NODE

TECHNICAL FIELD

The invention relates generally to a method and network node for measuring an uplink channel from a wireless device comprising a plurality of antennas.

BACKGROUND

In mobile communication networks, performance can generally be improved if the characteristics of the transmission are adapted to the characteristics of the radio channel. Even though there are techniques which adapt without much prior information about the channel, e.g. Incremental Redundancy (IR) used together with some Automatic Repeat Request (ARQ) scheme etc., the performance can still be improved, if the transmission is better adapted to the channel characteristics.

Often the term link adaptation is used when referring to the selection of modulation and coding scheme to be used for a certain transmission in an attempt to adapt the characteristics of the transmission to the characteristics of the channel.

In Long Term Evolution (LTE) networks, there are different uplink

transmission modes defined. Depending on the uplink transmission mode being used, Closed Loop Spatial Multiplexing (CLSM) might be supported as a transmission scheme for a Physical Uplink Shared Channel (PUSCH). The term Closed Loop (CL) implies the need of Channel State Information (CSI) for the transmission scheme to operate efficiently.

In uplink (UL) communication from the wireless device (called User

Equipment, UE, in LTE)) to the network, CSI can be obtained by measuring the received channel directly, as opposed to the downlink where the corresponding information must rather be obtained via Channel Quality Indicators (CQI), determined in a wireless device, and sent to the eNodeB (eNB). When measuring the channel directly, known reference signals are typically used for measurements.

The CSI obtained by measuring in the UL may contain several different aspects of the channel. When using CLSM as transmission scheme, also information about a suitable precoding matrix (PM) and/ or suitable rank for the transmission is part of what is estimated for the channel in order to achieve efficient transmission using this transmission scheme.

One way to achieve the estimation of the UL channel is by using Sounding Reference Symbols (SRS). What differentiates the SRS from most alternatives is that it can be configured to enable measurement of channel aspects not present, or not possible to extract, from let's say the PUSCH or other reference signals. In the case of LTE and Multiple Input Multiple Output (MIMO) configurations with CLSM for PUSCH, SRS provides a possibility to measure the channel on a per antenna basis, no matter what transmission scheme or other transmission parameters are used for the transmission of the PUSCH from that wireless device.

Using SRS, for the purpose of measuring the channel on a per antenna basis, in conjunction with CLSM in the UL may seem ideal, given the discussion above. There are, however, disadvantages with this solution. Firstly, activating SRS in a cell comes with a cost, which then needs to be regained by the features utilizing SRS. If not, SRS could preferably remain turned off all together. Secondly, the SRS resources available are limited and to be shared by all wireless devices in need of them, but each wireless device needs separate SRS resources. For a feature like CLSM requiring quite intensive SRS, including SRS for each antenna to operate efficiently, the resource consumption in terms of SRS resources quickly becomes extensive. In case of resource shortage, the only way to obtain more resources for SRS is to reduce the amount of PUSCH resources even further, which obviously comes with an increased cost for SRS on cell level. This may result in reduced cell throughput, whereby all wireless devices within the cell pay the price for SRS.

As the support for CLSM from the wireless device side is expected to grow, just like the amount of networks with multiple antennas, the use of MIMO based transmission schemes, using different types of channel aware precoding or spatial multiplexing, is expected to grow. Along with that also the resource shortage and/ or overhead from SRS is expected to be more of a problem.

SUMMARY

An object is to eliminate or at least alleviate the problems of the prior art discussed above.

According to a first aspect, it is presented a method for measuring an uplink channel from a wireless device comprising a plurality of antennas. The method is performed in a network node and comprises the steps of:

determining a need of measuring the uplink channel from the wireless device to the network node; selecting a precoding matrix in dependence of the need of measuring of the uplink channel; transmitting a command to the wireless device to use the precoding matrix for uplink transmission; and measuring the uplink channel. It has been found that selecting an appropriate precoding matrix for measuring improves the measuring. Moreover, the measuring can be performed on real data (not reference symbols) whereby impacts on throughput are small or even negligible, e.g. compared to using SRS.

The step of selecting a precoding matrix may comprise, when, in the step of determining, it has been determined that there is a need of measuring the uplink channel, selecting a precoding matrix of a larger order than would have been the case when there is not a need of measuring the uplink channel. Using a higher order precoding matrix provides a better measurement of the uplink channel than a lower order precoding matrix, since the antennas of the wireless device are used better, e.g. being better distinguishable and separated. The step of measuring may comprise measuring the uplink channel using sounding reference symbols. This can be a complement to the precoding matrix selection explained above and gives a more complete measurement.

