Control signaling and resource mapping for coordinated transmission

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Patent Application 61/585,556, filed Jan. 11, 2012, whose disclosure is incorporated herein by reference.

FIELD OF THE DISCLOSURE

The present disclosure relates generally to wireless communication, and particularly to methods and systems for signaling in wireless communication systems.

BACKGROUND

In some Multiple-Input Multiple-Output (MIMO) communication systems, multiple cells use Cooperative Multipoint (CoMP) transmission schemes for coordinating downlink MIMO transmissions to User Equipment (UEs). Third Generation Partnership Project (3GPP) Long Term Evolution-Advanced (LTE-A) systems, for example, use or contemplate the use of multiple CoMP modes such as Dynamic Point Selection (DPS), Dynamic Point Blanking (DPB), Cooperative beamforming (CB) and Joint Transmission (JT).

The CoMP modes used in LTE-A are specified, for example, in “3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Coordinated multi-point operation for LTE physical layer aspects (Release 11),” 3GPP TR 36.819, version 11.0.0, September, 2011, which is incorporated herein by reference. When using CoMP, the cooperating cells typically configure their transmissions based on channel feedback provided by the UEs.

The description above is presented as a general overview of related art in this field and should not be construed as an admission that any of the information it contains constitutes prior art against the present patent application.

SUMMARY

An embodiment that is described herein provides a method, including receiving in a mobile communication terminal signals from a group of cells that cooperate in a Coordinated Multipoint (CoMP) transmission scheme. Signaling information is received from a first cell in the group. The signaling information is indicative of a first pattern of time-frequency Resource Elements (REs) used by the first cell for transmitting reference signals, and is further indicative of a second pattern of the REs used by a second cell in the group for transmitting the reference signals. A third pattern of the REs, which are available for receiving data from the cells, is derived in the terminal from the signaling information that is indicative of the first and second patterns. The data from the cells is received in the terminal in one or more of the REs in the third pattern.

In some embodiments, receiving the signaling information includes receiving an indication of a time-frequency shift of the second pattern, and deriving the third pattern includes determining the third pattern based on the shift. In an embodiment, receiving the signaling information includes receiving an indication of a number of symbols used by the second cell for transmitting a control channel prior to the second pattern, and deriving the third pattern includes determining the third pattern based on the indication.

In another embodiment, receiving the signaling information includes receiving an index that enumerates multiple possible combinations of the first and second patterns, and deriving the third pattern includes determining the third pattern based on the index. In yet another embodiment, receiving the signaling information includes receiving a list of the cells in the group, or a measurement list of the cells in a measurement group over which the terminal is to perform signal measurements.

In an example embodiment, deriving the third pattern includes including in the third pattern only the REs that do not belong to any of the first and second patterns. In a disclosed embodiment, deriving the third pattern includes selecting, based on the signaling information, a subset of the REs in which to receive the data from the cells in a given CoMP mode. In some embodiments, receiving the signaling information includes receiving an indication of at least one RE that is to be skipped in deriving the third pattern, and deriving the third pattern includes omitting the at least one RE from the third pattern.

In an embodiment, deriving the third pattern includes omitting from the third pattern the REs of a given symbol upon detecting that the first cell or the second cell transmits one or more of the reference signals in the given symbol. In another embodiment, receiving the signaling information includes receiving one or more of a dynamic signaling and a semi-static signaling.

There is additionally provided, in accordance with an embodiment that is described herein, a communication apparatus including a transceiver and a processor. The transceiver is configured to receive signals from a group of cells that cooperate in a Coordinated Multipoint (CoMP) transmission scheme, and to receive, from a first cell in the group, signaling information which is indicative of a first pattern of time-frequency Resource Elements (REs) used by the first cell for transmitting reference signals, and is further indicative of a second pattern of the REs used by a second cell in the group for transmitting the reference signals. The processor is configured to derive from the signaling information that is indicative of the first and second patterns, signaled by the first cell, a third pattern of the REs that are available for receiving in the transceiver data from the cells, and to control the transceiver to receive the data from the cells over one or more of the REs in the third pattern.

