WO2013064190A1

WO2013064190A1 - Out-of-group interference measurement in wireless communication systems

Info

Publication number: WO2013064190A1
Application number: PCT/EP2011/069370
Authority: WO
Inventors: Matthew Webb; Yiwei Fang; Timothy Moulsley
Original assignee: Fujitsu Limited
Priority date: 2011-11-03
Filing date: 2011-11-03
Publication date: 2013-05-10

Abstract

A communications network comprises a group of two or more in-group antennas associated with one or more base stations and a user equipment, wherein the in-group antennas are arranged to co-operate in downlink transmission to the user equipment; transmission from the in-group antennas is co-ordinated so that their downlink transmission in particular transmission resources is muted; and wherein the user equipment is configured to measure a downlink channel in the muted transmission resources.

Description

Out-Of-Group interference Measurement in Wireless Communication Systems

Field of the Invention

The present invention relates to wireless communication systems, for example systems based on the 3GPP Long Term Evolution (LTE) and 3GPP LTE-A groups of standards. The invention is particularly suitable for application in mobile communications systems (such as LTE or IEEE 802.16 WiMAX systems) which may allow coordinated downlink transmission from more than one co-operating cell (or a single cell with more than one antenna) to a given mobile terminal.

Background of the Invention Wireless communication systems are widely known in which base stations (BSs) communicate with user equipments (UEs) (also called subscriber or mobile stations) within range of the BSs.

The geographical area covered by a base station is generally referred to as a cell, and typically many BSs are provided in appropriate locations so as to form a network covering a wide geographical area more or less seamlessly with adjacent and/or overlapping cells. (In this specification, the terms "system" and "network" are used synonymously). In more advanced systems, the concept of a cell can also be used in a different way: for example to define a set of radio resources (such as a given bandwidth around a carrier centre frequency), with an associated identity which may be used to distinguish one cell from another. The cell identity can be used for example in determining some of the transmission properties of communication channels associated with the cell, such as using scrambling codes, spreading codes and hopping sequences. A cell may also be associated with one or more reference signals (see below), which are intended to provide amplitude and/or phase reference(s) for receiving one or more communication channels associated with the cell. Therefore, it is possible to refer to communication channels associated with a cell being transmitted from or by the cell (in the downlink), or transmitted to a cell (in the uplink), even if the transmission or reception is actually carried out by a base station. Typically, in an FDD system, a downlink cell is linked or associated with a corresponding uplink cell. However, it should be noted that it would in principle be possible to organise a communication system which may have cell-like features without explicit cells being defined. For example, an explicit cell identity may not be needed in all cases. Thus cells are not an essential feature of a wireless network as discussed herein.

Each BS divides its available time and frequency resources in a given cell, into individual resource allocations for the user equipments which it serves. The user equipments are generally mobile and therefore may move among the cells, prompting a need for handovers of radio communication links between the base stations of adjacent cells. A user equipment may be in range of (i.e. able to detect signals from) several cells at the same time, but in the simplest case it communicates with one "serving" cell. For some purposes a BS may also be described as an "access point" or a "transmission point".

Modern wireless communication systems such as LTE and LTE-A are hugely complex and a full description of their operation is beyond the scope of this specification. However, for assisting understanding of the inventive concepts to be described later, some outline will be given of some of the features of LTE which are of particular relevance in the present invention. As an aside, references herein to LTE are taken to include later versions of LTE, such as LTE A, where not specifically excluded. OFDM and OFDMA

OFDM (Orthogonal Frequency Division Multiplexing) is one known technique for transmitting data in a wireless communication system. An OFDM-based communications scheme divides data symbols to be transmitted among a large number of subcarriers, hence the term frequency division multiplexing. Data is modulated onto a subcarrier by adjusting its phase, amplitude, or both phase and amplitude. The "orthogonal" part of the name OFDM refers to the fact that the spacings of the subcarriers in the frequency domain are specially chosen so as to be orthogonal, in a mathematical sense, to the other subcarriers. In other words, they are arranged along the frequency axis such that the sidebands of adjacent subcarriers are allowed to overlap but can still be received without inter-subcarrier interference. As the user equipments will receive the same signals at slightly different timings, in other words with a certain delay spread, each OFDM symbol is preceded by a cyclic prefix (CP), which is used to effectively eliminate inter-symbol interference. Further, OFDM enables broadcast services on a synchronized single frequency network with appropriate cyclic prefix design (see below). This allows broadcast signals from different cells to combine over the air, thus significantly increasing the received signal power and supportable data rates for broadcast services.

When individual subcarriers or sets of subcarriers are assigned to different user equipments, the result is a multi-access system referred to as OFDMA (Orthogonal Frequency Division Multiple Access), as used in WiMAX, LTE and LTE-A for the downlink - in other words for communication from base stations to user equipments. By assigning distinct frequency/time resources to each user equipment in a cell, OFDMA can substantially avoid interference among the users served within a given cell. In OFD A, users (called UEs in LTE) are allocated a specific number of subcarriers for a predetermined amount of time. An amount of resource consisting of a set number of subcarriers and OFDM symbols is referred to as a resource block (RB) in LTE. RBs thus have both a time and frequency dimension. Allocation of RBs is handled by a scheduling function at the base station (an eNodeB in an LTE-based system). An RB can be considered to be composed of resource elements (Res) each consisting of one sub-carrier with a duration of one OFDM symbol. In LTE, each eNodeB may have a plurality of antennas, and may serve multiple cells. One eNodeB may be considered to comprise one or more BSs. Moreover, there may be distinct uplink and downlink cells (in the remainder of this specification, the term "cell" can be assumed to mean at least a downlink cell). Incidentally, the wireless network is referred to as the Έ- UTRAN" (Evolved UMTS Terrestrial Radio Access Network) in LTE. The eNodeBs are connected to each other, and to higher-level nodes, by a backhaul network, e.g. the core network or Evolved Packet Core (EPC).

Frame Structure and Resource Blocks In a wireless communication system such as LTE or WiMAX, data for transmission on the downlink is organised in OFDMA frames each divided into a number of sub-frames. Various frame types are possible and differ between FDD and TDD for example. Frames follow successively one immediately after the other, and each is given a system frame number (SFN). Figure 1 shows a generic frame structure for LTE, applicable to the downlink, in which the 10 ms frame is divided into 20 equally sized slots of 0.5 ms. A sub-frame SF consists of two consecutive slots, so one radio frame contains 10 sub-frames.

Figure 2 shows a so-called downlink resource grid for the duration of one downlink slot. One downlink slot consists of Nsymb OFDM symbols in general. To each symbol, the above- mentioned cyclic prefix (CP) is appended as a guard time, as shown in Figure 1.

Nsymb depends on the cyclic prefix length. The generic frame structure with normal cyclic prefix length contains Nsymb = 7 symbols as illustrated in Figure 2. Additionally, an extended CP is defined in order to cover large cell scenarios with higher delay spread, and for MBMS transmission (see below). The transmitted signal in each slot is described by a resource grid of sub-carriers and available OFDM symbols, as shown in Figure 2. Each element in the resource grid is called a resource element (RE) and each resource element corresponds to one symbol. OFDMA allows access by multiple UEs to the available bandwidth as already mentioned. Each UE is assigned a specific time-frequency resource. The data channels are shared channels, i.e. for each transmission time interval of 1 ms, a new scheduling decision is taken regarding which UEs are assigned to which time/frequency resources during this transmission time interval. The basic scheduling unit for allocation of resources to the UEs is called a resource block (RB). As shown in Figure 2, one resource block is currently defined as 7 consecutive OFDM symbols in the time domain (or 6 with extended CP) and 12 consecutive sub-carriers in the frequency domain. The resource block size is the same for all system bandwidths, therefore the number of available physical resource blocks depends on the bandwidth. Several resource blocks may be allocated to the same UE, and these resource blocks do not have to be adjacent to each other. Scheduling decisions are taken at the base station (eNodeB). The scheduling algorithm has to take into account the radio link quality situation of different UEs, the overall interference situation, Quality of Service requirements, service priorities, etc.