The method may further comprise the steps of: determining a rank for the uplink channel in dependence of a result from the step of measuring the uplink channel, the rank indicating a number of independent data streams to use on the uplink channel; and transmitting a command to the wireless device to use the rank for uplink transmission. In other words, the measuring can result in a switch to a higher rank, which can give significant throughput improvements.

The step of determining a rank may also comprise considering a result from the step of measuring the uplink channel using sounding reference symbols.

The method may further comprise the steps of: determining a new precoding matrix for the uplink channel in dependence of a result from the step of measuring the uplink channel; and transmitting a command to the wireless device to use the new precoding matrix for uplink transmission. In other words, one result of the measuring can be to switch to a precoding matrix which is better suited to the current radio characteristics.

The step of determining a new precoding matrix may also comprise considering a result from the step of measuring the uplink channel using sounding reference symbols.

The step of determining a need of measuring an uplink channel may comprise determining that channel measurements in the network node for the uplink channel is older than a threshold age. In other words, when measurements are old enough, it is determined that the channel

measurements described above are needed.

The step of determining a need of measuring an uplink channel may comprise determining that a measurement of the uplink channel using sounding reference symbols is not available within a threshold time frame. In this way, the uplink channel is measured using the selection of precoding matrix when there will not be any SRS available for a long time.

The method may further comprise the step, prior to the step of determining a need to measure, of: obtaining a signal measurement. In such a case, the step of determining a need of measuring an uplink channel, the determination is based on the signal measurement. For example, SINR (signal to interference plus noise ratio) can be used in the evaluation of the need to measure.

According to a second aspect, it is presented a network node arranged to measure an uplink channel from a wireless device comprising a plurality of antennas. The network node comprises: a processor; and a memory storing instructions that, when executed by the processor, causes the network node to: determine a need of measuring the uplink channel from the wireless device to the network node; select a precoding matrix in dependence of the need of measuring of the uplink channel; transmit a command to the wireless device to use the precoding matrix for uplink transmission; and measure the uplink channel.

The instructions to select a precoding matrix may comprise instructions to, when the instructions to determine determines that there is a need of measuring the uplink channel, select a precoding matrix of a larger order than would have been the case when there is not a need of measuring the uplink channel.

The instructions to measure may comprise instructions to measure the uplink channel using sounding reference symbols.

The network node may further comprise instructions to: determine a rank for the uplink channel in dependence of a result from the instructions to measure the uplink channel, the rank indicating a number of independent data streams to use on the uplink channel; and transmit a command to the wireless device to use the rank for uplink transmission. The instructions to determine a rank may also comprise instructions to consider a result from the instructions to measure the uplink channel using sounding reference symbols.

The network node may further comprise instructions to: determine a new precoding matrix for the uplink channel in dependence of a result of the instructions to measure the uplink channel; and transmit a command to the wireless device to use the new precoding matrix for uplink transmission.

The instructions to determine a rank may also comprise considering a result from the instructions to measure the uplink channel using sounding reference symbols.

The instructions to determine a need of measuring an uplink channel may comprise instructions to determine that channel measurements in the network node for the uplink channel is older than a threshold age.

The instructions to determine a need of measuring an uplink channel may comprise instructions to determine that a measurement of the uplink channel using sounding reference symbols is not available within a threshold time frame.

The network node may further comprise instructions to: obtain a signal measurement. In such a case, the instructions to determine a need of measuring an uplink channel comprises instructions to determine a need of measuring based on the signal measurement.

The word 'plurality' in the description and claims is to be interpreted as meaning 'more than one'.

Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to "a/an/the element, apparatus, component, means, step, etc." are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, step, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is now described, by way of example, with reference to the accompanying drawings, in which:

Fig l is a schematic diagram illustrating an environment where embodiments presented herein can be applied;

Fig 2 is a schematic diagram illustrating how MIMO is used in an

embodiment of the wireless device of Fig l; Fig 3 is a schematic graph illustrating throughput for various radio conditions in uplink communication shown of Fig l;

Figs 4A-B are flow charts illustrating methods for measuring an uplink channel from a wireless device of Figs 1 and 2;

Fig 5 is a schematic diagram showing some components of the network node of Fig 1;

Fig 6 is a schematic diagram showing functional modules of the network node of Figs 1, 2 and 5; and

Fig 7 is a schematic graph illustrating SRS usage in the system of Fig 1, as used by the methods illustrated in Figs 4A-B. DETAILED DESCRIPTION

The invention will now be described more fully hereinafter with reference to the accompanying drawings, in which certain embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout the description.

