Abstract:
A method of operating a wireless communication system (FIG.  4 ) is disclosed. The method includes receiving a plurality of reference signals from a respective plurality of transceivers ( 402 ). Each of the plurality of reference signals is measured to produce a respective plurality of channel state information (CSI) measurements ( 404 ). An aggregated channel quality indicator (CQI) is calculated from measuring the plurality of reference signals ( 406 ). The aggregated CQI is transmitted to at least one transceiver of the respective plurality of transceivers ( 408 ).

Description:
[0001]    This application is a continuation of prior application Ser. No. 13/851,949, filed Mar. 27, 2013, which claims the benefit under 35 U.S.C. §119(e) of Provisional Appl. No. 61/615,984, filed Mar. 27, 2012 (TI-72062PS2), which is incorporated herein by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present embodiments relate to wireless communication systems and, more particularly, to operation of a Coordinated Multi-Point (CoMP) communication system in which a user equipment (UE) simultaneously communicates with plural base stations (eNB). 
         [0003]    With Orthogonal Frequency Division Multiplexing (OFDM), multiple symbols are transmitted on multiple carriers that are spaced apart to provide orthogonality. An OFDM modulator typically takes data symbols into a serial-to-parallel converter, and the output of the serial-to-parallel converter is frequency domain data symbols. The frequency domain tones at either edge of the band may be set to zero and are called guard tones. These guard tones allow the OFDM signal to fit into an appropriate spectral mask. Some of the frequency domain tones are set to values which will be known at the receiver. Among these are channel state information reference signals (CSI-RS). These are reference signals that are useful for channel measurement at the receiver. In a coordinated multi-point (CoMP) communication system these channel state reference signals are not precoded and are generated by a pseudo-random sequence generator as a function of the UE cell ID. In the Long Term Evolution (LTE) system of Releases 8, 9, and 10 for conventional point-to-point communication, the cell ID is not explicitly signaled by the eNB but is implicitly derived by the UE as a function of the primary synchronization signal (PSS) and secondary synchronization signal (SSS). To connect to a wireless network, the UE performs a downlink cell search to synchronize to the best cell. A cell search is performed by detecting the PSS and SSS of each available cell and comparing their respective signal quality. After the cell search is performed, the UE establishes connection with the best cell by deriving relevant system information for that cell. Similarly, for LTE Release 11 the UE performs an initial cell search to connect to the best cell. To enable multi-point CoMP operation, the connected cell then configures the UE by higher-layer signaling with a virtual cell ID for each CSI-RS resource associated with each respective base station involved in the multi-point CoMP operation. The UE generates the pseudo-random sequence for each CSI-RS resource as a function of the virtual cell ID. 
         [0004]    Conventional cellular communication systems operate in a point-to-point single-cell transmission fashion where a user terminal or equipment (UE) is uniquely connected to and served by a single cellular base station (eNB or eNodeB) at a given time. An example of such a system is the 3GPP Long-Term Evolution (LTE Release-8). Advanced cellular systems are intended to further improve the data rate and performance by adopting multi-point-to-point or coordinated multi-point (CoMP) communication where multiple base stations can cooperatively design the downlink transmission to serve a UE at the same time. An example of such a system is the 3GPP LTE-Advanced system. This greatly improves received signal strength at the UE by transmitting the same signal to each UE from different base stations. This is particularly beneficial for cell edge UEs that observe strong interference from neighboring base stations. 
         [0005]      FIG. 1  shows an exemplary wireless telecommunications network  100 . The illustrative telecommunications network includes base stations  101 ,  102 , and  103 , though in operation, a telecommunications network necessarily includes many more base stations. Each of base stations  101 ,  102 , and  103  (eNB) is operable over corresponding coverage areas  104 ,  105 , and  106 . Each base station&#39;s coverage area is further divided into cells. In the illustrated network, each base station&#39;s coverage area is divided into three cells. A handset or other user equipment (UE)  109  is shown in cell A  108 . Cell A  108  is within coverage area  104  of base station  101 . Base station  101  transmits to and receives transmissions from UE  109 . As UE  109  moves out of Cell A  108  into Cell B  107 , UE  109  may be handed over to base station  102 . Because UE  109  is synchronized with base station  101 , UE  109  can employ non-synchronized random access for a handover to base station  102 . UE  109  also employs non-synchronous random access to to request allocation of uplink  111  time or frequency or code resources. If UE  109  has data ready for transmission, which may be traffic data, a measurements report, or a tracking area update, UE  109  can transmit a random access signal on uplink  111 . The random access signal notifies base station  101  that UE  109  requires uplink resources to transmit the UE&#39;s data. Base station  101  responds by transmitting to UE  109  via downlink  110  a message containing the parameters of the resources allocated for the UE  109  uplink transmission along with possible timing error correction. After receiving the resource allocation and a possible timing advance message transmitted on downlink  110  by base station  101 , UE  109  optionally adjusts its transmit timing and transmits the data on uplink  111  employing the allotted resources during the prescribed time interval. Base station  101  configures UE  109  for periodic uplink sounding reference signal (SRS) transmission. Base station  101  estimates uplink channel quality indicator (CQI) from the SRS transmission. 
