WO2010061302A2

WO2010061302A2 - Antenna configuration for co-operative beamforming

Info

Publication number: WO2010061302A2
Application number: PCT/IB2009/054826
Authority: WO
Inventors: Timothy J. Moulsley; Lingyang Song; Matthew P. J. Baker; Ludovicus M.G.M. Tolhuizen
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2008-11-03
Filing date: 2009-10-30
Publication date: 2010-06-03
Also published as: WO2010061302A3

Abstract

The present invention relates to a method for signaling a precoding vector to be used by a secondary station during a beamforming reception comprising a plurality of streams from at least two different cells in a network, comprising for each stream, a primary station transmitting an indication of the precoding vector and an indication of a corresponding cell

Description

ANTENNA CONFIGURATION FOR CO-OPERATIVE BEAMFORMING

FIELD OF THE INVENTION

The present invention relates to a method of communication in a communication system like a mobile communication system, for example UMTS, LTE or LTE Advanced.

More specifically, the invention relates to a method of communication using cooperative beamforming, where the beamforming is obtained by using primary station antennas from different cells, and where pre-coding codebooks for co-operative beamforming between different cells are used.

BACKGROUND OF THE INVENTION

In a cellular telecommunication system, a plurality of user terminals within a cell communicate with a primary station. With the subsequent generations of cellular systems, the achievable data rate has been keeping increasing. In advanced systems such as UMTS and LTE, multi-antenna transmission/reception techniques variously described as, MIMO, precoding or beamforming are supported for transmissions from a single cell to a mobile terminal. Thanks to the spatial selectivity of the beamforming mode, such transmission modes have enabled an important increase of the achievable data rate and of the range of communication, while maintaining the average interference level.

In order to achieve the beamforming, a typical transmitting station having an antenna array applies a set of complex coefficients (forming a precoding matrix or precoding vector) to a signal transmitted from its respective antennas, so that the transmission stream is spatially directed towards a receiving station. However, reception of such a beamformed transmission may require the communication of this set of complex coefficients between the transmitting station and the receiving station. In implementations of such systems, precoding codebooks are defined. These precoding codebooks may be viewed as a way of describing precoding matrix (or precoding vector) of the channel coefficients or precoding weights in a compact way, thereby reducing the amount of required signalling for indicating the precoding. These codebooks also enable the user terminal (defined in LTE as a User Equipment or UE) to report to the network a preferred precoder for downlink transmission, in the form of an index to codebook entry. In this case, the preferred precoder is a set of complex coefficients to be applied to transmit antennas of the base station (defined in LTE as an eNodeB). Similarly, precoding codebooks may also be used by the base station to signal the precoding used for a transmission to the user terminal. This enables the user terminal to derive an appropriate phase/amplitude reference from common pilot symbols for demodulation of each downlink transmission.

In systems such as UMTS and LTE, multi-antenna transmission/reception techniques variously described as, MIMO, precoding or beamforming are supported for transmissions from a single cell to a mobile terminal. Precoding codebooks are defined, which enable the UE to report a preferred precoding index for downlink transmission. Typically the same codebook may be used to signal to a UE the precoding vector or matrix which is actually applied in the downlink by a base station. This enables the UE to derive an appropriate reference signal from common pilot symbols for demodulation of downlink transmissions.

In LTE, this signalled codebook index is referred to as PMI (Precoding Matrix Indicator). Typically, the same codebook may be used on one hand to signal to a user terminal the precoding vector or matrix which is actually applied in the downlink by a base station and on the other hand to feed back the preferred precoding matrix by the user terminal to enable a phase/amplitude reference to be derived. Alternatively, the reference(s) may be provided by precoded reference symbols (i.e. dedicated reference symbols or DRS).

Recently, it has been proposed to use cooperative beamforming, i.e. beamforming using antennas from multiple cells or multiple base station sites (under the description of CoMP or Co-operative Multi-Point transmission). Such systems are introduced on Figure 1. In such a system, a user terminal 110 within a serving cell 101a communicates in normal (i.e. single cell) mode with a primary station 100a. In normal beamforming mode, the primary station 100a applies a set of precoding weights to the signal to be transmitted from its transmit antennas 104a to create a spatial stream 105a directed towards the user terminal 110.

