmission rank includes, for each CSI-RS resource, a specific value ( _r1 , _r2 , ..., _rk ) _commonly determined according to the common transmission rank r. Here, different transmission ranks are determined for different CSI-RS resources within set 40 according to the common transmission rank, and are included in CSI report 50C (formed collectively by _CSI1 , _CSI2 , ..., _CSIK ), but the different transmission ranks are not independent.

In one example, a different transmission rank (r ₁ , r ₂ , ..., r _k ) is determined for each CSI-RS resource as the minimum value between the common transmission rank r _common and the maximum possible transmission rank for the CSI-RS resource. This is shown in equation (2) below.

r _k =min(r _common , r _max,k ) Equation (2)

In this example, NT _,k represents the number of antenna ports corresponding to CSI-RS resource k, and _rcommon is the common transmission rank determined based on subset 41 of CSI-RS resources 42 as described above. The maximum transmission rank rmax _,k cannot be higher than NT _,k (and is also limited by the number of receive antennas or the number of layers that wireless terminal 100 can decode). Here, the rank _rk assumed for CSI-RS resource k in the (CoMP) reporting set will be upper bounded according to equation (2).

In the embodiment of Figure 4, _rk will be used by the receiving network node 200 when determining the precoder and CQI for CSI-RS resource k. In general, the rank rk assumed for CSI-RS resource _k is a function of the common transmission rank ( _rcommon ). The dependence on the channel of other CSI-RS resources is therefore only by virtue of and conditioned on this parameter, and the rest of the precoding (e.g., precoder with a fixed number of columns) and channel quality determination is performed independently for each CSI-RS resource k.

In one or more embodiments, the maximum transmission rank is determined by the number of antenna ports used to transmit to the wireless terminal. In such embodiments, the maximum rank r _max,k in equation (2) can be replaced by NT _,k . In LTE Rel-10, for example, the number of antenna ports used can be 1, 2, 4, or 8. By contrast, in the embodiments of Figures 12-13, the receiving network node 200 can instead implement equation (2) based on the received common transmission rank. Therefore, in those embodiments, only the common transmission rank or an equivalent representation need be fed back instead of individual r _k values.

Referring again to FIG. 14 , in one example, in addition to determining different transmission ranks for different CSI-RS resources within a set based on a common transmission rank, the WT determines a precoder and a channel quality indicator (CQI) for each CSI-RS resource within the set based on the transmission rank determined for that CSI-RS resource. Thus, CQI ₁ and PMI ₁ may be determined by the WT based on r ₁ for "CSI-RS resource 1." In this example, CSI 50C is generated by the WT to include the precoder and CQI determined for each CSI-RS resource within the set.

The term "subset" is used herein in its general sense to refer to a portion of the set 40 of K CSI-RS resources. This is in contrast to the mathematical or technical meaning of the term, in which a subset can be the same as a set. In mathematical terms, "subset" as used herein is actually a "proper subset." In any case, as noted above, the number of CSI-RS resources in a subset is a single CSI-RS resource in the CSI-RS resources according to one or more embodiments.

In some embodiments, the CSI-RS resources for inclusion in the subset are predetermined. In other embodiments, the network 30 or the wireless terminal 100 intelligently calculates or otherwise determines which CSI-RS resources to include in the subset. In either case, the CSI-RS resources included in the subset are determined by the wireless terminal in some embodiments based on signaling received from the wireless communication network 30 (e.g., from the network node 20).

In one or more embodiments, the determining step (block 304) is optional, as it may or may not be performed by the wireless terminal. Thus, in some embodiments, the determination may be performed, for example, by a network node, which then informs the wireless terminal 100 of the common transmission rank. With this in mind, FIG15 illustrates a method 350 according to one or more embodiments in which the wireless terminal 100 receives reference symbols on a set 40 of K CSI reference symbol (CSI-RS) resources 42 (block 352), generates CSI feedback according to the common transmission rank for the set of CSI-RS resources (block 354), and transmits the CSI feedback to the communication network (block 356).