In some embodiments, a mobile communication terminal includes the disclosed communication apparatus. In some embodiments, a chipset for processing signals in a mobile communication terminal includes the disclosed communication apparatus.

The present disclosure will be more fully understood from the following detailed description of the embodiments thereof, taken together with the drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram that schematically illustrates a MIMO communication system that uses CoMP, in accordance with an embodiment that is described herein;

FIGS. 2 and 3 are diagrams showing allocations of time-frequency Resource Elements (REs) in a MIMO communication system that uses CoMP, in accordance with embodiments that are described herein; and

FIG. 4 is a flow chart that schematically illustrates a method for downlink reception in a mobile communication terminal, in accordance with an embodiment that is described herein.

DETAILED DESCRIPTION OF EMBODIMENTS

Embodiments that are described herein provide improved methods and systems for signaling in communication systems that use CoMP. In the disclosed embodiments, a group of cooperating cells transmit downlink MIMO signals to mobile communication terminals in multiple time-frequency Resource Elements (REs). In the embodiments described herein, the cells transmit the signals using Orthogonal Frequency Division Multiplexing (OFDM) modulation, in accordance with the LTE or LTE-A specifications, but the disclosed techniques can be used with various other communication protocols.

For a given terminal, one of the cells in the group is defined as an anchor cell (also referred to as primary cell or serving cell) and the other cells are defined as secondary cells. Among other functions, the anchor cell is responsible for transmitting signaling information to the terminal. Typically, each cell transmits reference signals (e.g., Common Reference Signals—CRS) in accordance with a respective periodic pattern of REs in time and frequency. The patterns used for transmitting the RSs may differ from one cell to another. Thus, only the REs that do not belong to any of the patterns are free for transmitting downlink data from the cells to the terminal.

In some embodiments, the anchor cell transmits to the terminal signaling information, which is indicative of the time-frequency RE patterns that are used by the different cells in the group for transmitting reference signals. In other words, the signaling information provided by the anchor cell indicates not only the pattern used by the anchor cell itself, but also the patterns used by the secondary cells. The terminal receives the signaling information, and uses it to identify the REs that are available for receiving downlink data from the cells. The terminal then receives the downlink data using one or more of these identified REs.

The disclosed techniques enable the terminal to identify which REs are available for data reception, by decoding only the signaling of the anchor cell, even when different cells transmit reference signals in different RE patterns (e.g., when different cells use different cell IDs). Several examples of reference-signal patterns and signaling schemes for various CoMP modes are described herein.

FIG. 1 is a block diagram that schematically illustrates a Cooperative Multipoint (CoMP) communication system 20, in accordance with an embodiment that is described herein. In the present example, system 20 operates in accordance with the Third Generation Partnership Project (3GPP) Long Term Evolution-Advanced (LTE-A) specifications. In alternative embodiments, system 20 may operate in accordance with any other suitable communication protocol in which cells coordinate transmission with one another, such as, for example, WiMAX.

In the embodiment of FIG. 1, system 20 comprises a mobile communication terminal 24 (referred to in LTE-A terminology as User Equipment—UE) and three cells 28 (base stations) denoted CELL1, CELL2 and CELL3. The terms cell, base station, eNodeB and Transmission Point (TP) are used interchangeably herein. The choice of a single UE and three cells is made purely by way of example. In real-life configurations, system 20 typically comprises a large number of cells, some of which may be collocated, and a large number of terminals. Each UE 24 comprises, for example, a cellular phone, a wireless-enabled computing device or any other suitable type of communication terminal.

Cells 28 cooperate with one another in transmitting precoded (i.e., beamformed) signals to UEs 24. Depending on the CoMP mode or on other factors, the cells may cooperate in beamforming, beam activation and deactivation, transmission scheduling or other tasks. A group of cells that cooperate in this manner, such as CELL1, CELL2 and CELL3, is referred to as a cooperating set. In various embodiments, cells 28 may use CoMP modes such as DPS, DPB, JT, CB, and possibly alternate between different modes over time.

