HK1190862B - Point-dependent resource symbol configuration in a wireless cell - Google Patents

Description

Point-dependent resource symbol configuration in a wireless cell

RELATED APPLICATIONS

Priority of united states provisional application 61/440923, filed on 9/2/2011, the entire contents of which are incorporated herein by reference.

Technical Field

The present invention relates generally to control of devices in a wireless communication network, and more specifically to techniques for allocating and using reference signals in a network with heterogeneous cell deployments.

Background

The third generation partnership project (3GPP) is continuing to develop a fourth generation wireless network technology known as Long Term Evolution (LTE). Improved support for heterogeneous network operation is the ongoing specification of release 10 of 3GPP LTE, and other improvements are discussed in the context of new features of release 11. In heterogeneous networks, a mix of cells of different sizes and overlapping coverage areas are deployed.

An example of such a deployment is seen in the system 100 shown in fig. 1, where several pico cells 120, each having a respective coverage area 150, are deployed in a larger coverage area 140 of a macro cell 110. The system 100 of fig. 1 implies a wide area wireless network deployment. However, other examples of low power nodes, also referred to as "points" in heterogeneous networks, are home base stations and relays. In this document, a node or point in a network is often referred to as being of some type, such as a "macro" node or a "pico" point. However, unless explicitly stated otherwise, this should not be understood as an absolute quantification of the role of a node or point in the network, but as a convenient way of discussing the role of different nodes or points with respect to each other. Thus, for example, the discussion regarding macro and pico cells may be equally applicable to interactions between micro cells and femto cells.

One purpose of deploying low power nodes, such as pico base stations, in a macro coverage area is to improve system capacity through cell division gain. In addition to improving overall system capacity, this approach also allows users to be provided with a wide area experience of ultra-high speed data access throughout the network. Heterogeneous deployments are particularly effective for covering traffic hotspots, i.e. small geographical areas with high user density. These areas can be served by pico cells as an alternative deployment to more dense macro networks.

The most basic means of operating a heterogeneous network is to apply frequency separation between different layers. For example, the macro cell 110 and the pico cell 120 shown in fig. 1 can be configured to operate on different non-overlapping carrier frequencies, thus avoiding any interference between layers. By not having macro-cell interference towards underlay cells, cell splitting gain is achieved when all resources can be used simultaneously by underlay cells.

One disadvantage of operating layers on different carrier frequencies is that it can lead to inefficiencies in resource utilization. For example, if there is low level activity in the pico cell, it may be more efficient to use all carrier frequencies in the macro cell and then essentially turn off the pico cell. However, the partitioning of the carrier frequencies across layers in this basic configuration is typically done in a static manner.

Another way to operate a heterogeneous network is to share radio resources between layers. Thus, by coordinating transmissions across the macro cell and underlay cells, two (or more) tiers can use the same carrier frequency. This type of coordination is referred to as inter-cell interference coordination (ICIC). In this way, some radio resources are allocated to the macro cell for a given time period, while the remaining resources can be accessed by the underlay cells without interference from the macro cell. This resource partitioning can change over time to accommodate different service requirements, depending on the cross-layer service conditions. This way of sharing radio resources across layers can be made more or less dynamic depending on the implementation of the interface between the nodes, in contrast to the static allocation of carrier frequencies described previously. In LTE, for example, an X2 interface is specified in order to exchange different types of information between base station nodes for coordination of resources. An example of such an information exchange is that the base station can inform other base stations that it will reduce the transmit power on certain resources.

Time synchronization between base station nodes is typically required in order to ensure that cross-layer ICIC will operate efficiently in heterogeneous networks. This is particularly important for time domain based ICIC schemes, where resources are shared in time on the same carrier.

Orthogonal Frequency Division Multiplexing (OFDM) technology is a key fundamental component of LTE. As is well known to those skilled in the art, OFDM is a digital multicarrier modulation scheme that employs a large number of closely spaced orthogonal subcarriers. Each subcarrier is individually modulated using conventional modulation techniques and channel coding schemes. In particular, 3GPP specifies Orthogonal Frequency Division Multiple Access (OFDMA) for downlink transmissions from a base station to a mobile terminal and single carrier frequency division multiple access (SC-FDMA) for uplink transmissions from a mobile terminal to a base station. Both multiple access schemes permit the available subcarriers to be allocated among several users.

SC-FDMA techniques employ specially formed OFDM signals and are therefore often referred to as "precoded OFDM" or Discrete Fourier Transform (DFT) spread OFDM. Although similar in many respects to conventional OFDMA techniques, SC-FDMA signals provide reduced peak-to-average power ratio (PAPR) compared to OFDMA signals, thus allowing the transmitter power amplifier to be operated more efficiently. This in turn promotes more efficient use of the limited battery resources of the mobile terminal. (SC-FDMA is described more fully in "Single Carrier FDMA for Uplink Wireless Transmission" by Myung et al (IEEE Vehicular technology Magazine, Vol.1, No.3, 9.2006, pages 30-38)).

The basic LTE physical resource can be seen as a time-frequency grid. This concept is illustrated in fig. 2, which fig. 2 shows a plurality of so-called subcarriers divided in the time domain into the frequency domain with a frequency spacing Δ f of the OFDM symbol intervals. The individual elements of resource grid 210 are referred to as resource elements 220 and correspond to one subcarrier during one OFDM symbol interval on a given antenna port. One of the unique aspects of OFDM is that each symbol 230 begins with a cyclic prefix 240, the cyclic prefix 240 being essentially a reproduction of the last portion of the symbol 230 fixed to the beginning. This feature minimizes problems from multipath for a large number of radio signal environments.

In the time domain, LTE downlink transmissions are organized into radio frames, typically ten milliseconds each, each consisting of ten equally sized subframes of one millisecond duration. This is shown in fig. 3, where an LTE signal 310 includes several frames 320, each divided into ten subframes 330. Not shown in fig. 3, each sub-frame 330 is further divided into two slots, each of which has a duration of 0.5 ms.

The LTE link resource is organized as a "resource block" defined as a time-frequency block having a duration of 0.5 milliseconds corresponding to one slot and containing a bandwidth of 180 kHz corresponding to 12 contiguous subcarriers having a spacing of 15 kHz. Resource blocks are numbered in the frequency domain, starting with 0 from one end of the system bandwidth. Two time-consecutive resource blocks represent a resource block pair and correspond to a time interval according to which scheduling operates. Of course, the exact definition of resource blocks may vary between LTE and similar systems, and the inventive methods and apparatus described herein are not limited to the numbers used herein.