Referring now to FIG. 16 , a method 400 implemented by a network node (e.g., network node 200) for adapting a transmission rank for transmissions between a wireless device and a wireless communication network is shown. The network node transmits reference symbols to the wireless terminal 100 on a set of CSI-RS resources (block 402). The network node later receives a CSI report from the wireless terminal that includes a common transmission rank ( _rcommon ) determined based on the reference symbols transmitted on a subset 41 of the CSI-RS resources 42 in the set 40 (block 404), wherein the common transmission rank in the CSI report is common to the set of CSI-RS resources. That is, the network node extrapolates or otherwise applies the common transmission rank to the CSI-RS resources 42 outside the subset 41, even though the common transmission rank was determined based only on the CSI-RS resources 42 in the subset 41. Based on this determination, the network node performs rank adaptation for the set of CSI-RS resources based on the common transmission rank (block 406).

In one or more embodiments, the received CSI report includes a common transmission rank _rcommon (e.g., see Figures 12 and 13). In one or more of these embodiments, the network node determines different transmission ranks for different CSI-RS resources within the set based on the reported common transmission rank _rcommon . In this case, instead of the wireless terminal 100 determining individual ranks (r ₁ , r ₂ , r ₃ , ..., r _k ) based on _rcommon and then reporting those individual ranks, as shown in Figure 14, the network node itself determines those individual ranks based on the common transmission rank _rcommon reported by the terminal 100. In one example, as described above with respect to equation (2), the individual rank for each CSI-RS resource is determined to be the minimum transmission rank between the common transmission rank and the maximum possible transmission rank for the CSI-RS resource. Therefore, equation (2) can be calculated by the wireless terminal side or the network node, depending on what is included in the CSI report 50. In one or more other embodiments, the network node determines the transmission rank for each CSI-RS resource in the set to be the common transmission rank based on the only transmission rank included in the CSI feedback for the set being the common transmission rank.

In one example, a network node transmits to a wireless terminal an indication of the CSI-RS resources in the set to be included in the subset. As in the method of FIG10 , the number of CSI-RS resources in the subset may be a single CSI-RS resource in the CSI-RS resources. The network node may dynamically switch between a coordinated multipoint (CoMP) transmission scheme and a non-CoMP transmission scheme based on reporting CSI only for a single CSI-RS resource within the subset (i.e., if the subset includes a single CSI-RS resource). This method of transitioning from CoMP to non-CoMP advantageously incurs very low signaling overhead.

Such signaling indicating what CSI-RS resources are to be included in the subset 41 may be performed semi-statically via higher layer signaling (e.g., RRC or MAC elements) or more dynamically via a physical layer control channel (e.g., PDCCH or other form of downlink control). The subset may also be determined implicitly via a predetermined rule referencing the configuration of what CSI-RS resources are to be measured on (CoMP measurement set) and for what CSI-RS resources to be reported (CoMP reporting set). Such a rule may, for example, state that the first L CSI-RS resources in the (CoMP) measurement/reporting set configuration message are to form the rank-determining CSI-RS resource subset. The rank-determining subset of CSI-RS resources may instead be determined by the WT rather than by the network. The subset may be selected to include the CSI-RS resources with the corresponding strongest long/short term channels.

