WO2013128426A1

WO2013128426A1 - Feedback reduction for coordinated multipoint transmission

Info

Publication number: WO2013128426A1
Application number: PCT/IB2013/051663
Authority: WO
Inventors: Helka-Liina MÄÄTTÄNEN; Mihai Enescu; Kari HÄMÄLÄINEN
Original assignee: Renesas Mobile Corporation
Priority date: 2012-03-02
Filing date: 2013-03-01
Publication date: 2013-09-06
Also published as: GB2499849A; GB2499849B; GB201203721D0

Abstract

Apparatus for use in coordinated multipoint transmission in a terminal device, the apparatus comprising a controller module arranged to measure channel quality for a plurality of transmission points, wherein the plurality of transmission points comprises a serving transmission point serving the device and at least one non-serving transmission point, calculate, at least on basis of the measured channel quality, channel quality indications for each of the plurality of transmission points, compare the channel quality indicators pertaining to the non-serving transmission points with the channel quality indicator pertaining to the serving transmission point, select, for feedback transmission together with the channel quality indicator pertaining to the serving point, at least one channel quality indicator pertaining to a non-serving point which fulfills a predetermined condition, and feed back the at least one selected channel quality indicator pertaining to a non-serving point and the channel quality indicator pertaining to the serving point.

Description

FEEDBACK REDUCTION FOR COORDINATED MULTIPOINT TRANSMISSION

Technical Field

The present disclosure relates to devices, methods, computer software and computer program products configured to achieve improvements in feedback for coordinated multipoint transmission arrangements, e.g. configured to be deployed in a scenario for mobile communication, which scenario comprises a plurality of transmission points, each comprising 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.

Background

Mobile data transmission and data services are constantly making progress. With the increasing penetration of such services, data throughput and transmission reliability attract more and more attention.

Under one aspect, investigation is made in scenarios for mobile communication which comprise a plurality of transmission points, each comprising 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 disclosure 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 DL (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 the desired signal and to mitigate inter-cell interference. According to 3GPP, 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 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-1 1 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 is 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 on CSI-RS. 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. During a 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, used in uplink measurement may be taken into account when reaching further agreements on the above. "

Another RAN meeting stipulated that CSI feedback for CoMP uses at least a per-CSI-RS-resource feedback.

Generally, current activities in CoMP related research target designing common feedback for the CoMP schemes. In current TR 36.819, a baseline feedback is individual per-point feedback with or without complementary inter-point feedback.

Thus the per-CSI-RS-resource CSI feedback has been taken as the baseline assumption.

In relation to CoMP, the feedback in a single point mode consists of a 2 bit rank report (e.g. rank indicator RI), 4 bit precoding matrix index (PMI) and a channel quality indicator (CQI). When RI=1, thus rank 1 reporting, there is one CQI made of 4 bits. When RI>1, there are two CQIs each made of 4 bits, or one 4 bit CQI and one 3 bit relative CQI. The feedback (from a terminal such as a user equipment UE towards a network transceiver device such as an evolved NodeB eNB) may be frequency selective or non-frequency selective and it may be reported in an aperiodic (on Physical Uplink Shared Channel, i.e. PUSCH) or in a periodic mode (on Physical Uplink Control Channel, i.e. PUCCH and/or PUSCH). Aperiodic reporting which enables also frequency selective CSI-reporting is triggered by appropriate downlink signalling from eNB to UE. A rank is always reported wideband. The existing (up to Release 10) CSI feedback modes in the aperiodic feedback mode are described in TABLE 1, i.e. the table below.

No PMI Sincjte PMI Mwitip!e PMX

Wideband Mode (wideband CQI)

r ^{U E} selected Mode 2- 0 Mode 2 -2

Fee¾ba¾ Csub and CQI)

type Higher Layei Mode 3-0 Mode 3- 1

config red

subband CQI)

TABLE 1: CSI feedback modes in the aperiodic feedback mode

Transmission mode 9 in LTE / LTE-A supports:

- Modes 1-2, 2-2, 3-1, if the UE is configured with PMI/RI reporting and a number of CSI-RS ports > 1;

- Modes 2-0, 3-0, if the UE is configured without PMI/RI reporting or a number of CSI-RS ports=l

Mode 3-2, while not adopted yet in the specification, has been extensively investigated. Its inclusion in the standard is imminent for Release 11.