However, in general, resource blocks may be dynamically assigned to mobile terminals and may be assigned independently for the uplink and downlink. Depending on the data throughput needs of the mobile terminal, the system resources allocated thereto may be increased by allocating resource blocks across several sub-frames or across several frequency blocks, or both. Thus, the instantaneous bandwidth allocated to the mobile terminal during scheduling can be dynamically adapted in response to varying conditions.

For scheduling of downlink data, the base station transmits control information in each subframe. This control information identifies the mobile terminal to which the data is directed and the resource blocks in the current downlink subframe carrying the data for each terminal. The first, two, three or four OFDM symbols in each subframe are used to carry this control signaling. In fig. 4, a downlink subframe 410 is shown in which three OFDM symbols are allocated to a control region 420. The control region 420 is primarily comprised of a control data unit 434, but also includes a plurality of reference symbols 432 used by the receiving station to measure channel conditions. These reference symbols 432 are interspersed between predetermined locations throughout the control region 420 and data symbols 436 in the data portion 430 of the sub-frame 410.

Transmissions in LTE are dynamically scheduled in subframes, where a base station transmits downlink assignments/uplink grants to certain mobile terminals (user equipments or UEs in 3GPP terminology) via a Physical Downlink Control Channel (PDCCH). The PDCCH is transmitted in the control region of the OFDM signal, i.e., in the first OFDM symbol of each subframe, and spans all or almost all of the entire system bandwidth. A UE that decodes a downlink assignment carried by the PDCCH knows which resource elements in the subframe contain data for that particular UE. Similarly, upon receiving an uplink grant, the UE knows on which time-frequency resources it should transmit. In the LTE downlink, data is carried by a Physical Downlink Shared Channel (PDSCH), and in the uplink, the corresponding channel is called a Physical Uplink Shared Channel (PUSCH).

LTE also employs multiple modulation formats, including at least QPSK, 16-QAM, and 64-QAM, as well as advanced coding techniques so that data throughput can be optimized for any of a variety of signal conditions. Depending on the signal conditions and the desired data rate, an appropriate combination of modulation format, coding scheme, and bandwidth is selected, typically to maximize system throughput. Power control is also used to ensure an acceptable error rate while minimizing interference between cells. In addition, LTE uses a hybrid arq (harq) error correction protocol, in which, after receiving downlink data in a subframe, the terminal attempts to decode it and reports to the base station whether the decoding was successful (ACK) or unsuccessful (NACK). In case of unsuccessful decoding attempts, the base station is able to retransmit the erroneous data.

Disclosure of Invention

In several embodiments of the present invention, channel state information reference symbols (CSI-RS) are transmitted using different CSI-RS resources on different transmission points in the same cell, while the configuration of CSI-RS measurement resources is done on a UE-specific basis. The selection of measurement resources to be used is determined by the network based on the nature of the channel from the transmission point to the UE of interest. As the UE moves around in the cell, the network tracks the channel properties and reconfigures the CSI-RS resources measured by the UE to correspond to the resources of one or more "closest" transmission points.

More specifically, a method is provided for collecting Channel State Information (CSI) feedback in a wireless network cell containing a heterogeneous deployment of transmission points, i.e., a plurality of geographically separated transmission points including a shared cell identifier, i.e., a primary transmission point having a first coverage area and one or more secondary transmission points each having a corresponding coverage area at least partially within the first coverage area. The methods may be performed at one or more nodes in the radio access network, e.g. at a control node connected to each transmission point via a signalling interface.

In one example method, a network receives CSI feedback from a mobile station, the CSI feedback based on measurements of first CSI reference symbols (CSI-RS) transmitted on first CSI-RS resources from a first one of transmission points. The network then detects that the mobile station has approached a second one of the transmission points that is different from the first one of the transmission points. In some cases, such detection is performed by measuring one or more uplink transmissions from the mobile station at a second one of the transmission points and evaluating channel strength based on the measurements. The measured uplink transmissions may include, for example, one or more of Sounding Reference Signals (SRS), Physical Uplink Control Channel (PUCCH) transmissions, and Physical Uplink Shared Channel (PUSCH) transmissions.

The network then reconfigures the mobile station to measure the CSI-RS on the second CSI-RS resource; the CSI-RSs are transmitted from a second one of the transmission points. Finally, the network again receives CSI feedback from the mobile station, this time based on measurements of a second CSI-RS transmitted on a second CSI-RS resource from a second one of the transmission points.

In some embodiments, for example in a CoMP scenario, the network can configure the mobile station to also measure CSI-RS on a third CSI-RS resource transmitted from a third one of the transmission points at the same general time that CSI-RS is also transmitted from the second transmission point. In this case, the CSI feedback received from the mobile station is also based on measurements of the third CSI-RS and is thus useful for characterizing the composite channel between the mobile station and two (or more) different transmission points.

Another example process for collecting Channel State Information (CSI) feedback begins with transmitting CSI-RSs from all several points in a cell for a given time interval. In many cases, the CSI-RS will be transmitted approximately simultaneously from all points in the cell, i.e. from all points sharing the same cell-id, but this is not strictly necessary.

A subset of CSI-RS resources for CSI feedback from the mobile station is selected based on channel strength measurements corresponding to the mobile station and the transmission points. The mobile station is then configured to provide at least CSI feedback of the selected CSI-RS resource for use in evaluating a channel between the mobile station and one or several of the transmission points.

In some cases, selecting the subset of CSI-RS resources is based on measurements of uplink transmissions at one or more of the transmission points. This measured uplink transmission may again include one or more of a Sounding Reference Signal (SRS), a Physical Uplink Control Channel (PUCCH) transmission, and a Physical Uplink Shared Channel (PUSCH) transmission. In other cases, the subset of CSI-RS resources is based on measurement data sent by the mobile station to one or more of the transmission points. In still other cases, a combination of both information sources may be used.

Also described are devices for performing the various processes described herein, including systems of transmitting nodes, corresponding control units and corresponding mobile stations in a wireless network. Of course, the present invention is not limited to the above features and advantages. Indeed, those skilled in the art will recognize additional features and advantages of the invention upon reading the following detailed description, and upon viewing the accompanying drawings.

Drawings

Figure 1 shows several pico cells covered by macro cells.

Fig. 2 shows an OFDM time-frequency resource grid.

Fig. 3 shows a time domain structure of an LTE signal.

Fig. 4 shows features of an LTE downlink subframe.

Fig. 5 shows the mapping of CSI-RS to LTE resource grid for two, four and eight antenna ports.

Fig. 6 shows the difference between uplink and downlink coverage in a mixed cell scenario.

Fig. 7 illustrates the use of inter-cell interference coordination in downlink subframes of a heterogeneous network.

Figure 8 shows a heterogeneous cell deployment with individual cell identification for each point.

Figure 9 illustrates a heterogeneous cell deployment in which cell identities are shared between macro and pico points in the coverage area of the macro point.