Reference Signals

To facilitate measurements of the radio link properties by UEs, and reception of some transmission channels, reference signals are embedded in the downlink sub-frame as transmitted from each antenna of an eNodeB or more accurately for LTE/LTE A, "antenna port". The term "antenna port" is sometimes preferred when referring to transmissions from multiple antennas, since it is possible for multiple physical antennas to jointly transmit a given signal and thus act as a single antenna port. Each base station (and thus each cell) of an eNodeB may comprise a plurality of antenna ports.

In case of two transmit antenna ports in LTE, therefore, reference signals are transmitted from each antenna port. The reference signals on the second antenna are offset in the frequency domain by three sub-carriers, and to allow the UEs to accurately measure the radio link properties, nothing is transmitted on the other antenna at the same time-frequency location of reference signals.

The reference signals provide an amplitude and/or phase reference for allowing the UEs to correctly decode the remainder of the downlink transmission. In LTE reference signals can be classified into a cell-specific (or common) reference signal (CRS), an MBSFN reference signal used in BMS, and a user equipment-specific reference signal (UE-specific RS). There are also positioning reference signals, and the UE-specific reference signals are also commonly termed demodulation reference signals, DM-RS.

The CRS is transmitted to all the UEs within a cell and used for channel estimation. The reference signal sequence, which spans the entire downlink cell bandwidth, depends on, or implicitly carries, the cell identity or "cell ID". As a cell may be served by an eNodeB having more than one antenna port, respective CRS are provided for each antenna port and the locations of CRSs depend on the antennal port. The number and location of CRSs depends not only on the number of antenna ports but also on which type of CP is in use.

Figure 3 shows the CRS signal structure for 1 ,2 and 4 antenna ports across to resource blocks The MBSFN reference signal can be transmitted in sub-frames allocated for MBSFN transmission (see below).

A UE-specific reference signal is received by a specific UE or a specific UE group within a cell. UE-specific reference signals are chiefly used by a specific UE or a specific UE group for the purpose of data demodulation.

CRSs are transmitted in all downlink sub-frames in a cell supporting non-MBSFN transmission. If a sub-frame is used for transmission with MBSFN, only the first few (0, 1 or 2) OFDM symbols in a sub-frame can be used for transmission of cell-specific reference symbols.

CRSs can be accessed by all the UEs within the cell covered by the eNodeB regardless of the specific time/frequency resource allocated to the UEs. They are used by UEs to measure properties of the radio channel - so-called channel state information or CSI - with respect to such parameters as a Channel Quality Indicator, CQI .

LTE-A (LTE-Advanced) introduces further reference signals including a Channel State Information reference signal CSI-RS (see below), and the expanded UE-specific demodulation reference signal DM-RS, (not to be confused with demodulation reference signals transmitted on the uplink by the UEs). These additional signals have particular application to beamforming and MIMO transmission techniques outlined below. Channels

Several channels for data and control signalling are defined at various levels of abstraction within the network. Figure 4 shows some of the channels defined in LTE at each of a logical level, transport layer level and physical layer level, and the mappings between them. For present purposes, the channels at the physical layer level are of most interest.

On the downlink, user data is carried on the Physical Downlink Shared Channel (PDSCH). There are various control channels on the downlink, which carry signalling for various purposes including so-called Radio Resource Control (RRC), a protocol used as part of the above- mentioned RRM. In particular the Physical Downlink Control Channel (PDCCH), Physical Broadcast Channel (PBCH) and Physical Multicast Channel (PMCH) are provided. PDCCH is used to carry scheduling information - called downlink control information, DCI - from base stations (called eNodeBs in LTE) to individual UEs.

Meanwhile, on the uplink, user data and also some signalling data is carried on the Physical Uplink Shared Channel (PUSCH), and control channels include a Physical Uplink Control Channel, PUCCH, used to carry signalling from UEs including channel quality indication (CQI) reports, precoding matrix information (PMI), a rank indication (Rl) for MIMO (see below), and scheduling requests.

MIMO A technique called MIMO, where MIMO stands for multiple-input multiple-output, has been adopted in LTE due to its spectral efficiency gain, spatial diversity gain and antenna gain. MIMO schemes employ multiple antennae at the transmitter and/or at the receiver to enhance the data capacity achievable between the transmitter and the receiver. By way of example, in a basic 2x2 MIMO configuration there are two antennae at the transmitter and two antennae at the receiver. Likewise, a basic 4x4 MIMO configuration contains four antennae at the transmitter and four antennae at the receiver. There is no need for the transmitter and receiver to employ the same number of antennae. Typically, a base station in a wireless communication system will be equipped with more antennae in comparison with a UE (which may often be, for example, a mobile handset), owing to differences in power, cost and size limitations.

The term MIMO channel (or simply "transfer channel" or "channel") is commonly used to describe the frequency response of the transmitter-receiver radio link in a MIMO scheme. The MIMO channel may be represented mathematically as a matrix H , the individual elements of which represent the channel characteristics (for example, channel frequency response) for transmitting signals from one particular transmitting antenna to one particular receiving antenna. For example, the element H_{b a} of matrix H would represent the channel characteristics for transmitting signals from the a^th transmitting antenna of a BS to the 6^th receiving antenna of a UE.

It should be noted that, despite the name "multiple-input multiple-output", MIMO systems can operate (and indeed provide benefit) even if one of the transmitter and the receiver has only one antenna (SIMO, for single-input multiple-output or MISO, for multiple-input single-output). In fact, MIMO systems might technically be said to operate even where the transmitter and the receiver both have only one antenna (SISO, single-input single-output), although this situation might be considered a special (degenerate) case because the MIMO channel would then be represented by a scalar rather than a matrix and a number of the benefits otherwise achievable using MIMO may not be possible.

One use of the MIMO technique is for so-called transmit (Tx) diversity, where blocks of data intended for the same UE are transmitted via multiple transmitting antenna ports, the signals from which may follow different propagation paths.

"Diversity coding" refers to the process for generating signals for transmission in a transmit diversity system. The antenna ports may be Tx antennas of different eNodeBs or of the same eNodeB. In LTE, owing to limitations on the physical size and capabilities of UEs, transmit diversity is more applicable on the downlink than to the uplink. Only one receiving antenna port (Rx antenna) is needed at the UE, although two or more Rx antennas may be used to improve performance.

CoMP and MBMS Related to the above, it is possible to coordinate the MIMO transmissions among multiple antennas (or antenna ports) belonging to the same base station (i.e. coordinating transmissions within a single cell) or among multiple base stations (i.e. coordinating transmissions in adjacent or nearby cells) to reduce inter-cell interference and improve the data rate to a given UE. This is called coordinated multi-point transmission/reception or CoMP, and is a technique being considered for inclusion in LTE-A. Downlink schemes used in CoMP include "Coordinated Scheduling and/or Coordinated Beamforming (CS/CB)" and "Joint Processing/Joint Transmission (JP/JT)". An additional technique which may be employed is aggregation of multiple carriers (CA) to increase the available peak data rate and allow more complete utilisation of available spectrum allocations.

In CS/CB, data to a single UE is transmitted from one transmission point, but decisions regarding user scheduling (i.e. the scheduling of timings for transmissions to respective UEs) and/or beamforming decisions are made with coordination among the cooperating cells (or cell sectors). In other words, scheduling/beamforming decisions are made with coordination between the cells (or cell sectors) participating in the coordinated scheme so as to prevent, as far as possible, a single UE from receiving signals from more than one transmission point.