Modes 2-2, 3-1, 3-2, however, may for example result in a quite heavy feedback load if the frequency selective feedback is triggered for multiple CSI-RS- resources. 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 latter has been decided to be the baseline although the aggregated feedback option is still under investigation. An example of additional aggregated feedback is an inter-point phase that may be used to improve the per-point PMIs. The CQI may be per point or aggregated. The agreement "CSI feedback for CoMP uses at least per-CSI-RS- resource feedback" does not describe whether all feedback is "per point" or only PMI, or both PMI and CQI feedback. From the PMI point of view, things are quite clear. There are per point PMIs and possible combiner to represent additional aggregated feedback. The CQI is more complex to handle. One may compute one aggregated CoMP CQI in addition to the fallback CQI, which would be the serving cell single cell CQI, or there could be per point CQIs. Per point CQIs have the advantage that depending on the definition, it is argued that CoMP CQIs for e.g. JT or DPS could be derived from the per-point CQIs. However, having sub-band per point CQIs in addition to sub-band per point PMIs may significantly increase the feedback load.

CoMP gaining mechanisms can be roughly categorized in two mechanisms One mechanism is diversity; a UE may be served by more than one fixed eNB. In JT- CoMP the UE is served simultaneously by more than one eNB. In DPS, the selection diversity may give CoMP gains and UE may be served by two or more eNBs in a flexible manner in time and frequency domain.

Another mechanism is scheduling flexibility. When the scheduling entity has received feedback from UEs to several eNBs, the scheduling entity may have more flexibility to optimize the scheduling decisions, in a network centric manner. A cost of this may be that part of the feedback is unused for transmission, especially in DPS CoMP.

From contribution Rl-113729 it is known that studies are made in that a CQI for each point in the CoMP measurement set is reported, and interference outside the CoMP measurement set is considered while deriving CQI. Further, a TxD CQI based on CSI-RS for each point in the CoMP measurement set is reported, while a differential CQI may be considered.

From contribution Rl-113983, it is known that studies are made in terms of a comparison of different ways to define the per-cell CQIs.

From a contribution Rl-114049 it is known that studies are made in terms of a way to reduce the per cell feedback load by proposing to have some CQIs long term and absolute and other CQIs short term and relative.

From a contribution Rl-113766 it is known that studies are made in terms of the per-cell CQIs and how to derive the CoMP CQI based on those. As one option, a scheme has been proposed of per-cell CQIs and an additive feedback quantity that is a value derived from the interference levels experienced by the UE. Irrespective of the above proposals, there is still a need to further improve such systems.

Summary

According to first embodiments, there is apparatus for use in coordinated multipoint transmission in a terminal device, comprising a controller module which is arranged to:

measure channel quality for a plurality of transmission points, wherein the plurality of transmission points comprises a serving transmission point serving the device and at least one non-serving transmission point;

calculate, at least on the basis of the measured channel quality, channel quality indications for each of the plurality of transmission points;

compare the channel quality indicators pertaining to the non-serving transmission points with the channel quality indicator pertaining to the serving transmission point;

select, for feedback transmission together with the channel quality indicator pertaining to the serving point, at least one channel quality indicator pertaining to a non-serving point which fulfills a predetermined condition; and

feed back the at least one selected channel quality indicator pertaining to a non-serving point and the channel quality indicator pertaining to the serving point.

According to second embodiments, there is a method for use in in coordinated multipoint transmission in a terminal device, the method comprising:

measuring channel quality for a plurality of transmission points, wherein the plurality of transmission points comprises a serving transmission point serving the device and at least one non-serving transmission point;

calculating, at least on the basis of the measured channel quality, channel quality indications for each of the plurality of transmission points;

comparing the channel quality indicators pertaining to the non-serving transmission points with the channel quality indicator pertaining to the serving transmission point; selecting, for feedback transmission together with the channel quality indicator pertaining to the serving point, at least one channel quality indicator pertaining to a non-serving point which fulfills a predetermined condition; and

feeding back the at least one selected channel quality indicator pertaining to a non-serving point and the channel quality indicator pertaining to the serving point.

According to third embodiments, there is computer software adapted to perform the method of the second embodiments.

According to fourth embodiments, there is a computer program product comprising a non-transitory computer-readable storage medium having computer readable instructions stored thereon, the computer readable instructions being executable by a computerized device to cause the computerized device to perform the method of the second embodiments.

According to embodiments, there is provided apparatus for use in a terminal device, comprising:

means for measuring channel quality for a plurality of transmission points, wherein the plurality of transmission points comprises a serving transmission point serving the device and at least one non-serving transmission point,

means for calculating, based on the measured channel quality, channel quality indications for each of the plurality of transmission points,

means for comparing the channel quality indicators pertaining to the non- serving transmission points with the channel quality indicator pertaining to the serving transmission point, and

means for selecting, for feedback transmission together with the channel quality indicator pertaining to the serving point, at least one channel quality indicator pertaining to a non-serving point which fulfills a predetermined condition, and

means for feeding back the at least one selected channel quality indicator pertaining to a non-serving point and the channel quality indicator pertaining to the serving point.