Fig. 10 is a process flow diagram illustrating a method for collecting channel state information feedback in a heterogeneous cell deployment.

Fig. 11 is a process flow diagram illustrating another method for collecting channel state information feedback in a heterogeneous cell deployment.

Fig. 12 is a block diagram illustrating features of a node in a heterogeneous cell deployment.

Fig. 13 is a process flow diagram illustrating a method for providing channel state information in a heterogeneous cell deployment.

Fig. 14 is a block diagram illustrating components of an example mobile station.

Detailed Description

Various embodiments of the present invention are now described with reference to the drawings, wherein like reference numerals are used to refer to like elements throughout. In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of one or more embodiments. It will be apparent, however, to one skilled in the art that some embodiments of the invention may be practiced or carried out without one or more of these specific details. In other instances, well-known structures and devices are shown in block diagram form in order to facilitate describing the embodiments.

Note that although terminology from the 3GPP LTE and LTE-advanced specifications is used throughout this document to illustrate the present invention, this should not be taken as limiting the scope of the invention to only these systems. Other wireless systems that include or are adapted to include heterogeneous cell deployments may also benefit from utilizing the concepts contained herein.

Demodulation of transmitted data typically requires an estimate of the radio channel. In LTE systems, this is done using transmitted Reference Symbols (RS), i.e. transmitted symbols having values that are already known to the receiver. In LTE, cell-specific reference symbols (CRS) are transmitted in downlink subframes. In addition to aiding downlink channel estimation, CRS is also used for mobility measurements performed by UEs.

CRS is generally intended for use by mobile terminals in the coverage area. To support improved channel estimation, especially when multiple-input multiple-output (MIMO) transmission techniques are used, LTE also supports UE-specific RSs, which are directed to individual mobile terminals and are intended for channel estimation specifically for demodulation.

Fig. 4 shows how the mapping of physical control/data channels and signals can be done on resource elements in a downlink subframe 410. In the illustrated example, the PDCCH occupies only the first of the three possible OFDM symbols that make up the control region 420, so in this particular case the mapping of data can start at the second OFDM symbol. Since the CRS is common to all UEs in a cell, the transmission of the CRS cannot be easily adapted to suit the needs of a particular UE. This is in contrast to UE-specific RS, by which each UE is able to put its own RS in the data region 430 of fig. 4 as part of the PDSCH.

The length of the control region (one, two, or three symbols) for carrying the PDCCH can vary from subframe to subframe and is signaled to the UE in a Physical Control Format Indicator Channel (PCFICH). The PCFICH is transmitted in a control region at a location known to the terminal. Once the terminal decodes the PCFICH, it knows the size of the control region and at which OFDM symbol data transmission starts. Also transmitted in the control region is a physical hybrid ARQ indicator channel. This channel carries an ACK/NACK response to the terminal in order to inform the mobile terminal whether the uplink data transmission in the previous subframe was successfully decoded by the base station.

As mentioned above, CRS are not the only reference symbols available in LTE. According to LTE release 10, a new RS concept is introduced. Release 10 supports separate UE-specific RS for demodulation of PDSCH, as is provided specifically for measuring channels to generate Channel State Information (CSI) feedback from the UE. The latter type of reference symbols is referred to as CSI-RS. CSI-RSs are not transmitted in every subframe and they are generally more sparse in time and frequency than RSs used for demodulation. CSI-RS transmissions may occur every 5 th, 10 th, 20 th, 40 th or 80 th subframe, as determined by a periodicity parameter and a subframe offset, each of which is configured by Radio Resource Control (RRC) signaling.

A UE operating in connected mode can be requested by a base station to perform Channel State Information (CSI) reporting. This reporting can include, for example, reporting the appropriate Rank Indicator (RI) and one or more Precoding Matrix Indices (PMIs) and Channel Quality Indicators (CQIs) for a given observed channel condition. Other types of CSI are also contemplated, including explicit channel feedback and interference covariance feedback. The CSI feedback helps the base station with scheduling, including deciding which subframes and resource blocks to use for transmission and which transmission scheme and/or precoder should be used. The CSI feedback also provides information that can be used to determine the appropriate user bit rate for transmission, i.e., link adaptation.

In LTE, periodic and aperiodic CSI reporting is supported. In case of periodic CSI reporting, the terminal reports CSI measurements using a Physical Uplink Control Channel (PUCCH) based on a configured periodic time. For aperiodic reporting, CSI feedback is transmitted on a Physical Uplink Shared Channel (PUSCH) at a pre-specified time after receiving a CSI grant from a base station. With aperiodic CSI reporting, the base station can thus request CSI reflecting downlink radio conditions in a particular subframe.

A detailed illustration of which resource elements in a resource block pair can potentially be occupied by new UE-specific RSs and CSI-RSs is provided in fig. 5 for the case of two, four and eight transmitter antenna ports for CSI transmission. The CSI-RS superimposes two antenna ports on two consecutive resource elements with orthogonal cover codes of length two. In other words, the CSI-RS are allocated in pairs, where two orthogonal codes of length two are transmitted simultaneously from a pair of antenna ports of the base station using the same pair of allocated resource elements.

In fig. 5, the CSI-RS resource elements are assigned numbers corresponding to antenna port numbers. In the left diagram corresponding to the case of two CSI-RS antenna ports, the possible locations of the CSI-RS are labeled as "0" and "1" corresponding to antenna ports 0 and 1.

As can be seen in fig. 5, many different CSI-RS patterns are available. For the case of two CSI-RS antenna ports, for example, where each CSI-RS pair is individually configurable, there are 20 different patterns in the subframe. When there are four CSI-RS antenna ports, two are assigned to a CSI-RS pair at a time, and thus the number of possible patterns is ten. For the case of eight CSI-RS antenna ports, five patterns are available. For TDD mode, some additional CSI-RS patterns are available.

In the following discussion, the term "CSI-RS resource" is used. The CSI-RS resource corresponds to a specific pattern present in a specific subframe. Thus, two different patterns in the same subframe constitute two different CSI-RSI resources. Likewise, the application of the same CSI-RS pattern to two different subframes again represents two separate instances of CSI-RS resources, and thus the two instances are again considered to be different CSI-RS resources.

Any of the various CSI-RS patterns shown in fig. 5 may also correspond to so-called zero-power CSI-RS, also referred to as muted REs. The zero-power CSI-RS is a CSI-RS pattern whose resource elements are silent, i.e., there is no transmitted signal on those resource elements. These silence patterns are configured with a resolution corresponding to a four antenna port CSI-RS pattern. Thus, the minimum unit of configurable silence corresponds to four REs.

