WO2016140603A1

WO2016140603A1 - Network node and method for configuring adaptive feedback channel structures

Info

Publication number: WO2016140603A1
Application number: PCT/SE2015/050242
Authority: WO
Inventors: Sairamesh Nammi
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 2015-03-04
Filing date: 2015-03-04
Publication date: 2016-09-09

Abstract

A method performed by a network node, for configuring Channel State Information, CSI, in a User Equipment (UE). The network node is a Multiple Input Multiple Output (MIMO) network node which serves the UE. The network node receives a signal from the UE, which signal comprises a message comprising channel state information, CSI. The network node determines whether or not a Precoding Matrix Index, PMI, feedback is required from the UE, based on the received signal. The network node further sends an indication to the UE, which indication indicates to the UE to send CSI with PMI to the network node when PMI is determined to be required, and which indication indicates to the UE to send CSI without PMI to the network node when PMI is determined not to be required.

Description

NETWORK NODE AND METHOD FOR CONFIGURING ADAPTIVE FEEDBACK

CHANNEL STRUCTURES

TECHNICAL FIELD

Embodiments herein relate to a network node and a method therein. In particular, it relates to a method for configuring an adaptive feedback channel structure of a UE.

BACKGROUND

Communication devices such as User Equipment (UEs) are enabled to

communicate wirelessly in a cellular communications network or wireless communication system, sometimes also referred to as a cellular radio system or cellular networks. The communication may be performed e.g. between two UEs, between a UE and a regular telephone and/or between a UE and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the cellular communications network. In some embodiments the non-limiting term user equipment (UE) is used. It refers to any type of wireless device that communicates with a radio network node in a cellular or mobile communication system. Examples of UE are target device, device to device (D2D) UE, machine type UE or UE capable of machine to machine (M2M) communication, PDA, iPAD, Tablet, mobile terminals, smart phone, laptop embedded equipped (LEE), laptop mounted equipment (LME), USB dongles etc. UEs may further be referred to as wireless terminals, mobile terminals and/or mobile stations, mobile telephones, cellular telephones, laptops, tablet computers or surf plates with wireless capability, just to mention some further examples. The UEs in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle- mounted mobile devices, enabled to communicate voice and/or data, via the RAN, with another entity, such as another wireless terminal or a server.

The cellular communications network covers a geographical area which is divided into cell areas, wherein each cell area being served by a network node. A cell is the geographical area where radio coverage is provided by the network node.

The network node may further control several transmission points, e.g. having Radio Units (RRUs). A cell can thus comprise one or more network nodes each controlling one or more transmission/reception points. A transmission point, also referred to as a transmission/reception point, is an entity that transmits and/or receives radio signals. The entity has a position in space, e.g. an antenna. A network node is an entity that controls one or more transmission points. The network node may e.g. be a base station such as a Radio Base Station (RBS), eNB, eNodeB, NodeB, B node, or BTS (Base Transceiver Station), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. In some

embodiments the non-limiting term radio network node or simply network node is used. It refers to any type of network node that serves a UE and/or is connected to other network node or network element or any radio node from where the UE receives a signal.

Examples of radio network nodes are Node B, base station (BS), multi-standard radio (MSR) node such as MSR BS, eNode B, network controller, radio network controller (RNC), base station controller (BSC), relay, donor node controlling relay, base transceiver station (BTS), access point (AP), transmission points, transmission nodes, RRU, RRH, nodes in distributed antenna system (DAS) etc.

Further, each network node may support one or several communication

technologies. The network nodes communicate over the air interface operating on radio frequencies with the UEs within range of the network node. In the context of this disclosure, the expression Downlink (DL) is used for the transmission path from the base station to the mobile station. The expression Uplink (UL) is used for the transmission path in the opposite direction i.e. from the UE to the base station.

In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), network nodes, which may be referred to as eNodeBs or even eNBs, may be directly connected to one or more core networks. In LTE the cellular communication network is also referred to as E-UTRAN.

3GPP LTE represents a project within 3GPP, with an aim to improve the UMTS standard. 3GPP LTE radio interface offers high peak data rates, low delays and increase in spectral efficiencies. The LTE ecosystem supports both Frequency Division Duplex (FDD) and Time Division Duplex (TDD). This enables the operators to exploit both paired and unpaired spectrum, since LTE has a flexibility in bandwidth as it supports 6 bandwidths 1.4 MHz, 3MHz, 5 MHz, 10 MHz, 15 MHz and 20 MHz. In the paired spectrum, a first range of frequencies is used DL transmission and a second range of frequencies is used for UL transmission. In an unpaired spectrum, both UL and DL can be carried on the same range of frequencies.

An LTE physical layer is designed to achieve higher data rates, and is facilitated by turbo coding and decoding, and higher order modulations, up to 64-QAM. The modulation and coding are adaptive and depend on channel conditions. Orthogonal Frequency Division Multiple Access (OFDMA) is used for the downlink, while Single Carrier

Frequency Division Multiple Access (SC-FDMA) is used for the uplink. The main advantage of such schemes is that the channel response is flat over a sub-carrier even though the multi-path environment may be frequency selective over the entire bandwidth. This reduces the complexity involved in equalization, as simple single tap frequency domain equalizers can be used at the receiver. OFDMA allows LTE to achieve its goal of higher data rates, reduced latency and improved capacity and coverage, with reduced costs for operators. The LTE physical layer supports Hybrid Automatic Repeat Requests (H-ARQ), power weighting of physical resources, uplink power control, and Multiple Input Multiple Output (MIMO). By using multiple parallel data streams transmission to a single UE, data rate can be increased significantly.