FIG17 and FIG18 illustrate an example wireless terminal 100 and a corresponding network node operable to implement a novel method for reporting channel conditions. Referring to FIG17 , a wireless terminal 100 is shown that includes a transceiver 110, a memory 130, and a processor 120 including one or more processing circuits. The one or more processing circuits may, for example, include one or more microprocessors, microcontrollers, digital signal processors, application-specific integrated circuits (ASICs), and the like. In one example, the wireless device 100 is operable to implement the method 300 or 350 in one or more of the various embodiments of the method 300 or 350 described above. Thus, in one or more embodiments, the one or more processing circuits are configured to receive reference symbols on a set of channel state information reference symbol (CSI-RS) resources via the transceiver 110 and (optionally) determine a common transmission rank (e.g., also referred to as a common rank impact parameter) for the set of CSI-RS resources based on the reference symbols received on a subset of those CSI-RS resources. The one or more processing circuits are further configured to generate CSI feedback based on the common transmission rank. In some embodiments, generating CSI feedback in this regard requires generating a CSI report to include the common transmission rank. In other embodiments, by contrast, generating CSI feedback involves generating feedback to include different transmission ranks for different CSI-RS resources in the set according to the common transmission rank, i.e., without including the common transmission rank itself in the CSI feedback (at least in the same sense). In any case, the one or more processing circuits are also configured to transmit the CSI feedback to the communication network via transceiver 110. In at least some embodiments, the one or more processing circuits are configured by executing instructions stored in memory 130.

18 , a network node 200 is shown, comprising a transceiver 210 configured to transmit and receive messages to and from a wireless device 100 in a wireless communication network, and a processor 220 operatively coupled to the transceiver 210. The processor 220 comprises one or more processing circuits, which may include, for example, one or more microprocessors, microcontrollers, digital signal processors, application-specific integrated circuits (ASICs), and the like. In one example, the network node 200 is operable to implement the method 400 in one or more of the various embodiments described above. Thus, in one or more embodiments, the one or more processing circuits 220 are configured to: transmit reference symbols on a set of CSI-RS resources via the transceiver 210; and receive CSI feedback via the transceiver 210, the CSI feedback comprising a common transmission rank determined based on the reference symbols transmitted on a subset of the CSI-RS resources in the set, wherein the common transmission rank in the CSI report is common to the set of CSI-RS resources. In some embodiments, the processor 220 is operable to configure channel state reporting for the wireless terminal 100 by sending configuration information to the wireless device 100, the configuration information including an indication of two or more channel state information resources forming a reporting set for the wireless device and an indication of a subset of the channel state information resources in the reporting set to be used by the wireless device to determine a common rank impact parameter for the CSI-RS in the reporting set. In any case, the one or more processing circuits are further configured to perform rank adaptation for the set of CSI-RS resources based on the common transmission rank.

Note that while terminology from 3GPP LTE has been used in this disclosure to illustrate the present invention, this should not be construed to limit the scope of the present invention to only the aforementioned systems. Other wireless systems, including WCDMA, WiMax, UMB, and GSM, can also benefit from applying the concepts covered in this disclosure. The concepts presented are generally applicable to any type of reference signal, where a subset of RS resources determines the rank for each RS resource in that subset and outside that subset.

Furthermore, while CSI-RS resources have been described as including sets and subsets, it is possible that a "set" of CSI-RS resources may not include all CSI-RS resources for a given WT. For example, assume there are four CSI-RS resources for a WT and the WT reports two ranks. In one example, a first rank may be determined by CSI-RS resource 1 and be common to CSI-RS resources 1, 2, and 3, and a second rank may be determined by resource 4 and be common only to CSI-RS resource 4. Thus, a "set" of CSI-RS resources, as the term is used above, includes resources 1, 2, and 3, since those are the resources for which the subset is common—however, a "set" in this regard is part of a larger set that includes resource 4.

Therefore, the foregoing description and accompanying drawings represent non-limiting examples of the methods and apparatus taught herein. As such, the present invention is not limited by the foregoing description and accompanying drawings. Instead, the present invention is limited only by the appended claims and their legal equivalents.

