Abstract:
The invention relates to a method of feeding back information from a receiver to a transmitter, and also a corresponding receiver, transmitter, system comprising a receiver and transmitter, and computer program products for performing the steps of the receiver and transmitter respectively. The method comprises: receiving signals at the receiver from the transmitter over a wireless multiple-input-multiple-output channel; and, based on the received signals, transmitting back reports from the receiver to the transmitter including a report indicating a pre-coding matrix and a report indicating a rank of the pre-coding matrix. In the event that the report indicating the rank is not transmitted, the receiver instead uses a default rank to determine the report.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to the feedback of channel related information in a Multiple-Input Multiple-Output (MIMO) system. 
       BACKGROUND 
       [0002]    In a wireless communication system implementing link adaptation, a receiver such as a mobile terminal feeds back channel information to a transmitter such as a base station so that the transmitter can adapt its transmission to the receiver in dependence on channel conditions. 
         [0003]    MIMO refers to the use of multiple transmit antennas and multiple receive antennas for the transmission of a signal in order to improve performance in a wireless communication system. A highly schematised block diagram of a MIMO system is shown in  FIG. 1 . The system comprises a transmitter  2  having multiple antennas  6 ( 1 ) . . .  6 ( n ) and a receiver  4  having multiple antennas  8 ( 1 ) . . .  8 ( m ). For example, in a cellular communication system like the 3GPP Long Term Evolution (LTE) standard, the transmitter  2  may be a base station (e.g. eNode-B in the 3GPP terminology) and the receiver  4  may be a mobile terminal (user equipment or UE in the 3GPP terminology). The transmitter  2  transmits a signal on some or all of its antennas  6 , and the receiver  4  receives the signal on some or all of its antennas  8 . To achieve good closed-loop performance, the transmitter  2  may perform MIMO “pre-coding” whereby it uses channel information to determine the relative amplitude and phase with which to transmit the signal on each antenna. 
         [0004]    In general, this information has to be fed back from the receiver  4 . To reduce the amount of feedback overhead, a precoding matrix approach was proposed in D. Love and R. W. Heath, “Limited Feedback Precoding for Spatial Multiplexing Systems”, in Proc. IEEE Globecom 2003, pp. 1857-1861. The basic idea behind this approach is to quantize the MIMO channel using a codebook consisting of a set of pre-defined matrices. For each channel realization, the receiver  4  finds the best precoding matrix (according to some performance criteria) from the codebook shared between the receiver and the transmitter, and then feeds back only the index of this matrix to the transmitter. This index may be referred to as a precoding matrix indicator (PMI). 
         [0005]    Another piece of information that the receiver  4  feeds back to the transmitter  2  is the rank indicator (RI). This provides the rank of the channel matrix, which is defined as the number of linearly independent columns of the channel matrix. For example, a N T =4×N R =4 channel matrix can have rank equal to 4, 3, 2 or 1 (rank≦min (N T ,N R )). The rank of the channel also determines the size of the precoding matrix to be used by the transmitter, i.e., the number of columns of the precoding matrix. Depending on the channel rank, the transmitter  2  will consider a specific subset of the full precoding codebook. Therefore, the transmitter  2  needs to know what rank the received PMI is referring to. 
         [0006]    Further, in addition to the RI and PMI, the receiver  4  feeds back a channel quality indicator (CQI) to the transmitter  2 , indicative of some metric relating to the received quality on the downlink channel. The transmitter  2  can then also take this into account when adjusting its transmission to the receiver  2 , typically selecting the appropriate modulation scheme and code rate to match the receiver channel quality information. 
         [0007]    As illustrated schematically in  FIG. 2 , the downlink channel may be an Orthogonal Frequency Division Multiplexing (OFDM) channel comprising a plurality of frequency sub-bands  12 , with the sub-bands being grouped together into groups of sub-bands  14 . The feedback of the CQI information may be either frequency selective or non frequency selective. In the non frequency selective case, the receiver  4  simply feeds back a single wideband CQI for the whole channel. In the frequency selective case, the receiver  4  also feeds back a CQI for each of a plurality of groups of sub-bands  14 . 