In a multi-path environment, such a multiple access scheme also provides opportunities for performance enhancing scheduling strategies. Frequency Selective Scheduling (FSS) may be used to schedule a user over sub-carriers, or part of the bandwidth, that provides maximum channel gains to that user and avoids regions of low channel gain. Channel response is measured and a scheduler utilizes this information to intelligently assign resources to UEs over parts of the bandwidth that maximize the signal- to-noise ratios and spectral efficiency of the UEs. In other words, the end to end performance of a multi-carrier system like LTE relies significantly on sub-carrier allocation techniques and transmission modes. LTE allows for different opportunistic scheduling techniques.

MIMO is an advanced antenna technique to improve the spectral efficiency and thereby boosting the overall system capacity. The MIMO technique uses a commonly known notation (M x N) to represent MIMO configuration in terms number of transmit (M) and receive antennas (N). The common MIMO configurations used or currently discussed for various technologies are: (2 x 1), (1 x 2), (2 x 2), (4 x 2), (8 x 2) and (2 x 4), (4 x 4), (8 x 4), where according to the commonly known notation (2 x 1) represents 2 transmit antennas and 2 receive antennas. The configurations represented by (2 x 1) and (1 x 2) are special cases of MIMO.

MIMO systems may be used for achieving diversity gain, spatial multiplexing gain and beamforming gain, thereby MIMO systems may significantly increase the data carrying capacity of wireless systems.

Antenna mapping, may in general be described as a mapping from the output of the data modulation to the different antennas ports. The input to the antenna mapping thus consists of the modulation symbols (QPSK, 16QAM, 64QAM, 256QAM etc.)

corresponding to the one or two transport blocks. To be more specific, there is one transport block per TTI except for spatial multiplexing, in which case there may be two transport blocks per TTI. The output of the antenna mapping is a set of symbols for each antenna port. The symbols of each antenna port are subsequently applied to the OFDM modulator - that is, mapped to the basic OFDM time-frequency grid corresponding to that antenna port.

An E-UTRAN cell is defined by certain signals which are broadcasted from a network node such as an eNB. These signals comprise information about a cell which may be used by UEs in order to connect to the network through the cell. The signals comprise reference and synchronization signals which the UE uses to find frame timing and physical cell identification as well as system information which comprises parameters relevant for the whole cell.

A UE needing to connect to the network must thus first detect a suitable cell, as defined in 3GPP TS 36.304 v11.5.0. This is performed by measuring on received reference signals sent by neighboring cells, also referred to as "listening" for a suitable cell. The suitable cell is commonly the cell with best quality of signal. Listening for a suitable cell may comprise searching for reference signals transmitted from the network node in an OFDM subframe. When a suitable cell is found the UE performs random access, according to a system information for the cell. This is done in order to transmit a Radio Resource Control (RRC) connection setup request to the network node. Assuming the random access procedure succeeds and the network node receives the request, the network node will either answer with an RRC connection setup message, which acknowledges the UEs request and tells it to move into RRC connected state, or an RRC connection reject, which tells the UE that it may not connect to the cell. In RRC connected state the parameters necessary for communication between the network node and the UE are known to both entities and a data transfer between the two entities is enabled.

To facilitate handover to other cells, each network node may store cell identities that are supported by the other network nodes in an address database, in order to know how to contact the network node of potential target cells for handover. Each network node serving a cell typically stores in a data base which cells it has neighbor relations to, i.e. which of the cells in the area UEs often perform handover to. The cell's neighbor relations will hereafter be referred to as the cell's Neighbor Cell List (NCL). Figure 1 shows a typical message sequence chart for downlink data transfer in LTE. From the pilot or reference signals, the UE typically computes channel estimates and then computes the parameters needed for Channel-State Information (CSI) reporting. The CSI report may e.g. comprise Channel Quality Indicator (CQI), Precoding Matrix Index (PMI) and rank information (Rl). The rank is the number of simultaneously transmitted streams that the Ml MO channel can support.

The CSI report may be sent to the network node via a feedback channel e.g. via Physical Uplink Control Channel (PUCCH), which is a periodic CSI reporting, or via Physical Uplink Shared Channel (PUSCH), which is an aperiodic CSI reporting. A scheduler of the network node uses this information when choosing parameters for scheduling of this particular UE. The network node sends the scheduling parameters to the UE in the downlink control channel called Physical Downlink Control Channel (PDCCH). After that actual data transfer takes place from the network node to the UE.

Downlink reference signals are predefined signals occupying specific resource elements within the downlink time-frequency grid. The LTE specification includes several types of downlink reference signals that are transmitted in different ways and used for different purposes by the receiving UE:

Cell-Specific Reference Signals (CRS) are transmitted in every downlink subframe and in every resource block in the frequency domain, thus covering the entire cell bandwidth. The cell-specific reference signals may be used by the terminal for channel estimation for coherent demodulation of any downlink physical channel except for Physical Multicast Channel (PMCH) and for Physical Downlink Shared Channel (PDSCH) in the case of transmission modes 7, 8, or 9, as defined in 3GPP TS 36.212 ver. 12.3.0. The different transmission modes are defined in 3GPP 36.213 ver. 12.4.0. The cell- specific reference signals may also be used by the UE to acquire CSI. Finally, UE measurements on cell-specific reference signals may be used as the basis for cell- selection and handover decisions.