Claims (33)

1. A method (300) implemented by a wireless terminal for reporting Channel State Information (CSI) to a wireless communication network, comprising: Receive (302) reference symbols on the set of Channel State Information Reference Symbol (CSI-RS) resources; Based on the reference symbols received on a subset of those CSI-RS resources, a common transmission rank for the set of CSI-RS resources is determined (304), wherein a CSI feedback associated with one CSI-RS resource inherits a function of the rank of a CSI feedback associated with another CSI-RS resource. (306)CSI feedback is generated based on the common transmission rank; and The CSI feedback is transmitted (308) to the communication network. 2. The method of claim 1, wherein the generation includes generating the CSI feedback to include the determined common transport rank. 3. The method according to claim 1, wherein the generation comprises: The transmission rank for each CSI-RS resource within the set is determined as the common transmission rank; and The common transmission rank is included once or multiple times in the CSI feedback. 4. The method of claim 1, wherein the generation comprises: Different transmission ranks are determined for different CSI-RS resources within the set based on the common transmission rank; and The different transmission ranks are included in the CSI feedback. 5. The method of claim 4, wherein determining different transmission ranks for different CSI-RS resources comprises determining, for each CSI-RS resource within the set, the transmission rank for the CSI-RS resource as the minimum of the following ranks: The common transmission rank; and The maximum possible transmission rank for the CSI-RS resource. 6. The method according to any one of claims 1-2 and 4-5, further comprising: Different transmission ranks are determined for different CSI-RS resources within the set based on the common transmission rank; and For each CSI-RS resource within the set, a precoder and a channel quality indicator (CQI) are determined based on the transmission rank determined for that CSI-RS resource; The generation includes generating the CSI feedback to include the precoder and the CQI determined for each CSI-RS resource within the set. 7. The method according to any one of claims 1-5, wherein the subset comprises only a single CSI-RS resource among the CSI-RS resources within the set. 8. The method according to any one of claims 1-5, wherein the function of the rank includes the rank itself. 9. The method according to any one of claims 1-5, further comprising: The CSI-RS resources included in the subset are determined based on signaling received from the wireless communication network. 10. A wireless terminal (100) operating for reporting Channel State Information (CSI) to a wireless communication network, the wireless terminal (100) comprising a transceiver (110) and one or more processing circuits (120), the one or more processing circuits (120) being configured to: Reference symbols are received on the set of Channel State Information Reference Symbols (CSI-RS) resources via the transceiver (110); Based on the reference symbols received on a subset of those CSI-RS resources, a common transmission rank for the set of CSI-RS resources is determined, wherein the CSI feedback associated with one CSI-RS resource inherits a function of the rank of the CSI feedback associated with another CSI-RS resource. CSI feedback is generated based on the common transmission rank; and The CSI feedback is transmitted to the communication network via the transceiver (110). 11. The wireless terminal of claim 10, wherein the one or more processing circuits (120) are configured to generate the CSI feedback to include the determined common transmission rank. 12. The wireless terminal of claim 10, wherein the one or more processing circuits (120) are configured to generate the CSI feedback by: The transmission rank for each CSI-RS resource within the set is determined as the common transmission rank; and The common transmission rank is included once or multiple times in the CSI feedback. 13. The wireless terminal of claim 10, wherein the one or more processing circuits (120) are configured to generate the CSI feedback by: Different transmission ranks are determined for different CSI-RS resources within the set based on the common transmission rank; and The different transmission ranks are included in the CSI feedback. 14. The wireless terminal of claim 13, wherein the one or more processing circuits (120) are configured to determine different transmission ranks for different CSI-RS resources by determining, for each CSI-RS resource within the set, the transmission rank for the CSI-RS resource as the minimum value among the following ranks: The common transmission rank; and The maximum possible transmission rank for the CSI-RS resource. 15. The wireless terminal according to any one of claims 10-14, wherein the one or more processing circuits (120) are further configured to: Different transmission ranks are determined for different CSI-RS resources within the set based on the common transmission rank. For each CSI-RS resource within the set, a precoder and a channel quality indicator (CQI) are determined based on the transmission rank determined for that CSI-RS resource; and The CSI feedback is generated to include the precoder and the CQI determined for each CSI-RS resource within the set. 16. The wireless terminal according to any one of claims 10-14, wherein the subset comprises only a single CSI-RS resource from the CSI-RS resources within the set. 17. The wireless terminal according to any one of claims 10-14, wherein the function of the rank includes the rank itself. 