         [0008]    In the current 3GPP LTE standard, the rank indicator (RI), precoding matrix indicator (PMI) and channel quality indicator (CQI) are typically reported periodically from the UE to the eNode-B. This periodic reporting is based on a control signalling in the form of a set of parameters transmitted by the network via the eNode-B to the UE, which determine the periodicity of the different reports for a given feedback mode [3GPP TS 36.213, “Technical Specification Group Radio Access Network: Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Layer Procedures (Release 8)”, V8.3.0, May 2008, Section 7.2.2]. 
         [0009]    For the non-frequency selective periodic CQI modes, the UE reports in different uplink reporting instances a) RI and b) wideband CQI/PMI for the modes with PMI report or only wideband CQI for the modes with no PMI report [3GPP TS 36.213, “Technical Specification Group Radio Access Network: Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Layer Procedures (Release 8)”, V8.3.0, May 2008, Table 7.2.2-3]. 
         [0010]    For the frequency selective periodic CQI modes, the UE reports in different uplink reporting instances a) RI, b) wideband CQI/PMI for the modes with PMI report or only wideband CQI for the modes with no PMI report, and c) frequency selective CQI in terms of multiple sub-band CQIs [3GPP TS 36.213, “Technical Specification Group Radio Access Network: Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Layer Procedures (Release 8)”, V8.3.0, May 2008, Table 7.2.2-3]. 
         [0011]    The control signalling from the eNode-B to the UE may be transmitted on the Primary Downlink Control Channel (PDCCH) and the RI, PMI and CQI reports fed back from the UE to the eNode-B may be signalled on the Primary Uplink Control Channel (PUCCH). An example of the RI, PMI and CQI information sent on the PUCCH  20  is illustrated schematically in  FIG. 3   a . Here, the PUCCH  20  comprises the sequential transmission in time on a plurality of reporting instances  22 ( t ),  22 ( t+ 1), 22(t+2), etc. Here, the first reporting instance comprises a report of the RI, the second reporting instance comprises a report of the wideband PMI and wideband CQI, and by way of example the next four reporting instances comprise respective reports of the sub-band CQI values for each of four groups of sub-bands  14 . Following the sub-band CQI reports, the sequence of an uplink reporting instance including the wideband PMI and CQI report followed by four reporting instances including sub-band CQI reports is repeated. That sequence may be repeated a number of times periodically, and after that the whole sequence may be repeated again periodically starting with another RI report and so on. The actual RI, PMI and CQI values reported will be updated with each periodic repetition on the relevant reporting instances. Note that  FIG. 3   a  shows an example of a frequency selective report, but it will be understood that a non-frequency selective report would contain the same sequence of RI, PMI and CQI reports, except that it would not include the sub-band CQI values. 
         [0012]    However, in some cases the UE may for certain reasons not transmit on one or more reporting instances  22  of the PUCCH  20 . If an RI, PMI and/or CQI report is scheduled for such a reporting instance  22 , then this RI, PMI and/or CQI report is said to be “dropped” and it will not be transmitted. There are also certain cases where a higher priority uplink transmission may cause an RI, PMI and/or CQI report to be replaced on a certain reporting instance  22 . More specifically, when the UE has any other higher priority control information to be transmitted on the PUCCH, it will need to replace any RI, PMI and/or CQI report scheduled on that reporting instance  22 . In such cases, the RI, PMI and/or CQI report is again said to be “dropped” from the reporting instance  22  in question. 