In a cooperative beamforming mode, a second primary station 100b in a neighboring cell 101b would use some of its antennas 104b to transmit in a cooperative way the same signal 105b (or another signal) as the one 105 a transmitted by the primary station 100a to the user terminal 110. The spatial stream now comprises two components 105a and 105b. As explained above, the user terminal 110 needs to feed back a channel state estimate based on measurements on received reference symbols. This estimate in this example is an indication of a preferred precoding matrix (or vector if there is only one transmission stream) in the form of a codebook index.

Typically, the specification for systems like LTE makes use of the term "antenna port", which is effectively a virtual antenna which may be derived by a linear combination of signals from one or more physical antennas. For convenience, we use the term "antenna", but this could also be understood as "antenna port"

Multi-antenna techniques such as beamforming, using antennas from multiple cells or multiple sites, could be of interest in LTE. However, it is not clear how codebooks should be defined for such cases, and it is not clear how existing (or new) codebooks designed for single cell use could be adapted for multi-cell use.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide with a method for providing codebooks that could be used for cooperative beamforming.

The invention comprises a method of mapping antennas from multiple cells to a codebook which may be suitable for a single cell, the definition of an "active beamforming set" a method of making measurements on candidate cells for co-operative beamforming.

In accordance with a first aspect of the invention, it is proposed a method for mapping antennas from multiple cells to a codebook which may also be used in a single cell.

In accordance with a second aspect of the invention, it is proposed a method for defining a set of cells which may potentially be used for co-operative beamforming.

In accordance with a third aspect of the invention, it is proposed a method for measurements by the UE on cells which could be candidates for co-operative beamforming. Each feature could be implemented independently.

In accordance with another aspect of the invention, a method is proposed for signaling a precoding vector to be used by a secondary station during a beamforming reception comprising a plurality of streams from at least two different cells in a network, comprising for each stream, a primary station transmitting an indication of the precoding vector and an indication of a corresponding cell.

In accordance with still another aspect of the invention, a primary station is proposed, the primary station operating in a first cell and comprising means for signaling a precoding vector to be used by a secondary station during a beamforming transmission to the secondary station, said beamforming transmission comprising a first data stream from the first cell and a second data stream from a further cell, the primary station being arranged for transmitting an indication of the precoding vector and an indication of the first cell.

In accordance with another aspect of the invention, a secondary station is proposed, the secondary station comprising means for receiving a beamforming transmission comprising at least a first data stream from at least a first cell and a second data stream from a second cell, and means for receiving a signaling message including an indication of the precoding vector and an indication of a corresponding cell.

These and other aspects of the invention will be apparent from and will be elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described in more detail, by way of example, with reference to the accompanying drawings, wherein:

Fig. 1; already described, is a block diagram of architectures that are being discussed in 3GPP.

Fig. 2 is a block diagram of a communication system in accordance with a first embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION This invention relates to a telecommunication system comprising a plurality of cells, each cell being controlled by a primary station, said primary station being able to communicate with secondary stations that are within the cell. Such a system will be detailed with reference to the system depicted on Figure 2.

As shown on Figure 2, a secondary station 410 contained in cell 401a communicates with a primary station 400a. In an exemplary embodiment of the invention, such a telecommunication system operates under the UMTS specifications or LTE specifications. Accordingly, the primary station 400 a may be an eNodeB and the secondary station 410 is a User Equipment (UE). A second cell 401b neighbouring cell 401a is represented on Figure 4. This cell 401b is controlled by a primary station 400b. On Figure 2, the primary stations 401a and 401b are represented as two different primary stations, however, it should be noted that it is possible that a single primary station may control a plurality of cells (usually three in current networks).

For the sake of clarity only one secondary station is represented, however, a plurality of secondary stations may be within a cell. Each primary station 400a and 400b comprises an antenna array 404a and 404b respectively having a plurality of antennas, in this example four antennas per cell. In accordance with a cooperative beamforming mode, the secondary station 410 may receive signals from its cell primary station 400a and from neighboring cells primary stations, in this example from primary station 400b. The primary stations 400a and 400b transmit to the secondary station 410 by using each set of respective precoding coefficients a respective data stream 4002a and 4002b and respective sets of reference symbols 4001a and 4001b. In this cooperative beamforming mode, the data streams are identical. Therefore the two data streams 4002a and 4002b can be considered as a single joint transmission. In another cooperative beamforming mode the data streams may be different. The reference symbols 4001a and 4001b may be used by the secondary station 410 for decoding efficiently the data streams 4002a and 4002b. On the basis of these reference symbols 4001a and 4001b, or on other reference symbols embedded in the downlink channels transmitted by the primary stations 400a and 400b, the secondary station 410 estimates channel coefficients in order to establish a preferred precoding matrix for the respective cells.