The purpose of the zero-power CSI-RS is to boost the signal-to-interference-and-noise ratio (SINR) of the CSI-RS in a given cell by configuring the zero-power CSI-RS in the interfering cell such that the otherwise interfering resource elements are silent. Thus, the CSI-RS pattern in a given cell matches the corresponding zero-power CSI-RS pattern in the interfering cell.

Boosting the SINR level of CSI-RS measurements is particularly important in applications such as coordinated multipoint (CoMP) or in heterogeneous deployments. In CoMP, the UE may need to measure channels from non-serving cells. Interference from a much stronger serving cell can make those measurements difficult-if not impossible. Zero-power CSI-RS is also needed in heterogeneous deployments, where the zero-power CSI-RS in the macro layer is configured to coincide with CSI-RS transmissions in the pico layer. This avoids strong interference from the macro node when the UE measures the channel of the pico node.

The PDSCH carrying data to the mobile station is mapped around the resource elements occupied by the CSI-RS and the zero-power CSI-RS, so it is important that both the network and the UE adopt the same CSI-RS and zero-power CSI-RS configurations. Otherwise, the UE may not be able to decode the PDSCH in the subframe containing the CSI-RS or its zero-power peer.

The above-mentioned CSI-RS is used for measurement of a downlink channel, i.e., a channel from a base station to a mobile terminal. In the uplink, so-called Sounding Reference Symbols (SRS) may be used to acquire CSI on the uplink channel from the UE to the receiving node. When SRS is used, it is transmitted on the last DFT-spread OFDM symbol of the subframe. The SRS can be configured for periodic transmission and dynamically triggered as part of an uplink grant. The primary purpose of SRS is to aid in uplink scheduling and link adaptation. However, for Time Division Duplex (TDD) LTE systems, SRS is sometimes used to determine the beamforming (beamforming) weights for the downlink by exploiting the fact that the downlink and uplink channels are the same when the same carrier frequency is used for both downlink and uplink (channel reciprocity).

When the PUSCH carries data of the uplink, the PUCCH is used for control. The PUCCH is a narrowband channel using a resource block pair, where two resource blocks are on opposite sides of the potential scheduling bandwidth. The PUCCH is used to convey ACK/NACK, periodic CSI feedback, and scheduling requests to the network.

Before an LTE terminal can communicate with an LTE network, it first has to find a cell in the network and acquire synchronization with the cell, a process known as cell search. The UE must then receive and decode the system information needed to communicate with and operate properly in the cell. Finally, the UE can access the cell through a so-called random access procedure.

To support mobility, a terminal needs to continuously search its serving cell and neighbor cells, synchronize with them, and estimate their reception quality. The reception quality of the neighboring cell, relative to the reception quality of the current cell, is then evaluated in order to determine whether a handover (for connected mode terminals) or cell reselection (for idle mode terminals) should be performed. For connected mode terminals, the handover decision is made by the network based on the measurement report provided by the terminal. Examples of such reports are Reference Signal Received Power (RSRP) and Reference Signal Received Quality (RSRQ).

The results of these measurements, possibly supplemented by configurable offsets, can be used in several ways. The UE can for example connect to the cell with the strongest received power. Alternatively, the UE can be assigned to the cell with the best path gain. Something in between these alternatives may be used.

These selection strategies do not necessarily result in the same selected cell for any given set of cases, since the base station output power is different for different types of cells. This is sometimes referred to as link imbalance. For example, the output power of a pico base station or relay node is typically about 30 dBm (1 watt) or less, while a macro base station can have an output power of 46 dBm (40 watts). Therefore, even in the vicinity of the pico cell, the downlink signal strength from the macro cell can be greater than the pico cell. From a downlink point of view it is generally better to select a cell based on the downlink received power, whereas from an uplink point of view it is better to select a cell based on the pathloss.

These alternative cell selection approaches are shown in fig. 6. The solid lines originating from each of the macro cell 110 and the pico cell 120 represent the received power at each point between the two cells. These lines intersect, i.e., are equal, at boundary 540. Accordingly, a UE in area 510 will see a stronger received signal from the pico cell 120 and will obtain the best downlink performance when it selects the pico cell 120. On the other hand, the dotted lines originating from the pico cell 120 and the macro cell 110 represent the path loss between the UE at a given point and the macro cell 110 or the pico cell 120. Because the path loss is not weighted by the transmitter output power, the lines intersect at some point halfway between the macrocell 110 and the picocell 120, as seen at the boundary 530. A UE outside the area 520 would then experience a lower path loss to the macro cell 110 than to the pico cell 120 and would thus achieve better uplink performance when it selects the macro cell 110. Due to this unbalanced situation, there is a portion of coverage area 520 outside of coverage area 510 where no cell is optimal for both downlink and uplink performance.

From a system perspective, it may often be better to have a given UE connect to the pico cell 120 in the above-described situations, even in certain situations where the downlink from the macro cell 110 is much stronger than the pico cell downlink. However, when the terminal operates in a region between uplink and downlink boundaries, i.e., a link imbalance region as shown in fig. 6, cross-layer ICIC will be required. Interference coordination across cell layers is particularly important for downlink control signaling. A terminal in the region between the downlink and uplink boundaries of fig. 6 and connected to the pico cell 120 may not be able to receive downlink control signaling from the pico cell 120 if the interference is not handled properly.

One way to provide cross-layer ICIC is shown in fig. 7. Interfering macro cells, which may cause downlink interference towards pico cells, transmit a series of subframes 710, but avoid scheduling unicast traffic in certain subframes 712. In other words, neither PDCCH nor PDSCH is transmitted in those subframes 712. In this way, it is possible to create low interference subframes that can be used to protect users of pico cells operating in areas of link imbalance.

To perform this approach, the macro base station (MeNB) indicates to the pico base station (PeNB) via backhaul interface X2 which subframes will not be used for scheduling users. The PeNB can then take this information into account when scheduling users operating in the link imbalance region, so that these users are scheduled only in subframes 722 aligned with low-interference subframes transmitted in the macro layer. In other words, these users are scheduled only in interference protected subframes. Picocell users operating in a downlink boundary, such as coverage area 510 of fig. 6, can be scheduled in all subframes, i.e., in protected subframes 722 and the remaining unprotected subframes in the series of subframes 720.

In general, by ensuring that the scheduling decisions in the two cell layers do not overlap in the frequency domain, data transmissions (but not control signaling) in different layers may also be separated in the frequency domain. This may be facilitated by exchanging coordination messages between different base stations. However, this is not possible for control signaling, since according to the LTE specification, control signaling spans the full bandwidth of the signal and therefore the time domain approach must be used.

A typical way to deploy a network is to have each different transmission/reception point provide a cell with different coverage than all other cells. That is, a signal transmitted from or received at a certain point is associated with a cell identifier (cell-id) that is different from cell identities for other nearby points. Typically, each of these points transmits its own unique signal for broadcast (PBCH) and synchronization channels (PSS, SSS).