DeModulation Reference Signals (DM-RS), which may sometimes also be referred to as UE-specific reference signals, are specifically intended to be used by UEs for channel estimation for PDSCH in the case of transmission modes 7, 8, or 9. The label "UE-specific" relates to the fact that each demodulation reference signal is intended for channel estimation by a single terminal. That specific reference signal is then only transmitted within the resource blocks assigned for PDSCH transmission to that terminal.

CSI reference signals (CSI-RS) are specifically intended to be used by terminals to acquire channel-state information (CSI) in the case when demodulation reference signals are used for channel estimation. CSI-RS have a significantly lower time/frequency density, thus implying less overhead, compared to the cell-specific reference signals.

In LTE, the uplink control channel carries information about HARQ-ACK information corresponding to the downlink data transmission, and channel state information. The channel state information typically consists of Rl, CQI, and PMI. Either PUCCH or PUSCH can be used to carry this information. Note that the PUCCH reporting is periodic and the periodicity of the PUCCH is configured by the higher layers, while the PUSCH reporting is aperiodic.

In LTE, the Physical Downlink Control Channel (PDCCH) carries information about the scheduling grants. Typically this consist of number of MIMO layers scheduled, transport block sizes, modulation for each codeword, parameters related to HARQ, sub band locations. SUMMARY

It is therefore an object of embodiments herein to enhance the performance in a wireless communications network.

According to a first aspect of embodiments herein, the object is achieved by a method performed by a network node for configuring Channel State Information, CSI, in a User Equipment, UE. The network node is a Multiple Input Multiple Output, MIMO, network node which serves the UE. The network node receives a signal from the UE. The signal comprises a message comprising channel state information, CSI. The network node determines whether or not a Precoding Matrix Index, PMI, feedback is required from the UE, based on the received signal. The network node further sends an indication to the UE. The indication indicates to the UE to send CSI with PMI to the network node when PMI is determined to be required, and which indication indicates to the UE to send CSI without PMI to the network node when PMI is determined not to be required. According to a second aspect of embodiments herein, the object is achieved by a method performed by a UE, for configuring Channel State Information, CSI. The UE is served by a MIMO network node and receives Demodulation Reference Symbols, DM-RS, from the network node for channel estimation. The UE sends a signal to the network node. The signal comprises a message comprising channel state information, CSI. The UE then receives an indication from the network node, which indication indicates whether to send CSI with or without PMI to the network node. The UE then sends CSI with PMI to the network node when this is indicated by the indication and sends CSI without PMI to the network node when this is indicated by the indication. According to a third aspect of embodiments herein, the object is achieved by a network node for performing the method of configuring Channel State Information, CSI, in a User Equipment, UE. The network node is configured to serve a UE. The network node is configured to send Demodulation Reference Symbols, DM-RS, to the UE for channel estimation. The network node is further configured to receive, from the UE, a signal comprising a message comprising channel state information, CSI. The network node is further configured to determine, based on the received signal, whether or not a Precoding Matrix Index, PMI, feedback is required from the UE. The network node is further configured to send, to the UE, an indication. The indication indicating to the UE to send CSI with PMI to the network node when PMI is determined to be required, and indicating to send CSI without PMI to the network node when PMI is determined not to be required.

According to a third aspect of embodiments herein, the object is achieved by a UE for performing the method of configuring Channel State Information, CSI, wherein the UE is served by a Ml MO network node. The UE is configured to receive Demodulation

Reference Symbols, DM-RS, from the network node for channel estimation. The UE is further configured to send a message to the network node, comprising channel state information, CSI. The UE is further configured to receive a message from the network node comprising an indication whether to send CSI with or without PMI to the network node. The UE is further configured to send CSI with PMI to the network node when this is indicated by the message, and configured to send CSI without PMI to the network node when this is indicated in the message.

By indicating to the UE not to send PMI during conditions when the PMI information is not used by the network node in order to determine which PMI to use for transmissions, the channel overhead and the transmission power is reduced in the uplink. Thereby the interference in the cell can be reduced, which enhances the performance in the wireless communications network BRIEF DESCRIPTION OF THE DRAWINGS

Examples of embodiments herein are described in more detail with reference to attached drawings in which: Figure 1 is a signaling diagram depicting a common downlink data transfer.

Figure 2 is a schematic block diagram illustrating embodiments of a wireless

communications network.

Figure 3 is a flowchart depicting embodiments of a method in a network node.

Figure 4 is a signaling diagram for a first embodiment.

Figure 5 is a signaling diagram for a second embodiment.

Figure 6 is a flowchart depicting embodiments of a method in a UE.

Figure 7 is a flowchart depicting a MIMO system according to a first embodiment herein.

Figure 8 is a control channel structure according to a second embodiment..

Figure 9 is a flowchart depicting a MIMO system according to a second embodiment. Figure 10 is a control channel structure according to a second embodiment.

Figure 1 1 is a schematic block diagram illustrating embodiments of a network node.

Figure 12 is a signaling diagram for the first embodiment.

Figure 13 is a further signaling diagram for the second embodiment.

Figure 14 is a schematic block diagram illustrating embodiments of a UE.

Figure 15 shows simulation results for a first embodiment.