On the other hand, in JP/JT, data to a single UE is simultaneously transmitted from multiple transmission points to (coherently or non-coherently) improve the received signal quality and/or cancel interference for other UEs. In other words the UE actively communicates in multiple cells and with more than one transmission point at the same time. From the viewpoint of the UE, it makes no difference whether the cells belong to different eNodeBs or to the same eNodeB.

In CA, discrete frequency bands are used at the same time (in other words, aggregated) to serve the same user equipment, allowing services with high bandwidth demands (up to 100MHz) to be provided. CA is a feature of LTE-A (LTE-Advanced) which allows LTE-A- capable terminals to access several frequency bands simultaneously whilst retaining compatibility with the existing LTE terminals and physical layer. CA may be considered as an complement to JP for achieving coordination among multiple cells, the difference being (loosely speaking) that CA requires coordination in the frequency domain and JP in the spatial domain. Figure 5 schematically illustrates the principles of CS/CB and JP downlink transmission schemes respectively, used in CoMP.

Joint Processing (JP) is represented in Figure 5(a) in which cells A, B and C actively transmit to the UE, while cell D is not transmitting during the transmission interval used by cells A, B and C.

Of less relevance to the present invention, coordinated scheduling and/or coordinated beamforming (CS/CB) is represented in Figure 5(b) where only cell B actively transmits data to the UE, while the user scheduling beamforming decisions are made with coordination among cells A, B, C and D so that the co-channel inter-cell interference among the cooperating cells can be reduced or eliminated.

As another example of co-operative transmission among base stations, MBMS (Multimedia

Broadcast Multicast Services) may be performed via multi-cell transmission. In case of multi-cell transmission, the cells and content are synchronized to enable for the terminal to combine the received signal from multiple eNodeBs. This concept is also known as a Single Frequency Network. The E-UTRAN can configure which cells are part of an Single Frequency Network for transmission of an MBMS service, so-called MBSFN operation. The MBMS traffic can share the same carrier with the unicast traffic or be sent on a separate carrier. For MBMS traffic, the above-mentioned extended CP is provided, allowing the UEs to combine the transmissions from the different eNodeBs, and in the case of sub-frames carrying MBSFN data, specific MBSFN reference signals are used as already mentioned. In MIMO scenarios, the UE will be expected to provide feedback to the network regarding how its transmissions should be formed in terms of, e.g., number of spatial layers, i.e. rank (Rl), precoding (PMI) and modulation and coding scheme (CQI). In the case of a coordinated multipoint (CoMP)-enabled network, this feedback information could be derived as a function of some, or all, of the channels between the multiple transmission points in the cooperating group of cells and the UE, as compared to the simple case of having only one transmission point and one associated channel to the UE.

Advanced CoMP schemes may also perform better if some more explicit information regarding the radio channel is available.

In the (Release 10) LTE A specifications, UE-specific channel state information reference signals (CSI-RS) are introduced which are defined for 1 , 2, 4 or 8 antenna ports of a cell and which have a much lower density in time and frequency than the Common Reference Symbols (CRS), and a hence much lower overhead. Their purpose is to allow improved estimation of the channel for feeding back RI/PMI/CQI and possibly other related parameters to the network. CSI- RS can thus be viewed as LTE-A's solution to channel estimation and feedback for high-rate PDSCH (downlink data transmission) scenarios. CSI-RS patterns in time and frequency can be configured by higher layers to allow considerable flexibility over which resource elements (REs) contain them.

Furthermore, to support CoMP operation, an LTE Release 10 UE can be configured with multiple CSI-RS patterns specific to its cell: · One configuration for which the UE shall assume non-zero transmission power for the

CSI-RS; and

• Zero or more configurations for which the UE shall assume zero transmission power for the CSI-RS. These configurations may simultaneous from the UE point of view. The purpose of the 'zero power CSI-RS patterns' is to ensure that a cell so-configured can safely be assumed by the UE to not transmit in the REs which will contain CSI-RS of the cells it is cooperating with in a CoMP scenario. Although CoMP is not directly supported by the LTE Release 10 specifications, knowledge of the presence of zero power CSI-RS patterns can be used by a Release 10 UE to mitigate the possible impact of CoMP on data transmissions using PDSCH.

Figure 6 shows a configuration ('Configuration 0') for an 8 antenna port system with normal cyclic prefix and non-zero transmission power. An example zero transmission power CSI-RS pattern would be to configure the UEs to assume zero power in the locations shown for Ports 15, 16, 17 and 18.

Using CSI-RS patterns, the channels in a co-operating group can be measured without interference from the serving cell.

LTE A (Release 10) also introduces time-domain (evolved) intercell interference coordination (elCIC) via almost blank subframe (ABS) configurations. These are signalled to a UE with a 40- bit bitmap telling the UE which subframes on a 40-subframe periodicity the UE can implicitly assume will contain a significantly different level (e.g. lower level) of interference power from other cells. The UE can then generate independent CSI reports for ABS and non-ABS subframes, reflecting the different interference conditions. The following signals would still be present in an ABS, if they would be present in a normal subframe: CRS, synchronization signals, PBCH, paging, and, if configured, CSI-RS. However, PDSCH and other control channels may be removed or transmitted with lower power, so the UE can assume the subframe contains significantly less power than a non-ABS would. ABSs can be formed from normal subframes, or MBSFN subframes in which case only the CRS in the first OFDM symbol are transmitted. An MBSFN ABS is thus 'blanker' than a normal ABS. ABS subframes, as currently defined in LTE, are intended to be to define resources with low transmitted power (i.e. low interference) from one or more non-serving cells. Their use can be to enable interference coordination in heterogeneous networks with cells of different sizes/transmit power to reduce limitations on a UE served by one cell due to interference from another cell. Generally speaking, ABS subframes can be seen as a way of protecting control channels from interference . This would be typically exploited in LTE by one eNodeB signalling to another eNodeB that certain subframes transmitted by a given cell can be assumed to be ABS, and therefore low interference levels can be expected in those subframes due to transmissions from that cell. Figure 7 show an ABS formed from a normal subframe, with blank resource elements, resource elements for CRS reference signals and resource elements for the physical broadcast channel (PBCH) and the primary/secondary synchronization signal (PSS/SSS). In LTE, REs in an ABS which are expected by the UE to be blank may still contain control channels or PDSCH. Time (milliseconds) is along the x-axis. The diagram includes 14 OFDM symbols covering two slots of a subframe. The resource grid here is the same size as shown in other figures.

Figure 8 show an ABS formed from an MBSFN subframe, in which the only non-blank REs are for CRS transmission

It is desirable to improve the performance of networks in which there is coordinated downlink transmission, for example from more than one co-operating cell or from a single cell with more than one antenna.

Summary of the Invention According to invention embodiments, there is provided a communications network including a group of two or more in-group antennas associated with one or more base stations and a user equipment, wherein the in-group antennas are arranged to co-operate in downlink transmission to the user equipment; transmission from the in-group antennas is co-ordinated so that their downlink transmission in particular transmission resources is muted; and wherein the user equipment is configured to measure a downlink channel in a muted transmission resource.