         [0013]    The 3GPP LTE standard allows the possibility of dropping the transmission of RI and wideband CQI or wideband CQI/PMI from a given reporting instance  22  for different reasons:
       An aperiodic CQI report on PUSCH is requested, which will be transmitted instead of the scheduled periodic CQI report on PUCCH.   A scheduling request (SR) needs to be transmitted by the UE, which will cause a drop of information on PUCCH.   A positive or negative acknowledgment (ACK/NACK) needs to be transmitted by the UE, which will cause a drop of information on PUCCH.   A UE Discontinuous Reception (DRX) inactive cycle will cause any uplink transmissions to be invalid (typically for power saving reasons).   RI and wideband CQUPMI collisions due to the RI offset parameter set to O=0 by the eNode-B, in which case the UE will drop the wideband CQI/PMI transmission.   In the presence of a measurement gap, the UE will drop all uplink transmissions overlapping with the gap.       
 
         [0020]    The missed transmission of this information in the uplink can cause a problem, because without the RI and/or PMI transmission, the CQI values sent on the following reporting instances have no meaning. In fact, all the RI/PMI/CQI reports are linked, and the wideband PMI is computed based on the reported rank while the sub-band CQI values are determined by the UE based on both the reported rank and precoding matrix. So the meaning of the reported PMI depends on the RI, and the meaning of the reported CQI depends on the RI and PMI. This implies that the eNode-B needs to know the correct RI in order to correctly interpret the reported PMI, and needs to know the correct RI and PMI in order to correctly interpret the reported CQI 
         [0021]    The current status of the LTE specification is to do nothing and accept losing the RI or PMI information in the presence of a drop of a scheduled RI or PMI transmission. 
         [0022]    A possible alternative solution is to configure the UE to reschedule the RI report by shifting it along in time to another reporting instance after the reporting instance at which it was originally scheduled. All subsequent reports are then also shifted along in time by the same number of reporting instances  22 . This means that under normal circumstances, the eNode-B should still receive the RI correctly in order to interpret the subsequent PMI and CQI reports. 
         [0023]    An example of this is illustrated in  FIG. 3   b , which shows the case of a DRX inactive cycle in which any uplink transmissions are invalid, or a measurement gap in which the uplink signal is not transmitted. Consider a scenario where a measurement gap or a UE DRX inactive cycle overlaps with a PUCCH reporting instance  22  containing an RI transmission, as depicted in the  FIG. 3   b  (the DRX/GAP period can last multiple WB/CQI reporting intervals, but for illustration only one WB/CQI interval is shown as overlapping the DRX/GAP period). Under the current status of the LTE specification, any reports in the DRX/GAP period would simply be dropped and not retransmitted. But, under the possible alternative solution, the RI report is re-scheduled to the next available reporting instance  22 ( t+ 4) immediately after the end of the DRX/GAP period, and the subsequent sequence of PMI and CQI reports is shifted along in time accordingly. 
         [0024]    Another example is illustrated in  FIG. 3   c , which shows the case where the UE receives data transmission from the eNode-B and in response must send back a positive acknowledgement signal ACK ora negative acknowledgement signal NACK to the eNode-B in the next reporting instance  22  of the PUCCH  20 . That means that the RI, PMI or CQI report that was scheduled for that reporting instance must be dropped, since the ACK has higher priority than the RI, PMI and CQI reports. Again, under the current status of the LTE specification, that report would simply be omitted altogether and not retransmitted. This would include the possibility an RI report being replaced by the ACK/NACK. But, under the possible alternative solution, the RI report would be re-scheduled to the next reporting instance  22 ( t+ 1) immediately after the ACK/NACK, with the subsequent sequence of PMI and CQI reports being shifted along in time accordingly. Similar comments apply to any higher priority transmission that the UE must make to the eNode-B, which will displace an RI report. 
         [0025]    Another alternative for the case of frequency-selective CQI report is to sacrifice one of the sub-band CQI reports every time a drop of RI or PMI transmission has occurred. Examples of this are illustrated in  FIGS. 3   d  and  3   e . In  FIG. 3   e  for example the next sub-band CQI report CQI 1  is deliberately omitted from transmission by the UE, and the eNode-B is configured to expect that CQI report CQI 1  to be dropped. Similarly in  FIG. 3   d , the sub-band CQI report CQI 3  is deliberately omitted from reporting instance  22 ( t+ 4), and the eNode-B is configured to expect that accordingly. 