Advantageous further developments are set out in respective dependent claims. According to further embodiments, there are provided computer program products comprising respective computer-executable components which, when the program is run on a computer, are configured to perform the above method embodiments, respectively. That is, such computer program products also encompass computer readable storage media comprising a set of computer-executable instructions which, when the program is run on a device (or on a processor or processing unit thereof which may be part of a controller or control unit or control module), such as a terminal UE and its processor, cause the device to perform the method embodiments. In particular, the above computer program product/products may be embodied as a computer-readable storage medium.

Accordingly, under embodiments, improvements in CoMP feedback arrangements are achieved. In particular, according to some embodiments, performance improvement in CoMP feedback scenarios is based on such methods, devices and computer program products which enable appropriate CQI feedback to be provided, and which contribute to an improved CQI reporting for CoMP. At least some embodiments involve providing a simple implementation of the improvements, which consist e.g. in diminishing the redundant feedback load and thereby enable balancing the tradeoff between scheduling flexibility and amount of feedback for it.

Thus, according to at least some embodiments, minimizing redundant feedback considering the baseline assumption of per-CSI-RS-resource is achieved. Also, taking into consideration other options such as aggregated feedback or additional aggregated feedback is enabled. An example of an aggregated feedback is an aggregated CQI that is used instead of the per-cell CQIs. An example of an additional aggregated feedback is a phase combiner fed back in addition to per cell PMIs.

In line with some embodiments, hence, there is achieved a reduction in uplink signalling (feedback) overhead. Though, the terminal UE calculates full CSI (e.g. CQI based on estimated channel) reports for all configured points. However, the UE hardware may be anyway dimensioned such that these calculations can be carried out in relation to the agreed concept of applying the per-point feedback. Brief Description of the Drawings

For a more complete understanding of embodiments of the present disclosure, reference is now made to the following descriptions taken in connection with the accompanying drawings in which:

FIGURE 1 illustrates one example of an overview of devices/entities involved; and

FIGURE 2 illustrates one example of a flowchart of a processing performed at a terminal device such as a UE,

Fig. 3 illustrates one example of a flowchart of a processing performed at a network transceiver device such as an eNB;

Fig. 4 is an illustration of selective CSI (or CQI, respectively) feedback, based on an evaluated condition per sub-band, in an example case of two transmission points,

Fig. 5 shows two tables comparing a CoMP feedback scenario according to at least an embodiment with prior arrangements, and

Fig. 6 is an illustration of selective CSI (or CQI, respectively) feedback, based on an evaluated condition per sub-band, in an example case of two transmission points, wherein the feedback of the non-serving point is derived under different transmission hypothesis.

Detailed Description

Example embodiments will be described herein below.

Generally, embodiments are implemented in a framework for a coordinated transmission from multiple points or antennas, as illustrated in rough outline in Fig. 1.

For the subsequent description and explanation of embodiments and concepts of embodiments, the following definitions / explanations shall support understanding:

In coordinated multipoint transmission CoMP, a CoMP scheme represents a set of transmission points used, e.g. "which" ones are used as well as "how" those are used (for example jointly used in coherent or non-coherent joint transmission "JT", or non-jointly used e.g. in DPS (where the assumption on the non-serving points in the measurement set can be as blanked/muted or transmitting interference) or CS/CB transmission, referred to as "non-JT"). Plural co-located antennas may constitute a transmission point, while a transmission point may, in some scenarios also be represented by a single antenna only. Transmission from a transmission point is effected within (physical) resources such as resource elements RE of (one or more) physical resource blocks PRB. Signals (transmitted from the eNB and received at the UE) used for CSI measurement / evaluation of the CoMP scenario are referred to as reference (signals or) symbols 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. Thus, a plurality of such reference symbols RS used for obtaining information on channel state(s) are referred to as CSI-RS. A plurality of REs (time slots) in time domain form so-called sub-frames or frames, respectively. The CSI-RS REs can be of zero-power or nonzero-power. Whilst the non-zero-power CSI-RS can be used for both signal and interference estimation, the zero-power CSI-RS are mainly intended for interference estimation. Other types of RS could be envisioned for estimating the signal and interference.

A set of those resources carrying CSI-RS signals may thus be representative for an underlying CoMP (transmission) scheme. When computing a parameter indicative of a quality of the underlying channel such as a channel quality indicator (CQI), a terminal UE assumes transmission from the respective set of CSI-RS resources (on which it bases its measurement and computation) according to the CoMP scheme assumption. A respective CSI-RS is thus present in plural, different sub-bands, and hence CQIs (or CSIs) are derived for respective CSI-RSs and are obtained per respective sub-band.