As illustrated on Figure 2, it may be possible to report a PMI (Precoding Matrix Indicator) for each co-operating cell, i.e. the user terminal transmits an indication of a first preferred precoding matrix 411a for the serving cell 401a to the primary station 400a and an indication of a second preferred precoding matrix 411b for the cell 401b to the primary station 400b. Thus, the base stations 400a and 400b may use different precoding in order to have a fine adjustment of the transmission beam. This means that the user terminal needs to feed back as many PMIs as there are cooperating cells. This may represent a great amount of signalling and overhead.

On Figure 2, it is shown that the secondary station transmits indications of precoding matrix 411a and 41 Ib respectively to cells 401a and 401b. It is however to be noted that bth messages may be transmitted to a single cell, then relaying the relevant precoding indications to the corresponding neighbouring cells.

In this example of a system like LTE, co-operative beamforming is carried out between two cells. The precoding is assumed to be carried out for each spatial stream by applying a precoding vector across a set of antennas, which belong to more than one cell. The vectors are contained in a codebook. Precoding is signalled for a spatial stream indicating the precoding vector, and the cell and antenna corresponding to each element in the vector. For each spatial channel, each type of information is indicated by a defined field (e.g. vector index from a codebook, cell from a list of cells, antenna from the set of antennas in a cell). An individual user may receive one or more spatial channels. This approach would have the advantage of allowing the selection of a subset of the antennas from a cell with more than one antenna.

In another embodiment in a system like LTE, co-operative beamforming is carried out between a minimum of two cells. In this case it is assumed that a cell engaged in co-operative beamforming uses all its antennas for that purpose. The precoding for a set of spatial channels is assumed to be carried out by applying a precoding matrix across a set of antennas, which comprises all those belonging to the beamforming cells. When an entry from a precoding codebook is signalled, additional information is also signalled indicating which cells are associated with the codebook entries. The UE is associated with a serving cell, and a set of additional cells is defined which can support co-operative beamforming (we denote this as "active beamforming set", which has some similarities to an "active set" in UMTS). As an example, the serving cell may be identified as "cell 0" and the three entries in the "active beamforming set" are cell 1, cell 2 and cell 3. Therefore, if a UE is located on the border between cell 0 and cell3, these two cells should be used for co-operative beamforming. In this case the additional information would indicate cells "0" and "3", this would be understood as indicating that the beamforming is carried out using all the antennas from cell 0 (the serving cell) and cell 3 (the 3rd entry in the active beamforming set). So if each cell has two antennas, the first two antennas would be from cell 0 and the second two antennas would be from cell 3. In this embodiment, according to the example, the appropriate codebook would be one designed for four antennas. An additional 2 bits of information would be required to indicate which two cells are included (6 ways of selecting 2 cells from 4). The same codebook could be used for beamforming from a single cell with 4 antennas. In an embodiment such as the previous one the UE is associated with a serving cell, the "active beamforming set" may be defined in different ways such as: UE specific signalling

• first few entries in the neighbour cell list

• first few entries in the neighbour cell list which are indicated as members of the "active beamforming set" (e.g. with a flag) In a variation of this embodiment, the co-operative beamforming always includes the serving cell and one additional cell from the active beamforming list. In this case it is only necessary to indicate which additional cell is included in the beamforming and the relevant codebook entry. In some embodiments more than one additional cell may be supported in co-operative beamforming operation and it may be necessary to indicate how many cells are included, increasing the number of bits, or requiring a variable length message.

For simplicity the number of antennas in all the co-operating cells would be the same. However this is not essential. For example, with a codebook designed for four antennas, if the serving cell has two antennas, co-operative beamforming can be carried out with two additional cells with one antenna each. So to support operation with different numbers of antennas in different cells, the UE will need to be able to reliably determine the number of antennas for the co-operating cells. This could be done for example, by measurement, or by signalling, e.g. additional information in the neighbour cell list (or relating to it).