The coordination to mute downlink transmission resources may result in reduction of transmission power from some or all of the antennas. At one extreme, at least one in-group antenna may transmit with reduced transmission power, or not transmit some or all transmissions (such as data and/or control signal transmissions). At the other extreme, all of the in-group antennas, or all but one of the in-group antennas may transmit with reduced transmission power, or not transmit some or all transmissions (such as data and/or control signal transmissions)

CoMP performance may be improved by using measurements not only of the channels within the cooperating group, but by also providing to the network some feedback regarding interference being received from outside the cooperating group - here termed Out-of-group interference' (OGI) or Out-of-cell interference' (OCI) for single cell embodiments. This OCI or OGI interference may be from other antennas in the same network and/or from antennas associated with other networks. In present releases of LTE and other technologies there is no particular provision for coordination among several cooperating cells (or antennas of a single cell, which may be geographically spaced) to permit the UE to make measurements of OGI/OCI without simultaneously receiving unwanted signals from other cells (or antennas) in the cooperating group of cells/antennas to which it is connected. Both the prior art schemes described above, using CSI-RS or ABS are inherently in-group configurations. For example, the two types of CSI- RS configuration can currently be used as follows:

• CSI-RS transmitted from each cooperating cell, using non-overlapping patterns, to allow each UE to measure the channel of its serving cell. · Zero-power CSI-RS patterns defined in each cell and corresponding to the CSI-RS transmissions from all the other cells, allowing each UE to measure the channels of the cells in the cooperating group.

This prior art does not give the UE the possibility of assuming it can safely take measurements interfered only by (or substantially only by) OGI at any particular time or frequency. The inventors have come to the realisation that a UE cannot simply try to measure the CSI-RS of all the out-of-group cells since it does not know their CSI-RS configurations in time and frequency. Thus, without good knowledge of the OGI, the performance gains identified in joint transmission and other such transmission and feedback schemes may not be easily available.

If it were possible to obtain "clean" measurements of OGI, system performance with CoMP could be improved, for example in delivering higher data rates to users; maintaining data rates and reducing transmission power levels; and allowing more accurate scheduling decisions to be taken thus improving resource efficiency. Therefore, techniques of invention embodiments for providing the LTE network (or other telecommunications network) with means to coordinate its transmissions in such a way as to allow the UE to obtain measurements of OGI, and to instruct the UE in the manner of doing so, are of significant interest.

As used herein, the term "resources" includes time and frequency (bandwidth) resources for wireless transmission.

Incidentally, the communications network may further include one or more out-of-group antennas; wherein the out-of-group antennas can interfere with downlink transmission from the in-group antennas to the user equipment; and wherein the channel measurement can be used to assess out-of-group interference created by the out-of-group antennas. That is, the in-group antennas may be a subset of network antennas which are network antennas and there may also be a subset of out-of-group antennas. Transmission from the in-group antennas may be coordinated so that measurement of the downlink channel in a muted transmission resource is not subject to in-group interference from at least one of the in-group antennas and may be subject to out-of-group interference from at least one of the out-of-group antennas.

The in-group antennas can operate in a communication network in which cells are defined. Alternatively, there may be no specific cell definition. If the network does use cells, all the in- group antennas may be associated with the same base station and together provide a single cooperating cell, and the user equipment measurement can be used to investigate out-of-group interference in the form of out-of-cell interference. As an aside, a particular set of antennas may be arranged to form a cell regardless of which base station they are nominally attached to.

In some advanced communication systems such as systems based on LTE, the "antenna" is not necessarily a physical antenna but a port corresponding to a plurality of physical antennas which are configured to transmit copies of the same transmission signal. Alternatively, the antenna may be a single physical antenna.

In many invention embodiments, the group is effectively a group of cells. In such embodiments, each antenna is associated with a single base station providing a single cell(although each base station can have more than one antenna), at least two of the in-group antennas are associated with different base stations, so that the grouping of the antennas leads to a group of cooperating in-group cells; the in-group cells are configured to co-operate in downlink transmission (of data) to the user equipment; the in-group cells are configured to co-ordinate their downlink transmission in particular transmission resources to be muted; and the user equipment measurement investigates out-of-group interference.

Thus, omitting the references specifically to antennas, which are not necessary when we are speaking of cells cooperating, some invention embodiments relate to a communications network comprising a group of cells provided by one or more base stations, and a user equipment, wherein the cells are configured as in-group cells to co-operate in downlink to the user equipment; the in-group cells are configured to co-ordinate their downlink transmission in particular transmission resources to be muted; and wherein the user equipment is configured to measure a downlink channel in a muted transmission resource. In this case, the communications network may further comprise one or more out-of-group base stations, each providing one or more out-of-group cells wherein the out-of-group cells can interfere with downlink transmission from the in-group cells to the user equipment; and wherein the channel measurement can assess out-of-group interference created by the out-of-group cells.

As mentioned in the introduction, the term "cell" in this specification is to be interpreted broadly. For example, it is possible to refer to communication channels associated with a cell being transmitted from or by the cell (in the downlink), or transmitted to a cell (in the uplink), even if the transmission or reception is actually carried out by one or more antennas or antenna ports of a base station. The term "cell" is intended also to include sub-cells, which could be sub-divisions of a cell based on using particular antennas or corresponding to different geographical areas within a cell. The cells may be associated with different base stations or with the same base station. The term "base station" itself has a broad meaning and encompasses, for example, an access point or transmission point.

Reference herein to coordinated muted downlink transmission includes reference to downlink transmission reduced or even entirely eliminated from some or all of the cooperating group members, that is cells or antennas or antenna ports, (where antenna ports are defined in the technology in question). In preferred invention embodiments, the downlink transmission in the muted transmission resources may be muted by a reduction of transmission power and/or by disallowing some or all transmission. For example, certain types of transmission may be disallowed (such as data transmission and/or certain types of control information transmission). Alternatively or additionally the power of the signals transmitted may be reduced, for example to below a certain threshold. In building up a picture of interference from out of group sources, it can be advantageous to determine noise and interference when there is no (or almost no) transmission from the group members. Thus transmission from some or all group members may be muted in the muted transmission resources and the downlink channel measurement may be measurement of noise and/or interference properties of a channel including any out-of-group interference. Alternatively or additionally, it can be advantageous to check for the effect of out-of-group interference on a reference signal transmitted from one or more group members without interference from any other group member. Therefore in other invention embodiments all transmission in the group can be muted in the muted transmission resources except for transmission of a reference signal (for example from a single group member) and the downlink channel measurement in a muted transmission resource (such as one or more REs) can include measurement of a channel transfer function including any out-of-group interference (and noise).

Some embodiments can combine these two alternatives, by implementing them at different times/frequencies.. As a reminder, a group member can be an antenna (or antenna port) in a single cell arrangement or in a system without cells having a group of cooperating antennas, or it may be a single cell in a group of cooperating cells.

In invention embodiments, resources can be divided in the time and/or frequency domain to provide muted transmission resources. In many communication systems, a frame based method is used for transmission. In this case, time can be divided into subframes of predetermined duration for downlink and uplink transmission and the particular transmission resources which are muted can comprise at least one muted downlink subframe. Not all of the subframe needs to be muted to create a muted downlink subframe. For example the muted downlink subframe may correspond to a subframe indicated by signalling to be an almost blank subframe (ABS or MBSFN) as described hereinbefore. The eNodeBs provide signalling to indicate the inclusion of such a subframe to the UE. Alternatively or additionally a PDSCH (data downlink) muted subframe can be defined (and so indicated) in which the base station (such as eNodeBs in LTE) of the cooperating group members are specifically required to mute downlink data transmission, for example PDSCH transmission may be disallowed. It should be noted that a subframe indicated to be an ABS may not, in fact, be blank in an RE which is assumed to be blank by the UE. For example, the UE may be at a cell edge and only cell-centre transmissions may be scheduled in the blank RE.

Preferably the user equipment is configured to measure a downlink channel in the muted downlink subframe in which the UE is not expecting (data) transmission from group members, but may be expecting a reference signal transmission. This portion of the muted downlink subframe may additionally or alternatively be one in which out-of-group interference may occur, for example from downlink data transmission.