Fig l is a schematic diagram illustrating an environment where embodiments presented herein can be applied. A mobile communications network 9 comprises a core network 3 and a radio access network comprising one or more network nodes 1 and optionally one or more radio network controllers (not shown). The network nodes 1, are here in the form of evolved Node Bs also known as eNBs but could also be in the form of Node Bs (NodeBs/NBs) and/or BTSs (Base Transceiver Stations) and/or BSSs (Base Station

Subsystems), etc. The network nodes 1 provide radio connectivity to a plurality of wireless devices 2. The term wireless device is also known as user equipment (UE), mobile terminal, user terminal, user agent, etc.

Each one of the network nodes 1 provides radio coverage in one or more respective radio cells. Uplink (UL) communication 6, from the wireless device 2 to the network node 1, and downlink (DL) communication 4, from the network node 1 to the wireless device 2 occur over a wireless radio interface. The radio conditions of the wireless radio interface (both UL 6 and DL 4) vary over time and also depends on the position of the wireless device, due to effects such as fading, multipath propagation, interference, etc. As is explained in more detail below, by adapting transmissions to current radio conditions, performance can be improved, e.g. by higher throughput.

The core network 3 provides access to central functions in the mobile communication network and connectivity to other communication networks 8. The mobile communications network 9 may e.g. comply with any one or a combination of LTE (Long Term Evolution), UMTS utilising W-CDMA (Wideband Code Division Multiplex), CDMA2000 (Code Division Multiple Access 2000), or any other current or future wireless network, as long as the principles described hereinafter are applicable. Fig 2 is a schematic diagram illustrating how MIMO is used in an

embodiment of the wireless device of Fig l. On the transmission side, the wireless device 2 comprises j antennas 28a-j each capable of transmitting a respective uplink signal 4a-j. The value of j is not important for the concepts presented herein as long as j is greater than one. On the input side, data 24 is provided which is to be transmitted over the uplink signals 4a-j. The data is split to i independent data streams (also known as layers) 25a-i in a layer mapping module 20, where i is grater than or equal to one. Rank is defined as the number of layers, i.e. the value of i, and can vary over time. A serving network node sends a command to the wireless device 2 defining what rank to be used for uplink transmissions. This command can be part of downlink control information.

Using a precoding module 22, the i layers 25a-i are then mapped to j transmission streams 26a-j, where j is greater or equal to i. In other words, it is not possible to use a rank which is greater than the number of antennas. The precoding module 22 uses a i x j precoding matrix to map the i layers to the j transmission streams. The precoding matrix is one of a number of predefined precoding matrices in a so called codebook. Since the precoding matrix is rank dependent, there is a codebook for each rank. For example, in LTE and 2x2 MIMO with CLSM for PUSCH, there are 7 different precoding matrices defined, of which one supports of dual layer transmissions.

The serving network node sends a command to the wireless device indicating which one of the predefined precoding matrices to be used. This command can be part of downlink control information. Fig 3 is a schematic graph illustrating throughput for various radio conditions in uplink communication shown of Fig 1. The horizontal axis represents uplink signal to interference plus noise ratio (SINR) and the vertical axis represents average throughput in the uplink. There are four lines

representing various uplink configurations. A first line 10 represents rank l operation without using precoding. In this situation, there is thus one layer and no precoding, which results in only one uplink transmission stream, i.e. only one antenna is used. No precoding is here to be interpreted as using an identity matrix. A second line n represents rank l operation with precoding. In this situation, there is still one layer. However, precoding is used to spread the single layer over two transmission streams, and thus two antennas, which results in better throughput compared to the first line representing rank ι operation without precoding. A third line 12 represents rank 2 operation without precoding. This means that two independent data streams (two layers) are fed to two antennas for uplink transmission without any precoding. Compared to the rank 1 operations, this is especially beneficial for higher SINR values. This is due to higher SINR values provides a better environment for multiple independent transmission streams to be received correctly at the receiver.

A fourth line 13 represents rank 2 operation with precoding and which adapts precoder and rank in dependence of uplink channel measurements. This gives the best throughput for a given SINR value (greater than about 25dB in this example). One conclusion that can be drawn from Fig 3, is that for high SINR values, a significant gain in throughput can be achieved by switching from rank one operation to rank two operation. Another conclusion that can be drawn is that the use of a good precoder can make a considerable difference in throughput. The choice of precoder matrix and rank would thus benefit from being dependent on the radio conditions for the uplink channel, whereby channel measurements are crucial for achieving a good throughput given the circumstances. Fig 4A is a flow chart illustrating a method for measuring an uplink channel from a wireless device 2 of Figs 1 and 2 according to one embodiment. The method is performed in the network node 1 of Fig 1. The method is executed for a particular wireless device and can be executed independently, in parallel, for a plurality of wireless devices.