Note that the parameter CQI is used herein as an example only and that other parameters may likewise be applicable in the framework of at least embodiments when applied to other standards in which identical, similar or corresponding parameters may be assigned a different name. In case a transmission point TP i is represented by a single antenna Tx i, a single respective CSI-RS i may be assigned thereto, and an evaluated channel will be representative for a channel established from that TP i (e.g. at an eNB or a remote radio head RRH under control of the eNB) towards the terminal. On the other hand, in case a transmission point TP i is represented by plural antennas, e.g. by 2 such as Tx i & Tx k, (or even more than two) , still a single respective CSI-RSJ may be assigned thereto, and an evaluated channel will be representative for a channel established from e.g. both (or the plurality) Tx i & Tx k (e.g. at an eNB or a remote radio head RRH under control of the eNB) towards the terminal. Still further, a scenario is possible in which a transmission point TP i is represented by one or more antennas Tx i, but plural CSI-RS_n,m may be assigned thereto. An evaluated channel will then be representative for a channel established from TP i (e.g. at an eNB or a remote radio head RRH under control of the eNB) but using resources CSI-RS_n,m towards the terminal.

Fig. 1 illustrates one example of an overview of devices/entities involved with reference to entities known from LTE /LTE-A. Other names may be assigned to similar entities in other standards, while as long as the functionality imparted thereto remains the same or substantially similar, embodiments as described herein below will remain applicable also to those other standards.

As shown in Fig. 1, a network device such as a transceiver device eNB

(reference numeral 10) is equipped with a control module Ctrl (reference numeral 11) as well as with at least one transmit antenna (e.g. as an example two are shown and labelled as Txl, Tx2). Note that the transmit antennas may also be used for reception (as an example illustrated as Txl/Rxl), or one or more separate reception antennas (not shown) can be provided to receive uplink transmissions (from a terminal UE (reference numeral 20) to the eNB (reference numeral 10)). However, for description purposes of the CoMP scenarios, focus is mode on the transmit antennas Tx (Txl, Tx2, ...). The eNB 10 is connectable to and may control a (single or plural) remote radio head RRH also equipped with at least one transmit antenna (RRH and its transmit antenna(s) not shown in Fig. 1). The eNB 10 and potentially the one or more RRH's are in communication with a terminal such as a user equipment UE (reference numeral 20) equipped with a transmit/receive antenna Tx/Rx as well as with a control module Ctrl (reference numeral 21).

The eNB 10 transmits data in downlink to the UE 20 in a coordinated multipoint transmission CoMP mode. The CoMP mode may comprise plural CoMP scenarios, or CoMP configurations, respectively, represented by respective CSI-RS resource configurations (non-zero-power or also zero-power CSI-RS configurations, or optionally further RS configurations (CSI-RS or other RS) for estimation and/or interference measurement). DL transmission comprises data and control channels (such as the PDSCH or PDCCH). Control channels carry control signals and/or configuration signals for control/configuration of the UE.

The UE 20 transmits data in uplink UL to the eNB 10 using data and control channels. Control channels in UL, at least in relation to some embodiments, serve to carry feedback signals from the UE 20 to the eNB 10. Signals fed back from the UE may comprise a rank indicator RI, and a precoding matrix indicator PMI, but at least a channel quality indicator CQI (and/or other channel state indicator CSI). Such CSI feedback (which typically consists of PMI/CQI/RI, but may also comprise other indications) may be computed with a CoMP assumption on the points being part of the measurement set, possible assumption options being JT, DPS with wihout blanking, coordinated beamforming. The UE 20 is configured to measure transmission channels in respective CoMP scenarios (represented by respective CSI- RS configurations) and to compute a parameter indicative of a quality of the underlying channel such as a channel quality indicator CQI. At least one such CQI parameter (determined at the UE 20 and selected at the UE 20 from plural determined ones) is fed back from the UE to the eNB. To this end, the UE is equipped with a measurement unit and processing unit (not separately shown in Fig. 1) which can be assumed to be part of the control module.

Fig. 2 illustrates an example of a flowchart of a processing performed at a terminal device (e.g. at a processor, module or chipset or any other subunit e.g. inserted in or connected to or associated to / in functional interaction with such terminal device) such as a UE (numeral 20 in Fig. 1). Prior to giving a more detailed description, it should be noted that in the following a "lower point" is defined as a point that has lower CQI or other gain or performance measure than the serving cell CQI (or considered measure), and "higher point" is defined as a point that has higher CQI or other performance measure than the serving cell CQI (or considered measure). Same or different CoMP scheme assumptions may be possible for the "lower" and "higher point" CQI. Whilst for example the "lower point" may assume single point transmission for the CQI computation, the "higher points" may assume various CoMP schemes. If higher ranks CQIs are reported, different CoMP scheme assumptions may be possible between ranks. The CQI/PMI feedback for "lower points" are more likely to become redundant than the feedback for "higher points". One option is that UE always sends feedback only for the "higher points" but then all scheduling flexibility is lost and fallback feedback to serving cell is not available.