An alternative embodiment (which may of course be combined with muted downlink subframes) provides a muted portion of a (normal) downlink subframe in a frame based system, rather than an entire subframe indicated as muted. This muted portion may be one or more muted resource elements in a LTE subframe.

Within the muted transmission resources, whether they are a muted portion of downlink subframe or a muted subframe of some type, there may be no transmission (to allow noise/ interference measurement) or there may be transmission of a reference signal to allow measurement of a channel transfer function including out-of-group interference. In one embodiment, each group member is configured to transmit a channel measurement reference signal, such as CSI-RS in LTE A or CRS in LTE, in separate resources within the muted transmission resources, to allow measurement of a channel transfer function including any out- of-group interference. The separate resources may be scheduled in advance of scheduling the muted transmission resources, to make sure that the muted resources are suitable positioned.

As mentioned previously, there may be no in-group data transmission in the muted transmission resources. For example, a group member may be configured not to transmit in resources reserved for a reference signal intended for channel measurement by another group member, such as CSI-RS or CRS in LTE, and thus the group member may be configured to provide zero- power CSI-RS or CRS resources as the muted transmission resources, to allow measurement of noise and out-of-group interference properties.

As an alternative to the known signals in LTE a specific group-wide pattern of zero-power interference measurement resources can be created. In this case, each group member may be configured not to transmit in the group-wide pattern of zero-power transmission resources, which are specifically reserved as the muted transmission resources for out-of-group interference measurement.

The reader will appreciate that the invention encompasses embodiments in which there are no cells, embodiments which refer to cooperation within a single cell and embodiments in which there is cooperation between a group of cells. The group of cells may be all within the control of a single eNodeB, in which case the eNodeB can carry out the coordination function. In other arrangements, the cells can be controlled by more than one eNodeB. In this situation, invention embodiments provide for coordination between the eNodeBs controlling the cooperating cells. Such coordination would normally be over the X2 interface. In one embodiment, the in-group cells are provided by more than one eNodeB; and the eNodeBs providing the in-group cells exchange signalling to control the in-group co-operation, preferably in the form of one or more patterns of (group-wide) muted transmission resources and/or one or more patterns of reference signal resources. The serving eNodeB/cell may also signal patterns to the UE, along with any supplemental information required to assess OGI. For example, the eNodeB wishing to enable OGI measurement in its cells may send an indication to each eNodeB within the cooperating group, receive one or more proposed patterns and/or other information from each eNodeB, derive a suitable pattern for use in all the cooperating cells, inform the cooperating eNodeBs of the pattern and then signal this or a pattern suitable derived from it (for example by RRC) to the UE required to make OGI measurements, together with any other information required. Of course the other eNodeBs may not cooperate at one or more stages of this procedure, in which case they could simply be considered not to be part of the cooperating group, at least for the purposes of the UE feedback. It can be advantageous to use signalling already known within a standard for additional purposes, such as this OGI determination. In some embodiments the inverse of a LTE ABS pattern may be used to indicate muted transmission resources. An additional indication could be used to indicate that these resources are to expect low interference.

In other embodiments RNTP signalling (relative narrowband transmit power signalling) may be used to indicate or request ('Reverse' RNTP signalling) at least one future power limit in specific resources (and optionally also specific resources with no such limit), thus defining muted transmission resources.

According to an embodiment of a further aspect of the invention there is provided a telecommunications method in a network comprising a group of two or more in-group antennas associated with one or more base stations and a user equipment, wherein the in-group antennas co-operate in downlink transmission to the user equipment; transmission from the in- group antennas is co-ordinated so that their downlink transmission in particular transmission resources is muted; and wherein the user equipment measures a downlink channel in a muted transmission resource (such as an RE) to investigate out-of-group interference.

According to an embodiment of a yet further aspect of the invention there is provided a user equipment in a communications network comprising a group of two or more in-group antennas associated with one or more base stations and the user equipment, wherein the user equipment is configured to receive cooperative downlink transmission from the in-group antennas; transmission from the in-group antennas is co-ordinated so that their downlink transmission in particular transmission resources is muted; and wherein the user equipment is configured to measure a downlink channel in a muted transmission resource created by coordinated transmission from the in-group antennas. According to an embodiment of a still further aspect of the invention there is provided an eNodeB in a communications network comprising a group of cells provided by more than one base station, at least one of the cells being associated with the eNodeB and another of the cells being associated with another eNodeB, and a user equipment, wherein the eNodeB is configured to control said at least one cell to co-operate with the other cells in the group in downlink transmission to the user equipment; the eNodeB is configured to control said at least one cell to cause particular transmission resources to be muted by co-ordinating its downlink transmission with the other cells in the group; and wherein the eNodeB is configured to exchange signalling messages with at least said another eNodeB to coordinate downlink transmission across the group of cells.

Computer program aspects of the invention relate to software (one or more computer programs) which when executed on a communications network cause it to carry out the method described above. The software may be provided on a non-transitory computer-readable medium which can be viewed as an article of manufacture.

In general, and unless there is a clear intention to the contrary, features described with respect to one aspect of the invention may be applied equally and in any combination to any other aspect, even if such a combination is not explicitly mentioned or described herein.

As is evident from the foregoing, many embodiments of the present invention involves signal transmissions between cells and user equipments in a wireless communication system. The cells are associated with one or more base stations. A base station may take any form suitable for transmitting and receiving such signals. It is envisaged that the base stations will typically take the form proposed for implementation in the 3GPP LTE and 3GPP LTE-A groups of standards, and may therefore be described as an eNodeB (eNB) (which term also embraces Home eNodeB) as appropriate in different situations. However, subject to the functional requirements of the invention, some or all base stations may take any other form suitable for transmitting and receiving signals from user equipments.

Similarly, in the present invention, each user equipment may take any form suitable for transmitting and receiving signals from base stations. For example, the user equipment may take the form of a subscriber station (SS), or a mobile station (MS), or any other suitable fixed- position or movable form. For the purpose of visualising the invention, it may be convenient to imagine the user equipment as a mobile handset (and in many instances at least some of the user equipments will comprise mobile handsets), however no limitation whatsoever is to be implied from this.

Brief Description of the Drawings

Reference is made, by way of example only, to the accompanying drawings in which:

Figure 1 illustrates a generic frame structure employed for the downlink in an LTE wireless communication system;

Figure 2 illustrates resource allocation within a frame; Figure 3 shows an arrangement of CRS reference signals in Resource Blocks;

Figure 4 shows relationships between various channels defined in LTE;

Figure 5 illustrates principles employed in Co-operative Multipoint Processing (CoMP as applied to cells);

Figure 6 shows arrangements of CSI-RS reference signals in Resource Blocks;

Figure 7 shows an Almost Blank Subframe (ABS);

Figure 8 shows a Multimedia Broadcast Single Frequency Almost Blank Subframe (MBSFN ABS);

Figure 9a shows an arrangement of a network and Figure 9b shows muted transmission resources according to an invention embodiment;

Figure 10 shows muted transmission resources and corresponding reference signal transmission according to an invention embodiment; and

Figure 11 is a flow chart of a general invention embodiment. Detailed Description

Figure 9a depicts a general cell-based embodiment and illustrates three cells (Cell A, Cell B and Cell C) forming a group of cells, surrounded by other cells of the same network (Cell D, Cell E, Cell F and Cell G). Cells A, B and C cooperate in downlink transmission to the UE shown and are labelled as in-group cells. Figure 9b shows transmission resources as a map of frequency against time. Shared muted transmission resources of invention embodiments for Cells A, B and C are provided. These are illustrated as covering the entire bandwidth available, or as isolated blocks of resources within the map. in this simple example, transmission is not allowed by any of the in-group cells, and noise/interference measurements can be made in the muted resources Figure 10 depicts a scenario in which one of the cells transmits a reference signal in the muted transmission resources. Interference in the received reference signal at the UE will mainly be due to out-of-group transmission.