         [0006]    While the preceding approaches provide steady improvements in wireless communications, the present inventors recognize that still further improvements in transmission of channel state information (CST) from the UE to the eNB are possible. Accordingly, the preferred embodiments described below are directed toward this as well as improving upon the prior art. 
       BRIEF SUMMARY OF THE INVENTION 
       [0007]    In a preferred embodiment of the present invention, there is disclosed a method of operating a wireless communication system. The method includes receiving a plurality of reference signals (CSI-RS) from a respective plurality of transceivers. Each of the respective plurality of reference signals is measured at the UE to produce a respective plurality of channel state information (CSI) estimates. An aggregated channel quality indicator (CQI) is calculated from the respective plurality of CSI estimates. The aggregated CQI is transmitted to at least one transceiver of the respective plurality of transceivers. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         [0008]      FIG. 1  is a diagram of a wireless communication system of the prior art; 
           [0009]      FIG. 2  is a diagram of a Coordinated Multi-Point communication system of the present invention; 
           [0010]      FIG. 3  is a diagram showing communication between a user equipment (UE) and a base station (eNB) according to the present invention; 
           [0011]      FIG. 4  is a flow chart showing channel state information (CSI) feedback according to a first embodiment of the present invention; 
           [0012]      FIG. 5  is a flow chart showing channel state information (CSI) feedback according to a second embodiment of the present invention; 
           [0013]      FIG. 6  is a time division multiplex diagram showing CSI feedback on the Physical Uplink Control Channel (PUCCH) according to the present invention; 
           [0014]      FIG. 7A  is a time division multiplex diagram showing CSI feedback on the Physical Uplink Control Channel (PUCCH) according to another embodiment of the present invention; and 
           [0015]      FIG. 7B  is a time division multiplex diagram showing CSI feedback flow on the Physical Uplink Control Channel (PUCCH) according to yet another embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0016]    Channel state information (CSI) feedback from user equipment (UE) to a base station (eNB) is essential for operating a coordinated multi-point (CoMP) LTE wireless communication system. This CSI feedback facilitates transmission parameter selection, beamforming, scheduling, interference alignment, and other factors necessary for an effective communication system. Accordingly, embodiments of the present invention employ channel state information reference signals (CSI-RS) to derive and feed back an aggregated channel quality indicator (CQI) and/or an aggregated precoding matrix indicator (PMI) to improve feedback from the UE to the eNB. 
         [0017]    The following abbreviations are used throughout the instant specification.
       eNB: E-UTRAN Node B or base station   UE: User Equipment   CSI: Channel State Information   CQI: Channel Quality Indicator   CSI-RS: Channel State Information Reference Signal   E-UTRAN: Evolved Universal Terrestrial Radio Access Network   PDCCH: Physical Downlink Control Channel   PDSCH: Physical Downlink Shared Channel   PUCCH: Physical Uplink Control Channel   PUSCH: Physical Uplink Shared Channel   CRS: Cell-specific Reference Signal   LTE: Long Term Evolution   DL: DownLink   UL: UpLink   PMI: Precoding Matrix Indicator   RI: Rank Indicator   RRC: Radio Resource Control   PRB: Physical Resource Block   QAM: Quadrature Amplitude Modulation   IRC: Interference Rejection Combining   MRC: Maximum Ratio Combining   BLER: Block Error Rate   DPS: Dynamic Point Selection   JT: Joint Transmission   MIMO: Multiple-Input Multiple-Output   SNR: Signal to Noise Ratio       
 
         [0044]    Traditional wireless networks operate in a point-to-point transmission manner where a LTE connects to and receives data from a single base station. For data transmission, the base station performs downlink scheduling in order to allocate different frequency resources for downlink transmission to different UEs, possibly using different code rates, QAM constellation sizes, transmit powers, and MIMO precoding vectors. Downlink scheduling at the eNB is enabled by knowledge of channel state information (CSI), which is measured and reported by the UE. In LTE, a CSI report comprises a set of MIMO transmission properties recommended by the UE based on the downlink channel measurement, including rank indicator, precoding matrix indicator, and channel quality indicator. Rank indicator (RI) denotes the number of data streams (layers) recommended for downlink transmission. The value of RI feedback can vary from 1 to the minimum of eNB