The CQIs described may be used as in one of the contributions (referenced above). Then, for example the serving cell CQI, could be long term and/or absolute and the other cell CQIs may be short term and/or relative, while the other point CQIs are sent only for "higher points."

Accordingly, an embodiment is as follows. When a UE is triggered to send feedback corresponding to more than one CSI-RS configured resource, UE sends feedback characterizing the serving point, and additionally for other non-serving points only when a measured quantity, like CQI, of those points is higher than that of the serving point. That decision may include a hysteresis (as outlined below in relation to Fig. 4).

For a "simple" scenario according to an example embodiment comprising only two transmission points, the other transmission point is fed back on those PRBs, where a measure such as CQI is higher, per sub-band, than that of the serving point.

For a less "simple" scenario according to another example embodiment with more than two transmission points, there exist options to send feedback for all those cooperating transmission points that can be classified as higher points, or for a subset of those. An example of such a subset is, when from a multitude of better points, only the best point feedback is sent. Alternatively, from a multitude of better points, only the "worst" is not fed back, or the two best are fed back, or the worst two are not fed back, or only those which are better than the average of all better points (better than the serving point) are fed back, or the like. Same or different CoMP schemes may be assumed for the best reported points.

The following may be summarized in this regard. In addition to the serving point frequency selective CQI, the UE constructs a frequency selective multi-point CQI by combining sub-band wise the best CQI sub-bands of the reported points. Feedback is saved by not sending the full frequency selective multi point CQIs due to not feeding back those CQI's for the lower points. Serving point feedback serves as fallback mode. Additional scheduling flexibility is given to the scheduling entity for those sub-bands where the feedback is doubled. A good tradeoff between amount and feedback and support for the two CoMP gaining mechanisms, diversity and scheduling flexibility is achieved.

According to an embodiment, there is also an option that the other point CQI is a more general CoMP CQI, which may be a per point CQI with different muting assumptions, or aggregated CQI for joint transmission. For this assumption, it is assumed that the nature of the CQI is signaled with an indication that may be wideband or per reported sub-band. The indication type may be for example a codeword from a CoMP scheme selection (CSS) codebook.

According to an embodiment, there is also an option that the other point CQI has different CoMP scheme assumption per rank, which may be a per point CQI with different muting assumptions, or aggregated CQI for joint transmission. For this assumption, it is assumed that the nature of the CQI is signaled with an indication that may be wideband or per reported sub-band. The indication type may be for example a codeword from a CoMP scheme selection (CSS) codebook or a codeword from a CoMP rank order indication (ROI).

As regards UE procedures in relation to some embodiments, reference is made to Fig. 2. The UE CSI feedback measurement and reporting functions, according to the example embodiment outlined below, for example as follows.

The UE process starts in a step S20. In a step S21 , the UE receives a measurement trigger or request for feedback measurement results from a eNB. In step S22, the UE measures the channel corresponding to multiple transmission points based on configured CSI-RS resources. In step S23, the UE calculates the RI and CQI/PMI for serving point. Also, the UE calculates (step S24) the other (non-serving) point CQI/PMI feedback based on the configured multi-point CSI-RS resource, and also optionally an inter-CSI-RS resource phase/amplitude information. Other CoMP feedback may be computed as well if deemed necessary.

Note that it may also be a further modification of an example embodiment that for example, the UE receives separate requests for calculation, but sends the feedback later on in the future, after some delay, e.g. if uplink capacity is not blocked or restricted by other UL signaling of higher priority.

In step S25, the UE compares, per sub-band, CQIs from other-than-serving- point (ex. CQIj, CQI k) with serving point CQI i, (e.g. with reference to Fig. 1 : CQIs of transmit antennas Txl, Tx2 are compared with CQI of antenna Tx3) (assuming that a transmission point is constituted by a single antenna only) and selects, in a step S26, on which sub-bands the other point or points' feedback is submitted. This may optionally involve a threshold such that for setting the CoMP feedback indicator bit it is required that CQIj,k > CQI i+Δ where Δ may be configured by the eNB to the UE. (And optionally, for resetting the feedback indicator to "0", i.e. no feedback, CQIJ,k < CQI i-Δ, can be configured, thereby achieving hysteresis. Note that the delta "+Δ" can be different from the "-Δ" and, insofar different thresholds may be applied. Also, different deltas may be configured for different sub-bands.) When a sub-band CQI for more than one point is better than the CQI for the fallback point (i.e. serving point), options exist to send feedback for all those cooperating transmission points that can be classified as higher points or for a subset of those (for example feedback is sent for the strongest point only). The UE sends in step S27 the feedback according to the decisions as taken before and described above. Then, the UE process returns to S21 and the UE is in a state in which it may receive a new measurement/feedback trigger.