Figure 16 shows simulation results for a second embodiment.

DETAILED DESCRIPTION Figure 2 depicts an example of a wireless communications network 100

according to a first scenario in which embodiments herein may be implemented. The wireless communications networklOO is a wireless communication network such as an LTE, E-Utran, WCDMA, GSM network, any 3GPP cellular network, Wimax, or any cellular network or system.

The wireless communications network 100 comprises a network node 110. The first network node 110 may be a transmission point such as a radio base station, for example an eNB, an eNodeB, or an Home Node B, an Home eNode B or any other network node capable to serve a wireless terminal such as a user equipment or a machine type communication device in a wireless communications network. The network node 1 10 serves a plurality of cells 130.

A UE 120 operates in the wireless communications network 100. The network node 1 10 may be a transmission point for the UE 120. The UE 120 may e.g. be a wireless terminal, a wireless device, a mobile wireless terminal or a wireless terminal, a mobile phone, a computer such as e.g. a laptop, a Personal Digital Assistant (PDA) or a tablet computer, sometimes referred to as a surf plate, with wireless capability, or any other radio network units capable to communicate over a radio link in a wireless

communications network. Please note the term wireless terminal used in this document also covers other wireless devices such as Machine to machine (M2M) devices.

Example of embodiments of a method in the network node 110 for configuring CSI in a UE 120 will now be described with reference to a flowchart depicted in Figure 3. The network node 110 is a Multiple Input Multiple Output, MIMO, network node which serves the UE 120. The method comprises the following actions, which actions may be taken in any suitable order. Dashed lines of a box in Figure 3 indicate that this action is not mandatory.

Action 301

The network node 1 10 receives a signal from the UE 120. The signal comprises a message comprising channel state information, CSI.

In a further embodiment herein, the message comprised in the signal received from the UE 120 may further comprise an indication of the battery life of the UE 120. Action 302

When the network node 110 has received the signal from the UE 120, the network node 1 10 determines whether or not a Precoding Matrix Index, PMI, feedback is required from the UE, based on the received signal.

In one embodiment herein, the network node 110 determines whether or not the PMI is required by performing a Doppler metric (Dm) estimation based on the received signal. If the Dm for the UE 120 is below a certain threshold, then PMI is determined to be required. If the Dm for the UE 120 is above the threshold, then PMI may be determined not to be required. For UEs 120 with slow Doppler, using the PMI sent by the UE 120 might give the best gains, but for UEs 120 with high Doppler, the PMI might be outdated and may therefore not be useful. Hence if the same PMI is used for high speed channels with DM-RS, the performance will be degraded and closed loop MIMO may perform inferior to that of open loop MIMO. In these cases it is beneficial that the network node chooses its own precoding matrix. Therefore, when the Dm, which is an indication of speed of the UE 120, is low, it is beneficial that the UE 120 sends PMI feedback, also referred to as using Type A configuration and when the DM is high, it is beneficial that the UE 120 does not send PMI feedback, which may also be referred to as using Type B configuration.

There are several methods to compute Dm. For example:

· Direct speed measurement: The network node 110 may compute the direct speed of the UE 120, e.g. by positioning or based on GPS at multiple intervals. The Dm may then be taken as an average of the individual speed measurements.

• Rate of change of uplink channel estimates: The network node 1 10 estimates the uplink channel. The rate of change of the uplink channel gives a measure of Dm. · Rate of change of channel quality information: The Dm may also be computed as

Dm = ACQI/AT,

where CQI is the channel quality information reported by the UE 120 at any given time interval and ACQI represents the rate of change of CQI over K, where K is the number of measurements.

In another embodiment herein, the network node 110 determines whether or not the PMI is required by determining the position of the UE 120 in a cell 130 based on the received signal. When the UE 120 is not within a certain distance from the network node 1 10, then PMI may be determined to be required. When the UE 120 is within a certain distance from the network node 1 10, then PMI may be determined not to be required. One criterion for determining whether to use Type A or Type B information may be to identify the location of the UE 120 in the cell. For example when the UE 120 is closer to the network node 110 it may use Type B reporting, since the UE 120 generally reports a high rank at the cell center. A high rank MIMO system may not require PMI feedback from the UE. Since the signal sent from the network node 1 10 on the channel will be the same as the signal received at the UE 120, the network node 110 may choose a suitable PMI on its own volition. Note that there are several methods for the network node to identify the UE location, for example using GPS measurements, using the reported CQIs Or using the mobility measurement reports, etc. The size of a Neighbor Cell List (NCL) may also be an indication whether the UE 120 is at the cell center or not. The network node 110 may further determine whether or not the PMI is required based on the power headroom of the UE 120. PMI may be determined to be required when the power headroom is below a certain threshold, and may be determined not to be required when the power headroom is above a certain threshold.

As shown in figures 15 and 16, which will be described below, by using Type B information, transmit power of the UE 120 may be reduced. Hence one criterion for determining whether to use Type A or Type B information may be the UE 120 power headroom. For example, if the power headroom is below certain threshold, then the network node 1 10 may determine that PMI is not required, i.e. decide to use Type B information. The power headroom information may be explicitly or implicitly informed to the network node 110 from the UE 120.