transmit antennas and UE receive antennas. Precoding matrix indicator (PMI) indicates the best precoding matrix that the UE recommends for downlink transmission. Channel quality indicator (CQI) is an indicator of the quantized signal-to-noise ratio which the UE is able to observe when the reported PMI and RI are used for hypothetical data transmission. In general, one CSI report comprises RI, PMI, and CQI, or a subset thereof. In a conventional wireless network, the reported CSI is per-point CSI corresponding to a single-cell channel with respect to the connected base station. UE selection of the PMI/CQI report is dependent on proprietary UE receiver implementation (e.g. MRC or IRC) and is transparent to the wireless standard. Ideally, the reported PMI/CQI should optimize a certain performance metric (e.g. maximum sum throughput) subject to a 10% BLER. This is also used in 3GPP RAN Working Group 4 for setting UE performance requirements for PMI/CQI. The legacy CSI report implicitly reflects both channel and interference components. That is, there is no separate feedback for channel and interference, respectively. Without loss of generality, the reported CQI can be denoted as a quantization of equation [1], 
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         [0000]    where H is the per-point channel, w is the PMI, I is the interference power, N is the noise power, u is the receiver equalizer, and ′ is the Hermitian operator. 
         [0045]    Referring to  FIG. 2 , there is a diagram of a coordinated multi-point (CoMP) wireless communication system according to the present invention. The communication system includes user equipment (UE)  200  and base stations (eNB)  202 ,  204 , and  206 . These base stations may be macro eNB, pico eNB, femto eNB, or other suitable transmission points (TP). For UE  200 , a plurality of CSI-RS resources is configured based on which the UE can measure in the downlink channel. Each CSI-RS resource can be associated by the E-UTRAN with a base station, a remote radio head (RRH), or a distributed antenna. UE  200  is configured by higher-layer RRC signaling with a specific virtual cell identifier (ID) for each CSI-RS resource. These virtual cell IDs are used by a pseudo-random sequence generator to generate the channel state reference signals (CSI-RS) corresponding to each CSI-RS resource. UE  200  receives each virtual cell ID from higher layer RRC signaling after establishing initial cell connection with the best cell. The CSI-RS from eNBs  202 ,  204 , and  206  are transmitted to UE  200  over wireless channels  208 ,  212 , and  210 , respectively. 
         [0046]    For CoMP, per-point CSI feedback is a baseline where the UE reports CSI of each base station separately. Since each base station is associated with a CSI-RS resource, this is equivalent to per-CSI-RS-resource feedback. Several different implementations of per-CSI-RS-resource feedback are possible. In one embodiment, per-point CQI and per-point PMI are reported for each configured CSI-RS resource. Alternatively, per-point CSI information is explicitly reported for a subset of base stations. For other base stations without explicit per-point CSI feedback, per-point CSI information can be inferred or estimated from other CSI reports (e.g. aggregated CQI) when available. It is possible that per-CSI-RS-resource CSI feedback comprises a subset of RI, PMI, and CQI information. In one embodiment, at least per-point PMI pertaining to legacy LTE definition is reported for each configured CSI-RS resource. Such PMI is an indication of the spatial characteristics for each CoMP measurement point and important for all CoMP transmission schemes. It could be used for single-point beamforming in dynamic point selection, for interference alignment in coordinated beamfomiing and scheduling, and for coherent and non-coherent beam combining in joint processing. In addition, per-point CQI is needed for all CoMP schemes to enable point selection, perform interference alignment in coordinated beamforming/scheduling and joint transmission. In one embodiment, per-point CQI is reported for each configured CSI-RS resource. This provides the maximum scheduling flexibility. With per-point CQI of all CSI-RS resources, the eNB scheduler is able to dynamically switch between different CoMP transmission schemes and/or dynamically fall back to single-point transmission, based on quickly changing system conditions such as cell loading, traffic type, or UE mobility. In another embodiment, per-point CCI is reported for one or a subset of CSI-RS resources. For instance, a UE-centric feedback for DPS may report CQI for the selected point plus a point selection indicator, while CQI for other points is not reported. For points without CQI feedback, CQI is either unavailable or has to be predicted by the base station from other feedback information (e.g. aggregated COD which reduces the accuracy of per-point CQI. 