Figure 11 is a flowchart including the basic steps of invention embodiments. A group of cells/antennas is configured in step S10 to coordinate their downlink transmission. In step S11 particular transmission resources are muted. In option step S12 a reference signal is transmitted by one member only of the group, and in step S13 the UE measures the downlink channel in the muted transmission resources. Since the transmission of reference signals in muted transmission resources is optional, the main possibilities for the UE may be: • Measure the channel (using reference signals not in muted resources), measure the out- of-group interference (in muted resources) or

• Measure the channel (using reference signals in muted resources), measure the out-of- group interference (in the same muted resources)

Specific Embodiments

The basic principle of these invention embodiments is to coordinate transmissions, or muting of transmissions, among the several co-operating cells and possibly several eNodeBs in a CoMP group such that certain resource elements (REs) experience reduced or eliminated transmissions from within the CoMP group. The invention coordinates time-domain and/or frequency domain DL transmissions for this purpose. A UE can then be configured by an eNodeB to make channel and/or OGI measurements in these coordinated REs. These measurements are those necessary to allow the UE to formulate feedback to the network incorporating DL precoding to help mitigate the effects of OGI (or to allow the network to determine suitable such DL precoding itself). The embodiments describe two main network scenarios: (i) all the cells belong to one eNodeB which is able to impose its coordination choices on its cells; and (ii) where multiple eNodeBs are cooperating and there must be some new messages exchanged over the X2 interface to reach a coordinated decision on which REs are to be made available for OGI measurements. The invention could be included in LTE specifications.

In general, unless otherwise indicated, the embodiments described below are based on LTE, where the network operates using FDD and comprises one or more eNodeBs, each controlling one or more downlink cells, each downlink cell having a corresponding uplink cell. Each DL cell may setve one or more termir^ (U^ in that serving cell. In order to control the use of transmission resources in time, frequency and spatial domains for transmission to and from the UEs, the eNodeB sends control channel messages (PDCCH) to the UEs. A PDCCH message typically indicates whether the data transmission will be in the uplink (using PUSCH) or downlink (using PDSCH), it also indicates the transmission resources, and other information such as transmission mode, number of antenna ports, data rate, number of codewords enabled. In addition PDCCH may indicate which reference signals may be used to derive phase reference(s) for demodulation of a DL transmission. Reference signals for different antenna ports, but occupying the same locations, are distinguished by different spreading codes. The cells in a CoMP group may co-operate in the transmission of signals (such as PDSCH) to a UE. One or more cells in the COMP group may become serving cells for a given UE.

In order for the eNodeB (or group of eNodeBs) to schedule efficient transmissions to UEs with appropriate transmission parameters and resources, each UE provides feedback on the DL channel state for one cell, or a group of serving cells to the eNodeB(s) controlling the serving cell(s) for that UE.

Coordinated ABSs for OGI measurement

In a first embodiment, a coordinating eNodeB configures all the cells within the CoMP group to have a common ABS pattern in addition to any other independent patterns they may already have been configured with. In these group-specific ABSs, the locations of which a signalled to the UEs, a UE can then make measurements of OGI by monitoring the REs corresponding to possible locations of PDSCH transmissions from cells outside the group. This reverses the existing uses of ABS where they are used to indicate muting of an interfering cell rather than indicating muting of the serving cell(s). To enable this, the following RRC signalling options (in TS 36.331 ) could be used:

1. Use the existing CSI measurement pattern (the csi-SubframePatternConfig-r10 IE) which allows two patterns for CSI measurements to be configured. One of these could correspond to group-wide muting of PDSCH. A given UE could then be configured to make measurements of OGI in those subframes. The other CSI pattern could be used for any other purpose, subject to the constraint that the two patterns are not permitted to overlap.

2. Specify a new, fourth, pattern in TS 36.331 which the UE is to use for specifically for measurements of OGI. This leaves the network free to use the existing patterns for any existing purpose and does not impose any inter-cell coordination requirements on the use of those patterns. It may be necessary to send the UE an additional indication of which pattern should be used for OGI measurements.

A UE making measurements of OGI (for example in terms of the received power level), in ABS as indicated above would combine this information with channel measurements made using CSI-RS in order to estimate the channel quality (for example, in terms of the achievable data transmission rate for the cells in the CoMP group, subject to the observed OGI).

As variation of this embodiment the OGI measurement is restricted to REs known to the UE not to contain specific signals transmitted by the serving cell(s): for example not containing control signals such as CRS, CSI-RS, PDCCH . These are signals which may still be present in the ABS even if the serving cell is transmitting a subframe indicated to be an ABS, so that avoiding them makes sure there is no transmission from the serving cell. As a further variation, the OGI measurement may be further restricted to REs known to the UE contain only specific signals from the out-of-group cells: for example PDSCH, which will provide a high level of OGI interference. The information on such restrictions may be signalled to the UE by RRC.

In an extension of this embodiment, there are several cooperating eNodeBs which each control one or more cells, so coordination between eNodeBs is implemented. In this case, an eNodeB wishing to enable OGI measurements in its cells:

1. Sends an Invoke Indication to each eNodeB controlling cells within the CoMP group. 2. Receives proposed ABS Information and Measurement Subset lEs from each eNodeB willing to cooperate.

3. Derives from (2) a suitable ABS pattern for use in all the cells for OGI measurement purposes.

4. Informs the cooperating eNodeBs of this pattern via the Usable ABS Pattern IE. 5. Signals this pattern by RRC to the UE(s) required to make OGI measurements, together with any other information required to make appropriate measurements, such as restrictions on particular REs.

Any eNodeB choosing not to configure ABS after (4) is considered not to be part of the CoMP group for the time being (at least for the purposes of the UE CSI feedback). A variation on this embodiment is to use MBSFN ABSs rather than normal ABSs. There would be less freedom to choose the ABS patterns in this case, since MBSFN subframes can only occur in subframes 1 , 2, 3, 6, 7 and 8.

Coordinated ABS and CSI-RS patterns

The second embodiment is like the first embodiment, except that a specific CSI-RS pattern is also configured for each of the cells in the CoMP group. These patterns are chosen such that the CSI-RS for all cooperating cells fall in the group-wide ABS which has also been configured. Since the CSI-RS can be arranged such that no other transmissions from within the group will be present in the REs containing CSI-RS, this allows the UE(s) to make measurements of CSI- RS which are interfered only by OGI. As a variation of this embodiment a common group-wide CSI-RS pattern is configured for all the cells in the CoMP group.

In the case of the extension of the first embodiment, where there is more than one cooperating eNodeB, this would require X2 interface exchange of the common group-wide CSI-RS pattern among all cooperating eNodeBs.

In a variation of this embodiment, and its extension, the CSI-RS pattern(s) is coordinated first and the group-wide ABS pattern is coordinated second, so as to position ABSs suitably in the time domain given a particular CSI-RS pattern or set of patterns.

In a further variation of this embodiment, MBSFN ABSs may be used instead of normal ABSs. Coordinated CSI-RS zero-power patterns

In a third embodiment, an eNodeB configures one or more CSI-RS zero-power patterns common to all cells in the CoMP group. The UE(s) can assume that only noise and OGI are present in these REs.

In the case of the extension of the third embodiment, like the extension to the first embodiment, where there is more than one cooperating eNodeB, this would require X2 interface exchange of all configured CSI-RS patterns and CSI-RS zero-power patterns among all cooperating eNodeBs.