In further embodiments herein, the network node 110 determines whether or not the PMI is required based on the battery life of the UE 120. When the battery life is above a certain threshold PMI may be determined to be required, and when the battery life is below a certain threshold PMI may be determined not to be required. Computing PMI may consume more UE 120 battery life due to increased processing as well as increase in transmit power. Therefore if the UE 120 battery life or available UE 120 battery power is below a threshold then the network node 110 may determine that PMI is not required, i.e. decide to use Type B reporting. The UE 120 may report its battery life status to the network node 1 10 either explicitly or implicitly.

In further embodiments herein, the network node 110 determines whether or not the PMI is required when the network node 1 10 sends Demodulation Reference Symbols, DM-RS to the UE 120. When sending DM-RS, there is no need to inform the selected precoding matrix there by saving the number of bits in the downlink control channel.

In further embodiments herein, the network node 110 determines whether or not the PMI is required based on a combination of two or more of the criteria described above.

Action 303

When the network node 110 has determined whether or not the PMI is required, the network node sends an indication to the UE 120. The indication indicates to the UE 120 to send CSI with PMI to the network node 1 10 when PMI is determined to be required, which may also be referred to as sending over a Type A feedback channel. When PMI is determined not to be required the indication indicates to the UE 120 to send CSI without PMI to the network node 1 10, which may also be referred to as sending over a Type B feedback channel.

In further embodiments herein, the indicating is performed using physical layer signaling. When using physical layer signaling, the network node 110 sends the signaling about the change of feedback channel structure. Embodiments herein are useful as they reduce the latency compared to the higher layer signaling. Figure 4 shows one method to send the signaling from network node 110. Assume that initially the UE sends the feedback channel using Type A feedback configuration. After few a TTIs, the eNode B checks the criteria. If the criterion is a pass then it sends the signaling through downlink control channel to change the feedback channel configuration. Note that this message can be sent using a separate field in the downlink control channel or by using an unused combination in the downlink control channel.

In further embodiments herein, the indicating is performed using higher layer signaling. Figure 5 shows an example of message sequence chart with higher layer signaling. Assume that the network node 110 is receiving the Type A feedback channel. The network node 1 10 periodically checks the selection criteria and if this is pass it sends a higher order signaling to the UE 120 to switch to the Type B channel structure.

Example of embodiments of a method performed by a UE 120 for configuring CSI in a UE 120 will now be described with reference to a flowchart depicted in Figure 6. The UE 120 is served by a Ml MO network node 1 10 and receives Demodulation Reference Symbols, DM-RS, from the network node 110 for channel estimation. The method comprises the following actions, which actions may be taken in any suitable order.

Dashed lines of a box in Figure 4 indicate that this action is not mandatory. Action 401

The UE 120 sends a signal to the network node 1 10, which signal comprises a message comprising channel state information, CSI.

In further embodiments herein, the message comprised in the signal sent from the UE 120 may further comprise an indication of the battery life of the UE 120. Action 402

After having sent the signal to the network node 110, the UE 120 receives an indication from the network node 110, which indication indicates whether the UE 120 should send CSI with or without PMI to the network node 110.

Action 403

When the indication indicates that the UE 120 should send CSI with PMI, the UE 120 sends CSI with PMI to the network node 1 10. When the indication indicates that the UE 120 should send CSI without PMI, the UE 120 sends CSI without PMI to the network node 1 10.

Figure 7 shows a conceptual diagram of a MIMO system with demodulation reference signal when the network node 1 10 has indicated to the UE 120 to send CSI with PMI. According to embodiments herein, such a system may be referred to as a Type A MIMO system. The transmitter (Tx) of the network node 1 10, transmits common reference signals namely either CRS or CSI-RS for channel sounding. A receiver (Rx) of the UE 120 estimates channel quality, which typically may be Signal-to-lnterference Ratio (SINR), from channel sounding, and computes the PMI, Rl and CQI for the next downlink transmission. This information is referred to as CSI. The UE 120 conveys this information through the feedback channel. As shown in Figure 8, the feedback channel carries information about HARQ-ACK and CSI which comprises Rl, CQI, and PMI. Note that it may comprise other parameters such as e.g. preferred sub bands. This feedback channel may be referred to as a Type A feedback channel.

For downlink data transmission, the network node 120 uses this information and chooses the precoding matrix as suggested by the UE, CQI and the transport block size etc. Finally, both the reference signal (DM-RS) and the data are multiplied by the precoding matrix selected by the network node and is transmitted. The UE receiver (Rx) estimates the effective channel, i.e. the channel multiplied by the precoding matrix, and demodulates the data.

Figure 9 shows a further conceptual diagram of a MIMO system with demodulation reference signal when the network node 1 10 has indicated to the UE 120 to send CSI without PMI. According to embodiments herein, such a system may be referred to as a Type B system. The transmitter (Tx) of the network node 110, transmits common reference signals namely either CRS or CSI-RS for channel sounding. The receiver (Rx) of the UE 120 estimates channel quality, which typically may be SINR, from channel sounding, and computes the preferred Rl and CQI for the next downlink transmission. The UE 120 may compute PMI or may not compute PMI. Similar to Type A MIMO system, the UE 120 will convey CSI to the transmitter (Tx) of the network node 110, but it will exclude 5 PMI as shown in Figure 10. Such a feedback channel may be referred to as a Type B feedback channel.

For downlink data transmission, the network node 1 10 uses this information for scheduling and chooses the precoding matrix on its own. This may for example be done, based on uplink measurements or angle of arrival. Finally, both the reference signal DM- 10 RS and the data are multiplied by the precoding matrix selected by the network node 110 and is transmitted to the UE 120. The UE 120 estimates the effective channel, i.e. the channel multiplied by the precoding matrix, and demodulates the data.