         [0026]    Yet another alternative would be to retransmit the dropped RI at the next opportunity, and to shift the subsequent sequence of PMI and CQI reports by one place, until the next wideband CQI/PMI reporting instance, thereby again sacrificing one of the sub-band CQI reports. 
         [0027]    When an RI report is dropped, the current state of the LTE specification causes a problem because the eNode-B will lose the information of an entire reporting interval between one RI and the next. 
         [0028]    However, the alternative solution discussed in relation to  FIGS. 3   b  and  3   c  is also problematic because it can lead to a misalignment between eNode-B and UE in the interpretation of the different reports. For example, if a control signalling from the eNode-B is not properly detected by the UE, perhaps due to a poor quality PDCCH, then the UE may miss the transmission of downlink data, and not report the corresponding ACK/NACK in the uplink. In this case, there may be a discrepancy between what the UE transmits and what the eNode-B expects to receive. So referring to  FIGS. 3   b  and  3   c  for example, the UE may transmit with the scheduling shown in the top row whilst the eNode-B expects to receive the scheduling shown in the bottom row. Thus the eNode-B&#39;s expectation will not be aligned with the UE&#39;s actual PUCCH transmission. 
         [0029]    The alternative solution of  FIGS. 3   d  and  3   e  reduces the impact of this misalignment problem to some extent. In  FIG. 3   e  for example, the misalignment will always be regained again by reporting instance  22 ( t+ 3), and in  FIG. 3   d  it will be regained by reporting instance  22 ( t+ 5). However, the situation in  FIGS. 3   d  and  3   e  is still problematic in another way because it requires one of the sub-band CQI reports to be sacrificed. 
         [0030]    It is an aim of the present invention to find an alternative solution to the problem of RI dropping. 
       SUMMARY 
       [0031]    According to one aspect of the present invention, there is provided a method of feeding back information from a receiver to a transmitter, the method comprising: transmitting signals from the transmitter to the receiver over a wireless multiple-input-multiple-output channel; based on the received signals, transmitting a plurality of reports back from the receiver to the transmitter in a periodic sequence of respective time intervals, the reports of each period comprising at least an indication of a pre-coding matrix and an indication of a rank of the pre-coding matrix; in response to an event, omitting the report comprising the rank indication from one of said periods; at the receiver, determining a subsequent report comprising an indication of a pre-coding matrix on the basis of a predetermined default rank, and transmitting that report to the transmitter; and at the transmitter, interpreting the indication of the pre-coding matrix without a report of a rank indication for said period by instead using the predetermined default rank, and using that interpretation to control a transmission of a subsequent signal to the receiver over the wireless multiple-input-multiple-output channel. 
         [0032]    By completely omitting the rank indicator report and instead using a default rank, rather than rescheduling the rank indicator report, the present invention provides improved reliability. In embodiments, it may also allow the possibility of using full frequency selective CQI information. 
         [0033]    According to another aspect of the present invention, there is provided a method of feeding back information from a receiver to a transmitter, the method comprising: receiving signals at the receiver from the transmitter over a wireless multiple-input-multiple-output channel; based on the received signals, sequentially transmitting a plurality of reporting instances back from the receiver to the transmitter including at least a report indicating a pre-coding matrix; in the event that a report indicating a rank of the respective pre-coding matrix is not transmitted back from the receiver to the transmitter before transmitting the report indicating a pre-coding matrix, then instead, at the receiver, determining the report indicating the pre-coding matrix using a default rank, and transmitting that report back to the transmitter. 
         [0034]    According to another aspect of the present invention, there is provided a method of feeding back information from a receiver to a transmitter, the method comprising: receiving signals at the receiver from the transmitter over a wireless multiple-input-multiple-output channel; based on the received signals, transmitting a plurality of reports back from the receiver to the transmitter in a periodic sequence, each period comprising a plurality of reports at respective time intervals, the reports of each period including an RI report and a PMI report, but at least an RI report in one of said periods being replaced with one of an ACK, NACK and SR or missed due to a DRX cycle; at the receiver, determining a PMI report of said period on the basis of a default rank, and transmitting that report back to the transmitter. 