         [0047]    Since per-point CQI is derived under single-point transmission hypothesis, it is likely to be less accurate for CoMP link adaptation such as JT, where signals from multiple transmission points are combined either coherently or non-coherently at the UE receiver. In contrast, aggregated CQI aims to improve the link adaptation accuracy of CoMP joint transmission. With this scheme, an aggregated CQI is calculated by the UE to reflect the downlink SNR when all base stations jointly transmit data to the UE. Assume a CoMP measurement set comprising K points, where per-point PMI is reported for each point. UE  200  receives the composite signal y in equation [2] from K transmission points. Here H is the channel state and v i  is the precoding hypothesis for each of K eNBs. In the example of  FIG. 2  K=3, but in a practical CoMP network K may be greater or less than 3. In one embodiment, aggregated CQI is derived assuming precoding with v k  on the k-th measurement point (e.g. k-th CSI-RS resource), where v k  is the PMI feedback corresponding to the k-th measurement point. Essentially, such an aggregated CQI corresponds to incoherent CoMP-JT beamforming with the following received signal y. 
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         [0000]    In another embodiment, aggregate CQI is derived assuming precoding with e jθ     k    v k  on the k-th measurement point (e.g. k-th CSI-RS resource), where v k  is the PMI feedback, and θ k  is the inter-point co-phasing feedback corresponding to the k-th point. Essentially, such an aggregated CQI corresponds to coherent CoMP-JT beamforming with the following received signal y in equation [3]. 
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         [0000]    The aggregated CQI reflects a boosted SNR value when all transmission points jointly transmit data to the UE. It is possible that the aggregated CQI may be larger than the summation of per-point CQIs. It is also possible to report multiple aggregated CQIs, each of which is derived under different CoMP transmission set hypotheses. For instance, assume the CoMP measurement set has three transmission points (TP1, TP2, and TP3). The UE may report an aggregated CQI corresponding to each combination of two points in the measurement set and/or report an aggregated CQI corresponding to the entire CoMP measurement set. Multiple aggregated CQIs, if reported on the PUCCH channel, can be time-division multiplexed on different PUCCH transmissions at different time instances. Otherwise, if multiple aggregated CQIs are to be reported on the PUSCH channels, they can be transmitted in the same PUSCH transmission, or transmitted in different PUSCH transmissions. 
         [0048]    UE  200  transmits the aggregated CQI to primary eNB  202  over channel  214 . UE  200  may optionally transmit the aggregated CQI over channels  216  and  218  to eNBs  206  and  204  of the CoMP network. 
         [0049]    The aggregated CQI is computed by the UE based on the downlink quality associated with the aggregated channel over M transmission points along with their respective preceding hypotheses. The M transmission points are a subset within set size K. When M=K, only one aggregated CQI is reported corresponding to all transmission points of the set K. Alternatively, when M&lt;K, there are 
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         [0000]    possibilities. In this case, it is possible to report either a few or all of the respective CQIs. 
         [0050]    There are multiple ways to configure the aggregated CQI information by the eNB. The aggregated CQI mainly targets link adaptation for coherent/non-coherent joint transmission but is not required for coordinated beamforming/scheduling and dynamic point selection. From this perspective, aggregated CQI can be configured UE-specifically by higher layer signaling. On the other hand, whether aggregated CQI should be explicitly reported also depends on the decision of per-point feedback. As one possibility, the UB may report per-point CQI for all points plus aggregated CQI. As another possibility, the UE may report per-point CQI for one or a subset of points plus aggregated CQI. 
         [0051]    Turning now to  FIG. 3 , there is a diagram showing communication between user equipment (UE)  300  and a base station (eNB)  320  according to the present invention. UE  300  may be a cell phone, computer, or other wireless network device. UE  300  includes a processor  306  coupled to a memory  304  and a transceiver  310 . Processor  306  may include several processors adapted to various operational tasks of the UE including signal processing and channel measurement and computation. The memory stores application software that the processor may execute as directed by the user as well as operating instructions for the UE. Processor  306  is also coupled to input/output (I/O) circuitry  308 , which may include a microphone, speaker, display, and related software. Transceiver  310  includes receiver  312  and transmitter  314 , suitable for wireless communication with eNB  320 . Transceiver  310  typically communicates with eNB  320  over various communication channels. For example, transceiver  310  sends uplink information to eNB  320  over physical uplink control channel PUCCH and physical uplink shared channel PUSCH. Correspondingly, transceiver  310  receives downlink information from eNB  320  over physical downlink control channel PDCCH and physical downlink shared channel PDSCH. 