The concept of "points" is largely used in conjunction with techniques for coordinated multipoint (CoMP). In this context, a point corresponds to a set of antennas that cover 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 having one or more antennas all intended to cover a similar geographical area. Different points often represent different sites. When the antennas are sufficiently geographically separated and/or have antenna patterns pointing in sufficiently different directions, they correspond to different points. From a scheduling perspective, techniques for CoMP need to introduce correlation in scheduling or transmission/reception between different points, in contrast to conventional cellular systems that operate a certain point more or less independent of other points.

In fig. 8, a typical strategy of one cell-id per point is shown for a heterogeneous deployment where multiple low-power (pico) points 120 are placed in the coverage area of a higher-power macro point 110. In this deployment, the pico node transmits a different cell identifier than the cell identifier "cell-id 1" transmitted by the macro cell 110, i.e., "cell-id 2", "cell-id 3", and "cell-id 4". Note that similar principles are clearly also applicable to typical macro-cellular deployments, where all points have similar output power and are perhaps placed in a more conventional manner than is the case for heterogeneous deployments.

One alternative to a typical deployment strategy is to instead have all UEs in the geographical area depicted by the coverage of the high power macro point provided with signals associated with the same cell-id. In other words, from the UE perspective, the received signals behave as if they came from a single cell. This is shown in fig. 9. Here, all pico nodes 120 transmit the same cell identifier "cell-id 1" that is also used by the overlay macro cell 110.

Note that in both fig. 8 and 9, only one macro-point is shown; other macro points typically use different cell-ids (corresponding to different cells) unless they are co-located at the same site (corresponding to other sectors of the macro site). In the latter case of several concurrent macro points, the same cell-id may be shared across the concurrent macro points and those pico points corresponding to the union of the coverage areas of the macro points. The synchronization, BCH, and control channels are transmitted from the high power point, while data can be transmitted to the UE from the low power point as well by using a shared data transmission (PDSCH) depending on the UE specific RS.

This approach has beneficial effects for those UEs that are capable of receiving PDSCH based on UE-specific RS, while UEs that only support CRS of PDSCH must be satisfied to use only transmissions from high power points, and thus will not benefit from deployment of additional low power points on the downlink. This latter group may include at least all release 8 and 9 UEs used in FDD LTE systems.

The single cell-id approach to heterogeneous and/or hierarchical cell deployment is suitable for situations where fast backhaul communication exists between points associated with the same cell identifier. A typical scenario is where a base station serves one or more sectors of the macro-level and has a fast fiber connection to Remote Radio Units (RRUs) performing the role of other points sharing the same cell-id. Those RRUs may represent low power points each having one or more antennas. Another example is when all points have similar power classes, where no single point has greater importance than the other points. The base station would then process the signals from all RRUs in a similar manner.

One significant advantage of the shared cell approach over the typical approach is that handover procedures between cells need only be invoked on a macro basis. Another important advantage is that interference from CRS can be greatly reduced, since CRS need not be transmitted from every point. There is also more flexibility in the coordination and scheduling between points, which means that the network can avoid the inflexible concept of relying on semi-statically configured low interference subframes as shown in fig. 7. The shared cell approach also allows the downlink to be separated from the uplink so that, for example, path loss based selection of reception points can be performed for the uplink without causing severe interference problems for the downlink, where the UE can be served by a different transmission point than the one used in uplink reception.

However, the shared cell-id approach presents some problems when CSI is involved. A single cell may now contain a large number of antennas, many more than one to eight transmit antennas of the LTE process for which CSI feedback is designed. In addition, the overhead due to transmitting CSI-RS tends to become large for many antennas used by a cell.

Furthermore, even when there are eight or fewer antennas sharing the same cell, the distributed arrangement of these antennas creates a composite channel to the UE that has the property of being a poor match to the design assumptions for the normal CSI feedback process, which are designed to match the channel characteristics that result when the antennas are limited to a single transmission point.

To address these issues, in several embodiments of the present invention, CSI-RS are transmitted using different CSI-RS resources on different transmission points in the same cell, while the configuration of CSI-RS measurement resources is done on a UE-specific basis, with the selection of one or more measurement resources being determined by the network based on the properties of the channel from the transmission point to the UE of interest. As the UE moves around in the cell, the network tracks the channel properties and reconfigures the CSI-RS resources measured by the UE to correspond to the resources of one or more "closest" transmission points.

At a high level, a system configured according to some embodiments of the invention includes the following features. First, a set of points are associated with the same cell, since signals transmitted from any one point in the set are associated with the same cell-id. For example, a given cell in a system may include two or more low power points in addition to a high power point. As previously mentioned, the coverage areas of the low power points may overlap or fall entirely within the coverage area of the high power point. A control node in the network configures a given UE operating in a cell to measure channel properties based on CSI-RS resources transmitted from one of the points when sufficiently close to that point. Here, "close" refers to the radio sense, because UEs close to the transmission point receive the transmitted signal well. Of course, "close" in a radio sense will typically, but not necessarily, coincide with "close" in a geographic sense.

However, when the UE moves closer to another point, the network configures the UE to instead measure the channel properties based on the CSI-RS transmitted from that other point using different CSI-RS resources. The selection of which CSI-RS resource (or resources) the UE should use at any given time may be made by the network based on measurements on the uplink signal (e.g., SRS, PUSCH, PUCCH, etc.) or based on CSI feedback from the UE (or other UEs), or some combination of both.

In some cases, for example in a coordinated multipoint (CoMP) scenario, a UE may be expected to measure CSI-RS resources corresponding to multiple points. In this case, the same general procedure is used, but it is mentioned in the discussion immediately above that the single "points" may each be replaced by a "set of points".

In more detail, for a CSI feedback mode using CSI-RS, the UE can be configured through higher layer signaling from the network in order to determine CSI-RS resources on which to measure. In various embodiments of the present invention, this configuration is UE-specific. Typically, the configuration of CSI-RS is performed in a cell-specific manner, such that all UEs served by the same cell obtain the same configuration and all UEs use the same CSI-RS resources for measurements. However, in the case of shared cell-id, UE measurements for CSI feedback need to be carefully controlled from the network in order to solve the CSI problem. Effective network control is achieved by configuring the CSI-RS in a UE-specific manner depending on which point or points in the cell contribute significantly to the received signal for a given UE.