         [0035]    According to further aspects of the invention, there are provided corresponding receivers, transmitters, and communication systems comprising transmitter and receiver. For each of the receiver and transmitter, there is also provided a corresponding computer program product comprising code which when executed on a processor of the receiver or transmitter respectively operates it to perform the method steps of the receiver or transmitter respectively. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0036]    For a better understanding of the present invention and to show how it may be carried into effect, reference will now be made by way of example to the accompanying drawings in which: 
           [0037]      FIG. 1  is a schematic block diagram of a wireless communication system, 
           [0038]      FIG. 2  is a schematic representation of an OFDM channel, 
           [0039]      FIG. 3   a  is a schematic representation of feedback from a UE on a PUCCH, 
           [0040]      FIG. 3   b  is another schematic representation of feedback on a PUCCH, 
           [0041]      FIG. 3   c  is another schematic representation of feedback on a PUCCH, 
           [0042]      FIG. 3   d  is another schematic representation of feedback on a PUCCH, 
           [0043]      FIG. 3   e  is another schematic representation of feedback on a PUCCH, 
           [0044]      FIG. 3   f  is another schematic representation of feedback on a PUCCH, 
           [0045]      FIG. 3   g  is another schematic representation of feedback on a PUCCH, 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0046]    According to a preferred embodiment of the present invention, a default RI value is assumed in case the RI report is dropped. Both the base station (eNode-B) and the mobile terminal (UE) assume a default RI value, preferably RI=1, in the case where the RI report on PUCCH is dropped. Thus, instead of rescheduling the RI report or omitting a sub-band CQI report as discussed in relation to  FIGS. 3   b - 3   e , the RI report itself is omitted and a default value used. 
         [0047]    This is illustrated schematically in  FIGS. 3   f  and  3   g . In  FIG. 3   f , a DRX inactive cycle or measurement gap lasts until reporting instance  22 ( t+ 3), causing the RI report to be dropped. However, the RI report is not rescheduled and does not displace any other report. Instead, the reports simply continue as previously scheduled from reporting instance  22 ( t+ 4) onwards. However, the UE computes those reports on the basis of a default RI value, preferably RI=1. The eNode-B is aware of the mechanism that has led to the drop of the RI transmission, and hence knows that it should use the default RI value to interpret subsequent MI and/or CQI reports, instead of relying on an actual RI report. That is to say, the eNode-B is pre-configured with the default-value. (Although note, in the case of  FIG. 3   f , the eNode-B will not be able to use the sub-band CQI reports at reporting instances  22 ( t+ 4) and 22(t+5), unless it is agreed to retransmit earlier the wideband CQI/PMI report.) 
         [0048]    Similarly, in  FIG. 3   g , a data transmission from the eNode-B requires the UE to transmit a response such as an ACK in the reporting instance  22 ( t ) in place of the RI report, causing the RI report to be dropped. Again, the RI report is not rescheduled and does not displace any other report. Instead, the reports simply continue as previously scheduled from reporting instance  22 ( t+ 1) onwards. 
         [0049]    Another RI dropping scenario would occur if any higher priority control information (aperiodic CQI report, SR, ACK/NACK or other) needs to be transmitted on the uplink PUCCH in place of a scheduled RI report, in which case that scheduled RI transmission is dropped for that reporting instance in favour of the required higher priority transmission. In this case, instead of the RI report, both the UE and eNode-B are again configured to use a default RI value, preferably RI=1. That is, the UE determines subsequent PMI and CQI reports relative to the default RI value, and in complement the eNode-B interprets the subsequent PMI and CQI reports using the default RI value. In this sense, both the UE and eNode-B “assume” a default RI value. 