In the present embodiment, system 20 comprises a central scheduler 32, which schedules the transmissions of the various cells to the various UEs, and calculates precoding vectors (i.e., sets of complex weights to be applied to the signals transmitted via the respective transmit antennas of the cells) to be applied by the cells when transmitting the CoMP transmissions, in an embodiment.

In some embodiments the central scheduler also selects the appropriate CoMP mode, and the cell or cells in the set that will transmit to a UE. For a given UE 24, one of the cells in the group is typically assigned by scheduler 32 to serve as an anchor cell or primary cell, and the other cells in the group are referred to as secondary cells. Among other functions, the anchor cell is responsible for transmitting signaling information to the UE.

Central scheduler 32 typically selects the CoMP mode, the transmitting cell or cells, and/or the precoding vectors, based on channel feedback that is received from one or more of the UEs. The UE typically calculates the channel feedback by performing signal measurements on reference signals that are transmitted by the cells. In the disclosed embodiments, the time-frequency positions of the reference signals is signaled to the UE using methods that are described in detail below. Using these techniques, the UE is able to identify which REs are occupied by reference signals and which REs are available for receiving data.

Central scheduler 32 is shown in system 20 of FIG. 1 as a separate entity solely by way of example. Alternatively, central scheduler 32 can be implemented as part of a cell or collocated with a cell among the cooperating set (cells 28), for example CELL1. In another embodiment, the function of the central scheduler is carried out in a decentralized fashion, without a single entity controlling the cells within the cooperating set.

In the embodiment of FIG. 1, UE 24 comprises one or more antennas 36, a downlink receiver (DL RX) 40, an uplink transmitter (UL TX) 44, and processing circuitry 48. Receiver 40 receives downlink signals from cells 28 via antennas 36. Processing circuitry 48 processes the received signals. Among other tasks, processing circuitry 48 extracts signaling information that is received from the anchor cell. The signaling information indicates the patterns of REs used by the various cells in the group for transmitting reference signals. From the signaling information, the UE derives the REs that are available for data reception, and uses one or more of these REs to receive data from cells 28.

In the context of the present patent application and in the claims, the term “data” refers to user data that is sent from the cells over data channels, and not to reference signals. In an example embodiment, the data is received in Physical Data Shared Channels (PDSCH), and the reference signals comprise Common Reference Signals (CRS), both specified in the LTE and LTE-A specifications. In alternative embodiments, any other suitable type of data and reference signals can be used.

In the present embodiment, processing circuitry 48 comprises a signaling extraction module 52, which extracts the signaling information from the received downlink signals, and derives the time-frequency locations of the REs occupied by reference signals and the REs available for data reception. A PDSCH decoding module 56 uses the locations of the REs available for data reception for decoding PDSCH in at least one of these REs. A measurement and feedback module 60 uses the locations of the REs occupied by reference signals to measure the channel response over the reference signals in at least one of these REs. Module 60 then calculates feedback information based on the channel measurements, and provides the feedback information to transmitter 44 for transmission to the cells.

The UE configuration seen in FIG. 1 is an example configuration, which is depicted in a highly simplified manner solely for the sake of clarity. In alternative embodiments, any other suitable UE configuration can be used. UE elements that are not mandatory for understanding of the disclosed techniques have been omitted from the figure for the sake of clarity.

In various embodiments, some or all of the elements of UE 24, including receiver 40, transmitter 44 and processing circuitry 48, are implemented in hardware, such as implementing receiver 40 and/or transmitter 44 using one or more Radio Frequency Integrated Circuits (RFICs), or implementing processing circuitry 48 using one or more Field-Programmable Gate Arrays (FPGAs) or Application-Specific Integrated Circuits (ASICs). In alternative embodiments, various elements of UE 24 are implemented in software, or using a combination of hardware and software elements. In some embodiments, some or all of the elements of UE 24 are implemented in a signal-processing chipset.

In some embodiments, various UE elements, such as various elements of processing circuitry 48, are implemented in a programmable processor, which is programmed in software to carry out the functions described herein. The software may be downloaded to the processor, in whole or in part, in electronic form, over a network, for example, or it may, alternatively or additionally, be provided and/or stored on non-transitory tangible media, such as magnetic, optical, or electronic memory.

