Mobile data transmission and data services are constantly making progress. With the increasing penetration of such services, data throughput and transmission reliability find more and more attention.
Under one aspect, investigation is made in scenarios for mobile communication which comprise a plurality of transmission points, each constituted by a set of at least one transmit antenna, for transmitting data to another device in a coordinated transmission from at least a subset of the plurality of transmission points.
It should be noted that concepts outlined in connection with the present invention are generally independent of any particular communication standard; rather, they are generally applicable to a variety of compatible standards. In order to properly describe the concept(s), however, for explanatory purposes only and without any intention to limit the applicability of the concept(s) introduced in the specification to a particular standard, those concept(s) are described with reference to an example scenario. As the example scenario, LTE (Long Term Evolution) and/or LTE-A (LTE-Advanced) was chosen for the network infrastructure.
That is, e.g. in the 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) and LTE-Advanced (LTE-A), single cell single-user (SU-) and multi-user (MU-) multiple-input multiple-output (MIMO) network performance is interference-limited, especially at the cell edge.
Therefore, introduction of the coordinated multipoint (CoMP) transmission/reception technology has been considered, where in downlink (from a network device such as an eNB (evolved NodeB) towards a terminal such as a user equipment UE), multiple points co-operate in scheduling and transmission in order to strengthen desired signal and mitigate inter-cell interference. According to e.g. the 3GPP technical report on CoMP, TR36.819, a point is defined as a set of geographically co-located transmit antennas and the sectors of the same site correspond to different points. It should be noted that a cell is formed by one or multiple points.
The above mentioned CoMP TR was approved after a recent RAN meeting. The agreed CoMP working item definition proposes the following focus for the CoMP work during a subsequent release (e.g. Rel-11):
“The work for specifying CoMP support in Rel-11 should focus on                Joint transmission (JT)        Dynamic point selection (DPS), including dynamic point blanking        Coordinated scheduling/beamforming (CS/CB), including dynamic point blanking”.        
In joint transmission (JT) CoMP, two or more points transmit simultaneously to a CoMP user. Dynamic point selection (DPS) on the other hand refers to a scheme where the transmission point is switched according to changes in signal strength. In coordinated beamforming/scheduling (CB/CS), in turn, the scheduling decisions of neighbor points are coordinated in order to reduce interference. In principle, all schemes may include blanking/muting which means that one or more transmission points are blanked/muted (switched off or not used for transmission) to decrease the interference.
The agreed CoMP working item targets specification of intra-cell and inter-cell DL CoMP schemes which operate in homogeneous and heterogeneous configurations. Four main scenarios have been studied so far:
intra-site (scenario 1),
inter-site with high power remote radio head (RRH) (scenario 2),
low power RRH within the coverage of the macro cell, without and with the same cell ID (scenarios 3 and 4, respectively).
CoMP working item addresses both frequency division duplex FDD and time division duplex TDD. Hence, unified solutions should be targeted, as it is always the case in LTE specifications.
CoMP is intended to improve the performance of cell edge users, as especially at cell edge the performance is interference limited. A CoMP measurement set is formed by M cells/points for which the UE is measuring channel state information. The reporting set has been limited to N cells/points defining the number of points for which CSI feedback is reported. A common assumption has been that the CoMP reporting set is formed by two to three points. Also the CoMP reporting set could be equivalent to the CoMP measurement set. The number of points involved in CoMP scheme (cooperation set) does not need to be signaled to the UE or mentioned in specifications but is left for network implementation. The point from which the UE would receive transmission in single-cell mode is defined as the serving point.
In Release 10, different reference signals (RS) were defined for CSI estimation and data demodulation purposes. Namely, channel state information reference symbols (CSI-RS) and demodulation reference symbols (DM-RS).
Such reference symbols are assigned to (specific) physical resource elements RE within physical resource blocks PRB. A resource element RE is represented by a time slot and a frequency (bandwidth) assigned to it within the frequency-time domain. A plurality (defined number) of resource elements in frequency/bandwidth domain form a physical resource block PRB (in frequency domain), and a plurality of PRBs are present within a channel.
PDSCH (Physical downlink shared channel) resource element muting is also specified, allowing for multi-cell channel estimation. The baseline feedback has been agreed to be implicit feedback which consists of rank indicator (RI), precoding matrix index (PMI) and a channel quality indicator (CQI). Hence, the UE estimates the channel, selects rank and PMI and calculates the post-processing (after receiver) SINR (signal to interference noise ratio) and derives the CQI based on that. CQI may be seen as indicative of the post processing SINR. Release 10 feedback operates per point. The CoMP specific flavors are that a UE may receive CSI-RS resources from more than one point and it is possible to design aggregated (over multiple CSI-RS resources) or per point (per CSI-RS resource) feedback. The per-point PMIs may be improved by a combiner feedback that may be an inter-point phase and/or amplitude value.