To perform the method actions for configuring CSI in a UE 120 described above in

15 relation to Figure 3, the network node 1 10 may comprise the following arrangement

depicted in Figure 11. As mentioned above the network node 1 10 is configured to serve one or more UEs 120 and is configured to send Demodulation Reference Symbols, DM- RS, to the UE 120 for channel estimation.

The network node 1 10 comprises a radio circuitry 501 to communicate with UEs

20 120, a communication circuitry 502 to communicate with other network nodes and a processing unit 503.

The network node 1 10 is configured to, e.g. by means of a receiving module 504 being configured to, receive a signal comprising a message comprising CSI from the UE 120. The network node 1 10 is further configured to, or comprises a determining module

25 505 configured to, determine whether or not a PMI feedback is required from the UE, based on the received signal. The network node 1 10 is further configured to, or comprises a sending module 506 configured to, send an indication to the UE 120, indicating to the UE 120 to send CSI with PMI to the network node 1 10 when PMI is determined to be required, and to send an indication indicating to send CSI without PMI to the network

30 node 110 when PMI is determined not to be required.

The network node 110 may further be configured to, e.g. by means of the determining module 505 further being configured to, determine whether or not the PMI is required by performing Doppler metric, Dm, estimation based on the received signal. The 35 network node is configured to determine that PMI is required when the Dm for the UE 120 is below a certain threshold, and to determine that PMI is not required when the Dm for the UE 120 is above the threshold. The determining module 505 may be comprised in the processing unit 503. In a further embodiment herein the network node 1 10 may be configured to, e.g. by means of the determining module 505 further being configured to, determine whether or not the PMI is required based on the position of the UE 120 in the cell 130. The network node 1 10 may be configured to determine that PMI is required when the UE 120 is not within a certain distance from the network node 1 10, and to determine PMI not to be required when the UE 120 is within a certain distance from the network node 110. The determining module 505 may be comprised in the processing unit 503.

The network node 1 10 may further be configured to, e.g. by means of the determining module 505 further being configured to, determine whether or not the PMI is required based on the power headroom of the UE 120. The network node 1 10 may be configured to determine that PMI is required when the power headroom is below a certain threshold, and to determine PMI not to be required when the power headroom is above a certain threshold. The determining module 505 may be comprised in the processing unit 503.

In a further embodiment herein, the network node 1 10 may be configured to, e.g. by means of the determining module 505 further being configured to, determine whether or not the PMI is required based on the battery life of the UE 120. The network node 110 may be configured to determine that PMI is required when the battery life is above a certain threshold, and to determine PMI not to be required when the battery life is below a certain threshold. The determining module 505 may be comprised in the processing unit 503.

In further embodiments herein, the network node 110 may be configured to, e.g. by means of the sending module 506 further being configured to, send the indicating using physical layer signaling. The sending module 506 may be comprised in the radio circuit 501. In this method, the network node sends the indication about the change of feedback channel structure. This method has the advantage that it reduces the latency compared to the higher layer signaling. Figure 12 shows another example of how the physical layer signaling, also depicted in figure 4, can be used to change the configuration of the feedback channel structure. Assume that initially the UE 120 sends the feedback channel using Type A feedback configuration as configured by the RNC. Say after few TTIs, the network node 110 checks the criteria. If the criterion is a pass then the network node 110 sends the signaling through HS-SCCH order to change the feedback channel configuration.

In further embodiments herein, the network node 1 10 may be configured to, e.g. by means of the sending module 506 further being configured to, send the indicating using higher layer signaling.

Figure 13 shows another example of higher layer signaling, also depicted in figure 5, using two nodes for example in HSDPA according to further embodiments herein. A network node, such as e.g. a RNC, configures the UE 120 with one type of reporting after N TTI by sending a message comprising a Radio Resource Control (RRC) configuration towards the UE 120. The message comprising the configuration is forwarded to the UE 120 by the network node 110. The network node 110 checks the criteria for switching from the Type A to the Type B channel structure and if this is a pass the network node 1 10 informs the RNC to change the feedback channel configuration. The RNC sends the change of feedback channel structure message using a RRC (re) configuration message towards the UE 120 via the network node 110.

The embodiments herein for managing configuring Channel State Information (CSI) in a User Equipment (UE) 120 may be implemented through one or more processors, such as the processing unit 503 in the network node 110 depicted in Figure 11 , together with computer program code for performing the functions and actions of the embodiments herein. The program code mentioned above may also be provided as a computer program product, for instance in the form of a data carrier carrying computer program code for performing the embodiments herein when being loaded into the in the network node 110. One such carrier may be in the form of a CD ROM disc. It is however feasible with other data carriers such as a memory stick. The computer program code may furthermore be provided as pure program code on a server and downloaded to the network node 110.

The network node 110 may further comprise a memory 506 comprising one or more memory units. The memory 506 is arranged to be used to store obtained information, measurements, data, configurations, schedulings, and applications to perform the methods herein when being executed in the network node 110.

To perform the method actions for configuring Channel State Information (CSI) 5 described above in relation to Figure 6, the UE 120 may comprise the following

arrangement depicted in Figure 14. As mentioned above the UE 120 is served by a Ml MO network node 1 10 and is configured to receive Demodulation Reference Symbols, DM-RS, from the network node 110 for channel estimation.