In a determine need to measure step 30, it is determined whether there is a need to measure the uplink channel from the wireless device 2 to the network node 1. This step can be considered to be a trigger to perform the rest of the steps of the method. In one embodiment, this involves determining that channel measurements in the network node 1 for the uplink channel are older than a threshold age. In other words, the previous channel measurements being old can trigger a need to measure.

In one embodiment, this involves determining that a measurement of the uplink channel using SRS is not available within a threshold time frame. In other words, if there has not been SRS measurement recently and a new SRS is not due for a while, this can trigger a need to perform a measurement. Since the method is performed in the network node, it is able to look into the future to evaluate when the next SRS measurement for the wireless device is due. This is explained in more detail with reference to Fig 7 below.

In a select PM step 32, a precoding matrix is selected in dependence of the need of measuring of the uplink channel.

Optionally, when it has been determined that there is a need of measuring the uplink channel, a precoding matrix is selected which is of a larger order than would have been the case when there is not a need of measuring the uplink channel, i.e. a precoding matrix with a greater number of rows.

This forces the wireless device to use a larger order precoding matrix, which can then be evaluated in a later measurement by the network node (see a measure step 36 below). It is to be noted that even if the higher order precoding matrix is not the optimal choice, throughput is most likely not going to be zero. However, using the higher order precoding matrix provides a better measurement of the channel between the various antennas.

Also, using a higher order precoding matrix provides a better measurement of the uplink channel than a lower order precoding matrix, since the antennas of the wireless device are used better, e.g. being better distinguishable and separated. Hence, the purpose of utilising the precoding matrix of higher order for measuring the channel is that this effectively separates the individual spatial streams, i.e. transmission paths in the spatial domain of the channel when using multiple antennas on the transmitter and receiver side. For a low order precoding matrix, or let's say a precoding matrix of order one, the individual spatial streams of the channel are combined in the air and are not easily distinguishable. That is also the reason why a rank one channel is not suitable for transmission of multiple streams of data. In general, what we want to do is to update measurements of the uplink channel, e.g. in a channel matrix, and when updating these measurements, it should ideally be possible to measure the different spatial streams of the channel individually, since the spatial streams may not all share the same trend since the last update. To see that separation a precoding matrix of higher order is preferred. In a transmit command to use PM step 34, a command is transmitted to the wireless device to use the precoding matrix for uplink transmission which was determined in the select PM step 32. This command can e.g. be part of downlink control information.

In a measure step 36, the uplink channel is then measured. The wireless device uses the precoding matrix as commanded and the measurement of the uplink channel is updated. As explained in more detail below, these new measurements may then result in actions to change the precoding matrix and/ or rank to better utilise the current channel conditions. In one embodiment, the measuring also comprises measuring the uplink channel using SRS. This gives a more complete picture of the uplink channel, even if it comes at the resource cost of using SRS.

Fig 4B is a flow chart illustrating a method for measuring an uplink channel from a wireless device 2 of Figs 1 and 2 according to one embodiment. The method of Fig 4B is similar to the method of Fig 4A and only steps which are new or modified compared to the method of Fig 4A are described here. As with the method illustrated in Fig 4A, this method is executed for a particular wireless device and can be executed independently in parallel for a plurality of wireless devices.

In an optional obtain signal measurement step 29, a signal measurement is obtained. For example, the signal measurement can be the SINR for the uplink channel, which often is available for other purposes in the network node. In the determine need to measure step 30, when a signal measurement is obtained, the determination is based on the signal measurement. For example, looking at Fig 3 again, it is of greater need to measure when rank 1 is used and the SINR is high, as this could result in a switch to rank 2 which would then result in significantly higher throughput. In an optional determine rank step 38, a rank is determined for the uplink channel in dependence of a result from the measure step 36. Optionally, this comprises considering a result from the measure step 36, using SRS. For example, the result of the measurement can be to switch to a higher rank, which can result in a significant increase in throughput. See e.g. the difference in throughput in Fig 3 between the rank 1 lines 10,11 and the rank 2 lines 12, 13 above 30 dB SINR.

When a new (i.e. different from the current) rank is determined in the determine rank step 38, a command is transmitted to the wireless device to use the new rank for uplink transmission. In an optional determine new PM step 42, a new precoding matrix is determined for the uplink channel in dependence of a result from the measure step 36. Using the new measurements, it could, e.g. be determined to use a new precoding matrix of the same order or a new precoding matrix of a higher order.

Optionally, this comprises considering a result from the measure step 36, using SRS.