In some embodiments, different cells 28 in the cooperating group transmit downlink signals using respective different cell identifiers (cell IDs). Therefore, transmission parameters that are derived from the cell ID will also differ from cell to cell. One of the parameters derived from the cell ID is the pattern of REs, in time and frequency, used by the cell for transmitting Common Reference Signals (CRS).

Thus, in some embodiments, different cells 28 in the CoMP group transmit CRS in respective different patterns of REs. In such a scenario, the choice of REs that are available for transmitting PDSCH to the UEs is limited to the REs that do not belong to any of the CRS patterns of the cells.

FIG. 2 is a diagram showing allocations of time-frequency REs in system 20, in accordance with an embodiment that is described herein. In the present example, the CoMP group comprises two cells 28 (Transmission Points—TP), referred to as TP#1 and TP#2. Both cells transmit downlink signals using normal LTE-A sub-frames.

The diagram at the top-left of the figure shows the time-frequency RE allocation in the downlink signal transmitted by TP#1. The horizontal axis represents the time domain (OFDM symbols) and the vertical axis represents the frequency domain (OFDM sub-carriers). The same pattern is typically repeated periodically in time and frequency.

In the signal of TP#1, the first three OFDM symbols of each transmission Time Interval (TTI) are allocated for a Physical Downlink Control Channel (PDCCH) 68A. The PDCCH REs are dotted in the figure. A pattern of REs 72A (checkered in the figure) is used for transmitting Common Reference Signals (CRS). The remaining REs (clear in the figure) are available for transmitting PDSCH 76A.

(Note that Common Reference Signals (CRS) are typically also transmitted in the region of PDCCH 68A. FIG. 2 does not show CRS in 68A for the sake of clarity. CRS in the PDCCH region are typically not used in PDSCH extraction in accordance with the 3GPP specifications. The same simplification is also applied in the other diagrams in FIG. 2.)

The diagram at the top-right of the figure shows the time-frequency RE allocation in the downlink signal transmitted by TP#2, which is the present example has a different cell ID than TP#1. In the signal of TP#2, the first four OFDM symbols of each TTI are allocated for a PDCCH 68B, a pattern of REs 72B is used for transmitting CRS, and the remaining REs are available for transmitting PDSCH 76B.

As can be seen in the figure, TP#1 and TP#2 differ from one another in the number of symbols allocated to PDCCH (and therefore in the start time of the CRS pattern), as well as in the CRS pattern itself. It can also be seen that CRS patterns 76A and 76B of TP#1 and TP#2 can be viewed as a certain time-frequency shift of some baseline pattern. In other words, CRS patterns 76A and 76B can be produced by applying different time-frequency shifts to the same baseline pattern of CRS. In some embodiments, these properties are used for defining the signaling information that is transmitted to UE 24, as will be explained below.

The diagram at the bottom of the figure shows an overlay of the top-left and top-right diagrams. REs in OFDM symbols 68C are occupied by PDCCH in at least one of the TPs. REs 72C are occupied by CRS in at least one of the TPs. The remaining REs 76C (the clear REs in the diagram) are not occupied by PDCCH or CRS in any of the TPs, and are therefore available for PDSCH transmission. Scheduler 32 will thus typically schedule PDSCH transmissions in one or more of REs 76C.

In the present example, TP#1 serves as the anchor cell. In some embodiments, the anchor cell transmits to UE 24 signaling information, which is indicative of the CRS patterns of TP#1 and TP#2 (patterns 72A and 72B). Signaling extraction module 52 in processing circuitry 48 of UE 24 extracts the signaling information from the downlink signal of TP#1, and derives the locations of REs 76C that are available for PDSCH. Module 52 provides the locations of REs 76C to PDSCH module 56. In an embodiment, module 52 also derives the locations of REs 72A and 72B, i.e., the locations of the CRS REs per TP, and provides this information to measurement and feedback module 60.

The example of FIG. 2 refers to two TPs, one serving as an anchor cell and the other serving as a secondary cell. Generally, however, the disclosed techniques can be used with larger CoMP groups that comprise multiple secondary cells. In such embodiments, the anchor cell transmits to the UE signaling information that is indicative of the CRS patterns of the anchor cell and the multiple secondary cells.