For example, each transmission point may transmit using its own CSI-RS resource (as given by the CSI-RS pattern in the subframe, periodicity, and subframe offset). As the UE approaches a particular transmission point, the relative strength of the channel from the different transmission points to the UE is evaluated. Based on this evaluation, the network decides when to reconfigure the UE to measure CSI-RS on the particular CSI-RS being used by the particular transmission point. The network may obtain the channel strength from measurements of uplink signals including SRS, PUCCH, PUSCH or from multiple CSI-RS resource CSI feedback if such feedback is supported in LTE.

Thus, the CSI-RS resources for which measurements are to be made are configured by the network in a UE-specific manner in the cell, such that the selected resources are determined primarily based on the transmission point that each UE hears best. As the UE moves between transmission points, the network tracks the channel properties and reconfigures the CSI-RS resources for the UE to correspond to the resources that are "closest" to the transmission points.

This CSI-RS reconfiguration procedure may also be applicable when CoMP is employed. In order to support efficient coordination between points, the UE needs to feed back CSI corresponding to channels formed between the UE and the plurality of transmission points. As one example, the UE may be configured such that CSI corresponding to two or three strongest channels or transmission points is fed back. Instead of configuring only one CSI-RS resource for the UE of interest, the network now needs to configure multiple CSI-RS measurement resources in the cell. The network needs to monitor the radio conditions of the point associated with the UE and when the radio conditions of the UE change, the network reconfigures one or more resources with the aim that the UE makes measurements of the point of interest (i.e. the point that the UE hears sufficiently well). As for the non-CoMP CSI-RS case, measurements of the uplink signal and its strength at different reception points may be used as a basis for decision on the CSI-RS resource measurement set.

Alternatively, the UE may be configured to make measurements on a larger set of CSI-RS resources, after which a subset of those CSI-RS resources is selected for actual CSI feedback. Thus, the optimal CSI-RS resource measurement subset is determined by a larger set of actual measurements. This measurement on a larger set is of course performed by the UE. However, the selection of the best set of CSI-RS measurements for evaluating the channel conditions can be performed by the UE or by the network. In the latter case, the UE sends measurements corresponding to a larger set of CSI-RS resources to the network, which then instructs the UE on which CSI-RSs to measure. In the former case, the UE needs to send CSI for only a smaller set of resources.

Using the techniques disclosed herein can help ensure that effective CSI feedback is obtained from the UE that matches the channel characteristics. Without these transmission and configuration strategies, the UE may feed back CSI that is poorly matched to the main part of the channel from the transmission point, causing a loss of array gain and difficulties in performing multi-user MIMO scheduling.

Given the above details of UE-specific configuration of CSI-RS resources, it will be appreciated that fig. 10 and 11 illustrate a generalized process for collecting Channel State Information (CSI) feedback in a wireless network cell containing a heterogeneous deployment of transmission points, i.e., including a plurality of geographically separated transmission points sharing a cell identifier, and including a primary transmission point having a first coverage area and one or more secondary transmission points each having a corresponding coverage area at least partially within the first coverage area. The illustrated method is performed at one or more nodes in the radio access network, e.g. at a control node connected to each transmission point via a signalling interface.

As shown in block 1010 of fig. 10, the network receives CSI feedback from the mobile station based on measurements of first CSI reference symbols (CSI-RS) transmitted on first CSI-RS resources from a first one of the transmission points. The network then detects that the mobile station has approached a second one of the transmission points that is different from the first one of the transmission points. This is shown as block 1020. Although several techniques for detecting that a mobile station is approaching a particular transmission point are possible, the term "close" is generally intended herein to mean that the mobile station is close in the radio signal sense, as the radio channel between the mobile station and the transmission point is improving. Thus, in some cases, such detection may be performed by measuring one or more uplink transmissions from the mobile station at a second one of the transmission points and evaluating the channel strength based on the measurements. The measured uplink transmissions may include one or more of Sounding Reference Signals (SRS), Physical Uplink Control Channel (PUCCH) transmissions, and Physical Uplink Shared Channel (PUSCH) transmissions.

The network then reconfigures the mobile station to measure the CSI-RS on the second CSI-RS resource, as shown at block 1030; these CSI-RSs are transmitted from a second one of the transmission points, as shown at block 1040. Finally, as shown at block 1050, the network again receives CSI feedback from the mobile station, this time based on measurements of a second CSI-RS transmitted on a second CSI-RS resource from a second one of the transmission points.

The process shown in fig. 10 is not limited to measurements of CSI-RS transmitted by only a single transmission point at any given time. In some embodiments, for example in a CoMP scenario, the network can configure the mobile station to also measure CSI-RS on a third CSI-RS resource transmitted from a third one of the transmission points at the same general time that CSI-RS is also transmitted from the second transmission point. In this case, the CSI feedback received from the mobile station is also based on measurements of the third CSI-RS and is thus useful for characterizing the composite channel between the mobile station and two (or more) different transmission points.

Fig. 11 illustrates a closely related process for collecting Channel State Information (CSI) feedback in a wireless network cell similar to that described above. This process begins, as shown at block 1110, with transmitting CSI-RS from all several points in the cell for a given time interval. In many cases, the CSI-RS will be transmitted approximately simultaneously from all points in the cell, i.e. from all points sharing the same cell-id, but this is not strictly necessary.

A subset of CSI-RS resources for CSI feedback from the mobile station is selected based on channel strength measurements corresponding to the mobile station and the transmission point, as shown in block 1120. The mobile station is then configured to provide at least CSI feedback for the selected CSI-RS resource, as shown at block 1130, for use in evaluating a channel between the mobile station and one or more of the transmission points.

In some cases, selecting the subset of CSI-RS resources is based on measurements of uplink transmissions at one or more transmission points. This measured uplink transmission may again include one or more of a Sounding Reference Signal (SRS), a Physical Uplink Control Channel (PUCCH) transmission, and a Physical Uplink Shared Channel (PUSCH) transmission. In other cases, the subset of CSI-RS resources is based on measurement data sent by the mobile station to one or more transmission points. In still other cases, a combination of both information sources may be used.

Other embodiments of the inventive techniques disclosed herein include wireless systems, including a primary node and one or more secondary nodes, corresponding to the methods and techniques described above. In some cases, the above-described methods/techniques will be implemented in a system of transmitting nodes, such as shown in detail in fig. 12.

The system shown in figure 12 includes a macro node 110, two pico nodes 120, a UE 130, and an O & M node 190. The macro node 110 is configured to communicate with the pico node 120 and the O & M node 190 via an inter-base station interface 1204, the interface 1204 comprising appropriate network interface hardware controlled by software executing a network interface protocol. Macro node 110 includes a receiver 1202 and a transmitter 1206 that communicate with UE 130; in some cases, the receiver 1202 may also be configured to monitor and/or measure signals transmitted by the pico node 120. The receiver circuitry 1202 and the transmitter circuitry 1206 use known radio processing and signal processing components and techniques that are typically in accordance with a particular telecommunications standard, such as the 3GPP LTE-advanced standard. Since various details and engineering tradeoffs associated with the design of interface circuits and radio transceiver circuits are well known and not necessary for a complete understanding of the present invention, additional details are not shown herein.