Fig. 3 illustrates an example of a flowchart of a processing performed at a network transceiver device such as an eNB. In brief, eNB procedures in relation to embodiments encompass that the eNB uses the report received in the feedback from the UE for example as follows. Namely, the eNB or the scheduling entity performs scheduling based on the available per sub-band feedback. This may contribute to enhancement and performance improvement in CoMP scenarios from the UE and from the network perspective.

In more detail, according to one example embodiment in this regard, the process starts for the eNB in a step S30. In a step S31 , eNB sends a measurement trigger to a terminal UE. The trigger may be periodic, e.g. every 60 ms, every second or the like. The trigger may be aperiodic based on e.g. a predefined schedule for measurement at specific times. In some embodiments, aperiodic triggers may be issued based on events prevailing/occuring in the communication between the eNB and the UE. In a step S32, the eNB receives feedback reports, per sub-band, from the UE and/or UEs. The reports contain CQI (or CSI), and optionally also PMI and RI as parameters. In a step S33, the eNB applies those reports (values/parameters received) in scheduling. This may imply that eNB forwards the reports to a scheduling entity of the network. Once scheduling is completed, the eNB transmits in CoMP mode as scheduled, S34. If in a step S35 it is determined that a scheduling timer expired, e.g. in case of periodic or aperiodic pre-scheduled scenario, or in case a measurement trigger event occurred, the flow returns from S35 back to S31 and a measurement trigger is sent to the UE. If not (NO in S35), the flow returns back to S34 and the eNB continues to send in CoMP mode as previously scheduled.

Fig. 4 is an illustration of the selective CSI (or CQI, respectively) feedback, based on an evaluated condition per sub-band, in an example case of two transmission points.

The table (Fig. 4 indicates when CSI and/or CQI feedback is to be sent from UE for both transmission points, a serving one and a non-serving (other) one to the eNB (or scheduling entity of the network). The UE measures N sub-bands, i.e. sub- bands 1, 2, N-2, N-l, N. In the example, for a transmission point 1 which is assumed to be the serving transmission point, a quality measure S I is measured. S I is used (as represented in Fig. 4 by an arrow— >) to derive a corresponding quality indicator feedback such as CQI l . As indicated in Fig. 4, for the serving point, in each sub-band, CSI is fed back. In addition, in the example, for a transmission point 2 which is assumed to be a non-serving transmission point, a quality measure S_2 is measured. S_2 is used (as represented in Fig. 4 by an arrow — >) to derive a corresponding quality indicator feedback such as CQI 2. As indicated in Fig. 4, for the non-serving point, in each sub-band, CSI is fed back case by case, in the example shown, in sub-bands 3,— , N-l and N. The selective (conditional) feedback of a CSI parameter per sub-band for non-serving transmission points is determined based on a test that the measured qualities (based on S_l, S_2, respectively, and represented by CQI l, CQI 2) meet a predetermined condition. Namely, CSI feedback for transmission point 2 is only accomplished or allowed in case CQI_2>CQI_1 is true. In the illustrated example in Fig. 4, this condition is fulfilled only in the listed example sub-bands, so that the last column in the table in Fig. 4 merely lists, as a flag indicating this condition, a flag value of "1" (CQI_2>CQI_1 "True") in those sub- bands, while it lists a flag value of "0" (CQI 2>CQI 0 "False", i.e. CQI 2<=CQI 1) in other sub-bands.

Measurements to obtain S2, SI for deriving CQI 2, CQI l, respectively, can be conducted repeatedly e.g. periodically or based on other fixed (aperiodic) schedules or trigger events.

In this regard, as an option, the above outlined decision can be based on hysterisis. That is, e.g. only in case S_2>S_1 is met in for example two consecutive measurements, the flag indicative of S_2>S_1 is set to indicate "true". Likewise, this is applicable in the other direction. That is, a flag having been set to "1" is only reset to zero (then preventing feedback of the CSI/CQI value for the non-serving transmission point in that respective sub-band) in case that CQI_2>CQI_1 is not met (to indicate "false") in for example two consecutive measurements.

The above "hysteresis" (or "persistency") can be combined with a hysteresis in terms of value as mentioned above. For example, it is checked whether CQI_2>CQI_l+delta in order to report CSI for CQI 2. Other modifications outlined above can be combined herewith.

Fig. 5 shows two tables comparing a CoMP feedback scenario according to some embodiments with prior arrangements. These tables present the amount of feedback for three feedback options. The upper table (Fig. 5(a)) is for two cooperating points, S_i is the serving point and S J is the cooperating point. The lower table (Fig. 5(b)) is for three cooperating points, S_i is the serving point and S J and S_k are the cooperating points. N describes noise and interference outside the CoMP set. The first row describes the per cell CSI feedback option. The second row describes an option where the UE feeds back only the strongest point.