In some embodiments, the signaling information further indicates the identities of various sets of cells 28 that are related to the CoMP scheme. The sets may comprise, for example, a Radio Resource Management (RRM) set of cells for which the UE is to perform RRM measurements (e.g., Reference Signal Received Power—RSRP), a Channel State Information (CSI) measurement set of cells for which the UE is to measure CSI, a CSI feedback set of cells for which the UE is to report CSI, and/or a transmission set of cells that transmit to the UE. In an embodiment, the signaling of the various CoMP sets is carried out in accordance with the following table:

In an embodiment, the CoMP transmission set is transparent to the UE, in which case the transmission set is not signaled explicitly. In alternative embodiments, any other signaling scheme can be used for signaling the various CoMP sets. Dynamic signaling may be signaled, for example, using Downlink Control Information (DCI).

In other embodiments, the transmission set is signaled explicitly to the UE, for example using a bit-map in which “1” indicates that the respective TP belongs to the transmission set, and “0” indicates that the TP does not belong. For example, consider the group of four CSI-RS resources {1, 3, 4, 7}. The transmission set {1, 3} can be signaled to the UE using the bit-map [1 1 0 0]. CSI-RS resources that do not belong to the transmission set are nevertheless included in the bit-map, so that the UE is able to derive the PDSCH resource mapping correctly.

In various embodiments, the signaling information may be formatted using different formats. In an example embodiment, the signaling information indicates, per TP, the number of CRS ports, the time-frequency shift of the CRS pattern of each CRS port relative to some baseline pattern position, and the number of symbols allocated to PDCCH (and therefore in the start time of the CRS pattern.

In some embodiments, the signaling further indicates the type of downlink sub-frame used by each TP. A given TP, for example, may use Multimedia Broadcast Single Frequency Network (MBSFN) sub-frames in which the first two OFDM symbols are allocated to PDCCH, and CRSs are not transmitted at all. If all TPs transmit using MBSFN, sub-frames overhead is reduced and all REs other than the first two OFDM symbols are available for PDSCH. In an embodiment, the signaling information indicates the MBSFN configuration (e.g., whether or not MBSFN is used) per TP.

In some embodiments, the various parameters listed above are signaled explicitly. In an example embodiment, each parameter is separately assigned a certain number of bits in the signaling information. In another example embodiment, the possible combinations of parameters are enumerated in advance in a manner that is agreed upon between the UEs and the cells. The signaling information in this embodiment comprises an index that indicates the applicable combination of parameters. This scheme is more efficient in terms of signaling overhead.

In other embodiments, the PDCCH length is signaled explicitly and the other parameters are tied a-priori to the Channel State Information Reference Signal (CSI-RS) configuration. In the latter embodiment, the signaling information indicates the PDCCH length and the CSI-RS configuration, and the UE determines the other parameters (e.g., CRS pattern shift) from the CSI-RS configuration. In emerging versions of the LTE-A specification, the PDSCH RE mapping is represented jointly with quasi-co-location information in a single field denoted “PDSCH RE Mapping and Quasi-Co-Location Indicator” (PQI). In the present context, PQI is also considered signaling information that is indicative of the pattern of REs used by a cell.

In some embodiments, the use of a certain CoMP mode (e.g., JT or DPS) is restricted to certain sub-frame configurations of the different cells in the cooperating group. In these embodiments, processing circuitry 48 applies a certain CoMP mode only when the sub-frame configurations of the various cells are suitable for that mode.

Consider, for example, the JT CoMP mode (also referred to as Joint Processing—JP) in which multiple cells transmit the same data to the UE simultaneously. The REs to be used for JT should be clear of PDCCH and CRS in all the configurations of all the cooperating cells. Thus, in an example embodiment, the cells and the UE use JT only in MBSFN sub-frames (which do not contain CRS at all and in which it is known that the first two OFDM symbols are assigned for PDCCH).