The network node 1 10 comprises a radio circuitry 601 to communicate with UEs 10 120 and a processing unit 602.

The UE 120 is configured to, e.g. by means of a sending module 603 being configured to, send a message to the network node 110, which message comprises channel state information (CSI). The UE 120 is further configured to, e.g. by means of a receiving module 604 being configured to, receive a message from the network node 15 1 10 comprising an indication whether to send CSI with or without PMI to the network node 110. The UE 120 is further configured to, e.g. by means of the sending module 604 being configured to, send CSI with PMI to the network node 1 10 when this is indicated by the message received from the network node 1 10, and to send CSI without PMI to the network node 110 when this is indicated in the message received from the network node 20 110.

The embodiments herein for managing configuring Channel State Information (CSI) in a UE 120 may be implemented through one or more processors, such as the

processing unit 602 in the UE 120 depicted in Figure 12, together with computer

25 program code for performing the functions and actions of the embodiments herein. The program code mentioned above may also be provided as a computer program product, for instance in the form of a data carrier carrying computer program code for performing the embodiments herein when being loaded into the in the UE 120. One such carrier may be in the form of a CD ROM disc. It is however feasible with other data carriers such as a

30 memory stick. The computer program code may furthermore be provided as pure program code on a server and downloaded to the UE 120.

The UE 120 may further comprise a memory 605 comprising one or more memory units. The memory 605 is arranged to be used to store obtained information,

35 measurements, data, configurations, schedulings, and applications to perform the methods herein when being executed in the UE 120. Those skilled in the art will also appreciate that the receiving module 604 and sending module 603 described above may refer to a combination of analog and digital circuits, and/or one or more processors configured with software and/or firmware, e.g. stored in the memory 605, that when executed by the one or more processors such as the processing unit 602 as described above. One or more of these processors, as well as the other digital hardware, may be included in a single Application-Specific Integrated Circuitry (ASIC), or several processors and various digital hardware may be distributed among several separate components, whether individually packaged or assembled into a system-on-a-chip (SoC).

Figure 15 shows a Bit Error Rate (BER) plot for a 4 Tx antenna system with no rank restriction. The Type A feedback channel needs to code 11 information bits, i.e. 4 bits for CQI, 3 bits for differential CQI and 4 bits for PMI. These 11 bits are encoded using (20, 1 1) block code as defined in TS 36.212. ver.12.3.0 For Type B feedback channel the feedback channel only needs to decode 7 information bits, i.e. 4 bits for CQI and the 3 bits for differential CQI. These 7 bits are encoded using a (20,7) block encoder as defined in TS 36.212, ver.12.3.0. It may be observed from the figure 16 that the Type B gives a gain around 2 dB at BER of 0.01. Note that the figure shows the BER comparison for Additive White Gaussian Noise (AWGN) channels. The gain will be more in fading channels.

Figure 16 shows the BER plot for a 4 Tx antenna system with rank restriction equal to 1. In this case the Type A feedback channel has 8 information bits to encode, i.e. 4 bits for CQI and 4 bits for PMI. These 8 bits are encoded using a (20, 8) block encoder as defined in TS 36.212 ver.12.3.0. For Type B feedback channel the input bits are 4. These 4 bits are encoded using (20,4) block code as defined in TS 36.212 ver. 12.3.0.

It can be observed from the above figure that the Type B gives a gain around 2 dB at BER of 0.01. Note that the figure shows the BER comparison for AWGN channels. The gain will be more in fading channels.

When using the word "comprise" or "comprising" it shall be interpreted as non- limiting, i.e. meaning "consist at least of".

The embodiments herein are not limited to the above described preferred embodiments. Various alternatives, modifications and equivalents may be used.

Therefore, the above embodiments should not be taken as limiting the scope of the invention, which is defined by the appending claims. The embodiments herein are described in particular for LTE/LTE-A. The

embodiments may however also be applicable to any RAT or multi-RAT system where the UE operates using multiple carriers e.g. LTE FDD/TDD, WCMDA/HSPA, GSM/GERAN, Wi Fi, WLAN, WiMax, CDMA2000 etc.

The embodiments may be applicable to single carrier as well as to multicarrier (MC) or carrier aggregation (CA) operation of the UE. CA may also be referred to as "multi- carrier system", "multi-cell operation", "multi-carrier operation", "multi-carrier" transmission and/or reception.

Note that the solutions outlined equally applies for Multi Radio Access Bearers (RAB) on some carriers. In Multi-RABs data and speech may be simultaneously scheduled.

Claims

1. A method performed by a network node (1 10), for configuring Channel State

Information, CSI, in a User Equipment, UE, (120) wherein the network node (1 10) is a Multiple Input Multiple Output, MIMO, network node which serves the UE (120), the method comprising:

- receiving (301) a signal from the UE (120), which signal comprises a

message comprising channel state information, CSI,

- determining (302) whether or not a Precoding Matrix Index, PMI, feedback is required from the UE, based on the received signal,

- sending (303) an indication to the UE (120), which indication indicates to the UE (120) to send CSI with PMI to the network node (1 10) when PMI is determined to be required, and which indication indicates to the UE (120) to send CSI without PMI to the network node (1 10) when PMI is determined not to be required.