Interference measurement patterns In the third embodiment, it may be necessary to configure several CSI-RS zero power patterns in order to provide enough REs in the correct locations for the UE to make suitable OGI measurements. This could result in a number of bitmaps being exchanged to mute the total set of REs required, with possibly some redundancy among them. Therefore, in a fourth embodiment of the invention, a new zero-power interference measurement pattern is defined (which is not a CSI-RS zero-power pattern and therefore possibly in addition to any CSI-RS zero-power pattern) which configures a set of REs in which the UE can assume no transmission, like a zero-power CSI-RS pattern but not constrained to the CSI-RS REs, and without the constraint to a four-port pattern. This results in signalling a single bitmap rather than potentially multiple bitmaps as in the third embodiment. There would be some implicit constraints on where these REs could be, e.g. CRS and PSS/SSS locations (among others) would need to be avoided. In the case where there is more than one cooperating eNodeB, this would require X2 interface coordination of the group-wide interference measurement pattern among all cooperating eNodeBs.

Co-ordinated CSI-RS patterns and CSI-RS zero-power patterns In a fifth embodiment each CSI-RS pattern transmitted by a cell within the CoMP group is arranged to coincide with an otherwise group-wide CSI-RS zero-power pattern (similar to the second embodiment where the ABS are replaced by CSI-RS zero power patterns). This means that CSI-RS can be measured with only OGI present.

Muted-PDSCH subframes In the first and second embodiments, the UE is configured with suitable ABS pattern(s). However, there is no mandatory requirement at the eNodeB to mute PDSCH in these subframes - it is merely an assumption the UE can make for measurement purposes. Therefore, in a sixth embodiment, a new type of subframe (which is not an ABS subframe and therefore possibly in addition to any ABS subframe) is defined where PDSCH shall be muted at the eNodeB. These subframes are used in a pattern-based manner analogous to ABS, and in the case of this invention, a CoMP group-wide common muted-PDSCH pattern is agreed. This PDSCH-muting pattern is then signalled to the UE for the purpose of measuring OGI. The subframe pattern for this muting can be defined and signalled in specifications in a way analogous to that for existing ABS, and agreement among multiple cooperating eNodeBs arrived at in a way analogous to that in the extension of the first embodiment.

As a variation of this embodiment, the muted PDSCH are arranged to coincide with a coordinated group-wide CSI-RS pattern (or patterns). One cell could signal CSI_RS at a time. CSI-RS could be transmitted at the same time, but in different frequency resources for the different cells, or otherwise distinguished, for example by orthogonal spreading codes. This pattern need not be explicitly signalled to the UE, since the scheduler can avoid transmitting PDSCH in subframes with CSI-RS present, the locations of which the UE will have been configured with. In the case of multiple cooperating eNodeBs, the scheduler at each of them can operate independently in this respect.

Inverse ABS configuration The current use of an ABS pattern is to define subframes with low interference from other cells.

In a seventh embodiment, this could be applied in reverse (otherwise like the first and second embodiments), such that the inverse of the ABS pattern corresponds to subframes with low interference from the serving cell (or cooperating group). If a UE is aware of an ABS pattern used for coordinating interference from outside the serving group, it may be assumed that other frames have low interference from within the serving group.

If a pattern of subframes is signalled to a UE, an additional indication could be used to indicate if these subframes should be assumed to have low (or perhaps high) interference from the cooperating group. Alternatively, the UE assumption could be pre-determined by the system specification.

Reverse RNTP configuration

In an eighth embodiment, an indicator based on LTE Rel-8 RNTP (Relative Narrowband Transmit Power) is circulated over the X2 interface. Currently, an RNTP bitmap sent from one eNodeB to another over X2 indicates specific RBs where transmission power from the first eNodeB will not exceed a configured level and other RBs where no promise to limit transmission power is made. This could already be used to configure a UE to make OGI measurements in the limited-power RBs, although some new signalling from the eNodeB to a UE to indicate that OGI measurements are to be conducted in the relevant RBs could be needed. The first eNodeB then signals to the UE(s) over RRC that OGI measurements can or shall be conducted in the indicated RBs. It could further signal to the UE(s) the RRNTP threshold it has set, allowing the UE(s) to take some account of the in-group interference which may affect their OGI measurements. When the first eNodeB decides that OGI measurement requirements according to the signalled bitmap have been met, it can signal an all-zero bitmap to release the constraints. In a variation on this, a single-bit indicator can be sent in the LOADJNFORMATION message to indicate that the OGI measurement bitmap has now expired.

In this embodiment, a 'reverse' RNTP (RRNTP) request is made by a first eNodeB to at least one neighbouring eNodeB, to indicate, on a frequency-domain per-RB basis, which RBs the first eNodeB wishes to configure a UE to measure OGI in and therefore where reduced transmission power from the neighbours is requested (rather than Offered' as with Rel-8 RNTP). This could be supplemented by an indication of transmission power threshold it prefers not to be exceeded by any cooperating eNodeB in the indicated RBs. In other RBs, no particular request regarding transmission power limitation by the neighbouring eNodeBs is made. The eNodeBs in the CoMP group (including the first eNodeB) are free to use any transmission powers in the indicated RBs up to the indicated threshold.

The first eNodeB then signals to the UE(s) over RRC that OGI measurements can or shall be conducted in the indicated RBs. It could further signal to the UE(s) the RRNTP threshold it has set, allowing the UE(s) to take some account of the in-group interference which may affect their OGI measurements. When the first eNodeB decides that OGI measurement requirements according to the signalled bitmap have been met, it can signal an all-zero bitmap to release the constraints. In variation on this, a single-bit indicator can be sent in the LOADJNFORMATION message to indicate that the OGI measurement bitmap has now expired. The existing Rel-8 high-interference indicator (HII) and overload indicator (01) are not fully suitable for this purpose since they indicate only that interference will be or is being experienced, respectively, in a particular RB rather than to make a request and indicate a desired transmission power limit.

If several eNodeBs make concurrent requests, then any cooperating eNodeB is effectively limited to the lowest concurrent RNTP threshold. It may not be possible for a given eNodeB to meet the lowest threshold, but it may be able to meet some of the higher concurrent thresholds.

Therefore, in an extension of this embodiment, each cooperating eNodeB sends an indication to the first eNodeB as to whether it will comply with the RNTP restriction request. This could be an existing Rel-8 RNTP bitmap and threshold indicating where it will in fact apply what RNTP limit, or a simple yes/no indication for the entire request. On receiving all such indications within a certain time, the first eNodeB can decide whether enough compliance has been offered and thus whether to signal measurement capability to the UE(s).

In a further variation, the first eNodeB may further signal to those neighbouring eNodeBs which indicated compliance with its request, whether it has instructed the UE(s) to make OGI measurements in the indicated RBs, i.e. whether their compliance is actually required. Alternatively, it can simply send per-RB bitmap which cancels the previous if insufficient compliance has been offered.

Using CRS for OGI Measurements

In the previous embodiments it has been assumed that the UE uses CSI-RS for channel measurements and for CoMP. However, similar embodiments are possible where common reference symbols (CRS) are used instead.

CRS are defined to LTE (Release 8) but are also an essential feature for later Releases. The channel transfer function could not be measured with CRS for both the serving cell and other cells (including cells in the cooperating group). This is facilitated if the CRS for different cells have different frequency offsets. This can be achieved, for example, by suitable choice of cell IDs, although the available number of different frequency offsets is limited. Therefore if muted resources (e.g. resource elements) are configured, in particular subframes, these could be arranged such that the same resources are muted in all the cooperating cells (or for all the cooperating antennas). This would allow out-of-group interference to be measured in these resources. If muted resources coincide with CRS, then the corresponding channel transfer functions could be measured, subject only to interference from outside the group. Non-cell embodiment

In general, a UE may receive signals from one or more antenna ports, which each may be formed by the network from one or more physical antennas. Each physical antenna is controlled by one eNodeB, and each eNodeB may control any number of physical antennas. An antenna port may be formed from physical antennas controlled by one or more eNodeBs.