When a new precoding matrix is determined in the determine new PM step 42, a transmit command to use new PM step 34' is performed to transmit a command to the wireless device to use the new precoding matrix for uplink transmission.

Fig 5 is a schematic diagram showing some components of the network node 1 of Fig 1. A processor 50 is provided using any combination of one or more of a suitable central processing unit (CPU), multiprocessor, microcontroller, digital signal processor (DSP), application specific integrated circuit etc., capable of executing software instructions stored in a memory 54, which can thus be a computer program product. The processor 50 can be configured to execute the method described with reference to Figs 4A-B above.

The memory 54 can be any combination of read and write memory (RAM) and read only memory (ROM). The memory 54 also comprises persistent storage, which, for example, can be any single one or combination of magnetic memory, optical memory, solid state memory or even remotely mounted memory.

A data memory 53 is also provided for reading and/or storing data during execution of software instructions in the processor 50. The data memory 53 can be any combination of read and write memory (RAM) and read only memory (ROM). The memory 54 may also comprise persistent storage, which, for example, can be any single one or combination of magnetic memory, optical memory, solid state memory or even remotely mounted memory. The network node 1 further comprises an I/O interface 52 for communicating with the core network and optionally with other network nodes.

The network node 1 also comprises one or more transceivers 51, comprising analogue and digital components, and a suitable number of antennas 55 for radio communication with wireless devices within one or more radio cells, optionally using remote radio units and/ or sectors. The processor 50 controls the general operation of the network node 1, e.g. by sending control signals to the transceiver 51 and receiving reports from the transceiver 51 of its operation. In one embodiment, the I/O interface 52 is directly connected to the transceiver 51, whereby data to and from the core network is directly routed between the I/O interface 52 and the transceiver 51.

Other components of the network node 1 are omitted in order not to obscure the concepts presented herein.

Fig 6 is a schematic diagram showing functional modules of the network node 1 of Figs 1, 2 and 5. The modules can be implemented using software instructions such as a computer program executing in the network node 1 and/or using hardware, such as application specific integrated circuits, field programmable gate arrays, discrete logical components, etc. The modules correspond to the steps in the methods illustrated in Figs 4A-B. A signal measurement obtainer 60 obtains a signal measurement, which can be used by a measuring need determiner 61 described in more detail below. This module corresponds to the obtain signal measurement step 29 illustrated in Fig 4B.

The measuring need determiner 61 determines whether there is a need of measuring the uplink channel from the wireless device to the network node. This module corresponds to the determine need to measure step 30 illustrated in Figs 4A-B. l6

A PM selector 62 selects a precoding matrix in dependence of the need of measuring of the uplink channel. This module corresponds to the select PM step 32 illustrated in Figs 4A-B.

A transmitter 63 transmits a command to the wireless device to use the precoding matrix for uplink transmission. Also, when a new rank is determined by a rank determiner 65, the transmitter 63 transmits a command to the wireless device to use the rank for uplink transmission. Furthermore, when a new precoding matrix is determined by a rank determiner new PM determiner 66, the transmitter 63 transmits a command to the wireless device to use the rank for uplink transmission. This module corresponds to the transmit command to use PM step 34 illustrated in Figs 4A-B as well as the transmit rank step 40 and transmit command to use PM step 34' illustrated in Fig 4B.

A measurer 64 measures the uplink channel. This module corresponds to the measure step 36 illustrated in Fig 4A and 4B

The rank determiner 65 determines a rank for the uplink channel in dependence of the measurer 64. This module corresponds to the determine rank step 38 illustrated in Fig 4B.

The new PM determiner 66 determines a new precoding matrix for the uplink channel in dependence of the measurer 64. This module corresponds to the determine new PM step 42 illustrated in Fig 4B.

Fig 7 is a schematic graph illustrating SRS usage in the system of Fig 1, as used by the methods illustrated in Figs 4A-B. The horizontal axis represents time. Here ti is a current time, SRSi is a previous SRS and SRS ₂ is the next SRS, i.e. in the future. This defines a time period 17 to the previous SRS and a time period 18 to the next SRS.

This is optionally used in the determine need to measure step 30 of Figs 4A- B. Using the time periods 17, 18, it can be determined when the next SRS is predicted to occur. If either one, or a sum, of the time periods 17, 18 is greater than a threshold value, this can trigger a need to measure.

Using the embodiments presented herein, the need to use SRS for uplink channel measurements is greatly reduced. This improves channel

measurements both in quality and frequency, whereby better adaptation to current conditions (e.g. using precoding matrix and/or rank) is achieved.

The invention has mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the invention, as defined by the appended patent claims.