The macro node 110 also includes processing circuitry 1210, the processing circuitry 1210 including one or more microprocessors or microcontrollers and other digital hardware, which may include Digital Signal Processors (DSPs), dedicated digital logic, or the like. Either or both of the microprocessor and the digital hardware may be configured to run program code stored in memory 1220 along with stored radio parameters. Again, because various details and engineering tradeoffs associated with the design of baseband processing circuits of mobile devices and wireless base stations are well known and not required for a complete understanding of the present invention, additional details are not shown herein. However, several functional aspects of the processing circuitry 1210 are shown, including a measurement unit 1212, a control unit 1214, and a configuration unit 1216. Configuration unit 216 controls radio transmitter 1206 to transmit CRS, CSI-RS, PDSCH, etc., under the control of control unit 1214, which control unit 1214 also manages communications with other nodes via inter-BS interface circuitry 1204. The control unit 1214 also evaluates the data obtained from the measurement unit 1212, e.g. channel state information and/or load information, and controls inter-base station communication and transmitter configuration accordingly.

The program code stored in the memory circuit 1220, which 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., includes program instructions for executing one or more telecommunications and/or data communications protocols, as well as instructions for performing all or portions of one or more of the techniques described above in several embodiments. In some embodiments, the radio parameters stored in memory 1220 may include one or more predetermined tables or other data used to support these techniques.

The pico node 120 may comprise very similar components and functional blocks as shown in the macro node 110, with the corresponding control unit being responsible for receiving control instructions from the macro node 110 (or another pico node 120) and configuring the pico node's transmitter circuitry accordingly.

In some embodiments, to facilitate the techniques described above, the macro node 110 may act as a control node because the macro node 110 performs all or part of one of the methods shown in fig. 10 and 11 or variations thereof. In other cases, the pico node 120 may act as a similar control node. In still other embodiments, control functionality may be split between two or more physical nodes that together act as a control node to perform the above-described techniques for collecting channel state information in a heterogeneous cell deployment. Thus, the term "control node" as used herein is not limited to a single piece of equipment at a single physical location, but may also represent a collection of network equipment operating together.

Still further embodiments of the inventive techniques described herein include a mobile station implemented method operating in a heterogeneous cell deployment as described above. In particular, fig. 13 illustrates an example of one such process for providing Channel State Information (CSI) feedback in a wireless network cell that includes a plurality of geographically separated transmission points sharing a cell identifier, the transmission points including a primary transmission point having a first coverage area and one or more secondary transmission points each having a corresponding coverage area wholly or substantially within the first coverage area.

The illustrated method begins, as shown in block 1310, by measuring, in a mobile station, signals transmitted from two or more transmission points in a cell. The generated measurements are sent to at least one transmission point, as shown in block 1320. The mobile station then receives configuration information from the at least one transmission point instructing the mobile station to measure CSI reference symbols (CSI-RS) from two or more CSI-RS resources corresponding to the at least two transmission points. This is shown at block 1330. The mobile station measures CSI-RSs from two or more CSI-RS resources, as shown at block 1340, and sends CSI feedback to at least one transmission point based on the measured CSI-RSs, as shown at block 1350.

An example of a mobile station configured to perform the method of fig. 13 or a variant thereof is shown in fig. 14. Of course, not every detail of the mobile station design is shown, but several components relevant to the current technology are shown. The illustrated device includes radio circuitry 1410 and baseband and control processing circuitry 1420. The radio circuitry 1410 includes receiver circuitry and transmitter circuitry that uses known radio processing and signal processing components and techniques that are typically in accordance with a particular telecommunications standard, such as the 3GPP LTE and/or LTE-advanced standards. Again, because various details and engineering tradeoffs associated with the design and implementation of such circuits are well known and not required for a complete understanding of the present invention, additional details are not shown herein.

The baseband and control processing circuits 1420 include one or more microprocessors or microcontrollers 1430 and other digital hardware 1435 which may include Digital Signal Processors (DSPs), special purpose digital logic, and the like. Either or both of the microprocessor 1430 and the digital hardware 1435 may be configured to execute program code 1445 stored in memory 1440 in conjunction with radio parameters 1450. Again, because various details and engineering tradeoffs associated with the design of baseband processing circuits of mobile devices are well known, additional details are not shown here.

Program code 1445 stored in memory circuitry 1440, which 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., includes program instructions for executing one or more telecommunications and/or data communications protocols as well as instructions for performing one or more of the techniques described above in several embodiments. Radio parameters 1450 include various configuration parameters and parameters determined from system measurements (e.g., channel measurements), and may include configuration data indicating which CSI-RS should be measured and measurement data resulting from such measurements.

Accordingly, in various embodiments of the invention, processing circuitry, such as baseband and control processing circuitry 1420 of fig. 14, is configured to perform one or more of the techniques described above for providing Channel State Information (CSI) feedback in a wireless network cell including a plurality of geographically separated transmission points sharing a cell identifier. As described above, this processing circuitry is configured with suitable program code stored in one or more suitable memory devices that implement one or more of the techniques described herein. Of course, it will be understood that not all of the steps of these techniques need be performed in a single microprocessor or even in a single module.

Examples of several embodiments of the invention are described in detail above with reference to the accompanying drawings of specific embodiments. As it is, of course, not possible to describe every conceivable combination of components or techniques, those of ordinary skill in the art will recognize that the present invention may be implemented in many different ways than those specifically described herein without departing from essential characteristics of the invention. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.

Claims (21)

1. A method for collecting channel state information, CSI, feedback in a wireless network comprising a plurality of geographically separated transmission points sharing a cell identifier, the method comprising receiving (1010) CSI feedback from a mobile station, the CSI feedback being based on measurements of a first CSI-RS transmitted on a first CSI reference symbol, CSI-RS, resource from a first one of the transmission points, characterized in that the method further comprises:

detecting (1020) that the mobile station has approached a second one of the transmission points different from a first one of the transmission points;

reconfiguring (1030) the mobile station to measure CSI-RS on a second CSI-RS resource transmitted from a second one of the transmission points; and

receiving (1050), from the mobile station, CSI feedback based on measurements of a second CSI-RS transmitted on the second CSI-RS resource from a second one of the transmission points.

2. The method of claim 1, wherein the transmission point comprises: a primary high power transmission point having a first coverage area; and one or more secondary low power transmission points each having a coverage area within or substantially within the first coverage area.