Table 1 summarizes the feedback and channel estimation options for each CoMP scheme.
TABLE 1Feedback for different CoMP schemesJTDPS + mutingCS/CB + mutingFeedback1) Per pointPer pointPer pointPMI/CQI (+PMI/CQI +PMI/CQI orcombiner)point selection +CS/CB +2) Per point PMIpossiblemuting specific(+combiner) +mutingadditionalaggregated CQI +indicationfeedbackserving pointCQI3) AggregatedPMI/CQI +serving pointPMI/CQIChannelPer point CSI-RSPer point CSI-Serving pointestimationor aggregatedRSCSI-RS +single CSI-RSpossibly otherpatternpoint CSI-RS
During a recent RAN meeting, the following working assumption was agreed:
“Definition: “CSI-RS resource” here refers to a combination of “resourceConfig” and “subframeConfig” which are configured by higher layers.
Standardize a common feedback/signaling framework suitable for scenarios 1-4 that can support CoMP JT, DPS and CS/CB. Feedback scheme to be composed from one or more of the following, including at least one of the first 3 sub-bullets:                feedback aggregated across multiple CSI-RS resources        per-CSI-RS-resource feedback with inter-CSI-RS-resource feedback        per-CSI-RS-resource feedback        per cell Rel-8 CRS-based feedback        
Note that use of SRS sounding reference signal (SRS) used in uplink measurement may be taken into account when reaching further agreements on the above.”
The CoMP problem relates mostly to the CQI feedback. The CQI is used by the eNB to perform adaptive modulation and coding which means the transmission rate is adapted based on channel conditions. Accuracy of the CQI value affects greatly on the system performance, especially if the CQI is overestimated and too high transmission rate is assigned which is not supported by the actual radio link. The CQI depends on the transmission hypotheses made by UE at a given time. For example:                When reporting an aggregated JT CQI, UE assumes combined transmission from N points to the UE,        When reporting a DPS CQI without muting, UE assumes transmission from a selected transmission point and interference from other transmission points,        When reporting a DPS CQI with muting, UE assumes transmission from one point and zero interference from points that are assumed to be muted,        When reporting CQI for CS/CB, UE assumes transmission from one point and reduced interference or muting from other transmission points.        
In addition to the transmission hypothesis, the CQI value depends on the hypothesis of the interference. The exact interference level depends on the exact scheduling decisions and used PMIs in the other cells at time of transmission. It follows that the level of the exact interference is not known at the UE at the time when CQI is evaluated. The UE is not aware of a scheduling decision of any eNB without signaling, and moreover, the scheduling decisions affecting the experienced interference level are not yet even made, and thus cannot be signaled even if that kind of signaling would be possible. For the case of CS/CB, the coordination between points reduces the level of interference but the level of interference reduction is typically not known by the UE.
In a recent RAN1 meeting, several companies contributed aspects related to CoMP CQI. Out of these, one did not discuss the CQI derivation details. Others presented equations on how the eNB can derive the CQI for joint transmission from per point CQIs with a given assumption on interference. The assumptions on how the UE would estimate the interference level for the per-point CQIs was not considered.
A further contribution discusses measurement objectives as follows “Observation 1: for each CoMP scheme, considering the coordination method, the interference should “Include signals from all the points/cells outside of the transmission set or coordination set, Consider the actual resource elements causing interference to the PDSCH, Not include signals from the point/cell the UE assumes as transmitting the PDSCH, Not include signals from the point/cell with blanking on some or all resource elements”.
But no concrete scheme for implementation is discussed/presented.
A more related prior art addressing estimation of interference level for deriving a CQI can be found in a previous standard. The energy per resource element (EPRE) is assumed to be different for the CSI-RS from which the channel is estimated and for the PDSCH where the data is transmitted. As stated in TS36.213, V.10.3.0, section 7.2.5, a parameter P_c is defined that indicates the ratio between PDSCH EPRE and CSI-RS EPRE and it is signaled to the UE. The parameter is referenced as p-C-r10 in TS36.331 v.10.3.0 in section 6.3.2. This parameter is part of the PhysicalConfigDedicated information element used to specify the UE specific physical channel configuration on a transmission carrier. This parameter is needed for the CQI derivation in current system, and the parameter is user and transmission frequency specific.
Assuming that UE estimates signal power from the CSI-RS, P0, and other cell interference and noise power N, the final CQI is based on the SINR=P_c P0/N. Thus the current definition of P_c is applied to the “useful” (payload) signal transmission to the UE.
Thus, existing and/or discussed systems for CoMP still lack fully appropriate feedback from terminals so as to properly estimate/take into account also interference experienced by the terminals.
Thus, there is still a need to further improve such systems.