18. A method (400) implemented by a network node for adapting the transmission rank of a wireless device to a wireless communication network, comprising: Transmit (402) reference symbols on the set of Channel State Information Reference Symbols (CSI-RS) resources; Receive (404) CSI feedback, the CSI feedback including a common transmission rank determined based on the reference symbols transmitted on a subset of the CSI-RS resources in the set, wherein the common transmission rank in the CSI feedback is common to the set of CSI-RS resources, and wherein a CSI feedback associated with one CSI-RS resource inherits a function of the rank of a CSI feedback associated with another CSI-RS resource; and (408) rank adaptation is performed on the set of CSI-RS resources based on the common transport rank. 19. The method of claim 18, further comprising determining the transmission rank for each CSI-RS resource in the set as the common transmission rank based on the fact that only the transmission rank included in the CSI feedback for the set is the common transmission rank. 20. The method of claim 18, further comprising determining different transmission ranks for different CSI-RS resources within the set based on the common transmission rank. 21. The method of claim 20, wherein determining different transmission ranks for different CSI-RS resources comprises determining, for each CSI-RS resource within the set, the transmission rank for the CSI-RS resource as the minimum of the following ranks: The common transmission rank; and The maximum possible transmission rank for the CSI-RS resource. 22. The method according to any one of claims 18-21, wherein the subset comprises a single CSI-RS resource among the CSI-RS resources within the set. 23. The method of claim 22, further comprising dynamically switching between a coordinated multipoint CoMP transmission scheme and a non-CoMP transmission scheme based on CSI feedback for the individual CSI-RS resources within the set. 24. The method according to any one of claims 18-21, further comprising: Indications for the CSI-RS resources to be included in the subset will be transmitted within the set. 25. The method according to any one of claims 18-21, wherein the function of the rank includes the rank itself. 26. A network node (200) operating for adapting transmission rank between a wireless device and a wireless communication network, the network node comprising a transceiver (210) and one or more processing circuits (220), the one or more processing circuits (220) being configured to: Reference symbols are transmitted over the set of Channel State Information Reference Symbols (CSI-RS) resources via the transceiver (210); CSI feedback is received via the transceiver (210), the CSI feedback including a common transmission rank determined based on the reference symbols transmitted on a subset of the CSI-RS resources in the set, wherein the common transmission rank in the CSI feedback is common to the set of CSI-RS resources, and wherein a CSI feedback associated with one CSI-RS resource inherits a function of the rank of a CSI feedback associated with another CSI-RS resource; and Rank adaptation is performed on the entire set of CSI-RS resources based on the common transport rank. 27. The network node of claim 26, wherein the one or more processing circuits (220) are configured to determine the common transmission rank for each CSI-RS resource in the set based on the only transmission rank included in the CSI feedback for the set being the common transmission rank. 28. The network node of claim 26, wherein the one or more processing circuits (220) are configured to determine different transmission ranks for different CSI-RS resources within the set based on the common transmission rank. 29. The network node of claim 28, wherein the one or more processing circuits (220) are configured to determine different transmission ranks for different CSI-RS resources by determining, for each CSI-RS resource within the set, the transmission rank for the CSI-RS resource as the minimum of the following ranks: The common transmission rank; and The maximum possible transmission rank for the CSI-RS resource. 30. The network node according to any one of claims 26-29, wherein the subset includes a single CSI-RS resource among the CSI-RS resources within the set. 31. The network node of claim 30, wherein the one or more processing circuits (220) are further configured to dynamically switch between a coordinated multipoint CoMP transmission scheme and a non-CoMP transmission scheme based on CSI feedback for the individual CSI-RS resource within the set. 32. The network node according to any one of claims 26-29, wherein the one or more processing circuits are further configured to: Indications for the CSI-RS resources to be included in the subset will be transmitted within the set. 33. The network node according to any one of claims 26-29, wherein the function of the rank includes the rank itself.