         [0050]    Another scenario would be that the RI parameter offset O, signalled by higher layers and denoting the interval between RI and WB CQI/PMI reports happens to be zero. This leads to a collision between the RI and WB CQI/PMI reports. In that case, as stated by the 3GPP specifications, the WB CQI/PMI is dropped, and the RI is still transmitted. Under these circumstances, in the case of frequency selective CQI, the SB CQI become useless since they cannot be correctly interpreted by the eNodeB. In contrast, the preferred solution will keep the transmission of the WB CQI/PMI in place by using a default rank value (RI=1). All the following WB CQI/PMI and SB CQI will be computed and transmitted based on the default rank value 
         [0051]    In addition, in a preferred scheme the UE may retransmit the wideband CQI and PMI report when the wideband CQI/PMI transmission is dropped. That is, if a DRX inactive cycle or measurement gap overlaps with a scheduled wideband PMI and CQI report, or if any other higher priority control information needs to be transmitted on the uplink PUCCH in place of a scheduled wideband PMI and CQI report, then that scheduled wideband PMI/CQI transmission is dropped for that reporting instance and may be transmitted on a subsequent reporting instance, preferably the next reporting instance. This may involve a subsequent sub-band CQI report being omitted, analogously to the omission in  FIGS. 3   d  and  3   e.    
         [0052]    The preferred scheme is summarised as follows.
       In the case where the RI transmission on PUCCH is dropped, both UE and base station (eNode-B) assume a default RI value RI=1.   In the case where the wideband CQI or the wideband CQI/PMI transmission on PUCCH is dropped, then:
           a) for non-frequency selective CQI report modes, do nothing (since the wideband CQI or wideband CQI/PMI will anyway be transmitted at the next reporting instance);   b) for frequency selective CQI report modes, do nothing and wait for the next wideband CQI/PMI reporting instance, or retransmit the wideband CQI or the wideband CQI/PMI in place of a single sub-band CQI report, and then go back to the normal reporting instants.   
               
 
         [0057]    The above solution guarantees a default mode of operation that is agreed between the UE and the e-Node-B. With the approach based on a default rank mode, both the UE and the e-Node-B can safely rely on a fallback transmission mode in case of drop of information. This is in contrast to the techniques of the prior art, where RI retransmission forces the eNode-B to use only part of the frequency selective CQI report, which implies reduced information for frequency selective scheduling in the current CQI/PMI reporting cycle. 
         [0058]    In addition, the proposed solution has the advantage of simplicity and the advantage of not requiring a specific additional operation mode necessary for the retransmission of dropped RI and/or PMI as proposed in the prior art. 
         [0059]    The above solution is general, and does not depend on the kind of event causing the UE to drop an RI/PMI/CQI transmission. The solution provides improved reliability (preferred default mode is based on rank  1 ), and the possibility of using full frequency selective CQI information. 
         [0060]    It will be appreciated that the above embodiments have been described only by way of example. For instance, although the above has been described in terms of a UE and eNode-B, the present invention can apply to any kind of mobile terminal and base station, or most generally any system of wireless transmitter and receiver in which the receiver feeds back information to the transmitter. Further, although the above has a preferred application to 3GPP LTE standards, it may have an application to other wireless communications systems: the terms pre-coding matrix indicator or PMI, rank indicator or RI, and channel quality indicator or CQI, or similar, are not intended to refer to their specific definitions under any one particular standard. In general, pre-coding matrix can refer to any matrix determining the amplitudes and phases with which to transmit a signal on the antennas of a communication system having multiple transmit and receive antennas, and rank can refer to the rank of any channel matrix. Similarly, channel quality indicator can in general refer to any metric relating to the received quality on the downlink channel, whose interpretation when fed back to the transmitter is dependent on the rank and/or pre-coding matrix. Furthermore, where the above refers to reporting instances, it will be understood that this may refer to time intervals of one or more uplink sub-frames or any other time transmission instances of any uplink channel. Other applications and configurations may also be apparent to the person skilled in the art given the disclosure herein. The scope of the invention is not limited by the described embodiments.