2. The method according to claim 1 , wherein determining whether or not the PMI is required comprises performing a Doppler metric, Dm, estimation based on the received signal, and wherein PMI is determined to be required when the Dm for the UE (120) is below a threshold, and PMI is determined not to be required when the Dm for the UE (120) is above the threshold.

3. The method according to claim 1 or 2, wherein determining whether or not the PMI is required comprises determining the position of the UE (120) in a cell (130) based on the received signal, and wherein PMI is determined to be required when the UE (120) is not within a certain distance from the network node (1 10), and PMI is determined not to be required when the UE (120) is within a certain distance from the network node (1 10).

4. The method according to any of the previous claims, wherein determining whether or not the PMI is required is based on the power headroom of the UE (120), and wherein PMI is determined to be required when the power headroom is below a certain threshold, and PMI is determined not to be required when the power headroom is above a certain threshold.

5. The method according to any of the previous claims, wherein determining whether or not the PMI is required is based on the battery life of the UE (120), and wherein PMI is determined to be required when the battery life is above a certain threshold, and PMI is determined not to be required when the battery life is below a certain threshold.

6. The method according to claim 5, wherein the message comprised in the signal received from the UE (120) further comprises an indication of the battery life of the UE (120).

7. The method according to any of the previous claims, wherein determining is

performed when the network node (1 10) sends Demodulation Reference Symbols, DM-RS to the UE (120).

8. The method according to any of the previous claims, wherein the indicating is

performed using physical layer signaling.

9. The method according to any of the claims 1 to 5, wherein the indicating is

performed using higher layer signaling.

10. A method performed by a UE (120), for configuring Channel State Information, CSI, wherein the UE (120) is served by a MIMO network node (1 10), and wherein the UE (120) receives Demodulation Reference Symbols, DM-RS, from the network node (1 10) for channel estimation, the method comprising:

- sending (401) a signal to the network node (110), which signal comprises a message comprising channel state information, CSI,

- receiving (402) an indication from the network node (1 10), which indication indicates whether to send CSI with or without PMI to the network node (1 10), and

- sending (403) CSI with PMI to the network node (1 10) when this is indicated by the indication, and sending CSI without PMI to the network node (1 10) when this is indicated by the indication.

1 1. The method according to claim 10, wherein the message comprised in the signal sent from the UE (120) further comprises an indication of the battery life of the UE (120).

12. The method according to any of the claims 10-11 , wherein the message comprised in the signal sent from the UE (120) further comprises an indication of the battery life of the UE (120).

13. A network node (110) for performing the method of configuring Channel State Information, CSI, in a User Equipment, UE, wherein the network node (1 10) is configured to serve a UE (120) and wherein the network node (1 10) is configured to send Demodulation Reference Symbols, DM-RS, to the UE (120) for channel estimation, the network node (110) further being configured to:

- receive (301), from the UE (120), a signal comprising a message comprising channel state information, CSI,

- determine (302), based on the received signal, whether or not a Precoding

Matrix Index, PMI, feedback is required from the UE,

- send (303), to the UE (120), an indication indicating to the UE (120) to send CSI with PMI to the network node (110) when PMI is determined to be required, and indicating to send CSI without PMI to the network node (110) when PMI is determined not to be required.

14. The network node (1 10) according to claim 13, the network node further being configured to determine whether or not the PMI is required by performing Doppler metric, Dm, estimation based on the received signal, wherein the network node (1 10) is configured to determine that PMI is required when the Dm for the UE (120) is below a threshold, and to determine PMI not to be required when the Dm for the UE (120) is above the threshold.

15. The network node (110) according to claim 13 or 14, the network node (1 10)

further being configured to determine whether or not the PMI is required based on the position of the UE (120) in the cell (130), wherein the network node (1 10) is configured to determine that PMI is required when the UE (120) is not within a certain distance from the network node (110), and to determine PMI not to be required when the UE (120) is within a certain distance from the network node (1 10).

16. The network node (1 10) according to any of the claims 13 to 15, the network node (1 10) further being configured to determine whether or not the PMI is required based on the power headroom of the UE (120), wherein the network node (1 10) is configured to determine that PMI is required when the power headroom is below a certain threshold, and to determine PMI not to be required when the power headroom is above a certain threshold.

17. The network node (1 10) according to any of the claims 13 to 16, the network node (1 10) further being configured to determine whether or not the PMI is required based on the battery life of the UE (120), wherein the network node (1 10) is configured to determine that PMI is required when the battery life is above a certain threshold, and to determine PMI not to be required when the battery life is below a certain threshold.

18. The network node (1 10) according to any of the claims 13 to 17, the network node (1 10) further being configured to perform the indicating using physical layer signaling.

19. The network node (1 10) according to any of the claims 13 to 18, the network node (1 10) further being configured to perform the indicating using physical layer signaling.

20. A UE (120) for performing the method of configuring Channel State Information, CSI, wherein the UE (120) is served by a MIMO network node (1 10), and wherein the UE (120) is configured to receive Demodulation Reference Symbols, DM-RS, from the network node (1 10) for channel estimation, the UE (120) further being configured to:

- send (401) a message to the network node (110), comprising channel state information, CSI,

- receive (402) a message from the network node (1 10) comprising an

indication whether to send CSI with or without PMI to the network node (1 10), and

- send (403) CSI with PMI to the network node (110) when this is indicated by the message, and

- send (403) CSI without PMI to the network node (1 10) when this is indicated in the message.