         [0052]    Base station  320  includes a processor  326  coupled to a memory  324 , a symbol processing circuit  328 , and a transceiver  330  via bus  336 . Processor  326  and symbol processing circuit  328  may include several processors adapted to various operational tasks including signal processing and channel measurement and computation. The memory stores application software that the processor may execute for specific users as well as operating instructions for eNS  320 . Transceiver  330  includes receiver  332  and transmitter  334 , suitable for wireless communication with UE  300 . Transceiver  330  typically communicates with UE  300  over various communication channels. For example, transceiver  330  sends downlink information to UE  300  over physical downlink control channel PDCCH and physical downlink shared channel PDSCH. Correspondingly, transceiver  330  receives uplink information from UE  300  over physical uplink control channel PUCCH and physical uplink shared channel PUSCH. 
         [0053]      FIG. 4  is a flow chart showing channel quality indicator (CQI) feedback according to a first embodiment of the present invention. Operation begins with UE initialization  400  when the EU enters the CoMP configuration. The UE determines a primary eNB and synchronizes with other suitable eNBs as indicated by the CoMP configuration. The UE determines the virtual cell ID for each CSI-RS resource. At block  402 , the UE determines CSI-RS sequence for plural CSI-RS resources. The UE measures  404  per-point CSI from each of the CSI-RS resources. The UE calculates  406  an aggregated CQI from the plural CSI-RS resources. The UE subsequently transmits  408  the per-point CSI and aggregated CQI to the primary eNB. Responsively, the primary eNB selects and transmits  410  appropriate communication parameters to the UE. At block  412 , the UE communicates with the plurality of eNBs subject to the received communication parameters. 
         [0054]      FIG. 5  is a flow chart showing precoding matrix indicator (PMI) feedback according to a second embodiment of the present invention. Operation proceeds as previously described with respect to  FIG. 4 . At block  500 , however, the UE transmits per-point PMI hypotheses and an aggregated CQI report to the primary eNB. Responsively, the primary eNB selects and transmits  410  appropriate communication parameters to the UE. At block  412 , the UE communicates with the plurality of eNBs subject to the received communication parameters. 
         [0055]    Referring to  FIG. 6 , there is a time division multiplex diagram showing per-point CSI and aggregated CQI feedback on the Physical Uplink Control Channel (PUCCH) according to one embodiment of the present invention. A first per-point CSI 1  measurement is transmitted at  600  followed by a second per-point CSI 2  at  602 . Here, the subscript indicates a particular per-point CSI-RS source in the CoMP configuration. Other per-point CSI measurements (not shown) are subsequently transmitted followed by an aggregated CQ report at  604 . At blocks  606  and  608 , a second set of per-point measurements of CSI 1  and CSI 2  are respectively transmitted followed by a second aggregated CQI report  610 . 
         [0056]      FIG. 7A  is a time division multiplex diagram showing CSI feedback on the Physical Uplink Control Channel (PUCCH) according to another embodiment of the present invention. Here, a first per-point CSI 1  measurement is transmitted at  700 . A second per-point CSI 2  is transmitted at  702  together with an aggregated CQI report. Other per-point CSI measurements (not shown) may also be transmitted. A second set of per-point measurements is then transmitted beginning with CSI 1  at block  704 . Next per-point measurement CSI 2  is transmitted at  706  together with a second aggregated CQI report. 
         [0057]    Turning now to  FIG. 7B , there is a time division multiplex diagram showing PMI feedback flow on the Physical Uplink Control Channel (PUCCH) according to yet another embodiment of the present invention. Here, a first per-point PMI 1  hypothesis and CQI 1  measurement are transmitted at  710 . A second per-point PMI 2  hypothesis is transmitted at  712  together with an aggregated CQI report. Other per-point PMI hypotheses (not shown) may also be transmitted. A second set of per-point hypotheses is then transmitted beginning with PMI 1  and per-point CQI 1  report at block  714 . Next, per-point hypothesis PMI 2  is transmitted at  716  together with a second aggregated CQI report. 
         [0058]    Still further, while numerous examples have thus been provided, one skilled in the art should recognize that various modifications, substitutions, or alterations may be made to the described embodiments while still falling with the inventive scope as defined by the following claims. Other combinations will be readily apparent to one of ordinary skill in the art having access to the instant specification.