The third row illustrates results and/or effects obtained when some embodiments are implemented, where the other points are reported in addition to the serving point only when those points are stronger than the serving cell.

The first columns describe the feedback for rank 1 and the second columns show the feedback for rank 2. As can be seen from the tables, as regards feedback load (in uplink), the feedback data load in relation to this example embodiment is between the basic per cell feedback and this UE selected feedback scheme, where the network has no flexibility in scheduling.

Thus, from the above example it becomes visible that feedback load can be reduced while still preserving scheduling flexibility in example embodiments.

Fig. 6 is an illustration of the selective CSI (or CQI, respectively) feedback, based on an evaluated condition per sub-band, in an example case of two transmission points, wherein the feedback of the non-serving point is derived under different transmission hypothesis. Fig. 6 is similar to Fig. 4, with the difference being that in this example as shown, the feedback of the non-serving point (transmission point 2) is derived under e.g. assumption of joint transmission, JT, (cf. sub-band 3 and N-1) or dynamic point selection, DPS (cf. sub-band and N). Hence, a predetermined condition for feeding back may not only reside in the value/result of the comparison of CQI_2>CQI_1, but may also reside in a determined assumption of the transmission scheme.

Note that the predetermined condition may be set per sub-band, e.g. for sub- band 3 and N-1, as illustrated, feedback will only occur if JT was determined as an assumed transmission scheme, and e.g. for sub-bands "..." and N, feedback will occur only in case of DPS being determined as an assumed transmission scheme. Note further that in at least one example, (not shown in Figs. 4 or 6) also the predetermined condition which is reflected in that the non-serving point has a channel quality indicator indicating a better channel quality compared to that of the serving point, can be set or configured sub-band-wise, e.g. differently for different sub-bands. For example, the threshold "delta" mentioned above can be different among sub- bands.

Some embodiments may be implemented in software, hardware, application logic or a combination of software, hardware and application logic. The software, application logic and/or hardware generally reside on control modules or modems, in general circuitry. In an example embodiment, the application logic, software or an instruction set is maintained on any one of various conventional computer-readable media. In the context of this document, a "computer-readable medium" may be any media or means that can contain, store, communicate, propagate or transport the instructions for use by or in connection with an instruction execution system, apparatus, or device, such as a computer or smart phone, or user equipment.

As used in this application, the term 'circuitry' refers to all of the following:

(a) hardware-only circuit implementations (such as implementations in only analog and/or digital circuitry) and

(b) to combinations of circuits and software (and/or firmware), such as (as applicable):

(i) to a processing system or combination of processor(s) or

(ii) to portions of processor(s)/software (including digital signal processor(s)), software, and memory(ies) that work together to cause an apparatus, such as a mobile phone or user equipment or any other terminal, or network entity such as a server, to perform various functions) and

(c) to circuits, such as a microprocessor(s) or a portion of a microprocessor(s), that require software or firmware for operation, even if the software or firmware is not physically present.

This definition of 'circuitry' applies to all uses of this term in this application, including in any claims. As a further example, as used in this application, the term "circuitry" would also cover an implementation of merely a processor (or multiple processors) or portion of a processor and its (or their) accompanying software and/or firmware. The term "circuitry" would also cover, for example and if applicable to the particular claim element, a baseband integrated circuit or applications processor integrated circuit for a mobile phone (terminal) or a similar integrated circuit in a server, a cellular network device, or other network device.

The present disclosure relates in particular but without limitation to mobile communications, for example to CoMP enabled environments under WCDMA, LTE, WIMAX and or WLAN and can advantageously be implemented in user equipments or smart phones, or personal computers connectable to such networks as well as in network devices such as eNBs. That is, it can be implemented as/in chipsets to such devices, and/or modems thereof.

If desired, the different functions discussed herein may be performed in a different order and/or concurrently with each other. Furthermore, if desired, one or more of the above-described functions may be optional or may be combined.

Although various embodiments are set out in the independent claims, other embodiments comprise other combinations of features from the described embodiments and/or the dependent claims with the features of the independent claims, and not solely the combinations explicitly set out in the claims.

It is also noted herein that whilst the above describes example embodiments, these descriptions should not be viewed in a limiting sense. Rather, there are several variations and modifications which may be made without departing from the scope of the present disclosure as defined in the appended claims.