3. The method of claim 1 or 2, wherein detecting that the mobile station has approached a second one of the transmission points comprises measuring uplink transmissions from the mobile station at the second one of the transmission points and evaluating channel strength based on the measurements.

4. The method of claim 3, wherein the measured uplink transmissions comprise at least one of Sounding Reference Signals (SRS), Physical Uplink Control Channel (PUCCH) transmissions, and Physical Uplink Shared Channel (PUSCH) transmissions.

5. The method of claim 1 or 2, further comprising:

configuring the mobile station to also measure CSI-RS on a third CSI-RS resource transmitted from a third one of the transmission points; and

transmitting a third CSI-RS from a third one of the transmission points on the third CSI-RS resource concurrently with the transmission of a second CSI reference symbol, CSI-RS, from a second one of the transmission points;

wherein the CSI feedback received from the mobile station is further based on measurements of the third CSI-RS.

6. The method of claim 1 or 2, further comprising:

transmitting (1110) CSI-RS from each of the transmission points using at least one different CSI-RS resource for each of the transmission points;

selecting (1120) a subset of the CSI-RS resources for CSI feedback from a mobile station based on channel strength measurements corresponding to the mobile station and the transmission point; and

configuring (1130) the mobile station to provide CSI feedback for the selected CSI-RS resource.

7. The method of claim 6, wherein selecting the subset of CSI-RS resources is based on measurements of uplink transmissions at one or more of the transmission points.

8. The method of claim 7, wherein the measured uplink transmissions comprise at least one of sounding reference signals, SRS, physical uplink control channel, PUCCH, transmissions and physical uplink shared channel, PUSCH, transmissions.

9. The method of claim 6, wherein selecting the subset of CSI-RS resources is based on measurement data transmitted by the mobile station to the wireless network.

10. A method for providing channel state information, CSI, feedback in a wireless network comprising a plurality of geographically separated transmission points sharing a cell identifier, the method comprising measuring (1310), in a mobile station, a first CSI-RS transmitted on a first CSI reference symbol, CSI-RS, resource from a first one of the transmission points, and sending (1320) measurements from the measurements to the wireless network, characterized in that the method further comprises:

receiving reconfiguration information from the wireless network when approaching a second one of the transmission points that is different from the first one of the transmission points, the reconfiguration information instructing the mobile station to measure CSI-RS on second CSI-RS resources transmitted from the second one of the transmission points;

measuring (1340) CSI-RS transmitted from a second one of the transmission points on the second CSI-RS resource; and

sending (1350) CSI feedback from the mobile station based on measurements of second CSI-RSs transmitted from a second one of the transmission points on the second CSI-RS resource.

11. A control unit for use in a wireless network having a wireless network cell including a plurality of geographically separated transmission points sharing a cell identifier, the control unit comprising:

a network communication circuit (1204) configured to transmit and receive control information to and from a plurality of transmission points; and

a processing circuit (1210) configured to receive channel state information, CSI, feedback from a mobile station, the CSI feedback based on measurements of first CSI-RS transmitted on first CSI reference symbol, CSI-RS, resources from a first one of the transmission points;

characterized in that the processing circuit (1210) is further configured to:

detecting that the mobile station has approached a second one of the transmission points that is different from the first one of the transmission points;

reconfiguring the mobile station to measure CSI-RS on a second CSI-RS resource transmitted from a second one of the transmission points; and

receiving CSI feedback from the mobile station, the CSI feedback based on measurements of a second CSI-RS transmitted on the second CSI-RS resource from a second one of the transmission points.

12. The control unit of claim 11, wherein the processing circuit (1210) is configured to detect that the mobile station has approached a second one of the transmission points by using measurements of uplink transmissions from the mobile station made at the second one of the transmission points and evaluating channel strength based on the measurements.

13. The control unit of claim 12, wherein the measured uplink transmissions comprise at least one of sounding reference signals, SRS, physical uplink control channel, PUCCH, transmissions and physical uplink shared channel, PUSCH, transmissions.

14. The control unit of any of claims 11 to 13, wherein the processing circuit (1210) is further configured to:

configuring the mobile station to also measure CSI-RS on a third CSI-RS resource transmitted from a third one of the transmission points; and

control a third transmission point of the transmission points to transmit a third CSI-RS on the third CSI-RS resource simultaneously with a transmission of a second CSI-RS from a second transmission point of the transmission points;

wherein the CSI feedback received from the mobile station is further based on measurements of the third CSI-RS.

15. The control unit of any of claims 11 to 13, wherein the processing circuit (1210) is further configured to:

control all of the transmission points to transmit CSI-RS, wherein at least one different CSI-RS resource is used by each of the transmission points;

selecting a subset of the CSI-RS resources for CSI feedback from a mobile station based on channel strength measurements corresponding to the mobile station and the transmission point;

configuring the mobile station to provide CSI feedback of the selected CSI-RS resource.

16. The control unit of claim 15, wherein the processing circuit (1210) is configured to select the subset of CSI-RS resources based on measurements of uplink transmissions at one or more of the transmission points.

17. The control unit of claim 16, wherein the measured uplink transmissions comprise at least one of sounding reference signals, SRS, physical uplink control channel, PUCCH, transmissions and physical uplink shared channel, PUSCH, transmissions.

18. The control unit of claim 15, wherein the processing circuit (1210) is configured to select the subset of CSI-RS resources based on measurement data transmitted by the mobile station to one or more of the transmission points.

19. The control unit according to any of claims 11 to 13, wherein the control unit is part of a master transmission node.

20. The control unit of any of claims 11 to 13, wherein the control unit is part of one of the secondary transmission nodes, and wherein the network communication circuitry is further configured to transmit and receive control information to and from the primary transmission node.

21. A mobile station configured to provide channel state information, CSI, feedback in a wireless network comprising a plurality of geographically separated transmission points sharing a cell identifier, the mobile station comprising:

a radio circuit configured to receive a signal transmitted from the wireless network; and

a processing circuit (1420);

the processing circuit (1420) is configured to:

measuring a first CSI-RS transmitted from a first one of the transmission points on a first CSI reference symbol, CSI-RS, resource and received by the radio circuitry;

transmitting measurement data from the measurements to the wireless network using the radio circuit;

characterized in that the processing circuit (1420) is configured to:

when approaching a second one of the transmission points different from a first one of the transmission points, receiving reconfiguration information from the wireless network instructing the mobile station to measure CSI-RS on second CSI-RS resources transmitted from the second one of the transmission points, wherein the first and second CSI-RS resources are associated with the shared cell identifier;

measuring, using the radio circuitry, CSI-RS transmitted from a second one of the transmission points on the second CSI-RS resource; and

sending, using the radio circuitry, CSI feedback based on measurements of a second CSI-RS transmitted on the second CSI-RS resource from a second one of the transmission points.