In another example embodiment, the cells and the UE use JT only when the anchor cell and secondary cells have the same number of PDCCH symbols (same PDCCH length). In yet another embodiment, the cells and the UE use JT only when the PDCCH length of the secondary cells does not exceed the PDCCH length of the anchor cell. In these embodiments, the REs occupied by CRS are skipped by all the cooperating cells when transmitting PDSCH using JT. In these embodiments, the transmission set is typically signaled explicitly.

As another example, consider the DPS CoMP mode in which only a single dynamically-selected cell transmits downlink signals to the UE at any given time. DPS is less sensitive to CRS pattern mismatch and PDCCH length mismatch than JT. Nevertheless, the mapping and starting position of the PDSCH is not known to the UE. The PDSCH starting position is given by max(c_s,c_d), wherein c_s and c_d denote the PDSCH starting positions of the anchor cell and of the dynamically-selected transmitting cell, respectively.

In an example embodiment, for DPS, the signaling information jointly encodes the CRS pattern time-frequency shift and the PDCCH length. The control information is transmitted by the anchor cell, the PDSCH is scheduled dynamically, and the Radio Resource Control (RRC) layer is configured to inform the UE of the PDSCH scheduling. In an embodiment, the control information is transmitted by the anchor cell, and the signaling information provides explicit indication of the cell ID. The UE is thus able to derive the PDCCH length and CRS pattern shift from the signaling information.

FIG. 3 is a diagram showing allocations of time-frequency REs in system 20, in accordance with another embodiment that is described herein. In this example, TP#1 and TP#2 transmit normal sub-frames, and the sub-frame configurations of TP#1 and TP#2 are the same as in the example of FIG. 2 above. In the example of FIG. 3, however, the UE skips an OFDM symbol entirely if that OFDM symbol comprises a CRS of any of the TPs. Therefore, as can be seen in the diagram at the bottom of the figure, the UE regards the entire fifth, eighth and twelfth OFDM symbols as not available for receiving data.

Generally, the signaling information may comprise a rate matching indicator that is indicative of the REs that are to be skipped by the UEs when identifying REs that are available for PDSCH. The rate matching indicator may be defined using zero-power CSI-RS, which are configured jointly for multiple TPs and are not UE-specific.

In other embodiments, the anchor cell transmits normal sub-frames and the secondary cells transmit MBSFN sub-frames. The actual sub-frame configuration (e.g., PDCCH length and CRS pattern shift) of the anchor cell is signaled to the UE. This sort of configuration incurs small signaling overhead and no CRS pattern mismatch.

In yet another embodiment, TP#1 and TP#2 transmit using non-coherent JT. In this mode, the two TPs transmit independent data streams to the same UE simultaneously. The port allocation for Demodulation Reference Signals (DMRS) ensures that the two streams are assigned orthogonal ports. In this embodiment, Downlink Control Information (DCI) allocation rank-2 is used, and the same PDSCH scrambling is applied to both streams.

FIG. 4 is a flow chart that schematically illustrates a method for downlink reception in terminal 24, in accordance with an embodiment that is described herein. The method begins with DL RX 40 of terminal 24 receiving a CoMP downlink signal from an anchor cell and one or more secondary cells, at a reception operation 80. As part of the downlink signal, DL RX 40 receives signaling information from the anchor cell, at a signaling reception operation 84. The signaling information is indicative of the CRS patterns used by the anchor cell and the secondary cell or cells. The signaling information is provided to signaling extraction module 52 in processing circuitry 48 of terminal 24.

Module 52 uses the signaling information to derive the pattern of REs that are available for PDSCH, at a PDSCH RE identification operation 88. PDSCH module 56 decodes the PDSCH in one or more of the REs derived by module 52, at a decoding operation 92.

Although the embodiments described herein mainly address 3GPP LTE/LTE-A, the methods and systems described herein can also be used in other applications, such as in IEEE 802.11/Wi-Fi, IEEE 802.16/WiMAX, and any other distributed antenna system (DAS).

It is noted that the embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and sub-combinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art. Documents incorporated by reference in the present patent application are to be considered an integral part of the application except that to the extent any terms are defined in these incorporated documents in a manner that conflicts with the definitions made explicitly or implicitly in the present specification, only the definitions in the present specification should be considered.