List of exemplary acronyms used in this document:

CB coordinated beamforming

CoMP coordinated multipoint transmission

CS coordinated switching

CSI-RS channel state information reference signal

CSS CoMP scheme selection

CQI channel quality indicator

DL downlink DPS dynamic point selection

JT joint transmission

PUSCH physical uplink shared channel

PUCCH physical uplink control channel

PDCCH physical downlink control channel

PMI precoding matrix information

ROI rank order indicator

RPvC radio resource control

RSRP reference signal received power

RI Rank Indicator

SRS Sounding Reference Signal

TS Technical Specification

The present disclosure proposes methods, apparatuses/devices, computer software and computer program products related to improvements in feedback in coordinated multipoint transmission arrangements. With regard to an apparatus, an apparatus for use in coordinated multipoint transmission in a terminal device is proposed, the apparatus comprising a controller module which is arranged to measure channel quality for a plurality of transmission points, wherein the plurality of transmission points comprises a serving transmission point serving the device and at least one non-serving transmission point, calculate, at least on the basis of the measured channel quality, channel quality indications for each of the plurality of transmission points, compare the channel quality indicators pertaining to the non- serving transmission points with the channel quality indicator pertaining to the serving transmission point, and select, for feedback transmission together with the channel quality indicator pertaining to the serving point, at least one channel quality indicator pertaining to a non-serving point which fulfills a predetermined condition, and feed back the at least one selected channel quality indicator pertaining to a non-serving point and the channel quality indicator pertaining to the serving point.

Claims

1. Apparatus for use in coordinated multipoint transmission in a terminal device, comprising a controller module which is arranged to:

2. Apparatus according to claim 1 , wherein the predetermined condition reflects that the non-serving point has a channel quality indicator indicating a better channel quality compared to that of the serving point.

3. Apparatus according to claim 2, wherein the predetermined condition reflects that the non-serving point has a channel quality indicator that is better by a first threshold compared to that of the serving point.

4. Apparatus according to claim 2 or 3, wherein the predetermined condition reflects that the non-serving point has a channel quality indicator that is better compared to that of the serving point in more than one consecutive measurement.

5. Apparatus according to any preceding claim, wherein the controller module is arranged to select a subset of those channel quality indicators pertaining to non-serving points which fulfill the predetermined condition.

6. Apparatus according to any preceding claim, wherein the controller module is arranged to de-select a channel quality indicator pertaining to a non-serving point which no longer fulfills the predetermined condition.

7. Apparatus according to any preceding claim, wherein the predetermined condition is configured per sub-band and comprises a determined assumption on a transmission scheme.

8. Apparatus according to any preceding claim, comprising a transceiver module arranged to communicate with another device,

wherein the controller module is arranged to report feedback to the another device, responsive to the another device's coordinated transmission from at least a subset of a plurality of transmission points, each transmission point comprising a set of at least one transmit antenna, wherein each subset with resources allocated thereto defines a respective coordinated transmission scheme.

9. Apparatus according to any preceding claim, wherein the terminal device comprises a user equipment.

10. A method for use in in coordinated multipoint transmission in a terminal device, the method comprising:

calculating, at least on the basis of the measured channel quality, channel quality indications for each of the plurality of transmission points; comparing the channel quality indicators pertaining to the non-serving transmission points with the channel quality indicator pertaining to the serving transmission point;

selecting, for feedback transmission together with the channel quality indicator pertaining to the serving point, at least one channel quality indicator pertaining to a non-serving point which fulfills a predetermined condition; and

11. A method according to claim 10, wherein the predetermined condition reflects that the non-serving point has a channel quality indicator indicating a better channel quality compared to that of the serving point.

12. A method according to claim 11 wherein the predetermined condition reflects that the non-serving point has a channel quality indicator that is better by a first threshold compared to that of the serving point.

13. A method according to claim 11 or 12, wherein the predetermined condition reflects that the non-serving point has a channel quality indicator that is better compared to that of the serving point in more than one consecutive measurement.

14. A method according to any of claims 10 to 13, comprising selecting a subset of those channel quality indicators pertaining to non-serving points which fulfill the predetermined condition.

15. A method according to any of claims 10 to 14, comprising de-selecting a channel quality indicator pertaining to a non-serving point which no longer fulfills the predetermined condition.

16. A method according to any of claims 10 to 15, wherein the predetermined condition is configured per sub-band and comprises a determined assumption on a transmission scheme.

17. A method according to claim 10, comprising:

communicating with another device; and

reporting feedback to the another device, responsive to the another device's coordinated transmission from at least a subset of a plurality of transmission points, each transmission point comprising a set of at least one transmit antenna, wherein each subset with resources allocated thereto defines a respective coordinated transmission scheme.

18. Computer software adapted to perform the method of any of claims 10 to 17.

19. A computer program product comprising a non-transitory computer- readable storage medium having computer readable instructions stored thereon, the computer readable instructions being executable by a computerized device to cause the computerized device to perform a method according to any of claims 10 to 17.