For any of the embodiments described above, the one or more eNodeBs forming the antenna ports for a UE exchange suitable information with all eNodeBs in the CoMP group to establish the coordination of transmission resources as appropriate for the embodiment. The signalling to indicate this to a UE is then sent over the antenna ports which that UE expects to receive the relevant signals from.

In a variation of this, a particular set of antenna ports may be arranged to form a cell, for the purposes of transmission to at least one UE.

General

In summary, invention embodiments provide for downlink (DL) coordination of resource use and reference signal patterns among the several cells/antennas and/or eNodeBs operating in a coordinated multipoint (CoMP) group. This coordination is so that a UE is able to measure the interference arising from outside of the CoMP group without these measurements being disturbed by transmissions from within the group. According to preferred embodiments, the cooperating cell(s) and/or eNodeBs agree coordination so as to reduce or eliminate their in-group transmissions in a common set of resource elements (REs) across the ciroup. The UE can then use these resources to make Out-of-Group Interference (OGI) measurements to provide enhanced channel state feedback to the network. Intercell interference coordination (ICIC) is already in LTE standards but only to avoid interference from other cells than the serving cell, whereas the signaling of embodiments is to mute the serving cell(s) in particular REs and arrive at this coordination on a CoMP group-wide basis. Having controlled the transmission power in specific resources, it may be then (additionally or separately) advantageous to position certain reference signals in these resources so that measurements of them subject only to OGI may be conducted. CoMP schemes which utilize feedback of OGI information to the network are known to have improved data rates, or alternatively reduced transmission power, since the transmission characteristics (e.g. precoding) can be better adapted to the interference environment. The invention provides a UE with the ability to obtain a significantly improved measurement of out- of-group interference (OGI) than would currently be possible in a single cell or multi-cell scenario.

The invention embodiments have been described with reference to LTE FDD, but could also be applied for LTE TDD, and the principles applied to other communication systems.

The general principles of the embodiments (with the exception of the eighth, reverse RNTP configuration) could also be used for the case where the cooperating group consists of only one cell since even in that case, measurements of OGI" at the UE may still be useful.

Claims

Claims:

1. A communications network including a group of two or more in-group antennas associated with one or more base stations and a user equipment, wherein

the in-group antennas are arranged to co-operate in downlink transmission to the user equipment;

transmission from the in-group antennas is co-ordinated so that their downlink transmission in particular transmission resources is muted; and wherein

the user equipment is configured to measure a downlink channel in a muted transmission resource.

2. A communications network according to claim 1 , wherein all the in-group antennas are associated with the same base station and together provide a single cooperating cell, and the user equipment measurement investigates out-of-group interference in the form of out-of-cell interference.

3. A communications network according to claim 1 or 2, wherein each antenna is an antenna port corresponding to a plurality of physical antennas which are configured to transmit copies of the same transmission signal.

4. A communications network according to claim 1 or 3, wherein

each antenna is associated with a single base station providing a single cell, at least two of the in-group antennas are associated with different base stations, so that the grouping of the antennas leads to a group of co-operating in-group cells;

the in-group cells are configured to co-operate in downlink transmission to the user equipment;

the in-group cells are configured to co-ordinate their downlink transmission in particular transmission resources to be muted; and wherein

the user equipment measurement investigates out-of-group interference.

5. A communications network according to any of the preceding claims, wherein the downlink transmission in the muted transmission resources is muted by reduction of transmission power and/or by disallowing some or all transmission.

6. A communications network according to any of the preceding claims, wherein

transmission from all group members is muted in the muted transmission resources and the downlink channel measurement in a muted transmission resource is measurement of noise and interference properties of a channel including any out-of-group interference and; and/or wherein

transmission from all the group members is muted in the muted transmission resources except for transmission of a reference signal and the downlink channel measurement is measurement of a channel transfer function including any out-of-group interference.

7. A communications network according to any of the preceding claims, being

configured to use a frame-based method for transmission, in which time is divided into subframes of predetermined duration for downlink and uplink transmission, and in which the particular transmission resources comprise at least one muted downlink subframe.

8. A communications network according to claim 7, wherein the muted downlink subframe is indicated to be an almost blank subframe, ABS or MBSFN ABS, as defined in LTE A, or a PDSCH muted subframe in which the eNodeB or eNodeBs of the co-operating group members are specifically required to mute PDSCH transmission, and preferably wherein the UE measures a downlink channel in a portion of the muted downlink subframe in which the UE is not expecting transmission from group members except for any reference signals and/or in which the UE is expecting out-of-group interference from downlink data transmission.

9. A communications network according to any of the preceding claims, being

configured to use a frame-based method for transmission, in which time is divided into subframes of predetermined duration for downlink and uplink transmission, and in which the particular transmission resources comprise at least one muted portion of a downlink subframe, preferably at least one muted resource element in an LTE subframe.

10. A communications network according to any of the preceding claims, wherein each group member is configured to transmit a channel measurement reference signal, such as CSI-RS in LTE A or CRS in LTE, in separate resources within the muted transmission resources, to allow measurement of a channel transfer function including any out-of-group interference.

11. A communications network according to any of the preceding claims, wherein a group member is configured not to transmit in resources reserved reference signal intended for channel measurement by another group member, such as CSI-RS in LTE A or CRS in LTE, to provide zero-power CSI-RS or CRS resources as the muted transmission resources, to allow measurement of noise and out-of-group interference properties.

12. A communications network according to any of the preceding claims, wherein each group member is configured not to transmit in a group-wide pattern of zero-power transmission resources, which are specifically reserved for out-of-group interference measurement.

13. A communications network according to claim 4 or any claim when dependent thereon, wherein the in-group cells are provided by more than one eNodeB; and wherein the eNodeBs providing the in-group cells exchange signalling to control the in-group co-operation, preferably in the form of one or more patterns of muted transmission resources and/or one or more patterns of reference signal resources.

14. A communications network according to any of the preceding claims wherein the inverse of an LTE A ABS pattern is used to indicate muted transmission resources, and/or wherein LTE Relative Narrowband Transmit Power RNTP signalling is used to indicate or request at least one future power limit in specific resources, thus defining muted transmission resources.

15. A telecommunications method in a network comprising a group of two or more in-group antennas of one or more base stations and a user equipment, wherein

the in-group antennas co-operate in downlink transmission to the user equipment;

the user equipment measures a downlink channel in the muted transmission resources.

16. A user equipment in a communications network including a group of two or more in-group antennas of one or more base stations and the user equipment, wherein

the user equipment is configured to receive cooperative downlink transmission from the in-group antennas;

the user equipment is configured to measure a downlink channel in the muted transmission resources created by coordinated transmission from the in-group antennas.

17. An eNodeB in a communications network including a group of cells provided by more than one base station, at least one of the cells being associated with the eNodeB and another of the cells being associated with another eNodeB, and a user equipment, wherein

the eNodeB is configured to control said at least one cell to co-operate with the other cells in the group in downlink transmission to the user equipment; the eNodeB is configured to control said at least one cell to cause particular transmission resources to be muted by co-ordinating its downlink transmission with the other cells in the group; and wherein

the eNodeB is configured to exchange signalling messages with at least said another eNodeB to coordinate downlink transmission across the group of cells.