In order to allow the network to use co-operative beamforming effectively, it is necessary that the network has knowledge of the channel characteristics of the radio paths from each cell to the UE. In one embodiment of this feature the UE makes measurements of the channels in each of the cells in "active beamforming set". The measurements could comprise, for example, one or more of:

• Average signal power from a cell to the UE (based on reference symbols) • Average SNR or SIR or SINR of transmissions from a cell to the UE (based on reference symbols)

• transmission rate from a cell to the UE

• transmission parameters (e.g. modulation and coding scheme) from a cell to the UE. • Preferred cells for co-operative beamforming (and estimated achievable data rate)

• Preferred cell(s) assuming that one cell is the serving cell

• Preferred precoding vectors/matrix (for preferred cells)

• Preferred antennas for co-operative beamforming (and estimated achievable data rate)

• Preferred precoding vectors (for preferred antennas)

• CQI (channel quality information) , expressed as an achievable transmission rate (or equivalent)

• Transmission rank The UE reports one or more of these measurements to the network.

In other embodiments, the UE derives quality reports from one or measurements, where a quality report corresponds to a beam created by multiple cells. Such a quality report may comprise, for example, one or more of:

• transmission rate for a beam • SNR, SIR or SINR for a beam

• transmission parameters (e.g. modulation and coding scheme) for a beam. One or more such quality reports are reported to the network. The invention is applicable to systems using co-operative beamforming between cells which may include LTE -Advanced. The cells may be located a single base station site, or on different sites, for example femto-cells implemented by fibre radio techniques.

In the present specification and claims the word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. Further, the word "comprising" does not exclude the presence of other elements or steps than those listed.

The inclusion of reference signs in parentheses in the claims is intended to aid understanding and is not intended to be limiting.

From reading the present disclosure, other modifications will be apparent to persons skilled in the art. Such modifications may involve other features which are already known in the art of radio communication.

Claims

1. A method for signaling a precoding vector to be used by a secondary station during a beamforming reception comprising a plurality of streams from at least two different cells in a network, comprising for each stream, a primary station transmitting an indication of the precoding vector and an indication of a corresponding cell.

2. The method of claim 1 , wherein the secondary station is associated with a reference cell.

3. The method of claim 2, wherein the indication of the cell corresponding to the precoding vector is based on the reference cell.

4. The method of claim 2 or 3, wherein the primary station first transmits the indication of the precoding vector associated to the reference cell, and then transmits the indication of the precoding vector associated to at least one other cell.

5. The method of claim 4, wherein the indication of the at least one other cell is done by higher layer signaling.

6. The method of any of the preceding claims, further comprising the network generating an active beamforming list of cells that are able to perform a cooperative beamforming transmission, and selecting at least two cells from the list to perform cooperative beamforming transmission with the secondary station.

7. The method of claim 6, wherein the active beamforming list is signaled to the secondary station independently.

8. The method of claim 6, further comprising signalling to the secondary station the neighbor list of cells neighboring a reference cell to which the secondary station is associated, and wherein the neighbor list is ordered so that the cells of the active beamforming list are at a predetermined position in the neighbor list.

9. The method of claim 6 or 8, further comprising flagging the cells of the active neighboring list that are currently engaged in a cooperative beamforming operation mode.

10. The method of any of the preceding claims, wherein the number of antennas used for the beamforming is predetermined.

11. The method of any of the claims 6 to 10, wherein the secondary station performs channel quality measurements between each cell of the active beamforming list and the secondary station.

12. The method of claim 11, wherein for at least one cell of the active beamforming list the secondary station estimates of the data rate that could be reached in a beamforming reception with the considered cell.

13. The method of claim 12, wherein the secondary station signals a report representative of the estimated reachable data rate for the considered cell.

14. A primary station operating in a first cell, the primary station comprising means for signaling a precoding vector to be used by a secondary station during a beamforming transmission to the secondary station, said beamforming transmission comprising a first data stream from the first cell and a second data stream from a further cell, the primary station being arranged for transmitting an indication of the precoding vector and an indication of the first cell.

15. A secondary station comprising means for receiving a beamforming transmission comprising at least a first data stream from at least a first cell and a second data stream from a second cell, and means for receiving a signaling message including an indication of the precoding vector and an indication of a corresponding cell.